Chain-Length-Dependent Termination of Styrene Bulk Homopolymerization Studied by SP-PLP-EPR

Single-pulse pulsed-laser polymerization in conjunction with time-resolved quantitative EPR detection of radicals, SP-PLP-EPR, has been carried out in the early period of styrene polymerizations between 73 and 135 degrees C using dicumyl peroxide as the photoinitiator. To enhance signal-to-noise quality, in addition to styrene-H-8, the fully deuterated monomer, Sty-d8, has been investigated. The chain-length dependence of the rate coefficient for termination of two radicals of chain length i, k(t)(ii), is adequately represented by the composite model. The power-law exponents for short-chain and long-chain radicals are obtained as alpha(s) = 0.51 +/- 0.05 and al = 0.16 +/- 0.05, respectively, with the two ranges being separated by the crossover chain length, ic = 30 +/- 10. The termination rate coefficient for two radicals of chain length unity, k(t)(1,1,) scales with temperature as does monomer fluidity. From k(t)(i,i) deduced via SP-PLP-EPR, chain-length-averaged termination rate coefficients, < k(t)>, have been estimated, which are close to the reported experimental values.