Handbook of Polymer Synthesis Second Edition Episode 10 pps

Polymerization in Solution The monomer is soluble in numerous solvents; however, the polymer precipitatesfrom most of these solvents at about 15% conversion during radical polymerization

in 1848 [3] Among its typical reactions he recognized that upon standing the fluid acrolein

is spontaneously converted to a white, solid, infusible, and insoluble product he calleddisacryl Later this substance has been proven to be the result of a spontaneouspolymerization [4–8] But it was not before the early 1940s that the career of acrolein as a

‘key compound’ in organic chemistry began [9,10] It is mainly used in the production

of D,L-methionine and acrylic acid In polymer chemistry, however, none of the acroleinhomopolymers has until now achieved technical significance, although the monomer

is difunctional and highly reactive, and the polymers are susceptible to modificationreactions [11–13]

The oldest method for the preparation of acrolein, the acid-catalyzed thermolysis ofglycerol (dehydration) at about 190
C, is still used today to obtain acrolein on alaboratory scale [3]:

ð1Þ

By support of KHSO4the yield can be enhanced up to 50% [14] Further possibilitiesare the reaction of gaseous propene with a suspension of HgSO4 in aqueous sulfuricacid [15]:

ð2Þ

or the pyrolytic cleavage of 2,3-dihydropyrane [16,17]:

ð3Þ

The first efficient and profitable manufacturing process for acrolein was established

by Degussa AG, Germany, in 1942 [8,18–20] It depends on the gas-phase condensation(addition and dehydration) of formaldehyde with acetaldehyde at 300 to 320
C In thepresence of alkaline silica gel catalysts yields as high as 82% were achieved

ð4Þ

In 1945, at the same time that the Shell Company commercialized the pyrolysis ofdiallyl ether [21], acrolein production began

ð5ÞWith the supply of large amounts of propene in the 1950s the search began to find

a system for its direct oxidation with molecular oxygen to yield acrolein Attempts withcuprous oxide marked the beginning of the technical development of alkene oxidation

in the gas phase by metal oxide catalysts [22] But this system showed weak points inthe conversion (20%) [23,24] and in the selectivity, with the consequence that most ofthe propene added had to be recycled and many side products had to be removed Thedevelopment and introduction of the bismuth molybdate/bismuth phosphomolybdatesystem (Sohio, 1957) as a catalyst [25–27] and the following application for propene

oxidation opened the door to problem control Specifically, for the system BiPMo12O52

catalyst on a SiO2support, a reasonable selectivity (maximum 72%) could be observed.However, the propene conversion (57%) was still low By a further development towardmodern multicompound metal oxide catalysts [28] the propene conversion could be raisedfrom 90 to 98% with a maximum yield of 80 to 90% The main side product (ca 5 to 10%)

is acrylic acid, which can be removed by distillation

Examples of catalysts are:

FeMoBiCoNiP oxide [29] (Nippon Kayaka), FeMoBiCoNiPK oxide [30] (NipponKayaka), FeMoBiCoNiPSm oxide [31,32] (Degussa), MoBiFeCoWKSNaLi oxide [33](Nippon Shokubai), MoBiFeP oxide [34] (Farbenwerke Hoechst)

Common conditions for a good performance are:

300 to 400
C reaction temperature, 1.5 to 3.5 s contact time, 5 to 8 vol% propeneconcentration, 150 to 250 kPa inlet pressure, 1 : 10 to 20 : 1% molar ratio propene/air/gaspassed over a solid catalyst of suitable shape

B Radical Polymerization

Acrolein, a member of the family of the polymerizable 2-alkenales and 2-alkenones,

is provided with an extraordinary tendency for polymerization Therefore, it may only bestored in the presence of a stabilizer (e.g., hydrochinone) in the absence of light, air, andmoisture because of spontaneous polymerization Even small amounts of initiator have theability to force acrolein polymerization radically, anionically, or cationically, partly in anexplosive manner According to the existing reaction conditions and the catalysts used,

it is possible to attain polymers of completely different shapes with characteristic features[9,13,35]

Radical polymerization prinicipally proceeds across the vinyl function [1,2-addition;Scheme (6a)], whereas ionic polymerization yields products mainly by an addition at thecarbonyl group [3,4-addition; Scheme (6b)] However, the third possibility, 1,4-additionacross the a,b and C,O double bond, is a subordinate process [Scheme (6c)] [12,36,37]

ð6acÞ

Because of the polymerization across one of the two double bonds in acroleinpolymers, the corresponding function remains pendant at the polymer backbone and isaccessible to derivation reactions or for analytical purposes [9,37].

