Photosensitizers for free radical polymerization initiation resins and method of making the same

A photoinitiator composition that includes (a) 1-aryl-2-alkyl-1,2-ethanedione and (b) a rigid 1,2-dione in a weight ratio of (a):(b) in a range of about 1:20 to about 20:1 is disclosed. The photoinitiator composition can be used in a photocurable dental composition in an amount sufficient to achieve a degree of double-bond conversion of at least 50%. Methods for making the photocurable dental composition are also disclosed.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention generally relates to whiter photoinitiated dental resins and
 composites, and to methods for making same. More specifically, the
 invention relates to light-cure dental compositions including at least one
 photopolymerizable monomer and a photoinitiation system including (a) a
 1-aryl-2-alkyl-1,2-ethanedione and (b) a rigid 1,2-dione in a weight ratio
 of (a):(b) in a range of about 1:20 to about 20:1, wherein the mixture is
 present in an amount sufficient to achieve a degree of double-bond
 conversion (DC) of at least 50%.
 2. Brief Description of Related Technology
 There is a consensus that improved conversion of double bonds during
 photopolymerization is critical for the optimization of mechanical
 properties (Ferracane and Greener, 1986; Ferracane et al., 1997;
 Peutzfeldt and Asmussen, 1996, 1992), biocompatibility (Rietschel, 1986),
 and color stability (Imazato et al., 1995) of light-activated dental
 resins. Photopolymerization implies both the light-induced increase of
 molecular weight by monomer to polymer conversion, as well as crosslinking
 of developing or preexisting macromolecules.
 Photopolymerization reactions commonly are initiated by free-radicals
 formed by photosensitizers (also referred to hereinafter as
 photoinitiators). These photosensitizers typically posses a carbonyl group
 having a non-bonding electron capable of being promoted into the .pi.*
 anti-bonding orbital by absorption of light of the quantum mechanically
 allowed wavelength. This electron promotion leads to production of a pair
 of free radicals, either by intramolecular cleavage (e.g., as with benzoin
 ethers such as benzoin methyl ether) or by proton abstraction from a
 labile site (e.g., an .alpha.-alkylamine group on amines such as
 N,N-dimethylamino ethylmethacrylate) with photosensitizers such as
 camphorquinone (CQ). Generally, photoinitiators should absorb and
 photoinitiate polymerization reactions in the visible light spectrum, such
 as at a wavelength of about 470 namometers (nm) for CQ.
 Proton abstraction can be made highly efficient by formation of a complex
 between the photoexcited sensitizer and an electron-donating (reducing)
 agent, such as a tertiary amine. The complex is referred to as an
 "exciplex." Proton abstraction occurs within this exciplex which then
 breaks down to form free radicals (Oster and Yang, 1968; Hutchison and
 Ledwith, 1974; Ledwith, 1977). Aliphatic compounds containing two or more
 vicinal carbonyl groups have also been used as photosensitizers. Diacetyl,
 for example, absorbs in both the near-ultraviolet and blue regions of the
 spectrum (up to 467 nanometers (nm)) and has been used as a sensitizer for
 polymerization of methyl methacrylate with visible light (Gladyshev and
 Rafikov, 1962).
 Even though a variety of compounds can act as initiators in the visible
 light region (Pummerer and Kehlen, 1933; Oster 1954, 1958; Oster et al.,
 1959; Fouassier, 1993), most investigations concerning dental resins have
 utilized CQ as a photosensitizer (Linden, 1993; Taira et al., 1988) in
 combination with a variety of reducing agents (Kubo, 1989; Nikaido, 1989;
 Kadoma and Imai, 1990; Yoshida and Greener, 1993). CQ is an alpha
 dicarbonyl that absorbs light having a wavelength of about 468 nm and,
 therefore, forms a very effective photoinitiator system when combined with
 an electron donor. However, CQ is inherently yellow, which causes problems
 in color matching. This, in turn, places practical limits on the
 concentration of CQ in a dental resin and, consequently, limits the degree
 of polymerization and depth of cure that can be attained. Efforts to
 improve the curing system have investigated the use of alternative
 photosensitizers (Peutzfeldt and Asmussen, 1992, 1992; Peutzfeldt, 1994,
 1994; Peutzfeldt and Asmussen, 1996, 1996), alternative amines (Cohen and
 Chao, 1968; Antonucci and Venz, 1987), and alternative curing devices
 (Tarle et al., 1995; Puppala et al. 1996). Peutzfeldt and Asmussen (1996,
 1996), seeking to increase crosslinking via additional components in the
 resin system, reported that diacetyl (2,3-butanedione) and propanal
 improved several properties in peroxide/amine initiated resins. Earlier,
 diacetyl had been reported to generate free radicals upon absorption of
 photons.
