Photoinitiation of multifunctional acrylates via ferrocene-alkyl chloride charge transfer complexes

The propensity for ferrocene-alkyl chloride charge transfer complexes (CTCs) to photoinitiate free-radical polymerization of multifunctional acrylates was determined using photodifferential scanning calorimetry. Also, the effects of varying ferrocene (ferrocene, methoxyferrocene, and cyanoferrocene) and alkyl chloride (dichloromethane and benzyl chloride) derivatives were evaluated with regard to the overall polymerization rate and conversion. Furthermore, relative polymerization rates of traditional free-radical Type I and Type II photoinitiators were compared to those of the ferrocene-alkyl chloride CTCs. Semi-empirical quantum mechanical analysis of the complexation reaction was performed using PM3, indicating a thermodynamic preference of complexations involving benzyl chloride, and corroborated the reported complexation mechanism. In order to explain the varying polymerization rates, the association constants for each complex were determined, whereupon complexation of each ferrocene derivative with dichloromethane was found to be more facile than similar complexation with benzyl chloride due to steric considerations. Substituent effects were more pronounced for the benzyl chloride complexes relative to those involving dichloromethane where steric constraints caused deviation from the expected effect. Thus, the cyanoferrocene-benzyl chloride CTC was determined to be the most effective photoinitiator examined with regard to semi-empirical analysis, complexation kinetics, and polymerization rate.