Handbook of Polymer Synthesis Second Edition Episode 15 pdf

Carothers, p ¼ extent of reaction is the fraction of bonds which have been formed between the monomers of the system, i.e., the ratio ofthe actual number of bonds at the given moment to

is available and considerable technological input is made to design the network in order tomatch the demands.

This contribution to chemical crosslinking does not include the use of electronbeam or g-irradiation These methods have some advantages over the use of chemicalcrosslinking agents as they do not leave behind toxic, elutable agents Also it does notinclude peroxide initiated radical crosslinking of saturated polymers which proceedsrandomly by hydrogen abstraction from chain segments and coupling reactions of theseradical sites

This contribution does also not include ‘physical’ almost reversible crosslinkingdue to microphase separation of block copolymers, to strong hydrogen bonding or to ionicinteractions or to crystallite formation

Any formation of a polymer network starts from monomeric, oligomeric or polymericindividual molecules which react in solution, in melt or in the solid state It is necessarythat at least a small fraction of these molecules has a functionality f  3 to undergo bondformation with another individual From each individual molecule may emanate zero to fbonds to neighboring molecules and thus this molecule may participate in the formation

of a large cluster of molecules which is called a macromolecule In the so-called sol–geltransition, an infinitely large macromolecule is formed This infinitely large macromolecule

is called a gel whereas a collection of finite clusters is called a sol independently fromthe fact that the gel may be formed by crosslinking the molecules in the solid state A gelusually coexists with a sol: the finite clusters are then trapped in the interior of the gel.Gelationis the phase transition from a state without a gel to a state with a gel, i.e., gelationinvolves the formation of an infinite network [1,2,5,6–9].

The conversion factor p (see W H Carothers, p ¼ extent of reaction) is the fraction

of bonds which have been formed between the monomers of the system, i.e., the ratio ofthe actual number of bonds at the given moment to the maximally possible number of suchbonds Thus, for p ¼ 0, no bonds have been formed and all monomers remain isolated1-clusters In the other extreme, p ¼ 1, all possible bonds between monomers have beenformed and thus all monomers in the system have clustered into one infinite network, with

no sol phase left Thus for small p no gel is present whereas for p close to unity one suchnetwork exists The gel is, in fact, considered as one molecule Therefore, there is in general

a sharp phase transition at some intermediate critical point p ¼ pc, where an infinite clusterstarts to appear: a gel for p above pc, a sol for p below pc This point p ¼ pcis the gel pointand may be the analog of a liquid–gas critical point: For p below pc, only a sol is presentjust as for T above Tconly a supercritical gas exists But for p above pc, sol and gel coexistwith each other; similarly for T below Tcvapor and liquid coexist at equilibrium on thevapor pressure curve However, we do not assert that these thermal phase transitionsand gelation have the same critical behavior Also, in gelation there is no phase separation:Whereas the vapor is above the liquid, the sol is within the gel The liquid–gas transition is

a thermodynamic phase transition whereas gelation deals with geometrical connections(i.e., with bonds) At least in simple gelation models the temperature plays only a minorrole compared with its dominating influence on the thermodynamic phase transitions.Such simple gelation theories often make the assumption that the conversion p alonedetermines the behavior of the gelation process, though p may depend on temperature T,concentration c of monomers, and time t

Early theoretical approaches to the gel-formation [1–4] as the Flory–Stockmayertheory do not take into account several aspects which naturally occur as the individualmolecules grow to form the gel, such as cyclic bond formation, excluded volume effectsand steric hinderance The Flory–Stockmayer theory assumes that in the gelation processeach bond between two individual monomeric, oligomeric or polymeric molecules isformed randomly Thus this theory assumes point-like monomers This apparently isnot the case when already existing macromolecules are crosslinked, i.e., in vulcanizationreactions as well as in copolymerization reactions of macromolecules with the

Probably most network structures obtained by copolymerization reactions ofbifunctional monomers and larger fractions of monomers with a higher functionalityare inhomogeneous, consisting of more densely crosslinked domains embedded in a lessdensely crosslinked matrix, often with fluent transitions.

