Handbook of Polymer Synthesis Second Edition Episode 4 pdf

Formula and History The esters of acrylic and methacrylic acid, whose polymerization reactions are described in this chapter, are unsymmetrically substituted ethylenes of the general for

Polymers of Acrylic Acid, Methacrylic Acid,

Maleic Acid and their Derivatives

Oskar Nuyken

Technische Universita¨t Mu¨nchen, Garching, Germany

(This section was prepared by O Nuyken, G Lattermann, H Samarian, U Schmelmer,

C Strissel, L Friebe.)

This section is supposed to be a review of the background and possibilities of acrylateand methacrylate polymerization with a main focus on recent developments Additionalinformation and examples are given in the first edition of this book [1]

A Introduction

1 Formula and History

The esters of acrylic and methacrylic acid, whose polymerization reactions are described

in this chapter, are unsymmetrically substituted ethylenes of the general formula

ð1Þ

with R ¼ H for acrylates and R ¼ CH3for methacrylates The substituents R0may be of agreat variety: from n-alkyl chains to more complicated functional groups In the followingchapters these compounds are generally named acrylic esters, although in literature, esters

of other a-substituted acrylic acids (e.g., R ¼ –CN, –Cl, –C2H3) are sometimes included inthis term

The first report of a polymeric acrylic ester was published in 1877 by Fittig andPaul [2] and in 1880 by Fittig and Engelhorn [3] and by Kahlbaum [4], who observedthe polymerization reaction of both methyl acrylates and methacrylates But it remained

to O Ro¨hm [5] in 1901 to recognize the technical potential of the acrylic polymers Hecontinued his work and obtained a U.S patent on the sulfur vulcanization of acrylates

in 1914 [6] In 1924, Barker and Skinner [7] published details of the polymerization of

methyl and ethyl methacrylates In 1927 [8], based on the extensive work of Ro¨hm, thefirst industrial production of polymeric acrylic esters was started by the Ro¨hm & HaasCompany in Darmstadt, Germany (since 1971, Ro¨hm GmbH, Darmstadt) After 1934,the Ro¨hm & Haas Co in Darmstadt was able to produce an organic glass (Plexiglas) by

a cast polymerization process of methyl methacrylate [9] Soon after, Imperial ChemicalIndustries (ICI, England), Ro¨hm & Hass Co (United States), and Du Pont de Nemoursfollowed in the production of such acrylic glasses Nowadays poly(methyl methacrylate)(PMMA) as homo- or copolymer exceeds by far the combined amount of all otherpolyacrylic esters produced [10]

The most common procedure for the technical synthesis of the monomer methylmethacrylate (MMA) is the reaction of acetone cyanhydrine with water and methanol inthe presence of concentrated sulfuric acid [11]:

ð2Þ

ð3Þ

Many other processes and reactions of the monomer synthesis are described sively in literature [12–14] For different acrylic esters, especially on a laboratory scale, thealcoholysis of the corresponding acid chlorides as well as direct esterification reactions ofmethacrylic acid, but also transesterification reactions of MMA, are often preferred [13–15].The physical properties of various monomers are well summarized in literature [16,17]

exten-3 Reactions

Acrylic esters have two functional groups, where reactions occur: the ester group andthe double bond Reactions on the ester group are carried out under conditions thatprevent polymerization of the double bond (i.e., the use of polymerization inhibitorsand low reaction temperatures are necessary) Typical reactions of the ester function are:saponification, transesterification, aminolysis, and Grignard reaction [10,17] Reactions

of the double bond beside polymerization reactions are Diels-Alder reaction; Michaeladdition; and addition of halogens, dihalocarbenes, hydrogen halogenides, alcohols,ammonia and amines, nitroalkanes, or sulfur compounds such as hydrogen sulfide ormercaptanes [10,17]

Most acrylates are polymerized by both radical and anionic initiations, with theformer being the more commonly used In all cases the heat of polymerization must becarefully controlled to avoid runaway reactions The values of the heat of polymerizationfor selected methacrylates are listed in literature [18] In general, the rate of polymeriza-tion and the average molar mass must be controlled by the initiator and monomerconcentration and the reaction temperature In all cases the use of high-purity monomers

is important for proper polymerization conditions Therefore, the removal of inhibitors

is necessary Phenolic inhibitors such as hydroquinone, 4-methoxyphenol, or aromaticamines are usually removed by alkaline or acidic extraction [11,19] Otherwise, the

monomers are distilled from inhibitors of low volatility such as dyes (methylene blue,phenothiazine), aromatic nitro or copper compounds.

To prevent inhibition by dissolved oxygen, acrylic monomers must be carefullydegassed before polymerization [19] After the polymerization step, the isolation ofthe product is often necessary Depending on the polymerization technique, this may

be achieved by different procedures (e.g., precipitation, spray drying, breakdown of

a colloidal system, etc.) Purification of soluble polymers can be achieved by repeatedcycles of precipitation, or in the case of water solubility, by dialysis The removal ofsolvent may often be very difficult because of strong polymer–solvent interactions.Therefore the polymer is slightly heated above Tg under high vacuum, spray-dried, orfreeze-dried Freeze-drying with benzene, dioxane, or water results in a very dry, highlyporous material

1 Bulk Polymerization

In contrast to acrylic monomers the bulk polymerization of methacrylic esters is veryimportant in manufacturing sheets, rods, tubes, and molding material by cast moldingtechniques [9–11] Three important properties are characteristic of the bulk polymerization

of acrylates First, a strong volume contraction, being relatively high compared with othermonomers, occurs during the polymerization reaction (see Table 1) It may be overcomeeither by using ‘prepolymers’ (i.e., solution of polymers in their monomers, usuallyprepared by bulk polymerization until a desired viscosity level of the mixture [20]) or byforming rigid polymer networks even at low conversion through cross-linking agents.Second, the polymerization process is accompanied by a considerable reaction heat (seeTable 1), which is higher for acrylates than for methacrylates Therefore, after 20 to 50%conversion, causing an increased viscosity of the system, a drastic autoacceleration processmay be possible, known as gel or Trommsdorff effect [11,21,22] Thus it is necessary toregulate very carefully heat removal during the polymerization in bulk Third, at highconversion, branching and cross-linking reactions, leading finally to insoluble networks,may occur [23–25] This is due to chain transfer involving abstraction of hydrogen fromthe polymer chain, subsequent branching, and combining two branch radicals

Bulk polymerization is commonly started by radical initiators such as azocompounds and peroxides; however, some examples of thermal self-initiation of bulk

Table 1 Shrinkage and reaction heat of various methacrylates.a

a The percent shrinkage can be calculated by using the following equation:

% shrinkage ¼ 100  (D p D m )/D p (D m ¼ monomer density at 25
C; D p ¼ polymer

density at 25
C.

polymerization of MMA [27] and octylacrylate [28] are described For MMA, whichcannot form a Diels-Alder adduct, diradicals are believed to play a role in the thermalinitiating mechanism [29–31].

Different descriptions of general procedures for the bulk polymerization of acrylates(sheets, molding material) are given in Refs [12] and [19] The bulk polymerization ofg-alkoxy-b-hydroxypropylacrylates is described in Ref [32] Bulk atom transfer radicalpolymerization is reviewed in Ref [33]

2 Solution Polymerization

Several general disadvantages of bulk polymerization (removal of the reaction heat,shrinkage, nonsolubility of the resulting polymer in the monomer, side reactions inhighly viscous systems such as the Trommsdorff effect or chain transfer with polymer)are responsible for the fact that many polymerization processes are carried out in thepresence of a solvent A homogeneous polymerization occurs when both monomerand polymer are soluble in the solvent When the polymer is insoluble in the solvent, theprocess is defined as solution precipitation polymerization Other heterogeneous polymeriza-tion reactions in liquid–solid or liquid–liquid systems such as suspension or emulsionpolymerizations are described later Conventional solution polymerization is comparedwith solution precipitation polymerization for the synthesis of acrylic resins in Ref [34]

In homogeneous systems including inert solvents, the reaction rate decreases withdecreasing monomer concentration In solution precipitation polymerization, kinetics maydeviate from that in homogeneous solution

In nearly every polymerization system the influence of the solvent on the course ofthe reaction is important Thus chain transfer reactions with active chain ends occur inradical polymerization The solvent can also influence the stereoregularity of the product

in anionic polymerizations The boiling range of the solvents should correspond to that

of the monomers and to the decomposition temperature of the initiators Thus commonpolymerization temperatures are often between 60 and 120
C (under reflux of the solvent)

A general procedure for the radical homopolymerization of acrylates in solution is given

in Ref [35]

Not only acrylic esters that have intermediate solubility in water due to additionalhydroxy or amino groups can be polymerized in water, but also conventional acrylicmonomers with a relatively low water solubility (MMA: 15 g/L at room temperature) [36]can be polymerized in water Acrylate monomers of intermediate solubility in water, such

as hydroxyalkyl acrylates and methacrylates or aminoalkyl acrylates or methacrylates,undergo free-radical polymerization with a variety of initiator systems Both monomerclasses have been reviewed in the literature [37] Highly soluble monomers such as2-sulfoethyl methacrylates or the corresponding alkali salts are easily polymerized to highmolar mass by hydrogen peroxide in aqueous solution [38] Anionic initiation has beenaccomplished in a variety of solvents, both polar and nonpolar

Isolation and purification of the product is performed, for example, by addition of anonsolvent, leading to polymer precipitation or by removal of the solvent by spray drying

or by freeze drying in benzene, dioxane, or water Polymer precipitation should bequantitative However, PMMA with a degree of polymerization less than 50 is still solubleeven in methanol; thus petroleum ether is necessary to precipitate the low-molar-massPMMA [39] Numerous solvents and nonsolvents for polymers are reviewed in Refs [40]and [41]

In industrial processes it is sometimes advantageous to have a strong solvent–polymer interaction Thus solution polymerization is often performed for applications inwhich the solvent remains present (e.g., in protective coatings, adhesives, and viscositymodifiers).

