| Abstract: | Chain transfer to polymer in emulsion polymerizations of acrylate monomers and vinyl acetate has been studied using 13C NMR spectroscopy to elucidate the chemistry by which chain transfer occurs and to quantify the mol% branches resulting from the reaction. In emulsion polymerizations of n-butyl acrylate, ethyl acrylate and methyl acrylate, chain transfer to polymer proceeds via abstraction of hydrogen atoms from backbone tertiary C-H bonds and typically gives rise to 2-4 mol% branches in the polymers obtained at complete conversion, the level of branching increasing with reaction temperature. For these acrylates, there is no evidence for a significant difference between the extent of chain transfer to polymer. In emulsion polymerizations of vinyl acetate, chain transfer to polymer proceeds mainly via H-abstraction from methyl side-groups, though there is a small contribution from abstraction at backbone tertiary C-H bonds. The levels of branching that result are substantially lower than in acrylate emulsion polymerizations, typically being in the range 0.6-0.8 mol% in the polymers obtained at complete conversion. The level of branching increases with temperature and as the degree of monomer starving (and hence instantaneous conversion) increases. Emulsion copolymerization of vinyl acetate with a small amount (5-20 wt%) of n-butyl acrylate gives rise to a significant increase in the level of branching (to values around 1.3-1.6 mol%), which results predominantly from H-abstraction of backbone tertiary C-H bonds in n-butyl acrylate repeat units by propagating radicals with vinyl acetate end units. |
