Dent Mater 8:324-326, September,1992
Effect of phenolic compounds on the polymerization of methyl methacrylate S. Fujisawa 1, Y. Kadoma 2
IFaculty of Dentistry, ~Institute for Medical and Dental Engineering, Tokyo Medical and Dental University, Tokyo, Japan Abstract. The inhibitory effects of six phenolic compounds and two analogues on the polymerizationof methyl methacrylate (MMA) by azobisisobutyronitrile (AIBM) were examined. Differenbal scanning calorimetry was used to determine the induction period (IP) and initial rate of polymerization (IRP). The IP values (minutes) decreased in the order: eugenol> thymol> hydroquinone>cresol> guaiacol> phenol>>> safrole> methol. The IRP values of all compounds tested ranged from 0.8 to 1.2 percent per minute, except for hydroquinone (0.2 percent per minute). Phenolic compounds inhibited the polymerization of MMA by scavenging radicals, and eugenol was the most potent inhibitor. INTRODUCTION Eugenol is a phenol derivative that is widely used in combination with zinc oxide as a pulp capping agent, temporary cement, and root canal filling cement. Phenol, cresol, and thymol are sometimes used as dentin disinfectants and in endodontic therapy. Phenolic compounds react with free radicals, thereby inhibiting the polymerization of methacrylate monomers. Thus, these compounds are known to be incompatible with the resin-restorative system (Reisbick and Brodsky, 1971; Grajower et al., 1974; Lingard et al., 1981; Phillips, 1982; Marshallet al., 1982; Millstein and Nathanson, 1983). However, the kinetics of free-radical polymerization of methacrylate in the presence of phenolic compounds has not been sufficiently investigated. In the present study, the inhibitory effects ofsixphenolic compoundswith respect to the induction period and initial rate of polymerization for the methyl methacrylate (MMA)-AIBMsystem were evaluated by differential scanning calorimetry (DSC). Benzoyl peroxide (BPO) is widely used as an initiator of the resin system in dentistry. The composition of BPO is strongly influenced by its free radicals. The addition ofhydroquinone to BPO decreases the rate of decomposition,while additives of amines increases it. Also, BPO readily decomposes in the presence of phenol and cresol (Nozaki and Bartlett, 1946; Bartlett and Nozaki, 1947). BPO induces decompositionin the initiation of polymerization. Therefore, it is difficult to study the kinetics of the radical-scavenging effects of phenolic compounds on the free-radical polymerization of methacrylates. Therefore, for this investigation, the kinetics of the inhibitory effects were studied using the MMA-AIBM system. MATERIALS AND METHODS The compounds used in this study are listed in Table 1, along with their chemical structures. Eugenol was used after 324 Fullsawa & Kadoma/Effect of phenols on polymenzatlon
purification by high performance liquid chromatography. MMA (Tokyo Kasei Chemical Company, Tokyo, Japan) was purified by distillation and AIBM (Tokyo Kasei Chemical Company) was recrystallized from methanol. The experimental resins consisted of MMA, AIBM, and a phenolic additive. The concentrations of AIBM and each phenolic compound in MMA were 1.0 and 3.0 Mol %, respectively. About 10 ~L of the experimental resin was loaded into an aluminum sample container and then sealed by applying pressure. The container was placed in a differential scanning calorimeter (model DSC 3100, MAC Science Corporation, Tokyo, Japan), kept at 70°C, and the thermal changes due to polymerization were recorded for 4 h. The induction period (IP) and final conversion were calculated from the DSC thermograms. RESULTS/DISCUSSION Table 2 and Figs. 1 and 2 show the polymerization of MMA by AIBM in the presence of phenolic compounds and their