Methods: Ethoxylated bisphenol A dimethacrylate (BisEMA; control) was combined with mono-, di- or tetra-thiols: dodecanethiol (DDT), methyl-mercaptopropionate (MMP), 1,6-hexanedithiol (HDT), or pentaerythritol-tetramercaptopropionate (PETMP). A 10:1 molar ratio of methacrylate to thiol functional groups was established in all formulations. Real-time near-infrared (RT-NIR) spectroscopy provided methacrylate reaction kinetics under standardized photopolymerization conditions. Flexural strength/modulus measurements were used to determine polymeric mechanical strength properties. Dynamic polymerization stress development was monitored by a tensometer coupled with RT-NIR.
Results: Incorporation of thiol CTA significantly (P<0.002) enhanced BisEMA conversion: control 68.0±0.2%, DDT 88.6±1.0%, MMP 83.6±0.3%, HDT 75.9±1.2%, PETMP 70.6±0.8%. Thiol additives eliminated autoacceleration and decreased the maximum reaction rate but also postponed the onset of autodecceleration to much higher conversion (~50-70%) compared with the control (<20% conversion), which is strong evidence for delayed network development. The unusual thiol-induced kinetic profiles effectively allow higher conversion to be achieved in the same time as the control. Mechanical properties for thiol-methacrylate polymers were improved (P<0.001). For the control, MMP, HDT and PETMP materials, the flexural strength was 60.8±6.8, 101.7±8.7, 88.9±8.5 and 87.7±3.9 MPa and flexural modulus was 1.58±0.19, 3.11±0.16, 2.61±0.14 and 2.55±0.02 GPa, respectively. The polymerization stress for the same series was 1.82±0.06, 1.95±0.09, 1.48±0.10 and 1.30±0.02 MPa, respectively, with all thiol-containing materials showing delayed onset of stress development.
Conclusions: Minor amounts (3-7 wt%) of thiol CTA substantially altered dimethacrylate reaction kinetics and network formation. A monothiol additive, which increases the number of isolated chain ends, provided the greatest increase in both conversion and mechanical strength while a tetrathiol, which introduces a novel multifunctional crosslink to the methacrylate network structure, best achieved the clinically desirable goal of limiting polymerization stress.
Support: NIH/NIDCR-1R01DE018233