 More recently, Makinen and Makinen (1982) and Inano et al. (1983) reported
 that several types of enzymes are photoxidized and inactivated in
 broad-spectrum visible light in the presence of a variety of
 alpha-dicarbonyl compounds such as 2,3-butanedione and
 1-phenyl-1,2-propanedione. In visible light, 1-phenyl-1,2-propanedione
 inactivated enzymes more rapidly than the other diketones tested.
 Thus, there is a need in the art for new photoinitiator systems for forming
 photo-cured dental resins which photopolymerize with higher efficiency and
 with less yellowing.
 SUMMARY OF THE INVENTION
 The invention provides a photoinitiator system for use in forming
 photo-cured dental resins comprising a mixture of (a) a
 1-aryl-2-alkyl-1,2-ethanedione and (b) a rigid 1,2-dione in a weight ratio
 of (a):(b) in a range of about 1:20 to about 20:1.
 The invention also provides a photocurable dental composition comprising at
 least one photopolymerizable monomer and a photoinitiator system
 comprising a mixture of(a) a l-aryl-2-alkyl-1,2-ethanedione and (b) a
 rigid 1,2-dione in a weight ratio of (a):(b) in a range of about 1:20 to
 about 20:1, wherein the mixture is present in an amount sufficient to
 achieve a degree of double-bond conversion (DC) of at least 50%.
 The invention further provides a method of preparing resin systems suitable
 for use in restorative dentistry and other biomedical applications. The
 method includes combining at least one photopolymerizable monomer with a
 photoinitiator system comprising a mixture of (a)
 1-aryl-2-alkyl-1,2-ethanedione and (b) a rigid 1,2-dione in a weight ratio
 of (a):(b) in a range of 1:20 to about 20:1, wherein the mixture is
 present in an amount sufficient to achieve a degree of double-bond
 conversion of at least 50%.
 Further aspects and advantages of the invention may become apparent to
 those skilled in the art from a review of the following detailed
 description, taken in conjunction with the drawings, examples, and the
 appended claims. While the invention is susceptible of embodiments in
 various forms, described hereinafter are specific embodiments of the
 invention with the understanding that the present disclosure is intended
 as illustrative, and is not intended to limit the invention to the
 specific embodiments described herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 1-phenyl-1,2-propanedione (PPD) has an aromatic group on one side of the
 dicarbonyl and a methyl group on the other. The inventors have discovered
 that PPD is a photosensitizer suitable for use with dental resins, with
 similar or better efficiency than CQ. This was verified experimentally
 (Park and Rawls, 1998; Chae and Sun, 1998), and the inventors also found
 that PPD appeared to act synergistically with CQ to increase monomer
 conversion to polymer. A study was then carried out to explore the
 synergistic effect as a means of developing an improved
 photopolymerization system.
 The invention provides a photoinitiator system for use in forming
 photo-cured dental resins. The photoinitiator system includes a mixture of
 a 1-aryl-2-alkyl-1,2-ethanedione and a rigid 1,2-dione in a weight ratio
 of about 1:20 to about 20:1, preferably about 1:16 to about 16:1, more
 preferably about 1:8 to about 8:1.
 The invention also provides a photo-cured dental composition which includes
 at least one monomer photopolymerizable by the aforementioned
 photoinitiator system which is present in an amount sufficient to achieve
 a degree of double-bond conversion (DC) of at least 50%, preferably at
 least 55%, and more preferably at least 60%.
 In a preferred embodiment, the composition can also include a reducing
 agent, such as, for example, a dialkylarylamine, a tertiaryalkylamine, or
 a mixture thereof Suitable dialkylarylamines for use in the invention
 include, but are not limited to, N,N-cyanoethylmethylaniline (CEMA),
 N,N-dimethyl-paratoluidene, and mixtures thereof. Suitable
 tertiaryalkylamines for use in the invention include, but are not limited
 to, N,N-dimethylamino ethylmethacrylate, N,N-diethylamino
 ethylmethacrylate, and mixtures thereof.
