CA1213699A - Dental restorative composition - Google Patents

Abstract

Abstract of the Disclosure A polymerizable dental composition containing methacrylate monomer(s) and free radical polymerization catalyst for said monomer(s), the monomer(s) having been treated with a sulfonated cation-exchange resin in free acid form prior to contact thereof with said catalyst or during contact thereof with said catalyst but prior to free radical liber-ation by said catalyst, to improve the stability of the monomer(s) -catalyst system against polymerization, particularly under ambient conditions.

Description

i~l3~

BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to compositions benefi-cially adapted for dental restorative and repair applications and particularly to such compositions having good stability on standing under varying conditions over extended time periods.
DISCUSSION OF THE PRIOP~ ART
Polymerizable dental compositions based on the use of polymerization catalyst, e.g. free radical initiator, and methacrylate type monomer, such as the bisphenol A-glycidyl methacrylate reaction product described in Bowen, U.S. Patent 3,066,112, and commonly referred to as BIS-GMA, provide valuable base materials for a wide variety of dental restorative and repair procedures, such as fillings, pit and fissure sealing and the like. Generally, such compositions are used according to a prescribed chronology whereby contacting of polymerization cata-lyst activator therefor in the presence of polymerizable monomer occurs only at the time of actual use by the dentist. Materials are selected and dosage controlled to insure fairly rapid poly-merization upon such contacting to produce a solid mass po]ymer-izate within the oral cavity. To facilitate handling and manipulation, the compositions may be supplied to the dentist in the form of a plurality of viscous pastes which enable uniform blending of the compositions with a minimum of effort.
As described in the prior art, the foregoing may be effected by providing physically separate, filler-loaded compos-itions containing respectively, monomer/catalyst, ~21365'9 monomer/activ~-tor, etc. ~he compositions as made available commercially are separately packaged.
Despite the precaution exercised by mamlfacturers, it is found in practice that undesired polymer~zation of the monomer composition on standing nevertheless occurs fairly rapidly and to a disturbing degree prlor to the time of contacting same wlth the activator composition. Such undesired polymerization is noted despite the inclusion of various in-hibitors in the rnonomer compositions to react with adventitious free radicals that might induce polymerization. Polymerizates formed from such de-stabilized monomer compositions are invariab-ly of inferlor structural integrity, color, etc. Moreover, hardening of the monomer composition, a necessary consequence of pre-polymerization, may well render the affected monomer entirely unsuitable for dental use. The economics may thus be prohibitive. To a~oid the premature polymerization problem, it is common practice for the dentist to refrigerate the com-ponents of the polymer-forming composition prior to use.
¦ Remedial techniques heretofore provided in mitigation of the foregoing are varied. Thus, it is found that monomer de-stabilization can be effectively retarded somewhat by using a specific type of free radical catalyst, e.g., one having superior thermal stability. However, such an approach assumes that thermal influences may in all cases be the source of the problem, i.e., the higher temperatures accelerate or initiate the free radical producing reaction of the catalyst. Moreover, the range of catalyst selection is greatly reduced according to this method. Other techniques allege the use of rather high loadings of filler, e.g., silica, to achieve adequate 3G storeageability. However, inorganic fillers of the type lZ~ 9 normally used in the compositions described may well retard the curing or polymerization of methacrylate monomers as pointed out in the published literature. This can be counter-acted, appar-ently by the use of silane coupling or keying agents which pur-portedly function to at least partially restore curability or polymerizability of the monomer. It is of utmost importance that the effects of the means resorted to for suppressing un-desired pre-polymerization of monomer are not significantly manifested at the time of monomer/catalyst activator contacting, at which time fairly rapid and efficient polymerization is imperative.
Although the aforedescribed is but partially represent-ative of relevant technology on the subject, it is apparent that effective implementation of the remedial process may be diffi-cult, requiring a careful and precise balancing of numerous factors which in and of themselves may present further problems.
In accordance with the present invention, it has been discovered that the monomer stability problem as described attaches principally to the nature of the monomer as supplied for use in formulating the composition rather than any inter-action which might occur between and among materials employed as co-ingredients in the monomer composition.
The present invention is based upon the surprising discovery that methacrylate monomer, and particularly that of the BIS-GMA type generally as commercially supplied, when ad-mixed with catalyst particularly of the peroxide and hydro-peroxide types, has limited shelf life and the mixture more often than not polymerizes under storage conditions before use rendering the product unsaleable for restorative purposes. Such spurious reactivity obtains under even moderate environmental conditions, such as those prevailing during normal storage. Thus, in a laboratory test using commercial BIS-GMA as ~Z136~g the monomer, catalyst added samples thereof, not treated in accordance with the invention, give polymer after but one day's standing at room temperature.
Whether instability is due to a contaminant and what the source might be is open to question; thus, it may have been introduced at some point in the preparation and/or treatment of the methacrylate monomer and/or precursor materials.
Distillative purification of bisphenol A type methacrylate monomers in particular is known to be difficult due to their high boiling nature; hence, the possibility of a high boiling contaminant with the material is not precluded.
Whatever the reason, it has been found that the treatment of the methacrylate monomer with an ion-exchange material and particularly a sulfonic acid type cation-exchange resin in acid form greatly minimizes any tendency of the monomer to polymerize particularly when admixed with catalyst and especially of the free radical peroxide type.
Accordingly, the present invention provides a polymerizable composition stabilized against premature activation of catalyst component comprising at least one meth-acrylate monomer having 2 to 4 polymerizable olefinic double bonds and free radical-liberating polymerization catalyst capable when activated of initiating polymerization of said monomer, said catalyst being present in amount sufficient to achieve a predetermined rate and/or degree of polymerization, said monomer having been treated prior to or during contact thereof with said catalyst with a sulfonic acid type cation-exchange resin in free acid form.
The invention also provides a process for the preparation of the composition defined above which comprises treating a methacrylate monomer composition containing at least lZ1365'~

