NATIONAL BUREAU OF STANDARDS REPORT

10 060

Progress Report

on CRYSTALLINE DIMETHACRYLATE MONOMERS

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

3 11.05-1 1-31 10561 June 30, 1969

NBS REPORT

10 060

P rogress Report

on CRYSTALLINE DIMETHACRYLATE MONOMERS

by R. L. Bowen*

* Research Associate from the American Dental Association

in the Dental Research Section, National Bureau of

Standards, Washington, D. C. 20234

This investigation was supported in part by Research Grants

DE02494-0 1 and DE02494-02 to the American Dental Association from the National Institute of Dental Research and is part of the dental research program conducted by the National

Bureau of Standards in cooperation with the Council on Dental

Research of the American Dental Association; the Dental Research Division of the U. S. Army Medical Research and Development Command; the Dental Sciences Division of the School of

Aerospace Medicine, USAF; and the Veterans Administration.

IMPORTANT NOTICE NATIONAL BUREAU OF STA

for use within the Government. E and review. For this reason, the whole or in part, is not authoriz Bureau of Standards, Washington

the Report has been specifically p

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

accounting documents intended

j bjected to additional evaluation

isting of this Report, either in Office of the Director, National

the Government agency for which

lies for its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

(Abstract)

CRYSTALLINE DXMBTHACRYLATE MONOMERS

Certain dimethacrylate monomers can be prepared and

purified by recrystallization. Upon mixing, the

crystals liquify by forming a ternary eutectic. The

colorless oily liquid is suitable for use in composite

formulations

.

1

Previously, BIS-GMA1 (a dimethacrylate monomer, the reaction

product of an epoxy resin and methacrylic acid or of 4,4*-

isopropy lidenediphenol and glycidyl methacrylate) 2was

developed for use as a binder for reinforcing fillers.

This dimethacrylate monomer, shown in Figure 1, has had

widespread use in current commercial dental composite

restorative materials. However, BIS-GMA is not completely

color-stable and is too viscous for use without being

thinned with methyl methacrylate or other comonomers of

low viscosity. Furthermore, it cannot be purified by

distillation or by crystallization since it is a mixture

of high-molecular-weight optical isomers.

This report describes the synthesis and properties

of three dimethacrylate monomers that were synthesized

in order to reduce the viscosity, improve the color and

storage stability and possibly other properties associated

with the resin that forms the binder of composite restorative materials. The monomers, with melting points slightly

above room temperature , form a liquid when combined in

2

ternary eutectic proportions. Their purification by

recrystallization provides a means of attaining desired

purity and of facilitating production control.

This paper does not deal with properties of the

polymer or composite materials prepared from these

monomers. It encompasses only the synthesis of the

monomers and the formulation of a liquid ectectic

monomer based on them.

Materials and Methods*

The three monomers , bis ( 2-methacry loxyethy 1 ) isophthalate, (MEI)j bis (2-methacry loxyethyl) terephthalate,

(MET), and bis (2-methacry loxyethy 1) phthalate (MEP) were

condensation products of 2 -hydroxyethyl methacrylate (HEMA)

and the corresponding acid chlorides of the phthalic

acid position isomers. Most of the compounds that were

used are listed in Table 1. They were used as received.

Certain commercial materials and equipment are identified

in this paper to specify adequately the experimental procedure. In no instance does such identification imply

recommendation or endorsement by the National Bureau of

Standards or that the material or equipment identified is

necessarily the best available for the purpose.

3

SYNTHESIS OF MEI.—-The synthesis was carried out in

a three-neck round-bottom flask placed in a cooling

bath, equipped with stirrer, thermometer, dried nitrogen

inlet, and a dropping funnel. BHT (butylated hydroxytoluene? 2 , 6-di-t-buty 1-4-methyl phenol) and di-b-butyl

sulfide, each equivalent to 0.1% of the theoretical

yield of monomer were placed in the flask. To this

was added 194 ml of pyridine (2.4 mols) and 305 ml of

HEMA (2.4 raols) with stirring. Dried nitrogen was used

as the reaction atmosphere and the cooling bath maintained

the temperature within the range of 10-3 5° C throughout

the reaction. Isophthaloyl chloride (203 gmy 1 mol),

dissolved in 112 gm of acetone, was added dropwise to

the stirred reagents in the reaction vessel. As the

reaction proceeded, a white precipitate (pyridinium

chloride) formed. The acid chloride solution was

added over a four-hour period. The mixture after standing

overnight without stirring was vacuum filtered to remove

crystals of pyridinium chloride. The residue on the filter

was rinsed with additional acetone and then discarded.

