Chain-length dependent termination in pulsed-laser polymerization.: 5 -: The evaluation of the rate coefficient of bimolecular termination kt for the reference system styrene in bulk at 25°C

Following earlier suggestions the values for the rate coefficient of chain termination k(1) in the bulk polymerization of styrene at 25 degrees C were formally calculated a) from the second moment of the chain-length distribution (CLD) and b) from the rate equation for laser-initiated pseudostationary polymerization (both expressions originally derived for chain-length independent termination) by inserting the appropriate experimental data including the rate constant of chain propagation k(p). These values were treated as average values, <(k(t)(m))over bar> and <(k(t)*)over bar>, respectively. They exhibited good mutual agreement, even the predicted gradation (<(k(t)(m))over bar> < <(k(t)*)over bar> by about 20%) was recovered. The log-log plot of <(k(t))over bar> vs. the number-average degree of polymerization of the chains at the moment of their termination yielded exponents b of 0.16-0.18 in the power-law <(k(t))over bar> = A.<(P-n)over bar>b-,A ranging from 2.3 x 1.0(8) to 2.7 x 10(8) L.mol(-1).s(-1). These data are only slightly affected if termination is not assumed to occur by recombination only and a small contribution of disproportionation is allowed for.