Mechanisms and kinetics of nitroxide-controlled free radical polymerization

The bulk polymerization of styrene at 125 °C in the presence of a PS-TEMPO adduct was studied with respect to the polymerization rate and the concentration of free TEMPO as a function of time, where PS is polystyrene and TEMPO is 2,2,6,6-tetramethylpiperidinyl-1-oxy. The results were perfectly consistent with the proposed kinetic scheme which assumes the existence of a stationary state with respect to both polymeric and nitroxyl radical concentrations and predicts that the polymerization rate of the nitroxide-mediated system is independent of the adduct concentration, being equal to the polymerization rate of the adduct-free system, i.e., the rate of thermal polymerization in the case of the styrene system studied here. The equilibrium constant K for the PS-TEMPO reversible reaction was estimated to be 2.1 × 10^-11 mol L^-1 on the basis of the dilatometric and electron spin resonance data. This value of K was indicated to be large enough to set the system under control. This work thus shows that in order for the "living" radical polymerization mediated by a stable nitroxyl radical (SNR) to proceed successfully, a constant supply of initiating radicals (by, e.g., thermal initiation) to make up for the loss of polymer radicals due to irreversible termination is essential as well as the frequent reversible combination of polymeric and nitroxyl radicals. The total number of initiating radicals to be supplied in this way may be small compared with the number of polymer-SNR adducts so that they have no important influence on the molecular weight and its distribution of the product.