Low volume shrinkage of polymers by photopolymerization of 1,1-bis(ethoxycarbonyl)-2-vinylcyclopropanes†
Low volume shrinkage of polymers by photoring-opening polymerization of 1,1-bis(ethoxycarbonyl)-2-vinylcyclopropane (VCP) and its methyl substituted derivate 1,1-bis(ethoxycarbonyl)-2-(prop-1-en-2-yl)-cyclopropane (Me-VCP) is presented. The aim of this study was to evaluate the photopolymerization behavior and structure of the corresponding polymers from VCP and its methyl substituted derivates 1,1-bis(ethoxycarbonyl)-2-(prop-1-en-2-yl)-cyclopropane (Me-VCP) and 1,1-bis(ethoxycarbonyl)-2-methyl-2-(prop-1-en-2-yl)-cyclopropane (DiMeVCP). VCP monomers were polymerized using camphorquinone (CQ) as a photo-initiator either in a binary or ternary photo-initiator system. The binary systems were formulated with ethyl 4-dimethylaminobenzoate (EDMAB) in relation to the monomer and 1 mol% of CQ. The ternary system was a mixture of 1 mol% of CQ, 2 mol% of EDMAB and 2 mol% of DPIHFP. Ternary photo-initiator systems showed high polymerization rates leading to high conversion in a short photo-activation time. One of the important findings is the formation of higher amounts of 1,5-type ring-opened units and lower amounts of cyclobutane-containing units in photopolymerization, which can make VCP an attractive component of dental composites, as the volume shrinkage is proportional to the 1,5-ring opening. Earlier it was observed that thermal free radical polymerization of VCPs gives higher amounts of cyclobutane structures compared to 1,5-type ring-opened units in the corresponding polymers, causing VCP to be excluded from the list of promising dental composites, as the volume shrinkage was higher than expected.