The chain-length dependence of the termination rate coefficient, k_t, in methyl acrylate (MA) and dodecyl acrylate (DA) radical polymerization has been determined via the single pulse pulsed-laser polymerization near-infrared reversible addition-fragmentation chain transfer (SP-PLP-NIR-RAFT) technique. Polymerization is induced by a laser SP and the resulting decay in monomer concentration, c_M, is monitored via NIR spectroscopy with a time resolution of microseconds. A RAFT agent ensures the correlation of radical chain length and monomer-to-polymer conversion. The obtained rate coefficients for termination of two radicals of approximately the same chain length, i, are represented by power-law expressions, k_t(i,i) propor. to i~(-alpha). For both monomers, composite model behaviour of k_t(i,i) showing two distinct chain length regimes is observed. The exponent alpha_s referring to short chain lengths is close to unity, whereas the exponent alpha_1, which characterizes the chain-length dependency of large radicals, is slightly above the theoretical value for coiled chain-end radicals. The crossover chain length, i_c, which separates the two regions, decreases from MA (i_c = 30) to DA (i_c = 20). The results for MA and DA are consistent with earlier data reported for butyl acrylate. There appears to be a correlation of alpha_s and i_c with chain flexibility.
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