Radical polymerization occurs exclusively across the vinyl function The remainingpendant formyl groups form hydrates and acetales without effort by intra- and inter-molecular condensation The following structure elements are able to arise, including thecharacteristic tetrahydropyrane rings [38–40]:

ð7ÞDue to numerous chain cross-linkings by actetal groups, radically manufacturedacrolein polymers are insoluble in water and in organic solvents They decompose above

200
C without fusing The polymerization itself is carried out in bulk, in aqueous solution,and in organic solvents The Polymer precipitates from the solution and can be removed

by filtration [11] To start the polymerization the following initiators are used: inorganicperoxides [41], organic peroxides [42,43], azo compounds [42,43], redox initiators [43–45],g-rays and others [46–49]

1 Polymerization in Bulk

The first spontaneous curing of acrolein observed was also the first polymerization inbulk [3] Later, this observation was examined more closely [4–6,50–52] Furthermore, aslow light- or g-ray-initiated polymerization is possible, yielding highly cross-linked glassyproducts [46,53,54] By means of AIBN or peroxides as initiators an explosive course

of the reaction is observed that causes problems in the carriage of the reaction heat [42,55].Therefore, working with only small amounts is recommended

Ag(I), Fe(II), and Tl(III) compounds, Na2SO3, NaNO2, and polyacrolein hydroxysulfonicacid [13,41,42,56,57] It is favorable to add the reducing agents to the aqueous solution ofthe oxidizing agent and acrolein.

3 Polymerization in Emulsion

A very favorable way to obtain acrolein polymers having molar masses of some100,000 g/mol is by emulsion polymerization [43,58–62] In oil–water emulsions thewater-soluble addition compounds of sulfuric acid (respectively, SO2) and polyacrolein areused as very suitable emulsifiers to produce stable polymer dispersions The emulsionpolymerization is started by water-soluble redox initiators The acrolein polymerscontaining adsorbed or chemical bond SO2serve as reducing agents Together with air incombination with oxygen donors [e.g., Fe(NO3)39H2O, H2O2, K2S2O8], a powerful redoxsystem is designed [63–65] Further examples are the systems K2S2O8/AgNO3 [60,61],

K2S2O8/(NH4)2SO4–Fe(II) compounds, and K2S2O8/Na2SO3[55] Other soluble polymers,such as gelatine, PVA, or methyl cellulose, combined with sulfuric acid or SO2, alsoaccomplish the double function of emulsifier and reducing agent [63,64] Polymerization inthe inverse emulsion (water–oil) has also been described [66,67] Aliphatic and aromatichydrocarbons make up the continuous phase, and acrolein exists in the aqueous phase

4 Polymerization in Solution

The monomer is soluble in numerous solvents; however, the polymer precipitatesfrom most of these solvents at about 15% conversion during radical polymerization.Molecular weights up to 100,000 g/mol and aldehyde contents above 65% can beachieved when the polymerization is carried out in polar solvents such as DMF,g-butyrolactone, or pyridine by means of hydroperoxides and nitrous acid derivatives

as redox catalysts [68] Deviations from this behavior are observed if DMF is used

as solvent and the polymerization is initiated by AIBN A microgel is formed here;after 16% conversion the clear reaction solution turns into a transparent gel [69].Polymerization in the presence of methanol initiated by means of azo compounds

or peroxides does yield soluble poly(acrolein), presumably because of the polymer’smolecular weight [70]

5 Radiation-Induced Polymerization

Bulk polymerization of acrolein under the influence of g-rays yields a highly cross-linkedglassy polymer, which is completely insoluble in organic solvents and also in aqueoussulfuric acid Gamma-ray-induced polymerization in solution, especially in water, is muchfaster than in bulk [46–48,54] Investigations of radiation-induced polymerizations in bulk

or in aqueous solution by means of a 60Co source yielded microspheres of different sizecontaining reactive formyl functions [49,71,72]

6 Solubilization of the Polymers

To solubilize the products of radically induced acrolein polymerization, the followingprocedures are used

Disproportionation of the aldehyde and acetale groups pending on the polymerbackbone by means of sodium hydoxide solution (Cannizzaro reaction) [73–75]:

ð8Þ

Formation of water-soluble addition products by the action of sodium bisulfite andaqueous sulfurous acid [76–78]:

ð9Þ

By dialysis of the primary addition product, the following equilibrium can be forced

to the right side yielding water-soluble, SO2-free acrolein hydrate [79]:

200
C were obtained which were soluble in organic solvents but insoluble in sulfurous acid[80,81] Structural analysis of the polymer’s repetition units gave rise to the assumption thatchain growth occurs mainly across the carbonyl group (3,4-polymerization, -structure units) [37,81] Furthermore, there is addition across the vinyl function (1,2-addition) and across both functional groups (1,4-addition) [82,83] The latter takes placeonly on a very small scale Consequently, copolymers are formed that contain the followingstructure elements: , partly in block arrangement (n þ m ¼ 1; m ¼ 0.7 to 0.8) [84]

