Abstract
A number of polyfunctional methacrylates and acrylates have been synthesized to find an improved monomer system for the dental restorative resin. However, there are many difficulties in the reasonable design of monomer structure and the determination of improved monomer system, because the relationship between chemical structure of polyfunctional monomer and crosslinking reaction is not well known. In this study, the relationship between monomer structure and crosslinking reaction was mainly investigated on five ethyleneglycol dimethacrylates, three alkyleneglycol dimethacrylate and two dental monomers (Bis-GMA, UDMA). That is, the rate and extent of polymerization, the efficiency of crsslinking and the unsaturated double bond such as residual monomer and pendant double bond in the cured product of dimetha-crylates were studied. Homopolymerization of the dimethacrylates was isothermally carried out at 70, 80, 90, 100 or 110℃ under nitrogen and air atmospheres by using a DSC (Differential Scanning Calorimeter). Bis-GMA and UDMA were isothermally copolymerized at 90℃ with the ethyleneglycol dimethacrylates and the alkyleneglycol dimethacrylates in the nitrogen atmosphere. The residual monomer in the cured resin was extracted with methanol and quantitatively analysed by HPLC (High-Performance Liquid Chromatography). The results are summarized as follows : 1) The crosslinking reaction was mainly affected by the monomer structure, the temperature and the atmosphere of polymerization and the monomer composition in copolymerizasion. 2) In all of the dimethacrylates except for Bis-GMA, the reactivity of these monomer in isothermal bulk-polymerization varied with the chain length between two methacryloyloxy groups. That is, the extent of polymerization (Ep), the efficiency of crosslinking (Ec) and the rate of polymerization (Tp) increased, and the quantities of both the residual monomer (Rm) and pendant double bond (Dp) decreased, with increasing the chain length. 3) UDMA, TrEGDMA, TeEGDMA and NEGDMA containing a long aliphatic chain showed high Ep and Ec, low Rm and Dp, while the lowest reactivity was observed in Bis-GMA having bis-phenol A of a rigid frame work, among the dimethacrylates tested. 4) Polymerization of the dimethacrylates was inhibited by atmospheric oxygen, and the extent of inhibition increased with decreasing their chain length as well as their viscosity. And the inhibition by oxygen in the use of alkyleneglycol dimethacrylate was slightly stronger than that in ethyleneglycol dimethacrylates having similar chain length. 5) The reactivity in copolymerization varied with the monomer composition in many cases. However, in copolymerization of Bis-GMA with the dimethacrylates having the number of chain members less than eight, the reactivity was scarcely changed at Bis-GMA concentrations of more than 30 mol %. 6) TrEGDMA and TeEGDMA were found to be effective monomers for copolymerization with Bis-GMA or UDMA. 7) Unsaturated pendant double bonds were more abundant than residual monomers in each of the cured products, the former being two to five times of the latter in the homopolymer as well as in the copolymers. 8) In copolymerization, the monomer-ratio in the residual mixed monomers was nearly the same as the monomer-ratio first fed for copolymerization. 9) The glass transition temperature of the cured resin increased with decreasing the chain length between the two methacryloyloxy groups, and poly EGDMA and poly NPGDMA showed the highest glass transition temperatures among the cured products of dimethacrylates tested.
