Free-Radical Propagation Rate Coefficients via n-Pulse Periodic Polymerization

Abstract

Aspects of applying n‐pulse periodic initiation in pulsed laser polymerization/size‐exclusion chromatography (PLP/SEC) experiments are studied via simulation of molecular weight distributions (MWDs). In n‐pulse periodic PLP/SEC, sequences of n laser pulses at successive time intervals Δt1 up to Δtn are periodically applied. With the dark time intervals being suitably chosen, n‐modal MWDs with n well separated peaks occur. The n‐pulse periodic PLP/SEC method has the potential for providing accurate propagation rate coefficients, kp. Among several measures for kp, the differences in molecular weights at the MWD peak positions yield the best estimate of kp under conditions of medium and high pulse laser‐induced free‐radical concentration. Deducing kp from n dark time intervals (corresponding to n regions of free‐radical chain length) within one experiment at otherwise identical PLP/SEC conditions allows addressing in more detail a potential chain‐length dependence of kp. Simulations are compared with experimental data for 2‐pulse periodic polymerization of methyl methacrylate.

Determination of the Mode of Radical Termination from Pulsed Laser Polymerization Experiments in the Presence of Retardation and Chain Transfer to Agent

“…This error is expected to take into account the dispersion in δ associated with the accuracy in estimation of L 0 , L 2 , and ω 2 from MMDs. Taking into account that the errors in estimation of α and λ (±0.05 and ±0.02, respectively), the final value for the mode of termination reads δ = 0.60 ± 0.03 for n ‐BMA polymerization at 30 °C.…”

“…The inhibition reaction considered is: where Z is the (unknown) retarding agent and k z is the associated rate coefficient. Analytical expressions for this case have been derived previously, and are used to simulate MMDs generated by PLP at the HTRL under varying levels of U Z = k z [Z] for n ‐BMA polymerization at 30 °C, as presented in Figure a. All the distributions are given in the form of w (log M ) = M 2 n ( M ), where n ( M ) is the number MMD generated by polymerization during t d .…”

“…The values of α and λ for long-chain radicals are taken to be 0.2 (Barth et al [ 29 ] ) and 0.07 (assumed to be the same as for MMA polymerization in work by Nikitin et al [ 32 ] ), respectively. Values for δ estimated from experimental MMDs using Equation 18 are compared below with those estimated assuming no chain-length dependence (Equation 11 ).…”

Bimolecular Free-Radical Termination at Low Conversion

“…They have performed a number of simulations of PLP, using both chain-length-independent termination rate coefficients, employing generating functions (232)(233)(234)(235), as well as chain-length-dependent termination rate coefficients (220 posed two new methods for the recovery of chain-length-dependent k t information, one based on rate measurements and the other on MWD measurements (220). They have performed a number of simulations of PLP, using both chain-length-independent termination rate coefficients, employing generating functions (232)(233)(234)(235), as well as chain-length-dependent termination rate coefficients (220 posed two new methods for the recovery of chain-length-dependent k t information, one based on rate measurements and the other on MWD measurements (220).…”

Radical Chain Polymerization

“…Unlike the rotating sector method, the PLP-SEC method allows the measurement of k p directly without the need to couple it to the termination rate constant. The PLP-SEC method, like the rotating sector method, involves a non-steady-state photopolymerization [Beuermann, 2002;Beuermann and Buback, 2002;Kornherr et al, 2003;Nikitin et al, 2002]. Under pulsed laser irradiation, primary radicals are formed in very short times ($10 ns pulse width) compared to the cycle time ($1 s).…”