Is the rate constant of chain propagationkp in radical polymerization really chain-length independent?

Abstract

A careful investigation of the kp data obtained from pulsed-laser polymerization at different pulse separations t0 in a lot of systems has revealed that kp exhibits a slight but significant decrease when t0 is increased, corresponding to an about 20% decrease of kp extending over several hundreds in degree of polymerization. Transformation of this integral effect to individual chain-lengths reduces this range, of course, but still shows more than one hundred propagation steps to be concerned. This is interpreted in terms of a decrease of the monomer concentration at the site of propagation caused by the segments already added to the growing chain.