3 Suspension Polymerization

The term suspension polymerization, often also called aqueous suspension polymerization

or pearl or bead polymerization, means a process where liquid monomer dropletsare suspended in an aqueous phase under vigorous stirring This process can be regarded

as a bulk polymerization within the monomer droplets, where the polymerization heatcan easily be dissipated by the surrounding water To prevent the coalescence of thedroplets, the presence of suspension stabilizers or suspending agents is necessary.Two classes of suspension stabilizers are known [42,43]:

1 Water-soluble polymeric compounds These can be natural or modified naturalproducts such as gelatine, starch, or carbohydrate derivatives such as methylcellulose, hydroxyalkyl cellulose, or salts of carboxymethyl cellulose Syntheticpolymers such as poly(vinyl alcohol), partially hydrolyzed poly(vinyl acetate),sodium salts of poly(acrylic acids), methacrylic acids, and copolymers thereofare widely used in quantities between 0.1 and 1% related to the aqueous phase

2 Powdery inorganic compounds Earth alkaline carbonates, sulfates, phosphates,aluminum hydroxides, and various silicates (talc, bentonite, Pickering emulga-tors) are used in quantities between 0.001 and 1% The initiator systems are thesame as for radical bulk or solution polymerization processes (e.g., peroxides orazo compounds) A typical recipe is given in Ref [44]

3 Nonaqueous dispersion polymerization is defined as the polymerization of amonomer, soluble in an organic solvent, to produce an insoluble polymer whoseprecipitation is controlled by an added stabilizer or dispersant The resultingstable colloidal dispersion ensures good dissipation of the polymerization heat.Stabilization of the polymeric particles is generally achieved by a lyophilicpolymeric additive

PMMA is mostly homo- or copolymerized in aliphatic hydrocarbon dispersions,using different rubbers, polysiloxanes, long-chain polymethacrylates, or different blockand graft copolymers as stabilizers An interesting variant of the dispersion polymeriza-tion of acrylates is carried out in supercritical carbon dioxide [45,46] Transition-metal-mediated living radical suspension polymerization is discussed in Ref [47] Commonradical initiators are described in Refs [48] and [49] The entire field is reviewed extensively

temperatures and redox systems [e.g., Fe(III) salts, cumene hydroperoxide] for temperature polymerizations.

low-Three types of surfactants are known: (1) electrostatic (anionic or cationic)low-molecular mass surfactants; (2) steric stabilizers such as poly(vinyl alcohol), or acombination of (1) and (2); and (3) electrosteric stabilizers such as polyelectrolytes.Furthermore, many other additives (protecting agents, cosolvents, chain transfer agents,buffer systems, etc.) are often necessary The entire field is reviewed in Ref [51],comprising the special kinetics of particle growth and formation, particle size, andmolecular mass distribution

Various emulsion polymerization procedures for the thermal and redox initiation

of acrylic monomers are given in Refs [52] and [53] Methyl, ethyl, and n-butyl acrylatesand methacrylates are found to form high-molecular-mass compounds quite easilythrough a plasma-induced emulsion polymerization system [54] Emulsions are thermo-dynamically unstable, although they often may have an appreciable kinetic stability.The use of a co-emulsifier (e.g., long-chain alkanes, alkanol or ammonium salts, or blockcopolymers of ethylene and propylene oxide) can produce microemulsions They arethermodynamically stable systems, exhibiting an average particle size of about 100 nm [55].Thus transparent microemulsions of MMA can be obtained which have been photo-polymerized together with a photosensitizer [56] The field of microemulsion is reviewed

in Ref [57]

A emulsifier-free emulsion polymerization of acrylates is possible by the use of2-hydroxyethyl methacrylate [58] Acrylate block copolymers (P(MMA-b-MAA)) wereused as surfactants in emulsion polymerization of acrylate monomers [59]

5 Irradiation Polymerization

Irradiation-induced bulk polymerization can be divided into two types: solid-statepolymerization and polymerization in the liquid state, classified as follows:

1 UV light: the initiation process is thought to occur via a free-radical mechanism

2 g-radiation: the induced polymerization process involves free radicals or ionicspecies, depending on monomer, temperature, dose rate etc [60]

3 Electron-beam, x-ray, or ion-beam radiation

Since most of the monomers do not produce initiating species with a sufficientlyhigh yield upon UV exposure, it is necessary to introduce a photosensitive initiator Thephoto initiator (PI) will start the polymerization upon illumination Thus, the PI plays akey role in light-induced polymerization for it absorbs the incident light and generatesreactive radicals or ions and it controls the reaction rate and the depth of cure profilewithin the sample There are various photoinitiators used in UV-curing applicationswhich can be classified into three categories, depending on the way the initiating speciesare generated:

that undergo homolytic C–C bond cleavage upon UV exposure with formation

of two radical fragments like benzoin ether derivatives, hydroxyalkylphenones,a-amino ketones, morpholinoketones (MoK) and bisacylphosphine (BAPO)from Ciba Specialty [61] Phosphine oxides undergo fast photolysis to generatenon-colored products (Scheme 4) Their higher initiation efficiency is caused by

a disaggregation that is fast, as the rate of initiation is directly related to the rate

of the PI photolysis

ν

ð4Þ

2 Radical generation by hydrogen abstraction: some photoinitiators tend

to abstract a hydrogen atom from a H-donor molecule via an exciplex,

to generate a ketyl radical and the donor radical The H-donor radical initiatesthe polymerization, the inactive ketyl radical disappears by a radical couplingprocess (5) This type of photoinitiators includes benzophenone and thiox-anthone

ν

ð5Þ

3 Cationic photoinitiators: like protonic acids

Oxygen as an initiatior in photo-initiated free-radical polymerization and linking of acrylates is reviewed in Ref [62]

cross-Methyl methacrylate does not appear to polymerize in the solid state upon simple

UV radiation [63,64] However, under pressure sufficiently high to solidify the monomer

at a relatively high temperature or in a ‘solid solution’ in paraffin wax, polymerizationwas found to be possible It is remarkable that the g-radiation-induced solid-statepolymerization is influenced significantly when the polymerization proceeds in tunnelclathrates [1]

Another possibility for irradiation-induced solid state polymerization is that inmono- or multilayers Thus acrylates or methacrylates with different long-chain estergroups are polymerized by UV light, g-radiation, or electron-beam radiation [65–67] Themajority of the examples given in the literature for irradiation-induced bulk polymeriza-tion deal with monomers in the liquid state as pure compounds Some examples are givenfor polymerization in the presence of inclusion compounds or related polymer matrices(see Refs [60,68–72]) Another possibility has been described as photopolymerization of

an oriented liquid crystalline acrylate [73]

Photo- or radiation-initiated bulk polymerization of acrylates is often used for theproduction of thick coatings or sheets Demonstration experiments are given in Refs [12]and [19] For many purposes (e.g., photocoating, embedding media, etc.) casting resinsoften contain multifunctional cross-linking compounds [74,75] A review of the chemistry

of photoresists, reacted by UV, eximer laser (deep UV), x-ray, electron-beam, andionbeam irradiation is given in Ref [76] In general, most industrial processes use a largevariety of copolymerization reactions

Besides the above noted polymerization techniques photocuring is a special processthat transforms a multifunctional monomer into a crosslinked macromolecule by achain reaction initiated by reactive species generated by UV irradiation [77] Three basiccomponents are needed for photocuring:

1 The already mentioned photoinitiator;

2 A functionalized oligomer, which by polymerizing will constitute the backbone

of the three dimensional polymer network formed;

3 A mono or multifunctional monomer, which acts as reactive diluent and willthus be incorporated into the network

UV-curable resins of acrylate and methacrylate monomers gained great commercialsuccess because they offer high reactivity and the possibility of creating a large variety ofcrosslinked polymers with tailormade properties On the other hand there are problemslike early gelation of the irradiated sample and mobility restrictions of the reactive sitesduring the preceding reaction and also with increased monomer functionality Novelacrylate monomers seem to circumvent these problems Very promising results have beenobtained by introducing a carbamate or oxazolidone group into the structural unit of amonoacrylate [77] As shown by the RTIR profiles, the light-induced polymerization wasfound to occur faster than with typical monoacrylates or diacrylate monomers

The UV-cured polymers based on the novel acrylate monomers show someadvantages: completely insolubility in organic solvents which makes these very reactivephotoresists well suited for imaging applications; high crosslink density; good resistance tomoisture, strong acids, weathering and thermal treatment [78]

Photopolymerization in micellar systems is useful for the synthesis of polymersdisplaying high molecular weights [57] The model of photopolymerization used todescribe a micellar polymerization does not differ from the one in bulk or solutionphotopolymerization [79]

6 Plasma Polymerization

A general introduction to the field of plasma polymerization is given in Ref [31] Theplasma used in polymerization processes is the low-temperature plasma or low-pressureplasma, which is usually created by an electric glow discharge caused by, for example,

microwave power sources There are two general methods in use to polymerize puremonomers First, in plasma-state polymerization the plasma reacts directly within thevapor phase of a monomer, resulting in the vacuum deposition of polymers [31,80,81].Here the course of the initiation reaction depends on the bombardment of the monomer

by excited species such as radicals, ions, metastable particles, and on the absorption of UVradiation emitted by the different excited species Concerning the UV-induced part ofplasma polymerization, the propagation will be maintained by a free-radical mechanism.Acrylic monomers are not described as undergoing such processes

The second way, plasma-induced polymerization, is characterized by the formation

of initiating species under the influence of a plasma and subsequent polymerization inthe condensed phase One possibility for the initiation process is that it can take place byexposing liquid monomers to a plasma of different gases (helium, argon, nitrogen, NO,CO2, O2, CF4) [82] for several minutes The presence of radical initiators, photo-initiators,and photosensitizers can influence the course of the polymerization reaction [83–86].This technique is used to polymerize thin films for coating purposes

Another possibility in plasma-induced polymerization is to expose the vapor phaseover a liquid monomer [31,87], volatile initiator, or monomer solution to the plasma forseveral seconds only Chain propagation occurs in the liquid phase during a longer period

of postpolymerization in the absence of plasma The unique feature of this way of induced polymerization is that the formation of initiating species takes place in the gasphase, presumably creating diradicals with a very long lifetime [31] In most cases the molarmass increases with reaction time (i.e., conversion) This is not the case in conventionalfree-radical polymerization, although the tacticity of the resulting acrylic polymerscorresponds to that observed in free-radical polymerization Some similarities of polymercharacteristics (gel permeation chromatography, thermogravimetry, differential scanningcalorimetry) can be observed between plasma-induced and thermal polymerization, theinitiation process of the latter also being caused by diradicals

1 Free Radical Polymerization

The kinetic scheme of this type of polymerization is equivalent to other classicalvinyl polymerizations, including initiation, propagation, chain transfer, and termination(Scheme 6)

Propagation: P1.þn 1 M !kp Pn.Chain transfer: Pn.þM !kc,M PnþM.

Pn.þL !kc, L PnþL.Termination: Pn.þPm. !kt,r PnPm Recombination

Pn.þPm. !kt, d PnþPm Disproportionation

ð6Þ

Common solvents include toluene, ethyl acetate, acetone, and 2-propanol Theboiling range of the solvents should correspond to that of the monomers and to the

decomposition temperature of the initiators Thus common polymerization temperaturesare often between 60 and 120
C (under reflux of the solvent).

Most common initiators are compounds decomposing to starting radicals bythermolysis The main classes for both organic and aqueous media systems are reviewedaccording to the following main groups:

1 Azo and peroxy like azobisisobutyronitrile (AIBN) and dibenzoyl peroxide(BPO) initiators [88]

2 Redox initiators such as peroxide tertiary amine systems or those based onmetals or metal complexes [89]

3 Ylide initiators such as b-picolinium-p-chlorophenacylide or others [90] Thisinitiating system is especially interesting with respect to alternating copolymers

of MMA

4 Thermal iniferters [91], a class of initiators that not only can start a polymericchain but can also undergo a termination reaction by chain transfer (initiator,transfer agent, chain terminator) The resulting end group is thermally orphotochemically labile, being able to undergo reversible homolysis to regenerate

a propagating radical These materials have been applicated in the synthesis ofblock and graft copolymers

General conditions for a successful application of radical initiators are [92]:

1 The initiator decomposition rate must be reasonably constant duringthe polymerization reaction The ‘cage effect’ (recombination of initiatorradicals before starting a polymer chain) should be small, which is generallymore the case with azo compounds than with peroxides

2 Side reactions of the free radicals (e.g., hydrogen abstraction with dialkylperoxides and peresters) should be reduced

3 In addition to initiators, accelerators and chain transfer agents are sometimesused Thus, with accelerators (often redox activators (e.g., ZnCl2 [93], cobaltsalts, tertiary amines [94]), the reaction temperature can be drastically reduced;with chain transfer agents the average molar mass of the resulting polymer can

be regulated

Concerning the growing radicals in polymerization reactions, they can be studieddirectly by ESR spectroscopy as in the case of triphenylmethyl methacrylate and MMA[95] In the latter case it was concluded that there are two stable conformations ofthe propagating radicals The steric effect of the a-methyl group of MMA is not onlyresponsible for the comparatively low heat of the polymerization reaction, but also for acertain control of the propagation steps Therefore, in radical solution polymerizationthe polymethacrylates exhibit in most cases a favored syndiotacticity

With respect to the termination mechanism in radical acrylate polymerization, someresults are reviewed in Ref [96] In MMA polymerization the preferred terminationmechanism is solvent dependent (e.g., disproportionation is being favored in benzene).For alkyl acrylates termination involves predominantly combination

As mentioned earlier, a general procedure for the radical homopolymerizaiton

of acrylates in solution is given in Ref [35] With a-substituted acrylates other thanmethacrylates, isotacticity is somewhat enhanced [97]

Tacticity of acrylate or methacrylate polymers obtained by radical initiators is animportant matter of research, as it influences the physical properties of the acrylatepolymers: for example, the higher the syndiotacticity, the higher the glass transition

temperature (atactic PMMA: Tg¼105 C [68]; highly syndiotactic PMMA: Tg¼123 C[97]) The polymerization of MMA by redox initiation within solid particles ofstereoregular PMMA affects the configuration of chains [68,94,97–99] There is a greaterconfigurational disorder in the resulting product than with the PMMA obtained throughbulk polymerization without a stereoregular PMMA matrix.