analogues. The effect of the phenolic compounds (thymol, eugenol, phenol, p-cresol, guaiacol, and hydroquinone) on the induction period (IP) was greater than that of non-phenolic compounds (safrole and L-menthol). The presence of oxygen retards polymerization because oxygen as a diradical reacts with cyanoisopropyl radicals of AIBM (Schulz and Henrici, 1956). Thus, even polymerization in the control was slightly inhibited; even though the reaction was carried out in a sealed DSC pan, it contained a small amount ofoxygenbecause it was sealed in open air. As shown in Table 2, phenolic compounds inhibited the free-radical polymerization of MMA. The presence of such inhibitors markedly affects the IP. Fig. 1 shows the polymerization of MMA by AIBM with safrole, guaiacol, and eugenol. Polymerization of MMA appeared to be slightly inhibited by safrole and was markedly inhibited by guaiacol and by eugenol. The IP values for safrole, guaiacol, and eugenol were approximately 7, 57, and 83 min, respectively (Table 2). The difference in IP value between eugenol and guaiacol appears to be due to the presence of an allyl group (CH2CH:CH~) in eugenol (Table 1), since radicals produced in the allyl group have a degradative chain-transfer action (Laible, 1958). As was demonstrated by the high IP value for eugenol in comparison with that for safrole, a phenolic hydroxy group is essential for scavenging AIBM free radicals (Table 1). As Fig. 2 shows, the polymerization of MMA by AIBM was inhibited by L-menthol, phenol, th~znol, and hydroquinone
100Ir
TABLE 1: CHEMICALSTRUCTUREOF THE PHENOLICCOMPOUNDS
AND THEIRANALOGUES Compound
Manufacturer/Location ChemicalStructure
Eugenol
Kobayashl KoryoCo Tokyo, Japan
(OH)OCH3[1,3] 06H3-< CH2CH.CH2[4]
Guamcol
Kanto ChemtcalCo Tokyo, Japan
C6H4(OH)(OCH3)[12]
Safrole
Wako Pure Chemical Co CH2CHCH211] Tokyo, Japan 06H3"< 02CH213,4]
Phenol
Koso Chemtcal Co Tokyo, Japan
p-Cresol
Wako Pure Chemical Co CH3CsH4OH
Thymol
Da,chl Pure Chemical Co (OH)(CH3)[1,3] Tokyo, Japan CeH3-< CH(CHa)2[6]
L-Menthol
8o[
/
i
Wako Pure Chemical Co
,,/"
s
//
//
G
,/J'f/'~'~'-- E
/
/
/
0
/
/
30
60
90
120 Time,in)
150
180
210
240
Fig. 1. Polymerization of MMA by AIBM wtth safrole (S}, guaiacol (G), and eugenol (E), control (N) had no additive.
10(
....... M ---"-:Z_Z . . . .
8(
~H2-- ~H2 CH3-CH
j"
,,,""
C6HsOH
.............
N///
~
...... -N....... ~ . / ~ . ~
p
c
CH-CH(CHa) 2
I
CH2 - CHOH Hydroqumone
DaichlPure Chemical
Co C6H4(OH)2[para]
°
/
/
TABLE 2: POLYMERIZATIONOF METHYLMETHACRYLATEBY AIBM
AT 70°C IN THE PRESENCEOF PHENOLICCOMPOUNDSAND THEIR ANALOGUES Inducbon Penod (IP) (mm)
InMtlalRateof Polymenzatlon(IRP) (% mm)
Control
36
1 09
Thymol
67 0
1 07
Eugenol
82 6
0 84
Phenol
47 6
0 95
Saffole
6B
1.10
L-Methol
37
1 23
Hydroqumone
64 3
0 21
Guamcol
57 4
0 95
p-Cresol
62 6
1 00
Compound
Computatmnal errors for IP and IRP < ca 5%
and also in the control. The IP value for L-menthol was similar to the control value (Table 2), which indicates that L-menthol without a phenolic hydroxy group did not act as an inhibitor. As Figs. 1 and 2 show, after induction, the polymerization of the test compounds proceeds in a similar manner except for hydroquinone. Hydroquinone has been reported to retard the polymerization of MMA by AIBM (Bevington and Ghanem, 1958). The findings in this study also showed that hydroquinone inhibits the MMA-AIBM polymerization system. It is common commercial practice to add a small amount (approximately 0.006 percent) of hydroquinone to the methacrylate monomer to act as an inhibitor during storage.
.
30
60
90
.
.
120 150 Time(rain}
.
180
210
240
Fig. 2. Polymerization of MMA by AIBM with L-menthol (M), phenol (P), p-cresol (C), thymol (T), and hydroquinone (H), control (N) had no phenol additive.