 The invention further provides a method of preparing resin systems suitable
 for use in restorative dentistry and other biomedical applications. The
 method includes combining at least one photopolymerizable monomer with a
 photoinitiator system comprising a mixture of
 1-aryl-2-alkyl-1,2-ethanedione and a rigid 1,2-dione in a weight ratio of
 1:20 to about 20:1 in an amount sufficient to achieve a degree of
 double-bond conversion of at least 50%, preferably at least 55%, and more
 at least 60%.
 Suitable 1-aryl-2alkyl-1,2-ethanediones are characterized by compounds of
 formula:
EQU R.sub.n --R.sub.A --C(O)--C(O)--R.sup.1
 where R.sub.A is an aryl group such as phenyl, napthyl, anthracenyl, or the
 like; each R is the same or different and is a C.sub.1 to C.sub.20
 hydrocarbyl group including, without limitation, alkyl, aryl, alkaryl,
 aralkyl or cycloalkyl groups, and n is an integer equal to the number of
 substitutable sites on the given aryl group, e.g., for phenyl, n is 5; for
 napthyl, n is 7; R.sup.1 is a C.sub.1 to C.sub.12 alkyl group; and
 --C(O)--C(O) is an ethane dione group.
 Suitable rigid 1,2-diones include, but are not limited to,
 bicyclo-[2.2.1]-heptane-diones such as CQ.
 Suitable polymerizable monomers include, but are not limited to, vinyl
 esters, aromatic compounds and vinyl nitriles. Suitable vinyl esters
 include, but are not limited to, vinyl acetate and esters of acrylic acid
 having the structure CH.sub.2.dbd.CH--COOR, where R is a C.sub.1 to
 C.sub.20 alkyl, aryl, alkaryl, aralkyl or cycloalkyl group such as methyl
 acrylate, ethyl acrylate, and isopropyl acrylates and n-, iso- and
 tertiary-butyl acrylates. Other polymerizable monomers are described in
 U.S. Pat. Nos. 3,998,712; 4,038,350; 4,089,762; 4,352,853; 4,522,694;
 4,525,256; 4,777,190; 4,826,888; 5,472,991, and the respective disclosures
 of which are incorporated herein by reference.
 Preferably the photopolymerizable monomer is a diacrylate, a triacrylate,
 or a mixture thereof Suitable diacrylates for use in the invention
 include, but are not limited to,
 2,2-Bis-[4(2-hydroxy-3-mathacryloxy-propyloxy)phenyl]propane ("BisGMA"),
 2,2-Bis-[4-(2-ethoxy-3-mathacryloxy-propyloxy)phenyl]propane ("BisEMA"),
 urethane dimethacrylate ("UDMA"), triethyleneglycol dimethacrylate
 ("TEGDMA"), ethylene glycol dimethacrylate ("EGDMA"), and mixtures thereof
 Suitable triacrylates for use in the invention include, but are not
 limited to, trimethylolpropane triacrylate, trimethylolpropane
 trimethacrylate, and mixtures thereof.
 EXAMPLES
 The following examples are provided to illustrate the invention but are not
 intended to limit the scope of the invention.
 A BisGMA (30 mol %)-UDMA (40 mol %)-TEGDMA (30 mol %) monomer mixture was
 used for all resin formulations (Table 1). To make the materials
 light-curing, varying amounts of CQ and/or PPD were dissolved in the
 monomer mixtures along with N,N-cyanoethylmethylaniline as the reducing
 agent (0.2 wt. %/). All sample manipulations were carried out under
 filtered orange fight.
 TABLE I
 Function Compound Batch number Supplier
 Resin BisGMA 73425 Polyscience Inc. U.S.A.
 Monomer UDMA 721079 Ivoclar
 TEGDMA 295893 790 Fluka Chemical Corp.
 Photosensitizer CQ 06724AW Aldrich Chem. Co.
 PPD 07911BR Aldrich Chem. Co.
 Reducing CEMA 42407 Pfaltz & Bauer, Inc.
 Agent
 Seventeen groups, three specimens each, were tested in which PPD and CQ in
 the monomer system were each varied from 0.0 to 3.2 wt. % in the presence
 of the other, with the total CQ+PPD concentrations limited to 3.4 wt. %.
 Two factors were investigated for their effect on degree of conversion
 (DC): type of photosensitizer (CQ or PPD) and ratio of photosensitizers
 (PPD/CQ).
