New developments of polymeric dental composites

Prog. Polym. Sci. 26 (2001) 535±576

www.elsevier.com/locate/ppolysci

New developments of polymeric dental composites Norbert Moszner*, Ulrich Salz Ivoclar AG, Research and Development, FL-9494 Schaan, Liechtenstein Received 1 December 2000; accepted 27 January 2001

Abstract The currently used commercial restorative composites contain a mixture of various cross-linking dimethacrylates, glass- and/or silicon dioxide ®llers, and a photoinitiator system. They are cured by irradiation with visible light. New developments of polymeric composites for restorative ®lling materials are mainly focused on the reduction of polymerization shrinkage, and improvement of biocompatibility, wear resistance and processing properties. This can be partially achieved by using new tailor-made monomers and optimized ®ller particles. This review describes the polymeric chemical aspects of the application of new monomers, e.g. cyclic monomers, liquid-crystalline monomers, ormocers, branched monomers, compomers or Bis-GMA analogues or substitutes for restorative composites. In addition, the contribution of new ®ller-technologies for the improvement of restorative composites is discussed. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Dental monomers; Restorative composite; Ring-opening polymerization; Liquid-crystalline monomers; Dendrimers; Ormocers; Radiopacity; Nanoparticles; Polymerization shrinkage; Bioactivity; Composite reinforcement

Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 536 1.1. General description of dental restoratives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 536 1.2. Improvements of dental composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 538 * Corresponding author. E-mail address: [email protected] (N. Moszner). Abbreviations: AEMA, 2-Acryloyloxyethyl methacrylate; AMA, Allyl methacrylate; APTES, (3-Aminopropyl)triethoxysilane; AIBN, Azobisisobutyronitrile; Bis-GMA, 2,2-Bis[4-(2-hydroxy-3-methacryloyloxypropyl)phenyl]-propane; BOE, Bicyclic orthoester; DTBP, Di-tert-butylperoxide; EBPDMA, Ethoxylated Bis-GMA; HEMA, 2-hydroxyethyl methacrylate; DECHC, 3,4-Epoxycyclohexanemethyl-3,4-epoxycyclohexane carboxylate; D3MA, Dodecanediol dimethacrylate; EBPA, Ethoxylated poly(isopropylidenediphenol) resin; GMA, Glycidyl methacrylate; GDMA, Glycerol dimethacrylate; IPTES, (3-isocyanatopropyl)triethoxysilane; 4-META, 4-Methacryloyloxyethyl trimellitate anhydride; MMA, Methyl methacrylate; mp, Melting point; MOD, Mesial-occlusal-distal; Mw, Weight-average molecular weight; PETMP, Pentaerythritol tetra(3mercaptopropionate); SOC, Spiro orthocarbonate; SOE, Spiro orthoesters; Tg, Glass transition temperatures; TES, Triethoxysilane; TEOS, Tetraethoxysilane; TMXDI, a,a,a 0 ,a 0 -Tetramethyl-m-xylylene diisocyanate; TEGDMA, Triethyleneglycol dimethacrylate; UDMA, 1,6-Bis-[2-methacryloyloxyethoxycarbonylamino]-2,4,4-trimethylhexane; VL, Visible light 0079-6700/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0 0 7 9 - 6 7 0 0 ( 0 1 ) 0 0 00 5 - 3

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N. Moszner, U. Salz / Prog. Polym. Sci. 26 (2001) 535±576 2. Development of new monomers for dental restoratives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Ring-opening monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Spiro orthocarbonates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Cyclic ethers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3. Cyclic acetals and allyl sul®des . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4. Vinylcyclopropanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Liquid crystalline, branched and dendritic monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Liquid-crystalline monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Branched and dendritic monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Monomers for compomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Ormocers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Bis-GMA analogues and substitutes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.1. Fluorinated Bis-GMA analogues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.2. Bis-GMA substitutes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6. Radiopaque monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Improvements of dental restoratives based on their ®ller components . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Bioactive restoratives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Nanoparticles, composite reinforcement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Reduction of shrinkage stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Radiopaque ®llers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Trends to improve the processing of dental restoratives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Condensable/¯owable composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Special additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

539 540 540 543 545 548 550 550 552 553 555 561 561 562 565 567 567 568 569 570 571 571 572 572 573

1. Introduction 1.1. General description of dental restoratives Tooth-shaded dental restorations are becoming more and more popular. For the restoration of anterior lesions, as well as for the supply of smaller and medium-sized defects in the posterior region, direct composite ®lling materials are used. For larger defects, prefabricated ceramic restorations are attached to the tooth structure with composite-based cements in combination with the adhesive technology. For reasons of better comprehensibility, this review will not concentrate on the adhesive technology, but only on polymer-based restoratives. Per de®nition, a composite is a mixture of several components; in the case of dental ®lling composites, a mixture of an organic matrix and inorganic ®llers. Usually, the organic matrix is based on methacrylate chemistry, whereas especially cross-linking dimethacrylates like 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropyl)phenyl]propane (Bis-GMA), ethoxylated Bis-GMA (EBPDMA), 1,6-bis-[2-methacryloyloxyethoxycarbonylamino]-2,4,4-trimethylhexane (UDMA), dodecanediol dimethacrylate (D3MA) or triethyleneglycol dimethacrylate (TEGDMA) are used [1,2] (Fig. 1). By free-radical polymerization of the matrix monomers, a three-dimensional network is formed. The selection of the monomers strongly in¯uences the reactivity, viscosity and polymerization shrinkage of the composite paste, as well as the mechanical properties, water uptake, and swelling by water uptake of the cured composite. The polymerization shrinkage of low molecular monomers is more pronounced

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Fig. 1. Dimethacrylates mostly used in dental composite ®lling materials.

compared to that of high molecular monomers (Table 1). However, high molecular monomers are very viscous (Table 2). There is a correlation between the polymerization shrinkage, ®ller load and the viscosity of the composite. Therefore, special, favorable mixtures of high molecular monomers and reactive diluents in combination with different ®llers are used in dental composites. Due to the major in¯uence of the ®llers on the physical properties, the classi®cation of dental ®lling composites is based on the type of®ller used and the particle size Table 1 Polymerization shrinkage (DVp) of dental monomers Monomer

r mon a (g/cm 3)

r poly b (g/cm 3)

DVp (%)

TEGDMA UDMA Bis-GMA

1.072 1.110 1.151

1.250 1.190 1.226

214.3 26.7 26.1

a b

r mon ˆ density of monomer. r poly ˆ density of polymer.

Table 2 Correlation between the molecular weight and the viscosity of monomers Monomer

Molecular weight (g/mol)

Viscosity (mPa´s)

TEGDMA UDMA Bis-GMA

286 470 512

100 5,000±10,000 500,000±800,000

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Fig. 2. Classi®cation of composite ®lling materials.

thereof (Fig. 2) [3]. In general, two types of composites are on the market, i.e. micro®ll and hybrid composite ®lling materials. Micro®ll composites are based on nano-®llers with a particle size in the range of 10± 250 nm, whereas a differentiation between homogeneous and heterogeneous micro®lls is made. For better handling and higher load, heterogeneous micro®ll composites contain prepolymer particles based on a homogeneous micro®ll material. The inorganic part of hybrid composites is composed of about 70± 80% glass ®llers and 20±30% nano®llers. Table 3 shows the composition of a ®ne particle hybrid-composite (Tetric ceram/Vivadent) and a heterogeneous micro®ll composite (Heliomolar/ Vivadent). 1.2. Improvements of dental composites The clinical performance of ®lling materials depends very much on the indication of the restoration. The stress exerted on the restoration in the posterior region is much more pronounced than in the anterior region. According to literature [4] the average life cycle of hybrid composite posterior MOD restorations placed in general practices is four years, compared to eight years for amalgam restorations. To improve the clinical performance of composite ®lling materials, a lot of investigations are currently being Table 3 Composition of a ®ne particle hybrid and a heterogeneous micro®ll composite

Monomer mixture Fillers Filler particle size Classi®cation

Tetric ceram (Vivadent)

Heliomolar (Vivadent)

Bis-GMA, UDMA, TEGDMA Ba-silicate glass, Ba-¯uoro-silicate glass, ytterbium tri¯uoride, Zr/Si-mixed oxide 40 nm±3.5 mm Fine particle hybrid composite

Bis-GMA, UDMA, D3MA pyrogenic silicon dioxide, prepolymer, ytterbium tri¯uoride 40±250 nm (SiO2, YbF3) Heterogeneous micro®ll composite

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conducted. The main topics are as follows: ² ² ² ²

reduction of the polymerization shrinkage to improve marginal adaptation and avoid recurrent caries; release of ¯uoride or other substances to reduce recurrent caries; improvement of mechanical properties; improvement of biocompatibility by reducing the elution of components.

