Bootstrap effect in copolymerization

Prior to Harwood s work, the existence of a Bootstrap effect in copolymerization was considered but rejected after the failure of efforts to correlate polymer-solvent interaction parameters with observed solvent effects. Kamachi, for instance, estimated the interaction between polymer and solvent by calculating the difference between their solubility parameters. He found that while there was some correlation between polymer-solvent interaction parameters and observed solvent effects for methyl methacrylate, for vinyl acetate there was none. However, it should be noted that evidence for radical-solvent complexes in vinyl acetate systems is fairly strong (see Section 3), so a rejection of a generalized Bootstrap model on the basis of evidence from vinyl acetate polymerization is perhaps unwise. Kratochvil et al." investigated the possible influence of preferential solvation in copolymerizations and concluded that, for systems with weak non-specific interactions, such as STY-MMA, the effect of preferential solvation on kinetics was probably comparable to the experimental error in determining the rate of polymerization ( 5%). Later, Maxwell et al." also concluded that the origin of the Bootstrap effect was not likely to be bulk monomer-polymer thermodynamics since, for a variety of monomers, Flory-Huggins theory predicts that the monomer ratios in the monomer-polymer phase would be equal to that in the bulk phase. 

Kaim, A. and Oracz, P, Penultimate model in the study of the bootstrap effect in the methyl methacrylate-acrylamide copolymerization system, Polymer, 38, 2221 (1997). 

Copolymerization models based upon a Bootstrap effect were first proposed by Harwood and Semchikov" (see references cited therein). Harwood suggested that the terminal model could be extended by the incorporation of an additional equilibrium constant relating the effective and bulk monomer feed ratios. Different versions of this so-called Bootstrap model may be derived depending upon the baseline model assumed (such as the terminal model or the implicit or explicit penultimate models) and the form of equilibrium expression used to represent the Bootstrap effect. In the simplest case, it is assumed that the magni-... 

Based upon the above studies, it may be concluded that there is strong evidence to suggest that Bootstrap effects arising from preferential solvation of the polymer chain operate in many copolymerization systems, although the effect is by no means general and is not likely to be significant in systems such as STY-MMA. However, this does not necessarily discount a Bootstrap effect in such systems. As noted above, a Bootstrap effect may arise from a number of different phenomena, of which preferential solvation is but one example. Other causes of a Bootstrap effect include preferential solvation of die chain end, rather than the entire polymer chain, or the formation of non-reactive radical-solvent or monomer-solvent complexes. In fact, the Bootstrap model has been successfully adopted in systems, such as solution copolymerization of STY-MMA, for which bulk preferential solvation of the polymer chain is unlikely. For instance, both Davis and Klumperman and O Driscoll adopted die terminal Bootstrap model in a reanalysis of die microstructure data of San Roman et al. for the effects of benzene, chlorobenzene and benzonitrile on the copolymerization of MMA-STY. 

The effects of solvent on reactivity ratios and polymerization kinetics have been analyzed for many copolymerizations in terms of this theory.98 These include copolymerizations of S with MAH,"7 118 S with MAA,112 S with MMA,116 117 "9 121 S with HEMA,122 S with BA,123,124 S with AN,103415 125 S with MAN,112 S with AM,11" BA with MM A126,127 and tBA with HEMA.128 It must, however, be pointed out that while the experimental data for many systems are consistent with a bootstrap effect, it is usually not always necessary to invoke the bootstrap effect for data interpretation. Many authors have questioned the bootstrap effect and much effort has been put into finding evidence both for or against the theory.69 70 98 129 "0 If a bootstrap effect applies, then reactivity ratios cannot be determined by analysis of composition or sequence data in the normal manner discussed in Section 7.3.3. 

It was in article [52] where the main reason responsible for the above-mentioned peculiarities was explicitly formulated and substantiated. Its authors related these peculiarities with partitioning of monomer molecules between the bulk of a reaction mixture and the domain of a growing polymer radical. This phenomenon induced by preferential sorption of one of the monomers in such a domain is known as the bootstrap effect. This term was introduced by Harwood [53], because when growing a polymer radical can control under certain conditions its own microenvironment. This original concept enabled him to interpret many interesting features peculiar to this phenomenon. Particularly, he managed to qualitatively explain the similarity of the sequence distribution in copolymerization products of the same composition prepared in different solvents under noticeable discrepancies in composition of monomer mixtures. 

In an apparently homogeneous solution, macromonomers, possibly together with the resulting graft copolymers, may lead to some structure formation such as micelle or looser association, which may in turn change the apparent reactivities due to some specific solvation or partition of the monomers around the active sites. Such a bootstrap effect [52] maybe responsible for some complicated dependency of the apparent reactivities on the monomer concentration and composition in radical copolymerization of 29 with n-butyl acrylate [53]. 

Early work indicated that the nature of the reaction medium had no effect on the course of free radical copolymerizations in homogeneous reaction systems. More recent studies have not always supported this conclusion and it has been suggested that a bootstrap effect may be operating whereby there is a partitioning of the comonomers between the bulk of the reaction medium and the polymerization locus (i.e., the macroradical end) [29]. 

In fact, recent theoreticaP and experimental studies of small radical addition reactions indicate that charge separation does occur in the transition state when highly electrophilic and nucleophilic species are involved. It is also known that copolymerization of electron donor-acceptor monomer pairs are solvent sensitive, although this solvent effect has in the past been attributed to other causes, such as a Bootstrap effect (see Section 13.2.3.4). Examples of this type include the copolymerization of styrene with maleic anhydride and with acrylonitrile. Hence, in these systems, the variation in reactivity ratios with the solvent may (at least in part) be caused by the variation of the polarity of the solvent. In any case, this type of solvent effect cannot be discounted, and should thus be considered when analyzing the copolymerization data of systems involving strongly electrophilic and nucleophilic monomer pairs. 

It was reported by Barb in 1953 that solvents can affect the rates of copolymerization and the composition of the copolymer in copolymerizations of styrene with maleic anhydride [145]. Later, Klumperman also observed similar solvent effects [145]. This was reviewed by Coote and coworkers [145]. A number of complexation models were proposed to describe copolymerizations of styrene and maleic anhydride and styrene with acrylonitrile. There were explanations offered for deviation from the terminal model that assumes that radical reactivity only depends on the terminal unit of the growing chain. Thus, Harwood proposed the bootstrap model based upon the study of styrene copolymerized with MAA, acrylic acid, and acrylamide [146]. It was hypothesized that solvent does not modify the inherent reactivity of the growing radical, but affects the monomer partitioning such that the concentrations of the two monomers at the reactive site (and thus their ratio) differ from that in bulk. 

---