Copolymerization donor-acceptor monomer pairs, model studie

Several studies on the reactivities of small radicals with donor-acceptor monomer pairs have been carried out to provide insight into the mechanism of copolymerizations of donor-acceptor pairs. Tirrell and coworkers " reported on the reaction of n-butyl radicals with mixtures of N-phcnylmalcimidc and various donor monomers e.g. S, 2-chloroethyl vinyl ether),. lenkins and coworkers have examined the reaction of t-butoxy radicals with mixtures of AN and VAc. Both groups have examined the S-AN system (see also Section 7.3.1.2). In each of these donor-acceptor systems only simple (one monomer) adducts are observed. Incorporation of monomers as pairs is not an important pathway i.e. the complex participation model is not applicable). Furthermore, the product mixtures can be predicted on the basis of what is observed in single monomer experiments. The reactivity of the individual monomers (towards initiating radicals) is unaffected by the presence of the other monomer i.e. the complex dissociation model is not applicable). Unless propagating species are shown to behave differently, these results suggest that neither the complex participation nor complex dissociation models apply in these systems. 

The kinetics of copolymerization and the microstructure of copolymers can be markedly influenced by the addition of Lewis acids. In particular, Lewis acids are effective in enhancing the tendency towards alternation in copolymerization of donor-acceptor monomer pairs and can give dramatic enhancements in the rate of copolymerization and much higher molecular weights than are observed for similar conditions without the Lewis acid. Copolymerizations where the electron deficient monomer is an acrylic monomer e.g. AN, MA, MMA) and the electron rich monomer is S or a diene have been the most widely studied." Strictly alternating copolymers of MMA and S can be prepared in the presence of, for example, dictliylaluminum scsquichloridc. In the absence of Lewis acids, there is only a small tendency for alternation in MAA-S copolymerization terminal model reactivity ratios are ca 0.51 and 0.49 - Section 7.3.1.2.3. Lewis acids used include EtAlCT, Et.AlCL ElALCL, ZnCT, TiCU, BCl- LiC104 and SnCL. 

While there is clear evidence for complex formation between certain electron donor and electron acceptor monomers, the evidence for participation of such complexes in copolymerization is often less compelling. One of the most studied systems is S-.V1 Al I copolymerization/8 75 However, the models have been applied to many copolymerizations of donor-acceptor pairs. Acceptor monomers have substituents such as carboxy, anhydride, ester, amide, imide or nitrile on the double bond. Donor monomers have substituents such as alkyl, vinyl, aryl, ether, sulfide and silane. A partial list of donor and acceptor monomers is provided in Table 7.6.65.-... 

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