Abstract
The chain-length-dependent nature of the termination reaction in radical polymerization (RP) renders the overall termination rate coefficient, , a complex parameter in the usual situation where the radical chain-length distribution is non-uniform. This applies also for the activation energy of termination, E(a)(), which we subject to detailed mechanistic investigation for the first time. The experimental side of this work measures E(a)() for the dilute-solution, low-conversion, chemically initiated homopolymerization of styrene (ST), methyl methacrylate (MMA), butyl methacrylate, and dodecyl methacrylate. Values of 25-39 kJ mol(-1) are obtained, consistent with strong chain-length-dependent termination (CLDT) for short chains. On other hand, the reanalysis of analogous bulk polymerization data for ST and MMA finds E(a)() values of 18-24 kJ mol(-1), consistent with weak CLDT for long chains. Both these results are as expected from the so-called composite model for CLDT. A simple analytic framework for understanding and predicting E(a)() values is presented for the standard RP situation of continuous initiation. All the results of this work can be rationalized via this framework, which clearly establishes that E(a)() is determined by far more than just the E(a) of radical diffusion. This framework is extended to activation energy for the number-average degree of polymerization, E(a)(DP(n)), which we measure and successfully scrutinize via our CLDT model. In the final section of this work, we make interesting, testable predictions about E(a)() and/or E(a)(DP(n)) in various RP systems of different natures to those studied here, most notably, systems involving acrylates, continuous photoinitiation, or dominant chain transfer.