On the kinetics of photopolymerization: a theoretical study.

Abstract

In dye-sensitized polymerization, some paradoxical phenomena have been found, such as an anomalous decrease of reaction rate with decrementing thicknesses of the gelling layers. By mathematical modelling, and experimental verification, it has been found that high incorporation efficiencies (> 95% conversion of monomers into the growing polymer) can only be obtained at the correct levels of dye in the gelling solution and by using the proper light intensity. Paradoxically, if levels of sensitizer or incident light intensities are too high, the rate of photosensitizer consumption is too high, as compared with the rate of monomer incorporation, so that dye depletion occurs prior to chain elongation, and the reaction suddenly comes to a stop. In methylene-blue catalysis, the curve correlating the monomer incorporation with the catalyst concentration exhibits a maximum, indicating poor conversions both below and above a critical catalyst level. When correctly used, photopolymerization still offers unique advantages over chemical (persulfate) polymerization, such as absence of oxidizing conditions and control at the onset of reaction, while guaranteeing high conversion efficiencies (> 95%).