ANTHRAQUINONE PHOTOINITIATORS FOR FREE-RADICAL POLYMERIZATION - STRUCTURE DEPENDENCE ON PHOTOPOLYMERIZATION ACTIVITY

The photophysical, photochemical and photopolymerisation properties ha ve been studied for four derivatives of 2-substituted anthraquinone. P hotopolymerisation activity is highly dependent on the configuration o f the lowest triplet excited state. Fluorescence and phosphorescence a nalysis indicates efficient intersystem crossing to the triplet state and the pi-pi or n-pi* nature of the lowest excited triplet state, Th e four derivative possess either an amide or an ester type substituent . The amide derivatives possess an excited triplet state with pi-pi c onfiguration, whereas the ester derivatives exhibit an n-pi excited t ripler state. Photoinduced polymerisation of lauryl acrylate monomer h as been recorded by differential scanning photocalorimetry. Without an amine cosynergist, absolute quantum yields of photopolymerisation sho w that the anthraquinones with an ester type substituent and possessin g an n-pi configuration are more active. In the presence of a tertiar y amine cosynergist the anthraquinone derivates with an amide substitu ent possessing a longer wavelength pi-pi triplet state show enhanced activity. Nanosecond and microsecond hash photolysis studies and absol ute quantum yields of photoreduction have been related to their photop olymerisation activity. The results indicate the presence of semi-anth raquinone and radical anion intermediates in the photoreduction step. The type of photoreduction reaction occurring is dependent on the natu re of the excited state. The two types of derivative react predominant ly via either direct hydrogen atom abstraction or electron transfer fo llowed by proton transfer. The predominant method by which each molecu le undergoes photoreduction has been demonstrated by steady state irra diations and flash photolysis studies. Copyright (C) 1996 Elsevier Sci ence Ltd.