MOLECULAR-WEIGHT DISTRIBUTIONS IN NITROXIDE-MEDIATED LIVING FREE-RADICAL POLYMERIZATION - KINETICS OF THE SLOW EQUILIBRIA BETWEEN GROWING AND DORMANT CHAINS

A kinetic approach, recently developed to calculate the effect of exch ange between a dormant and an active species in group transfer polymer ization (GTP), has been applied to living free radical polymerization moderated by nitroxide stable free radicals. A general solution for th e molecular weight distribution as a function of conversion has been d erived. The solution depends only on the rate constants for propagatio n, the trapping of the growing chains by nitroxide radical, and the re lease of the growing chain. The general form of the solution is the sa me in GTP and the stable free radical-mediated polymerization (SFRP), except for the definition of the constants. Using the measured and kno wn experimental rate constants for SFRP and fitting to the only unknow n rate constant, that for reversible chain termination by the nitroxid e radical, allow a quantitative prediction of the molecular weight dis tribution with conversion for bulk free radical living polymerization of polystyrene. In this way a good fit to the experimental polydispers ity is obtained over a wide range of polymerization conditions. The ca lculated rate for the reversible termination is reasonable compared to known nitroxide-trapping reactions but is over 3 orders of magnitude slower than for a diffusion-controlled reaction, on the same order as the rate constant for polystyrene radical-radical termination. Neverth eless, because of the excess nitroxide present, trapping is fast enoug h to ensure a high rate of exchange of growing and dormant chains, res ulting in narrow polydispersities at high conversion. The very good fi t to this model indicates that neither initiation nor termination are important to the conversion dependence of the molecular weight distrib ution, as neither were taken into account in the kinetic model. This i s further support for the current understanding of the mechanism and k inetics of the SFRP process. The polydispersity in the bulk living fre e radical polymerization mediated by nitroxide is controlled by the ex change rate between the growing and dormant chains. At high conversion where the rate of polymerization is high, there can be some irreversi ble chain termination and some autopolymerization.