This article provides an overview of the current investigations based on new polymer chemical approaches in these ®elds. 2. Development of new monomers for dental restoratives The monomer matrix systems of dental ®lling composites, which are currently mostly based on a mixture of dimethacrylates, have to ful®l a number of basic requirements, as far as the reactivity, stability or toxicity of the monomers used and the properties (strength, stiffness and stability) of the formed polymer network (Fig. 3) are concerned. In this context, the development of new monomers for restorative ®lling materials is predominantly motivated by the will to overcome the main shortcomings of resin composites, i.e. marginal leakage due to polymerization shrinkage and insuf®cient abrasion resistance. The objective is to create composites, which can assume new functions, such as ¯uoride ion release or exhibiting an anti-cariogenic effect or anti-plaque action. Furthermore, cross-linking monomers are synthesized that improve the mechanical and processing properties of the composite. Such composites demonstrate reduced water up-take, as well as radiopacity or self-adhesion to dentin or enamel. An informative review on dental monomer systems used was presented by Peutzfeldt [2]. In the following part, developments of dental monomers are described. The main focus is placed on the polymer-chemical aspects of ring-opening monomers with the potential of low shrinkage, cross-linking

Fig. 3. Basic requirements for monomers in restorative composites.

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Fig. 4. Examples of Bailey's basic structures for expanding monomers [5].

monomers with a new architecture (mesogenic units, hyperbranched structures or nanoparticles), and acidic monomers used in compomers. 2.1. Ring-opening monomers 2.1.1. Spiro orthocarbonates About 20 years ago, Bailey [5] patented the polymerization of polycyclic ring-opening monomers, such as spiro orthocarbonates (SOC), spiro orthoesters (SOE) or bicyclic orthoesters (BOE) (Fig. 4). These monomers show a near-zero-shrinkage or expansion during polymerization and are useful for the production of strain-free composites, high strength adhesives or precision castings. Bailey's concept of the double ring-opening polymerization, ®rst reported in 1972 [6], initiated many important fundamental studies and some applied investigations on the synthesis and ring-opening polymerization of expanding monomers [7,8]. The ®rst example of an SOC investigated in a dental resin formulation was the crystalline 3,9-dimethylene-1,5,7,11-tetraoxaspiro [5.5] undecane 1 (Fig. 5) [9]. The monomer was used as a slurry in a free-radical polymerizable resin composition based on conventional dimethacrylates (BisGMA, ethyleneglycol dimethacrylate). Unfortunately, the crystalline SOC 1 was partially left undissolved and unreacted in the resin cured at ambient temperature. Therefore, the free-radically polymerizable SOC monomers 2±10 were synthesized for dental application. These monomers demonstrated melting points below room temperature depending on the substituents and the ring-size of the SOCs (Fig. 6) [10±12]. The polymerization behavior of SOCs 2±5a/b was investigated using the pure monomers and combinations with Bis-GMA, EBPDMA or TEGDMA. For the free-radical homopolymerization of SOC 2 in the presence of di-tert-butylperoxide (DTBP) at 1308C, it was found, that the polymerization took place

Fig. 5. Double ring-opening polymerization of SOC 1.

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541

Fig. 6. Liquid SOC 2±5a/b and 6±7 investigated in dental formulations [10±13].

via several competing pathways. In addition to the desired double ring-opening (path A), a single ringopening with a concomitant elimination of a cyclic carbonate molecule (path B), and a non-ring-opening vinyl polymerization (path C) occurred (Fig. 7). The degree of ring-opening varied between 45 and 85%. In comparison to the monofunctional SOCs 2 and 3, the 2,3-(bismethylene)-substituted SOC 4 was more

Fig. 7. Proposed mechanism for the free-radical polymerization of SOC 2.

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Fig. 8. Designed SOCs 11 and 12 for free-radical ring-opening polymerization [14].

reactive. However, in a mixture with the dimethacrylate EBPDMA it did not demonstrate any storage stability. Another alternative to enhance the reactivity of SOC monomers was to append a readily polymerizable group to the spirocyclic unit. This was realized with the SOC-substituted methacrylate 5a/b, which was synthesized as a mixture of isomers 5a and 5b [12]. The hybrid monomer 5a/b could be polymerized under free-radical and cationic conditions and showed the highest conversion for both the methacrylate and spiro methylene groups with a mixed cationic/free-radical initiator system. The freeradical polymerization of the low viscous SOCs 9 and 10 was investigated in bulk in the presence of DTBP at 1308C and in solution in the presence of azobisisobutyronitrile (AIBN) at 658C in comparison to methyl methacrylate [13]. The difunctional SOC 10 was more reactive than monomer SOC 9. However, the spectroscopic investigation of the formed poly(9) and poly(10) showed that primarily a vinyl polymerization with only a low degree of ring-opening reactions occurred. This ®nding was also con®rmed by the polymerization shrinkage of SOC 10 of about 12.5%, which was similar to that of TEGDMA …DV ˆ 14:3%†; a frequently used dental diluent dimethacrylate. In order to improve the freeradical ring-opening tendency of SOCs, various designed SOCs were synthesized, for example, based on seven-membered rings and using radical stabilizing benzyl groups [14]. The most promising monomers 11 and 12 (Fig. 8) showed a degree of ring-opening of 89 and 42%, respectively. Unfortunately, SOCs 11 and 12 were crystalline compounds. Generally, the evaluation of SOCs in dental composite formulations showed some disadvantages in the application of the methylene-substituted SOC-monomers for free-radical cured

Fig. 9. Disadvantages and consequences of the incorporation of methylene substituted SOCs in free-radical cured dental composite.

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Fig. 10. Cationic polymerizable SOCs 13±16 evaluated for dental composites [19,20].

composites. These disadvantages are summarized in Fig. 9. The main disadvantages are the low reactivity for free-radical additions and the sensitivity against water and acidic compounds. The usefulness of SOCs as expanding monomers was ®rst demonstrated for high-strength industrial composites based on the cationic ring-openig-polymerization [15]. In this context, the most promising expanding SOC candidates were six-membered SOCs, because the ®ve- and seven-membered SOCs were reported [16] to polymerize with the elimination of small molecules in the presence of a cationic initiator. Based on these ®ndings, the cationic polymerizable, non-methylen-substituted, six-membered SOCs 13±16 (Fig. 10) were evaluated as a component of photo-curable epoxy resins [17±21]. The crystalline SOC 13 (trans/trans-2,3,8,9-di(tetramethylene)-1,5,7,11-tetraoxaspiro [5.5] undecane, mp: 778C) was polymerized in the presence of a cationic photoinitiator (4-octyloxyphenyl)-phenyliodonium hexa¯uoroantimonate by irradiation with UV light under ring-opening with an expansion of about 3.5 vol% [17,18]. In contrast, the in¯uence of 30 wt% SOC 13 on the shrinkage of a UV-light cured three-component epoxy mixture based on diglycidyl ether of bisphenol A, 3,4-epoxycyclohexanemethyl-3,4-epoxycyclohexane carboxylate (DECHC), and vinylcyclohexene dioxide (weight ratio: 5:4:1) was neglectably small [19]. In this context, it is worth to mention that the pure epoxy mixture showed a polymerization shrinkage of only 0.3 vol%. For example, the SOCs 14±16 were more reactive in a mixture of DECHC and poly(THF) (80/20) compared to SOC 13. However, the addition of SOC to the initial epoxy/polyol mixture resulted in a signi®cant decrease in reactivity of the UV-light-cured mixture [20]. 2.1.2. Cyclic ethers In recent years, the application of cationic photo-polymerizable epoxy-monomer based compositions for dental ®llings have found increasing attention in patent applications [22±26]. Particularly cross-linking cycloaliphatic epoxy compounds were of interest, because they demonstrate signi®cantly lower shrinkage than dental methacrylate resins. Moreover, these epoxy resins were reactive enough to be cured by cationic photopolymerization in an acceptable time frame and to an adequate depth using a dental visible light (VL) source. Fig. 11 shows one example of the composition of a proposed VL-cured composite that contained a mixture of two cationic polymerizable diepoxides: cycloaliphatic epoxide 3,4-epoxycyclohexyl-methyl-3,4-epoxycyclohexanecarboxylate 17, which was more reactive than glycidyl compounds, and the diglycidyl ether of bisphenol A, which improved the mechanical properties of the cured composite. The poly(THF) as polyol in¯uenced the physical properties in the permanently cured composition and the speed

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Fig. 11. Composition (wt%) of a composite based on VL-curable epoxy resins [23].

of photocure. Camphorquinone 18, which is actually the most frequently used photoinitiator in VL-curing dental ®lling materials, acted as the VL-sensitizer, while diphenyliodonium hexa¯uoroantimonate 19 was used as the photoinitiator. Patented variations of these basic epoxy resin compositions mainly concern the synthesis of new cycloaliphatic diepoxides [25], for example, epoxide 20, the application of other cationic photoinitiators, such as aromatic sulfonium or ferrocenium salts [25], and the addition of accelerating low basic amines, such as ethyl 4-dimethylaminobenzoate [24] 21 (Fig. 12). In addition to epoxy resins, oxetanes were evaluated for dental application [26,27]. The reactivity of oxetanes is mainly controlled by the ring stress and the basicity of the ring-oxygen. The ring stress of oxetanes is similar to that of oxiranes. However, oxetanes demonstrate a higher basicity. Therefore, photoiniators, such as sulfonium and iodonium salts, can be used for the cationic polymerization of oxetanes. The ring-opening polymerization of oxetanes was also characterized by a signi®cantly lower shrinkage compared to methacrylates (Fig. 13). From the investigated oxetanes, the hydroxygroup containing monomer 22, possessed the highest polymerization rate, whereas esters and urethanes were either non-polymerizable or reacted only at very low rates. In addition, it was found [28] that the polymerization reactivity of oxetanes was substantially affected by the type of atmosphere used. In an air atmosphere, oxetane photopolymerization showed an induction period, while it was greatly accelerated in nitrogen. Generally, composites based on cyclic ethers showed some advantages in comparison to dimethacrylate materials (Fig. 14). In spite of these advantages, cationic ring-opening polymerization of cyclic ethers currently also creates a number of problems, which have not been completely solved up to date.

