Thiol-methacrylate networks and its application in novel hydrogels

Abstract

Thiol-ene systems have attracted considerable recent interest because they display efficient, versatile and selective “click” reactions. In this study, thiol-methacrylate networks based on a tetrafunctional thiol (PTMP) and dimethacrylate monomers were prepared by both photopolymerization and amine-catalyzed Michael addition reaction. The progress of the polymerization reaction was monitored by FTIR and Raman spectroscopy. The cured materials were characterized by measuring the glass transition temperature, the flexural modulus and the compressive strength. Innovative hydrogels based on PTMP and a water- soluble dimethacrylate monomer (LBisEMA) were prepared by both visible light photopolymerization and Michael addition reaction. Depending on the synthesis method employed significant differences in the degree of swelling were observed. LBisEMA–PTMP hydrogels prepared by the Michael addition reaction catalyzed by propylamine resulted in the highest water uptake.Hydrogels prepared by visible light photopolymerization contained unreacted thiol groups because of a faster homopolymerization reaction of the methacrylate groups. No significant effect of the PTMP/LBisEMA molar ratio on the water sorption capacity of the photocured hydrogel was observed. These trends are explained in terms of a balance between the mass fraction of hydrophilic groups and the crosslinking density of the network.Silver nanoparticles can be easily incorporated into thiol-ene systems because of the stabilizing properties of thiol functional groups. These polymer nanocomposites are very attractive for the preparation of biomaterials or dermatological patches with improved biocompatibility. Silver nanoparticles were prepared in PTMP by in situ reduction of silver nitrate with 2,6-di-tert-butyl-p-cresol. In this system, the thiol monomer acts as both stabilizing agent and reactive solvent. Mixtures of PTMP containing silver nanoparticles and a bifunctional methacrylate monomer were photoactivated with 2,2-dimethoxy-2- phenylacetophenone or camphorquinone (CQ) and then photopolymerized by irradiation with UV or visible light respectively. Visible light photopolymerization is commonly carried out using the CQ-amine photoinitiator system. CQ displays an intense dark yellow color which bleaches under irradiation giving as a result colorless polymers. However, we found that in thiol-ene systems CQ is regenerated through hydrogen transfer reactions between ketyl radicals and thiyl radicals. This feature must be taking into account when considering the long-term color stability of the derived cured materials.