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 α s and α l 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 α s values are close to each other: 0.65 ± 0.10 ( n -BMA) and 0.56 ± 0.10 ( t -BMA). Both α l values are found to be 0.20 ± 0.05, which is close to the theoretical value of α l = 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.