Photochemical study and photoinitiation activity of macroinitiators based on thioxanthone

In this work we studied the photopolymerization of methyl methacrylate using polymeric initiators based on thioxanthone chromophore. As co-initiator were used low molecular weight tertiary amines and bound to a polymer chain. The photoinitiation efficiency of these systems was compared with that of the corresponding low molecular weight analogue. A higher efficiency was obtained with the system comprising the thioxanthone bound to a polymer chain and the free amine. The results carried out with low molecular weight TX and the polymeric co-initiator showed that the efficiency is independent of the amount of amine units in the polymer chain. The photophysics of the different systems show that the polymer chain does not affect the emission characteristics of the thioxanthone chromophore. Bimolecular rate constants for the singlet and triplet quenching of thioxanthone by the amine were determined using fluorescence and laser flash photolysis techniques. The dependence of the photoinitiation efficiency with the amine concentration was simulated from the measured quenching rate constants, and shows that the active radicals are produced from the interaction of the ketone triplet with the amine. The ketyl radical yield was slowly higher for the thioxanthone bound to the polymer. The difference between the photoinitiation activity of the polymeric system and the low molecular weight models is explained in terms of the effect of the polymer chain on the reactivity of the amino radicals. (C) 2002 Elsevier Science Ltd. All rights reserved.