polymerization of multifunctional methacrylates and acrylates

Polymerization of multifunctional methacrylates and acrylates

Koji Miyazaki and Takashi Horibe Department of Dental Materials and Deuices, Fzikuoko Dental College, 700 Ta, Sawara-ku, Fukuoka 814-01, japan

The cross-linking reaction of 15 dimethacrylates, one trimethacrylate and five diacrylates was studied by means of differential scanning calorimetry (DSC) and high performance liquid chromatography (HPLC) to investigate the relationship between the polymerization characteristics and the chemical structure of these monomers. The amount of pendant double bonds (Dp) and the efficiency of cross-linking (Ec) were calculated from both the extent of polymerization (Ep) and the amount of residual monomer (Rm) obtained by DSC and HPLC analyses, respectively. The Ep and Ec values of various monomers increased with an increase in the number of chain members between the functional groups, while Rm and Dp values decreased.

The dimethacrylates consisting of aliphatic chains polymerized more readily than those containing aromatic units. High Ep was obtained in the aliphatic dimethacrylates and diacrylates with more than eight chain members and the aromatic dimethacrylates with more than 20 chain members among the monomers examined. These results depend presumably on the flexibility of the functional groups in the polymer network. With most dimethacrylates and diacrylates, high Ep was also observed in thermal scanning polymerization compared to isothermal polymerization at 90°C. The diacrylates showed a high rate of polymerization compared to the corresponding dimethacrylates.

INTRODUCTION

The chemical composition and the chemical structure of a monomer in a dental restorative resin depend upon the physical and chemical properties of the cured products. Therefore, a number of multifunctional methacrylates have been synthesized to find an improved monomer system for dental restorative However, there are many difficulties in the design of a monomer structure and the development of the improved monomer system, since the relationship between the chemical structure of multifunctional monomer and the polymerization mechanism is not well understood. In copolymerization of vinyl monomer with a multifunctional one, the number of cross-linkages produced is not equivalent to the number of multifunctional molecules employed. In recent years, the chemical ana lys iP and the polymer i~a t ion~,~ ,~ of commercial composite materials have been studied to reveal the relationship between the chemical composition and polymerization. We have also studied homopolymeri~ation~~'~ and copolymeri-

Journal of Biomedical Materials Research, Vol. 22, 1011-1022 (1988) 0 1988 John Wiley & Sons, Inc. CCC 0021-93041881111011-12$04.00

1012 MIYAZAKT AND HORIBE

zation" of various dimethacrylates by means of differential scanning calorimetry (DSC) and high performance liquid chromatography (HPLC). The purpose of this study is to find an improved monomer system for dental restorative resins. In the present article, the relationship between the chemical structure of multifunctional methacrylates and acrylates and the polymerization characteristics, e.g., the extent of polymerization (Ep), the amount of residual monomer (Rm) and pendant double-bond (Dp), and the efficiency of cross-linking (Ec), is described.

MATERIALS AND METHODS

The abbreviation and commercial sources of the 22 multifunctional monomers are shown in Table I. All the monomers were used as received. As an initiator benzoyl peroxide (BPO) was mixed with the monomers at 0.005 molar fraction against one of the vinyl double bond of monomers employed. In use of viscous monomers, such as bis-GMA and urethane dimethacrylate (UDMA), air bubbles formed by mixing with BPO powder were removed under the reduced pressure.

TG method

A thermo-balance (Model TG-30, Shimazu, Kyoto, Japan) was used to observe a weight change of the monomers during polymerization. The monomer (about 15 mg) was placed in the DSC aluminum pan (64 X

1.5 mm), carefully weighed, and quickly transferred to the sample holder in the apparatus. Thermogravimetric analysis was conducted from room temperature to 200°C under nitrogen atmosphere. TG analysis was made twice for each monomer investigated.

