An. Nikitin et Av. Evseev, CHAIN-LENGTH-DEPENDENT TERMINATION RATE-CONSTANT - EFFECT ON MOLECULAR-WEIGHT DISTRIBUTIONS FORMED BY PULSED-LASER RADICAL POLYMERIZATION, Macromolecular theory and simulations, 6(6), 1997, pp. 1191-1210
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.