one methacrylate monomer having 2 to 4 polymerizable olefinic double bonds with a sulfonic acid type cation exchange resin in free acid form, followed by or simultaneous with contact of said monomer composition with said catalyst.
As touched upon previously, the stability problem is especially acute with monomer compositions containing BIS-GMA, this substance being found, surprisingly, to undergo polymeriza-tion after but one day at room temperature in the presence of ~e.g. 2% based upon monomer) cumene hydroperoxide, a thermally stable ca-talyst. Conversely, commercial inhibitor-containing methyl methacrylate containing from 1-2% benzoyl peroxide undergoes polymerization after a few days at 25C, but exhibits significantly greater stability as would be expected, in the presence of the more thermally stable cumene hydroperoxide and t-butyl hydroperoxide. Based on ten hour half lives, the recommended temperature for use of benzoyl peroxide is about 73C and for cumene hydroperoxide and t-butyl hydroperoxide, 160C and 170C, - 4a -121369~

respectively. BIS-GMA is accordingly somewhat anomalous in that it undergoes fairly rapi~ premature polymerization even with thermally stable catalyst materials.
~he foregoing, not only serves to point out the importance of the possible disparate nature of the contaminant materials, but in addition, the disparate effects exhibited by the contaminants on different catalyst materials. When using BIS-GMA as the sole or principal monomer, prolonged contacting of monomer and catalyst prior to treatment is to be avoided whereas more leeway existæ with monomers such as methyl metha-crylate. In the former case, treatment with the ion-exchange resln should preferably be before contact Or monomer and catalyst .
The sulfonic acid cation-exchange re~ins are well known and commercially available in a wide variety of forms. Particular-ly preferred herein are the sulfonated, crosslinked, poly-styrene resins such as that available commercially as DOWE~50 W-X8 in the form of beads (1.9 meq/ml, ~ form wet - 5.1 meq/ml dry). Other suitable sulfonic acid cation-exchange resins include DOWEX 50W-X2, DOWEX 50W - X4 and DOWEX 50W- X10 (Dow Chemical Co.) as well as Amberlite*IR 120, Amberlyst*15 (Rohm & Haas) and Rexyn*lOl(H) of Fisher Chem. Co.~ the latter characterized as 4.6 meq/ml - dry basis. Cation exchan~e resins are most effectively used to treat the mon~mer prior to contact thereof with the catalyst, i.e. as a pretreatment.