DSC method

A differential scanning calorimeter (Model DSC-30, Shimazu, Kyoto, Japan) was used to measure the heat of polymerization of monomers tested. The monomer (about 15 mg) was placed in the DSC aluminum pan, carefully weighed, and quickly transferred to the sample holder in the apparatus. For a reference, the corresponding monomer of almost the same weight was previously polymerized by thermal scanning mode from room temperature to 200°C under nitrogen atmosphere and then placed on the reference holder. A stream of nitrogen (100 mL/min) was passed through the instru- ment for a period of 5 min before each run to stabilize the apparatus. The polymerization was carried out either at a constant temperature (90°C) or by thermal scanning (lO"C/min) from room temperature to 200°C under nitrogen atmosphere at a flow rate of 50 mL/min (DSC curve-1). The DSC analysis was immediately repeated on the polymerized sample used (DSC

TABLE I Investigated Materials

Name Abbreviation Source

Ethylene glycol dimethacrylate Diethylene glycol dimethacrylate Triethylene glycol dimethacrylate Tetraethylene glycol dimethacrylate Nonaethylene glycol dimethacrylate Neopentyl glycol dimethacrylate 1,3-buthylene glycol dimethacrylate 1,&hexame thylene glycol dimethacryla te Urethane dimethacrylate* 2,2 bis[.l-methacryloxy phenyllpropane 2,2 bis[4-methacryloxyethoxy phenyl]

2,2 bis[4(3-methacryloxy 2-hydroxy

2,2 bis[methacryloxy polyethoxy



Trimethylol propane himethacrylate*** Neoprntyl glycol diacrylate l,&hexamethylene glycol diacrylate Polyethylene glycol diacrylate*+'* Polyethylene glycol diacrylate**** Polyethylene glycol diacrylate****

propane

propoxy)phenyl] propane

phenyllpropane'"

phen yl]propane'+

phenyl]propane**

EGDMA DEGDMA TrEGDMA TeEGDMA NEGDMA NPGDMA BGDMA HGDMA UDMA BPDMA Bis-MEPP

Bis-GMA

TMPTMA NPGDA HGDA A-200 A-400 A-600

1 1 1 1 1 1 1 1 2 3 4

2

1

1

1

Sources; 1: Shin Nakamura Chem. Co. (Wakayama, Japan), 2: Kulzer (West Germany),

*Dimethacryloxyethyl trimethylhexamethylene diurethane. **The fractions of the Bis-MP26EPP (Shin Nakamura Chem. Co.) were isolated by

*'*Trimethacryla te. ****A-200 (n = 4), A-400 (n = 9), A-600 (n = 14).

3: Tokyo Kasei (Tokyo, Japan), 4: Ivoclar (Liechtenstein).

means of HPLC.

curve-2). The polymerization was conducted in an open aluminum pan since the use of a sealed pan gave complicated results.' The heat of polymerization (AH) was determined from the area between the two DSC curves (see Fig. 1) and was corrected by the heat of fusion of standard indium (3.265 + 0.002 Kcal/mol) in DSC and DTA analyses. The Ep was defined by the following equation,' where AH and AM are the heat of polymerization and the number of moles of monomers tested, and Hm is the heat of polymerization of methyl methacrylate (13.10 Kcal/mol). l2 The AM value is mul- tiplied by the number of 2 or 3 corresponding to the monomers with bi- or trifunctional groups, respectively.

AH

All E , values are the average of two separate analyses. The E , values of five diacrylates were calculated by use of the average value of the heat of poly-

1014 MIYAZAKI AND HORIBE

I

I

I

0 5 10 15 20

Time ( m i n )

Figure 1. A typical DSC curve exemplified by bis-MP,,, EPP. The monomer of about 15 mg containing BPO as initiator was polymerized by isothermal mode at 90°C under nitrogen atmosphere.

merization of three monoacrylates, i.e., methyl acrylate, ethyl acrylate, and butyl acrylate. These values of three monoacrylates are represented in Table 11.