Besides the inhomogeneity due to a non-uniform distribution of crosslinks,other inhomogeneities due to pre-existing orders, network defects (unreacted groups,intramolecular loops and chain entanglements) or inhomogeneities due to phaseseparation during the crosslinking process may contribute to network structures [7]

It may be concluded therefore that network inhomogeneity is a widespread structuralphenomenon of crosslinked polymers

For any existing polymer network the most important parameters are the crosslinkdensity, the functionality of the crosslinks, that is the number of elastic network chainstied to one given crosslink, the number of dangling chains (with only one end attached tothe network), molecular weight and molecular weight distribution of the elastic chains inthe network, the number of loops and the number of trapped entanglements

Polymerization reactions comprising monomers of the A–B plus Aftype (with f > 2) in thepresence of B–B monomers will lead not only to branching but also to a crosslinked polymerstructure Branches from one polymer molecule will be capable of reacting with those ofanother polymer molecule because of the presence of the B–B reactant Crosslinking can bepictured as leading to the structure I in which two polymer chains have been joined together(crosslinked) by a branch The branch joining the two chains is referred to as a crosslink

A crosslink can be formed whenever there are two branches that have differentfunctional groups at their ends, that is, one has an A group and the other a B group.Crosslinking will also occur in other polymerization reactions involving reactants withfunctionalities f greater than two These include the polymerizations

A A þ Bf!

A A þ B B þ Bf !

AfþBf!

In order to control the crosslinking reaction so that it can be used properly it

is important to understand the relationship between gelation and conversion, that isconsumption of monomers and/or functional groups, that is also called extent of reaction.Two general approaches have been used to relate the extent of reaction at the gel point tothe composition of the polymerization system based on calculating when Xn and Xw,respectively, reach the limit of infinite size

The first one considering the gel point when the number average degree ofpolymerization Xnbecomes infinite Xn! 1in a polycondensation reaction was given bythe pioneer W H Carothers himself [12] This approach is based on the simple assumptionthat the reactive groups in the system only are consumed by chemical reaction; nobranching or cyclization events are taken into account If the average functionality of allfunctional groups present in the system of two monomers A and B in equimolar amounts

is named favg, the average functionality of a mixture of monomers is the average number offunctional groups per monomer molecule and is given by

favg¼X

Nifi X

Ni

which of course is the general formula to calculate the average specifics of a great number

of individuals Thus for a system consisting of 2 moles of lycerol (a triol, f ¼ 3) and 3 moles

of adipic acid (a diacid, f ¼ 2), the total number of functional groups is 12 per 5 monomermolecules, and favgtherefore simply is 12/5 or 2.4 For a system consisting of equimolar

This is the so-called Carothers equation which relates the degree of polymerization to thenumber of molecules present in the polymerizing system From combination of both theseequations it follows that

At the gel point the number average degree of polymerization Xnbecomes infiniteand therefore the second term in the previous equation is zero Thus, the critical extent ofreaction pcat the gel point is given by

pc¼2=favg

This equation allows us to calculate the extent of reaction to which the reaction has to bepushed to reach the onset of gelation in the reaction mixture of reacting monomers fromits average functionality

In the example given above of reacting a dibasic acid, adipic acid, with atrifunctional alcohol, glycerol, which is of the type A2B3, we have to take 2 moles ofglycerol and 3 of adipic acid, or 5 altogether, containing 12 equivalents and favg¼12/5 ¼2.4 Then at Xn¼ 1, p ¼ 2/2.4 and the limit of reaction will be 5/6 ¼ 0.833 This, in fact,represents the maximum amount of reaction that can occur before gelation under anydistribution of combinations, provided only, that the reaction is all intermolecular

Flory [1,2] and also Stockmayer [3,4] used a statistical approach to derive an sion for predicting the extent of reaction at the time where gelation will occur by calculatingwhen Xw approaches infinite size This statistical approach in its simplest form assumesthat the reactivity of all functional groups of the same type is the same and independent ofmolecular size and shape It is further assumed that there are no intramolecular reactionsbetween functional groups on the same molecule such as cyclization reactions

expres-For the ease of demonstration how the branching reaction in a step-growthpolymerization reaction of A–A þ B–B þ Afmolecules proceeds, Flory has used a simplepicture to sketch the branching procedure which at some critical point finally leads togelation [13]