2~ According to this method, contamlnants removab7e by cationic exchange treatment including those of a cationic nature are at least substantially removed from the monomer, i.e. physically extracted therefrom. Impurities present in the cation exchange resin as commercially supplied may be removed therefrom by acetone wash followed by oven drying before contacting same with *Trade Mark l`

I lZ13~i~9 the methacrylate monomer. Such contacting may be by conven-tional technique, for example by simply adding the ion exchange beads (e.g. 16 to 100 mesh, preferably 20-50 mesh) to a solution of the methacrylate monomer, agitating the mixture for the required period and filtering to separate the beads.
Contacting is maintalned until contaminant is at least substan-tially removed, the latter determined by storage stability tests.
For example~ in a laboratory run a solution mixture of 8 part~
of cation exchange resin with 52 parts o~ a BIS-GMA containing monomer composition provided a substantially contaminant-free monomer product after 24 hours standing with agitation. Alter-natively, monomer decontamination can be achieved adequately by passing the monomer through a suitable column of the ion exchange resin in known manner.
Methacrylate monomer materials useful herein are well known in the art. The preferred materials generally include monomers having a central portion containing at least one aromatic ring and at least two acrylic end groups. Of thls type~ BIS-GMA is particularly preferred and in preferred embodiments constitutes at least about 50~ by weight o~ the total monomer composition. The commercial BIS-GMA available from Freeman Chemical Co. under the trademark NUROL is an example o~ materials useful herein.

- 6.-Methacrylate monomers particularly useful in this invention are those represented by the following general formulae:

[(M A )n Ar~2 B (M - A - OCO)2Ar I II

(M - A)m CR P M2R' (M - A - OCO - NH)2R

III IV V

fH2 M
CH - M' VI

wherein M is methacryloyloxy, i.e. CH2 = C(CH3)COO-;
M' is methacryloyloxy or hydroxyl; A is alkylene having 1-3 carbon atoms, such as methylene, propylene, isopropylene, hydroxyalkylene having 1-3 carbon atoms, such as hydroxy-methylene, 2-hydroxypropylene or acetoxyalkylene having 3-5 carbon atoms in the alkylene group such as 2-acetoxypropylene,

3-acetoxyamylene etc.; n is 1-4 preferably l or 2; m is 2 or 3 and p is l or 2 with the proviso that the sum of m and p is 4;
R is hydrogen, methyl, ethyl or -A-M wherein A and M are pre-viously described; Ar is phenylene, e.g., o-phenylene, m-phenylene or p-phenylene, alkyl substituted phenylene, e.g., tolylene or 5-t-butyl-m-phenylene or cycloaliphatic having 6 to lO carbonatoms such as 1,3-cyclohexylene; B is " ~C \ wherein R4 and R5 are independently hydro-6~9 gen, alkyl, e.g. Cl to C4, or substituted alkyl; and R' is alkylene having 2 to 12 carbon atoms such as ethylene, dodecyl-ene, etc. or -R2~0-R t ~ ~ wherein R is alkylene having 2 or 3 carbon atoms such as ethylene, propylene or isopropylene and x is zero to ~; and R3 is phenylene, tolylene, methylene-bis-phenylene or alkylene having 2 to 12 carbon atoms.
Monomers having the above formulae are well known and generally commercially available materials. Alternately, they are readily provided by conventional synthetic routes, for example, by reacting a phenolic compound such as diphenolic acid, phloroglucinol or bisphenol A with glycidyl methacrylate in the presence of various tertiary amines and/or phosphines or by reacting methacrylic acid with an epoxide-containing compound such as the diglycidyl ether of a bisphenol. Some o~ these monomers also are made by reacting appropriate alcohols with methacrylic acid, metacrylyl chloride or methacrylic anhydride.
Illustrative monomers having these formulae include:

CH2=C(CH3)COOCH2CH2-OCO - ~ COOCH2CH20COC(CH3) CH2;

CH2=C(CH3)-COO- ~ -CH20CO-- ~ -OCOC(CH3) = CH2 C[CH20COC(CH3) = CH2]4;
CH3CH2C(-cH2-o-c Il_CH12)3;

CH2= C(CH3)COO(CH2)40COC(CH3)=CH2;
CH2= C(CH3)COOCH2CH20CH2CH20CH2CH20COC(CH2)=CH2, CH2=C(CH3) COOCH2CH(OH)CH2-O- ~ -o-cH2~cH(oH)cH2ococ(cH3) = (CH2) CH2= C(CH3)CO ~ C(CH2)2 ~ O-COC(CH3)=CH2;