In a water-free medium chain growth polymerization can be initiated by numerousmetal-organic or basic compounds, such as trityl sodium [81], butyl lithium [80,81],naphthyl sodium [80,81], benzophenone potassium [81], sodium methoxide [80,81], lithiumorganocuprates [85] and rhodium(I) complexes [86] or ammonia [87], tert-phosphines[80,88], aliphatic amines [89], cyclic amines [90], and aromatic amines (pyridine [91,92],

imidazole [93,94], N-ethylimidazole [95]) The reaction temperatures range from  60 C to

þ25
C, whereby the reaction rates as well as the properties of the products (composition)are influenced Higher temperatures lead to products having a higher content of aldehydeside groups and a lower content of vinyl side groups Weaker bases and solvents with lowerpolarity also favor the formation of polymers with aldehyde side groups [81] Acroleincan be polymerized by alkali cyanides in polar solvents such as THF or DMF [96,97]

At reaction temperatures below  10
C, only 3,4-connected products were obtained

2 Cationically

Few sources describe the cationic acrolein polymerization in bulk or in homogeneoussolution [7,12,42,80,98] Using trifluoroborane-diethyl ether or triethyloxonium-tetra-fluoroborate as initiators carbonyl and vinyl group containing polymers are obtained atreaction temperatures ranging from  80
C to room temperature The carbonyl content ofthese polymers varies from 9 to 15 mol% For this polymerization polar solvents such

as nitromethane or nitrobenzene are favorable When the polymerization is stopped atlow conversion soluble products (cf in 1,4-dioxane, CHCl3, THF, pyridine) are obtained.Adding tert-amines during the last step of the polymerizations results in the highest content

of carbonyl polymerization [9] At higher conversions or at prolonged storage the productsbecome cross-linked and insoluble All these products soften between 80 and 120
C

2 Cellulose dispersed in an acrolein solution (solvent: water, ethanol, acetone,ether, or benzene) was treated with g-radiation of a60Co source at 40 to 43
C

In addition to the formation of a network of cellulose, homopolymerization

of acrolein was observed Homopolymerization of acrolein could be avoided ifcellulose was treated with gaseous acrolein at a pressure of 103torr beforeradiation [106]

3 Acrolein was grafted onto poly(ethylene) which was exposed to electron beams.The remaining aldehyde groups could be transformed into hydrazone, oxime,and oxyacid units [107]

3 Oxidative Copolymerization

Acrolein and acrylic acid were copolymerized in aqueous H2O2solution at 60 to 90
C toform poly(aldehyde carbon acids) The Cannizzaro reaction took place if an aqueous

solution or suspension of this polymer material was treated with aqueous NaOH.The aldehyde functions disproportionated into carboxylate and alcohol groups to formpoly(hydroxy carboxylates) [108,109].

4 Anionic Copolymerization

Acrolein was anionically copolymerized with acryl amide and methyl vinyl ketone(r1¼2.02, r2¼0.06) at 0
C in THF with imidazole as an initiator [110] Copolymeriza-tions of acrolein with various aldehydes (e.g., acetaldehyde and benzaldehyde) werecarried out in THF at  30
C with NaCN as initiator [111]

5 Block Copolymers

1 Living oligomers of butadiene were functionalized by the addition of acrolein

or ethylene oxide and then treated with acrolein to yield block copolymers Thehomopolymerization of acrolein could not be avoided [112,113]

2 Short poly(acrolein) blocks were formed, if a,o-disodium oligobutadiene(initiated with sodium naphthalene in THF at  40
C) was treated with

Table 1 Parameters of the radical copolymerization

Methacryl nitrile 0.72 0.06 1.20 0.08 50 AIBN Dioxane [101]Methyl acrylate 0 7.7 0.2 20 K2S2O8þAgNO3 Water [100,101]

Vinyl acetate 3.33 0.1 0.1 0.05 20 K2S2O8þAgNO3 Water [100]

a pH 3.

b pH 5.

c pH 7.

acrolein Homopolymerization of acrolein did not take place The acrolein unitscould be cross-linked after an UV cure to form a poly(acrolein) network that can

be used as photo-polymer layers to prepare negative printing plates [114]

6 Graft Copolymerization

Acrolein could be grafted onto imidazole-containing polymers [poly4(5)-vinylimidazole)

or copolymers of 4(5)-vinylimidazole with acryl amide, styrene, 1-vinyl-2-pyrrolidone,4-vinylpyridine, acrylates, and methyl vinyl ketone] in ethanol or an ethanol–watermixture at 0
C under nitrogen [115–117]

