Buback, M.; Müller, E.; Russell, G. T.
SP-PLP-EPR Study of Chain-Length-Dependent Termination in Free-Radical Polymerization of n-Dodecyl Methacrylate, Cyclohexyl Methacrylate and Benzyl methacrylate: Evidence of ´Composite´-Behavior
Journal of Physical Chemistry A 2006, 110, 3222-3230.
 

The chain-length dependence of the termination rate coefficient in n-dodecyl methacrylate (DMA), cyclohexyl methacrylate (CHMA), and benzyl methacrylate (BzMA) bulk free-radical homopolymerizations at ambient pressure and at temperatures from -20 to 0 °C is deduced using the recently developed technique of SP-PLP-EPR: pulsed-laser polymerization (PLP) in which time-resolved EPR measurement of radical concentration, c_R, is made following each single pulse (SP) of an excimer laser. The decay of c_R results from termination of radicals of almost identical size. Their chain length, i, increases linearly with time, t, after applying a SP. The rate coefficient, k_t^i,i, for termination of two radicals of size i is determined by fitting the experimental c_R vs t data. This process demonstrates that (at least) two power-law exponents are necessary to describe k_t^i,i over the extended chain-length range of i = 1 to 1000. This is consistent with the so-called "composite model", which uses power-law exponents α_S and α_L to describe termination of radicals either shorter or longer, respectively, than a crossover chain length, i_c. The fourth parameter obtained from fitting the SP-PLP-EPR data with this model is k_t^1,1, the termination rate coefficient for two radicals of degree of polymerization 1. Previous DMA experiments are reanalyzed while new experimental results are reported and analyzed for CHMA and BzMA. The parameter values for CHMA and BzMA termination at 0 °C are almost identical-k_t^1,1 ≈ 3 × 10⁷ L mol^-1 s^-1, α_S ≈ 0.50, i_c ≈ 90, and α_L ≈ 0.21-and they are close to those for DMA at 0 °C: k_t^1,1 ≈ 1 × 10⁷ L mol^-1 s^-1, α_S ≈ 0.64, i_c ≈ 50, and α_L ≈ 0.18. The results fully support the composite model in that the chain-length dependence is more pronounced for shorter than for longer radicals, i.e., α_S > α_L. Moreover, the power-law exponent that characterizes termination of long-chain radicals is close to the theoretical value of α_L = 0.16. In fact all parameter values-including the small differences between DMA and CHMA/BzMA-are more-or-less in accord with expectations based on polymer dynamics. Furthermore, our results suggest that termination of methacrylate radicals with large cyclic or long n-alkyl substituents may be affected by steric shielding of the radical functionality.