Dent Mater 8:270-273, July, 1992
Evaluation of methylene lactone monomers in dental resins J. W. Stansbury, J. M. Antonucci Dental and Medical Materials Group, PolymersDivision, National Institute of Standards and Technology, Gaithersburg, MD, USA
Abstract. a-Methylene-~,-butyrolactone (MBL), which can be described as the cyclic analog of methyl methacrylate, exhibits greater reactivity in free radical polymerizations than conventional methacrylatemonomers. Unfilledresin formulations composed of Bis-GMA/MBL or Bis-GMNTEGDMA/MBLwerelight-cured. The effect of the more reactive methylene lactone monomer on mechanical properties and the degree of conversion of the polymers was examined. The infrared absorption bands for the carbon-carbon double bonds of MBL and the methacrylate monomers are well resolved and allow the conversion of each component to be calculated individually. The incorporation of a small amount of MBL (5 w/o) to Bis-GMA significantly increased the conversion; however, additional MBL (10to 30 w/o) did not further increase the Bis-GMA conversion level. This appears to indicate an incompatibility between MBL and the bulky Bis-GMA monomer, Addition of 10 w/o MBL to Bis-GMA/TEGDMA (7:3) resulted in a cured resin with 71% methacrylate and 75% overall conversion efficiencies compared with the 57% conversion of the control formulation. The diametral tensile and the transverse strengths were approximately 10% greater for the MBL resin compared with the Bis-GMNTEGDMA control; however, these differences were not statistically significant. The synthesis and polymerization of several substituted methylene lactones was also studied. INTRODUCTION Considerable attention has been directed toward the study of a-methylenelactonesbymedicinalandnaturalproductchemists. This interest is primarily a result of the variety of biological activity responses, including cytotoxic, antitumoral and bactericidal properties (Hoffmann and Rabe,1985) which these compounds have been shown to possess. The sustained interest in the a-methylene lactones has led to the development and refinement of a number of synthetic routes to these compounds (Petragnani et al., 1986; Grieco, 1975). The resultant availability has opened access to non-medical applications which require larger-scale quantities. The simplest example of these compounds is a-methylene-~/butyrolactone (MBL; Fig. 1) which is the cyclic analog of methyl methacrylate (MMA). ExaminationofMBLasamonomerinfreeradicalpolymerizations (Ueda et al., 1982) demonstrated several properties of both the monomer and the corresponding polymer that are of interest in resin-based composite applications. The MBL monomer is a colorless liquid of low viscosity but has a boiling point that is more than 100°C higher than MMA. This difference is derived from the cyclic structure which makes 270 Stansbury & Antonucci/Methylene lactone monomers in dental resins
MBL much more polar than MMA. As a result, MBL has almost no odor and very low volatility compared with MMA. Another consequence of the greater polarity of MBL is that it serves as an excellent solvent for oligomers and non-crosslinked polymers. This is analogous to the comparison of the solvent properties of diethy] ether, which is often used to precipitate polymers and tetrahydrofuran, its cyclic counterpart, which is an excellent solvent for a wide range ofpolymers. Poly(MBL) produced by free radical initiation has a glass transition temperature (%)of195°C (Akkapeddi,1979a) as compared with the (105°C) ~i'~of atactic poly(MMA) (Lee and Rutherford, 1975). The linear polymer also displays excellent solvent resistance as evidenced by its lack of solubility in common organic solvents such as chloroform and tetrahydrofuran. The high Tg and low solubility ofpo]y(MBL) are indicative of significant structural rigidity of the polymer chain. This can be attributed to the conformationally rigid ]actone rings which do not have the rotational freedom of the pendant groups in poly(MMA). In addition to these favorable properties, copolymerization studiesofMBLwithMMAhavedemonstratedthattheformer is a significantly more reactive monomer (Akkapeddi,1979b). This increased reactivity of MBL compared with methacrylate monomers can be ascribed to a number offactors related to the cyclic lactone structure. First, the double bondin MBLis more accessible due to fewer steric interactions with neighboring groups. Further, the five-membered lactone ring is very nearly planar, which maintains maximum resonance stabilization of the radical formed upon addition to the double bond. Finally, due to ring strain andthe fixed s-cis conformation, the exocyclic carbon-carbon double bond of MBL is at higher energy than the vinyl group of MMA. The relief of this strain energy upon addition to the double bond provides additional driving force for the polymerization of MBL. The restrictions imposed on the use of resin-based composites in the oral environment include the necessity for near ambientpolymerizationtemperatureswhileprovidingacured material with an acceptably high modulus. In trying to satisfy these divergent criteria, the major problem encountered has been the fairly low degree of conversion obtained. For the current commercial dental composites, only about 60% of the available vinyl groups are converted to polymeric linkages using standard curing procedures (Ruyter,1985). The presence of significant residual unsaturation not only decreases the potential physical properties of the cured resin,but also introduces the problems of color instability (Asmussen, 1983)
H2c_
_o
Hzc
o ~
--
/ /0\ H2C~, CH2
H3C/
\0 / CH~ s-cis
~
c~3 HzC O c--c H3C/ \\O s-trans
MBL
MMA
Fig.1. Structures of a-methylene-?-butyrolactone (MBL) and methyl methacrylate (MMA; in the s-cis and the favored s-trans conformations).
o II
H2C O \kc_// / \ H2~ o I 5 Br C2It
+
R/C~' R'
z~
H2C. .0 \XC__C// =
/ \ H2C\c,,.o
R/ \R' 1: R=phenyl,R'= H 2: R,R'= -(CH2)5-
Fig.2. Synthesisof ?-substituted c¢-methylene-y-butyrolactonesfromaldehydesand ketones. Monomer 1= c~-methylene-y-phenyl-?-butyrolactone. Monomer 2 = 3.
methylene-l-oxaspiro[4.5]decan-2-one, o II
/~H2
~ C fC~'CH3 ~OH~'
2)1)SOC~|K2CO3. .
