The pivotal role of the nitroxide concentration in bulk living polymerization of styrene was studied between 115 and 135 °C, using in situ electron spin resonance spectroscopy (ESR) to follow the concentration of the TEMPO stable free radical during the polymerization. Molecular weight and conversion were also followed on the same reaction mixtures using gel permeation chromatography and thermogravimetric analysis, respectively. While molecular weights were linear with conversion, to high conversion, there was an increase in the polymerization rate with time:  nonideal behavior for a living polymerization. However, the TEMPO concentration also shows a slow decay as polymerization proceeds. Using the current mechanistic model, which predicts a polymerization rate inversely proportional to TEMPO concentration, this changing concentration was incorporated into the kinetic analysis. Except for low conversion in the lowest temperature polymerization, correction for the TEMPO concentration resulted in ideal, constant polymerization rate constants. While increasing the initial TEMPO concentration decreases the rate of polymerization dramatically, the corrected rate is independent of initial TEMPO concentration, again consistent with the current mechanism. From these corrected polymerization rates, the activation energy for the release of TEMPO from the growing chain end was estimated as 82 kJ/mol, considerably less than the previously observed value of 130 kJ/mol for the release of TEMPO from styrene 1-mers. Using TEMPO as a probe of irreversible chain termination, ESR shows that irreversible chain termination up to 75% conversion is limited to less than 2 chains in a hundred. It is concluded that the TEMPO-mediated polymerization is a living polymerization under the conditions of this study. To aid in the understanding of these living polymerizations that are based on reversible termination, a new term has been defined, the germination efficiency, which describes the yield of living chains in terms of the reversible terminating agent.

The autopolymerization of styrene, styrenic derivatives, and styrene/(meth)acrylate comonomer mixtures in the presence of stable nitroxide free radicals has been shown to be a “living” process. Molecular weight can be controlled by varying the ratio of vinyl monomer to TEMPO and low-polydispersity materials are obtained. Significantly, a definite incubation period is observed during these polymerizations, and the length of this incubation period increases with increasing amounts of TEMPO. The structures of the in situ generated unimolecular initiators which are formed during this incubation period correspond to those expected from a Mayo mechanism for the autopolymerization of styrene. The isolated and purified adducts, 4 and 5, were shown to be effective unimolecular initiators leading to low-polydispersity, controlled molecular weight polymers.

A kinetic approach, recently developed to calculate the effect of exchange between a dormant and an active species in group transfer polymerization (GTP), has been applied to living free radical polymerization moderated by nitroxide stable free radicals. A general solution for the molecular weight distribution as a function of conversion has been derived. The solution depends only on the rate constants for propagation, the trapping of the growing chains by nitroxide radical, and the release of the growing chain. The general form of the solution is the same in GTP and the stable free radical-mediated polymerization (SFRP), except for the definition of the constants. Using the measured and known experimental rate constants for SFRP and fitting to the only unknown rate constant, that for reversible chain termination by the nitroxide radical, allow a quantitative prediction of the molecular weight distribution with conversion for bulk free radical living polymerization of polystyrene. In this way a good fit to the experimental polydispersity is obtained over a wide range of polymerization conditions. The calculated 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. Nevertheless, because of the excess nitroxide present, trapping is fast enough to ensure a high rate of exchange of growing and dormant chains, resulting in narrow polydispersities at high conversion. The very good fit to this model indicates that neither initiation nor termination are important to the conversion dependence of the molecular weight distribution, as neither were taken into account in the kinetic model. This is further support for the current understanding of the mechanism and kinetics of the SFRP process. The polydispersity in the bulk living free radical polymerization mediated by nitroxide is controlled by the exchange rate between the growing and dormant chains. At high conversion where the rate of polymerization is high, there can be some irreversible chain termination and some autopolymerization.

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