Compositional Heterogeneity in Functional Copolymers

The copolymer composition equation only provides the average composition. Not all chains have the same composition. There is a statistical distribution of monomers determined by the reactivity ratios. When chains are short, compositional heterogeneity can mean that not all chains will contain all monomers. 

In early w ork, while compositional heterogeneity was recognized and could be predicted, it was difficuU to measure. Now, methods such as GPC combined with NMR and/or MALDI, GPC coupled with FTIR and two dimensional HPLC or can provide a direct measure of the composition dislribiition. 

Chain compositional heterogeneity is of particular relevance to functional copolymers which find widespread use in the coatings and adhesives industries.In these applications, the functional copolymer and a crosslinking agent are applied together and are cured to form a network polymer. The functional copolymers are based on functional monomers with reactive groups (e.g, OH). It is desirable that all copolymer molecules have a functionality of at least tw o. Nonfunctional polymer will not be incorporated and could plasticize the network or be exuded from the polymer. Monofunctional polymers are not involved in crosslink formation and will produce dangling ends. 

Various factors are important in determining the composition and molecular weight distribution of multicomponent copolymers e.g. monomer reactivity ratios, reaction conditions). Stockmaycr was one of the first to report on the problem and presented formulae for calculating the instantaneous copolymer composition as a function of chain length. Othershave examined the variation in copolymer composition with chain length by computer simulation. One method of ensuring a functionality of at least one is to use a functional initiator or transfer agent. 

The influence of selectivity in the initiation, termination or chain transfer steps on the distribution of monomer units within the copolymer chain is usually neglected. Galbraith et provided the first detailed analysis of these factors. They applied Monte Carlo simulation to examine the influence of the initiation and termination steps on the compositional heterogeneity and molecular weight distribution of binary and ternary copolymers. Spurling et extended this 

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