Model copolymerization reactions. Direct observation of a "penultimate effect" in a model styrene-acrylonitrile copolymerization

Forty years after the publication of the classical kinetic scheme for radical copolymerization, there is still debate concerning the factors that control composition and comonomer sequence in some copolymer chains. Interest in this problem arises not only from an intrinsic desire to understand the copolymerization process but also from the realization that alternative kinetic models of copolymerization predict very different copolymer chain structures. For example, Hill, O’Donnell, and O’Sullivan have shown in their analysis of styrene-acrylonitrile copolymerization that the best terminal, penultimate, and complex-participation models predict very similar copolymer compositions but very different comonomer sequences; in a copolymer prepared from an equimolar monomer mixture, the number fraction of styrene sequences which consist of an isolated styrene unit is predicted to be 0.74 for the terminal model, 0.61 for the penultimate model, and 0.18 for the unrestricted complex-participation model. Clearly the synthesis of well-defined copolymer structures requires a precise understanding of copolymerization mechanism.