Capek et al polymerized various alkyl acrylates, methyl (MA), ethyl (EA), butyl(BA), hexyl (HA) and 2-ethylhexyl (EHA) acrylate, and alkyl methacrylates in micro-emulsion [100] Microemulsion polymerizations of BA and EHA reached in a short time aconversion close to 100% In case of PMMA the polydispersity index varied from 2 to 4.This can be taken as evidence that the chain transfer events contribute to the terminationmechanism [57]

Cyclic acrylates are known to undergo ring-opening polymerizations according tothe following scheme:

ð7Þ

For several examples with different R1and R2, it was shown that polymerization inbulk gave a copolymer of the structure given above Quantitative ring opening occurred(n ¼ 0) when this reaction was carried out in t-butylbenzene at 140
C [101]

Mathias et al explored the chemistry of functional methacrylates and developed

a one-step, inexpensive entry via the hydroxymethyl derivatives [102] The radicalpolymerization of the esters of alpha-hydroxymethylacrylate (RHMA) and the etherdimers were carried out in solution or bulk They developed a mild, general synthesis

of the ester of alpha-hydroxymethylacrylate 1,4-Diazabicyclo[2.2.2]octane (DABCO) wasthe catalyst for the addition of formaldehyde and activated vinyl monomers (Scheme 8)

ð8Þ

These alcohol monomers provide a versatile entry to a multitude of multifunctionalpolymers Derivatization before and after polymerization allows incorporation of variousfunctional groups such as ester, ether, thioether, amine, and siloxy groups

Isolated RHMA could be readily converted to the ether in high yield by heating neatwith amine base The ethers were found to be excellent crosslinking agents for organic-soluble monomers such as styrene and commercial acrylates The hydrolyzed diacid and its

salt provided crosslink sites for water-soluble monomers such as acrylic acid In addition,the ester derivatives of the diacrylate ethers underwent cyclopolymerization (Scheme 9).

ð9Þ

The unexpected dimerization of the alcohol monomers provides new materialscapable of cyclopolymerization, crosslinking organic and water-soluble polymers, andMichael polyaddition with dithiols and diamines

In free-radical copolymerization of two monomers the relationship between thecomposition of the copolymer and the initial monomer mixture is ruled by the monomerreactivity ratios r1 and r2 These ratios are related to an individual system of givencomonomers, initiator, and temperature [103] They are summarized in Ref [104] fornumerous systems

To estimate the reactivity ratios of new comonomer pairs, their Q and e values,

as summarized in Ref [105], can be compared The Q and e values are a measure of thereactivities and the polarities in a copolymer system A special solvent effect has beendescribed in the radical copolymerization of optically active acryloyl-D-phenylglycinemethyl ester with MMA or MA in D- or L-ethylmandelate as optically active solvent.The rate of polymerization was higher in the D-ester [106]

The copolymerization behavior of the different acrylates and methacrylates is largelyindependent of the nature of the ester group if there are no important interactions withthe monomer or solvent Thus copolymerization reactions between different acrylates orbetween methacrylates yields uniform products in the monomer mixing proportion [107].The reactivity ratios for the copolymerization of methacrylates (M1) with acrylates (M2)are given in a first approximation as r1¼2.0 and r2¼0.3 [107] If chemical uniformcopolymers are desired, the reaction should be stopped at a low conversion value (5%)[108] On the other hand, the sequence distribution can be controlled by, for example,changing the addition time of one of the monomers [109] Despite their structuralsimilarities, different methacrylates or acrylates are often incompatible [107] A typicalrecipe for the preparation of a suspension copolymerization of ethyl acrylate andMMA and of an acrylic solution terpolymerization of 2-ethylhexyl acrylate, MMA, andhydroxyethyl methacrylate is described in Ref [110] Among the numerous comonomers,styrene and a-methyl styrene are the most important for industrial purposes, as lightfastness and chemical resistance of the acrylics can be combined with the higher heatresistance of the styrene compounds Those copolymers are produced by bulk, solution,

or suspension techniques [111]

In principal, all homopolymerization techniques can be applied to randomcopolymerization For radical copolymerizations numerous examples have been describedbefore Some selected typical examples of other polymerization methods are listed inRefs [112–127] Methods for the radical and anionic copolymerization of MMA withstyrene are given in Ref [128].

The following examples of alternating copolymerization are given in the literature:

1 MMA with styrene through photopolymerization in the presence of borontrichloride, ethyl boron dichloride, or aluminum tribromide [129]

2 MA or MMA with styrene in the presence of ethylaluminum sesquichloride [130]

3 MMA with styrene, initiated by b-picolinium-p-chlorophenacylide [90]

4 MA with isobutylene, initiated by a complex system of Al(ethyl)Cl2and benzoylperoxide [131]

Polar side groups are useful to improve the adhesion of copolymers on surfaces, toreduce incompatibility with other polymers and to modify the solubility of polymers, or tosynthesize graft copolymers Common functional monomers for free-radical copolymer-ization with acrylic monomers are listed in Table 2 [132]

Side groups can be introduced by polymer-modification reactions; for example,

a hydroxy group can be converted to halides, tert-amino, nitro, sulfane, and disulfanegroups and to heterocyclic units [107] Acrylic monomers, with C ¼ C double-bondcontaining side groups, can be used for radical and anionic crosslinking

Acrylates and methacrylates of bi- and polyfunctional alcohols are often used forthe direct crosslinking copolymerization Common diols used to obtain relevant diestersare glycol, 1,4-butanediol, glycerol, 2,2-bis(hydroxymethyl)-1-butanol, oligo(glycolethers), and oligo(1,2-propane diol ethers) Allyl and vinyl ester are particularlyinteresting, due to the different reactivity of both polymerizable double bonds A typicalrecipe for the radical cross-linking of acrylamide, 2-hydroxyethyl methacrylate, andethylene dimethacrylate to a copolymer gel is given in Ref [133]

Radical techniques are also used for the synthesis of graft polymers The graftingpolymerization of MMA or its mixture with other comonomers from diene unitscontaining rubbers, in bulk or suspension [134–140], and from a terpolymer of styrene,MMA, and t-butylperoxy acrylate [141] Furthermore, redox reactions of OH-containingpolymers, such as poly(vinyl alcohol) [142–144] or poly(hydroxyethyl methacrylate) [144],but also natural products such as cellulose [145,146] or gelatine [147] with, for example,

Ce4þ are used to graft MMA side chains Otherwise, hydroxyl functions in starch havebeen reacted with methacrylic anhydride Subsequently, MMA was radically grafted fromthese sites [148] Other monomers, such as methacrylonitrile or styrene, have been graftedradically from a copolymer of MMA and an azo side group containing methacrylate [149]

Table 2 Common functional monomers for free-radical copolymerization with acrylic monomers

Carboxyl Acrylic acid, methacrylic acid, itaconic acid

Amino 2-t-butylaminoethyl methacrylate, 2-dimethylaminoethyl methacrylateHydroxyl 2-Hydroxyethyl methacrylate, 2-hydroxyethyl methacrylate

n-Hydroxymethyl n-Hydroxymethyl acrylamide

True radical ‘grafting onto’ reactions have not been described for PMMA since radicalrecombination does not occur separately On the other hand, ‘grafting onto’ functionalgroups with reasonable transfer constants are described for poly(vinyl chloride) orchlorinated rubber [150] or for a poly(diethylamino methacrylate) backbone [151].

2 Anionic Polymerization

‘Living’ anionic polymerization was first discovered by Szwarc et al in the fifties [152], andsince then, a lot of work has been done in this field as anionic polymerization allows aprecise control of the molecular mass and results in a narrow molecular mass distribution.Additionally, the tailoring of block copolymers is possible [153–155] The living character

of anionic polymerization and the higher reaction rates, compared with free radicalpolymerization, especially in polar solvents, necessitate special experimental techniques.They are well described in Refs [156–158]

Anionic initiation has been accomplished in a variety of solvents, both polarand nonpolar Typically, initiation can proceed by electron transfer reactions from alkali

or alkaline earth metals, polycyclic aromatic radical anions, or alkali and magnesiumketyls The other possibility includes the nucleophilic addition of organometalliccompounds to the monomers Related monofunctional initiators comprise alkylderivatives of alkali metals or organomagnesium compounds such as Grignard reagents.Difunctional species are alkali derivatives of a-methylstyrene tetramer or the dimer of1,1-diphenylethylene An overview of the initiation process in carbanionic polymerization

is given in Ref [159]

Ester compounds of the acrylic acid are polymerizable anionically only in certaincases, mostly with only partial conversion The polymerization of methacrylic esters,however, proceeds with minor problems The need for strong purification of themonomers, the in general required low reaction temperature, and the tendency for thecarbonyl group to participate in the polymerization, particularly during the initiationstage, are serious handicaps for its commercial application Considering these difficultiesand the big interest especially in block copolymers containing methacrylic esters, it is

no surprise that permanent efforts were devoted to the development of a ‘perfectly’controlled polymerization of these monomers in terms of molecular characteristics like themolecular mass and the molecular mass distribution, regio- and stereoselectivity and thedesign of block copolymers

As far as the stereoregularity is concerned, studies of various types of initiation showthat methacrylates could be polymerized to give as well as isotactic, syndiotactic atacticpolymers Numerous physical properties are tacticity dependent: for example, the rate ofwater absorption is higher for syndiotactic than for isotactic polymer [97], the transitiontemperatures of liquid crystalline methacrylic polymers can be specifically influenced[160–162], and the miscibility of polymer blends is changed [163–165] In general, thestereoregularity depends on the solvent used, the initiator, and the reaction temperature.Reviews have provided an overview concerning analysis, properties and reactivities ofpolymers with respect to their tacticity [97,166,167]

Highly isotactic PMMA can be formed in nonpolar solvents with lithium-basedinitiators or some Grignard reagents [97] A laboratory recipe for isotactic PMMA(>96%) with narrow molecular mass distribution through polymerization of MMA intoluene with t-butyl-MgBr is given in Refs [97,156,168] The polymerization proceeds in aliving manner as the molecular mass increases direct proportionally with the conversionand the result is a highly isotactic polymer with narrow molecular mass distribution

