Chain Length Dependent Termination Rate Coefficients of Methyl Methacrylate (MMA) in the Gel Regime:  Accessing <i>k</i><sub>t</sub><i><sup>i,i</sup></i><sup></sup> Using Reversible Addition-Fragmentation Chain Transfer (RAFT) Polymerization

The termination rate coefficient, <i>k</i><sub>t</sub><i><sup>i</sup></i><sup>,</sup><i><sup>i</sup></i><sup></sup>, for propagating chains of near equal length, <i>i</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., <i>c</i>*), and was found to range from 15 to 30% conversion depending on the <i>M</i><sub>n</sub>. It was further shown that <i>c</i>* also corresponded with the gel onset conversions for vinyl acetate and methyl acrylate. The chain length dependence of <i>k</i><sub>t</sub> in the gel regime scaled as α<sub>gel</sub>(<i>x</i>) = 1.8<i>x</i> + 0.056, suggesting that reptation alone does not play a role in our system. A composite model was then derived to accurately describe <i>k</i><sub>t</sub><i><sup>i</sup></i><sup>,</sup><i><sup>i</sup></i><sup></sup> for chain lengths up to 3200 and conversions up to 70%. The <i>k</i><sub>t</sub><i><sup>i</sup></i><sup>,</sup><i><sup>i</sup></i><sup></sup> 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 <i>k</i><sub>t</sub><i><sup>i</sup></i><sup>,</sup><i><sup>i</sup></i><sup></sup>(<i>x</i>) data can be determined.