Copolymerization complex dissociation model

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. 

In these models, the complex formed by the monomer pair competes with the individual monomer molecules for the propagation reaction with the radicals. There are two variations of this approach in the complex participation model, the pair of monomers form a complex and are added to the chain radical [106-109]. On the other hand, in the complex dissociation model, the complex participates in the propagation process, but dissociates upon reaction and only one of the monomers is added to the chain [101, 103]. Although there is ample experimental evidence for the existence of such complexes in these copolymerizations (such as the bright colors associated with them) [76], it is questionable whether the complexes actually participate in the propagation step [76]. Additionally, for several years, Hall and Padias have accumulated experimental and theoretical evidence that refutes the validity of the models based on complex participation [76, 77]. Both the complex participation and the penultimate models were combined in the so-called comppen model [110]. 

The various copolymerization models that appear in the literature (terminal, penultimate, complex dissociation, complex participation, etc.) should not be considered as alternative descriptions. They are approximations made through necessity to reduce complexity. They should, at best, be considered as a subset of some overall scheme for copolymerization. Any unified theory, if such is possible, would have to take into account all of the factors mentioned above. The models used to describe copolymerization reaction mechanisms arc normally chosen to be the simplest possible model capable of explaining a given set of experimental data. They do not necessarily provide, nor are they meant to be, a complete description of the mechanism. Much of the impetus for model development and drive for understanding of the mechanism of copolymerization conies from the need to predict composition and rates. Developments in models have followed the development and application of analytical techniques that demonstrate the inadequacy of an earlier model. 

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