4

A clear yellow filtrate containing the monomer product

was obtained.*

The filtrate was diluted with 300 ml of chloroform

and washed three times in a separatory funnel using

500 ml of distilled water for each wash. The resin

phase was evaporated down under vacuum to remove

chloroform and water to approximately constant weight.

23 5® q A yellow liquid with a refractive index of n Q

«

1.5087 was obtained. The yield of this impure monomer

was slightly over 90% of the theoretical. To this was

added additional BHT and di-t-butyl sulfide (0.1% of

each) . The monomer was diluted with an equal volume

of methanol and then cooled, with stirring, to a low

temperature utilizing a dry ice bath. At a low temperature (about -20° c) the crystals formed spontaneously

* It might be possible to crystallize the monomer

directly from this acetone solution by stirring

and cooling it to a low temperature (e.g. -20*C

or below) with the aid of a dry-ice (solid carbon

dioxide) bath, although this was not attempted.

5

and developed rapidly. This slurry of crystalline monomer

was then suction filtered, the crystals of monomer being

retained on the filter paper.*

The crystals obtained were redlssolved In warm methanol

and recrystall tzed three times.

SYNTHESIS OP MET. -This compound was synthesized in

essentially the same manner as MEI f with the following

exceptions. Since the terephthaloyl chloride is less

soluble in acetone, a larger quantity of acetone was

required as a solvent and the mixture was warmed slightly.

SYNTHESIS OF MEP.——This monomer was synthesized in

essentially the same manner as were MEI and MET, with

the exception that no acetone was used in the synthesis

since the phthaloyl chloride is liquid at room temperature and could therefore be added dropwlse to the

reaction flask.

* The filtrate can then be concentrated and cooled again

to obtain additional crystallite monomer and this process

repeated to obtain maximum yield of crystalline product.
/
6

DETERMINATION OF EUTECTIC COMPOSITION. After the

individual melting points of the three recrystallized

monomers had been determined, the mixed melting point

(first sign of melting) was determined for the binary

systems MEI-MET, MEI-MEP and MET-MEP. The proportions

of the binary eutectics were calculated from the theoretical3 relationship: X! » 100 (T3 -Te) Where xx is the

T1+T3

2Te

mole percent of lower-melting component, Tx is the

melting point of the lower-melting component, Ta is

the melting point of the higher-melting component, and

Te is the eutectic temperature (first sign of melting

of the mixture).

The three binary diagrams were then incorporated

into a triangular ternary diagram to help predict the

composition of the ternary eutectic. The approximate

ternary eutectic composition was determined by trial

and error, 3 starting from the composition corresponding

to the intersection of the lines normal to the sides of

the triangle at the theoretical binary eutectic compositions.

7

Stabilizers and a polymerization accelerator4 ” 8 as

given in Table 2 were added to the approximate ternary

eutectic mixture. This formulation (Table 2) was sent

to an independent commercial laboratory for preliminary

toxicity evaluation.

Results

After the MEI was recrystallized three times from

methanol, it was a colorless white solid having a melting

point of 42. 3-43. 3 ®C. After melting, the metastable

liquid had a refractive index of n^*^° c ** 1.5116.

After the MET was recrystallized four times as

previously described, the white crystalline monomer

had a melting point of 50-53° C, and the refractive

index of the metastable liquid (measured immediately

after melting) was n

4^*^ ^ ” 1.5135. The yield after t

the first crystallization was greater than 55%.

After four recrystallization*, the colorless

white crystals of MEP had a melting point of 39-40°

and the refractive index of the metastable liquid

- tt -

(measured immediately after melting of some of these

22 3®

crystals) was n

Q « 1,5095. The yield was approximately 80% of theoretical at the time of the first

crystallization.