A A þ B B þ Af!Að f 1 ÞAðB BA AÞnB BA Að f 1 Þ

The center unit inFigure 1 is given by the segment to the right of the arrow withthe two Afat the end as branching sites Infinite networks are formed when n number ofchains or chain segments give rise to more n chains through branching of some of them.The criterion for gelation in a system containing a reactant of functionality f is that at least

one of the ( f  1) chain segments radiating from a branch unit will in turn be connected toanother branch unit (note: f is not identical to favgused by Carothers [12]) The probabilityfor this occurring is simply 1/( f  1) and the critical branching coefficient ac for gelformation is

ac¼1ð f  1ÞWhen a( f  1) equals 1, a chain segment will, on average, be succeeded by a( f  1)chains Of these a( f  1) chains a portion a will each end in a branch point so that

a2( f  1)2more chains are created The branching process continues with the number ofsucceeding chains becoming progessively greater through each succeeding branchingreaction

If all groups (of the same kind) are equally reactive, regardless of the status of othergroups belonging to the same unit, the probability PAthat any particular A group hasreacted equals the fraction of the As which have reacted; similarly, PBis defined If r is theratio of all A to all B groups, then

PB¼rPAFigure 1 Schematic representation of a trifunctionally branched three-dimensional polymermolecule [13]

It will depend on the analytical circumstances which of the unreacted groups, A or B, is theone to determine which of the equations will be used.

The observed pcvalues as in many other similar systems fall approximately midwaybetween the two calculated values The Carothers equation [12] gives a high value for pc.The experimental pc values are close to but always higher than those calculated fromthe Flory equation [13] Two reasons can be given for this difference: first the occurence ofintramolecular cyclization and second unequal functional group reactivity Both factorswere ignored in the theoretical derivations for p

Although both the Carothers and statistical approaches are used for the practicalprediction of gel points, the statistical approach is the more frequently employed Thestatistical method is preferred, since it theoretically gives the gel point for the largest sizedmolecules in a size distribution

Some theoretical evaluations of the effect of intramolecular cyclization on gelationhave been carried out [6,16,17] The main conclusion is that, although high reactantconcentrations decrease the tendency toward cyclization, there is at least some cyclizationoccurring even in bulk polymerizations Thus, even after correcting for unequal reactivity

of functional groups, one can expect the actual pcin a crosslinking system to be larger than

a calculated pcvalue

Table 1 Gel point for polymers containing tricarboxylic acid [13]

Extent of reaction at gel point ( pc)

r ¼[CO2H]/[OH] r Calculated from [12] Calculated from [13] Observed

IV CROSSLINKING — CONCEPT

Among all crosslinking strategies which are used to synthesize polymer networks, threedifferent classes are in common application:

1 One-shot crosslinking of multifunctional monomers or copolymerization withdifunctional monomers,

2 two-stage crosslinking via prepolymers,

3 crosslinking of high molecular weight polymers

Into the first category of crosslinking strategies fall the formation of divinylbenzene) resins, the methacrylic resins and some others, and among those also

poly(styrene-co-a smpoly(styrene-co-all frpoly(styrene-co-action of the so-cpoly(styrene-co-alled microgels In generpoly(styrene-co-al, these resins poly(styrene-co-are formed of monomerswhich in linear polymerization lead to thermoplastic polymers such as poly(styrene),polyacrylics or methacrylics a.s.o High glass transition temperature of the linearpolymers and high melt viscosity makes it unattractive to process premade linearthermoplastics prior to a second step of crosslinking reaction Incorporation of pendantC–C– double bonds into the linear chains by copolymerization with small quantities

of a difunctional monomer and thereby avoiding early stage crosslinking is difficult tohandle and such polymers would be very sensitive to undergo uncontrolled networkformation

One-shot crosslinking of multifunctional monomers and copolymerization therefore

is limited to the radical induced copolymerization of styrene and some derivatives withdivinylbenzene or of methacrylates with ethyleneglycol dimethacrylate as crosslinker

in suspension polymerization to form densely crosslinked polymer beads for applicationssuch as ion exchange resins, Merrifield resins, polymer supports for chemical reagentsespecially with the aspect of combinatorial syntheses

