  The kinetics of the free-radical polymerization of isobornyl methacrylate
(IBoMA) at 80C, initiated by benzoyl peroxide (BPO), was determined by
differential scanning calorimetry (DSC), using sample pans previously sealed in
air or in nitrogen. The polymerization was arrested by vitrification at a final
conversion close to 0.80. The kinetics in nitrogen could be fitted using
initiation and propagation rate constants reported in the literature, a
functionality of the termination rate constant controlled by translational
diffusion based on the free-volume model, and an efficiency factor for the
initiator decomposition that decreased with conversion in the medium-high
conversion range. The presence of oxygen led to a significant decrease in the
polymerization rate, an effect that was enhanced when decreasing the sample
size (increasing the surface-to-volume ratio). The original finding was the
presence of two maxima in the isothermal polymerization rate (a double gel
effect), a fact that was confirmed by replicating experiences in two different
DSC devices and changing the amount of initiator in the formulation. The first
maximum in the polymerization rate was explained by the decrease of the
diffusion rate of macromolecular radicals to the oxygen-rich boundary, an
effect that starts at low conversions. The second maximum was related to the
decrease in the solubility/diffusion of oxygen in the reaction medium, a
phenomenon that is particularly severe at high conversions.