The following data are only theoretical for the

binary and approximate for the ternary systems but are

presented since they might be useful in practical applications. Binary eutectics:

36% MET - 64% ME1 , 31°C

45% MEI - 55% MBP, 26°C

68% MEP - 32% MET, 29.5°C

Ternary eutectic:

47% MEI, 38% MEP, and 15% MET, (^23«C)

This ternary mixture is a colorless oily liquid

at room temperature; it solidified under refrigeration

(about 5°C)

TOXICITY.—-The summaries of the toxicity results (on the

formulation given in Table 2) are as follows* The monomer

formulation was evaluated for acute oral toxicity by

gastric intubation to groups of two adult male albino

9

rats at graded dosage levels of 1.0, 3.16, 10.0, and

31.6 ml/kg of body weight. The acute oral median lethal

dose of the material for albino rats was 31.6 ml/kg of

body weight? therefore, it is considered to be relatively

harmless by the oral route.

The test material was also evaluated for acute eye

Irritation by a single application of 0.1 ml of the

test material into one eye of each of three albino

rabbits. No irritation was produced in one eye, and

only slight conjuctival redness was produced in two eyes

which subsided by 72 hours or day 4. The test material

is considered to be essentially nonirritating to

rabbit eyes.

A considerable advantage of the ternary eutectic

mixture prepared from the three dimethacrylate crystalline

monomers as a binder for seinforeing fillers lies in the fact

that its viscosity permits its use without the addition of

thinners or diluents which may contribute undesirable properties. Having molecular weights of 390, the monomers of

the liquid are nonvolatile and require no volatile additives.

Their structural formulas are shown in Figure 2 . Compared

10

with BIS—GMA it is reasonable to expect that the introduction of electron-withdrawing substituents into the

aromatic ring will result in improved resistance to oxidation

and subsequent color formation. Electron-withdrawing substituents deactivate aromatic nuclei thereby diminishing

oxidation. Electron-donors, such as the alkyl and ether

groups on the BIS-GMA rings, have the opposite effect.

Information on MEI was not found in the literature. A

compound corresponding to MEP has been described. 9 The acidcatalyzed condensation product of methacrylic acid, ethylene

glycol, and phthalic acid, and of ethylene glycol monomethacrylate

and phthalic anhydride was reported. 10 11 Both were reported

to have *» 1.500 and D^° « 1.18? no mention was made of

crystallization.

Crystalline products melting between 30 and 53° C have

been reported to result from the reaction of bis- 3-hydroxyethyl

terephthalate with the acid chloride of methacrylic acid. 18

A correction of a previous report13 should be made. It

was stated that a liquid mixture of MEI and MET did not

crystallize on standing at room temperature. This binary
/
11 -

mixture did crystallize at a later date, having been metastable for a considerable period of time.

Although the work reported here is part of a larger

program encompassing the development of an esthetic

adhesive restorative material, 14 there is no reason to

believe that the monomer system reported here has adhesive

characteristics. To obtain adhesion, appropriate coupling

agents that have been referred to previously, 14 are indicated.

Properties of composite materials utilizing the

formulation given in Table 2, are being investigated.

Conclusions

Improved purity of nonvolatile monomers cam be

achieved readily when the molecular characteristics

are such as to facilitate recrystallization. The

bis(2-methacryloxyethyl) esters of phthalic, isophthalic

and terephthalic acids have melting points, slightly above

room temperature and they form a ternary eutectic mixture

12

that is liquid at room temperature. This colorless

slightly viscous liquid appears to be suitable for

formulation as the binder for composite materials for

use in industry and in dentistry.

- 13 -

Referenceg

1. Bowen, R. L. : Properties of a Silica-Reinforced

Polymer for Dental Restorations, JADA 66 857-64,

1963.

2. Bowen, R. L. t Dental Filling Material Comprising

Vinyl Silane Treated Fused Silica and a Binder

Consisting of the Reaction Product of Bis Phenol

and Glycidyl Acrylate, US Patent 3,066,112, 1962.

3. McCrone, W. C-, Jr.* Fusion Methods in Chemical

Microscopy , New York, Interscience, 1957, p

4. Bowen, R. I*., and Argentar, H. : Color Formation in

Methacrylate Accelerator Systems, IADR Program and

Abstracts of Papers (p 38), July 1965 (abstract).

5. Bowen, R. L., and Argentar, H. s Diminishing

Discoloration in Methacrylate Accelerator

Systems, JADA 75*918-923, 1967.

