M. Buback, M. Egorov, V. Kaminsky

Modeling the Effects of Primary Radicals in Free-Radical Polymerization

Macromol. Theory Simul., 11, 128-135 (2002)

The effects of non-ideal initiator decomposition, i.e., decomposition into two primary radicals of different reactivity toward the monomer, and of primary radical termination, on the kinetics of steady-state free-radical polymerization are considered. Analytical expressions for the exponent n in the power-law dependence of polymerization rate on initiation rate are derived for these two situations. Theory predicts that n should be below the classical value of 1/2. In the case of non-ideal initiator decomposition, n decreases with the size of the dimensionless parameter (k_tz /k_dz) r_ink_t, where k_tz is the termination rate coefficient for the reaction of a non-propagating primary radical with a macroradical, k_dz is the first-order decomposition rate coefficient of non-propagating (passive) radicals, r_in is initiation rate, and k_t is the termination rate coefficient of two active radicals. In the case of primary radical termination, n decreases with the size of the dimensionless parameter k_t,s r_in^1/2/k_p,s M r_t,l^1/2, where k_t,s is the termination rate coefficients for the reaction of a primary (short) radical with a macroradical, k_t,l is the termination rate coefficients of two large radicals, k_p,s is the propagation rate coefficient of primary radicals and M is monomer concentration. As k_t is deduced from coupled parameters such as k_t /k_p, the dependence of k_p on chain length is also briefly discussed. This dependence is particularly pronounced at small chain lengths. Moreover, effects of chain transfer to monomer on n are discussed.