Into the second category of crosslinking strategies fall the processes of preparingpolymer networks which make use of prepolymers These are two-stage processes in which

in the first stage, overhelmingly in step-growth polymerization reactions, prepolymersare prepared with molecular weight mostly ranging from 1 to 6  103which are soluble inorganic solvents, fusible and have low melt viscosity The second stage curing is achievedeither by heat — thermosetting — or, when necessary, by the addition of appropriatecuring agents Most prominent examples are epoxy resins, phenol-formaldehyde resins,unsaturated polyesters, and the polyurethane networks

Into the third catagory fall the vulcanization reactions of elastomers These polymers

A General Classification of Prepolymers [18]

Curing reactions applied to epoxy prepolymers, unsaturated polyesters, resoles, andnovolacs make use of three general classes of prepolymers which are distinguished by thenumber and location of sites of functional groups available for subsequent crosslinking reac-tions These three general classes have been defined as discussed in the following sections

1 Random prepolymers Random prepolymers are those built up from tional step-growth monomers which have been reacted randomly and which arecapable of forming crosslinked polymers directly Monomer conversion in thefirst-stage polymerization reaction for the formation of these prepolymers isstopped short and kept below the critical conversion at which networkformation would occur Crosslinking in the second-stage, step-growth poly-merization reaction is achieved simply by heating to carry the original reactionpast the critical conversion For this reason, the term thermoset is applied tothese prepolymers, and these are exemplified by the phenol-formaledehyderesole resins and the glycerol polyesters The term structoset has been applied tothe other two classes of prepolymers to distinguish them from the thermoset typebecause in the other two classes the second-stage crosslinking reaction requiresthe addition of a catalyst or monomer, and generally proceeds by a reactiondifferent from the first-stage reaction

polyfunc-2 Structoterminal prepolymers.Structoterminal prepolymers are those in which thereactive sites are located at the ends of the polymer chains These first-stagepolymers give maximum control of the length and type of chain in the finalnetwork polymer The epoxy prepolymers may be considered examples ofthis class if the second-stage reaction occurs overwhelmingly through reaction ofthe terminal epoxide functional groups If the aliphatic hydroxyl groups alongthe chain in epoxy prepolymers become significantly involved in the crosslinkingreaction, then these polymers are more properly included in the third class ofprepolymers

3 Structopendant prepolymers Structopendant prepolymers are those in which thecrosslink sites are distributed in either a regular or random order along the chain.Examples of this class are the unsaturated polyesters and the novolac resins

Phenol-formaldehyde condensates were among the first synthetic polymeric materials onthe market It was Baekeland at the beginning of the 20th century who in 1907 defined thedifferences between basic or acidic reaction conditions and the different molar ratios onthe reaction procedure and the resulting molecular structure He was able to manufacture

a thermosetting resin and made applications for a patent [19] (Bakelite)

Most phenolic resins are heat hardenable or thermosetting The resin may bedelivered to the user ready to be cured or it may be in the temporarily thermoplasticnovolac form to which a hardener, commonly hexamethylenetetramine–urotropin, will beadded The major categories of uses for phenolics are

Coatings

Industrial bonding resins

The latter includes resins for grinding wheels and coated abrasives, laminating, plywoodadhesives, glass wool thermal insulation and bonded organic fiber patting, foundry sandbonding, wood waste bonding, and other miscellaneous applications.

The base-catalyzed first-step reaction of phenol ( f ¼ 3, because reaction can take place intwo ortho and one para position) and formaldehyde ( f ¼ 2) with an excess of formaldehyde

of about 15 mol% closely resembles an aldol addition and yields mixtures ofmonomolecular methylolphenols and also dimers, trimers and the correspondingpolynuclear compounds according to a generalized reaction scheme given in (1b) Incommercial processes formaldehyde is added in aqueous solution Sodium hydroxide,ammonia and hexamethylenetetramine–urotropin, sodium carbonate, calcium-,magnesium-, and barium-hydroxide and tertiary amines are used as catalysts After thehydroxybenzyl alcohol has been formed in the first step, the condensation steps toform oligomers are likely to be a Michael type of addition to a base-induced dehydrationproduct of the hydroxybenzyl alcohol Detailed studies have been presented by Martin [20]and Megson [21]

