Reducing polymerization shrinkage stress of dental resins by new aromatization-driven chain transfer agents

Reducing polymerization stress without compromising the conversion rate or mechanical properties has been an important goal for developing new dental resin materials. To this, aromatization-driven addition-fragmentation chain transfer agents (ACTAs) were designed with high chain transfer rate and low polymerization inhibition effect. These new agents were synthesized and evaluated as polymerization regulators for methacrylate-based dental resin. The ACTAs were obtained via a two-step synthesis from anthrone, and characterized by NMR and mass spectroscopy. A typical resin composite, consisting of BisGMA, TEGDMA and photoinitiation system CQ / EDMAB, was modified by varying amount of an ACTA. Shrinkage stress and flexural properties were tested using a universal testing machine, and double bond conversion was evaluated by Fourier transform infrared spectroscopy.The chemical structures of ACTAs were confirmed by NMR and mass spectroscopy. The resin samples containing ACTAs exhibited substantially reduced shrinkage stress (almost no stress detected at 1.0 wt% incorporation of ACTA). The kinetics of ACTA-regulated polymerization provided support for the hypothesized addition-fragmentation mechanism. The double bond conversion was almost not affected for samples with 1.0 wt% or less ACTA. The addition of ACTA slightly reduced the flexural strength and modulus within the acceptable range. New chain transfer agents based on aromatization mechanism-driven fragmentation were synthesized. The new agents achieve shrinkage stress reduction while maintaining sufficient double bond conversion and mechanical properties, at 0.1wt.%, 0.2wt.%, 0.5wt.% and 1.0wt.% of addition.