~i)o
II o
\\ o 3
Fig.3. Synthesis of methylene phthalide (monomer 3) from 2-acetylbenzoic acid.
and accelerated degradation (McKinney and Wu, 1983). The use of a more reactive diluent monomer appears to be a practical means of addressing some ofthese deficiencies. This investigation was devised to evaluate the potential of MBL as a comonomer ingredient in Bis-GMA-based dental resin formulations. In addition, the synthesis and characterization of various substituted methylene lactones and their corresponding polymers was undertaken,
MATERIALSAND METHODS The MBL monomer used here was commercially obtained (Aldrich Chem. Co., Milwaukee, WI, USA) and was supplied with approximately 2% BHT included. The monomer was separated from the inhibitor by vacuum distillation (bp = 33°C/0.2 torr). Experimental resin formulations of MBL and Bis-GMA (2,2-bis-[4-(2-hydroxy-3-methacryloxypropoxy)phenylene]propane; Freeman Chemical Corp., Port Washington, WI, USA) or ternary mixtures including triethylene glycol dimethacrylate (TEGDMA; Esschem, Essington, PA, USA) were prepared. Diametral tensile strength (DTS), four-point bending transverse strength (TS)and degree of cure were evaluated for the experimental and control resins. The formulations, unless otherwise noted, contained 0.15% (by weight) camphorquinone (CQ) and 0.17% ethyl 4-dimethylaminobenzoate (EDMAB) as the photo-initiator system. Test specimens of the various compositions were obtained by irradiating 3 x 6 mm cylinders for DTS and 2 x 2 x 25 mm bars for TS for 20 s on the upper and lower surfaces with a visible light source (Prismetics Lite, L.D. Caulk, Div. of Dentsply, Milford, DE, USA). The specimens were stored for 24 h in 37°C water prior to testing according
to the procedures outlined in ADA Specification 27 for DTS and ISO Specification 4049 for TS. Statistical significance of the results was determined at the 95% confidence level. The degree of conversion was determined on thin films of the unfilled resins cured between Mylar sheets for 30 s and then stored for 24 h at 37°C. Fourier transform infrared spectroscopy was then used to compare the monomers with the corresponding cured films in duplicate according to the method described by Ferracane and Greener (1984).
Over the several month period that the experimental resins were stored at room temperature, there were no incidents of premature polymerization. Therefore, although MBL is a more reactive monomer than conventional methacrylates, this does not appear to present any significant storage stability problem. The substituted a-methylene lactone monomers were synthesized according to the method of Ohler e/al. (1970) as shown in Fig. 2 This procedure required the use of ethyl ~1983). Methylene phthalide (monomer 3, Fig. 3) was synthesized following the
bromomethylacrylicacid(Ramarajanetal.,
procedure of Liu and Howe (1983). MBL and the substituted methylene lactone monomers were homopolymerized with 1 mole % azobis(isobutronitrile) at 60°C for 2 h. Solubility studies with these linear polymers were conducted in chloro-
form(CHCl:JandN, N-dimethylformamide(DMF).RESULTS A series of Bis-GMA-based resin formulations with MBL concentrations ranging from 0 to 30% by weight were prepared. The composition and the evaluation of DTS and degree of conversion for the light-cured unfilled resins are given in Table 1. A crosshead speed of 20mm/min was utilized to obtain brittle fracture of these materials. As seen in Fig. 4, the exocyclicdoublebondofMBLabsorbsatasignificantlyshorter wavelength (1666 cm-1) in the infrared spectrum compared with a methacrylate vinyl absorption (1637 cml). This difference in the frequency of the absorptions permits the calculation of the conversion values for each of the monomers individually as well as the overall degree of cure. The conversions shown in Table 1 are averages from the analyses of duplicate specimens with uncertainties of approximately_+ 1%. The MBL monomer was incorporated into a conventional Bis-GMA/TEGDMA resin composition at a 10 weight % level for further evaluation. Table 2 provides the compositions of the experimentaland controlresins andthe results ofthe DTS, TS and degree of cure analyses for these fight-cured unfilled materials. Results from the synthesis and homopolymerization of a number of methylene lactones are given in Table 3 along with solubility properties of the various polymers. DISCUSSION The viscous nature of Bis-GMA makes it an impractical monomer to use alone at room temperature. The addition of 5 weight % MBL to the Bis-GMAyields a resin which, although stiff, was not inconvenient to handle. In contrast, the resin composed of 30 weight % MBL was very fluid and probably represents the lower viscosity limit that can be reasonably manipulated for applications such as pit and fissure sealants. As shown in Table 1, the DTS values of the light-cured unfilled resins improved with increasing MBL content to a maximum at 20 % MBL. Duncan's multiple comparisons test was applied to the DTS data for the series of resins. This Dental Materials~July 1992 271
TABLE 1: PROPERTIESOF Bis-GMNMBLLIGHT-CURED UNFILLED RESINS Resin Composition DiametralTensile Degreeof Conversion, MBL in Bis-GMA Strength, MPa % weight % mole % mean + s.d.(n) MBL Bis-GMA Total
0 5 10
0 22 37
36.1+2.8* (5) 39.7+1.2 (5) 48.6+_2.4 (5)
100 97
20 57 55.6+2.7 (5) 88 30 69 51.4+2.8 (5) 84 * Resin activated with 0.2 w/o CQ and 0.7 w/o EDMAB.
