Chain transfer to polymer in free-radical bulk and emulsion polymerization of vinyl acetate studied by NMR spectroscopy

摘要

Chain transfer to polymer in free-radical bulk and emulsion polymerization of vinyl acetate has been studied using C-13 NMR spectroscopy. The poly(vinyl acetate) (PVAc) spectra have been fully interpreted in terms of structural features arising from (i) normal and inverted repeat unit placements, (ii) chain transfer to polymer, and (iii) in the case of emulsion polymerization, hydrolysis of acetate side groups. Although evidence was obtained for chain transfer to polymer via H-abstraction from both backbone tertiary C-H bonds and methyl side groups, the latter make the dominant contribution. The spectral assignments facilitated calculation of the mole percent branches and the mole percent of inverted repeat units in the PVAc samples produced, the latter quantity taking values of 1.44 +/- 0.24 mol % independent of conversion in the emulsion polymerizations. The absence of an initiator effect in. the 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 overall conversion. However, because the emulsion polymerizations proceed almost exclusively within the latex particles and at high instantaneous conversions, the levels of branching in the PVAc produced (final levels = 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 temperature of emulsion polymerization from 60 to 70 degrees C led to an increase in the mole percent branches but had no measurable effect on the mole percent of inverted repeat units. [References: 30]