7 Cationic Copolymerization

Cationic copolymerization of acrolein with styrene took place in methylene chloride,toluene, and 1-nitropropane with bortrifluoride-etherate as a catalyst at differenttemperatures ( 78
C to 0
C) [118]

E Modification Reactions of Poly(acrolein)

1 Radically Polymerized Acrolein (Redox Poly(acrolein))

Redox poly(acrolein) is one of the most reactive polymers and susceptible to a number

of modification reactions that lead to high conversions under mild conditions [9,11,37].Containing one pendant aldehyde function per repetition unit – either free or masked –poly(acrolein) possesses functional groups and can react basically in the following ways[37,39,40]:

(a) As a Polymeric Monoaldehyde(i.e., after the pyran rings’ cleavage, the aldehydefunctions developed react independent of each other) Examples are oxidations [119](e.g., with peracetic acid) and reductions [120,121] [e.g., to poly(allylalcohol)] of the C,Ogroup, or reactions with alcohols to acetales [122], amines to imines [39,123],hydroxylamine to oximes [124], or phenylhydrazine to hydrozones [39,123] The latterserve for the quantitative determination of the aldehyde group content

(b) In Condensation Reactions Representative reactions are aldol condensation[125,126] with formaldehyde taking place at the polymers’ a-carbons, and Knoevenagelcondensation [40,127] with C,H acidic compounds (e.g., malodinitrile)

ð11Þ

ð12Þ

(c) As a Polymeric Dicarbonyl Compounds For reasons of their masking in theform of pyran rings, reactions are favored in which two adjacent carbonyl functions areinvolved The intramolecular disproportionation reaction by Cannizzaro serves as a well-known example Under the action of alkali and due to the proximity and reactivity of thealdehyde groups, polymers with pendant hydoxymethyl (CH2OH) and carboxylate(COO) groups are formed [73–75].

ð13Þ

(d) As a Polymeric Semiacetate The semiacetale hydroxy groups are able toperform characteristic reactions without cleaving the pyran ring structure (e.g., thioladdition) [128]

ð14Þ

Because of the insolubility of redox poly(acrolein) [129], modification reactions mustalways start in heterogeneous systems and lead to soluble products gradually The alreadypresented water-soluble products of the reaction between poly(acrolein) and Na2SO3or

H2SO3[76–78] are still better precursors for modification reactions than is native redoxpoly(acrolein) They permit a reaction performance in homogeneous media

Apart from conversions with low-molecular-weight compounds, soluble andinsoluble redox poly(acrolein) can react with high-molecular-weight substrates Connec-tions with the following in vivo and in vitro occurring polymers are good examples of thatbehavior: poly(vinyl alcohol) [130,131], cellulose [130–132], proteins [130,131,133,134](e.g., collagen, gelatine [135]), enzymes [130,136,137], lectins [138,139], erythrocytes[140–142] and lymphocytes [140], leukemia cells [140,142], antibodies [133,143,144], andmetal complexing agents [145]

2 Anionically Polymerized Acrolein

Due to the high portion of pendant vinyl groups, the following reactions of this polymermaterial are possible:

1 Co- and graft polymerization with vinyl and acryl monomers in the form of atwo-step copolymerization process [146]

2 Autoxidation of the double bond and a subsequent connection with thepolymers’ remaining aldehyde functions [81]

3 Light-induced cross-linking across the vinyl group [147]

F Applications and Economic Aspects

The statement that acrolein homopolymers do not find technical applications does nothold for copolymers The already introduced poly(aldehyde carbon acids) (trade namePOC, Degussa, Germany) are strong complexing agents [108,109,148–151] They are able

to form complexes with cations such as Naþ, Mg2þ, Ca2þ, Fe3þ, Mn2þ, Cu2þ[152] (alsoreversible), with gaseous ammonia [11], with peroxides for stabilization purposes, orwith amino acids The material is used in water treatment as a water softener in detergents

or rinsing agents, and as a supported sequestering agent showing rising complexingactivity with increasing aldehyde content The ability to bind amino acids is utilized in thedetermination of the C-terminated end in proteins [153]

On a laboratory scale, acrolein homo- and copolymers are tested as polymericreagents, polymeric complexing agents, and polymeric carriers Poly(acrolein) micro-spheres can easily be bound to antibodies, proteins, and drugs containing primary aminogroups in a single step under physiological pH [71,72,139–145,154,155] Aldehyde groupsreact under mild conditions with primary amino groups forming the corresponding imino(Schiff base) linkage Reaction with sodium cyanoborhydride as reducing agent forms astable –CH2–NH– linkage [134]

ð15Þ

In this way poly(acrolein) particles may play an important role as immunoreagentsfor biological research

(This section was prepared by O Nuyken, R Bayer and J Bayer.)

