Photopolymerization kinetics of dimethacrylates using the camphorquinone/amine initiator system

Abstract

The kinetics of photopolymerization of dimethacrylates with the camphorquinone (CQ)/amine pair has been studied by isothermal d.s.c. and u.v.-vis. spectroscopy. In accordance with the proposed initiation mechanism of electron and proton transfer from the amine to the triplet CQ ketone, only amines with an abstractable proton on the α-carbon undergo significant rates of polymerization. However, in contrast to other studies of exciplex formation from triplet state ketones and quenchers, only a poor correlation was observed between the rate of radical formation (measured in terms of the maximum polymerization rate) and ionization potential. In general, the efficiency of amines in enhancing initiation was: tertiary > secondary > primary amine. A kinetic scheme for CQ consumption and radical formation was in satisfactory agreement with the experimentally observed rate dependence on the radiation intensity and on the CQ and amine concentrations. For low amine concentrations, the polymerization rate had a half order dependence on concentration, but at intermediate levels, the rate was independent of amine concentration. The polymerization was slightly retarded at high amine concentrations. The influence of CQ concentration and radiation intensity on the polymerization rate generally fitted the theoretical half power relationship, however evidence for primary radical termination and pseudo first-order termination was found at high and low initiation rates, respectively.