(Table 3) In case of polymerization of ethyl (EMA) and n-butyl (n-BuMA) methacrylatesunder the same conditions, a bimodal molecular mass distribution was observed Thesimilar isotacticity in both fractions, indicates the existence of two types of active species[169] The addition of (CH3)3Al to the polymerization of EMA recently has been found

to have the beneficial effect of allowing the synthesis of highly isotactic PEMA with lowpolydispersity [167]

Rather high syndiotactic PMMA in general can be achieved in polar solvents [e.g.,with bulky alkyllithium compounds in THF at –78
C (85%)] [97] In addition, certaintypes of Grignard reagents [e.g., 3-vinylbenzyl-MgCl in THF at –110
C (livingpolymerization)] were used successfully for the preparation of highly syndiotactic (97%)PMMA [170] Contrary to the above-mentioned rule, highly syndiotactic PMMA (9–8%)with small molecular mass distribution has been described in apolar solvents, too [e.g.,with the complex catalyst t-butyl-Li/Al(alkyl)3in toluene at –78
C] [171]

More recently, atactic living anionic polymerization has been achieved by usingt-C4H9Li and bis(2,6-di-t-butylphenoxy)methylaluminum [MeAl(ODBP)2] (Al/Li ¼ 5)

in toluene at low temperature [172,173] Thereby, the role of MeAl(ODBP)2 is thestabilization of the propagating species and activation of the monomer by coordination

As the stereospecificity of the polymerization strongly depends on the polymerizationconditions, e.g., the ratio of the initiator components in the binary initiator system,combinations of t-C4H9Li and MeAl(ODBP)2 can also provide stereoregular statisticalcopolymers of methacrylates acrylates [174,175] as well as stereoregular block copolymersand block copolymers [176,177] via living polymerization Replacement of the methylgroup in MeAl(ODBP)2 by other alkyl groups (Scheme 10) resulted in an increase

of syndiotacticity with the size of the alkyl rest as it is shown in Table 4polymerization

of ethyl methacrylate (EMA) with t-C4H9Li and alkylaluminum bisphenoxide (molarratio ¼ 1:3) in toluene at 78
C for 24 h) [178]

MMA 10.0 mmol, toluene 5.0 mL;bDetermined by VPO;cDetermined by SEC;dDetermined by 1 H-NMR;

e MMA 20.0 mmol, toluene 10.0 mL.

this kind of polymerization is often limited by the occurrence of side reactions, including(1) the attack of the initiator at the carbonyl double bond of the monomer or polymer,(2) chain transfer of a-situated protons, (3) 1,4-addition via the enolate oxygen instead of1,2-addition through the carbanionic centers [see Scheme 11], and (4) coordination of thecounterion of the active centers with carbonyl groups Additionally, the ion pairs tend toaggregate into much less active dimers and higher agglomerates However, despite thosecomplications, it is possible to obtain polymers of narrow molecular mass distributionand ‘ideal’ polymerization kinetics under appropriate conditions [179]:

ð11Þ

Therefore, several partially successful strategies have been developed to avoid thementioned side reactions One of these is the so called ligated anionic polymerization(LAP) The basic concept of LAP is the use of suitable ligands, which are able tocoordinate at the active initiating or propagating ion-pairs The three major functions

of the ligands are (1) to promote a new complexation equilibrium, with ion-pairs and/oraggregates, preferably leading to a single stable active species, (2) to modulate the electrondensity at the metal enolate ion-pair and thereby influencing stability and reactivity, and(3) to protect the ion-pair by effecting a steric hindrance, and thus avoiding back-bitingreactions of the growing anion [180] Two efficient classes of ligand systems have beeninvestigated quite recently:

m-type ligands, such as alkali metal tert-alkoxides [181,182], aluminum alkyls[183,184] and some inorganic lithium salts [185]

m/s-type dual ligands, such as lithium 2-methoxyethoxide (MEOLi) [186],lithium 2-(2-methoxyethoxy) ethoxide (MEEOLi) [187,188] and lithium ami-noalkoxide [189]

Tert-alkoxides, especially lithium tert-butoxide (t-BuOLi), have been used by Vlcˇek

et al in complex initiator systems with alkali metal ester enolates, such as ethyla-lithioisobutyrate MMA [181], t-butyl acrylate [190], 2-ethylhexyl acrylate [191] have

Table 4 Polymerization of EMA with t-C4H9Li and alkylaluminum bisphenoxide

been prepared with beneficial effects of the additive, but at least a 10-fold excess of theadditive with respect to the initiator was necessary to reach low PDIs An overview is given

in Ref [192] When aluminum alkyls are used as m-type ligands for MMA polymerization

in toluene a fairly broad molecular mass distribution is observed Adding Lewis bases asco-solvents, such as methyl pivalate and diisooctyl phthalate resulted in the synthesis ofsyndiotactic PMMA with low polydispersity, even at 0
C [183] Various lithium saltshave been investigated as additives in anionic polymerization of MMA Thereby, LiCl wasshowed to have a favourable effect on the anionic polymerization, as the initiator efficiencyhas been kept high and polymers with narrow molecular mass distributions have beenobtained This effect was remarkable only when less sterically hindered initiators likea-methyl styrene have been used [193] Substitution of LiCl by LiClO4 as m-type ligandresulted in the synthesis of well defined polymethacrylates due to the better solubility inhydrocarbons [185]

Lithiated alkoxyalkoxides, bidentate ligands of the m/s-type (see Scheme 12), havebeen intensively investigated and they restricted the tendency for back-biting reactions byforming strong complexes with the end of the ‘living’ chain Due to this higher stabilizingefficiency, they provide excellent control over polymerization of acrylates as well asmethacrylates at low temperatures in THF and toluene Best results for MMApolymerization were obtained with MEEOLi when the polymerization was performed

at very low temperatures in a moderately polar solvent (toluene/THF mixture) [194].The same observation was made for the polymerization of butyl acrylate [195] Theoutstanding role of toluene as solvent for MMA polymerization in the presence ofmonolithium alkoxyalkoxides has been shown by Mu¨ller et al [196]

Recently, polydentate dilithium alkoxides (dilithium triethylene glycoxides)(Scheme 12) have been shown to be suitable additives for the polymerization of methylmethacrylates, as they provide high initiator efficiencies and narrow molecular weightdistributions (1.1–1.3) The addition of dilithium triethylene glycoxide to the anionicpolymerization of MMA (THF, (1,1-diphenylhexyl)lithium as initiator) resulted in thesynthesis of well controlled polymers even at relatively high temperatures This beneficialeffect could be assigned to a better coordination with the enolate ion pairs, thus slowingdown the polymerization rates (Table 5)[197]

ð12Þ

Several reviews of anionic polymerization of methacrylates and acrylates in thepresence of stabilizing additives have been published in the last years [198–200].Additionally, mechanistic studies of the propagating species have been investigated[198,200–203]

Another quite recently developed method for the controlled polymerization ofmethacrylates via anionic polymerization is the screened anionic polymerization(SAP), investigated by Haddleton et al The systems are based on lithium aluminumalkyl/phenoxide initiators, which are synthesized in situ following the equation shown

in Scheme 13 The polymerization was proved to have a ‘living’ nature by sequentialmonomer addition experiments [204–206].

ð13Þ

a End-functional polymers and copolymers One advantage of living anionicpolymerization is the availabilty of telechelic polymers [207] and macromonomers, whichare of specific interest for the preparation of comb-like (if monofunctional) and network(if difunctional) structures [208,209] In addition, due to its ‘living’ nature, anionicpolymerization provides a versatile synthetic route for the synthesis of a wide range ofwell defined polymer structures Thereby, the steadily increasing capability of LAP offersnumerous possibilities, e.g., for the preparation of block copolymers

Fully methacrylic triblocks, containing a central rubbery poly(alkyl acrylate) blockand two peripheral hard poly(alkyl methacrylate) blocks, are potential substitutes forthe traditional styrene-diene-based thermoplastic elastomers (TPEs), which have relativelylow service temperatures Fully methacrylic triblock copolymers are able to coverservice temperatures due to the varying Tg from 50
C (poly(isooctyl acrylate)) to

190
C (poly (isobornyl methacrylate) [210] Poly(methyl methacrylate)-b-poly(n-butylacrylate)-b-poly(methyl methacrylate) triblock copolymers, which are precursors forpoly(methyl methacrylate)-b-poly(alkyl acrylate)-b-poly(methyl methacrylate) via selectivetransalcoholysis, have been synthesized by a three-step sequential polymerization ofMMA, tert-butyl acrylate (t-BuA), and MMA in the presence of LiCl as stabilizing ligand[211,212] Various diblock copolymers, such as poly(methyl methacrylate)-b-poly(n-butylacrylate) and poly(methyl methacrylate)-b-poly(n-nonyl acrylate), have been synthesized

Table 5 Anionic polymerization of MMA in THF at various temperatures using DPHLi asinitiator in the presence of DLiTG.b

a DPHLi ¼ (1,1-diphenylhexyl)lithium (initiator).

b DLiTG ¼ dilithium triethylene glycoxide (additive).

c 10 3 M n,calc ¼ (moles of monomer/moles of initiator)  100.

d Determined by SEC.

via LAP with lithium 2-(2-methoxyethoxy) ethoxide (MEEOLi) and lithium and low polydispersities have been observed (1.20–1.35) Sequential anionicpolymerization of MMA and n-BuA in the absence of MEEOLi resulted in polymers withmolecular masses, significantly differing from the calculated values, and with broadermolecular mass distributions (PDI ¼ 2.65) [188] Additionally, the synthesis of acrylatediblock copolymers was investigated in the presence of tert-alkoxides, such as t-BuOLi[192] Stereoregular block polymers and block copolymers are also described in literature[176,177].

diphenylmethyl-Besides, polystyrene/polyacrylate [193,213] and polydiene/polyacrylate [214] blockcopolymers have been synthesized via LAP Thereby, the addition of stabilizing ligands,such as t-BuOLi and LiCl, provided narrow molecular mass distributions of the resultingpolymer

3 Polymerization by Complex Initiators

In this section polymerization reactions in the presence of organometallic systems aresummarized Recent work by Yasuda et al [215] has revealed the potential of rareearth metal, [SmH(C5Me5)2]2or LnMe(C5Me5)2(THF) (Ln ¼ Sm, Y and Lu), to initiatepolymerization of polar and nonpolar monomers in a living fashion(Table 6).Polymerswith high molecular mass and narrow polydispersity can be obtained with high yield.The initiation mechanism was discussed on the basis of x-ray analysis of the 1:2 adduct

of [SmH(C5Me5)2]2with MMA An eight-membered ring intermediate is formed whichstabilizes the enol chain end, also allowing insertion of monomer Afterwards the chainend coordinates to the metal in an enol form, while the penultimate MMA unitcoordinates to the metal at its C ¼ O group (Scheme 14)

(mm ¼ 97 %, Mn¼500,000, Mw/Mn¼1.12) was for the first time obtained quantitatively

by the use of [(Me3Si)3C]2Yb (Scheme 15) [215]

ð15Þ

Polymerization of acrylic esters, i.e methyl acrylate, ethyl acrylate, butylacrylate, and tert-butyl acrylate, initiated by rare earth metal complexes were non-stereospecific [216]

Various block copolymerizations of hydrophobic and hydrophilic acrylates werealso investigated, i.e., ABA type triblock copolymerization of MMA/BuA/MMA, triblockpolymerization of MMA/EtA/EtMA, and block copolymerization of MMA/TMSMA

