Photochemistry and Photopolymerization Activity of Water-Soluble Benzophenone Initiators

Abstract

Quantitative photophysical, photochemical and photopolymerisation data are presented on five novel water soluble benzophenone photoinitiators. Phosphorescence quantum yields, triplet lifetimes and transient formation on conventional flash photolysis correlate with the ability of the initiators to photoinduce the polymer isation of 2-hydroxyethylmethacrylate and a commercial monoacrylate resin in aqueous media. The results indicate that the lowest excited triplet state of the initiator is abstracting an electron from the tertiary amine cosynergist probably via a triplet exciplex followed by hydrogen atom abstraction. This is confirmed by a detailed analysis on the effect of oxygen, pH and the ionisation potential of the amine on transient formation and photopolymerisation. Using photocalorimetry a linear correlation is found between the photopolymerisation quantum yields of the initiators and their photoreduction quantum yields in aqueous media. One of the photoinitiators, 4-methylsulphoniumbenzophenone sodium salt, exhibited a photoreduction quantum yield of 1.5 which is associated with secondary processes due to the reaction of the alkylamino radicals with a ground-state ketone molecule.In previous papers we reported on the photochemistry and photopolymer isation activity of a series of water-and oil-soluble thioxanthone derivatives (1-6).In that work, all the thioxanthone structures were found to operate essentially by a primary photochemical process of electron abstraction via an exciplex with an amine co-synergist. However, whereas the oil soluble structures were found to operate solely through their lowest excited triplet state (1,2,6) the water-