 The DC was determined using an FTIR spetrophotometer (Midac Series M, Midac
 Co., Costa Mesa, Calif.). A small amount of the formulated resin monomer
 was placed between two potassium bromide (KBr) disks (zero seconds cure
 time) and scanned in the FTIR in the transmission mode, with 20 scans at a
 resolution of 1 wavenumber (cm.sup.-1). After the IR spectral scan, the
 monomer mixture between the transparent KBr disks was irradiated for 20,
 40, 60, 120, 240, and 480 seconds with an Optilux model 401 visibile light
 curing unit (Demetron, Danbury, Conn.; I=800 mW/cm.sup.2). After each
 exposure time, the specimens were again scanned for their FTIR spectrum.
 The number of remaining double bonds was determined by a method described
 by Ruyter and Gyorosi (1976). Remaining unconverted double bonds were
 calculated by comparing the ratio of aliphatic C.dbd.C absorption at 1637
 cm.sup.-1 to the aromatic carbon-carbon stretching band at 1608 cm.sup.-1.
 The aromatic band remains constant during the polymerization reaction and
 serves as an internal standard. The DC was determined by subtracting the
 percentage of residual aliphatic C.dbd.C bond from 100%(FIG. 1).
 All experiments were carried out in triplicate, and the results were
 analyzed by analysis of variance followed by pairwise multiple comparisons
 using the Student-Newman-Keuls' multiple range comparison test, with
 p=0.05 as the level of significance.
 To identify the light absorption range of each photosensitizer, the spectra
 of CQ and PPD were recorded with a UV/Vis spectrophotometer (Beckman
 DU-600, Beckman Coulter, Inc., Fullerton Calif.). Each photosensitizer was
 dissolved to concentrations of 10 mM in hexane. These spectra were
 compared with the spectral output distribution of the Optilux model 401
 curing light used in the experiments (courtesy of Demetron Research Corp.,
 Danbury, Conn.).
 Comparison of the yellowness of resin samples containing combinations of
 the two photosensitizers was also undertaken. Resins with the same
 formulation used for degree of cure, were prepared with PPD/CQ (wt. %):
 0.0/1.8, 1.8/0.0, 3.2/0.2, and 0.213.2. Specimens were placed in a PTFE
 mold (1.2 mm thick, 15 mm diameter) between two glass slides and cured for
 60 seconds. Blind comparisons of the color of test specimens were carried
 out by three observers, having normal vision, by visual inspection in
 bright diffuse daylight. Samples were viewed against a white background
 for no longer than 2 seconds and rated according to the following scale;
 0=no difference, 1=little perceptible difference, and 2=clearly
 perceptible difference. The color perception test was quantified using a
 calorimetric comparison, and carried out utilizing specimens of the same
 size and PPD/CQ concentrations, and measured with a color difference meter
 (Tokyo Denshoku TC-6FX, Japan).
 FIG. 1 shows a characteristic FTIR spectrum of an experimental resin
 containing PPD photosensitizer (0.2 wt. % in monomer system) with 0.2 wt.
 % CEMA, before and after curing (8 minutes irradiation). By irradiation,
 the peak at 1637 cm.sup.-1 caused by carbon-carbon double bond stretching
 is considerably reduced, demonstrating polymerization.
