Investigation of polymer networks for dental fillings formed by photochemically induced atom transfer radical polymerization of bifunctional methacrylates

Abstract

Homogeneous and therefore tough photopolymer networks are highly required in high-performance applications, such as dental materials. In this work, such networks were formed from dimethacrylates by using photochemically induced atom transfer radical polymerization (photoATRP). Therefore, an equimolar mixture of diurethane dimethacrylate (UDMA) and decanediol dimethacrylate (D3MA), which are the monomers commonly used in dental systems, was selected as a matrix resin. PhotoATRP conditions were first optimized to obtain in a reasonable time a similar conversion compared to that of a free radical polymerization (FRP) system. The effects of ligand type, initiator efficiency, and initiator concentration were screened. A real-time near-infrared (RT-NIR) photorheology device was used to determine gel point, double-bond conversion, and shrinkage stress in situ. A system containing 0.5 mol % ethyl α-bromophenylacetate initiator (EBPA), 200 ppm of CuBr₂, and 10 equiv tris[2-(dimethylamino)ethyl]amine ligand (Me₆TREN) led to a conversion of 68% after 900 s at 50 mW cm⁻². Microstructural characterization of the prepared network was also performed using positron annihilation lifetime spectroscopy (PALS) proving formation of a more homogeneous network in the case of photoATRP compared to the FRP. The mechanical properties of the photoATRP network were evaluated by tensile and Dynstat impact tests showing improved toughness of the cross-linked materials compared to the FRP one. UV stability test also showed a low degree of coloring for the materials prepared by photoATRP. Therefore, photoATRP can provide cross-linked materials with high double-bond conversion, homogeneous network, good mechanical properties, and high UV stability in a few minutes.