ð1Þ

Such mixtures, whose exact composition depend on the phenol–formaldehyde ratioand the reaction conditions employed, are termed resoles or resole prepolymers Theresoles are generally neutralized or made slightly acidic before the second-stage reaction isaccomplished by heating The second-stage polycondensation and crosslinking takes place

by the formation of methylene and dibenzyl ether linkages between the benzene rings toyield a network structure of type I The relative importance of the methylene and etherbridges is not well established, although both are definitely formed Higher reactiontemperatures favor the formation of the methylene bridges

Heat curing of resols usually is carried out at temperatures in the range 130–200
C.Below 150
C the formation of dibenzyl ether bridges is predominant whereas at highertemperatures methylene bridge formation is favored This was nicely shown by theinvestigations of Ka¨mmerer et al who carried out polycondensation reaction of 2,6-bis(hydroxymethyl-4-methylphenol to the corresponding poly(benzyl ether) [24] withmolecular weights ranging from 2500 to 20,000

Although at lower temperatures only water is liberated but also water andformaldehyde at temperatures above 150
C [25], the water to formaldehyde ratio is not

an exact measure of the ratio of benzyl ether to methylene bridge formation, because it isknown that the yield of isolable formaldehyde is considerably less than the theoreticalyield [21]

If curing is carried out above 180
C in the presence of air, some oxidation reactiontakes place which gives a reddish color to the final product Quinone structures areresponsible for the color and researchers were able even to isolate quinone methidesformed in pyrolysis reactions [26]

The reaction between phenol and formaldehyde under strongly acidic conditions can beregarded as an electrophilic substitution reaction, route (b) in Scheme 1 [28] The catalystsmost frequently used are sulfuric acid, oxalic acids or p-toluene sulfonic acid By theaddition of a proton to formaldehyde a hydroxymethylene carbenium ion is formed which

undergoes an electrophilic hydroxyalkylation reaction mostly in the o-position of phenol.From this o-methylol phenol compound water is eliminated by reaction of the methylolgroup with a proton thus yielding a benzylium type carbenium ion which then undergoesvery fast alkylation reaction of a second phenol molecule in the o-position with thegeneration of a new proton [20–22,27] Continued methylolation and methylene bridgeformation by these reactions leads to the formation of polynuclear compounds ofconsiderable complexity Under strongly acidic conditions, methylol substitution andmethylene-bridge formation both occur predominantly at p-positions [29] The pH mostfavorable for the formation of the o-products is between 4 and 5.

Novolacs require an auxiliary chemical crosslinking agent The most widely usedcrosslinker is hexamethylenetetramine, and the products in this curing reaction areinfluenced by the molar ratio of phenol nuclei to hexamethylenetetramine At a phenolnucleus to hexamethylenetetramine ratio of 6 : 1, the products turn out to contain little oreven no nitrogen, and the reaction appears to an almost entirely one of methylene-bridgeformation At a mole ratio 0.5 : 1 or higher, nitrogen enters into the product, and thenitrogen content of the products can come close to 10% with the amount of ammoniaevolved proportionately decreased

The reaction of curing is not clear It is known that under controlled conditionsphenol and hexamethylenetetramine form a crystalline salt of the stiochiometriccomposition C6H12N43C6H5OH [30] which, when heated, evolves ammonia with theformation of an insoluble, infusible polymer [31] In the presence of water, hexamethyl-enetetramine hydrolyzes with the formation of two moles of dimethylolamine DMA, onemole of formaldehyde and two moles of ammonia Water is ubiquitious in novolacs andtherefore under basic reaction conditions in the presence of tert and sec amines and alsoammonia as shown in the chart, methylene bridges are formed by entering formaldehydeinto the reaction With increasing amounts of hexamethylenetetramine, the benzylaminetype bridges become predominant.

