Ring-Opening Dental Resin Systems Based on Cyclic Acetals

Chapter 15

Ring-Opening Dental Resin Systems Based on Cyclic Acetals 1

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Β. B. Reed , Jeffrey W. Stansbury , and Joseph M. Antonucci 1

Paffenbarger Research Center, American Dental Association Health Foundation, Gaithersburg, MD 20899 Dental and Medical Materials Group, Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD 20899

Downloaded by MIT on June 1, 2013 | http://pubs.acs.org Publication Date: November 30, 1993 | doi: 10.1021/bk-1994-0540.ch015

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For monomers of comparable size,ring-openingpolymerization results in less shrinkage than that which accompanies 1,2-vinyl addition polymerization. Two monomer types were synthesized, nonvinyl (NVCA) and vinyl (VCA) cyclic acetals. The goals of this study were to assess the potential for reduced shrinkage throughfreeradical ring -opening polymerization of NVCA and VCA type monomers, and to test the mechanical strength of dental resin composites formulated with these novel monomers. Homo- and copolymerizations were conducted with several NVCAs and VCAs to evaluate their potential as comonomers in dental polymeric composites. Composite specimens were formulated with PBMD, a VCA derived from terephthaldehyde, and EBPADM, an ethoxylated bisphenol A dimethacrylate, and tested for their mechanical strength. Three control formulations were tested, one containing 100% EBPADM, the second containing PBD, a NVCA derived from terephthaldehyde, and the last containing p-dimethoxybenzene (DMOB). The composites containing PBMD and PBD gave mechanical strength values similar to the EBPADM control, while the control containing DMOB had much lower strength.

Polymerization shrinkage in dental resin composite systems causes numerous undesirable results: internal stresses, micro-cracks, debonding at the filler particle-resin interface that leads to accelerated degradation through reduced mechanical strength and abrasion resistance, and external loss of adhesion that produces marginal gaps at the composite-tooth interface and ultimately results in secondary carries and staining. To counteract the shrinkage caused when chain growth polymerization occurs, monomers capable of free radicalring-openingpolymerization have been designed (1). Ringopening monomers have the potential for polymerization with less volume change than noncyclic vinyl monomers such as methyl methacrylate or styrene. During 1,2-vinyl addition polymerization, monomer units go from Van der Waals to covalent bond distances. In contrast, duringring-openingpolymerization, volume contraction is offset as some covalent bonds are cleaved to give near Van der Waal bond distances (Figure la and lb).

0097-6156/94/0540-0184$06.00/0 © 1994 American Chemical Society

In Polymers of Biological and Biomedical Significance; Shalaby, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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In this investigation two classes of monomers were synthesized, nonvinyl cyclic acetals and vinyl cyclic acetals (NVCA and VCA, respectively (Figure 2)). A l l monomers were evaluated for their ability to homo- and copolymerize under free radical conditions. The monomers that displayed sufficient reactivity to warrant further study were then incorporated into dental composite resin systems. The diametral tensile strength (DTS) of several experimental composites were determined and compared with various controls.


Materials and Methods Unless otherwise stated, the reagents used in the syntheses of NVCA and VCA monomers and the monomers used in the copolymerization studies were obtained commercially (Aldrich Chemical Co.) and used as received. All monomers and noncrosslinked polymers were characterized by interpretation of their respective *H and proton-decoupled C NMR spectra which were obtained on a JEOL GSX-270(FT) spectrometer operated at 270 and 68.1 MHz, respectively. All NMR samples were analyzed in CDCI3. IR spectra were obtained on a Ratio Recording Perkin Elmer 1420 instrument controlled through a data station. Nonvinyl cyclic acetal synthesis was carried out according to the following general scheme (Figure 3). Stoichiometric amounts of the appropriate aryl aldehyde and diol were combined in a single-neck, round bottom flask with toluene as the solvent and 1 mole % of p-toluene sulfonic acid (PTSA), recrystalized from methanol, as the catalyst. The flask wasfittedwith a condenser and a Dean-Stark trap to collect, by azeotropic distillation, the water generated in the reaction. An inert atmosphere (argon) was maintained throughout all steps of the procedure. After refluxing for several hours, the mixture was allowed to cool to room temperature. The mixture was then extracted three times with a 5 wt/vol% of an aqueous sodium bicarbonate solution. The organic layer was dried with anhydrous sodium sulfate, filtered and concentrated under reduced pressure via rotary evaporation. PBD lH NMR: δ 4.00 (m, CH CH ), 5.80 (s, OCHO), 7.45 (s, aiom). C NMR: δ 65.0 (CH CH ) 103.0 (OCHO) 126.5 (arom C ,3,5,e) 1 3

