  The effect of a linear polymer dissolved in a reactive monomer on the
kinetics of free-radical polymerization is studied. The selected system was a
solution of polyisobutylene (PIB) in isobornyl methacrylate (IBoMA). Ternary
phase diagrams of PIB, IBoMA and poly(isobornyl methacrylate) (PIBoMA) were
built at 80C. They were shifted to lower conversions when increasing the molar
mass of PIB. Different PIBs exhibiting CPC at advanced conversions were
selected for the kinetic study performed employing differential scanning
calorimetry (DSC) at 80C. A simple kinetic model for free-radical
polymerizations describing the relevant termination rate constant in terms of
the free-volume theory, provided a consistent fitting of the polymerization
rates in the conversion range where the solution remained homogeneous.
Increasing the molar mass of PIB led to an increase in polymerization rate due
to the decrease in free volume and the corresponding decrease of the
termination rate. Increasing the amount of a particular PIB in the initial
formulation led to a less marked gel effect, explained by the smaller relative
variation of free volume with conversion. The dimensionless free volume of PIB
obtained from the kinetic model was found to increase with the volume
concentration of chain ends, as expected. Under conditions where phase
separation took place at very low conversions, the overall polymerization rate
exhibited the presence of two maxima (gel effects), representing the
polymerization in two different phases. The first maximum was associated to the
polymerization taking place in the phase lean in PIB and the second maximum was
associated to the polymerization of the monomer that was initially fractionated
with PIB.