1 Properties and Structure

Crotonaldehyde (2-butenal, crotonic aldehyde, b-methacrolein) is a colorless, stronglacrimatory, and toxic liquid The mutagene potential of crotonaldehyde and its role incancerogenese has been investigated [156–159] It has a melting point of  69
C and aboiling point of 102.2
C Crotonaldehyde and water form an azeotrop containing 24.8%water and boiling at 84
C Other physical properties are given in Refs [160] and [161]and the literature cited therein Technical crotonaldehyde consists of two isomers,

where trans-crotonaldehyde has an occurance of more than 95%.

Radical anions generated from metal–organic compounds are another group ofinitiators Based on polarographic investigations and Hu¨ckel calculations it was shownthat the polymerization of crotonaldehyde with benzophenone radical anions proceeds viaformation of a complex radical anion of crotonaldehyde [174] This complex accepts a

second electron from another benzophenone radical anion and a dianion is built that iscapable to grow and to build up the polymer chain This mechanism was corroborated byisolating 2,20-diphenyl-3-methyl-5-hydroxytetrahydrofurane from the solution [175] Itsformation can be explained by the following mechanism:

ð19Þ

The dianion is able to grow a polymer chain

Numerous initiators have been reported to be used in anionic polymerization ofcrotonaldehyde Some are shown inTable 2

Varying the conditions of the polymerization (initiators, temperature, solvent, etc.),polymers with different structures can be prepared Anionic initiators are leading topolymers containing monomer units bonded together via C–C– or C–O-linkages

ð20Þ

In the case of polycrotonaldehyde using sodium dihydronapthalide as initiator, both types

of linkages were found [184] An important change in the structure of the polymer chain

Table 2 Common initiators for anionic polymerization of crotonaldehyde

Various inorganic salts (e.g., K2CO3, NaNH2) [182,183]

was found by Rashkov et al [185] They compared the polymers started withhomogeneous initiators (e.g., potassium ethoxide) with heterogeneous initiators (e.g.,graphite inclusion initiators such as C8K) Homogeneous initiators cause polymerization

of the aldehyde groups even at low temperatures (e.g., 10 and 30
C) Heterogeneousinitiators inhibit the side reaction of the aldehyde groups so only the vinyl groupspolymerize even at high temperatures and concentrations of the monomer and/or initiator.The authors assume that the propagation of the active chain predominantly proceeds onthe surface of the heterogeneous initiator

Generally it was found that at low temperatures the polymerization of aldehydegroups proceeds to a larger degree [184,185]

A different polymer structure was obtained by using tert-phosphines as initiator.Koral [172,173] found a large amount of free carbonyl groups (conjugated andunconjugated) and ether groups together with a small hydroxyl concentration and someresidual unsaturation This structure results from a vinyl-type polymerization with

a simultaneous cyclization of some vicinal, pendant aldehyde groups The followingstructure is proposed:

ð21Þ

The anionic polymerization of crotonaldehyde was also carried out under highpressure with Et3N [186] It was found that the melting point and molar mass of thepolymer increase linearly with rising pressure or temperature

4 Cationic Polymerization and Field Polymerization

Cationic polymerization of crotonaldehyde is less important than anionic polymerization.With (EtO)3Al or (i-PrO)3Al as initiators, rather unstable polymers were obtained [187];with H3PO4 and PCl5only oil was formed [188] Polymerization of crotonaldehyde canalso be induced by high electric fields (several 107V/cm) [189] Field polymerization results

in the growth of organic semiconducting micro needles with side-chain cross-linking and

Pmax¼3

5 Step-growth Polymerization

The polymerization of crotonaldehyde and several amines (butylamine, ethylenediamine,triethylenetetramine, diethylenetriamine, hexamethylenediamine, aniline, melamine, anddiaminodiphenylmethane respectively diaminomaleonitrile) proceeds in two steps In thefirst step a Schiff-base-reaction between the aldehyde groups and the amino groups takeplace In the second step the vinyl groups disappear due to step-growth-polymerizationand lead to resins [190–196] The step-growth-polymerization of crotonaldehyde andalcohols like phenols and glycols leads to resins, too A review is given in Ref [160]

6 Copolymerization

Crotonaldehyde acts in copolymerization (with styrene, methacrylic esters, vinyl esters,vinylcaprolactam) as a retarder, and therefore only oligomeric products can be isolated[160,197]

up dental compositions [200] Citric acid were produced by hydrolysis of the polymericproduct of the lactonization of crotonaldehyde with ketene and water [201–203]

ð22Þ

The copolymerization of pyrrole, crotonaldehyde, and a polymerizable, organic acidleads to water-based resins or coating compounds [204]