 Tables II and III, below, contain the data for degree of conversion
 (+standard deviation) derived from these studies:
 TABLE II
 Groups
 (PPD/CQ
 by wt. % in
 monomer Curing Time (Seconds)
 system) 20 40 60 120 240
 480
 0.0/0.2 46.77(1.06) 50.25(3.42) 51.76(1.18) 55.96(2.63)
 55.53(0.54) 58.14(0.97)
 0.2/0.2 49.74(1.12) 54.38 66.84(2.04) 58.74(2.87)
 80.96(2.18) 60.81(2.98)
 0.4/0.2 52.10(1.79) 55.72(2.05) 57.99(1.24) 58.44(1.87)
 69.82(0.43) 61.24(0.94)
 0.8/0.2 52.1(1.05) 56.70(1.01) 58.69(0.85) 60.06(1.13)
 53.34(0.78) 63.81(1.47)
 1.6/0.2 54.70(0.85) 67.47(1.45) 58.84(1.35) 80.77(0.86)
 61.35(1.09) 63.68(1.02)
 3.2/0.2 57.85(1.03) 61.30(1.33) 52.96(0.77) 65.69(1.60)
 56.07(0.96) 68.67(0.70)
 0.2/0.0 41.28(5.92) 47.42(4.36) 48.54(2.40) 51.63(3.43)
 53.42(3.57) 55.72(3.36)
 0.2/0.2 49.74(1.12) 54.38(1.92) 55.94(2.04) 58.74(2.87)
 60.95(2.18) 60.61(2.98)
 0.2/0.4 52.66(1.78) 57.74(2.61) 69.21(2.17) 58.53(2.51)
 62.01(2.58) 62.37(2.38)
 0.2/0.8 55.57(0.58) 57.92(1.17) 60.01(0.71) 61.69(1.38)
 62.63(0.68) 63.24(2.07)
 0.2/1.6 57.83(1.03) 60.73(0.33) 62.45(0.84) 63.65(1.13)
 64.35(1.07) 65.02(0.89)
 02./3.2 54.59(1.38) 58.58(1.01) 59.52(0.82) 61.63(1.18)
 82.60(0.84) 63.28(0.65)
 0.1/0.1 41.77(2.70) 51.16(3.34) 53.80(1.00) 54.31(0.41)
 64.95(2.43) 57.34(0.86)
 0.0/0.2 46.77(1.06) 50.26(3.42) 51.76(1.18) 55.96(2.88)
 55.53(0.64) 58.14(0.97)
 0.2/0.0 41.28(5.92) 47.42(4.36) 48.54(2.40) 51.53(3.43)
 53.42(3.37) 55.72(3.38)
 0.2/0.2 49.74(1.12) 54.38(1.92) 55.94(2.04) 58.74(2.87)
 60.95(2.18) 60.81(2.98)
 0.0/0.4 51.33(0.86) 54.43(0.44) 55.92(1.07) 58.60(2.38)
 58.11(1.51) 58.71(1.30)
 0.4/0.0 50.48(1.03) 54.54(1.37) 56.15(1.48) 58.28(1.24)
 59.50(0.81) 60.53(0.85)
 0.0/1.8 53.13(1.14) 56.13(1.54) 58.29(0.89) 59.38(0.43)
 60.10(0.67) 61.43(0.45)
 1.8/0.0 50.96(0.97) 55.15(1.45) 68.03(2.23) 59.80(0.51)
 61.42(1.55) 61.45(0.82)
 1.6/0.2 54.70(0.85) 57.47(1.46) 59.84(1.35) 60.77(0.86)
 63.35(1.09) 63.66(1.02)
 0.2/1.6 57.83(1.04) 60.73(0.33) 62.45(0.94) 63.55(1.13)
 64.35(1.07) 65.02(0.89)
 0.9/0.9 50.92(0.52) 58.19(1.16) 61.48(0.60) 63.64(0.63)
 64.81(0.23) 65.34(0.45)
 TABLE III
 Groups DC conversion (%) at
 (PPD/CQ by wt. %) 2 minute-cure mean .+-. SD
 3.2/0.2 65.89 .+-. 1.60.sup.a
 1.6/0.2 60.77 .+-. 0.86.sup.b
 0.8/0.2 60.08 .+-. 1.13.sup.a
 0.2/0.2 58.74 .+-. 2.87.sup.b
 0.4/0.2 58.44 .+-. 1.87.sup.b
 0.0/0.2 55.96 .+-. 2.83.sup.b
 0.2/1.6 63.55 .+-. 1.13.sup.a
 0.2/0.8 61.69 .+-. 1.38.sup.a
 0.2/3.2 61.63 .+-. 1.18.sup.a
 0.2/0.4 59.53 .+-. 2.51.sup.a
 0.2/0.2 58.74 .+-. 2.87.sup.a
 0.2/0.0 51.53 .+-. 3.43.sup.b
 0.9/0.9 63.64 .+-. 0.63.sup.a
 0.2/1.6 63.55 .+-. 1.13.sup.a
 1.6/0.2 60.77 .+-. 0.86.sup.b
 1.8/0.0 59.80 .+-. 0.61.sup.b
 0.0/1.8 59.38 .+-. 0.43.sup.b
 0.2/0.2 58.74 .+-. 2.87.sup.a
 0.4/0.0 58.28 .+-. 1.24.sup.a
 0.0/0.4 56.60 .+-. 2.36.sup.a
 0.0/0.2 55.96 .+-. 2.83.sup.a
 0.1/0.1 54.31 .+-. 0.41.sup.a
 0.2/0.0 51.53 .+-. 3.43.sup.a
 *Within each subgrouping, mean values designated with the same supersript
 letter are not statistically different (p &lt; 0.05).