32 54 54
32 60 64
52 55
66 71
TABLE 2: PROPERTIESOF LIGHT-CUREDUNFILLEDRESINS Resin Composition Bis-GMNTEGDMA Bis-GMNTEGDMA/MBL weight % 70 : 30 63 : 27:10
mole%
54 : 46
37 : 32:31
Diametral TensileStrength MPa mean + s.d. (n)
42.2+3.6 (6)
46.6+3.7 (5)
75.3+4.3 (6)
82.4+2.4 (5)
57
71 96 75
TransverseStrength
MPa mean+ s.d. (n) Conversion % methacrylate lactone total
57
combination of these factors. It was anticipated that a highly mobile and reactive monomer such as M B L might have a beneficial influence on the degree of cure of resin formulations. The well resolved IR absorptions associated with the exocyc]ic double bond in M B L and the conventional methacrylate double bond in Bis-GMA provide insight into the relative polymerization efficiencies of the two monomers. The pure Bis-GMA formulation, as expected, displays an extremely low degree of cure due to the immobility of both the monomer and the growing polymer network. While there is a large increase in the Bis-GMA conversion accompanying the incorporation of 5% MBL into the resin, it is somewhat surprising that the methacrylate conversion becomes constant at this level despite the further decrease in resin viscosity as the MBL content continues to rise. This plateau in the methacrylate conversion limits the overall conversion to a gradual increase as additional M B L was included in the resin. High overall degrees of conversion have been obtained for Bis-GMA resins modified with MMA (Ruyter and Gyorosi, 1976). The observed differences in conversion for the two monomers may indicate a strong preference for the MBL radical species to add to another MBL monomer unit rather than the methacrylate groups of Bis-GMA. This could result in a stiff polymer matrix surrounding and isolating the more sluggishly reacting Bis-GMA double bonds. The reactivity ratios calculated by Akkapeddi (1979b) from the copolymerization of MBL (M1) with MMA(M2) are r 1 1.67 and r 2= 0.60. Although the a-methylene lactone monomer is more reactive, the product of these values (r,r 2 = 1.02) indicates that MBL would be expected to form an ideal, random copolymer with MMA. This same compatibility does not appear to exist with MBL and BisGMA under these ambient temperature, light-cured polymerization conditions. Since the MBL monomer appeared to be best utilized at relatively low concentrations, it was added to a conventional resin composition of Bis-GMA/TEGDMA (7:3 by weight) to give an experimental formulation of Bis-GMA/TEGDMA/ MBL (63:27:10). The results outlined in Table 2 indicate an approximate 10% enhancement in physical properties as measured by the DTS and TS of light-cured unfilled resins with M B L compared with the control; however, these differences were not statistically significant. The effect of MBL addition on degree of conversion of the unfilled Bis-GMA/TEGDMA resin was also evaluated. In this case, the Bis-GMA and TEGDMA methacrylate double bonds overlapped on the 1637 cm-1 IR absorption band while the MBL conversion could again be calculated separately. The degrees of conversion shown in Table 2 indicate a large increase in methacrylate polymerization efficiencyconnected with the addition of l0 weight % of MBL. This consideration, along with the nearly complete polymerization of the MBL ingredient, produces a very high overalllevel of conversion for the experimental resin. Therefore, the utilizationofaflexible comonomer such as TEGDMA, in conjunction with MBL and Bis-GMA, appears to successfully compatibilize the different monomer types. In addition to the evaluation of MBL, the synthesis and properties of some substituted methylene lactones were also examined. The substituted lactones are of interest since they offer: 1)ameanstoincreasemolecularweightofthemonomer, thereby reducing the amount of polymerization shrinkage; 2) the ability to influence properties of the monomers and the =
TABLE 3: PROPERTIESOF METHYLENELACTONEMONOMERS
ANDPOLYMERS Polymer Solubility* CHCI3 DMF DMF
Monomer
Yield.%
mp, °C
(25oc)
(50oC) (100oc)
MBL 1 2
# 66 54
liquid 52~54 liquid
+
+ +
+ + +
3
78
56-57
-
-
-
* + = soluble - = insoluble # Obtained commercially,
indicated a slight but significant increase in strength for the 5% MBL resin over Bis-GMA alone. The 10 and 30% MBL resins produced an intermediate group and the 20% MBL resin provided a material that was significantly stronger than the rest. It is well documented that the addition of a diluent comonomer to Bis-GMA greatly enhances the mobility of free monomer or pendant, partially reacted monomer units to engage other reactive sites within the growing polymer network (Ferracane and Greener, 1986). The result is a higher degree of conversion which has been correlated with an increase in mechanical strength properties of the cured resin (Asmussen, 1982). The reduction in viscosity induced by MBL addition is no doubt responsible for a large portion of the observed improvement in DTS. However, the incorporation of the rigid lactone rings on the polymer backbone may also contribute to this result. The decrease in the DTS for the resin with the highest MBL concentration may be a result of segmented copolymer formation, excessive polymerization shrinkage, a reduction in the cross-link density or a 272 Stansbury & Antonucci/Methylene lactone monomers in dental resins
l ~,, f~~k~
identified in this paper for adequate definition of the experimental procedure. In no instance does such identification imply recommendationor endorsement by the National Institute of Standards and Technology, or that the material or equipment is necessarily the best available for the purpose.