Fig. 12. Components for improved epoxy resin compositions.

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Fig. 13. Volume shrinkage during the cationic photopolymerization of oxetanes 22±24 in the presence of bis-[4-(diphenylsulfonio)-phenyl]-sul®de-bishexa¯uorophosphate [27].

2.1.3. Cyclic acetals and allyl sul®des Among the heterocyclic monomers that show little or no volume changes upon polymerization, cyclic ketene acetals have found great interest, since these monomers can undergo a free-radical or cationic ring-opening polymerization [29]. A ®rst evaluation of various 1,3-dioxolanes (Fig. 15) as monomer components of VL-cured dental composites based on EBPDMA with the photoinitiator system 18/21 was carried out by Reed [30]. In contrast to the non-vinyl cyclic acetals 25 and 26, the composite based on a mixture of EBPDMA with the difunctional cyclic vinyl acetal 28 showed a tensile strength similar to that of the control example with EBPDMA only. In this context, a free-radical ring-opening mechanism under the formation of poly(alkylene-ether-ketone)s was proposed for monomer 27 (Fig. 16). At temperatures higher than 808C, monomer 27 underwent a mixed polymerization mechanism involving the elimination of benzaldehyde [31]. 2-Phenyl-4-methylene-1,3-dioxolane 27 also underwent an exclusive cationic ring-opening polymerization in the presence of photoinitiators, such as tris(methylphenyl)sulfonium hexa¯uoroantimonate, 4-(decyloxyphenyl)phenyliodonium hexa¯uoroantimonate [32], and (h 5-2,4-cyclopentadiene-1yl) [1,2,3,4,5,6-h]-(1-methylethylbenzene]-iron (I)-hexa¯uorophosphine

Fig. 14. Advantages and problems of photo-curable composites based on the cationic ring-opening polymerization of cyclic ethers.

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Fig. 15. Non-vinyl 25/26 and vinyl 1,3-dioxolanes 27/28 evaluated for dental composites [30].

Fig. 16. Proposed mechanism of free-radical ring-opening polymerization of monomer 27 [30].

complex [33]. The glass transition temperatures (Tg) of the formed poly(alkylene-ether-ketone)s were in the range of 26±358C, which was too low for dental application. More promising polymers with a Tg between 52 and 678C were synthesized by the copolymerization of 27 with 28 [34]. Analogous to cyclic vinyl monomer 27, the seven-membered cyclic ketene acetal 2-methylene-1,3-dioxepane 29 underwent a nearly complete ring-opening polymerization and resulted in essentially pure poly(e-caprolactone) if polymerized with the free-radical initiator AIBN or with the photoinitiator benzoin isopropyl ether (Fig. 17) [35]. Furthermore, it was found [36] that the free-radical ring-opening polymerization of 29 was accompanied by intramolecular hydrogen transfer during propagation under the formation of a ¯exible, branched polyester. In order to increase the stiffness of the polyester, bicyclic 2-methylene-1,3dioxepanes 30a±g and 31 (Fig. 18) were synthesized, polymerized, and evaluated for dental application [37±39]. The bicyclic 2-methylene-1,3-dioxepanes primarily showed a free-radical ring-opening polymerization in the presence of AIBN or DTBP, while the cationic photopolymerization in the presence of a triarylsulfonium hexa¯uoroantimonate photoinitiator was mainly a vinyl polymerization. The volume changes during the free-radical polymerization of the bicyclic 2-methylene-1,3-dioxepanes were between 211.4% (30e) and 12.9% (30g). The volume expansion was attributed to the transition of the more dense structure of the crystalline monomer 30g to the less dense structure of the amorphous polyester formed. Unfortunately, the Tg of the formed polyesters varied between 28 and 358C, which was unacceptable for dental application. The 2-methylene-1,3-dioxepanes as electron-rich ole®nes were very sensitive against water and nucleophilic compounds, such as amines or alcohols. Furthermore,

Fig. 17. Mechanism of free-radical ring-opening polymerization of monomer 29 [35].

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Fig. 18. Bicylic 2-methylene-1,3-dioxepanes [38].

Fig. 19. Free-radical ring-opening polymerization of cyclic allyl sul®des.

composite pastes based on conventional dental glass ®llers and 2-methylene-1,3-dioxepanes were not storage-stable and tended to spontaneous hardening after some days. This was probably caused by the acidic silanol surface groups of the glass ®ller particles. Finally, 2-methylene-1,3-dioxepanes were signi®cantly less reactive than methacrylates, which resulted in an unacceptable increase of the curing times. In contrast to cyclic ketene acetals, cyclic allyl sul®des are stable in the presence of water. These monomers, for example, 6-methylene-1,4-dithiepane 32 or 3-methylene-1,5-dithiacyclooctane 33 (Fig. 19), readily survived the exposure to aqueous bases and acids [40]. The cyclic allyl sul®des underwent a free-radical ring-opening polymerization in the presence of AIBN leading to highly insoluble crystalline homopolymers of a high molecular weight (Mw approx. 500,000±700,000) with glass temperatures (Tg) of approx. 230 to 2508C and melting points (mp) of 100 2 1308C. Poly(32) and poly(33) showed a shrinkage of about 8.6 and 6.7%, respectively [40]. In this context, various liquid difunctional 6-methylene-1,4-dithiepanes, for example, monomers 34±36 [41] (Fig. 20), were synthesized. They formed cross-linked polymers and were, therefore, very interesting for use in composites. For the application of cyclic allyl sul®des in dental materials, the main problems were the signi®cantly

Fig. 20. Structure of liquid difunctional 6-methylene-1,4-dithiepanes 34±36.

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Fig. 21. Free-radical ring-opening polymerization of monomer 37.

lower reactivity of these monomers in comparison to methacrylates and the high ¯exibility of the amorphous structure of the formed polymers. 2.1.4. Vinylcyclopropanes 2-Vinylcyclopropanes are also well known [42] as low-shrinking free-radical polymerizable monomers. The free-radical ring-opening tendency of the relatively easily accessible 1,1-disubstituted 2vinylcyclopropanes is increased by both radical-stabilizing and electron-withdrawing groups. Therefore, the free-radical bulk polymerization of 1,1-diethoxycarbonyl-2-vinylcyclopropane 37 in the presence of AIBN at 608C yielded a polymer with almost exclusively 1,5-ring-opened repeating units A, whereas at higher temperatures, for example, at 808C (DBPO), polymers with additional units, probably cyclobutane rings B, were formed (Fig. 21) [43]. According to the ¯exibility of the formed polymer chains, poly(37) showed a Tg of only 408C, which was too low for the application in composites. The Tg of the ring-opened polymeric 1,1-disubstituted 2-vinylcyclopropanes depends on the nature of the substituents. However, bulky substituents also increase the melting points of the corresponding monomers. In order to ®nd a compromise between the melting behavior of the monomers and the Tg of the corresponding polymers, various symmetric and asymmetric 1,1-disubstituted 2-vinylcyclopropanes were synthesized and subjected to free-radical polymerization [44,45]. Compound 38, which formed polymers with a Tg of about 548C (Fig. 22), was found to be a useful monomer, due to its liquid behavior, in contrast to

Fig. 22. Tg of polymeric 1,1-disubstituted 2-vinylcyclopropanes.