In summary, the inhibitory effects of the monohydmc phenols tested were as follows: eugenol> thymol> cresol> guaiacol> phenol. The larger inhibitory effect of eugenol seems to be due to both its phenolic hydroxy group and allyl group. This kinetic study should help to determine the inhibitory effects of various compounds on the free-radical polymerization of methacrylate. ReceivedDecember12, 1991/AcceptedAugust 11, 1992 Address correspondenceand reprint requests to S Fuj]sawa Facultyof Dentistry TokyoMedmaland Dental Umvers]ty 1-5-45Yush]maBunkyo-Ku Tokyo,Japan REFERENCES
Bartlett PD, Nozaki K (1947). The decomposition of benzoyl peroxide in solvents (II). Ethers, alcohols, phenols and amines. J A m Chem Soc 69:2299-2306. Bevington JD, Ghanem NA (1958). The mechanism of retardation and inhibition in radical polymerizations (IV). Effects of diphenylpicrylhydrazine and a condensation product of acetone with phenylhydroxyamine on the sensitized polymerization of styrene. J Chem Soc 1958:2254-2259.
Dental Matenals/September 1992 325
Grajower R, Hirschfeld Z, Zalkind M (1974). Compatibility of a composite resin with pulp-insulating materials. A scanning electron microscope study. J Prosthet Dent 32:70-77. Laible RC (1958). Allyl polymerizations. Chem Revs 58:807843. Lingard GL, Davies EH, von Fraunhofer A (1981). The interaction between lining materials and compositeresin restorative materials. J Oral Rehab 8:121-129. Marshall SJ, Marshall GW, HarcourtJK(1982). The influence of various cavity bases on the micro-hardness of composites. Aust Dent J 27:291-295. Millstein PL, Nathanson D (1983). Effect of eugenol and
326 Fujisawa & Kadoma/Effect of phenols on polymenzatlon
eugenol cements on cured composite resin. JProsthet Dent 50:211-215. Nozaki K, Bartlett PD (1946). The kinetics of decompositionof benzoyl peroxide in solvent (I). J A m Chem Soc 68:16861692. Phillips RW (1982). Skinner's science of dental materials, ed 8. Philadelphia: Saunders Co., 166. Reisbick MH, Brodsky JF (1971). Strength parameters of composite resin. J Prosthet Dent 26:178-185. Schulz GV, Henrici G (1956). Reaction kinetics ofpolymerizationinhibitionby molecularoxygen(Experiments on methyl methacrylate). Makromol Chem 18/19:437-454.
Effect of phenolic compounds on the polymerization of methyl methacrylate S. Fujisawa 1, Y. Kadoma 2
IFaculty of Dentistry, ~Institute for Medical and Dental Engineering, Tokyo Medical and Dental University, Tokyo, Japan Abstract. The inhibitory effects of six phenolic compounds and two analogues on the polymerizationof methyl methacrylate (MMA) by azobisisobutyronitrile (AIBM) were examined. Differenbal scanning calorimetry was used to determine the induction period (IP) and initial rate of polymerization (IRP). The IP values (minutes) decreased in the order: eugenol> thymol> hydroquinone>cresol> guaiacol> phenol>>> safrole> methol. The IRP values of all compounds tested ranged from 0.8 to 1.2 percent per minute, except for hydroquinone (0.2 percent per minute). Phenolic compounds inhibited the polymerization of MMA by scavenging radicals, and eugenol was the most potent inhibitor. INTRODUCTION Eugenol is a phenol derivative that is widely used in combination with zinc oxide as a pulp capping agent, temporary cement, and root canal filling cement. Phenol, cresol, and thymol are sometimes used as dentin disinfectants and in endodontic therapy. Phenolic compounds react with free radicals, thereby inhibiting the polymerization of methacrylate monomers. Thus, these compounds are known to be incompatible with the resin-restorative system (Reisbick and Brodsky, 1971; Grajower et al., 1974; Lingard et al., 1981; Phillips, 1982; Marshallet al., 1982; Millstein and Nathanson, 1983). However, the kinetics of free-radical polymerization of methacrylate in the presence of phenolic compounds has not been sufficiently investigated. In the present study, the inhibitory effects ofsixphenolic compoundswith respect to the induction period and initial rate of polymerization for the methyl methacrylate (MMA)-AIBMsystem were evaluated by differential scanning calorimetry (DSC). Benzoyl peroxide (BPO) is widely