~...........
c:(.,....
Received October 25, 1991/Accepted April 10, 1992 Address correspondence and reprint requests to:
u z \
J.W. Stansbury Dentaland MedicalMaterialsGroup PolymersDivision NationalInstituteofStandardsand Technology Gaithersburg,MD20899USA
/
~ /
REFERENCES Akkapeddi MK (1979a). Po]y(a-methy]ene-y-butyrolactone) synthesis, configurational structure, and properties. Macromolecules 12:546-551. ,~v~uMRESS Akkapeddi MK(1979b). The free radical copolymerization of a-methylene-y-butyrolactone. Polymer 20:1215-1216. Fig.4. Pa~ial IR spectrum of Bis-GM~JMBLresin (3:1. upper). This composition A s m u s s e n E (1982). Restorative resins: Hardness and yields a 53:47 mixture of carbon-carbon double bonds based on the methacrylate strength vs quantity of remaining double bonds. Scand J (C=C~)and methylene lactone (C=CMeL)vinyl groups, respectively. The aromatic Dent Res 90:484-489. absorption band at 1600 cm-'was utilizedas the internalstandard in calculations of A s m u s s e n E (1983). Factors affecting the stability of restorthedegreeofconversion. ThelRspectrumofthecorrespondinglight-curedpolymer ative resins. Acta Odontal Scand 41:11-18. film(Iower)demonstratesthepreferentialconversionofthsMBLmonomercomponent FerracaneJL, GreenerEH(1984). Fouriertransforminfrared in this system. analysis of degree of polymerization in unfilled resins polymers via structural modifications and; 3) a technique for Methods comparison. J Dent Res 63:1093-1095. the preparation of bis(lactones) or multifunctional lactones Ferracane JL, Greener EH (1986). The effect of resin formucapable of polymerizing with cross-link formation, lation on the degree of conversion and mechanical properThe use of aldehyde or ketone startingmaterials with ethyl ties of dental restorative resins. J Biomed Mater Res a-bromomethylacrylate provides a convenient and versatile 20:121-131. route to the y-substituted a-methylene lactones (Fig. 2). Grieco PA (1975). Methods for the synthsis of a-methylene Benzaldehyde and cyclohexanone were used to obtain monolactones. Synthesis 1975:67-82. mers I and 2, respectively. Methylene phthalide, monomer 3, Hoffmann HMR, Rabe J (1985). Synthesis and biological can be easily prepared by another pathway (Fig.3) and proactivity of~-methylene-y-butyrolactones.AngewChemInt rides a 7-methylene lactone with an extended conjugation Ed 24:94-110. system. Lee WA, Rutherford RA (1975). In: Brandrup and Immergut, All the methylene lactone monomers readily homo-polyeds. Polymer Handbook. 2rid ed. New York: Wiley, III-148. merized to give glassy, brittle materials. The solubility Liu KC, Howe RK (1983). 3'-Arylspiro[isobenzofuranparameters of the series of polymers (Table 3) was investil(3H),5'(4H)-isoxazol]-3-ones and their conversion to 2-(3gated with solvents that would normally dissolve non-crossarylisoxazol-5-yl)benzoates. J Org Chem 48:4590-4592. linked methacrylate polymers. The cyclohexane-fused McKinney JE, Wu W (1983). Effect of degree of cure on poly(lactone) obtained from monomer 2 displayed the least hardness and wear of three commercial dental composites. solvent resistance. In contrast, the more rigid polymers J Dent Res 62:285, Abstr. No. 1047. prepared from MBL and monomer 3 showed extreme resis- Ohler E, Reininger K, Schmidt U (1970). A simple synthesis tance to solvent attack. This property may be advantageous of (x-methylene-y-butyrolactones. Angew Chem Int Ed in the design of polymer matrices that are impervious to 9:457-458. chemical softening and thereby offer greater wear potential. Petragnani N, Ferraz HMC, Silva GVJ (1986). Advances in While not pursued here, dialdehyde, diketone or other the synthesisof(z-methylenelactones.Synthesis 1986:157similar multifunctional starting materials could be employed 183. with these synthetic procedures to produce multifunctional Ramarajan K, Ramalingam K, O'Donnell DJ, Berlin KD lactone monomers. The addition of cross-linking potential to (1983).Ethyl a-(bromomethyl)acrylate.Organic Synthesis the already high conversion and good solvent resistance 61:56-59. characteristics of methylene lactone polymers might provide Ruyter IE (1985). Monomer systems and polymerization. In: for new dental composite materials with enhanced perforVanherle G and Smith DC, eds. Posterior Composite Resin mance capabilities. Dental Restorative Materials. The Netherlands: Peter Szulc Publishing Co., 109-135. ACKNOWLEDGMENTS Ruyter IE, Gyorosi PP (1976). An infrared spectroscopic study This work was supported by Interagency Agreement 01-DE of sealants. Scand J Dent Res 84:396-400. 30001withtheNationalInstituteofDentalResearch,Bethesda, Ueda M, Takahashi M, Imai Y, Pittman CU (1982). RadicalMD 20892. initiated homo- and copolymerization of a-methylene-yCertain commercial materials and equipment are butyrolactone. JPolymSci, PolymChemEd20:2819-2828. T
T
r
- -
,
Dental Materials~July 1992 273
Evaluation of methylene lactone monomers in dental resins J. W. Stansbury, J. M. Antonucci Dental and Medical Materials Group, PolymersDivision, National Institute of Standards and Technology, Gaithersburg, MD, USA
