C-13 NMR spectroscopy studies of branching and sequence distribution in copolymers of vinyl acetate and n-butyl acrylate prepared by semibatch emulsion copolymerization

C-13 NMR spectroscopy has been used to study branching and sequence distrib utions in copolymers of vinyl acetate (VAc) and n-butyl acrylate (BA) prepa red by semibatch emulsion copolymerization. All copolymerizations proceeded via a seed stage that was carried out under monomer-flooded conditions, fo llowed by a growth stage that operated under monomer-starved conditions. Th e C-13 NMR spectra of the copolymers have been fully interpreted both in te rms of the structural features arising from chain transfer to polymer and t he repeat unit sequence distributions. The sequence distributions for copol ymers formed in the seed stage are in reasonable agreement with predictions from the standard terminal-unit first-order Markov statistical model, show ing that normal copolymerization kinetics operate under monomer-flooded con ditions. However, the sequence distributions for copolymers produced in the growth stage are closely represented by a random Bernoullian distribution, showing that copolymers with random repeat unit sequence distributions are formed under monomer-starved conditions. The only reasonable explanation o f this observation is that propagation is subject to diffusion control unde r monomer-starved conditions. The branching data reveal a synergistic effec t in which the inclusion of only small amounts of either monomer leads to d isproportionate increases in the level of branching, This is a consequence of the efficacy of H-abstraction at BA backbone tertiary C-H bonds by the h ighly reactive VAc-ended chain radicals. The results indicate that radicals with VAc end units abstract hydrogen atoms from BA repeat units about 7-8 times more rapidly than from VAc repeat units and that radicals with VAc en d units are about 3-4 times as effective in abstracting hydrogen atoms from BA repeat units than are radicals with BA end units. Copolymer composition drift occurs in the seed stage due to the monomer-flooded conditions and r esults in virtually all of the BA being consumed before the conversion is h igh enough for chain transfer to polymer to become significant; hence, bran ching arises almost exclusively from H-abstractions by radicals with VAc en d units. However, in the monomer-starved growth stage, radicals with BA end units and radicals with VAc end units both contribute to chain transfer to polymer in proportions that correspond to the composition of the comonomer feed; this observation again is consistent with diffusion control of propa gation under monomer-starved conditions.