Photopolymerization of methyl methacrylate: effects of photochemical and photonic parameters on the chain length

Abstract

The effects of photochemical and photonic parameters are investigated on the chain length of poly(methylmethacrylate) (PMMA) as formed by the free radical photopolymerization process. Different photoinitiators were tested for the photopolymerization of PMMA. The Type II system based on isopropyl thioxanthone (ITX)–ethyl 4-(dimethylamino)benzoate (EDB) gave the best result. The conversion reaches 80% in 20 min contrary to Type I photoinitiators. The decrease of the ITX concentrations from 3 wt% to 0.05 wt% allows the formation of higher chains of PMMA with M_w = 170 000 g mol⁻¹, thanks to the decrease in photogenerated radicals. However, there was almost no influence of light intensity on polymer chains. To optimize the chain length of photopolymerized PMMA, Triazine A (TA) was added to reduce the formation of ketyl radicals arising from the reaction of ITX with EDB. The mechanism of the reaction of this three-component photoinitiating system studied by LFP confirmed the creation of ketyl radicals and their oxidation by TA. Then, the use of this three-component system increases both the kinetics of polymerization and the final conversion by virtue of the decrease in ketyl radicals, which act as terminating agents, and a concomitant increase of initiating radicals as formed from the reduced TA. Moreover, thanks to these new sources of initiating radicals, photopolymerization can take place at lower intensity of light. With TA, it was found possible to photopolymerize under very soft conditions and to allow the formation of longer polymer chains with M_w = 200 000 g mol⁻¹ at 1 mW cm⁻².