HPLC method

A Waters HPLC system (Waters associates Inc, MA, USA) equipped with a high pressure pump (Model 6000), an autosampler (Model 710B) and a data module (Model 730) was used for quantitative determination of the amount of residual monomers in the cured products. A column (4.64 X 125 mm) was packed with 5 pm octadecyl silane-coated particles (Fine-sil ODs-5) by the slurry method for reverse-phase HPLC. Elution was performed by isocratic elution of 70-80% methanol at ambient temperature at a flow rate of 0.7- 1.0 mL/min and the effluent was monitored by a UV detector (Model 440) at 214 nm. The cured products obtained at isothermal conditions or by thermal scanning polymerization in the DSC were ground into a size of about 0.1 mm in diameter, and immersed in methanol with dibutyl phthalate (DBP: 0.05-

TABLE I1 The Heat of Polymerization of Acwlates

Materials H (Kcal/rnol)

Methyl acrylate Ethyl acrylate Butyl acrylate

Average

18.09 17.49 20.03 18.54

Polymerization was carried out under the isothermal mode (90°C) in the presence of benzoyl peroxide (0.5 wt%) as initiator.

POLYMERIZATION OF MULTIFUNCTIONAL MONOMERS 1015

0.3 mg/mL) as an internal standard. The methanol solution was stored in an air bath at 37°C for 7 days. The extracts were diluted about 3-4-fold with methanol, and were analyzed by HPLC. The amount of residual monomer (R,) in the cured product was calculated from the following equation, where S, is the sample weight (mg), RA and X a are the ratios of the peak area of monomer to DBP in a sample and the standard, respectively. C, is the concentration of DBP as an internal standard (mg/mL).

R A X Cs Ra x S, l<,(%) =

All R, values are the average of four separate analyses. D, and €' were obtained by the following equations.

D,(%) = 100 - (E, + X,)

E , (%) = E, - D, In this study, the efficiency of cross-linking is expressed as a percentage of

saturated vinyl double-bonds against whole vinyl double-bonds in the monomer examined.

RESULTS

Representative thermogravimetric curves of some of the investigated monomers are shown in Figure 2. A weight change was clearly observed around the point where polymerization was initiated which increased with a decrease in molecular weight of the monomer. However, this change did not have a major impact on the measurement of the heat of polymerization by DSC because the weight change of all monomers examined was quite small. It was 3.8% even in EGDMA which gave the largest weight change in monomers analyzed. In addition, the heat of vaporization of various organic compounds is generally lower than the heat of Polymerization of various methacrylates and acrylate rn~nomers . '~

E,, R,, D,, and E , values of 21 di- or trifunctional monomers are given in Tables I11 and 1V. There are no data of D, and E , for BPDMA, a unique solid type monomer, in Table IV. These two values could not be obtained from the calculation because the sum of E, and R, was less than 50%. This indicates that E , of BPDMA is considerably influenced by the endothermic fusion at the period of polymerization initiation, because the melting point of BPDMA (74°C) is close to the temperature of polymerization initiation. The results in Tables 111 and IV show that the large variation of these values depends on the chemical structure of monomers and polymerization conditions. E , values are widely distributed from 34.8 to 97.5%. The dimethacrylates and diacrylates containing aliphatic long chains, TrEGDMA, TeEGDMA, NEGDMA, UDMA, A-200, A-400, and A-600, showed high E , values while low E , values were observed with the monomers containing aromatic medium long chains, BPDMA, bis-MEPP, and bis-GMA among the monomers


TABLE I11 Extent of Polymerization (&), Amount of Residual Monomer ( R d , Pendant Double Bond (LIP), and Efficiency of Cross-Linking (€J.

EGDMA DEGDMA TrEGDMA TeEGDMA NEGDMA NPGDMA BGDMA HGDMA UDMA BPDMA Bis-MEPP B i s - G M A B ~ s - M P ~ , ~ EPP Bis-MP1,3 EPP B ~ S - M P ~ , ~ EPP TMPTMA NPGDA HGDA A-200 A-400 A-600

61.7* 77.0' 82.4' 86.8* 96.8' 60.1* 62.3* 76.8' 83.3* 34.8 54.8 44.7* 65.1 72.6 74.2 42.3 62.3 75.4 88.0 94.1 96.7

