Synthesis and photopolymerization of low shrinkage methacrylate monomers containing bulky substituent groups

Summary

Objectives

This study was conducted to determine whether novel photopolymerizable formulations based on dimethacrylate monomers with bulky substituent groups could provide low polymerization shrinkage without sacrifice to degree of conversion, and mechanical properties of the polymers.

Methods

Relatively high molecular weight dimethacrylate monomers were prepared from rigid bisphenol A core groups. Photopolymerization kinetics and shrinkage as well as flexural strength and glass transition temperatures were evaluated for various comonomer compositions.

Results

Copolymerization of the bulky monomers with TEGDMA show higher conversion but similar shrinkage compared with Bis-GMA/TEGDMA controls. The resulting polymers have suitable mechanical strength properties for potential dental restorative materials applications. When copolymerized with PEGDMA, the bulky monomers show lower shrinkage, comparable conversion, and more homogeneous polymeric network structures compared with Bis-EMA/PEGDMA systems.

Significance

The novel dimethacrylate monomers with reduced reactive group densities can decrease the polymerization shrinkage as anticipated, but there is no significant evidence that the bulky substituent groups have any additional effect on reducing shrinkage based on the physical interactions as polymer side chains. The bulky groups improve the double bond conversion and help maintain the mechanical properties of the resulting polymer, which would otherwise decrease rapidly due to the reduced crosslinking density. Further, it was found that bulky monomers help produce more homogeneous copolymer networks.

Introduction

All of the current dimethacrylate-based dental resins produce considerable volumetric shrinkage during polymerization. For dental composite restoratives, the addition of inert filler limits actual shrinkage in proportion to the volume fraction of the filler used. However, polymerization shrinkage is unavoidable for vinyl-based systems because of the density increase in the resin phase as covalent bonds are formed and mobility is decreased by conversion of monomer to polymer. In particular, the stress developed with the shrinkage is still a major cause of clinical failure of polymer-based dental restorative materials.[1] Many studies have been conducted to examine the potential flow of resins or composites either as a natural consequence of the polymerization process [2], [3], [4] or based on modified photo-curing protocols designed to reduce shrinkage stress [5], [6], [7]. It is postulated that a lower light intensity and/or a more flowable resin, will develop less stress in the final polymer for the same level of conversion. This reduction in stress implies a greater volumetric shrinkage and in particular, that more shrinkage occurs prior to significant stress development. According to Bowen, [8] volumetric shrinkage of 2% in a resin composite is sufficient to compromise the marginal integrity between the restorative materials and the tooth structure, and as a result, micro-leakage and restoration failure can occur. As dentin bonding systems have improved, the extent of shrinkage and stress development that can be tolerated has increased, but to maintain an ideal marginal adaptation, there should be no dimensional mismatch at the restorative-tooth interface and no internal stress in the composite. Therefore, while acknowledging that the control of stress is the ultimate goal, one approach to improve composite restoratives is to develop new resins that exhibit as little shrinkage as possible. Also, a significant reduction in polymerization shrinkage and shrinkage-related stress would benefit the application of resins and composites in various other fields, such as microelectronics encapsulants, protective coatings, microlithography and holographic data storage materials.

As polymerization shrinkage is directly related to the conversion of reactive double bonds, the concentration of double bonds in the monomer and the degree of conversion achieved both would affect the final shrinkage results [9]. Patel [10] investigated a wide range of methacrylate esters and showed that the molar volume change is fairly constant at about 22.5 cm³ per mole double bond consumed. In order to get lower shrinkage monomers, an obvious approach is to increase the molecular weight or molar volume of the monomers in order to decrease the concentration of the reactive double bonds. However, if only simple changes are introduced in the dimethacrylate structure, such as substituting poly(ethylene glycol) for triethylene glycol in the spacer group, lower shrinkage would be obtained at the expense of rapidly decreased glass transition temperature and mechanical properties of the resulting polymer [11]. Considerable efforts have been directed toward the development of low shrinkage (meth)acrylate monomers and there has been some success, mainly with multifunctional monomer designs, in achieving lower polymerization shrinkage along with acceptable polymer mechanical properties.[12], [13], [14]

Photocurable dental resins, based on the widely used Bis-GMA/TEGDMA resin (70:30 mass ratio) exhibit volumetric shrinkage of about 7.0% at typical conversions of approximately 60% [9]. The relatively low conversion raises biocompatibility concerns due to leachable monomer and photoinitiator [15], but it may also have a detrimental effect on the mechanical properties [16] because of the relatively low cross-linking density. If the activity of all double bonds were assumed to be equal, increasing the conversion in the polymer from 60 to 80% would decrease the leachable monomer from 16 to 4% [17]. The use of higher proportions of TEGDMA can increase the final conversion attained in dental resins, but with this comes increased shrinkage, which is directly related to both initial reactive group concentration and conversion.

This study seeks to develop and evaluate novel low shrinkage monomers based on conventional methacrylate polymerization chemistry. The approach involves the introduction of bulky side groups to increase the molecular weight and molar volume of the monomer as a means to reduce the initial double bond concentration. The primary objectives of this investigation are to determine the effect of the bulky groups on dimethacrylate copolymer photopolymerizations as well as to evaluate the potential of the substituents to engage as physical crosslinks to reinforce the resulting network structures. The practical goal is low shrinkage resins that maintain acceptable polymeric glass transition temperatures and mechanical properties.