CH2-c(CH3)coo-cH2cH(OH~cH2-o-- ~ -- ~ --ocH2cH(OH)cH2ococ(CH3) O~H2CH(OH)OCOC(CH3)=CH2 2 C(cH3)coo-cH2cH2ocoNH-- ~
NHCOOCH2CH20COC(CH3)=CH2 ICH
CH2=C(CH3)COO-CH2CH-OCONH-CH2CH2C-C-f-CH2-NHCOOfH-CH2-OCO-C(CH3) CH CH CH CH =CH2 Monomers having the formulae I, II, III and IV are preferred in the practice of this invention. Of these monomers, I, II and III are particularly preferred, monomers IV being employed more often in admixture with one or more of monomers I, II and III.
Other useful methacrylate monomers suitable for use in the practice of this invention include those having the following formulae wherein M and Ar are as previously described;

(MR40Ar)2C(CH3) 2 wherein R4 is isopropylene;

(MR50Ar)2 and (MR50)2Ar wherein R5 is 2-hydroxy-propylene; MA R6M wherein R6 is hyrdoxycyclopentyl or hydroxycyclohexyl, and A is 2-hydroxyethylene; and M2R8 wherein R8 is:

, ~ _ 9 _ {~

( A ) {~ C --~}
I

(B) -CH2~ CH2-(C) -CH2--~o~ 2 or (D) -CH2 ~ CH2-,~syj~ - 9 a ~3~i5'9 Generally these monomers are commercially available or readily prepared. Preparative details for many of these monomers are given in United States Patent Nos. 3,066,112; 3,721,644;
3,730,947; 3,770,811 and 3,774,305. A tertiary eutectic monomer mixture also suitable for use in this invention is described in United States Patent ~o. 3,539,526.
It is to be understood that mixture of two or more appropriate methacrylate monomers are within the scope of this invention. In fact, depending on the choice of monomers, mixtures are often highly desirable to optimize the character-istics of the resulting dental composition. Thus, it is preferred that the monomer or monomer blend have a viscosity of from about 100 to about 10,000 centipoises as determined using a Brookfield viscometer at 20 rpm at room temperature.
More viscous masses are conveniently handled at higher temperatures. As indicated, preferred mixtures contain at least about 50% by weight of BIS-GMA.
Preferred aliphate dimethacrylate monomers (also referred to as diluent monomers) and particularly for use in mixture with the BIS-GMA as described include: hexamethylene dimethacrylate (HMDMA), triethylene glycol dimethacrylate (TEGDMA) and polyethylene glycol dimethacrylate (PEGDMA).
According to a highly preferred embodiment, the monomer composition comprises a 1:1 mixture of BIS-GMA and HMDMA, this system having excellent stability at both room and elevated temperature (37C).
Free radical liberating catalysts useful herein include, generally, organic compounds which, when activated, liberate free radical species capable of initiating polymeriza-tion of the aforedescribed monomers to form solid MASpolymerizates having, in particular, in the case of Eiller--t~

~3~3~

containing systems good compressive strength, on the order of atleast about 25,000 to 30,000 p.s.i., and preferably at least about 35,000 to 40,000 p.s.i. Preferred materia~s are the organic peroxides and hydroperoxides such as benzoyl peroxide, cumene hydroperoxide, t-butyl hydroperoxide, p-meth~71 benzene hydroperoxide and diisopropyl hydroperoxide. Of these, benzoyl peroxide, cumene hydroperoxide and t-butyl hydroperoxide are especially preferred. The catalyst is generally present in amounts of about .5 to 5%, preferably 1 to 4% and most prefer-ably 1 to 3% by weight of total monomer. A particular advantageof the invention is that high catalyst loadings are permitted within the range given without deleteriously affecting stability.
A further and particularly valuable aspect of the invention is that filler is not necessary to achieve storage ability and is thus an optional ingredient in the instant compositions. Thus, good stability obtains in the monomer composition whether or not such material be present. When used, the amount of filler may range up to about 400% by weight of monomer. In view of the polymerization (curing) retardant effects of the filler, however, it is generally advisable to limit the amount thereof to less than 100% and preferably less than about 80% by weight of total composition.