Abstract. a-Methylene-~,-butyrolactone (MBL), which can be described as the cyclic analog of methyl methacrylate, exhibits greater reactivity in free radical polymerizations than conventional methacrylatemonomers. Unfilledresin formulations composed of Bis-GMA/MBL or Bis-GMNTEGDMA/MBLwerelight-cured. The effect of the more reactive methylene lactone monomer on mechanical properties and the degree of conversion of the polymers was examined. The infrared absorption bands for the carbon-carbon double bonds of MBL and the methacrylate monomers are well resolved and allow the conversion of each component to be calculated individually. The incorporation of a small amount of MBL (5 w/o) to Bis-GMA significantly increased the conversion; however, additional MBL (10to 30 w/o) did not further increase the Bis-GMA conversion level. This appears to indicate an incompatibility between MBL and the bulky Bis-GMA monomer, Addition of 10 w/o MBL to Bis-GMA/TEGDMA (7:3) resulted in a cured resin with 71% methacrylate and 75% overall conversion efficiencies compared with the 57% conversion of the control formulation. The diametral tensile and the transverse strengths were approximately 10% greater for the MBL resin compared with the Bis-GMNTEGDMA control; however, these differences were not statistically significant. The synthesis and polymerization of several substituted methylene lactones was also studied. INTRODUCTION Considerable attention has been directed toward the study of a-methylenelactonesbymedicinalandnaturalproductchemists. This interest is primarily a result of the variety of biological activity responses, including cytotoxic, antitumoral and bactericidal properties (Hoffmann and Rabe,1985) which these compounds have been shown to possess. The sustained interest in the a-methylene lactones has led to the development and refinement of a number of synthetic routes to these compounds (Petragnani et al., 1986; Grieco, 1975). The resultant availability has opened access to non-medical applications which require larger-scale quantities. The simplest example of these compounds is a-methylene-~/butyrolactone (MBL; Fig. 1) which is the cyclic analog of methyl methacrylate (MMA). ExaminationofMBLasamonomerinfreeradicalpolymerizations (Ueda et al., 1982) demonstrated several properties of both the monomer and the corresponding polymer that are of interest in resin-based composite applications. The MBL monomer is a colorless liquid of low viscosity but has a boiling point that is more than 100°C higher than MMA. This difference is derived from the cyclic structure which makes 270 Stansbury & Antonucci/Methylene lactone monomers in dental resins
MBL much more polar than MMA. As a result, MBL has almost no odor and very low volatility compared with MMA. Another consequence of the greater polarity of MBL is that it serves as an excellent solvent for oligomers and non-crosslinked polymers. This is analogous to the comparison of the solvent properties of diethy] ether, which is often used to precipitate polymers and tetrahydrofuran, its cyclic counterpart, which is an excellent solvent for a wide range ofpolymers. Poly(MBL) produced by free radical initiation has a glass transition temperature (%)of195°C (Akkapeddi,1979a) as compared with the (105°C) ~i'~of atactic poly(MMA) (Lee and Rutherford, 1975). The linear polymer also displays excellent solvent resistance as evidenced by its lack of solubility in common organic solvents such as chloroform and tetrahydrofuran. The high Tg and low solubility ofpo]y(MBL) are indicative of significant structural rigidity of the polymer chain. This can be attributed to the conformationally rigid ]actone rings which do not have the rotational freedom of the pendant groups in poly(MMA). In addition to these favorable properties, copolymerization studiesofMBLwithMMAhavedemonstratedthattheformer is a significantly more reactive monomer (Akkapeddi,1979b). This increased reactivity of MBL compared with methacrylate monomers can be ascribed to a number offactors related to the cyclic lactone structure. First, the double bondin MBLis more accessible due to fewer steric interactions with neighboring groups. Further, the five-membered lactone ring is very nearly planar, which maintains maximum resonance stabilization of the radical formed upon addition to the double bond. Finally, due to ring strain andthe fixed s-cis conformation, the exocyclic carbon-carbon double bond of MBL is at higher energy than the vinyl group of MMA. The relief of this strain energy upon addition to the double bond provides additional driving force for the polymerization of MBL. The restrictions imposed on the use of resin-based composites in the oral environment include the necessity for near ambientpolymerizationtemperatureswhileprovidingacured material with an acceptably high modulus. In trying to satisfy these divergent criteria, the major problem encountered has been the fairly low degree of conversion obtained. For the current commercial dental composites, only about 60% of the available vinyl groups are converted to polymeric linkages using standard curing procedures (Ruyter,1985). The presence of significant residual unsaturation not only decreases the potential physical properties of the cured resin,but also introduces the problems of color instability (Asmussen, 1983)
H2c_
_o
Hzc
o ~
--
/ /0\ H2C~, CH2
H3C/
\0 / CH~ s-cis
~
c~3 HzC O c--c H3C/ \\O s-trans
MBL
MMA
Fig.1. Structures of a-methylene-?-butyrolactone (MBL) and methyl methacrylate (MMA; in the s-cis and the favored s-trans conformations).
o II
H2C O \kc_// / \ H2~ o I 5 Br C2It
+
R/C~' R'
z~
H2C. .0 \XC__C// =
/ \ H2C\c,,.o
R/ \R' 1: R=phenyl,R'= H 2: R,R'= -(CH2)5-
Fig.2. Synthesisof ?-substituted c¢-methylene-y-butyrolactonesfromaldehydesand ketones. Monomer 1= c~-methylene-y-phenyl-?-butyrolactone. Monomer 2 = 3.
methylene-l-oxaspiro[4.5]decan-2-one, o II
/~H2
~ C fC~'CH3 ~OH~'
2)1)SOC~|K2CO3. .
