Chain transfer to polymer in emulsion copolymerizations

Chain transfer to polymer in emulsion copolymerizations of (i) vinyl acetat e (VAc) with n-butyl acrylate (BA) and (ii) BA/acrylic acid with (a) methyl methacrylate (MMA) and (b) styrene (S) has been studied using C-13 NMR spe ctroscopy to quantify the level of branching in the copolymers produced. Th e results reveal synergistic effects in which the inclusion of a small amou nt of comonomer leads to disproportionate changes in the level of branching . The data from the VAc/BA copolymerizations show that radicals with VAc en d units abstract hydrogen atoms from BA repeat units more frequently than f rom VAc repeat units and that radicals with VAc end units are more effectiv e in abstracting hydrogen atoms from BA repeat units than are radicals with BA end units. These effects lead to higher levels of branching in VAc/BA c opolymers than results from the corresponding homopolymerizations and are a consequence of the efficacy of hydrogen abstraction at BA backbone tertiar y C-H bonds by the highly-reactive VAc-ended chain radicals. In contrast, t he effect of introducing MMA or S to an acrylate polymerization is to dispr oportionately reduce the level of branching, with S being more effective th an MMA in moderating chain transfer to polymer. These effects arise because propagating radicals with MMA or S end-groups have longer lifetimes compar ed to those with acrylate end groups and have a very much lower tendency to abstract hydrogen atoms from acrylate repeat units.