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Fig. 23. Cross-linking 1,1-disubstituted 2-vinylcyclopropanes 40±44.

crystalline monomer 39 (mp 44±458C [46]). Based on these results, the synthesis of cross-linking 1,1disubstituted 2-vinylcyclopropanes 40±44 (Fig. 23) was developed [47±49]. Monomers 40 and 42±44 were more or less viscous liquids, whereas monomer 41 was a crystalline compound (mp 72±888C). The difunctional monomers 41 and 42 were polymerized in bulk in the presence of AIBN at 658C and resulted in hard transparent polymers, which were completely insoluble in organic solvents. In this respect, bulk polymerization was accompanied by a volume change of 11.0 (41) and 23.9% (42), respectively. The expansion in volume was explained with the transition of the denser structure of the crystalline monomer compared to the less compressed structure of the amorphous polymer formed. In a diluted chlorobenzene solution, the free-radical polymerization of 41 and 42 yielded soluble polymers with laterally bonded 2-vinylcyclopropyl groups. For composite application, it is important that the Tg of the polymers is .608C (Fig. 24). It was found [48] that the cross-linking vinylcyclopropanes were less reactive than methacrylates. Therefore, hybrid monomers 45±47 (Fig. 25) containing both a vinylcyclopropyl and a methacrylic group were synthesized. They were used to improve the network formation of mixtures of cross-linking vinylcyclopropanes and dimethacrylates [49,50]. The cross-linking vinylcyclopropane 42 showed a low oral toxicity (LD50 . 2000 mg/kg), no mutagenicity and a lower cytotoxicity than the commonly used cross-linking methacrylates. Moreover, the cross-linking vinylcyclopropanes were stable in the presence of humidity, acid or basic impurities, as

Fig. 24. Free-radical polymerization of cross-linking vinylcyclopropanes 41 and 42 in diluted solution.

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Fig. 25. Cross-linking hybrid vinylcylopropanes.

well as inorganic ®llers, such as silica or glass powder. These properties and the low volume shrinkage during the polymerization of the cross-linking vinylcyclopropanes make them attractive as monomer component in dental ®lling materials. Thus, monomer 42 was able to modify the shrinking kinetics of dental composites, yielding a reduced amount of marginal failure and microleakage in in-vitro experiments [39]. In general, it can be summarized that although the polymerization shrinkage has been one of the main shortcomings of resin-based composites since their introduction, the ring-opening polymerization of cyclic monomers has not been successfully achieved for commercial dental ®lling materials to date. This is mainly caused by the excessive basic requirements, which have to be ful®lled (Fig. 3) by the monomers used for composite restoratives. 2.2. Liquid crystalline, branched and dendritic monomers 2.2.1. Liquid-crystalline monomers In addition to the ring-opening polymerization of cyclic monomers, the idea to use pre-ordered, i.e. liquid-crystalline or branched cross-linkers, is a second basic concept to achieve a low-shrinking photopolymerization system. The favorable properties of these monomers with a tailor-made molecular structure are the relatively low viscosity and a lower polymerization shrinkage of the pre-ordered monomers compared to that of the corresponding ordinary linear monomers. Accordingly, it was shown [51,52] that the photoinitiated polymerization of liquid-crystalline diacrylates, for example, 1,4-phenylene bis[4-(6-acryloyloxy)hexyloxybenzoate]s, proceeded at a signi®cantly higher rate in the liquid-crystalline than in the crystalline state, led to a high degree of conversion of the acrylate groups, and was accompanied by a lower polymerization shrinkage than that commonly observed for conventional acrylates. Maximum reduction of shrinkage was achieved if the polymerization of the liquid-crystalline, cross-linking monomer resulted in an amorphous polymer network instead in a polymer network with a liquid crystalline structure. Unfortunately, almost all of the described di(meth)acrylates melted at temperatures higher than 808C, resulting in complicated curing conditions, as well as restricted practical usefulness. In order to circumvent these problems, it was necessary to synthesize new liquid-crystalline di(meth)acrylates, which exhibited a melting temperature near or below room temperature. This was achieved by modifying the spacer length, introducing suitable substituents in the mesogenic group, and varying the mesogenic group, respectively [53±55]. It was very delicate to achieve a balance between introducing pendant groups on the liquid-crystalline molecules to reduce the temperature range of the crystalline phase stability, and avoiding too much structural perturbation of the monomer structure, which may result in the loss of the liquid crystalline character.

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Fig. 26. At near room temperature liquid-crystalline di(meth)acrylates 48 and 49.

A successful example was diacrylate 48 (Fig. 26), which was a nematic liquid-crystal monomer that demonstrated a polymerization shrinkage of approx. 2.1 vol% [54]. For monomer 49 a volume contraction of about 1.3 vol% was measured during photopolymerization [53]. A decrease in the transformation temperature of liquid-crystalline monomers was also achieved by the preparation of a mixture of two liquid-crystalline monomers or of a solution of a low-melting crystalline monomer and a comonomer as the solvent. Finally, branched liquid-crystalline bismethacrylates were proposed [56±58] as an additional group of ambient-temperature liquid-crystalline cross-linking methacrylates. The general structure of these branched monomers consisted of a rigid linear bismethacrylate central unit, linked with mesogenic groups via ¯exible alkyl spacers. This principle is visualized by the bismethacrylates 50 and 51 in Fig. 27. The branched structure was expected to impede crystallization, yet not disable the mesophase formation. The branched monomer 50 exhibited a nematic phase between 18 and 678C, while monomer 51 showed both a stable smectic C p phase (15±348C) and a cholesteric phase (34±588C). The photopolymerization of the branched bismethacrylates at ambient temperature resulted in cross-linked anisotropic polymer networks without remarkable changes in the structure of the mesophase. In other words, the liquid-crystalline structure of the monomers was ªfrozen inº by the

Fig. 27. Branched ambient-temperature liquid-crystalline bismethacrylates 50 and 51.

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photoinduced cross-linking. In case of monomer 50, the volume shrinkage during photopolymerization was approx. 2.5%. In summary, ambient-temperature liquid-crystalline cross-linking monomers are very promising as matrix monomers for photopolymerizable composites due to their low polymerization shrinkage, the relatively low viscosity, and the high monomer conversion. However, additional components of a composite, such as isotropic liquid comonomers or ®llers, may in¯uence the liquid-crystal formation. Furthermore, the synthesis of a liquid-crystalline monomer is more expensive. Moreover, the resulting polymer network tends to be more ¯exible, which may impair the mechanical properties. 2.2.2. Branched and dendritic monomers In order to simplify the monomer synthesis, highly branched non-liquid-crystalline monomers were synthesized and evaluated for dental composites [59±61]. Such composites demonstrated low viscosity and low polymerization shrinkage at the same time. For example, the branched methacrylate 52 (Fig. 28), was synthesized by simple Michael addition of technical 3,(4),8,(9)-bis(aminomethyl)tricyclodecane and 2-(acryloyloxy)ethyl methacrylate. Monomer 52 (molecular weight: 931 g/mol) showed a very low viscosity h of only approx. 150 mPa´s compared to Bis-GMA (molecular weight: 512 g/mol; h : ca. 1000 Pa´s) and a low polymerization shrinkage of approx. 2.9% (Bis-GMA: 6.0%). According to these results, the polymerization shrinkage of a corresponding VL-cured composite based on a monomer mixture of 52 and Bis-GMA (3.5:1.5) with a ®ller content of approx. 72% was only 1.1%. Unfortunately, the ¯exural strength (52 MPa) and the modulus of elasticity (5960 MPa) of this composite were relatively low. These ®ndings were true for branched methacrylates with more ¯exible aliphatic spacers [59] in particular. Branched macromonomers [60] and hyperbranched polyester methacrylates [61] showed a similar behavior. In other words, they demonstrated low monomer viscosity and low polymerization shrinkage, as well as high ¯exibility of the formed polymer networks and poor mechanical properties of the corresponding composites. Recently [62,63], more promising results were obtained by starting from hyperbranched Boltorn w polyols (Perstorp, Sweden), which were modi®ed by the reaction with methacrylic anhydride or methacryloyl chloride. The addition of the obtained hyperbranched multimethacrylates to a mixture of Bis-GMA/TEGDMA (1:1) resulted in both an improvement of the mechanical properties and a decrease in the free monomer leaching. Furthermore, we found [64,65] that ¯exible dendritic methacrylates can be used to modify the rheological behavior of composites. The dendritic cross-linking multifunctional methacrylates were synthesized by the Michael addition of 2-(acryloyloxy)ethyl methacrylate with the aminofunctional poly(propyleneimine) dendrimers from DSM (Netherlands). Depending on the generation of the starting dendrimer, multifunctional methacrylates

Fig. 28. Branched low viscous tetramethacrylate 52.

