Termination kinetics of dibutyl itaconate free-radical polymerization studied via the SP-PLP-ESR technique

The termination kinetics of dibutyl itaconate (DBI) bulk polymerization was studied via SP-PLP-ESR single pulse-pulsed laser polymerization With time-resolved detection of free-radical concentration by electron-spin resonance, at temperatures between 0 and 60°C. As is characteristic of PLP experiments, termination rate coefficients, k_t(i,i), are measured for radicals of (almost) identical chain length (CL) i. CL-averaged (k _t), far chain lengths up to 200 monomer units, and also k _t ⁰ referring to termination of very small-size radicals are directly deduced from measured DBI radical concentration vs time traces. At 45°C, 〈k_t〉 is (3.4±0.6) · 10⁵ L · mol^-1 · s^-1 and k_t ⁰ is (7,2±1.0) · 10⁵ L · mol ^-1 · s^-1. Both rate coefficients are independent of monomer conversion up to the highest experimental conversion of 18%. The associated activation energies are E_A(〈k_t〉) = 23.0±3.2 kJ · mol^-1 and E_A(k _t ⁰) = 27.6±2-8 kJ · mol^-1, respectively. "Model-dependent" and "model-free" analyses of radical concentration vs time profiles indicate a pronounced CL dependence of k_t(i,i) for DBI radicals of moderate size, 5 < i < 50. The lowering of k_t(i,i) with CL corresponds to an exponent α close to 0.5 in a power-law expression k_t,(i,i) = k_t ⁰ · i^-a. At higher chain lengths the variation of k _t(i,i) with CL becomes weaker and may be represented by an α value of 0.16 or even below. These results are consistent with models according to which a varies to a larger extent at low CL and to a smaller extent at high CL with the crossover region between the two ranges being located somewhere around i = 100.