~i)o
II o
\\ o 3
Fig.3. Synthesis of methylene phthalide (monomer 3) from 2-acetylbenzoic acid.
and accelerated degradation (McKinney and Wu, 1983). The use of a more reactive diluent monomer appears to be a practical means of addressing some ofthese deficiencies. This investigation was devised to evaluate the potential of MBL as a comonomer ingredient in Bis-GMA-based dental resin formulations. In addition, the synthesis and characterization of various substituted methylene lactones and their corresponding polymers was undertaken,
MATERIALSAND METHODS The MBL monomer used here was commercially obtained (Aldrich Chem. Co., Milwaukee, WI, USA) and was supplied with approximately 2% BHT included. The monomer was separated from the inhibitor by vacuum distillation (bp = 33°C/0.2 torr). Experimental resin formulations of MBL and Bis-GMA (2,2-bis-[4-(2-hydroxy-3-methacryloxypropoxy)phenylene]propane; Freeman Chemical Corp., Port Washington, WI, USA) or ternary mixtures including triethylene glycol dimethacrylate (TEGDMA; Esschem, Essington, PA, USA) were prepared. Diametral tensile strength (DTS), four-point bending transverse strength (TS)and degree of cure were evaluated for the experimental and control resins. The formulations, unless otherwise noted, contained 0.15% (by weight) camphorquinone (CQ) and 0.17% ethyl 4-dimethylaminobenzoate (EDMAB) as the photo-initiator system. Test specimens of the various compositions were obtained by irradiating 3 x 6 mm cylinders for DTS and 2 x 2 x 25 mm bars for TS for 20 s on the upper and lower surfaces with a visible light source (Prismetics Lite, L.D. Caulk, Div. of Dentsply, Milford, DE, USA). The specimens were stored for 24 h in 37°C water prior to testing according
to the procedures outlined in ADA Specification 27 for DTS and ISO Specification 4049 for TS. Statistical significance of the results was determined at the 95% confidence level. The degree of conversion was determined on thin films of the unfilled resins cured between Mylar sheets for 30 s and then stored for 24 h at 37°C. Fourier transform infrared spectroscopy was then used to compare the monomers with the corresponding cured films in duplicate according to the method described by Ferracane and Greener (1984).
Over the several month period that the experimental resins were stored at room temperature, there were no incidents of premature polymerization. Therefore, although MBL is a more reactive monomer than conventional methacrylates, this does not appear to present any significant storage stability problem. The substituted a-methylene lactone monomers were synthesized according to the method of Ohler e/al. (1970) as shown in Fig. 2 This procedure required the use of ethyl ~1983). Methylene phthalide (monomer 3, Fig. 3) was synthesized following the
bromomethylacrylicacid(Ramarajanetal.,
procedure of Liu and Howe (1983). MBL and the substituted methylene lactone monomers were homopolymerized with 1 mole % azobis(isobutronitrile) at 60°C for 2 h. Solubility studies with these linear polymers were conducted in chloro-
form(CHCl:JandN, N-dimethylformamide(DMF).RESULTS A series of Bis-GMA-based resin formulations with MBL concentrations ranging from 0 to 30% by weight were prepared. The composition and the evaluation of DTS and degree of conversion for the light-cured unfilled resins are given in Table 1. A crosshead speed of 20mm/min was utilized to obtain brittle fracture of these materials. As seen in Fig. 4, the exocyclicdoublebondofMBLabsorbsatasignificantlyshorter wavelength (1666 cm-1) in the infrared spectrum compared with a methacrylate vinyl absorption (1637 cml). This difference in the frequency of the absorptions permits the calculation of the conversion values for each of the monomers individually as well as the overall degree of cure. The conversions shown in Table 1 are averages from the analyses of duplicate specimens with uncertainties of approximately_+ 1%. The MBL monomer was incorporated into a conventional Bis-GMA/TEGDMA resin composition at a 10 weight % level for further evaluation. Table 2 provides the compositions of the experimentaland controlresins andthe results ofthe DTS, TS and degree of cure analyses for these fight-cured unfilled materials. Results from the synthesis and homopolymerization of a number of methylene lactones are given in Table 3 along with solubility properties of the various polymers. DISCUSSION The viscous nature of Bis-GMA makes it an impractical monomer to use alone at room temperature. The addition of 5 weight % MBL to the Bis-GMAyields a resin which, although stiff, was not inconvenient to handle. In contrast, the resin composed of 30 weight % MBL was very fluid and probably represents the lower viscosity limit that can be reasonably manipulated for applications such as pit and fissure sealants. As shown in Table 1, the DTS values of the light-cured unfilled resins improved with increasing MBL content to a maximum at 20 % MBL. Duncan's multiple comparisons test was applied to the DTS data for the series of resins. This Dental Materials~July 1992 271
TABLE 1: PROPERTIESOF Bis-GMNMBLLIGHT-CURED UNFILLED RESINS Resin Composition DiametralTensile Degreeof Conversion, MBL in Bis-GMA Strength, MPa % weight % mole % mean + s.d.(n) MBL Bis-GMA Total
0 5 10
0 22 37
36.1+2.8* (5) 39.7+1.2 (5) 48.6+_2.4 (5)
100 97
20 57 55.6+2.7 (5) 88 30 69 51.4+2.8 (5) 84 * Resin activated with 0.2 w/o CQ and 0.7 w/o EDMAB.
