RADICAL CONCENTRATIONS IN FREE-RADICAL COPOLYMERIZATION OF MMA/EGDMA

Radical concentration histories for the bulk free radical copolymerization of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA) initiated with 0.3 wt% 2,2'-azobis(2-methyl-propionitrile) (AIBN) at 70°C over the entire composition range were measured with an on-line electron spin resonance spectrometer. Radical concentrations of 10-7-10-3moll-1 were observed. At high EGDMA levels, the radical concentration increases monotonously from the onset of polymerization. Even in the glassy state, the radical concentrations continue to increase significantly. Once high concentrations of radicals which are chemically bound to the crosslinked network form, they are stable to temperature increases above the glass transition temperature. These stable radicals can propagate with a newly added monomer to the swollen gel. For low EGDMA levels (<25wt%) there exist four stages of radical concentration change with reaction time (monomer conversion): (1) the radical concentration remains relatively constant for low monomer conversions; (2) at some intermediate conversion there is a dramatic rise; (3) after reaching a peak concentration there is a small decrease; (4) the concentration then increases gradually and levels off. The dramatic rise in the second stage is in agreement with the hypothesis that the autoacceleration of reaction rate in a free radical polymerization is due to an increase in the radical concentration. An analysis of the radical concentration rise rates reveals that the use of the quasi-stationary state hypothesis (QSSH) at high levels of crosslinking is not valid. These radical concentration measurements coupled with the corresponding conversion rate data give a direct estimate of the propagation constant (Kp). The propagation constant was found to fall slightly at low conversions, but to fall dramatically at some high conversion where the conversion rate approaches zero. This explains why the polymerization rate of a bulk free radical polymerization run at T<Tgp (the glass transition temperature of the polymer) falls to almost zero even though appreciable monomer and initiator still exist in the reacting mass. The fall in Kp is in turn due to a radical trapping effect, i.e. the radical centres are surrounded by a solid environment consisting of polymeric segments. Using the assumption that bimolecular termination at high conversions is controlled by 'propagation diffusion', with Kt∝Kp[M], the initiation efficiency (f) and the termination rate constant (Kt) were also calculated from the data. The initiation efficiency was found to fall almost simultaneously with the propagation rate constant. © 1990.