Rpn. Veregin et al., THE PIVOTAL ROLE OF EXCESS NITROXIDE RADICAL IN LIVING FREE-RADICAL POLYMERIZATIONS WITH NARROW POLYDISPERSITY, Macromolecules, 29(8), 1996, pp. 2746-2754
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.