32 54 54
32 60 64
52 55
66 71
TABLE 2: PROPERTIESOF LIGHT-CUREDUNFILLEDRESINS Resin Composition Bis-GMNTEGDMA Bis-GMNTEGDMA/MBL weight % 70 : 30 63 : 27:10
mole%
54 : 46
37 : 32:31
Diametral TensileStrength MPa mean + s.d. (n)
42.2+3.6 (6)
46.6+3.7 (5)
75.3+4.3 (6)
82.4+2.4 (5)
57
71 96 75
TransverseStrength
MPa mean+ s.d. (n) Conversion % methacrylate lactone total
57
combination of these factors. It was anticipated that a highly mobile and reactive monomer such as M B L might have a beneficial influence on the degree of cure of resin formulations. The well resolved IR absorptions associated with the exocyc]ic double bond in M B L and the conventional methacrylate double bond in Bis-GMA provide insight into the relative polymerization efficiencies of the two monomers. The pure Bis-GMA formulation, as expected, displays an extremely low degree of cure due to the immobility of both the monomer and the growing polymer network. While there is a large increase in the Bis-GMA conversion accompanying the incorporation of 5% MBL into the resin, it is somewhat surprising that the methacrylate conversion becomes constant at this level despite the further decrease in resin viscosity as the MBL content continues to rise. This plateau in the methacrylate conversion limits the overall conversion to a gradual increase as additional M B L was included in the resin. High overall degrees of conversion have been obtained for Bis-GMA resins modified with MMA (Ruyter and Gyorosi, 1976). The observed differences in conversion for the two monomers may indicate a strong preference for the MBL radical species to add to another MBL monomer unit rather than the methacrylate groups of Bis-GMA. This could result in a stiff polymer matrix surrounding and isolating the more sluggishly reacting Bis-GMA double bonds. The reactivity ratios calculated by Akkapeddi (1979b) from the copolymerization of MBL (M1) with MMA(M2) are r 1 1.67 and r 2= 0.60. Although the a-methylene lactone monomer is more reactive, the product of these values (r,r 2 = 1.02) indicates that MBL would be expected to form an ideal, random copolymer with MMA. This same compatibility does not appear to exist with MBL and BisGMA under these ambient temperature, light-cured polymerization conditions. Since the MBL monomer appeared to be best utilized at relatively low concentrations, it was added to a conventional resin composition of Bis-GMA/TEGDMA (7:3 by weight) to give an experimental formulation of Bis-GMA/TEGDMA/ MBL (63:27:10). The results outlined in Table 2 indicate an approximate 10% enhancement in physical properties as measured by the DTS and TS of light-cured unfilled resins with M B L compared with the control; however, these differences were not statistically significant. The effect of MBL addition on degree of conversion of the unfilled Bis-GMA/TEGDMA resin was also evaluated. In this case, the Bis-GMA and TEGDMA methacrylate double bonds overlapped on the 1637 cm-1 IR absorption band while the MBL conversion could again be calculated separately. The degrees of conversion shown in Table 2 indicate a large increase in methacrylate polymerization efficiencyconnected with the addition of l0 weight % of MBL. This consideration, along with the nearly complete polymerization of the MBL ingredient, produces a very high overalllevel of conversion for the experimental resin. Therefore, the utilizationofaflexible comonomer such as TEGDMA, in conjunction with MBL and Bis-GMA, appears to successfully compatibilize the different monomer types. In addition to the evaluation of MBL, the synthesis and properties of some substituted methylene lactones were also examined. The substituted lactones are of interest since they offer: 1)ameanstoincreasemolecularweightofthemonomer, thereby reducing the amount of polymerization shrinkage; 2) the ability to influence properties of the monomers and the =
TABLE 3: PROPERTIESOF METHYLENELACTONEMONOMERS
ANDPOLYMERS Polymer Solubility* CHCI3 DMF DMF
Monomer
Yield.%
mp, °C
(25oc)
(50oC) (100oc)
MBL 1 2
# 66 54
liquid 52~54 liquid
+
+ +
+ + +
3
78
56-57
-
-
-
* + = soluble - = insoluble # Obtained commercially,
indicated a slight but significant increase in strength for the 5% MBL resin over Bis-GMA alone. The 10 and 30% MBL resins produced an intermediate group and the 20% MBL resin provided a material that was significantly stronger than the rest. It is well documented that the addition of a diluent comonomer to Bis-GMA greatly enhances the mobility of free monomer or pendant, partially reacted monomer units to engage other reactive sites within the growing polymer network (Ferracane and Greener, 1986). The result is a higher degree of conversion which has been correlated with an increase in mechanical strength properties of the cured resin (Asmussen, 1982). The reduction in viscosity induced by MBL addition is no doubt responsible for a large portion of the observed improvement in DTS. However, the incorporation of the rigid lactone rings on the polymer backbone may also contribute to this result. The decrease in the DTS for the resin with the highest MBL concentration may be a result of segmented copolymer formation, excessive polymerization shrinkage, a reduction in the cross-link density or a 272 Stansbury & Antonucci/Methylene lactone monomers in dental resins
l ~,, f~~k~
identified in this paper for adequate definition of the experimental procedure. In no instance does such identification imply recommendationor endorsement by the National Institute of Standards and Technology, or that the material or equipment is necessarily the best available for the purpose.
