Effect of monomer feed rate on chain transfer to polymer in semibatch emulsion polymerization of vinyl acetate studied by NMR spectroscopy

The effect of growth-stage monomer feed rate on the extent of chain transfer to polymer in semibatch emulsion homopolymerizations of vinyl acetate has been studied by using C-13 NMR spectroscopy to quantify the mol % branches in the poly(vinyl acetate) formed. Three 37.5% final solids content emulsion homopolymerizations of vinyl acetate were carried out at 70 degrees C using identical seed stages, but different monomer feed rates of 0.35, 1.39, and 2.08% min(-1) in the growth stages. The polymerizations performed using the feed rate of 0.35% min(-1) proceeded under monomer-starved conditions throughout, and the mol % branches increased steadily with overall conversion. Use of the higher feed rates led to lower instantaneous conversions from the end of the seed stage (17% overall conversion) to about 40% overall conversion, from which point onward the polymerizations proceeded under monomer-starved conditions. During the period of reduced instantaneous conversion, the frequency of chain transfer to polymer was lower, and the cumulative mol % branches reduced. However, as the polymerization progressed further and high instantaneous conversions were recovered, the mol % branches increased steadily. Thus, the lower values of mol % branches for the final samples of poly(vinyl acetate) from the reactions run at the two higher monomer feed rates result directly from the lower frequency of chain transfer to polymer in the period of monomer flooding that followed the start of the monomer feeds. The variation of conversion and mol % branches for two polymerizations carried out under the same conditions but to different final solids contents (37.5 and 50%) were virtually identical and confirm that the variation of instantaneous conversion with overall conversion is the principal factor controlling the mol % branches.