,~

`` ~ 1213699 The inorganic particulate filler employed in the com-positions of this ~nvention include fused silica, quartz, crystalline silica~ amorphous silica, soda glass beads, bar-ium glass and other radiopaque glasses, glass rods, ceramic oxides, particulate silicate glass and synthetic minerals such as beta-eucryptite (LiAlSiO4), the latter having a negative co-efficient of thermal expansion It is also feasible to employ finely divided materials and powdered hydroxylapatite, although materials that react with silane coupling agents are preferred.
Small amounts of pigments to allow matching of the composition to various shades of teeth can be included. Suitable pigments include iron oxide black, cadmin yellows and oranges, fluores-cent zinc oxides, titanium dioxide, etc. The filler particles would be generally smaller than about 50 microns in diameter and preferably smaller than 30 microns. Unfilled compositions are particularly useful where the dental composition is intend-ed for use as a cotaing, margin sealant for margin restorations or adhesive.
As mentioned, keying or coupling agents likewlse optional for use herein, are particularly beneficial when using a silica ~iller~ since they tend to at least partly counteract the polymerization inhibiting effects of the filler thereby restoring an approximally equivalent measure of polymerizability.
The beneficial effects of the keying agents which are silane compounds are most evident with respect to the compressive stren~th characterizing the final dental polymerizate.
The silane coupling agents or keying agents are materials that contain at least one polymerizable double bond .

3GS~

to react with the methacrylate monomers. Examples of suitable coupling agents are gamma-methacryloxypropyl trimethoxy silane, vinyl trichlorosilane, tris(2 methoxyethoxy) sil~ne, tris (acetoxy) vinyl silane, 1 - N-(vinylbenzylaminoethyl) amino-propyl trimethoxysilane-3. The first named material is preferred for use with methacrylate monomers because of the similarity in reactivity of the double bonds.
The coupling agent may be simply added to the monomer composition containing the filler, there being no requirement for prior hydrolysis according to, for example the acid and alkaline hydrolysis techniques described in U.S. 3,0~6,112 though such procedure can be used. Thus, the filler (usually quartz) may first be slurried with an aqueous solution of the keying agent of such concentration than on drying, about .5% to 2% by weight of the agent is deposited on or reacted with the filler and thereafter blended with the monomer composition. Apparently, the latter procedure results in a manifold increase in the amount of silane which reacts with and/or becomes attached to the filler. However, the strength characteristics of dental polymerizates prepared with the present compositions compare favorably whichever procedure is used.
Formation of the dental polymerizate in the oral cavity is accomplished by mixing the present composition usually provided as a mass of paste-like consistency with an activator containing composition which may be bulked with filler of the type described. Suitable activators, as is known in the art, include without limitation substituted thioureas, e.g. acetyl thiourea, N,N-dimethyl para-toluidine and para-toluenesulfinic acid. Polymerization and consequent hardening occurs rapidly but over a sufficient interval enabling the dentist to expeditiously treat the area designated for dental repair with 1~3l3~

