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Thiol-ene photopolymers generally comprise a multifunctional olefin, polythiol and photinitiator to give rise to a stepwise polymerization upon being irradiated under actinic light, as shown in scheme 1 [1]. The fatal factors affecting the efficient chain propagation include following two processes: (1) the addition reaction of ene moieties to thiyl radicals (R-S·) generated by hydrogen abstraction (propagation 1) and (2) the hydrogen abstraction of carbon radical species from other polythiols to reproduce thiyl radicals (propagation 2). Both reactions should proceed rapidly before termination due to radical-radical coupling occurs. From these viewpoints, we have started to synthesize novel types of dendritic polyenes having 4-, 8and 16-terminated allyl moieties as polyolefin species (AL(n), n = 4, 8 and 16), aiming at improvement of the reactivity in the propagation reaction 1 [2]. Since dendrimers are highly branched, spherical polymers with uniform molecular weights [3], the addition reactions of the allyl units to the thiyl radicals should be markedly enhanced by the local concentration of terminal units at the dendrimer surfaces. Indeed, it was confirmed that the dendritic AL(n)-based thiol-ene photopolymers brought about the remarkable acceleration in UV curing rates as the increase in the number of the terminal allyl units (n). In this paper, we focus on novel types of dendritic polynorbornenes because norbornene is regarded as one of the most promising ene units in thiol-ene chemistry because of its enhanced ability to activate carbon radicals in propagation 2 (Scheme 1) due to the distorted structures [1]. We describe here the effect of the norbornene units located at the outermost position of the dendrimers on the photosensitivity of the thiol-ene UV curable materials.

Synthesis of Polynorbornene Dendrimers to Apply to Thiol-ene Photopolymers with Excellent Photosensitivity