In recent years metallocene complexes have also been successfully used aspolymerization catalysts for methyl methacrylate Collins et al and Soga et al reportedthat cationic zirconocene complexes catalyse the polymerization of MMA [217–220].But these metallocene complexes consisted of more components than the only metallocenecomplex Ho¨cker et al investigated some novel single-component zirconocene complexes

as catalysts for the stereospecific polymerization of MMA [221] MMA was polymerized

by the cationic bridged zirconocene complex [iPr(Cp)(Ind)Zr(Me)(THF)][BPh4] attemperatures between 20 and 20
C The polymerization led to mainly isotactic PMMAdue to an enantiomorphic site mechanism and a low polydispersity index (1.12–2.33) Also

it has been assumed that the polymerization mechanism is of living character

Ho¨cker et al synthesized another zirconocene complex for the polymerization

of highly isotactic PMMA, namely Me2CCpIndZrMe(THF)þ

BPh

4 (Scheme 16: showingboth isomers) [222]

ð16Þ

Table 6 Results of the organolanthanide initiated polymerization of alkyl methacrylates

They also polymerized MMA with Me2CCp2ZrMe(THF) BPh4 at low tures yielding syndiotactic PMMA [222] Investigating the polymerization mechanism

tempera-it was proposed that a methyl group of a zirconocenium cation is transferred to acoordinated MMA molecule The resulting cationic ester enolate complex is the activespecies It activates the growing chain end as a donor and at the same time an incomingMMA molecule as an acceptor Thus the catalyst symmetry controls the microstructure

of PMMA

Concerning copolymerization various nickel and palladium-based catalyst systemscopolymerize ethylene and acrylates or polar 1-olefins at low pressure [223] WithBrookhart’s bisimine palladium complex simultaneous copolymerization and branchingwas observed Both polar and non-polar side chains were obtained, the ester side chainscan be used as cure sites in branched polyethylene rubbers (Scheme 17)

ð17Þ

4 Metal-free Polymerizations

In 1988 Reetz et al introduced the concept of metal-free polymerization of acrylates,methacrylates and acrylonitrile [224,225] Metal-free initiators are salts consisting of acarbanion (A) having R4Nþas cationic counterions They are synthesized by the reaction

of neutral CHor NH-acidic compounds such as malonic acid esters, nitriles, sulfones,nitro-alkanes, cyclopentadiene, fluorene derivates, carbazoles and succinimide Water isremoved azeotropically using toluene

Scheme (19) shows some examples of the synthesized initiators

ð19Þ

Anion and cation are connected to each other via H-bonds This often leads todimers in solution and in the solid state These species are also called ‘supramolecularion pairs’.

These initiator systems are capable of initiating the polymerizations of n-butylacrylate, methyl methylacrylate and acrylonitrile (PDI 1.1–1.4; molecular mass 1,500–20,000 g/mole) But it must be mentioned that the metal-free polymerization is not areal living process Backbiting and Hofmann elimination occur to a small but significantextent [226]

Another approach to PMMA is the polymerization of MMA using iodo-malonates

in combination with (nBu)4Nþ

I(1:1) as initiators, a new initiator system which is specificfor methacrylate, i.e., acrylates are not polymerized (Scheme 20) [227]

ð20Þ

The molecular mass can be controlled (1,500–20,000 g/mole), polydispersity values

in the range of 1.2 to 1.7 could be achieved, however the control of tacticity is not possible.Zagala et al investigated the polymerization of methacrylates in the presence oftetraphenylphosphonium (TPP) ion at ambient temperature The polymerization appears

to have living character [228] In case of MMA number average molecular massesincrease linearly with conversion and molecular mass distributions are narrow (< 1.30).Results of1H,13C and31P NMR studies indicated the presence of phosphorylides formed

by the addition of the PMMA enolate anion to one of the phenyls of the TPP cation.Mu¨ller et al managed to synthesize another metal-free initiator, namely the salt of thetetrakis[tris(dimethylamino)-phosphoranylideneamino]phosphonium (Pþ5) cation with the1,1-diphenylhexyl (DPH) anion, by a metathesis reaction between Pþ5 chloride and1,1-diphenylhexyllithium (Scheme 21) [229]

ð21Þ

5 Group Transfer Polymerization

In 1983, Webster et al reported a new living polymerization method, called group transferpolymerization (GTP) [230] This process consists of a continuously catalysed Michaeladdition of a silyl ketene acetal onto a,b-unsaturated ester compounds, mainly acrylic

ester monomers During the polymerization, the silyl group is transferred to the monomer,thus generating a new ketene function:

ð22Þ

ð23Þ

Beside this transfer mechanism Mu¨ller has proposed an associative mechanism,

at least for cases involving certain GTP catalyst components [231]

GTP, reviewed briefly in Refs [156,232–234], is controlled by the stoichiometry

of initiator and monomer and shows the characteristics of a living polymerizationmechanism Consequently, polymers with a controlled molecular mass up to 100,000 and

a narrow molecular mass distribution are obtained As an advantage over classical livingpolymerizations (anionic), GTP proceeds smoothly at room temperature In general thereaction temperature lies between 100 and 120
C, but 0 to 50
C is preferred But GTPdoes not produce polymers having a high degree of stereoregularity

Beside the silyl ketene acetal shown above, all silyl derivatives that add to acrylicmonomers, subsequently producing ketene acetals, can initiate the GTP (e.g., Me3SiSMe,Me3SiSPh, Me3SiCR2CN, R2P(O)SiMe3) [156] Bifunctional bis(silyl ketene acetals),which are interesting for subsequent block copolymers, have also been used [235] Stannylketene acetals and the corresponding germyl compounds are also known as initiators,although they lead to a somewhat broader molecular mass distribution than do thecorresponding silyl derivatives [234,236,237] Collins has developed an associative grouptransfer-type polymerization for methyl methacrylate based on zirconocenes [238].GTP is catalyzed by two different classes of compounds:

1 Anionic catalysts work by coordination to the silicon atom; they are needed inonly small amounts (0.01% based on the initiator) and are used preferably formethacrylic monomers [232] The anionic moiety comprises fluoride, azide, andcyanide, but also carboxylates, phenolates, sulfinates, phophinates, nitrite, andcyanates [232,236,237] These anions are often used in combination with theircorresponding acids as biacetate, H(CH3COO)2

or bifluoride, (HF2)

Thecounterions are usually tetraalkyl ammonium or tris(dimethylamino)sulfonium,[(CH3)2N]3Sþ

(TAS) The most widely used catalysts are TASHF2 andTASF2SiMe3 [239] To accelerate the reactivity of potassium bifluoride,KHF2, a crown ether (18-crown-6)-supported polymerization has been carriedout [240]

2 Lewis acids activate the monomer by coordination to the carbonyl group [241].Lewis acids are used preferably for acrylate monomers [232] Common catalysts

are zinc halides and organoaluminum compounds (e.g., dialkylaluminum halidesand dialkylaluminum oxides) [234] Mercury compounds such as HgJ2,Hg(ClO4)2, or alkyl HgJ also catalyze GTP with good results [242,243].Detailed descriptions of polymerizations of MMA, ethyl acrylate, and butylacrylate with either anionic or Lewis acid catalysts are given in Refs [156] and [234].Various other monomers, including lauryl, glycidyl, 2-ethylhexyl, 2-trimethylsiloxyethyl,sorbyl, allyl, and 2-(allyloxy)ethyl methacrylates have been employed in GTP [234].Because of the milder conditions, this polymerization method is generally much moresuitable than the classical anionic polymerization for monomers with reactive functionalgroups.

GTP of MMA with Lewis acid catalysts were reported to give PMMA with aratio of 2:1 syndiotactic/heterotactic triads, while anionic catalysts such as bifluoridesalts lead to a ratio of 1:1 [241] The influence of the temperature on tacticity is shown

in TASHF2/THF systems: With decreasing temperature, syndiotacticity increases from50% to 80% [234,244,245] A comparison of the triad distribution for anionic and GTpolymerizations of MMA with the same counterions under the same conditions showsthat the tacticities of both polymerization types are consistent [97,246] Some selectedexamples of the influence of different polymerization parameters on tacticity are given

in Ref [245]

The living character and different characteristic possibilities during GTP allowespecially the synthesis of either telechelics or block and graft copolymers Suchcharacteristic possibilities are:

1 Functionalized initiators Their use leads to terminal functionalized polymers.Thus, with phosphorus-containing ketene silyl acetals, trimethylsilyl methylsulfide, trimethylsilyl cyanide, dimethylketene-bis(trimethylsilyl)acetal, ordimethylketene-2-(trimethylsiloxy) ethyltrimethyl silyl acetal, terminal phospho-ric acid groups, thiomethyl groups, and cyanide, hydroxy, or carboxyl groupsare readily introduced [234] Furthermore, the styrene end group can also beachieved [247]

2 End-capping reactions Reaction of the living end groups with bromine yields anX-bromo ester [248] With 4-(bromomethyl)styrene a styryl-ended macromono-mer is available [249] Benzaldehyde gives, after hydrolysis, terminal benzhydrylalcohol groups [234] Terminal monofunctional polymers (e.g., living PMMAwith one masked OH end group) can be converted into bifunctional polymers byreacting the living center with bifunctional coupling agents such as 1,4-bis(bromomethyl)benzene [250] Three- and four-star polymers are obtainedwhen corresponding multifunctional agents were applied [232]

3 Functionalized monomers Since GTP is a much milder process than anionicpolymerization, for example, numerous functionalized monomers can

be polymerized Thus trimethylsilyl and 2-(trimethylsiloxy)ethyl, allyl, loxy)ethyl, and 4-vinylbenzene MA give polymers with functional groups alongthe chain, which were used for further modifications (e.g., for the synthesis ofgraft copolymers) [232,234,251]

2-(ally-Concerning grafting techniques, in GTP acrylates are much more reactive thanmethacrylates Thus 2-methacryloxyethyl acrylate in the presence of ZnBr2is polymerizedexclusively to a polymer with pendant methacrylate groups capable of radical and GTP

‘grafting from’ polymerizations [73] Irradiation techniques have often been employed

to create active sites on polymer backbones Thus alkyl acrylates and methacrylateshave been grafted from poly(ethylene) [252–254], poly(alkyl methacrylates) [255], orcellophane [256].

6 Catalytic Chain Transfer Polymerization (CCTP)

CCTP has its origins in biochemistry where coenzyme B12is used to conduct many radical reactions Enikolopyan et al were the first who used analogues of B12 forpolymerization [257,258] Methacrylate was polymerized by a catalyzed chain transferusing a cobalt porphyrine AIBN was used as initiator Two possible reaction sequencesfor the ‘catalytic’ aspect of CCT are described in the following scheme:

The efficiency of the reagents is influenced by the stabilizing base ligands A number

of bases have been used to enhance the transfer process, ranging from Et3N, which has theweakest effect, to (MeO)3P, which has the strongest

7 Living Radical Polymerization

Despite the long-time research in the field of free radical polymerization, this merization technique has been believed to be beyond reach of the precision control thathas been achieved in ionic living polymerizations due to the prevention of chain transferand termination reactions Nevertheless, many efforts have been made to realize the samecontrol in radical polymerization reactions The common general principle of the recentlydeveloped controlled radical polymerization processes is the temporarily transformation

poly-of the radical growing ends into more stable covalent precursors, called dormant species.This dormant species and the active radical are in a dynamic and rapid equilibriumdominated by the covalent species, and thereby suppressing the bimolecular radicaltermination reactions As a result, linear increase of the number-average molecular mass

Mn of the prepared polymer with respect to conversion as well as narrow molecularmass distributions are observed Although many systems, such as polymerization in thepresence of organocobalt porphyrine complexes [262], were investigated, the two most

widely used are stable free radical polymerization (SFRP) and atom transfer radicalpolymerization (ATRP).