the composition. The resultant polymerizate possesses good strength, particularly compressive strength, with no detectable tendency to chip, flake or otherwise rupture. The polymerizate is completely non-toxic and devoid of any tendency to yield low molecular weight or other materials which might cause pulpal irritation and thus in all essential respects seems inert to the effects of fluids present in the oral cavity.
The polymerizable dental compositions prepared and formulated as herein described are stable for a period of months at room temperature there being no evidence of premature polymerization. Moreover, good stability is likewise evident at elevated temperature, with certain preferred compositions indi-cating good stability for extended periods at temperature on the order of 37C.
It is understood that the contaminant problem as described may vary in severity having reference to the type of methacrylate monomer used as well as the catalyst material.
Thus, an improved level of stability may obtain in a given instance with a combination of low or even hi~h contaminant monomer and relatively stable catalyst material. The importance of a catalyst in this respect is not totally discounted.
However, to the extend that the contaminant contributes to the dissociative reaction of the catalyst and consequent free radical generation, the treatment herein prescribed and compositions formulated in accordance therewith provide the described improvement in stability. Since materials capable of so affect-ing the catalyst do not in many, if not most, cases lend them-selves to precise chemical identification, it is not necessarily known which types of materials might so function; yet such materials may be present in methacrylate monomers here and as herein described due to the process of monomer preparation or other treatment. BIS-GMA in particular is a more dramatic ~3~9 example of a commercial material discovered by the present ap~licant to contain such de-stabilizing contaminants.
Purification treatment of the monomer including that which has been or is in contact with catalyst must be effected prior to the formation of free radicals in appreciable quanti-ties, i.e. quantities sufficient to initiate a rate of polymer-ization which produces significant polymer within a relatively short time. By "short time" is meant a period less than that considered normally incident to inventory holding. For the most part, the polymerizable composition would be used by the dentist within a few weeks after purchase.
Monomer de-stabilization would not be a problem, inso-far as the present compositions are concerned since they are stable under the storage conditions normally to be anticipated, well in excess of the dentist's holding or inventory perioc..
Considering also that the poiymerizable composition would probably be refrigerated prior to use by the dentist, even longer holding times would be proper. Moreover, blending of monomer and catalyst by the manufacturer would obviously be according to schedule and necessarily shortly prior to sale.
Inevitably, however, given a sufficiently long holding time and/
or high temperature environment gel formation in the monomer composition will occur. Thus, even a small free radical popu-lation, in time will cause gelling.
Throughout the present case, the language "prior to formation of free radicals" or "prior to formation of free radicals in appreciable quantities" is intended to have a significance consistent with the foregoing and is to be under-stood having reference thereto. Thus, treatment of a preformed mixture of catalyst and monomer, in accordance with one embodi-~' , 12~,C~9 ment of the invention, prior to appreciable free radical form-ation, simpl~ means that any free radical species present at the time of the treatment would not be sufficient to produce gelling of the monomer composition within the holding time requirements described.
The stability of commercial BIS-GMA and methyl metha-crylate (~MA) in the presence of organic peroxide and hydro-peroxide catalysts is compared as follows:
Portions of ~A (Rohm and Haas product containing 10ppm methyl ether of hydroquinone, MEHQ) are mixed with catalyst to provide 2% solutions of benzoyl peroxide (BP), cumene hydroper-oxide (CHP), and t-butyl hydroperoxide (TBH) respectively. The solutions are allowed to stand at ambient temperature (20-25C).
Portions of BIS-GMA (Freeman Chemical Co. product available under the trade name "NUPOL") are similarly compounded to provide 2% solutions respectively of the same catalyst materials and allowed to stand at 20-25C. The results obtained are tabulated as follows:
TABLE I
Gel Time (da~s) Catalyst Monomer _ TBH CHP
BIS GMA (a) 3 2 MMA 3 (a) (a) (a) no gelling after 9 days when experiment terminated.
Surprisingly, BIS-GMA appears to be more stable with TBH a less thermally stable catalyst, i.e. BP as compared with either TBH or CHP. As previously indicated, both TBH or CHP
are recommended for use at temperatures significantly higher than for BP and would be expected when TBH or CHP is dissolved in the monomers, to give more stability against polymerization than BP.
r - 16 -lZ~

The data for MMA are more in line with what would normally be expected, underscoring the rather anomalous stability aspect o:E
BIS-GMA in the presence of free radical catalyst. Gel time connotes the day on which gel formation is first observed to take place.
The following examples are for purposes of illustra-tion only and are not to be considered as limitative. All parts and percentages are by weight.
EXAMPLE I
To 52 parts of a homogeneous 1:1 mixture of BIS-GMA, (NUPOL 46-4005 batch no. 124,793) and hexamethylene dimethacry-late (HMDMA)I from Sartomer Chemicals, lot no. PB 844 is added 8 parts of acetone-washed and oven-dried DOWEX 50 W-X 8 ion exchange beads (1.9MEQ/ml, H+ form). The mixture is agitated for 24 hours and then filtered through a sintered glass filter to remove the beads. To the monomer solution is added .026 part of MEHQ to replace any inhibitor possibly lost in the ion exchange treatment.
Portions of the thus treated monomer solution are used to make up solutions containing 2% BP and 4% CHP respective-ly. After 6 months standing at room temperature, these solutions exhibited no polymer formation at which time the experiment was terminated. Stability for the BIS-GMA composition is markedly improved as compared with the 2 day gel time for BIS-GMA in Table I for a 2% CHP solution. The greatly improved stability obtained for the BIS-GMA/HMDMA monomer composition in this example is even more surprising considering the high (4~) con-centration of CHP.

To 25 parts of the monomer composition obtained from the ion exchange treatment of Example I is first added 5~, based on monomer, of silane coupling agent and thereafter 75 ~2~36~9 parts of amorphous silica. The composition is stable (no gel formation observed) for over one year at room temperature.