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with 32 (second generation) to 128 methacrylic groups per molecule (®fth generation) were obtained. Surprisingly, all of the synthesized dendritic methacrylates were liquids. The methacrylated dendrimer of the ®fth generation, for example, demonstrated a viscosity of approx. 7 Pa´s, although the calculated molecular weight of this multifunctional monomer was about 30,700. This occurrence can be explained by both the high ¯exibility of the poly(propyleneimine) dendrimer structure and the well-known fact [66] that the intrinsic viscosity of dendrimers did not increase linearly to the molecular weight. According to the ¯exibility of the obtained dendritic monomers, almost every methacrylic double bond took part in the free-radical polymerization in the presence of AIBN. Starting from a mixture of these dendritic methacrylates (20%) and Bis-GMA (approx. 40%), UDMA (20%) and TEGDMA (20%), a composite paste with a ®ller content of approx. 80% was prepared. After the kneading procedure, the paste obtained was as ®ne-grained, dry-looking material. However, it becomes pasty with a ¯owable consistency under compressive or shear stress, and, therefore, can be processed like amalgam. The explanation of this rheological behavior is that the dendritic methacrylates acted as molecular sponge for the diluent monomer. Under compressive or shear stress, the dendrimers release the monomer. Basically, hyperbranched or dendritic methacrylates are very promising monomers for the preparation of low-shrinking composites, due to their relatively low viscosity and ef®cient incorporation into the formed polymer network. However, for a successful application in dentistry, those monomers, which will produce polymer networks with improved mechanical properties, have to be synthesized. 2.3. Monomers for compomers Compomers [67] are one kind of photocurable dental ®lling composite materials, also known as polyacid-modi®ed composite resins [68]. The term compomer is a combination of the words composite and glass ionomer and is used to describe water-free, single-component, light-cured composites consisting of acid-modi®ed dimethacrylate reinforced with silanized calcium-, strontium- or bariumaluminum-¯uorosilicate glass particles, which are well-known from glass ionomers [69]. Compomers were developed to improve the physical properties and the clinical handling of glass-ionomer cements, which were formed by the acid±base reaction of an aqueous polymeric acid and an ion-leachable glass [70]. The structure of the proposed monomers for compomers is characterized by the presence of at least two polymerizable methacrylic residues and acid groups. Examples are the bis(2-hydroxyethyl methacrylate) esters of butane 1,2,3,4-tetracarboxylic acid 53 or of the aromatic tetracarboxylic acids 54a±d (Fig. 29) [71±73]. Furthermore, bis(2-hydroxyethyl methacrylate) esters of cycloaliphatic or heterocyclic tetracarboxylic acids, for example, dimethacrylates 55 and 56, were used as compomer monomers (Fig. 30) [74]. Finally, compomers based on oligomeric polyacids, for example, the reaction product of oligomeric poly(acrylic acid) (Pn of about 10) with glycidyl methacrylate (GMA) were prepared (57, Fig. 31) [75]. Basically, these compomer monomers were able to react simultaneously with the methacrylate groups by free-radical polymerization and, by an acid±base neutralization reaction, with the cations released from the glass particles in the presence of water. However, in the absence of water the neutralization reaction did not occur. Therefore, the setting of compomers can only be achieved by VLinitiated polymerization of the monomers under the formation of a polymeric covalent network. In the presence of water from the environment, a limited acid±base reaction was observed [76]. This acid±base reaction of irradiated materials stored in water took place on the surface and reached a saturation point after approximately four weeks. Moreover, it was found by ESR spectroscopy [77], that the storage of VL-cured compomers in water or ethanol also in¯uenced the life cycle of the trapped free radicals.

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Fig. 29. Aliphatic and aromatic COOH-containing dimethacrylates for compomers.

All compomers showed a decrease in both compressive and ¯exural strength, which is caused by water induced degradation of the matrix±®ller interface in the compomers. Although compomers were developed to combine the favorable properties of composite resins (high values of mechanical properties, good clinical handling, and low effect of water on the material's stiffness) and glass-ionomer cements (no polymerization shrinkage, adhesion to the tooth structure, and

Fig. 30. Cycloaliphatic and heterocyclic COOH-containing dimethacrylates for compomers.

Fig. 31. Synthesis of GMA-modi®ed poly(acrylic acid).

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¯uoride release), their behavior is more similar to that of composite resins than that of glassionomers [78,79]. 2.4. Ormocers In addition to the classical composite ®lling materials, which are based on cross-linking dimethacrylates and inorganic ®llers, a new type of inorganic±organic hybrid dental materials, known as ormocers has been developed [80,81]. The aim was to reduce the polymerization shrinkage, and to improve the marginal adaptation, abrasion stability and biocompatibility. In vitro studies con®rmed that ormocers demonstrate a good abrasion resistance [82]. Their marginal adaptation is comparable to that of conventional composites in conjunction with the adhesive technology [83,84]. Improvement in biocompatibility can only be achieved if no diluting monomer, such as TEGDMA, is needed to reduce the viscosity of the corresponding condensate. The formation of ormocers, which means organically modi®ed ceramics, is as follows: starting from an alkoxy silane (Fig. 32) functionalized with a polymerizable group, hydrolysis and condensation led to an oligomeric Si±O±Si-nano-structure (Figs. 32 and 33) [85]. In addition to alkoxy silanes, other metal alkoxides, such as Ti-, Zr- or Al-alkoxides, can be condensated or cocondensated with alkoxy silanes [86,87]. These oligomers replace the conventional monomers in the composite. In a second step, a three-dimensional network is formed by the polymerization of the functional groups (Fig. 34). An example of a methacrylate-functionalized alkoxysilane used in dentistry is the reaction product 58 of (3-isocyanatopropyl)-triethoxysilane (IPTES) with glycerol dimethacrylate (GDMA) (Fig. 35) [81,85] or the carboxy-functionalized dimethacrylate alkoxysilane 60. 60 was prepared by the reaction of HEMA with 3-(methyldiethoxysilyl)propylsuccinic anhydride 59 (Fig. 36) [88]. The main problem is that the condensates of silane 58 or 60 show a very high viscosity, comparable to

Fig. 32. Chemical structure of ormocer silanes.

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Fig. 33. Hydrolytic condensation of a alkoxy silane under formation of Si±O±Si-nano-structures.

Fig. 34. Polymerization of methacrylate groups to form a 3-dimensional polymer network.

Fig. 35. Reaction of IPTES with GDMA.

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Fig. 36. Reaction of HEMA with 59.

that of Bis-GMA. To obtain high ®ller load and good handling properties of the composite, the condensate must be diluted with a low viscous monomer, such as TEGDMA. In this case, the biocompatibility requirements are no longer ful®lled. Recently, new cross-linking silanes were synthesized [89]. The resulting condensates demonstrated a viscosity in the range 1±15 Pa´s. By Michael addition of 2-acryloyloxyethyl methacrylate (AEMA) with (3-aminopropyl)triethoxysilane (APTES), the methacrylatefunctionalized amino silane 61 was obtained in a yield of 99% (Fig. 37). The reaction of APTES with the addition product 62 of succinic anhydride with GDMA resulted in silane 63, in which the dimethacrylate function is linked to the condensable function via an amide group (Fig. 38). The hydrolytic condensation of alkoxy silanes in the presence of ammonium ¯uoride (NH4F), for example, resulted in linear and branched oligomeric amorphous Si±O±Si structures. Under special hydrolysis±condensation conditions, oligomeric ring and cubic Si±O±Si structures (silsesquioxanes) are formed. For example, the hydrolysis±condensation of (3-methacryloxy)propyltrimethoxysilane in acetone catalyzed by n-propylamine produced a pale yellow liquid in a yield of @ 90% [90]. Sellinger et al. [91,92] synthesized polymerizable silsesquioxanes in two steps. After the synthesis of octahydridosilsesquioxane (HSiO1.5)8, the subsequent hydrosilylation reaction with propargyl methacrylate resulted in a distribution of di- to hexamethacrylate substituted cubes (see Fig. 39). In addition to polymerizable groups, such as methacrylates, rigid-rod LC moieties were also combined with silsesquioxane cubes to form low-viscosity LC-cube macromonomers [93,94]. Another approach to obtain an inorganic±organic composite via sol±gel reaction is to condensate

Fig. 37. Michael addition of AEMA with APTES.

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Fig. 38. Synthesis of dimethacrylate-functionalized 3-amidopropylsilane 63.

Fig. 39. Structure of an octahydridosilsesquioxane partial methacrylate functionalized.

tetra-alkoxysilane with polymerizable alkoxides which are polymerized or copolymerized simultaneously (Fig. 40) [95]. Wei et al. [96,97] adopted the method of ®rstly synthesizing the organic polymer matrix and, in a second step, cross-linking the inorganic±organic network by condensation (Fig. 41). The trialkoxysilyl functionalized polymer 65 was synthesized in two different ways: either by grouptransfer copolymerization (GTP) of MMA with allyl methacrylate (AMA), followed by the hydrosilylation reaction of copolymer 64 with triethoxysilane (TES) (Fig. 41, Scheme 1), or the hydrosilylation of

Fig. 40. In situ composite formation by simultaneous condensation and polymerization.

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Fig. 41. Synthesis of an inorganic±organic composite by copolymerization followed by cocondensation.

AMA and subsequent free-radical copolymerization with MMA (Fig. 41, Scheme 2). Afterwards, the trialkoxy silyl-functionalized polymer 65 was cocondensated with TEOS (Fig. 41, Scheme 3). The problem of methacrylate-functionalized silanes is that the polymerization shrinkage of their condensates is still remarkably high, which has been con®rmed by in vitro investigations. The marginal adaptation of ormocer composites is comparable to that of conventional composites [83,84]. This was the reason for combining non- or low-shrinking polyreaction systems with the sol±gel process. It is well known [98] that the thiol-ene polyaddition proceeds with a low volume contraction. Suitable norbornene silanes have already been synthesized [99] (example Fig. 42). The reaction of the corresponding condensates with pentaerythritol tetra(3-mercaptopropionate) (PETMP) showed a volume contraction of only 0.5% [100]. The resulting polymers are relatively ¯exible, which means that the corresponding composites showed low mechanical properties.

