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A group of monomers which has found commercial use axe the urethane dimethacrylates. The first type of urethane dimethacrylate to appear was synthesized from hydroxyalkyl methacrylates and diisocyanates (91), These monomers have molecular weights nearly equal to that of BisGMA, but are less viscous. The most commonly used monomer of this type, l,6-bis(methacrylyloxy-2-etlioxycarbonylamino)- 2,4,4-trimethylhexan (UEDMA = UDMA), is the reaction product of 2- hydroxyethyl methacrylate and 2,,4,4-trimethylhexamethylenediisocyanate (Fig, 11), This monomer has been used alone (e,g, Isocap®, Vivadent; Isopast®, Vivadent; Isomolar®, Vivadent) or in combination with other monomers, e,g.,, BisGMA and TEGDMA (e,g, Heliomolar®, Vivadent; Estic Microfill Composite®,, Kulzer; Estilux MicroflU®, Kulzer; Durafill Light-curing Composite®, Kulzer) As mentioned above, urethanes have also been synthesized by the reaction of the secondary hydroxyl groups of BisGMA with isocyanates to create less hydrophilic monomer systems. Thus, Nuva-Fil® (L,D. Caulk) as well as Ful-Fil® and Prisma-Fil® (L.D. Caulk) were based on oligomers synthesized from BisGMA and hexamethylene diisocyanate (Fig. 12) (32, 34). Another urethane dimethacrylate system was used in Fotofil® (Johnson & Johnson/ICI), the very first proprietary resin composite to be visible light activated (32,, 92, 93). As may be seen from Fig. 13, this monomer system contains di(urethanophenyl) methane groups, which are susceptible to photo-oxidation with formation of quinoid groups accompanied by intense yellowing (1). In a subsequent resin composite, Occlusin® (ICI), the sensitive and rigid di(urethanophenyl) methane group was substituted by a hexamethylene diurethane group,, which is more flexible because of reduced barriers to free rotation about the bonds (Fig. 14) (34). The increased flexibility has been suggested to explain the higher degree of conversion obtainable with Occlusin compared with Ful-Fil (34), The advantages of UEDMA have been reported to be the lower viscosity (approximately 11,000 mPa-s at 23°C; ASMUSSEN, personal communication) and a greater flexibihty of the urethane linkage, which may improve toughness (94). However, a general difference between resin composites based on UEDMA or other urethane dimethacrylates and conventional BisGMA-based resin composites is very difficult to deduce from existing in vitro and in vivo studies (95-103). One reason for this difBculty is that actual differeBces caused by dissimilar monomer systems are camouflaged by the fact that the materials differ in many other aspects, e.g., type and amount of filler, type and amount of initiators, and quality of silanization of the filler particles, the effect of which may be more determinative for the properties. This, however, does not imply that the choice of monomer system is insignificant. The results of in vitro studies, in which all other components than the monomer system were kept constant, indicate that resin composites based on UEDMA will have improved mechanical properties compared with resin composites based on BisGMA (13, 14, 104). Also, there are indications that higher degrees of conversion are obtainable with urethane dimethacrylates as compared with BisGMA: TEGDMA mixtures (16, 94). All other things being equal, this would result in improved biocompatibility of the resin composite. Fewer pendant methacrylate groups and less residua! monomer may be oxidized to formaldehyde or methacrylic acid. Furthermore, by use of urethane dimethacrylates, the risk of bisphenol A formation is nonexistent. Dimethacrylates with aromatic groups have been reported to produce rigid polymers, while dimethacrylates with aliphatic groups produce flexible polymers (81, 105). It has been theorized that dimethacrylates with "hard" segments (aromatic groups) and "soft" segments (ahphatic groups) in the same molecule will restilt in polymers with increased toughness (106). For the purpose of designing stich a polymer, a dimethacrylate with a polyurethane chain was synthesized from HEMA and a poljoirethane diisocyanate, imprecisely described as a "polyester urethane with propylene glycol and hexanediol" (106). The dimethacrylate, (PUDMA; Fig. 15), incorporated two phenylmethyl groups as hard segments and propylene glycol or polymethylene as a soft segment. When heat-cured, this homopolymer exhibited less water sorption, lower volume shrinkage, and greater deflection than did polymers of BisGMA or of TEGDMA, while maintaining acceptable flexural strength and modulus of elasticity. The high toughness of PUDMA was explained by the favorable combination of the rigidity of the aromatic ring and the flexibility of the long chain. However, it seems that the idea of combining hard and soft segments in one dimethacrylate was implemented already in the Fotofil® and Occlusin® tirethanes (Fig. 13） |
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