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1 3

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139.0 (arom Ci 4). f

Synthesis of a vinyl cyclic acetal monomer is similar to the synthesis of a NVCA but includes a dehydrohalogenation step as shown in Figure 4. Stoichiometric amounts of the appropriate aryl aldehyde and 3-chloropropane-l,2-diol were reacted and worked up as previously described. The chloromethyl cyclic acetal intermediate was dissolved in toluene and added slowly to three equivalents of potassium ieri-butoxide in toluene in a round bottom flask cooled in an ice bath. When the addition was complete, the mixture was allowed to warm to room temperature and stirred vigorously overnight. The dark reaction mixture was vacuum filtered through a paper filter covered with a small amount of silica gel. Thefiltratewas dried and concentrated as before. MPD l H NMR: δ 4.00 (s, C=CH ), 4.45 (s, =CH ), 4.60 (d d, CH ), 6.15 (s, OCHO), 7.40-7.55 (m, arom). C NMR: δ 67.5 (OCH C=), 78.5 (=CH ), 106.0 (OCHO), 126.5 (arom C4), 128.5 (arom C ,6), 130.0 (arom 03,5), 136.5 (arom Q ) , 156.0 (C=CH ). PBMD *H NMR: δ 4.00 (s, =CH ), 4.45 (s, =CH ), 4.55 (d d, OCH C=), 6.15 (s, OCHO), 7.55 (s, arom). 13c NMR: δ 67.0 (OCH C=), 78.5 (=CH ), 105.0 (OCHO), 126.5 (arom C , , ,6), 8 (arom C ) , 155.5 (C=CH ). Homopolymerizations of each monomer were attempted in bulk and in solution. Bulk polymerizations were conducted at 60°C for 24 h with vacuum degassed monomers containing 2,2'-asobis(isobutronitrile) (AIBN). Solution polymerizations were conducted by placing an aliquot of the monomer in a screw cap vial along with 0.5-1.0 wt% AIBN and «90 w/w% benzene. Argon was then bubbled through the mixture for 10 s before the cap was secured. The sealed vial was placed in the 60 C E

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13

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M

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POLYMERS OF BIOLOGICAL AND BIOMEDICAL SIGNIFICANCE

R

I


Van der Waal's Distance

H

Near Van der Waal's Distance

Figure la. Free radical ring opening of a NVCA type monomer to an ester intermediate.

R -

3

Near Van der Waal's Distance Van der Waal's Distance

Figure lb. Free radical ring opening of a VCA type monomer to a keto ether intermediate. NVCA

VCA

4-Methylene-2-phenyl1,3-dioxolane (MPD) 2,2'-(l,4-Phenylene)bisl,3-dioxolane(PBD)

P S *

•o

Ο

ο-

2,2'-( 1,4-Pheny lene)bis4-methylene-l,3-dioxolane (PBMD) Figure 2. Monomers used in this study.


