Composition of Butadiene-Styrene Copolymers Prepared by Emulsion Polymerization

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Abstract
The formation of a high-polymer molecule occurs by the successive additions of monomer molecules to a growing chain (chain propagation), the growing chain itself being made up of a number of monomer molecules with a radical derived from a monomer molecule on the end of the chain at which addition occurs. It was pointed out by Norrish and Brookman that chain propagation in the formation of a copolymer from two polymerizing monomers involves four different reactions; the growing chain may have a radical unit derived from either of two monomers at the end of the chain, to which either of two monomer molecules may add. In general it may be expected that the four growth processes will involve four different reaction-rate constants. This situation has been studied thoroughly from the theoretical point of view by Alfrey and Goldfinger, by Mayo and Lewis, by Wall, and by Simha and Branson. The theoretical treatment has been extended by Walling and Briggs to the copolymerization of an arbitrary number of monomers. A considerable amount of experimental data on copolymerizations of various pairs of monomers in homogeneous solution and the quantitative interpretation in terms of the concept given above have been presented. Prior to the comprehensive treatments of the composition of copolymers referred to above, Wall had shown that a relatively simple treatment could be given to the problem, on the assumption that only two rate constants are involved in the chain propagation, corresponding to the addition of either of two monomers to the growing chain This would mean that both radical types on the end of the growing chain would have the same relative reactivity with monomers. Wall has pointed out that such a copolymerization would be “ideal” in the same sense as a pair of liquids which follows Raoult's law. Mayo and Lewis, however, have stated that this “ideal” copolymerization behavior does not correspond to the experimental facts in the case of the bulk copolymerization of styrene and methyl methacrylate, or in the case of the copolymerization of maleic anhydride with certain ethylene derivatives.

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