CHAIN TRANSFER TO POLYMER IN FREE-RADICAL BULK AND EMULSION POLYMERIZATION OF VINYL-ACETATE STUDIED BY NMR-SPECTROSCOPY

Chain transfer to polymer in free-radical bulk and emulsion polymeriza tion of vinyl acetate has been studied using C-13 NMR spectroscopy. Th e poly(vinyl acetate) (PVAc) spectra have been fully interpreted in te rms of structural features arising from (i) normal and inverted repeat unit placements, (ii) chain transfer to polymer, and (iii) in the cas e of emulsion polymerization, hydrolysis of acetate side groups. Altho ugh evidence was obtained for chain transfer to polymer via H-abstract ion from both backbone tertiary C-H bonds and methyl side groups, the latter make the dominant contribution. The spectral assignments facili tated calculation of the mole percent branches and the mole percent of inverted repeat units in the PVAc samples produced, the latter quanti ty taking values of 1.44 +/- 0.24 mol % independent of conversion in t he emulsion polymerizations. The absence of an initiator effect in. th e bulk polymerizations shows that the propagating PVAc chain radicals are the principal hydrogen atom abstractors. In both bulk and emulsion polymerization, the mole percent branches increased steadily with ove rall conversion. However, because the emulsion polymerizations proceed almost exclusively within the latex particles and at high instantaneo us conversions, the levels of branching in the PVAc produced (final le vels = 0.61-0.75 mol %) were much higher than in the PVAc produced by bulk polymerization (final levels = 0.13-0.23 mol %). Increasing the t emperature of emulsion polymerization from 60 to 70 degrees C led to a n increase in the mole percent branches but had no measurable effect o n the mole percent of inverted repeat units.