8.0 6.0 2.4

<0.1 0.3

10.6 11.5 3.6 3.8

33.9 15.2 20.5 5.2 2.5 4.4

17.2 5.5 1.1

<0.1 <0.1 <0.1

30.3 17.0 15.2 13.2 2.9

29.3 26.2 19.6 12.9 31.3 30.0 34.8 29.7 25.3 21.4 40.5 32.2 23.5 12.0 5.9 3.3

31.4 60.0 67.2 73.6 93.Y 30.8 36.1 57.2 70.4 3.5

24.8 9.9

35.4 47.3 52.8

1.8 30.1 51.9 76.0 88.2 93.4

Polymerization was isothermally carried out at 90°C under nitrogen atmosphere. Ini- tiator: Benzoyl peroxide (0.005 molar fraction against one of the vinyl double bond in monomer). E, and R, values were obtained by the analyses on DSC and HPLC, respectively, and D, and €, were elucidated by the calculation using the €, and R,,, values (see text for detail).

*Data of previous report.'"

examined. E , values of cured products were similar to the tendency of the 15, values. On the other hand, the R, and D, values inversely correlated with the E, and E, values. High X, and D, values were observed with the monomers containing aromatic medium long chains, BPDMA, bis-MEPP, and bis-GMA. The X, values for all the cured products obtained by polymerization during thermal scanning were much smaller than those obtained by the isothermal polymerization at 90°C. The R, values of monomers except those of BPDMA and bis-GMA were less than 0.5%. In both isothermal and thermal scanning polymerization modes, the D, values of all monomers examined were larger than the R, values. It was found that there were high D, values in all monomers tested even at the thermal scanning polymerization. The highest D, value was 41.7% for TMPTMA, although its ability of polymerization was intermediate between those of BPDMA and bis-MEPP or bis-GMA. The high ability of polymerization was observed in the five diacrylates among the multifunctional monomers employed.


HGDMA (0.8)

NPGDMA (2.0)

i

EGDMA (3.81

50 100 150 200

Figure 2. Typical thermogravimetric curves. The weight change of monomers (sample weight: 15 mg) was measured by a thcrmobalance during the polymerization by thermal scanning from room temperature to 200°C under nitrogen atmosphere. Numbers in parentheses indicate the weight change in percentage.

Ternperature("C 1

TABLE IV Extent of Polymerization (Ep), Amount of Residual Monomer (Rm),

Pendant Double Bond (Dp), and Efficiency of Cross-linking (Ec)

Materials EP (%I Rm (%) Dp (70) Ec (70 )

EGDMA DEGDMA TrEGDMA TeEGDMA NPGDMA BGDMA HGDMA UDMA BPDMA Bis-MEPP Bis-GMA Bis-MP1,2EPP Bis-MP1,3EPP Bis-MP2,,EI'P TMPTMA NPGDA HGDA

71.0 82.1 91.3 93.5 76.6 74.5 78.7 97.5 45.9 68.9 80.0 92.8 94.5 89.5 58.3 81.3 87.1

0.1 0.3 0.3 0.2 0.2 0.3 0.3 0.1 1.9 0.2 4.5

<0.1 0.1 0.2

<0.1 <0.1 <0.1

28.9 17.6 8.4 6.3

23.2 25.2 21.0 2.4

30.9 14.7 7.2 5.4

10.3 41.7 18.7 12.9

-

42.1 64.5 82.9 87.2 53.4 49.3 57.7 95.1

38.0 66.1 85.6 89.1 79.2 16.6 62.6 74.2

-

The polymerizations were carried out at temperature scanning from room temperature to 200°C under nitrogen atmosphere (see TABLE 111 for other conditions).