used as an initiator of the resin system in dentistry. The composition of BPO is strongly influenced by its free radicals. The addition ofhydroquinone to BPO decreases the rate of decomposition,while additives of amines increases it. Also, BPO readily decomposes in the presence of phenol and cresol (Nozaki and Bartlett, 1946; Bartlett and Nozaki, 1947). BPO induces decompositionin the initiation of polymerization. Therefore, it is difficult to study the kinetics of the radical-scavenging effects of phenolic compounds on the free-radical polymerization of methacrylates. Therefore, for this investigation, the kinetics of the inhibitory effects were studied using the MMA-AIBM system. MATERIALS AND METHODS The compounds used in this study are listed in Table 1, along with their chemical structures. Eugenol was used after 324 Fullsawa & Kadoma/Effect of phenols on polymenzatlon
purification by high performance liquid chromatography. MMA (Tokyo Kasei Chemical Company, Tokyo, Japan) was purified by distillation and AIBM (Tokyo Kasei Chemical Company) was recrystallized from methanol. The experimental resins consisted of MMA, AIBM, and a phenolic additive. The concentrations of AIBM and each phenolic compound in MMA were 1.0 and 3.0 Mol %, respectively. About 10 ~L of the experimental resin was loaded into an aluminum sample container and then sealed by applying pressure. The container was placed in a differential scanning calorimeter (model DSC 3100, MAC Science Corporation, Tokyo, Japan), kept at 70°C, and the thermal changes due to polymerization were recorded for 4 h. The induction period (IP) and final conversion were calculated from the DSC thermograms. RESULTS/DISCUSSION Table 2 and Figs. 1 and 2 show the polymerization of MMA by AIBM in the presence of phenolic compounds and their analogues. The effect of the phenolic compounds (thymol, eugenol, phenol, p-cresol, guaiacol, and hydroquinone) on the induction period (IP) was greater than that of non-phenolic compounds (safrole and L-menthol). The presence of oxygen retards polymerization because oxygen as a diradical reacts with cyanoisopropyl radicals of AIBM (Schulz and Henrici, 1956). Thus, even polymerization in the control was slightly inhibited; even though the reaction was carried out in a sealed DSC pan, it contained a small amount ofoxygenbecause it was sealed in open air. As shown in Table 2, phenolic compounds inhibited the free-radical polymerization of MMA. The presence of such inhibitors markedly affects the IP. Fig. 1 shows the polymerization of MMA by AIBM with safrole, guaiacol, and eugenol. Polymerization of MMA appeared to be slightly inhibited by safrole and was markedly inhibited by guaiacol and by eugenol. The IP values for safrole, guaiacol, and eugenol were approximately 7, 57, and 83 min, respectively (Table 2). The difference in IP value between eugenol and guaiacol appears to be due to the presence of an allyl group (CH2CH:CH~) in eugenol (Table 1), since radicals produced in the allyl group have a degradative chain-transfer action (Laible, 1958). As was demonstrated by the high IP value for eugenol in comparison with that for safrole, a phenolic hydroxy group is essential for scavenging AIBM free radicals (Table 1). As Fig. 2 shows, the polymerization of MMA by AIBM was inhibited by L-menthol, phenol, th~znol, and hydroquinone
100Ir
TABLE 1: CHEMICALSTRUCTUREOF THE PHENOLICCOMPOUNDS
AND THEIRANALOGUES Compound
Manufacturer/Location ChemicalStructure
Eugenol
Kobayashl KoryoCo Tokyo, Japan
(OH)OCH3[1,3] 06H3-< CH2CH.CH2[4]
Guamcol
Kanto ChemtcalCo Tokyo, Japan
C6H4(OH)(OCH3)[12]
Safrole
Wako Pure Chemical Co CH2CHCH211] Tokyo, Japan 06H3"< 02CH213,4]
Phenol
Koso Chemtcal Co Tokyo, Japan
p-Cresol
Wako Pure Chemical Co CH3CsH4OH
Thymol
Da,chl Pure Chemical Co (OH)(CH3)[1,3] Tokyo, Japan CeH3-< CH(CHa)2[6]
L-Menthol
8o[
/
i
Wako Pure Chemical Co
,,/"
s
//
//
G
,/J'f/'~'~'-- E
/
/
/
0
/
/
30
60
90
120 Time,in)
150
180
210
240
Fig. 1. Polymerization of MMA by AIBM wtth safrole (S}, guaiacol (G), and eugenol (E), control (N) had no additive.
10(
....... M ---"-:Z_Z . . . .
8(
~H2-- ~H2 CH3-CH
j"
,,,""
C6HsOH
.............
