Mechanism and Kinetics of Nitroxide-Controlled Free-Radical Polymerization

The mechanism and kinetics of the polymerization of styrene in the presence of alkoxyamine (P-X) were discussed on the basis of experimental and computer-simulated data for a styrene/TEMPO system, where TEMPO is 2,2,6,6-tetramethylpipendmyl-1-oxy. It was confirmed that the stationary-state kinetics holds accurately except fo an initial short time. Namely, in this state the radical concentration [P*] and hence the rate of polymerization are determined solely by the balance of the initiation and termination reactions, just like in a nitroxyl-free system. ESR studies showed the existence of the reversible dissociation of P-X. Two mutually related but basically different methods to determined the activation rate constant k_act were described. It was shown that the spontaneous dissociation of P-X is the only main mechanism of activation in the styrene/TEMPO system. The polydispersity of the product is thus determined by the rate constant of dissociation k_d (= k_act) and polymerization time, and the chain length is determined by the number of P-X adducts and [P*]. Hence [X*] and the rate constant of combination (association) play no role (in the stationary state) but to maintain the stationary state. Other reactions such as initiation, termination and the thermal decomposition of P-X produce deviations from the behavior of an ideal living system. Since all these rate constants are available, details of the polymerization process and product characteristics can now be predicted.