 FIG. 2 shows the DC as a function of curing time when the total
 photosensitizer concentration was (A) 0.2 wt. %, (3) 0.4 wt. %, and (C)
 1.8 wt. %. The DC for each photosensitizer, used either alone or in
 combination, is shown for each concentration. The DC increases with
 sensitizer concentration and levels out after about 60 seconds for all
 groups (p&gt;0.05). Also, at each total concentration, the DC of the
 combination is either equal (See FIGS. 2-A and 2-B) to or greater than the
 DC for either CQ or PPD alone (p&lt;0.05, see FIG. 2-C).
 In FIG. 3, the DC is shown as a function of photosensitizer concentration,
 with each photosensitizer alone (see FIG. 3-A) or in combination with the
 other (see FIG. 3-B), holding the concentration of CEMA constant. For both
 CQ and PPD, the DC increases with concentration and plateaus at about 0.4
 wt. % (see FIG. 3-A). CQ-containing resins tend to reach this plateau at
 somewhat lower concentrations than PPD-containing resins. This may be due
 to greater overlap of the CQ absorption band (380-510 nm) with the output
 of the curing lamp (see FIG. 5). PPD alone as photosensitizer induces a
 DC, at both short and long exposure times, which is not significantly
 different from CQ alone (see FIG. 3-A). The effect of combining two
 photosensitizers, CQ and PPD, in the presence of a set amount of amine
 reducing agent (0.2 wt. % CEMA) is shown in FIG. 3-B. Degree of cure
 increased rapidly with increasing concentration of the second
 photosensitizer and reached a plateau at about 60% conversion. This
 occurred at a ratio of approximately 4:1 (at total photosensitizer
 concentrations of 1.0 wt. %) for both PPD/CQ and CQ/PPD. At total
 concentrations above 1.5 wt. %, increasing PPD concentration increased DC,
 while increasing CQ concentration depressed DC. These trends were seen at
 both short (20 seconds) and long (2 minutes) exposure times.
 FIG. 2 appears to show synergism, with the degree of conversion of CQ+PPD
 generally exceeding that for the same concentration of either alone. This
 result appears as a trend at photosensitizer concentrations of 0.2 and 0.4
 wt. %, and is significant (p&lt;0.05) at 1.8 wt. % (see FIG. 2-C). The
 synergistic effect is illustrated in FIG. 4, which shows the DC at
 different curing times as a function of CQ concentration in the total
 photosensitizer mixture. When CQ reached 89%, the highest DC values were
 attained at every exposure time. If the groups containing only PPD are
 compared to those having only CQ as photosensitizer, when the curing time
 is short (20 and 40 seconds), CQ produces a higher DC than PPD. However,
 as the curing time increases beyond 60 seconds, PPD produces higher DC
 values than CQ (see FIGS. 2-B, 2-C, and 4).
 FIG. 5 shows the spectral radiant flux (W/cm.sup.2 /nm) emitted by the
 Demetron 80 Watt lamp. The filtered region, used for curing, is indicated
 and the visible absorption spectra of 10 in solutions of CQ and PPD in
 hexane are also shown.
 The color perception results demonstrated that samples with high PPD levels
 (PPD/CQ=1.8/0.0 and 3.2/0.2) have perceptibly less depth of yellowness
 (difference rating=2) than the samples with high CQ levels (PPD/CQ=0.0/1.8
 and 0.2,3.2), respectively. The colorimetric comparison confirmed the
 color perception results, demonstrating that substituting PPD for CQ
 results in approximately a 20% reduction in the Yellowness Index over a
 wide range of concentrations of photosensitizer. These results are shown
 in Table IV (below) which contains a quantitative color comparison of
 resins photocured with mixtures of CQ and PPD, and graphically illustrated
 in FIG. 6.