8 Stable Free Radical Polymerization (SFRP)

In the 1990s the groups of Rizzardo and Georges reported a stable free radicalpolymerization process (SFRP) allowing the preparation of polystyrene with a narrowpolydispersity In the presence of stable free radicals, such as the mainly used 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), macromolecules based on styrene and styrenederivatives with well defined structures were synthesized [263,264]

In contrast, the extension of this promising polymerization process to acrylatesproved to be more challenging than expected Indeed, synthesis of random copolymers ofstyrene and low amounts of n-butyl acrylate provided high yields and narrow molecularmass distributions, but increasing the level of acrylate resulted in higher polydispersitiesand a lowering of conversion (Table 7) Additionally to random copolymerization, thismethod was applied for the synthesis of a poly(styrene-b-(styrene-co-n-butyl methacrylate)block copolymer [265]

The mechanism of SFRP [266] (Scheme 25) involves an equilibrium betweennitroxide-capped polymer chains and uncapped polymer chains Its success relies on theretention of the suitable amount of free nitroxide in the reaction to keep the propagatingpolymer radical chains at a concentration which allows the polymerization to proceed at asufficient rate but avoids bimolecular termination by coupling

Table 7 Effect of increased acrylate level in TEMPO-mediated stable free random

copolymerization with styrene

about 5% and a low molecular mass, generally below 4000, the careful control of theamount of oxygen allows to continue polymerization to higher conversions, rarelyexceeding 20% Similar results were obtained using initiator/nitroxide adducts for thecontrol of the initial amount of excess free nitroxide [268,269] (Scheme 26).

ð26Þ

In contrast, the performance of n-butyl acrylate polymerizations in the presence

of glucose as radical scavenger and reducing agent and sodium bicarbonate leads to apolymerization process with a ‘living’ nature to conversions around 60% and a molecularmass of approximately 30,000 in 6.5 h The living character of the poly(n-butyl acrylate)prepared in this manner could be proven by the formation of a block copolymer after theaddition of styrene As ene-diols were believed to be the active species hydroxyacetonewas used to substitute the glucose and the polymerization of n-butyl acrylate with BPOand hydroxy-TEMPO was performed with yields of 60–70% and molecular masses around60,000 were obtained after 8.5 h The living character can be demonstrated by theincremental increase in molecular mass during reaction time [270]

Another class of counter radicals, introduced by Mu¨llen et al and resulting in

a controlled polymerization of acrylates and methacrylates, are triazolinyl radicals

A molecular mass of ca 60,000 was achieved at a conversion around 35% [271] Besides,the addition of small amounts of camphorsulfonic acid (CSA) [272] and FMPTS[273,274] was examined Thereby, a reduction in the concentration of free nitroxideduring the polymerization to a level around 5  106M resulted in an improvement

of polymerization rates and consequently a higher molecular mass, but at a cost in thenarrowness of molecular mass distribution

Additionally, the synthesis of a wide range of block copolymers containingpoly(alkyl acrylates) was successfully performed by different groups in ‘living’ manner

by using N-oxyl radicals as radical stabilizing agents [275–281] In principle, first of all,

a nitroxide-terminated macroinitiator is synthesized Then a second monomer is addedand by heating, the relatively weak bond between the macroinitiator chain and thenitroxide end group is broken, which allows stable free radical polymerization of thesecond monomer to take place Several examples of synthesized polymers are presented

because of their potential application as surface active agents, pigment dispersants,flocculants, and compatibilizers in polymer blends after hydrolysis

As a solventless route to block copolymers the application of supercritical carbondioxide in the SFRP process was investigated This offers additional potential forproviding a higher complexity of macromolecular structures in the absence of organicsolvents Due to the increased diffusivity of monomer dissolved in the supercritical CO2and the plasticization of the polymer, the rate of polymerization of the second block can

be increased, and thereby, a one pot synthesis of block copolymers becomes possible[282,283]

9 Atom Transfer Radical Polymerization (ATRP)

Another interesting method of controlled radical polymerization has got its roots inorganic chemistry’s atom transfer radical addition (ATRA) [284,285], so named because itemploys atom transfer from an organic halide to a transition-metal complex to generatethe reacting radicals Extending this reaction to the synthesis of polymers, led to the atomtransfer radical polymerization (ATRP) [286] which was first developed by Matyjaszewski

et al Compared to other controlled radical systems, ATRP seems to be the most robustsystem due to its tolerance towards impurities (water, oxygen, inhibitor), and it can beused for a larger number of monomers Additionally, ATRP is a catalytic system, andtherefore, the polymerization rate can be easily controlled by the amount and activity ofthe catalyst

The control of the polymerization reaction afforded by ATRP is the result of theformation of ‘dormant’ alkyl (pseudo)halides This reduces the instantaneous con-centration of the active radicals and thereby suppresses bimolecular termination reactions.The reversible deactivation and activation leads to a slow, but steady growth of thepolymer chain with a well defined end group (Scheme 27) Control and properties ofthe synthesized polymers depend on the stationary concentration of active radicals andthe relative rates of propagation and deactivation When one or less than one monomerunit is incorporated into the polymer chain during one activation step, the polymeriza-tion is well controlled The ATRP equilibrium can be approached from both directions

in Scheme 27 Beginning with an alkyl halide and the lower valent metal complex, theprocess is called direct ATRP If a conventional thermal initiator like AIBN and thehigher valent metal complex are the starting materials, the polymerization process isnamed reverse ATRP [287]

ð27Þ

Table 8 Molecular mass and polydispersity of synthesized methacrylate block copolymers

a

2-(dimethylamino)ethyl methacrylate.

The molecular mass is controlled by the initial monomer-to-initiator ratio andmonomer conversion In case of well controlled polymerization molecular mass increasesdirect proportionally with conversion.

The multicomponent ATRP system consists of an initiator (alkyl (pseudo)halide,RX), a redox-active transition metal in its lower oxidation state (Mnt), ligands, adeactivator (XMnþ1t species) and the monomer ATRP is performed in bulk or in solution

at elevated temperatures [288] with the possible use of different additives One importantitem to regard is the fact that in ATRP one set of conditions cannot be applied to everymonomer class While neither polyacrylic nor poly(methacrylic) acid can be synthesizedwith currently available ATRP systems, because the monomers rapidly react with themetal complexes to form metal carboxylates, various acrylate esters can be polymerized byATRP (Scheme 28) [289] In analogy to these acrylate esters a wide range of methacrylateesters is expected to undergo ATRP

ð28Þ

a Catalyst System (Transition Metal and Ligands) Several transition metalsystems have been reported to control the radical polymerization of acrylic monomers.The metal is supposed to participate in a one-electron transfer redox cycle rather than atwo-electron process which would cause side-reactions like oxidative addition followed

by reductive elimination A higher affinity of the metal to group/atom X in comparisonwith the affinity to hydrogen and alkyl affinity should prevent transfer reactions (e.g., b-Helimination)

While ATRP of methyl acrylate was reported only for the copper catalyst system[290–292] methyl meth(acrylate) was also polymerized with copper [290,293–295],ruthenium/aluminum alkoxide [296,297], iron [298,299] and nickel [300–303] catalystsystems (Table 9).Thereby, it must be noted that in principle, the ruthenium-based systemproposed by Sawamoto et al requires the addition of Lewis acids, e.g., Al(O-i-Pr)3[297].Recent investigations showed, that the ‘half-metallocene’-type ruthenium(II) chlorideRu(Ind)Cl(PPh3)2 (Ind ¼ indenyl) led to a fast and well controlled polymerizationeven without the addition of Al(O-i-Pr)3, whereas in case of a polymerization withRu(Cp)Cl(PPh3)2 (Cp ¼ cyclopentadienyl), the addition of Al(O-i-Pr)3 is necessary Theactivity of Ru(II)-catalysts decreases in the order: Ru(Ind)Cl(PPh3)2>RuCl2(PPh3)2>Ru(Cp)Cl(PPh3)2[304]

In the case of the nickel(II)-bromide complexes, such as NiBr2(PPh3)2, additives likeAl(O-i-Pr)3should also be added to improve the control of polymerization [301], whereasfor NiBr2(Pn-Bu3)2 such additives are unnecessary [302] ATRP of MMA and n-BuAcatalyzed by NiBr2(PPh3)2 is reported with a reduced control of polymerization [301].Recently, it has been shown that increasing the monomer concentration is an interestingway to improve the polymerization rate while keeping the actual radical concentration low[304] The use of bis(ortho-chelated) arylnickel(II) complexes [299] as catalyst was alsoinvestigated and the polymerization of MMA without additional Lewis acids was shown

to be well controlled, whereas with the recently investigated zerovalent nickel complex,Ni(PPh3)4, the polymerization required the addition of Al(O-i-Pr)3[303] Possible catalystsystems are described in several reviews [289,305,306]

In addition to the metal ion, the halide ion has also got an influence on the kinetic

of ATRP by affecting the atom transfer equilibrium The use of copper bromide instead

of copper chloride leads to more rapidly decreasing polydispersities ( p-toluenesulfonylchloride/copper chloride ( p-TsCl/CuCl) conversion ¼ 25%, Mn¼8500, Mw/Mn¼2;p-TsCl/CuBr for the same conversion, Mn¼7800, Mw/Mn¼1.18 [294,295]) This can beassigned to the better efficiency of bromine in the deactivation step [307,308]

The ligand has got an influence on the ATRP by affecting the redox chemistry due

to its electronic effects, controlling selectivity by steric and electronic effects and bysolubilizing the catalytic system Thus, the use of bipyridine instead of 4,40-di-(5-nonyl)-2,20-bipyridine causes a lower control of the polymerization because of the reducedsolubility of the deactivator [309] In the case of bulk polymerization, well controlledpolymer structures are obtained, if substituted bipyridine is used Nonsubstitutedbipyridine as ligands in ATRP, were shown to allow ATRP in ethylene carbonate [310].Other effective p-accepting ligands like 2-iminopyridines [293] and some aliphaticpolyamines have also been described Replacing the bipyridine ligands with linear andtetraamines resulted in significantly faster and better controlled polymerization [311] Anexcess of triphenylphosphine to the polymerization of MMA via ATRP with NiBr2(PPh3)2

as catalyst has got a beneficial effect on the kinetic of this polymerization and results in alinear dependence of Mn on the monomer conversion [304]

b Alkyl (Pseudo)Halides In general, any alkyl halide with activating tuents on the a-C-atom, such as aryl, carbonyl, and allyl groups are potential ATRPinitiators The polyhalogenated compounds CCl4and CHCl3as well as compounds withweak R–X bonds, such as N–X, S–X, and O–X, can also be used as initiators for ATRP.The wide range and the role of the initiators for polymerization control have beendescribed in several reviews [288,289,306]

substi-The main role of the alkyl halide (RX) is to generate growing chains quantitatively.The structure of the alkyl group R preferably mimics the growing polymer chain.Therefore a-halopropionates are effective initiators for the ATRP of acrylates [288] Themain importance of the choice of the initiator for the polymerization of acrylates andmethacrylates is based on the requirement of a fast initiation to obtain molecular masscontrol A slow initiation results in higher molecular masses than predicted and a higherpolydispersity, which is specified in Table 9 for the polymerization of MMA [299] Similarresults were also observed for the phosphine-based Ni(II) complexes [301,302] and the Ni(0)complex Here, well controlled radical polymerization was only possible with bromideinitiators [303]

