THE PIVOTAL ROLE OF EXCESS NITROXIDE RADICAL IN LIVING FREE-RADICAL POLYMERIZATIONS WITH NARROW POLYDISPERSITY

The pivotal role of the nitroxide concentration in bulk living polymer ization of styrene was studied between 115 and 135 degrees C, using in situ electron spin resonance spectroscopy (ESR) to follow the concent ration of the TEMPO stable free radical during the polymerization. Mol ecular weight and conversion were also followed on the same reaction m ixtures using gel permeation chromatography and thermogravimetric anal ysis, respectively. While molecular weights were linear with conversio n, to high conversion, there was an increase in the polymerization rat e with time: nonideal behavior for a living polymerization. However, t he TEMPO concentration also shows a slow decay as polymerization proce eds. Using the current mechanistic model, which predicts a polymerizat ion rate inversely proportional to TEMPO concentration, this changing concentration was incorporated into the kinetic analysis. Except for l ow conversion in the lowest temperature polymerization, correction for the TEMPO concentration resulted in ideal, constant polymerization ra te constants. While increasing the initial TEMPO concentration decreas es the rate of polymerization dramatically, the corrected rate is inde pendent of initial TEMPO concentration, again consistent with the curr ent mechanism. From these corrected polymerization rates, the activati on energy for the release of TEMPO from the growing chain end was esti mated as 82 kJ/mol, considerably less than the previously observed val ue of 130 kJ/mol for the release of TEMPO from styrene 1-mers. Using T EMPO as a probe of irreversible chain termination, ESR shows that irre versible chain termination up to 75% conversion is limited to less tha n 2 chains in a hundred. It is concluded that the TEMPO-mediated polym erization is a living polymerization under the conditions of this stud y. To aid in the understanding of these living polymerizations that ar e based on reversible termination, a new term has been defined, the ge rmination efficiency, which describes the yield of living chains in te rms of the reversible terminating agent.