CHAIN-LENGTH-DEPENDENT TERMINATION RATE-CONSTANT - EFFECT ON MOLECULAR-WEIGHT DISTRIBUTIONS FORMED BY PULSED-LASER RADICAL POLYMERIZATION

For polymerization initiated by an arbitrary sequence of laser pulses a numerical technique for calculating molecular weight distributions ( MWDs) is developed, which takes into consideration the chain length de pendence of the termination rate constant k(t). The MWDs for methyl me thacrylate and styrene are calculated by use of alpha and k(0) values (for the law k(t)(i,j) = k(0)(i)(-alpha) of termination of radicals wi th chain length i) and averages (<(i, j)over bar>) (for rate constants k(t)(i,j) = k(0)(<(i, j)over bar>) of termination of radicals with di fferent degrees of polymerization) taken from the literature. The depe ndences of the overall termination constant [k(t)] on initiation param eters (pulse repetition rate (upsilon) and pulse intensity for initiat ion by periodic laser pulses) are presented. Two methods are proposed for alpha and k(0) determination: a) by experiments on polymerization with periodic laser pulses where monomer-to-polymer conversions per pu lse are determined for different upsilon; b) by experiments on polymer ization with packets of pulses where the constants k(p) (the rate cons tant of propagation), alpha and k(0) can be determined simultaneously from MWD. For both methods simple analytical equations are derived for evaluation of the constants. The limits of application of the methods are determined by use of the numerical technique for MWD calculation.