B Methacrolein

1 Properties and Reactions

Methacrolein (2-methylpropenal, methacrylaldehyde, 2-methylacrolein, a-methylacrolein)

is a colorless, sharp (stinging) smelling, flammable, highly reactive, and lacrimatory liquidwith a melting point of  81
C and a boiling point of 68.4
C Methacrolein and waterform an azeotrop containing 6.7% water and boiling at 63.9
C The solubility in water is6% at 20
C Other properties are described in Refs [205–207]

ð23Þ

The reactions of methacrolein are analogous in many respects to those of acrolein.Dimerization of methacrolein occurs similar to the dimerization of acrolein via Diels–Alder addition, where methacrolein reacts both as a diene and a dienophile [205] Bytreatment with alkali tri-, tetra-, and pentamers are formed by Michael addition [208,209]

By exposure to air, methacrolein forms peroxides and acids The peroxide groups can beincorporated into the polymer chain [208]:

ð24Þ

Avoiding air by storage under nitrogen and avoiding iron salts [210], no inhibitor(hydroquinone) is required.

2 Synthesis

The following methods are used to synthesize methacrolein:

1 Catalytic oxidation of isobutane with oxygen [211–216]:

For laboratory use methacrolein can also be prepared by heating of Mannichaldehydes [232].

3 Radical Polymerization

Methacrolein polymerization can be carried out in bulk, inorganic solvents, and in water.Heating methacrolein without initiator gives a brittle, yellow polymer which does notcontain any free aldehyde groups Contrary to this free aldehyde groups were found byinitiation with peroxo or azo compounds [233] The polymer was described as clear andglassy For the radical polymerization of methacrolein in organic solvents, peroxo or azocompounds were used [79] With ammonium peroxodisulfate in DMF, molar masses of5,000 up to 21,000 g/mol were reached [234] The polymerization is aqueous media can becarried out in different ways:

1 Precipitation polymerization with redox systems [235,236] or peroxo compounds[208,237] as initiators Using peroxo compounds, the resulting molar masses arerelatively low (maximally reached 30,000 g/mol) due to the monomer causingchain transfer [208] This effect is also known from other aldehydes

2 Suspension polymerization [238]

3 Emulsion polymerization [233,239–241] In an extended investigation of theredox system K2S2O8–Na2S2O5, Andreeva et al [240,242,243] found thatpolymerization in emulsion failed because the interaction of bisulfite ions withthe monomer at the double bond causes a deactivation of the initiator

Further, the effect of proton formation during polymerization follows theequation

It has been suggested that the polymerization of methacrolein initiated bybenzophenone or naphthalene anion radicals is accomplished by electron transfer[246,248] In the case of benzophenone it was shown by polarographic methods andHu¨ckel calculations that the complex radical anion with methacrolein (if it is formed) isunstable and dissociates into benzophenone and the radical anion of methacrolein.The initiation with heterogeneous inclusion compounds such as C8K, C16K, or C24K

is explained as follows [251]: The initiation is preceded by adsorption of the monomer onthe initiator surface The monomer molecule absorbs an electron and converts into

an anion radical The latter remains fixed on the initiator surface due to coulombic

interactions with the counterion After recombination of the radical ends, a dianion isformed that is suitable for propagation The propagating anion ends probably remainfixed on the initiator surface.

In general four types of linkages between the monomer units are possible and founddepending on the reaction conditions [249,250]:

1 Bonded via C–C-linkage:

The relation of the former two is very sensitive to the polymerization conditions

At low temperatures polymerization of the aldehyde groups proceeds to a larger degree.The formation of the dianion (36) was found by Rashkov et al [249] The authors showedwith Hu¨ckel calculations that the formation of tetrahydropyrane rings in the propaga-tion reaction of methacrolein is energetically favoured These calculations lead to theassumption that these rings are formed as a result of the interaction of dianion (36) withmethacrolein molecules

ð36Þ

If the polymerization of methacrolein is initiated by a typical anionic initiatorsuch as BuLi, the polymer obtained does not contain tetrahydropyrane rings [251].When sodium dihydronaphthalide at higher temperatures and higher monomerconcentrations is used the formation of the cyclic lactones (35) is observed Alsoside reactions of the aldehyde group take place and the polymers yield is decreased.Koton et al [250] assumed that the side reactions with aldehyde groups are due tothe catalytic effect of the propagating anionic ends, consisting of an alcoholate group .–CH–O–Mtþ Here the same result as in the polymerization of crotonaldehyde

is found By initiation with a typical anionic initiator such as KOEt, side reactionswith the aldehyde groups in the polymer proceed to a considerable extend even

at higher temperatures In contrast to that the aldehyde groups were not involved

in side reactions if heterogeneous initiators (e.g., graphite inclusion compounds)are used

Also methacrolein is the base for new types of monomers like (phenylsulfonyl)-1-aza-1,3-butadiene as described in [252], which can be polymerizedanionically

