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 dependence of the termination rate constant k(t). The MWDs for methyl methacrylate 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 with 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 different degrees of polymerization) taken from the literature. The dependences of the overall termination constant [k(t)] on initiation parameters (pulse repetition rate (upsilon) and pulse intensity for initiation 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 pulse are determined for different upsilon; b) by experiments on polymerization with packets of pulses where the constants k(p) (the rate constant 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.