Chain-Length-Dependent Termination in n-Butyl Methacrylate and tert-Butyl Methacrylate Bulk Homopolymerizations Studied via SP-PLP-ESR

The chain-length dependence of the termination rate coefficient, k t, of bulk homopolymerizations of n-butyl methacrylate (n-BMA) and tert-butyl methacrylate (t-BMA) at ambient pressure and temperatures between −30 and 60 °C has been studied via the single pulse−pulsed laser polymerization−electron spin resonance (SP-PLP-ESR) technique. The decay of radical concentration, c R, after laser SP initiation is monitored with a high time resolution of microseconds by ESR spectroscopy. Radical chain length, i, increases linearly with time t after applying the laser pulse. The experimental k t i,i values refer to rate coefficients for termination of two radicals of identical chain length i. The variation of k t i,i with chain length is adequately represented via the composite model proposed by Smith et al., in which two power-law expressions, k t i,i ∝ i −α, are contained with the exponents αs and αl referring to short-chain and long-chain radicals, respectively. The transition between the two regimes occurs at the crossover chain length, i c. The rate coefficients extrapolated for termination of two radicals of chain length unity, k t 1,1, are almost identical for n-BMA and t-BMA with an activation energy of E A(k t 1,1) ≈ 10 kJ mol−1. The αs values are close to each other: 0.65 ± 0.10 (n-BMA) and 0.56 ± 0.10 (t-BMA). Both αl values are found to be 0.20 ± 0.05, which is close to the theoretical value of αl = 0.16. The crossover chain lengths are i c ≈ 50 for n-BMA and i c ≈ 70 for t-BMA. The minor differences in composite-model parameter values of n-BMA and t-BMA are assigned to differences in chain mobility.