Chain length dependent termination rate coefficients of methyl methacrylate (MMA) in the gel regime: Accessing kti,i using reversible addition-fragmentation chain transfer (RAFT) polymerization

The termination rate coefficient, kti,i, for propagating chains of near equal length, i, was evaluated using the RAFT-CLD-T method over a wide range of chain lengths and up to a conversion of 70% for MMA polymerizations carried out in the presence of the RAFT agent, CPDB, at 80°C. We found that the conversion for the gel onset corresponded to the conversion at which polymer chains begin to overlap (i.e., c*), and was found to range from 15 to 30% conversion depending on the Mn. It was further shown that c* also corresponded with the gel onset conversions for vinyl acetate and methyl acrylate. The chain length dependence of kt in the gel regime scaled as agel(x) = 1.8x + 0.056, suggesting that reptation alone does not play a role in our system. A composite model was then derived to accurately describe kti,i for chain lengths up'to 3200 and conversions up to 70%. The kti,i, profiles for well-known termination models were tested and most gave unsatisfactory agreement with our experiments. Our model can be readily applied to any monomer provided accurate kti,i(x) data can be determined. © 2007 American Chemical Society