Chain Length Dependent Termination Rate Coefficients of Methyl Methacrylate (MMA) in the Gel Regime:  Accessing k_t^i,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 °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 α_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.