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, k(t)(i,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 degrees 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 M-n. It was further shown that c* also corresponded with the gel onset conversions for vinyl acetate and methyl acrylate. The chain length dependence of k(t) in the gel regime scaled as alpha(gel)(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 k(t)(i,i) for chain lengths up to 3200 and conversions up to 70%. The k(t)(i,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 k(t)(i,i)(x) data can be determined.