6. Birch, S. F., and Fidler, F. A.* Improvements in

or Relating to Production of Aromatic Amines, British

Patent 619,877 March 16, 1949.
t
14 -

7. Groggins, P. H. : Unit Processes In Organic Synthesis ,

(1st ed.

)

. New York, McGraw-Hill Book Co., Inc., 1935

p 291-2.

8. Fierz-David, H. E., and Blangey, L.x Fundamentals of

Dye Chemistry . New York, Interscience, 1949, p 133-4.

9. Hammond, R. : The Preparation of New Polymerizable

Organic Materials and Polymers thereof, British

Patent 595,881, 1947.

10. Berlin, A. A., Popova, G. L., and Isaeva, E. F.x

Condensation Telomer ization and a New Type of

Unsaturated Polyesters, Doklady Akad. Nauk SSSR.

123x282-4, 1958.

11. Berlin, A. A., Popova, G. L., and Isaeva, E. F.x

Investigation of Polymerization and Properties of

Mixed Polyesters of the Acrylic Series, Doklady

Akad Nauk SSSR. 126x327-9, 1959.

15 -

12. Mihailov, M. , and Boudevska, H- 1 Synthesis and

Polymerization of Polyesterroethacrylates of Terephthalic

and Furane-2 , 5-dicarboxy lie Acid, Conspt. Rend. Acad.

Bulgare Scl. . 18t31-4, 1965 (Eng.).

13. Bowen, R. L., and Argentar, H.i Dimethacrylate Monomers

for Preparing Composite Restorative Materials, I ADR

Program and Abstracts of Papers (p. 175), March 1967

(abstract)

.

14. Bowen, R. L. s Development of an Adhesive Restorative

Material, in Adhesive Restorative Dental Materials II,

University of Virginia Workshop, Public Health Service

Publications no. 1494, 1966, p 225-231

.

~ 2JB ~

TABLE 1

Materials Used in the Monomer Synthesis

« Material Grade Source

Acetone a 4iCo Allied Chera.

Chloroform •; Reag . ACS Allied Chem.

2-hydroxyethy 1

methacrylate (HBMA)

96% Rohm & Haas

Isophthaloyl chloride 98% rain. Hooker Chemical Corp.

Methanol ACS Fisher Sci. Co.

Phthaloyl chloride Pract

.

Matheson, Coleman

St Bell

Pyridine Reag. Baker Chemical Co.

Ter ephthaloy 1 chloride 93/4 min. Hooker Chemical Ccrp.
»
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i4

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44 44 ©M *

44G

GO 0 -A O 44 G © O U O i4r4 • ft 43 ft In

reeponse

to

interest

in

this

compound 4

.®

Dr.

Charles

W.

Taylor

(3M

Co.)

synthesized

the

amine

bya

method®" 8

utilizing

relatively

inexpensive

starting

materials

and

kindly

furnished

usa

sample.

This

was

the

source

of

the

accelerator

used

in

the

formulation

described

here.

The

material

is

now

commercially

available

from

Kastman

Chem«

Products,

Inc.

H % H

O O A c /—A o o

Hg odtoCHa 0

X ft < ) OCHs -C^CHa O&OfSl?

CH, H

7 t

N==/ H CH,

B*

BIS-GMA

0

H^COCC"CHs

MMA

Pig. i_—Assumed structural formula of BIS—GHA**1, The

aster isles Indicate asymetric carbon atoms that result

in a number of stereoisomers. Methyl methacrylate (MIA)

is shown for comparison.

o o

h2 c=ccoc2 h4 oc

CH3

ME I

O 0

C0C2 H4 0CC=CH2 CH3

0 o h2 c=ccoc2 h4 oc- CH3

o o

C0C2 H4 0CC=CH2 CH3

MET

o o

H2 C=CC0C2 H4 0C

ch3

o o

C0C2 H4 0CC=CH2 CH3

MEP

Fig. 2 Structural formulas of three dimethacrylate

monomers. Bis (2-methacry loxyethy 1) isophthalate, (ME I

)

bis (2-methacryloxyethyl)terephthalate, (MET), and

bis (2-methacry loxyethy l)phthalate, (MEP) were each

readily purified by crystallization.

USCOMM-NBS-DC
'