3-methyl-N-5 Cationic Polymerization

Cationic initiators (BF3-etherate, SnCl4, or AlCl3) are used to form soluble polymers withfree aldehyde groups [253] Tertiary phosphines in the presence of secondary alcohols atlow temperatures are used by other authors [233]

Also an unsaturated cyclic acetal (2-isopropenyl-4-methylene-1,3-dioxolane), which

is formed from methacrolein and epichlorhydrine, can be polymerized via cationic polymerization [254]

produces copolymers with sulfite groups which leads to water soluble copolymers[270–272]

Copolymers of methacrolein are used as coating material for immuno assays[273]

III POLY(METHYL VINYL KETONE)

(This section was prepared by O Nuyken, A Riederer and S Bu¨chel.)

Vinyl ketones are an interesting class of monomers because various members ofthis group polymerize via a radical, anionic, and cationic mechanism Methyl vinyl ketone(MVK) – also named 3-butene-2-one – is its best examined representative The physicalproperties of poly(methyl vinyl ketone) (PMVK) depend on the polymerization conditionsand the degree of polymerization PMVK ranges from a viscous oil to a hard plastic orrubbery mass Polymers obtained with free radical initiators are amorphous materials withlow softening points (about 40 to 80
C) and poor thermal and chemical stability [274,275].The molecular weights are relatively low because of the lability of the protons in thea-position to the carbonyl groups

The polymers are soluble in the monomer and in numerous organic solvents, such asacetone, methyl ethyl ketone, tetrahydrofurane, dioxane, pyridine, or chloroform Theyare insoluble in aliphatic and aromatic hydrocarbons, carbon tetrachloride, ethyl ether,and water The reactivity of the carbonyl groups in homo- and copolymers obtainedfrom MVK allows many modification reactions PMVK itself has not found commercialapplications because of its instability, but great efforts have been made in synthesizingcopolymers with a wide range of physical properties: for example, the preparation of oil-and solvent-resistant rubbers with butadiene to replace styrene-butadiene rubbers orthe preparation of crosslinked resins by treating MVK-butyl acrylate copolymer withhydrazine or using a divinyl compound as comonomer [275] Crystalline products areobtained by anionic polymerization with some organometallic compounds They aresoluble in formic acid and show melting points of 140 to 160
C [276]

MVK, or systematically 3-butene-2-one, was first synthesized in 1906 by heatingb-chloroethylketone with diethylaniline [277]:

ð37Þ

Alternative synthetic routes are described below

1 Hydration of vinylacetylene in the presence of mercury salts [278]:

ð38Þ

2 Oxidation of 1-butene (formation of an olefin–mercury–salt complex and itsdecomposition with acid [278]:

ð39Þ

3 Thermal dehydration of b-ketoalcohol catalysed by weak acids [279]:

ð40Þ

4 Reaction of acetone with formaldehyde in the gas phase passing over lead zeolite

or alkali metal hydroxide-impregnated silica gel at 200 to 300
C [280]:

ð41Þ

5 Mannich reaction of acetone, formaldehyde, and diethylamine hydrochloridefollowed by pyrolysis at 150 to 210
C under reduced pressure [281]:

ð42ÞThe Mannich reaction is generally the method of choice Some physical properties ofMVK are summarized in Table 3

B Radical Polymerization

The radical polymerization of MVK is initiated by almost any common free-radicalinitiator in bulk, solution, emulsion, or suspension Marvel and Levesque [283] havepolymerized MVK in bulk with 0.5% benzoyl peroxide as intiator at 50
C and found a1,5-diketone structure, indicating a head-to-tail arrangement of the soluble polymer

ð43Þ

For producing polymers with good color stability, azobisisobutyronitrile (AIBN)

is favored All other catalysts leave residues or degrade the polymer during the

polymerization [275] UV- or g-irradiation can start the bulk polymerization [284].Therefore, quinone is added to the monomer for long-term storage As already mentioned,bulk polymerization is feasible, but better results are obtained in solvents such ascyclohexane or petroleum ether, which dissolve the monomer but not the polymer(precipitation polymerization) These polymers show not only higher rates of polymeriza-tion but also higher molecular weights, which has been attributed to the reduction oftermination relative to the propagation rate [285].