Fig. 42. Example of a norbornene-functionalized silane.

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Fig. 43. Examples of spiro orthoester and oxetane silanes.

Ring-opening polymerization is another way to reduce polymerization shrinkage. Appropriate functionalized silanes have also been synthesized [86,94,101]. Fig. 43 shows examples of a spiro orthoester silane 66 and two different oxetane-functionalized silanes 67 and 68. The ring-opening of these systems occurs in a suf®cient manner only under cationic polymerization conditions. However, cationic polymerization is very sensitive to humidity, which may be a problem in dental application. Finally, a number of vinylcyclopropane group-containing silanes were synthesized [102], for example, silanes 69 and 70 (Fig. 44), which polymerize under radical conditions by complete ring-opening. However, the disadvantage of the vinylcyclopropanes described so far is their low reactivity. Composite ®lling materials based on the ormocer technology that are currently available on the market are not pure ormocer systems. Diluent monomers are used to adjust the viscosity of the condensate. To avoid elusion of unreacted substances, a biocompatible formulation should be at least free of monomer. A substantial improvement of the physical properties, such as abrasion resistance, compared to conventional dental composites, was not achieved. A signi®cant reduction of the polymerization shrinkage with pure methacrylate systems is not possible. A combination with ring-opening polymerizable groups is necessary. In this case, cationic polymerizable functionalities have to be incorporated with the imponderabilities of polymerization inhibition by humidity in the oral cavity. Radical ring-opening vinylcyclopropanes are currently not reactive enough to be combined with the sol± gel systems.

Fig. 44. Examples of vinylcyclopropane group-containing silanes.

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2.5. Bis-GMA analogues and substitutes Presently, most of the commercial VL-curing restorative materials contain Bis-GMA (Fig. 1), which was synthesized by Bowen [103] from bisphenol A and GMA. The reason for this dominance of BisGMA is that this bulky, difunctional monomer shows a relatively low polymerization shrinkage ( < 6.0%), rapid hardening by free-radical photopolymerization, and low volatility. Furthermore, it leads to cured materials with good mechanical properties. However, the de®ciencies of Bis-GMA, i.e. high viscosity (1.0±1.2 kPa´s at 238C), water susceptibility, and the relatively low degree of double-bond conversion of light-cured Bis-GMA-based materials and their proneness to brittle fracture and wear, have stimulated the development of Bis-GMA analogues and substitutes. In the following, the recent developments of ¯uorinated Bis-GMA analogue monomers and monomers, which may substitute BisGMA, will be brie¯y discussed. 2.5.1. Fluorinated Bis-GMA analogues Fluorocarbon-containing polymers have low surface energies, are highly hydrophobic and display excellent resistance to softening to a wide range of chemicals. Furthermore, the potential resistance to staining and microbial attachment, as well as the generally good biocompatibility make ¯uorinated polymers very attractive for dental application. The novel semi-¯uorinated aromatic dimethacrylate monomers 71 and 72 (Fig. 45), which are structural analogues to Bis-GMA, were synthesized by ethoxy- or propoxylation of 4,4 0 -(hexa¯uoroisopropylidene)diphenol (hexa¯uorobisphenol A) and subsequent methacrylation of the reaction products with methacryloyl chloride [104]. Monomers 71 and 72 demonstrated a signi®cantly lower viscosity (0.8±1.3 Pa´s) and exhibited a higher conversion of methacrylate double bonds for the isothermal polymerization at 808C. Moreover, materials based on 71 and 72 showed a lower water uptake and thus a higher Vickers hardness value of the water saturated samples compared to the Bis-GMA-based materials [105]. The in¯uence of the monomer structure and ¯uorine content on resin and composite properties was studied systematically on the basis of dimethacrylate monomers with varying ¯uorine contents and distributions [106±109]. The ¯uorinated dimethacrylates were synthesized by the reaction of diepoxides with ¯uoroalcohols and conversion of the diol intermediates to dimethacrylates 73±80 (Fig. 46). Resin formulations based on dimethacrylates 73± 80 were prepared with equimolar proportions of 1,10-decamethylene dimethacrylate, and 0.2 wt% of 18 and 0.8 wt% 21 as the photoinitiators. Fused quartz (60 wt%) was mixed with the resins to provide photopolymerizable composite pastes. The results of the investigation of the VL-cured materials showed [108] that

Fig. 45. Structure of semi¯uorinated Bis-GMA analogues 71 and 72.

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Fig. 46. Structure of ¯uorinated monomers 73±80.

the use of bisphenol A or its ¯uorinated analogue as a core structure in the monomers provided composites with the highest mechanical strength. Moreover, the placement of ¯uorine in the extended per¯uoroalkyl chains did not decrease the water sorption and resulted in a lower mechanical strength compared to the use of alternate ¯uorinated aromatic terminal groups. Similar results were also obtained with various photocurable oligomeric ¯uoromonomers, for example, reaction products of ¯uorinated diepoxides and diols (Fig. 47), which especially contributed to the reduction of the polymerization shrinkage of the evaluated dental composites [109]. Finally, it should be mentioned that an alternative approach to the ¯uorinated Bis-GMA analogues is the use of ¯uorinated dimethacrylate-reactive diluents, for example, ¯uorinated triethyleneglycol dimethacrylate, which resulted in a decrease in the water absorption and polymerization shrinkage, respectively [110,111]. 2.5.2. Bis-GMA substitutes In the context of the discussion about the estrogenicity of bisphenol A, Bis-GMA impurities, or

Fig. 47. Structure of highly ¯uorinated multifunctional oligomers.

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Fig. 48. Bis-GMA substitutes on the basis of urethane dimethacrylates 81a±d.

degradation products [112], we synthesized the new urethane dimethacrylates 81a±d (Fig. 48) by the reaction of the commercially available a,a,a 0 ,a 0 -tetramethyl-m-xylylene diisocyanate (TMXDI) with OH-group-containing monomers, for example, 2-hydroxyethyl or hydroxypropyl methacrylate, which can be used to substitute Bis-GMA in dental composites [113]. TMXDI combines the advantageous properties of aliphatic (low tendency to discoloration) and aromatic (stiffness) diisocyanates. Therefore, the photopolymerization of urethane dimethacrylates 81a±d resulted in UV-light stable polymer networks with properties similar to those of Bis-GMA. Moreover, the mechanical properties of photocured materials based on these monomers were less in¯uenced by storage or boiling in water. Based on another approach of urethane chemistry, novel methacrylate carbamoyl isocyanurate resins were evaluated for use in dental restoratives [114]. These cross-linkers were synthesized by the step-bystep reaction of a triisocyanato-isocyanurate with one or more hydroxy-group-containing monomers. For example, the reaction of the isocyanurate derived from hexamethylene diisocyanate with 2-hydroxyethyl methacrylate and pentaerythritol trimethacrylate resulted in monomer 82 (Fig. 49). The physical properties of cured highly ®lled composites based on monomer 82 were superior to those of comparable conventional Bis-GMA based restoratives. A new family of multi-methacrylates with higher molecular weights and hydrophobicity was synthesized starting from ethoxylated poly(isopropylidenediphenol) resin (EBPA) by partial esteri®cation with methacryloyl chloride (Fig. 50) [115,116]. These multifunctional oligomers 83 demonstrated lower polymerization shrinkage and lower uptake of water. In addition, the mixtures of the multi-methacrylate with TEGDMA demonstrated photopolymerization characteristics comparable to those of the Bis-GMA/ TEGDMA control group. Similar results were obtained with oligomeric multi-methacrylates starting from a propoxylated poly(isopropylidenediphenol) resin [117]. A number of new ¯uorene-based dimethacrylates 84 (Fig. 51) were synthesized, which resulted in VL-cured matrix resins that exhibited higher, water saturated glass temperatures (59±638C) compared to Bis-GMA (538C) [118].

Fig. 49. Ethylenically unsaturated carbamoyl isocyanurate 82.

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Fig. 50. Synthesis of multi-methacrylate oligomers 83 based on EBPA polyols.

Fig. 51. Fluorene-based dimethacrylates 84.

Furthermore, the resins based on 84 showed a greater resistance to creep and fracture. Several dimethacrylates 85 with a more rigid structure and hydrophobic nature were derived from 3,3,5trimethylcyclohexan-1-one-phenol adducts (Fig. 52) [119]. They can also be used to reduce the water sorption and to improve the resistance to creep and wear of highly ®lled dental composites. In general, the substitution of Bis-GMA in restorative composites may contribute to substantial improvements of dental ®lling materials as far as the increase of the degree of double-bond conversion, the reduction of water sorption and the improvement of staining, creep and wear resistance are concerned.