 TABLE IV
 PPD/CQ
 Concentration
 (wt/% in Yellowness
 Monomer System) Index L* a* b* .DELTA.E
 0.0/1.8 69.67 75.33 -8.83 32.76 4.9943
 (0.74) (0.22) (0.61) (0.51)
 1.8/0.0 56.68 76.71 -9.2 28.01
 (0.31) (0.07) (0.05) (0.17)
 0.2/3.2 98.75 75.38 -13.73 46.99 9.5727
 (0.23) (0.19) (0.11) (0.05)
 3.2/0.2 77.52 74.75 -12.95 37.47
 (0.10) (0.15) (0.18) (0.10)
 Hunter L*a*b* Yellowness Index (Y.I.)
 The results show that PPD serves as an efficient visible light
 photosentitizer comparable to camphorquinone for initiation of dental
 resin polymerization (see FIGS. 1-4). Moreover, PPD and CQ act
 synergistically as photosensitizers (see FIGS. 2-4). First, PPD and CQ
 probably have different mechanisms of free radical formation. While CQ
 operates predominantly by proton abstraction by ketone (analogous to
 benzil (Ar--C(O)--C--Ar)), PPD, in analogy with benzoin
 (Ar--C(O)--C(OH)--Ar), can undergo both photocleavage and proton
 abstraction. Second, the combination of PPD and CQ absorb more of the
 available photon energy, due to absorbing over slightly different
 wavelength ranges (see FIG. 5). The more efficient utilization of photon
 energy will either reduce the required curing time and/or decrease the
 remaining unreacted groups. This blend of photosensitizers may also
 produce a better balance between surface cure and bulk cure.
 An interesting effect is observed at the higher concentrations of CQ and
 PPD, when the amine reducing agent is kept constant FIG. 3). DC is
 increased at higher PPD concentrations but is decreased at higher CQ
 concentrations (20 seconds and two minutes exposure times). This may be
 related to the different mechanisms utilized by PPD and CQ. While CQ is
 most efficient when it is able to form an exciplex (excited complex) with
 an electron donator, PPD is most efficient forming free radicals by the
 photocleavage route, which is independent of amine concentration. Thus,
 when the surplus of CQ is too high, not all excited molecules will find an
 amine to form an exciplex and many will return unreacted to the ground
 state. The samples will remain yellow from the excess of CQ, the
 excitation light source will be more strongly attenuated and free radical
 formation will be reduced. Another mechanism available to PPD may be
 additional polymerization via its ability to undergo keto-enol tautomeric
 transformations. Diketones may exist in the enol form to an appreciable
 extent, and also can participate in the crosslinking reactions and
 increase the DC at higher PPD concentrations.
 From FIG. 5 it would be expected that reducing the CQ (Lambda.sub.max
 =about 410 nm) shifts the hue to a less yellow shade, as verified by both
 a visual color comparison and a colorimetric test. The combination will
 also contribute to a reduction in chroma (from deep yellow to a pale
 yellow) when the total photosensitizer PPD & CQ) concentration is held
 constant. Because CQ is yellow, a limit to its concentration is imposed by
 aesthetic considerations (Taira et al., 1988). Thus, using PPD in
 combination with CQ will expand the practical limit of photosensitizer
 concentration. For a given DC, lower total concentration of
 photosensitizer is required. At any concentration, DC will be increased
 but more of the light will be absorbed close to the surface (Guthrie et
 al., 1986; Gatechari and Tiefenthaler, 1989). Therefore, from the
 standpoint of esthetics and depth of cure in deep restorations, after
 finding the PPD/CQ ratio for maximum DC in film samples, evaluating the DC
 by changing the total photoinitiator concentration is needed. Further, by
 matching the spectral distribution of the visible light curing unit to the
 absorption spectrum of the combined photosensitizers, enhanced
 photopolymerization should be achieved efficiently (Cook, 1982).
 Further studies show that 1-phenyl-1,2-propanedione itself is a
 photosensitizer of potential value as a less off-color sensitizer for
 visible light cured dental resins. Moreover, there is a synergism effect
 between CQ and PPD. This feature may be taken advantage of by optimizing
 the PD/CQ system for maximum efficiency resulting in both aesthetic and
 functional property advantages. Further investigation of the
 structure/property relationship of PPD-like compounds, and their effect in
 combination with CQ and other potential sensitizers, has potential for
 further improvements in photoinitiator efficiency.
 The foregoing description is given for clearness of understanding only, and
 no unnecessary limitations should be understood therefrom, as
 modifications within the scope of the invention may be apparent to those
 having ordinary skill in the art.