Kinetics of Conversion of Cyclic Enes/Thiol Copolymerized with BisEMA

Objectives: Thiol-enes alone or in conjunction with methacrylates have been introduced as low stress alternatives for dental composites. This study analyzes the influence of structurally varied strained cyclic enes combined with thiols of different functionalities and extended to reactions involving dimethacrylate resins. Methods: Bicyclo [2.2.1] hepta-2,5-diene (BCH) or 5-vinyl-2-norbornene (VN) were co-polymerized with mono- or tetra- functional thiols (methyl 3-mercaptopropionate (MMP) or (pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), respectively). The molar ratio of ene to thiol functional groups was kept at 1:1. For the dimethacrylate co-polymerizations, ethoxylated bisphenol A dimethacrylate (BisEMA, which accounted for 70 wt% of the formulation) was combined with PETMP with or without MMP or BCH. Kinetics of conversion was followed with near-IR (6165 cm-1: methacrylate; 6033 cm-1: BCH ene; 6110 cm-1 VN exocylic-ene) during photoactivation. Results: BCH combined with MMP and PETMP reached 31.0±1.3 and 39.4±3.4% conversion, respectively, while the reactivity of VN combined to MMP was much higher: 94.4±5.7%. VN/PETMP showed phase separation as monomers and were not tested. The addition of PETMP to BisEMA increased the final methacrylate conversion compared to the pure BisEMA control (91.2±0.5 and 58.6±0.4%, respectively). The further addition of BCH and VN led to conversion values of 78.2±1.8 and 89.7±1.1%, respectively. In combination with BisEMA, the vinyl peak from BCH was very weak and the one from VN did not seem to change with polymerization. Conclusions: Vinyl-norbornene groups were more reactive than BCH, both with the thiols alone and combined with BisEMA. In the presence of BisEMA, the ene contribution to network formation does not seem to be very significant, but at least one of the enes (BCH) reduced the system's reactivity. The addition of the tetrafunctional thiol led to a 36% increase in the dimethacrylate conversion. Support: NIH/NIDCR-1R01DE018233.