Presence of Two Maxima in the Isothermal Free-Radical Polymerization Rate of Isobornyl Methacrylate Retarded by Oxygen

E. R. SOULÉ; J. BORRAJO; R. J. J. WILLIAMS

MACROMOLECULES

AMER CHEMICAL SOC

Año: 2004 vol. 37 p. 1551 - 1557

0024-9297

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