Evaluation of initiator systems for controlled and sequentially curable free-radical/cationic hybrid photopolymerizations

Free-radical/cationic hybrid photopolymerizations of acrylates and epoxides were initiated using a three-component initiator system comprised of camphorquinone as the photosensitizer, an amine as the electron donor, and a diaryliodonium salt. Thermodynamic considerations revealed that the oxidation potential of the electron donor must be less than 1.34 V relative to SCE for electron transfer with the photoexcited camphorquinone to take place. This electron transfer leads to the production of the active centers for the hybrid polymerization (two radicals and a cation). Further investigation revealed that only a subset of electron donors that meet the oxidation potential requirement resulted in polymerization of the epoxide monomer; therefore, a second requirement for the electron donor (pK(b) higher than 8) was established. Experiments performed using a combination of electron donors revealed that the onset of the hybrid system's cationic polymerization can be advanced or delayed by controlling the concentration and composition of the electron donor(s). These studies demonstrate that a single three-component initiator system can be used to initiate and chemically control the sequential curing properties of a free-radical/cationic hybrid photopolymerization and is a viable alternative to separate photoinitiators for each type of polymerization. (c) 2005 Wiley Periodicals, Inc.