Cured novolacs show a more or less slightly yellow color There is some indication inthe literature that the benzylamine type bridges are converted to azomethines by hydrogenelimination under heating conditions applied in the curing reaction [20]

Bender et al found that the o,o0-compounds have a much more rapid cure rate thanisomeric ‘novolacs’ [23] The gel times for the 2,20, 4,40, and 2,40 isomers at 160
C havebeen reported to 60, 175, and 240 sec, respectively

Urea 1 ( f ¼ 4) and melamin 2, 2,4,6-triamino-1,3,5-triazin ( f ¼ 6) under basic or acidicconditions react with formaldehyde ( f ¼ 2) rather similar to the phenol–formaldehydereaction The reaction products are called aminoplastics

Polymerization of urea and formaldehyde in a 1.5 : 1 ratio in the first-stage reactionyields various methylolureas as prepolymers [32–36], which in a second-stage reaction arecured by heat (thermosetting) under neutral or slightly acidic conditions Control of theextent of reaction is achieved by pH (by the use of buffers) and temperature control.The reaction rate increases with increasing acidity [37,38] The prepolymer can be made atvarying pH levels depending on the reaction temperature Polymerization is stopped bybringing the pH close to neutral and cooling.

The second-stage, crosslinking reaction of the prepolymers under acidic conditionscauses the formation of a network containing principally a random mixture of linearand branched substituted trimethylenetriamine repeating units and, to some extent, alsomethylene ether bridges and methylene bridges [35,39] The latter are exclusively formedunder strongly acidic conditions [40]

The formation and crosslinking of random prepolymers from melamine, triamino-1,3,5-triazin, and formaldehyde follows in a similar manner [33,34,41–43], but,unlike urea, melamin readily forms polymethylol compounds with two methylol groups

2,4,6-on a single nitrogen atom Paper chromatographic separati2,4,6-on of the products ofthis reaction, in which an excess of formaldehyde greater than 2.1 was used, revealedthe presence of all possible methylol compounds from the monosubstituted to thehexasubstituted derivatives [44]

Epoxy resins as a class of crosslinked polymers are prepared by a two-step polymerizationsequence The first step which provides prepolymers, or more exactly: preoligomers,

is based on the step-growth polymerization reaction of an alkylene epoxide which contains

a functional group to react with a bi- or multifunctional nucleophile by which prepolymersare formed containing two epoxy endgroups In the second step of the preparation of theresins, these tetrafunctional (at least) prepolymers are cured with appropriate curingagents.Table 2compiles a representative selection of di- and multi-epoxides both as alkyland cycloalkyl epoxides and the most widely used curing reagents

The most widely used pair of monomers to prepare an epoxy prepolymer are 2,20bis(4-hydroxyphenyl)propane (referred to as bisphenol-A) and epichlorohydrin, theepoxide of allylchloride The formation of the prepolymer can be seen to involve twodifferent kinds of reactions The first one is a base-catalyzed nucleophilic ring-openingreaction of bisphenol-A with excess of epichlorohydrin to yield an intermediate b-chloroalcoholate which readily loses the chlorin anion reforming an oxirane ring Furthernucleophilic ring-opening reaction of bisphenol-A with the terminal epoxy groups leads tooligomers with a degree of polymerization up to 15 or 20, but it is also possible to preparehigh molecular weight linear polymers from this reaction by careful control of monomerratio and reaction conditions [45] The two ring-opening reactions occur almost exclusively

-by attack of the nucleophile on the primary carbon atom of the oxirane group [46].Depending on the conditions of the polymerization reaction, these low molecularweight polymers can contain one or more branches as a result from the reaction ofthe pendant aliphatic hydroxyl groups with epichlorohydrin monomer In most cases,however, the chains are generally linear because of the much higher acidity of the phenolichydroxyl group At high conversions, when the concentration of phenolic hydroxyl groupsdrops to a very low level, under the base-catalyzed reaction conditions formation andreaction of alkoxide ions become competitive and polymer chain branching may occur.Polymers of this type with molecular weight exceeding 8000 are undesirable because

of their high viscosity and limited solubility, which make processing in the stage, crosslinking-reaction difficult to perform The oligomers of the diglycidylether ofbisphenol-A (DGEBA) are the most commonly epoxy resins, therefore a great deal of

second-Table 2.

Aliphatic epoxy monomers and

pre-polymers (selection)

Curing agentsprim./sec Amines

tert Amines

Aliphatic-cycloaliphatic epoxy compounds Acid anhydrides

Polymerization catalystssuch as amine complexes of Lewis acids [65]

or diaryliodonium salts [66], photocrosslinking [67]

investigations with respect to the processibility behavior before crosslinking is focused onthis oligomer [47].