Fig. 52. Dimethacrylates 85 derived from 3,3,5-trimethylcyclohexanone-phenol-adducts.

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Fig. 53. Isopropyl dimethacryloyl isostearoyl titanate 86.

2.6. Radiopaque monomers Restorative materials used in the posterior region must demonstrate a certain degree of radiopacity to give dentists the possibility to detect secondary caries, marginal defects, overhangs or other imperfections on X-rays. Adequate radiopacity also permits the assessment of contact point adequacy and interproximal contour [120]. Further dental applications that require radiopaque materials are the X-ray location of swallowed or respired partial dentures [121], or the X-ray control of a root canal ®lling [122]. A radiopacity higher than that of dentin meets ISO standard 4049 for restorative composites. 1 Some studies have shown that composites should exhibit a radiopacity equal to or greater than that of natural enamel [123]. To detect excess of composite cements, a radiopacity of more than 300 Al% is required [124]. In general, the radiopacity of composite ®lling materials is achieved by ®llers that absorb or re¯ect X-rays. Most un®lled or low-loaded composites, such as ¯owable composites, do not ful®ll these requirements [125]. In this case, radiopacity must be achieved with a radiopaque polymer matrix. There are two different ways to obtain radiopaque polymer matrice, using either heavy metal-containing monomers or monomers with iodine-/bromine residues. Matsumura et al. [126] prepared a titaniumbased composite core build-up material by incorporating isopropyl dimethacryloyl isostearoyl titanate 86 (Fig. 53) as a coupling agent in combination with Bis-GMA, TEGDMA and 4-META. The radiopacity of the homopolymer was not described, only the radiopacity of a composite with pure titanium as the ®ller. The disadvantages of this metal-based monomer are the reduced mechanical properties. The same facts were found by using zinc or barium acrylate [127]. Iodine- or bromine-containing monomers are the most common sources for radiopaque polymer matrices. The halogen residue is mostly linked to an aromatic ring, such as the carbazole ring system (Fig. 54) [128] or to phenyl residue, as it is the case for 2,4,6-triiodophenyl methacrylate (88, Fig. 55) or 2-methacryloyloxyethyl 2,3,5-triiodobenzoate (89, Fig. 55) [129,130]. The problem of the monofunctional systems is their low reactivity. The cross-linking monomer 90 (Fig. 55) is faster reacting, due to its early gel formation [131]. The polymerization of a 1:1 mixture of the radiopaque monomer 90 and a non radiopaque methacrylate-cross-linking monomer results in a polymer with a radiopacity of about 200 Al% [131]. Anzai et al. [132] chose the cyclophosphazene ring-system (Fig. 56) to connect the halogenated aromatic part with the polymerizable, functional methacrylate group. From octachlorocyclotetraphosphazene P4N4Cl8, four chlorine residues were replaced by p-bromophenol and the residual chlorines by HEMA. Some of the monomers described have the potential to provide suf®cient radiopacity in low-®lled or un®lled resin systems for X-ray differentiation. Due to the fact that a larger amount of these monomers is 1

ISO 4049 1988.

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Fig. 54. (1,2,3,4,6,7,8-Heptabromo-9-carbazoyl)-a-butyl methacrylate 87.

Fig. 55. Radiopaque methacrylates 88±90.

necessary in the corresponding formulation, a combination with ring-opening functional groups is necessary for low polymerization shrinking systems. To obtain a better compatibility with the often moist tooth structure (for example in the case of dentin adhesives) more hydrophilic systems are required.

Fig. 56. Radiopaque cyclophosphazene 91.

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3. Improvements of dental restoratives based on their ®ller components 3.1. Bioactive restoratives The most frequent reason for the replacement of dental ®llings is recurrent caries [133,134], independent of the type of ®lling material. Plaque accumulates in gaps formed around the ®lling after a certain wear time, which in some cases results in the formation of secondary caries. One approach to reduce the amount of recurrent caries is to incorporate additional preventive effects into the ®lling material to avoid demineralization and support remineralization of the tooth structure adjacent to the restoration. Fluoride ions are known to alter the tooth structure (¯uorohydroxyapatite formation) and make it less soluble in acids produced by cariogenic bacteria [135]. Fluoride release can be achieved from the organic as well from the inorganic part of the composite restorative. Examples for ¯uoride ion-containing monomeres or polymers are described mainly in patent literature (Fig. 57). In most cases, ¯uoride release is achieved via the inorganic part of the composite. Barium- or strontium ¯uorosilicate glasses are often used [138]. In some cases, ¯uoride-release results from the addition of ¯uoride salts, such as strontium-[138], sodium ¯uoride, or potassium hexa¯uorotitanate [139]. The most investigated ¯uoride releasing restorative materials are the glass ionomers. Fluoride release depends very much on the solvent in which the testspecimens are stored and the corresponding pH value. The amount of ¯uoride released in deionized water is about two times higher than that released in arti®cial saliva [140]. At low pH values, more ¯uoride is released compared to neutral conditions [141]. That means when acids are produced by cariogenic bacteria, ¯uoride is released on demand. In many cases, ¯uoride-releasing restoratives can be recharged by oral hygiene ¯uoridating agents [142]. In addition to measuring the ¯uoride release, a lot of investigations concentrate on the effect of ¯uoride release on the adjacent tooth structure, either by investigating the ¯uoride uptake of enamel and dentin [143] or the effect on preventing demineralization or accelerating remineralization [144,145]. In contrast to the in vitro investigations, which con®rmed that particularly glass ionomers have a preventive effect against the demineralization of the tooth structure, a comparative study with the same in vitro and in vivo test setup showed no bene®t of glass ionomers [146] in vivo. These ®ndings are somehow con®rmed by the investigations on the clinical behavior of dental restoratives placed in general dental practices [133,134]. For that reason, dental restoratives with additional or bene®cial effects have been developed to prevent secondary caries formation. There are several possible ways leading to this aim. Release of substances with an antimicrobial effect to combat cariogenic bacteria, additional support of remineralization or buffering acids produced by cariogenic bacteria. Some glass ionomers show antibacterial properties against Streptococcus mutans and Lactobacilli

Fig. 57. Example of a ¯uoride-containing monomer 92 and a ¯uoride-releasing polymer 93 [136,137].

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Fig. 58. Antibacterial monomer 12-methacryloyloxydodecylpyridinium bromide 94.

[147]. The reason for this effect remains unclear. It may either be the ¯uoride release, the low pH value, or the effect of eluated metal ions, such as zinc, silver or aluminum ions. Silver ions also have been incorporated in silica glass ®llers prepared with the sol±gel method to provide composite ®lling materials [148] with antibacterial properties. Antimicrobial substances, such as chlorhexidine, have also been incorporated in restorative materials [149]. The problem with physically bounded antimicrobials is that the drug is released in high quantities in an initial burst and not on demand. Imazato et al. [150] synthesized an antibacterial monomer 94 (Fig. 58), which can be incorporated in a composite by means of copolymerization. The disadvantage is that such composites show only an antimicrobial effect in case of direct surface contact with bacteria. Human saliva exhibits a high caries preventive effect [151]. Due to the high content of calcium and phosphate ions the chemical equilibrium between demineralization and remineralization is shifted in the direction of remineralization. This natural mechanism of prevention has been mimicked by adding amorphous calcium phosphate (ACP) as ®ller to restorative composite materials. The mechanical properties of such composites are lower compared to those of the ACP-free contol [152]. Arti®cial caries lesion coated with ACP-®lled composite recovered 71% of the mineral loss [153]. In addition to a high concentration of calcium and phosphate ions, human saliva demonstrates an enormous buffer capacity to neutralize acids produced by cariogenic germs. A restorative ®lling material attempted to combine the remineralizing and buffering effect of natural saliva with the preventive effect provided by ¯uoride [154,155]. In conclusion, there is a strong clinical demand for materials that inhibit the formation of secondary caries. Bioactive materials have a high potential to ful®ll this requirement. However, there must be a balance between the release of active ingredients (ef®ciency) and the durability of the corresponding ®lling. Release on demand is one mechanism that helps in this respect. Beside the ¯uoride release, the disadvantage of these new concepts is that there are currently only few long-term clinical results, if at all. 3.2. Nanoparticles, composite reinforcement At present, a large number of investigations are being conducted to improve the properties of composite materials, such as abrasion resistance, rheological or mechanical properties. For example, in situ formed layered silicate nano®llers increase the strength and toughness of acrylic nanocomposites [156]. Organo-polysiloxane particles of 5±200 nm in diameter are also described as compact modi®ers for dental materials [157]. Due to the low interaction between the particles, an increased load of the corresponding composite was achieved with the consequence of reduced polymerization shrinkage. In dental composites, mostly particle-sized ®llers are used. The reinforcement of a composite is often

