Preparation and photopolymerization behavior of multifunctional thiol-ene systems based on hyperbranched aliphatic polyesters

A multifunctional polythiol based on hyperbranched polyester Boltorn™ H40 (thioglycolic actate of H40), and two multi-ene hyperbranched polyesters based on Boltorn™ H20 and P1000 (allyl ether terminal functionalized H20 and norbornene end-capped P1000) were synthesized and characterized by 1H NMR spectral analysis. The UV-induced photopolymerization behavior of multifunctional thiol-ene system, including those oligomers synthesized above, a tri-functional thiol, a tetra-allyl ether and a tri-norbornene, was real-time monitored with Photo-DSC in air. The polymerization rate (Rp) and functional group conversion were evaluated according to the Photo-DSC exotherms. It was found that the Rp and the conversion decreased with decreasing the molar ratio of [SH]/[CC] from 1:1 to 1:10. Moreover, the photopolymerization behavior of thiol-ene system was greatly influenced by the type of ene group and the functionality of the oligomers and monomers. The norbornene monomer exhibited much higher reactivity than allyl ether monomers even with less functionality. And for the same type of thiol-ene system, the functionality of a monomer determined its reactivity. The apparent rate constant for photopolymerization (kapp) was also calculated from the equation Rp = kapp [SH]x[CC]y to generally evaluate the integrative effect of all complex factors affecting thiol-ene photopolymerization.