1018

DISCUSSION

MIYAZAKI AND HORIBE

Various dimethacrylates have been widely used as a component of dental restorative materials. In recent years, a denture base resin consisting of a polyfunctional monomer, e.g., urethane dimethacrylate has also been developed. The reason for using the multifunctional monomers seems to be that they produce a strong and tough resin matrix. Dental composites consisting of polyfunctional monomers and inorganic filler or combined fillers with organic and inorganic phases showed excellent mechanical properties among the various restorative resins, However, they are not adequate for tooth restorations, because of insufficient marginal fracture toughnes~'~ and wear re~istance.'~ The residual unsaturated double bonds in cured products affect the chemical and biological properties such as color stabilityI6 and pulpal resp0n~e.l~ The unsaturated pendant double bond in polymeric matrix is susceptible to oxidative degradation. '* Since copolymerization with a cross-linker is terminated at the glass transition temperature (T,) of the cured p r~duc t , ' ~ it is recommended that polymerization be carried out at a higher temperature than T,. Thus the extent of polymerization (E,) is affected by the mobility of functional groups based on a chemical structure of multifunctional monomers and a polymerization temperature. In the dental restorative composites and the comonomer system with similar compositions, e.g., bis-GMA-TrEGDMA, it has been shown that E, is also influenced by the concentration of diluent, catalyst, and inhibitor and the kind of catalyst in the monomer systems.'8,2@ The E , is closely correlated with the dynamic mechanical properties and the thermal characteristics of cured products.'@

In this study, we mainly investigated the chemical structure of various multifunctional monomers and polymerization conditions to reveal the polymerization characteristics (€p, R,, D,, EJ. The relation of E,, R,, D,, and E , values in the cross-linked network polymers is shown in Figure 3. Con- sequently, these polymerization characteristics were closely correlated to both the monomer structure and the polymerization condition. Figures 4 and 5 show E,, X,, and D, values in relation to the number of chain members which refer to atoms consisting of a main chain between the two functional groups. An aromatic ring was taken as four carbon atoms. E,, R,, and D, values closely correlated with the chain members as shown in Figures 4 and 5. E , values of the monomers containing an aliphatic chain markedly increased with an increase in the chain members from 2 to 12, but at more than 12, E , values became constant (€,-1). E , values of monomers containing an aromatic chain, bisphenol A, also increased with in increase in the number of chain members, but these were generally lower than those of aliphatic dimethacrylates and diacrylates (4,-2). BPDMA showed the lowest E , value among the monomers tested. It can be assumed that the geometrical restriction affected the motion of unsaturated double bonds in the polymer. Therefore, high E, values were obtained with monomers having the framework extended by an ethylene glycol chain such as bis-MP1,3 EPP and bis- MP2,' EPP. The reactivity of pendant double bonds attached at the one end to the cross-linked network is ~rnaller'~ than that of the vinyl double bonds


Ep = 60 (60.11

M

M

t--o t--o

Dp.30 (29.3)

M

e--o M Rrn=lO

(1 0.6)

NPGDMA

Ep = 45 (44.7)

M Ec=lO (9.9) -

t--O Dp.35 (34.8) t--o -

Rrn.20 (20.5)

Bis-GMA

E p = 60 (65.1)

M

Ec-30 (35.4)

M

M Dp.30

(29.7)

M

e--o M Rm.70

(5.2)

Bis-MPwEPP

Figure 3. Schematic representation of extent of polymerization (E,,), amount of residual monomer (R,,,), and pendant double bond (D,) and efficiency of cross-linking (E,) in cross-linking network polymer, exemplified by NPGDMA, bis-GMA, and bis-MI',,, EPP. The numbers of E,, R,,, D,, and E, indicate the individual values in percentage and actual values obtained are presented in parentheses. 0, saturated double bond; 3, unsaturated double bond.

0 0 4 8 12 16 2 0 24 40

Number of Chain Members

Figure 4. The extent of polymerization (E,) and the amount of residual monomer ( R , ) in relation to the number of chain members between methacryloyloxy or acryloyloxy groups. One aromatic ring was counted as four chain members. Polymerization condition: isothermal mode at 90°C under nitrogen atmosphere. E,-1 and Rm-I: aliphatic monomers, €,-2 and R,-2: aromatic monomers. 0, Ethylene glycol dimethacrylates; 0, ethylene glycol diacrylates; 0, alkylene dimethacrylates; +, alkylene diacrylates; , aromatic dimethacrylates; 0, UDMA; A, TMPTMA *: Data of previous report.'"