As group X bromine and chlorine seem to work good, obviously this group migratesrapidly and selectively between the growing chain and transition metal Another class

of initiating molecules for the polymerization of acrylates and methacrylates aresulfonylchlorides, which were reported by Sawamoto [312] and Percec [313–315].Table 9 Polymerization of MMA by FeBr2/dNbipy with different initiators.a

Arenesulfonylchlorides are described as the first universal class of initiators for the tional polymerization of styrene(s), methacrylates and acrylates as they initiate quanti-tatively and fast regardless of the substituents of these three classes of monomers [313].

func-c Solvents and Additives Typically, ATRPs are performed in bulk, but solventsmay be used and are even necessary in case of polymers which are insoluble in theirmonomers Solvents used are mostly nonpolar such as benzene, p-xylene, p-dimethoxy-benzene and diphenyl ether Some polar solvents such as ethylene carbonate, propylenecarbonate and water were also used successfully [294,316] A wide range of additiveshas been investigated, to study their effects on ATRP Matyjaszewski et al showed thatmoderate concentrations of water, aliphatic alcohols and polar compounds have little or noinfluence upon copper-mediated ATRP [317], whereas the addition of amine and phosphineligands leads to an inhibition of ATRP [289] The addition of various phenols, in contrast,resulted in an acceleration of ATRP of MMA [318] Sawamoto et al investigated alcoholssuch as methanol, 2-butanol, and 2-methyl-2-butanol as solvents for ruthenium-mediatedMMA polymerization and they observed that molecular masses grew directly proportional

to conversion and that molecular mass distribution was narrow [319]

Living polymerization in water also led to polymers with a relatively narrowmolecular mass distribution (1.1–1.3) and molecular masses, which showed linear increasewith conversion, indicating the living character of this polymerization [320] Recently,Matyjaszewski et al reported both reverse and direct ATRP of n-butyl methacrylate in

an aqueous dispersed system via the miniemulsion approach, characterized by a linearincrease of the molecular mass with conversion and a narrow distribution of molecularmasses [321] The suspension-type process of living polymerization of MMA in waternot only led to well controlled and high molecular masses and low PDIs, but also thepolymerization proceeded without the addition of Al(O-i-Pr)3 and clearly faster thanATRP in organic solvents [322]

As an environmentally friendly alternative to organic solvent, the use of supercriticalcarbon dioxide has recently attracted considerable interest It offers additional advantages

as low solution viscosity and the fact of being effectively chemical inert Fluorinatedmethacrylates were successfully polymerized in supercritical carbon dioxide and the

‘living’ nature was examined by low PDIs and the synthesis of block copolymers [323]

d Temperature and Reaction Time As the energy of activation for the tion in ATRP is higher than that for termination, higher kp/ktratios and therefore bettercontrol of polymerization are achieved at higher temperatures But at elevated temperatureschain transfer and other side reactions are of increased significance Thus, optimaltemperature has to be found for each ATRP system, depending on monomer and catalyticsystem as well as on the targeted molecular mass and on the desired reaction time Due to thehigher reactivity of acrylate radicals relative to styryl radicals, ATRP of MMA is proceeded

propaga-at lower temperpropaga-atures (70–90
C) than that of styrene (110
C) [289]

e New Materials by ATRP A major advantage of ATRP is the fact, thatpolymers with complex topologies and compositions can be synthesized using a quitesimple polymerization technique The possibilities of obtaining controlled compositions

by ATRP have been reviewed by various authors [288,308,324]

Bifunctional initiators were employed to gain telechelics [304] and trifunctional andtetrafunctional initiators were used for the preparation of star polymers [325,326].Additionally, halide initiators are helpful for the preparation of end-functionalizedPMMAs via ATRP Thereby, ATRP provides a possibility to attach other functionalgroups to the chain extremities [304] Azide displacement reactions are a very successfulapproach of functionalizing the terminating end of the polymer chains [327]

Successfully homopolymerized methacrylate and acrylate monomers(H2C¼CRCOOR0, R ¼ H, CH3) are: R0¼Me, Et, n-Bu, t-Bu, ethylhexyl, 2-hydroxyethyl,glycidyl, fluoroalkyl [288] Because of relatively similar reactivity of various monomers inradical polymerization a wide range of random copolymers can be synthesized via ATRP[304,328,329] Due to the ‘living’ nature of the polymerization, the obtained randomcopolymers have very similar amounts of comonomers, whereas in conventional freeradical copolymerization, the composition of polymer chains within a sample is quitevariable from chain to chain [289] If the reaction medium is slowly alternated from onemonomer to another, a compositional gradient along the chain is observed This method

is called gradient copolymerization [330,331] Physical properties of these gradientcopolymers were found to be quite different from those of the corresponding block andrandom copolymers [332]

Block copolymers have been prepared using ATRP via two ways: by the sequentialaddition of a second monomer to the polymerization medium after nearly completeconsumption of the first monomer [333] or by the synthesis of isolated and purifiedhomopolymers with functional end-groups as macroinitiators [319] The latter way allowsthe preparation of ABA block copolymers when bifunctional initiators are used, whereasthe first method enables the synthesis of triblock copolymers by addition of the firstmonomer after consumption of the second monomer [333] When RuCl2(PPh3)3 andAl(O-i-Pr)3 are used as catalyzing system, Sawamoto et al showed that the secondpolymerization step proceeds at similar rates as the first polymerization step, and themethod resulted in polymers with significantly increased molecular masses and evennarrowed molecular mass distributions This indicates the complete retention of thechlorine end-groups and their suitability for the re-initiation of living polymerization [319].Additionally, organic–inorganic hybrid polymers were synthesized using ATRP.The potential use of poly(dimethylsiloxane) in block copolymers for applications such asthermoplastic elastomers and pressure sensitive adhesives, resulted in intensive research inthis field [334,335]

Several interesting polymer structures were obtained by combining differentpolymerization methods A combination of TEMPO-mediated stable free radical poly-merization with ATRP was investigated, and thereby, graft copolymers with polystyrenebackbones and poly(t-butyl methacrylate) grafts were synthesized [326] The synthesizedpolystyrene-precursors, containing suitable initiating groups for ATRP by copolymeriza-tion with p-(chloromethyl) styrene are called macroinitiators Matyjaszewski et al reportedthe successful transformation of carbocationic into ‘living’ radical polymerization, resulting

in block copolymers Thereby, no modification was necessary for the initiation of the secondpolymerization step [336,337] This procedure allowed the synthesis of a ABA-blockcopolymers with a cationically obtained middle block of polyisobutylene (PIB) flanked

by methacrylate blocks [337] The possiblities of the transformation of other livingpolymerizations to controlled radical polymerization have been reviewed in Ref [338]

(This section was prepared by O Nuyken, G Staufer and M Scha¨fer.)

A Introduction

Polyacrylamides (PAAm) and polymethacrylamides (PMAAm) are of great technical andacademical importance The wide range of industrial applications of PAAm and PMAAm

is due to their high water solubility The most important uses for the polymers are asflocculating agents for minerals, coal, industrial waste, and so on; additives in papermanufacturing; thickening agents; agents for water clarifying; and uses in oil recovery[339–346] In addition, several dozen actual or potential applications have been mentioned[347–355] Among its many applications, PAAm is most commonly used as a crosslinkedhydrogel in electrophoretic separations of biopolymers [356–365].

The first report of polyacrylamide (PAAm) was given in 1894 by C Moureu [366], whowas also the first (one year earlier) to synthesize the acrylamide monomer (AAm), startingfrom acryloyl chloride and ammonia as reactants Although acrylamide was known for

a long time, commercial production began in 1954 by hydration of acrylonitrile Thestarting point is the reaction of acrylonitrile with sulfuric acid and water at 100
C to formacrylamide sulfate Several processes have been developed to remove sulfate [367–369].More detailed information regarding synthesis, properties, and reactions of AAm is given

in the literature [347,348,370] Methacrylamide (MAAm) is prepared in a similar way frommethacrylonitrile or directly from acetone cyanohydrine, (CH3)2C(OH)CN [371–377].This reaction is now the most important synthesis of MAAm

The synthesis of AAm by enzymatic transformation is attracting increasingattention Microbial nitrile hydratase converts nitriles into AAm This method has beenapplied to the industrial kiloton-scale production of AAms [378]

C General Aspects of Polymerization

Homogenous polymerization of AAm is usually performed in aqueous solution Theradical polymerization leads to a linear polymer of the general structure [379,380] whereby

n varies between 20,000 and 300,000 Polymer made by using anionic initiators shows atotally different structure, called nylon-3 or poly(b-alanine) [347] (Figures 1 and 2).PAAm and PMAAm can also be obtained by polymer analogous step Polyacrylateesters can react with amines to yield PAAm and PMAAm Polyacrylonitrile can besaponified to a copolymer of acrylic acid salt and acrylamide

PAAm is a linear, white, odorless polymer that exhibits very low toxicity Theamorphous polymer shows a glass transition temperature of about 190
C measured byDTA [381], although higher temperatures obtained by TBA (torsion braid analysis) [382]

Figure 1 Polyacrylamide

Figure 2 Nylon-3

and lower temperatures [383,384] are given in the literature PAA may be crosslinked byimide formation at temperatures >100
C The polymer starts to decompose at 220
C,ammonia is evolved At 335
C the second decomposition region begins due to thebreakdown of the polymer backbone and the imides to form nitrile units PAAm is highlysoluble in water, whereas it is insoluble in all common organic solvents, such as methanol,chloroform, and tetrahydrofurane.