N///
~
...... -N....... ~ . / ~ . ~
p
c
CH-CH(CHa) 2
I
CH2 - CHOH Hydroqumone
DaichlPure Chemical
Co C6H4(OH)2[para]
°
/
/
TABLE 2: POLYMERIZATIONOF METHYLMETHACRYLATEBY AIBM
AT 70°C IN THE PRESENCEOF PHENOLICCOMPOUNDSAND THEIR ANALOGUES Inducbon Penod (IP) (mm)
InMtlalRateof Polymenzatlon(IRP) (% mm)
Control
36
1 09
Thymol
67 0
1 07
Eugenol
82 6
0 84
Phenol
47 6
0 95
Saffole
6B
1.10
L-Methol
37
1 23
Hydroqumone
64 3
0 21
Guamcol
57 4
0 95
p-Cresol
62 6
1 00
Compound
Computatmnal errors for IP and IRP < ca 5%
and also in the control. The IP value for L-menthol was similar to the control value (Table 2), which indicates that L-menthol without a phenolic hydroxy group did not act as an inhibitor. As Figs. 1 and 2 show, after induction, the polymerization of the test compounds proceeds in a similar manner except for hydroquinone. Hydroquinone has been reported to retard the polymerization of MMA by AIBM (Bevington and Ghanem, 1958). The findings in this study also showed that hydroquinone inhibits the MMA-AIBM polymerization system. It is common commercial practice to add a small amount (approximately 0.006 percent) of hydroquinone to the methacrylate monomer to act as an inhibitor during storage.
.
30
60
90
.
.
120 150 Time(rain}
.
180
210
240
Fig. 2. Polymerization of MMA by AIBM with L-menthol (M), phenol (P), p-cresol (C), thymol (T), and hydroquinone (H), control (N) had no phenol additive.
In summary, the inhibitory effects of the monohydmc phenols tested were as follows: eugenol> thymol> cresol> guaiacol> phenol. The larger inhibitory effect of eugenol seems to be due to both its phenolic hydroxy group and allyl group. This kinetic study should help to determine the inhibitory effects of various compounds on the free-radical polymerization of methacrylate. ReceivedDecember12, 1991/AcceptedAugust 11, 1992 Address correspondenceand reprint requests to S Fuj]sawa Facultyof Dentistry TokyoMedmaland Dental Umvers]ty 1-5-45Yush]maBunkyo-Ku Tokyo,Japan REFERENCES
Bartlett PD, Nozaki K (1947). The decomposition of benzoyl peroxide in solvents (II). Ethers, alcohols, phenols and amines. J A m Chem Soc 69:2299-2306. Bevington JD, Ghanem NA (1958). The mechanism of retardation and inhibition in radical polymerizations (IV). Effects of diphenylpicrylhydrazine and a condensation product of acetone with phenylhydroxyamine on the sensitized polymerization of styrene. J Chem Soc 1958:2254-2259.
Dental Matenals/September 1992 325
Grajower R, Hirschfeld Z, Zalkind M (1974). Compatibility of a composite resin with pulp-insulating materials. A scanning electron microscope study. J Prosthet Dent 32:70-77. Laible RC (1958). Allyl polymerizations. Chem Revs 58:807843. Lingard GL, Davies EH, von Fraunhofer A (1981). The interaction between lining materials and compositeresin restorative materials. J Oral Rehab 8:121-129. Marshall SJ, Marshall GW, HarcourtJK(1982). The influence of various cavity bases on the micro-hardness of composites. Aust Dent J 27:291-295. Millstein PL, Nathanson D (1983). Effect of eugenol and
326 Fujisawa & Kadoma/Effect of phenols on polymenzatlon
eugenol cements on cured composite resin. JProsthet Dent 50:211-215. Nozaki K, Bartlett PD (1946). The kinetics of decompositionof benzoyl peroxide in solvent (I). J A m Chem Soc 68:16861692. Phillips RW (1982). Skinner's science of dental materials, ed 8. Philadelphia: Saunders Co., 166. Reisbick MH, Brodsky JF (1971). Strength parameters of composite resin. J Prosthet Dent 26:178-185. Schulz GV, Henrici G (1956). Reaction kinetics ofpolymerizationinhibitionby molecularoxygen(Experiments on methyl methacrylate). Makromol Chem 18/19:437-454.
