Dye-sensitized photopolymerization of vinyl monomers in the presence of ascorbic acid-sodium hydrogen orthophosphate complex

Abstract

The kinetics of photopolymerization of vinyl monomers (methyl methacrylate, methylacrylate, and acrylonitrile) sensitized by the dyes acridine orange, dahlia violet, rose bengal and Pyronine G have been studied systematically. Systems containing dye, reducing agent, and monomer were irradiated with light of wavelengths 365 mμ, 546 mμ, and 405 mμ. The induction period was kept to a minimum by suitable experimental conditions. All experiments were conducted under deaerated conditions. The complex, AA-B, formed between ascorbic acid and sodium hydrogen orthophosphate (of the buffer) and reported earlier from our laboratory, played an important role in the production of the free radicals. The course of the polymerization reaction was followed by the measurements of rate of monomer disappearance -d[M]/dt (gravimetrically), rate of dye disappearance -d[D]/dt (spectrophotometrically), and the chainlengths n, of the polymer formed (viscometrically); light intensity, light wavelength, and concentrations of the dye, reducing agent, and monomer were the various variables for these measurements. A tentative scheme based on the excitation of the dye in the primary photochemical act and the interaction of the excited dye with the complex AA-B in the dark resulting in radicals (from ascorbic acid) which initiate vinyl polymerization is proposed, and the experimental results are found to conform to such a scheme. The values of various ratios of rate constants such as k_p/k_t^½, k_r/k₇, k_i/k₆ were calculated and are discussed.