n

A

0 4 a 12 16 20 24

Number of Chain Members

Figure 5. The extent of polymerization (L,) and the number of pendant double bonds (D,) in relation to the number of chain members between methacryloyloxy and acryloyloxy groups. Polymerizatlon conditions: thermal scanning mode from room temperature to 200°C under nitrogen atmosphere. (Symbols: see Figure 4.)

of monomers, and the reactivity decreases with an increase of E, value. The geometrical restriction was also observed in the polymerization of aliphatic dimethacrylates and diacryhtes with Iess than eight chain members. On all monomers employed, high E , values were obtained in the polymerization of the temperature scanning mode compared to the isothermal mode (90°C). This is the reason that the high reactivity of monomers resulted from the increase in diffusion rate of vinyl groups caused by raising the polymerization temperature,

R, and D, values were inversely proportional to E , values (R,-1, 2, Dp-l, 2) and decreased with an increase of the number of chain member. Low R, values were observed in the aliphatic monomers with more than eight chain members. Bis-GMA, often used as a main component of dental restorative resin, showed higher R, and D, values than those of UDMA though they have nearly the same chain length. In autopolymerization of commercial composites with bis-GMA-TrEGDMA, it was reported4r7r8 that a large number of unsaturated double bonds remained in the cured product. The increase in the mole fraction of diluent monomer (TrEGDMA) tends to decrease the amount of unsaturated double bonds2' This might be caused by the increase in the mobility of the copolymerization system. However, the addition of dimethacrylates containing a short aliphatic chain, NPGDMA, BGDMA, HGDMA, EGDMA, and DEGDMA could not affect appreciably the decrease of unsaturated double bonds in the copolymerization. D, values of whole monomers examined were higher than li, values in the polymerization at both the isothermal and the thermal scanning modes. This is because the mobility of pendant double bond attached to the cross-linked polymer network is lower than that of a residual monomer in the last stage of the p~lymerization. '~


The E , in the copolymerization of MMA with various dimethacrylates affected physical properties, e.g., wear resistance, tensile strength, impact strength, and temperature characteristics." In this study, it is possible that the E , involving a cross-linked point as a loop which is not effective for the mechanical property of cured product since the E , was expressed as a percentage of saturated vinyl double bonds at both ends of the monomers against the whole vinyl double bonds in the monomers.22 The E, was closely correlated with the E , and the dimethacrylates and diacrylates with long aliphatic chains, TrEGDMA, TeEGDMA, NEGDMA, UDMA, A-200, A-400, and A-600 showed high E , values ranging from 67.2 to 95.7%. The low E , values were observed with the monomers containing short aliphatic or aromatic chains, especially EGDMA, NPGDMA, BGDMA, TMPTMA, NPGDA, BPDMA, bis-MEPP, and bis-GMA.

In this study, a significant correlation between the polymerization characteristics (€,, X,, D,, E,) and the monomer structure, chain length, and framework between two functional groups was observed. The high extent of polymerization of matrix resin is required for dental composites. The high €,, E , and low X, were obtained on the multifunctional monomers containing long chains between functional groups. In this connection, the recently developed multifunctional oligomers4 are presumed to be effective monomers for the resin matrix of dental composites. On the contrary, the multifunctional monomers containing short aliphatic or aromatic chains are not useful as the base monomer or diluent of autopolymerized dental composites. However, the dynamic mechanical properties of these composites cannot be simply discussed on the basis of only the E , of cured products, because the relationship between the dynamic mechanical properties and the structural characteristics of dental composites is complicated. The relationship between the dynamic mechanical properties and the network structure of cross-linked polymers is now under investigation.

The authors would like to thank Dr. Kenji Kuromizu, Fukuoka Dental College and Dr. Kouichi Katsuki, Nihon Univ. School of Dentistry at Matsudo for the helpful suggestions during the preparation of this manuscript.

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Received November 2, 1987 Accepted April 12, 1988

polymerization of multifunctional methacrylates and acrylates

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