In contrast to PAAm, polymethacrylamide does not seem to have any great technicalimportance in applications Investigations of molecular models show that the presence

of the two substituents (–CH3and –CONH2) leads to an inhibition of rotation about theC–C bonds and to a highly rigid polymer [385] The solubility of PMAAm is very similar

to that of PAAm

D Radical Polymerization

Most commercially available polymers are made by radical initiators Polymerizationcan be initiated by all types of radical sources, such as peroxides [386,387], persulfates[388,389], azo compounds [390–392], redox systems [393–394], UV light [395–396], x- [397]

or g-radiation [398], electro- [399] or mechanochemically [400] The radical polymerizationshows a strong dependence on temperature, pH, monomer concentration, polymerizationmedium [392,401], and activators [392] Water leads to the protonation of themacroradical, which in turn leads to an increase in the reactivity This is reflected

in high values of the chain growth rate constant and therefore the high molecularweight [402] (Figure 3)

A change of solvent (THF, DMSO, DMF) or solvent mixtures (water–methanol,water–DMSO) leads to lower rates of propagation and reduces the molecular mass, bound

up with a prolongation of the reaction time to complete conversion The main reason forthat behavior is based on the insolubility of the polymer in the solvents used The reactionbecomes heterogeneous and the polymer precipitates The propagation rate increaseslinearly with increasing monomer concentration Normally, 10 to 30% solutions ofmonomer are used for polymerization At higher concentrations deviation from linearitytakes place, caused by higher viscosity of the reaction medium Similar behavior wasshown for the increase in initiator concentrations Up to 5  103mol/L the propagationrate increases At higher initiator concentrations polymerization rate decreases because of

a higher termination rate A survey of characterization methods of PAAm (PMAAm)

is given in the literature [403] The structure is varified by spectroscopic methods Thedetermination of molecular weight and especially of molecular weight distribution is quitedifficult Gel permeation chromatography is not usable for two main reasons: because ofthe lack of effective column packings and because water-soluble high-molecular-weightstandards are not available However, several other methods, including light scattering,sedimentation, and viscosimetry, are used successfully to determine the molecular mass ofPAAm

Figure 3 Growing Chain

E Anionic Polymerization

Solution polymerization of AAm (MAAm) at high pH, caused by strong bases, yields apolymer with a totally different structure, called poly-b-alanine or nylon-3 [404].Nylon-3 exhibits interesting properties like high capacity of moisture uptake and highcrystallinity The polymerization is normally carried out in polar solvents with strongbases such as sodium hydroxide as initiators and in the presence of an inhibitor forradical polymerization Polymerization reactions yield a spectrum of products withfractions soluble in pyridine (A), fractions soluble in water (B), and fractions solubleonly in solvents such as formic acid (C) Fraction A consists largely of monomerand dimer Both other fractions are crystalline polymers with high melting points(325
C for B and 340
C for C) Fractions B and C differ only in degree ofcrystallinity; C is more crystalline, which can be shown by x-ray measurements.Polymers with a molecular weight average of about 80,000 (light scattering in 90%formic acid) show typical polyamide behavior Wet spinning from formic acid orchloroacetic acid yields fibers, but the polymer has not been produced commerciallyuntil now Another interest in nylon-3 arises from its ability to adopt conformationssimilar to the characteristic a-helix of polypeptides [405] For the mechanism ofpolymerization, two principal routes are discussed [406,407]: The first method ofinitiation is the reaction of base (B) with the vinyl double-bond followed by hydrogentransfer

B + H2C CHCONH2 BCH2 CHCONH2 BCH2 CH2CONH

– –

The second method or initiation postulates an acid–base reaction:

The propagation step is described by:

H2C CHCONH + HC CHCONH2 H2C CHCONHCH2CHCONH2

The following experimental data establish why the second mechanism presented ispreferred by the authors:

1 An unsaturated dimer could be isolated from the reaction mixture

2 The carbanion of the dimer can abstract a proton from any other species It isnot necessary that the proton transfer in every propagation step is intramole-cular

3 The molecular weight distribution is very broad, which should be expected forpolymers formed by the chain transfer

Electro initiated polymerization of AAm solutions leads to PAAm at the anode andnylon-3 at the cathode A radical mechanism for anodic polymerization and an anionicmechanism for the polymerization at the cathode has been proposed [408,409]

AAm can be polymerized in solution, bulk, inverse emulsion, suspension or asprecipitation polymerization [340] The solution polymerization is the oldest and mostcommon method for production of high molecular weight PAAm and takes place asbatch and continuous process A 10 to 70% solution of deoxygenated monomer inwater polymerizes rapidly at low temperatures with all common radical initiators Thepolymerization is started by increasing the temperature to 40–80
C, depending on theinitiating system The monomer concentration is limited by the polymerization enthalpy,the rapid kinetic and the molecular mass of the desired polymer Therefore transfer agentslike isopropanol are often used to reduce molecular weight [347,348] Many authors haveshown that polymerization of AAm (MAAm) is strongly influenced by temperature,solvent, concentration of monomer and initiator, additives (inorganic salts, Lewis acids)and pH value [392,401] It could be shown that propagation rate increases with risingtemperature A maximum velocity of polymerization is reached at 50 to 60
C At highertemperatures the propagation rate decreases because of side reactions (intermolecularimidization) and higher rates of termination

Bulk polymerization can be divided into two types: polymerization in the solidphase and in the molten phase Bulk polymerization is interesting for the followingreasons: (1) polymerization of crystalline monomer may lead to crystalline andstereoregular polymers, and (2) impurities, such as solvent, catalyst, and initiator,may be avoided However, only the second reason is realistic since polymer obtained

by solid-state polymerization is amorphous and shows no tendency to crystallize Thecrystalline matrix is unable to exert any appreciable steric control Furtherinvestigations have shown that propagation takes place at the polymer–monomerinterface, controlled by local strains and defects in the crystal Polymerization in themolten monomer soon becomes heterogeneous because of insolubility of polymer in itsown monomer

AAm can be polymerized by ionizing radiation (x-, g-, or UV-radiation) Crystals areirradiated continuously during polymerization at temperatures between 0 and 60
C.Monomer can also be exposed to g-rays at about 80
C, then removed from the radiationsource and allowed to polymerize at higher temperature with a lower propagation rate

If a limiting conversion is reached at one temperature, chain ends are still reactive.Polymerization can be continued by warming up to higher temperatures Molecular weightincreases with time, a transfer reaction to monomer occurs only to a very limited extend,and reaction with oxygen can be neglected [410]

Inverse emulsion polymerization is used for the preparation of polymers withultrahigh molecular masses For this type of polymerization, the expression ‘dispersionpolymerization’ is often used in the literature [410] A concentrated monomer solution(about 40% monomer in water) is dispersed under intensive stirring in aliphatic oraromatic hydrocarbons in the presence of additives (emulsifiers, protective colloids).Polymerization can be initiated by either water-soluble or oil-soluble initiators [411–418].The advantage of this process is based on the constant viscosity of the reaction mixture,

as the increase of viscosity takes place only in the dispersed phase By the use of additives(tensides), the dispersion inverts when the emulsion is stirred into water Precipitationfrom the aqueous solution yields a polymer with ultrahigh molar mass The quality ofpolymer made by inverse emulsion polymerization is influenced by the following factors:(1) species and concentration of initiator, (2) species and concentration of additives(emulsifiers, protective colloids), (3) type of oil phase, and (4) particle size of the dispersedwater phase Because of the easy modification of all these parameters, much attention hasbeen given in recent years to water-in-oil emulsion polymerization of AAm and MAAm.For suspension polymerization the initial system is obtained by dispersion of anaqueous monomer solution in an organic liquid by mechanical stirring in the presence ofstabilizers [402] The dispersion medium may be represented by aromatic and aliphaticsaturated hydrocarbons The polymerization is initiated by water-soluble initiators, UV org-radiation The process occurs in droplets of an aqueous monomer solution (diameters0.1–5.0 mm) that act as microreactors [419,420]

Precipitation polymerization takes place in organic solvents or in aqueous organicmixtures, that serve as solvents for the monomer but as precipitates for the polymer.During the process the precipitation of the polymer takes places and polymerizationproceeds under heterogeneous conditions The advantage of precipitation polymerization

is that the medium never gets viscous and the polymer is easy to isolate and dry

The degree of hydrolysis in 10 M NaCl at 100
C becomes  95%, but alsodegradation of the macromolecule occurs.

Hofmann degradation of PAAm leads to polyvinylamine [421–424] (Figure 6).Polymers containing N-methylol groups can be synthesized by the Mannich reaction

of PAAm [425–428] (Figure 7)

In principal it is possible to synthesize AAm derivates first or to change the amidegroup by polymer analogous reaction Many P(M)AAm derivates are synthesized andcharacterized especially in biochemistry [429–433] This leads to a wide variety of, e.g.,antitumor agents [434,435], optical active polymers [436], biorecognizable polymers [437]and gels [438–440]

(This section was prepared by O Nuyken, T Volkel, and V.-M Graubner.)

A Introduction

ð29Þ

Figure 5 hydrolysis of PAAm at high pH

Figure 6 Hofmann degradation

Figure 7 Mannich rection

Acrylic and methacrylic acid (propenoic and 2-methylpropenoic acid) are the basiccompounds of a large number of derivatives, such as acrylonitrile, acrylamide, metha-crylamide, acrylic esters, and methacrylic esters Homopolymerization of these acids are

of minor technical importance; however, they are often used as comonomers to improvespecial polymer properties At room temperature glacial acrylic acid and methacrylic acidsare clear colorless liquids with sharp penetrating odors that resemble the odor of aceticacid At lower temperatures they freeze to colorless prismatic crystals [441] Acrylic acidtends to spontaneous polymerization, which can be explosive Therefore, an importantinhibitor for storage is hydroquinone monomethyl ether and the storage material has to bestainless steel, glass, or ceramic Rust can start polymerization To avoid separation of thestabilizer during crystallization, acrylic acid should be storaged above the melting point(13
C) Above 30
C, dimerization to 2-carboxyethyl acrylate proceeds (Scheme 30) Thestabilization and storage of methacrylic acid are analogous Some important physicalconstants of the monomers are listed in Table 10

ð30Þ

Poly(acrylic acid) and poly(methacrylic acid) are hygroscopic, brittle, colorless solidswith glass transitions of 106 [443] and 130
C [444], respectively Above 200 to 250
C theylose water and become insoluble cross-linked polymer anhydrides Poly(methacrylic acid)depolymerizes partially at this temperature The anhydride is not hydrolyzable by wateralone but by aqueous alkaline solutions at room temperature [443] Decomposition takesplace at about 350
C

Carefully dried polyacids (e.g., by freeze-drying) dissolve extraordinarily well inwater, even with high molar masses After rigorous drying the solvation rate decreases.Other solvents for these polyacids are dioxane, dimethylformamide, and lower alcohols;nonsolvents are acetone, ether, hydrocarbons, and the monomers The solubility ofpoly(acrylic acid) increases with temperature, while the solubility of poly(methacrylic acid)decreases [445] The solubility of the salts of the polyacids depends in a complex way on the

pH value and the counterions Alkali and ammonium salts are water soluble Polyvalentcations form in water-swellable gels The viscosity of aqueous solutions increases with theamount of polymer, to a constant value Due to this experimental fact, it is not easy tocalculate molar masses from the intrinsic viscosities [446]

Table 10 Physical properties of acrylic and methacrylic acids [442]

Concentrated aqueous solutions of poly(acrylic acids) are thixotropic [447],

of poly(methacrylic acid) are rheopectic [448] Acrylic acid and methacrylic acid easilycopolymerize together or with acrylic and methacrylic esters, acrylonitrile, vinylpyrroli-done, styrene, and others The copolymers are of technical importance Copolymers withfour to six different compounds are quite common

The simplest and most economical method for preparing polymers is polymerization

in aqueous solution To undergo the problems of handling of the solution and ofthe removal of the polymerization heat at higher molecular masses and concentrationsbiphasic systems are used: suspension polymerization, precipitation polymerization, etc.The latter can also be performed in aqueous solution by addition of acids or salts reducingthe solubility of the polymers Also suitable are organic solvents in which the monomersbut not the polymers are soluble Another method is reverse emulsion polymerization[449] Here one needs emulgators, which form small stable drops of monomers in an inertorganic solvent The size of the resulting polymer particles is variable Polymers andcopolymers of acrylic acid and methacrylic acid are also available by acid or basichydrolysis of polynitriles, polyesters, and polyamides Technical significance has the basichydrolysis of poly(acrylamide)

1 Manufacturing of Acrylic Acid [450]

1 The propylene oxidation process is very attractive because of the availability ofhighly active and selective catalysts and the relatively low costs of propylene Itproceeds in two steps: the first giving acrolein and the second, acrylic acid [451–454]

ð31Þ

2 In 1953 Walter Reppe [455] discovered the reaction of nickel carbonyl withacetylene and water to give acrylic acid In the commercial process nickelchloride is recovered and recycled to nickel carbonyl