The aged product obtained from Example 2 is mixed with an equal portion of a similar composition but omitting the CHP and adding 2% acetyl thiourea as a reductant. A rapid and complete cure is effected.
Other monomer compositions tested as described in the foregoing examples with similar stabilization results are as follows with parts indicated parenthetically:
BIS-GMA (71) HMDMA (29) BIS-GMA (71) lPEGDMA (29) BIS-GMA (50) 2TEGDMA (50) BIS-GMA (71) TEGDMA (29) 1 polyethylene glycol dimethacrylate 2 triethylene glycol dimethacrylate

Claims (23)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A polymerizable composition stabilized against premature activation of catalyst component comprising at least one methacrylate monomer having 2 to 4 polymerizable olefinic double bonds and free radical-liberating polymerization catalyst capable when activated of initiating polymerization of said monomer, said catalyst being present in amount sufficient to achieve a predetermined rate and/or degree of polymerization, said monomer having been treated prior to or during contact thereof with said catalyst with a sulfonic acid type cation-exchange resin in free acid form.

2. A composition according to claim 1 wherein at least about 50 weight percent of said methacrylate monomer contains at least one aromatic ring in its central portion.

3. A composition according to claim 1 wherein at least about 50 weight percent of said methacrylate monomer is the reaction product of bisphenol A
and glycidyl methacrylate.

4. A composition according to claim 3 containing up to about 50 weight percent hexamethylene dimethacrylate.

5. A composition according to claim 4 wherein said monomer comprises an approximate 1:1 mixture of the reaction product of bisphenol A and glycidyl methacrylate and hexamethylene dimethacrylate.

6. A composition according to claim 1 containing up to about 400% by weight based on the weight of said monomer of particulate inorganic filler.

7. A composition according to claim 6 wherein said filler is a siliceous material.

8. A composition according to claim 7 wherein said filler is silica.

9. A composition according to claim 8 containing up to 5 weight percent based on the weight of said monomer of silane coupling agent.

10. A composition according to claim 1 containing from about .5 to 5 weight percent based on weight of said monomer of free radical catalyst selected from the group consisting of benzoyl peroxide, cumene hydroperoxide, t-butyl hydroperoxide, p-methyl benzene hydroperoxide and diisopropyl benzene hydroperoxide.

11. A polymerizable dental composition stabilized against premature activation of catalyst component comprising at least one methacrylate monomer having 2 to 4 polymerizable olefinic double bonds, at least about 50 weight percent thereof compris-ing the reaction product of bisphenol A and glycidyl methacrylate, from about .5 to 5% based on weight of said monomer of free radical-liberating polymerization catalyst comprising an organic peroxide or hydroperoxide compound, from 0 to 400% based on weight of said monomer of particulate inorganic siliceous filler, and from 3 to 6% based on weight of said monomer of silane coupling agent, said monomer having been treated prior to contact with said catalyst with a sulfonic acid type cation exchange resin in free acid form.

12. A composition according to claim 11 wherein said monomer is an approximate 1:1 blend of the reaction product of bisphenol A and glycidyl methacrylate and hexamethylene dimethacrylate, and said silane coupling agent is present in an amount of about 5%.

13. A process for the preparation of the composition of claim 1 which comprises treating a methacrylate monomer composition containing at least one methacrylate monomer having 2 to 4 polymerizable olefinic double bonds with a sulfonic acid type cation exchange resin in free acid form, followed by or simultaneous with contact of said monomer composition with said catalyst.

14. A process according to claim 13 wherein said treatment of monomer composition with said cation exchange resin is prior to free radical liberation by said catalyst.

15. A process according to claim 13 wherein at least about 50 weight percent of said methacrylate monomer contains at least one aromatic ring in its central portion.

16. A process according to claim 13 wherein at least about 50 weight percent of said methacrylate monomer is the reaction product of bisphenol A and glycidyl methacrylate.

17. A process according to claim 16 wherein up to about 50 weight percent of said methacrylate monomer is hexamethylene dimethacrylate.

18. A process according to claim 17 wherein said monomer comprises an approximate 1:1 mixture of the reaction product of bisphenol A and glycidyl methacrylate and hexamethylene dimethacrylate.

19. A process according to claim 13 wherein up to about 400% by weight, based on the weight of said monomer, of particulate inorganic filler is included in the composition.

20. A process according to claim 19 wherein said filler is a siliceous material.

21. A process according to claim 20 wherein said filler is silica.

22. A process according to claim 21 wherein up to 5 weight percent of silane coupling agent, based on the weight of said monomer, is included in the composition.

23. A process according to claim 13 wherein there is included in the composition from about .5 to 5 weight percent based on weight of said monomer of free radical catalyst selected from the group consisting of benzoyl peroxide, cumene hydroperoxide, t-butyl hydroperoxide, p-methyl benzene hydroperoxide and diisopropyl benzene hydroperoxide.