Aliphatic-cycloaliphatic epoxy compounds (ACECs) contain different epoxy groups in themolecule: glycidyl, i.e., 2,3-epoxypropyl groups, and cycloaliphatic, i.e., 1,2-epoxycyclo-pentane or 1,2-epoxycyclohexane rings, for which molecules 3 and 4 are characteristic

Nevertheless, the sequential entering of different epoxy groups into the reaction,irrespective of the acidic or basic character of the curing agent, is a very important feature

of the crosslinking process of ACECs because it conditions the formation of a regularpolymer network [51]

The epoxy prepolymers are considered as structopendant prepolymers because of thependant aliphatic hydroxyl groups or as structoterminal prepolymers with respect to theterminal epoxy groups [52]

An acid anhydride as curing agent is bifunctional ( f ¼ 2) and crosslinking occursprimarily through the hydroxyl groups In this reaction, the prepolymer acts as astructopendant prepolymer Maleic anhydride introduces C–C– double bonds into theresin Mostly phthalic anhydride and pyromellitic anhydride are used

Anhydrides react initially with the hydroxyl groups in the prepolymers to form esters, and the generated carboxyl groups in this half-ester can condense with anotherhydroxyl group Also the reaction of the carboxyl group with an epoxy group is possible[53,54], but these reactions are much slower than the initial alcohol–anhydride reactionand are not shown in the above picture For these reasons dianhydrides are very effectivecrosslinking agents, and because of the great number of hydroxyl groups in theprepolymer, curing with dianhydrides can form very densely crosslinked, second-stagepolymers if used in relatively high concentrations

half-The prepolymer is a structoterminal prepolymer when amines are used ascrosslinkers Crosslinking in this case involves the base-catalyzed ring-opening of theoxirane groups Both primary and secondary amines are used as crosslinking agents [55].Since each N–H bond is reactive in this process, primary and secondary amine functionalgroups have a crosslinking functionality f equal to two and one, respectively A variety ofamines such as diethylene triamine ( f ¼ 5), triethylene tetramine ( f ¼ 6), m-phenylene-diamine ( f ¼ 4) and others are used as crosslinking agents The presence of other reactants

is required to foster this ring-opening reaction because the nucleophilic ring-openingreaction of an amine with an oxacyclopropane is not only accelerated by, but, in fact,requires the presence of an active proton-donor [56] Anhydrous diethylamine and

oxacyclopropane do not react, but the reaction proceeds readily in the presence ofcatalytic amounts of proton-donating agents like water, methanol or ethanol [57].Similarly, the reaction of epoxybenzylacetophenone with morpholine or with piperidine

in benzene or ether is extremely slow, but proceeds smoothly in methanol at roomtemperature [58] The reaction of phenyl glycidyl ether with diethylamine in the absence ofsolvents shows a sigmoidal rate curve, which can be attributed to the autocatalytic effect

of the hydroxyl groups in the product [59], while in proton-donating solvents the reaction

is greatly accelerated and the sigmoidal form of the rate curve disappears By protonation

of the oxacyclopropane oxygen, an intermediate oxonium ion is formed which facilitatesthe nucleophilic attack on the carbon atom In the case of the epoxy end groups of theprepolymers, this nucleophilic attack is exclusively directed to the sec carbon atom Phenolhas been found to be a particularly useful proton-donating accelerator And it has beenshown also that the reaction of oxacyclopropane with aniline in the presence of smallamounts of water [60] or acids [61] is proportional to the concentration of water or to thestrength of the acid Different mechanisms have been proposed by Smith [56], Tanaka [62],and King et al [63], but they have not yet been confirmed [64]

Structoterminal prepolymers with two isocyanate endgroups prepared by reaction ofpolyethers containing two hydroxyl endgroups with diisocyanates are the basis for theformation of polyurethane networks They can be made either in melt or in solution,but polyurethanes with melting points much above 200
C are difficult to prepare in meltbecause of the thermal instability of the urethane linkage above 220
C [68] The molecularweight of the prepolymers generally is in the range of 1–10  103