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.

No abstract

The reversible bond formation between cobalt(II) catalytic chain transfer agents and propagating radicals was studied using electron paramagnetic resonance and conventional kinetic measurements. It was found that this reversible cobalt−carbon bond formation has no significant effect on the catalytic chain transfer polymerization of methyl methacrylate but does affect the polymerization behavior of styrene. In both systems significant induction periods are observed which seem to disappear in the methyl methacrylate system but persist in the styrene system upon decreasing the initial concentration of the cobalt(II) complex. The overall rates of polymerizations are found to be readily described by “classical” free-radical polymerization kinetics, including a chain-length-dependent average termination rate coefficient. Furthermore, in contrast to the situation observed in methyl methacrylate polymerization where constant molecular weights are produced over the entire conversion range, it was found that the molecular weight in styrene increases with conversion until a constant molecular weight is obtained which is given by the Mayo equation. The kinetic behavior and the molecular weight evolution could simply be modeled by a reaction scheme providing a constant radical concentration and the presence of a chain transfer agent.

No abstract

Summary: The benzene solution polymerizations of tert‐butyl acrylate (tBA) and 2‐ethylhexyl acrylate (EHA) have been investigated with respect to the effects of mid‐chain radicals (MCRs) on the rate of polymerization, the total radical concentration, the apparent rate coefficients for propagation, termination, and β‐fragmentation, the contents of unsaturated end groups and branching, and the molecular weight distribution. The EHA polymerization involves a considerably higher concentration of MCR and is more significantly affected by MCR than the tBA polymerization. The MCR content as estimated by electron paramagnetic resonance (EPR) spectroscopy during photosensitized EHA polymerization was as high as 70% of the total radical concentration at 25 °C. However, no 1H and 13C NMR resonances due to unsaturated ends and branching, respectively, were detected for the poly(EHA) obtained at 25 °C. These findings indicate that a high MCR content does not directly correspond to significant MCR effects on the polymer structure. The rate constants for mutual reaction ($k'_{\rm t}$) and β‐fragmentation (kf) of MCR were estimated for the two assumed cases where MCR was consumed solely by (i) mutual reaction, and (ii) β‐fragmentation, based on the after‐effect of the photosensitized EHA polymerization monitored by EPR spectroscopy at 25 °C; $k'_{\rm t}$ = 3.5 × 103 L · mol · s−1 and kf = 4.2 × 10−2 s−1. These rate coefficients were compared with those for reactions of structurally similar radicals.