BisGMA

Compomers contain no water, but rather are mainly formulated from the same components as conventional composite resins. Typically this means macromonomers, such as bis-glycidyl ether dimethacrylate (bisGMA) or its derivatives and/or urethane dimethacrylate, blended with viscosity-reducing diluents, such as triethylene glycol dimethacrylate (TEGDMA). These polymer systems are filled with non-reactive inorganic powders, for example, quartz or a silicate glass [271]. 

Next-generation soft contact lenses, dental polymers, surface coatings, and similar materials are produced from compounds of varying structure and reactive functionality. For example, currently in development are new soft lenses that will be manufactured from monomers synthesized with dimethylsil-oxane backbones. The dimethylsiloxane backbone is terminated with a methacryloxy functionality that supplies the site for polymerization. The siloxane provides lens softness. Occasionally the functionality is formed on both ends of the monomer, resulting in undesired properties. The compound BisGMA is a monomer that is polymerized to form hard dental structures. In the monomer synthesis process impurities are coproduced that interfere with the polymerization. Finally, diacetone acrylamide used in a copolymerization process is another specialty monomer that is occasionally contaminated with difficult-to-remove impurities. These three monomers are quite reactive at modest temperature and cannot be purified by distillation. The three examples that are presented here derive from as yet unpublished research (Krukonis, 1982c). 

BisGMA resin is an advanced restorative material currently in use but it suffers from certain drawbacks. The chemical name of BisGMA is 2,2-bis(p2 -hydroxy-3 -methacryloxypropoxy)-phenylene-propane. The impurities present in the resin can limit its shelf life and can also interfere with the polymerization reaction to form impervious enamel-like structures. These impurities exhibit very low vapor pressures and cannot readily be removed by conventional distillation techniques. Similar to the soft-lens monomer, heating the mixture of BisGMA results in the premature polymerization of the resin. The ease of polymerization of BisGMA, although advantageous for its end use, can limit its ability to be purified. 

The BisGMA monomer is polymerized by either of two free-radical mechanisms a thermally induced peroxide initiation or a photoinduced a,b-diketone initiation. Many of the initiator formulations are proprietary to their respective manufacturers, but a typical peroxide initiator is benzoyl peroxide, which forms free radicals when the temperature is raised. The a,b-diketone initiator responds to incident light energy to form free radicals. 

In the peroxide formulation initiation, the organochlorine impurities that are present in commercial BisGMA accelerate the decomposition of the peroxides, a process that forms free radicals and reduces the stability and shelf life of the resin. In the a,b-diketone formulation, the organochlorine impurities complex with other additives in the formulation that are present to enhance free-radical formation. The presence of the impurities thus reduces the photoefficiency and negates the advantages of simple photoinitiated diketone curing. 

The problems with the chlorinized impurities notwithstanding, BisGMA is the superior dental material available today. If the impurity-induced limitations could be eliminated or reduced, the use of BisGMA could proliferate into... 

Figure 9.55 HPLC chromatogram of commercially available BisCMA (1) 1-chloro-isopropanol methacrylate (2) BisGMA and (3) diglyddyl ether of bisphenol-A.

The main monomer used is the substance generally known as bisGMA, but whose systematic chemical name is 2,2-bis[4-(2-hydroxy-3-methacrylyloxypropoxy)phenyl] propane (Fig. 3.1). Composites based on this substance were developed originally by Bowen in the early 1960s in an attempt to combine the chemistry of epoxy and methacrylate systems [9]. This monomer is a liquid of high viscosity and to prepare composite... 

Fillers also interfere with the transmission of light because of the difference in refractive index between them and the matrix phase. This causes refraction at the interface between the filler and the polymer [53,54], which results in altered light intensity and thus recued conversion of monomer to polymer. This effect is particularly severe in the so-called flowable formulations which contain relatively high levels of diluent molecules because the mismatch in the refractive index between these substances and the fillers is greater than that with bisGMA or UDMA [55]. 

The phenomenon of polymerization shrinkage in composite resins based on either bisGMA or UDMA is considered to be their most important deficiency... 

As well as conventional composites of the type based on bisGMA and/or UDMA and filled with silicate-based filler, there are now materials available that are essentially composites in that they comprise a polymeric matrix reinforced with finely divided filler. However, either the polymer system or the filler phase is of a different chemical composition from that of conventional composite resins. Three such materials are currently available, and these are the ormocers, the siloranes and the giomers. Their details are given in Table 3.3, and their characteristics are described in the following subsections. 

The constituent molecules of siloranes are large, as shown in Fig. 3.9, and have a reasonable viscosity, though less so that bisGMA. 

V. Qvist, K. Stoltze, J. Qvist, Cytotoxicity of a bisGMA dental composite before and after leaching in organic solvent, J. Biomed. Mater. Res. 25 (1989) 443-457. 

Neither of these materials contains any of the monomers used in conventional composites, particular bisGMA [30], In addition, it contains no bisphenol A or its derivatives, so that it is biocompatible for its intended use, and does not contribute to the burden of molecules in the environment that mimic the action of oestrogen. 

Figure 7 Methacrylates and acrylates widely used In medicine and dentistry (a) PMMA, (b) poly(acrylic acid), (c) pHEMA, (d) poly(2-phenylethyl methacrylate), and (e) 2,2-bls[4-(2-hydroxy-3-methacryloyolxypropoxy)phenyl]propane monomer (BIsGMA) (also called bisphenol A-glycidyl methacrylate).

PMMA and other methacrylate and acrylate polymers are widely used in dentistry. PMMA is used for dentures and root canal sealants. Polymers of 2,2-bis[4-(2-hydroxy-3-methacry-loyolxypropoxy)phenyl]propane (BisGMA), triethyleneglycol dimethacrylate (TEGDMA), and urethane dimethacrylate (UDMA) are used in dental composite resins, most commonly with a silica filler. Such composite resins are used for filling cavities, reshaping, and restoring teeth and for full and partial crowns. 

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