Requirements Associated with Studies into a Chain-Length Dependence of Propagation Rate Coefficients via PLP−SEC Experiments

Experiments directed toward the determination of free-radical propagation rate coefficients k<INF>p</INF> from pulsed laser initiated polymerizations (PLP) in conjunction with size-exclusion chromatography (SEC) were modeled using the program PREDICI. The simulations were carried out in order to investigate whether an experimentally observed variation of k<INF>p</INF> with the initiating laser pulse repetition rate is necessarily due to a chain-length dependence of k<INF>p</INF>. The modeling was performed for laser pulse repetition rates ranging from 0.5 to 50 Hz and primary radical concentrations being varied by more than 2 orders of magnitude. The resulting molecular weight distributions were very different in shape. The k<INF>p</INF> data derived from the simulated molecular weight distributions showed a variation of about 30% with chain lengths, although the same propagation rate coefficient was introduced into all the simulations. The experimental finding of k<INF>p</INF> being dependent on chain length induced by using different laser pulse repetition rates is not necessarily due to a chain-length dependence of k<INF>p</INF>. The modeling results show the importance of reporting all experimental details together with k<INF>p</INF> data derived from PLP−SEC experiments.