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oven for 24 h. Copolymerizations were conducted by transfering equimolar amounts of the experimental monomer and either methyl methacrylate (MMA) or styrene along with the AIBN initiator into a glass vial. The vial was degassed, vacuum sealed, and placed in the 60 C oven for 24 h. After 24 h the vial was removed from the oven, allowed to cool to room temperature, and the contents of the vial dissolved into a minimal amount of dichloromethane. The solution was added dropwise to «20 fold v/v hexane to precipitate polymers of significant molecular weight present in the sample. The precipitate was filtered, dried and dissolved into CDCI3 for analysis by H and C NMR. If the combined NMR spectra were not sufficient to determine the structure of the polymers, then IR spectral analysis was used to aid in the elucidation of the structure. The monomers that yielded homo- or copolymers were formulated into composites. Three comonomers, PBD, PBMD and p-dimethoxybenzene (DMOB), were combined individually with EBPADM, an ethoxylated bisphenol A dimethacrylate (Diacryl 101, Noury Chemicals), in various ratios to form the resin components of the composites. The initiator system used was 0.4 wt% camphorquinone (CQ) and 0.8 wt% ethyl 4-iV^V-dimethylaminobenzoate (4-EDMAB). The respective resins were mixed in a wt/wt powder to liquid ratio of 5 with a barium oxide containing glass powder (Corning Glass 7724), which was silanized according to a previously reported procedure (2). The resulting paste was deaerated under a slight vacuum (« 175 Pa) for 2 h. Samples were made by hand packing the composite paste into cylindrical stainless steel molds, 3 mm H by 6 mm D, which were then covered with 1 mm thick glass plates and afixed with spring clips. Each sample was irradiated with a visible light (Prismetics Lite, Caulk/Dentsply) for 60 s per side and placed in a 37 C oven for 15 min. They were then removed from oven, ejected from their molds and stored in deionized water at 37°C for 24 h before being tested. A universal testing machine (United Calibration Corp.) was used to measure the diametral tensile strength (DTS) of each sample at a crosshead speed of 1 cm/min. Five or more samples were testedfromeach composite formulation. The stress-strain curves and the broken samples were inspected to determine if the sample failure was primarily due to tensile stress without significant plastic deformation. e


l

1 3

e

Results and Discussion VCAs homopolymerized under the conditions previously described; therefore they were considered promising candidates for inclusion in the resin phase of composites. MPD yielded homopolymer which was dissolved in CDCI3 for NMR analysis. The most distinguishing features of its C NMR spectrum were peaks around 103 and 206 ppm. A peak at 103 ppm is corresponds to a cyclic acetal which indicates that 1,2addition polymerization occurred, leaving the ring intact. The peak at 206 ppm is characteristic of a ketone; which indicatesring-openingto the keto-ether after radical attack on the exomethylene group (Figure 5). PBMD, the difunctional analog of MPD, yielded crosslinked homopolymer, which could not be characterized with solution NMR. Presumably PBMD polymerized through similar mechanistic pathways as MPD. PBMD was formulated into composites because its difunctional nature offers the following possible advantages: (1) the potential for two rings to be opened during copolymerization, thus further counteracting the shrinkage caused by the dimethacrylate comonomer conversion (EBPADM), and (2) the potential for enhancing the degree of crosslinking, thereby strengthening the composite. It was hypothesized that a monomer of the first type (NVCA) could undergo free radical polymerization through abstraction of its tertiary hydrogen (Figure la). Results of this study indicate this did not occur to a measurable extent under the reaction conditions given. NVCAs proved to be not only stable under the circumstances described in the previous section but also unreactive even under harsher conditions (2 wt% ί-butylperoxide at 126 C for 24 h). Polymerization studies of the NVCA 1 3

e


POLYMERS OF BIOLOGICAL AND BIOMEDICAL SIGNIFICANCE


Figure 3. Synthesis of a NVCA type monomer.

Figure 4. Synthesis of a VCA type monomer.

103 ppm Figure 5. Polymerization pathway via ring-opening and vinyl addition.



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compounds with monofunctional comonomers such as M M A indicated no incorporation of the cyclic acetal or any ring-opened products into the polymer (PMMA). Therefore, PBD was considered an excellent candidate for the formulation of a negative control for the composite study; it is approximately the same size, shape, and molecular weight as PBMD, yet seemingly unreactive underfreeradical conditions. One composite formulation was made with 100 wt% EBPADM to serve as a positive control. A second negative control using DMOB as a nonreactive component for EBPADM was also formulated into a composite. DMOB was chosen because it has even less chemical potential for incorporation into the polymeric matrix than PBD. It contains no vinyl group or labile hydrogen, yet it has comparable size and molecular weight to both PBD and PBMD. The results of the DTS tests are shown in Table I. The values given are in MPa with the standard deviation indicated by the number in parentheses. All statistical information was generated with the general linear model program of the Statistical Analysis System software (3). Comparisons of the data were made with Duncan's Multiple Range Test (modified for unequal sample sizes) at p

Ring-Opening Dental Resin Systems Based on Cyclic Acetals

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