~...........
c:(.,....
Received October 25, 1991/Accepted April 10, 1992 Address correspondence and reprint requests to:
u z \
J.W. Stansbury Dentaland MedicalMaterialsGroup PolymersDivision NationalInstituteofStandardsand Technology Gaithersburg,MD20899USA
/
~ /
REFERENCES Akkapeddi MK (1979a). Po]y(a-methy]ene-y-butyrolactone) synthesis, configurational structure, and properties. Macromolecules 12:546-551. ,~v~uMRESS Akkapeddi MK(1979b). The free radical copolymerization of a-methylene-y-butyrolactone. Polymer 20:1215-1216. Fig.4. Pa~ial IR spectrum of Bis-GM~JMBLresin (3:1. upper). This composition A s m u s s e n E (1982). Restorative resins: Hardness and yields a 53:47 mixture of carbon-carbon double bonds based on the methacrylate strength vs quantity of remaining double bonds. Scand J (C=C~)and methylene lactone (C=CMeL)vinyl groups, respectively. The aromatic Dent Res 90:484-489. absorption band at 1600 cm-'was utilizedas the internalstandard in calculations of A s m u s s e n E (1983). Factors affecting the stability of restorthedegreeofconversion. ThelRspectrumofthecorrespondinglight-curedpolymer ative resins. Acta Odontal Scand 41:11-18. film(Iower)demonstratesthepreferentialconversionofthsMBLmonomercomponent FerracaneJL, GreenerEH(1984). Fouriertransforminfrared in this system. analysis of degree of polymerization in unfilled resins polymers via structural modifications and; 3) a technique for Methods comparison. J Dent Res 63:1093-1095. the preparation of bis(lactones) or multifunctional lactones Ferracane JL, Greener EH (1986). The effect of resin formucapable of polymerizing with cross-link formation, lation on the degree of conversion and mechanical properThe use of aldehyde or ketone startingmaterials with ethyl ties of dental restorative resins. J Biomed Mater Res a-bromomethylacrylate provides a convenient and versatile 20:121-131. route to the y-substituted a-methylene lactones (Fig. 2). Grieco PA (1975). Methods for the synthsis of a-methylene Benzaldehyde and cyclohexanone were used to obtain monolactones. Synthesis 1975:67-82. mers I and 2, respectively. Methylene phthalide, monomer 3, Hoffmann HMR, Rabe J (1985). Synthesis and biological can be easily prepared by another pathway (Fig.3) and proactivity of~-methylene-y-butyrolactones.AngewChemInt rides a 7-methylene lactone with an extended conjugation Ed 24:94-110. system. Lee WA, Rutherford RA (1975). In: Brandrup and Immergut, All the methylene lactone monomers readily homo-polyeds. Polymer Handbook. 2rid ed. New York: Wiley, III-148. merized to give glassy, brittle materials. The solubility Liu KC, Howe RK (1983). 3'-Arylspiro[isobenzofuranparameters of the series of polymers (Table 3) was investil(3H),5'(4H)-isoxazol]-3-ones and their conversion to 2-(3gated with solvents that would normally dissolve non-crossarylisoxazol-5-yl)benzoates. J Org Chem 48:4590-4592. linked methacrylate polymers. The cyclohexane-fused McKinney JE, Wu W (1983). Effect of degree of cure on poly(lactone) obtained from monomer 2 displayed the least hardness and wear of three commercial dental composites. solvent resistance. In contrast, the more rigid polymers J Dent Res 62:285, Abstr. No. 1047. prepared from MBL and monomer 3 showed extreme resis- Ohler E, Reininger K, Schmidt U (1970). A simple synthesis tance to solvent attack. This property may be advantageous of (x-methylene-y-butyrolactones. Angew Chem Int Ed in the design of polymer matrices that are impervious to 9:457-458. chemical softening and thereby offer greater wear potential. Petragnani N, Ferraz HMC, Silva GVJ (1986). Advances in While not pursued here, dialdehyde, diketone or other the synthesisof(z-methylenelactones.Synthesis 1986:157similar multifunctional starting materials could be employed 183. with these synthetic procedures to produce multifunctional Ramarajan K, Ramalingam K, O'Donnell DJ, Berlin KD lactone monomers. The addition of cross-linking potential to (1983).Ethyl a-(bromomethyl)acrylate.Organic Synthesis the already high conversion and good solvent resistance 61:56-59. characteristics of methylene lactone polymers might provide Ruyter IE (1985). Monomer systems and polymerization. In: for new dental composite materials with enhanced perforVanherle G and Smith DC, eds. Posterior Composite Resin mance capabilities. Dental Restorative Materials. The Netherlands: Peter Szulc Publishing Co., 109-135. ACKNOWLEDGMENTS Ruyter IE, Gyorosi PP (1976). An infrared spectroscopic study This work was supported by Interagency Agreement 01-DE of sealants. Scand J Dent Res 84:396-400. 30001withtheNationalInstituteofDentalResearch,Bethesda, Ueda M, Takahashi M, Imai Y, Pittman CU (1982). RadicalMD 20892. initiated homo- and copolymerization of a-methylene-yCertain commercial materials and equipment are butyrolactone. JPolymSci, PolymChemEd20:2819-2828. T
T
r
- -
,
Dental Materials~July 1992 273