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realized by incorporating ®bers or whiskers into a composite. Fiber-reinforced ®lling composites are only rarely described. The gradual addition of silanized short-cut glass-®bers to a Bis-GMA/TEGDMAbased composite resulted in an increase in the elastic modulus and a decrease in tensile strength [158]. Xu et al. [159,160] used ceramic single-crystalline whiskers as ®llers to reinforce composites. The ®ller mass fraction ranged from 0 to 70%. Compared to micro®ll and hybrid composites, the whisker-reinforced material demonstrated signi®cantly higher ¯exural strength. Whiskers exhibit some potential for the reinforcement of composite ®lling materials. However, especially the described whiskers [159,160] are not easily accessible. 3.3. Reduction of shrinkage stress Polymerization shrinkage of composite ®lling materials by itself is not the problem in restorative dentistry. Marginal gaps are the result of shrinkage stress that is build up at the interface of the ®lling material and the cavity walls. Polymerization contraction stress can be compensated to a certain extent by ¯ow after gelation of the composite [161]. Apart from the monomer composition of the composite and the polymerization kinetics, the amount of stress build up depends strongly on the geometry of the cavity. This can be expressed by the c-factor which is the ratio of bonded to non-bonded surfaces [162]. For example, porosity in an admixed two-component material reduces the shrinkage stress, since the non-bonded surface area is enlarged [163]. Porosity in the restorative presents the disadvantage of weakening the material and resulting in discoloration of the surface. The admix of non-bonded nanoparticles as described in Refs. [164,165] has the same effect as porosity with regard to the reduction of polymerization contraction stress. Usually, the surfaces of silicate ®llers are conditioned with methacrylate-functionalized silane (mostly g-methacryloxypropyl trimethoxy silane) to create a covalent bond between the ®ller particles and the organic matrix. Condon and Ferracane [164] silanized aerosil-type silicon dioxide with a non-methacrylate-functionalized silane to include non-bonded surfaces in the composite (see Fig. 59). Additional reduction of shrinkage stress can be achieved by incorporating methacrylated styrene± allyl alcohol copolymer [165]. However, only a slight reduction of the mechanical properties compared to the silanization with methacrylate-functionalized silane was observed [164,165]. Other attempts to reduce the polymerization shrinkage respectively shrinkage stress via the inorganic part of the composite are adding ammonia-modi®ed montmorillonite [166] (NH3/MMT) or using porous ®llers [167]. Composites containing ammonia-modi®ed montmorillonite increase their temperature during curing. Curing causes the NH3/MMT-particles to swell and counteract polymerization shrinkage. The addition of porous silicon dioxide glass particles with a particle size of 0.5±50 mm and porosity size in the range of 20±120 nm increases the abrasion resistance and reduces the polymerization shrinkage of composite ®lling materials [167]. In this way, a reduction of 50% of the linear polymerization shrinkage and an increase in abrasion resistance in the range of 100% was achieved. The effect of porous ®llers on the abrasion resistance was con®rmed by others [168]. However, the reduction of polymerization shrinkage could not be reproduced [169]. In conclusion, inorganic parts of composite ®lling materials possess a strong potential for reducing the polymerization contraction stress. Nevertheless, further in vitro and in vivo studies have to con®rm this potential.

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Fig. 59. Filler treated with methacrylate-functionalized and non-functionalized silane.

3.4. Radiopaque ®llers Radiopacity in dental composites is mainly achieved by means of ground glasses, which contain heavy metals, such as barium or strontium, which absorb or re¯ect X-rays. Micro®ll composites based on nano-scaled silicon dioxide were initially not radiopaque. Adding nano-scaled rare earth metal compounds, such as ytterbium tri¯uoride [170], resulted in radiopaque micro®ll composites, which ful®ll the requirements of ISO 4049. 1 Ytterbium tri¯uoride has been added to hybrid composites too [171], in order to gain more possibilities to increase the radiopacity above 300 Al%. In this way, the corresponding requirements for cements [124] can be ful®lled. Recently, other radiopaque nanoparticles, such as tantalum- [172] or zirconium oxide [173], have been incorporated into composites to improve the radiopacity or to obtain transparent materials [172]. Surface modi®cation with methacrylate silanes is less ef®cient for non-silicate ®llers [173]. Therefore, tantalum oxide nanoparticles were surface-functionalized with a phosphate methacrylate [172] (Fig. 60). Nanoparticles show the tendency of agglomeration. In this case, the refractive index of the particles has to be adjusted to the refractive index of the polymer matrix to achieve translucent materials. With the sol±gel process, mixed oxide particles can be prepared with a refractive index dependent on the ratio of the different metal ions [174].

Fig. 60. Tantalum oxide nanoparticle functionalized with a phosphate methacrylate.

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4. Trends to improve the processing of dental restoratives 4.1. Condensable/¯owable composites For a few years, composites with different consistencies have been on the market. In addition to the conventional composites with a creamy consistency, what is known as condensable and ¯owable composite ®lling materials are also available. The catchword `condensable' is somewhat marketing-driven and implies that the handling properties are comparable to those of amalgam. From a scienti®c view, composites are not condensable and do not have working and curing characteristics similar to amalgam. Based on this knowledge, the catchword `condensable' is now more and more replaced by the term `packable' [175]. The distinguishing handling properties of packable composites are reduced stickiness and increased viscosity compared to conventional composites. With these properties, packable composites can be placed in a manner somewhat resembling amalgam placement and therefore favors this type of material for posterior restoratios [176]. The composition of packable composites is similar to that of conventional composites, with the difference of a higher ®ller load. In some cases, the higher ®ller load is achieved by adding a certain amount of coarse ®llers or even ®ber fragments to the composite (see SEM, Fig. 61). As a result of the higher ®ller load, the ¯exural strength and -modulus are a little higher than those of conventional composites [177]. Flowable composites have a ®ller load in the range of 52±68 wt% and most of them are ®ne particle hybrids [178]. The ¯owable composites, initially introduced for the restoration of cervical defects, are generally predestinated for small cavities. Given their good wetting properties, they are also favorable for use as an initial layer in large cavities [178,179]. Due to the low ®ller load, the volumetric polymerization shrinkage of the ¯owable composites is higher than that of conventional or packable composites [180].

Fig. 61. SEM pictures of two commercially available packable composites.

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Fig. 62. Photochromic dye system added to dental composites.

4.2. Special additives Fluorescent dyes added to composites give them the appearance of natural tooth structure even under UV-light. Therefore, it is basically rather dif®cult to distinguish tooth-shaded composites from natural tooth structure. For esthetic reasons this fact is very important. However, if there is a need to remove a composite ®lling it is very hard for the dentist to distinguish between the ®lling and the tooth structure. One possibility for optical differentiation is to add a ¯uorescent dye to the composite, which absorbs light in the wavelength range 360±480 nm and a ¯uorescent maximum in the range of 480±600 nm [181]. In this case, light in the wavelength range of 360 480 nm has to be ®ltered out for visualization. Another approach is the use of photochromic dyes [182]. Photochromic dyes change their appearance under irradiation with light, for example, from colorless to red (see Fig. 62). Two systems are known, i.e. reversible and non-reversible system. In general, photochromic dyes should be reversible. In the special case of photochromic dental composites, some dyes lose their photochromic behavior during radical polymerization [182]. If a full-ceramic crown is cemented with a photochromic composite cement, the red-shaded cement can be easily distinguished during cementation and removed. After polymerization, the red shade disappears completely [182].

5. Conclusions The efforts to improve the clinical performance and the handling of restorative composite ®lling materials are mainly focused on the reduction of the polymerization shrinkage, as well as the improvement of biocompatibility, wear resistance, and the processing properties. The polymerization shrinkage, which impairs the marginal adaptation, can be reduced by ring-opening polymerization of cyclic monomers. Presently, the main disadvantage of the free-radical polymerizable cyclic monomers is the lower reactivity in comparison to methacrylates. In case of the cationic polymerizable cyclic monomers, it is necessary to increase the reactivity of the monomers and solve the problems concerning toxicology, as well as the sensitivity against bases and water. The contribution of liquid-crystalline, hyper-branched or dendritic monomers to the reduction of the polymerization shrinkage is more limited compared to that of cyclic monomers. Therefore, the combination of these monomer structures with ring-opening groups seems to be promising. Furthermore, these monomers may also help to improve the processing properties of the ®lling composites. Improved biocompatibility and wear resistance can be achieved with ormocers and ¯uorine-containing monomers. The incorporation of polymerizable cyclic groups is also very attractive for these matrix monomers. In some cases, non- or low-®lled resin systems are

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required. For this purpose, the radiopacity can be achieved by means of halogen-containing monomers or nanoparticles that absorb or re¯ect X-rays to a high degree. Bioactive composites are mainly designed to avoid or reduce recurrent caries. In addition to ¯uoride release, special ®llers or monomers are added to the composite either to reduce bacterial growth or to promote remineralization and avoid demineralization of the tooth structure. Reinforcement by ®bers or whiskers is currently under investigation. High- and low-viscosity composites extend the possibilities of dentists with regard to handling. Special ¯uorescent or photochromic dyes enable the temporary visualization of tooth-shaded composites during application. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]

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