An interesting initiator for MVK is N,N-dimethylaniline or N,N-diethylamine [286]

As weak bases they do not polymerize, for example, methyl methacrylate or methylacrylate

It seems to be a fact that the a,b-unsaturated carbonyl group is building up theinitiating species, which is proposed to be an electron transfer complex of the following type:

ð44Þ

Table 3 Physical properties of MVK

toxic lacrimatory, liquid

[274,275,278]

Since MVK is completely miscible with water, an emulsion-type polymerizationwithout additional emulsifiers is possible For the oxodisulfate-silver nitrate initiator,the following mechanism is suggested:

ð45Þ

This method allows polymerization at  15
C Molecular weights on the order of

4  105g/mol were observed [287] Higher-molecular-weight PMVK can be obtained bydecreasing the solubility of MVK in water by adding sodium chloride and an emulsifiersuch as potassium caproate [288] (Table 4)

C Ionic and Group Transfer Polymerization

1 Anionic Polymerization

A wide variety of typical anionic initiators is described for the polymerization of PMVK.Grignard reagents are used as well as complexes formed of alkylaluminum or alkylzinccompounds with alkali metal alkyls (so called ‘-ate complexes’) Alkali metal initiators andalkoxides are also described Some examples are given in Table 5

The ethyl derivatives of aluminum, cadmium, magnesium, and zinc yield highlycrystalline polymers Organometallic complexes such as magnesium diethyl cobaltchloride, which coordinate strongly with the polymer anion and the monomer, producewhite crystalline PMVK PMVK obtained by alkoxides, sodium naphthalene, andn-butyllithium are intensively colored because of a partially occurring aldol condensation[292] The mechanism of these reactions has been studied intensively It is assumed that

Table 4 Radical polymerization of MVK

n-BuLi reacts in three ways with MVK:

ð46Þ

III acts as the propagating species in the polymerization reaction

Compounds such as AlEt3 form ‘-ate complexes’, which react to a ‘conjugateaddition’ product (II) In this case propagation occurs via type II intermediates

More details about the polymerization mechanism by ‘-ate complexes’ are given inthe section on a,b-unsaturated ketones.

Grignard reagents such as n-BuMgBr react as follows [295]:

2 Cationic Polymerization

Cationic polymerization of MVK is certainly not the method of choice However, if borontrifluoride etherate was added to a monomer-carbon dioxide mixture in petroleum etherpolymerization was observed [298] Acid-catalyzed polarography of MVK in methanol isalso considered to be a cationic polymerization For the polymer an alternating ketone-ether copolymer structure was suggested [299,300] The following reaction mechanism is

proposed (Structure (51)):

3 Group Transfer Polymerization

A nonionic way of polymerizing MVK is the group transfer polymerization (GTP)with dimethylketene methyl trimethylsilyl acetal as intiator and the Me3SiF2 aniondelivered from tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF2SiMe3)

[(Me2N)3S–F2SiMe3] as catalyst [301]:

ð52Þ

Metallocene-catalysts were successfully applied as initiators for the GTP [302,318,319] Especially adducts of group 4 metallocene-enolates and tris(pentafluoro-phenyl)boranes lead to a rapid polymerization by means of group transfer polymerization

similar to styrene in its copolymerizability, as indicated by its parameters, e ¼ 0.7 and

Q ¼1.0

The Q-value is identical to that of styrene and e has the opposite polarity [302].Therefore, the expected equivalent incorporation of the two monomers in the copolymerwas found Some examples of comonomers with the corresponding r1–r2value pairs aregiven in Table 6

The initiators used in copolymerization are the same as those in tion Benzoyl peroxide is also used in grafting MVK onto poly(cis-1,4-isoprene) to givesurface coating materials [308] The copolymers are also able to undergo some polymer-analogous reactions such as the cross-linking of a n-butyl acrylate/MVK copolymer withsulfur/zinc oxide [294] resulting in disulfide cross-linkages

homopolymeriza-2 Copolymerization in the Presence of Lewis Acids

Despite the results of pure radical copolymerization, it is more difficult to producecopolymers of MVK with styrene under ionic conditions Only a small amount (about2%) of styrene is incorporated in the polymer if catalysts such as Et3Al, Et2Zn, and

Et2Cd are used [276] It was more attractive to copolymerize MVK with styrene undercatalysis of Lewis acids such as AlCl3, EtAlCl2, or ZnCl2 The products obtained are

1 : 1 copolymers Although these reactions run without any radical initiator, shown bythe addition of hydroquinone, the yield of copolymer can be increased in the presence

of traces of benzoyl peroxide [309] The copolymerization behavior of MVK can bechanged by complexation of the monomer with Lewis acids [310] The 2 : 1 complex(MVK)2ZnCl2 can be copolymerized with allyl benzene, which is not possible withoutZnCl2 [311]

In recent years, conductive poly(methyl vinyl ketone) homo- and copolymers wereprepared [321,322] Conductivity was achieved by reacting PMVK with a dopant solutioncontaining POCl3 During the reaction double bonds are formed, namely the PMVK

is partly converted into poly(acetyl-acetylene) Conductivities from 107to 109S cm1could be achieved [323], which varied drastically with the time of treatment with the

Table 6 Reactivity ratios of some comonomers