IE51153B1 - Stabilization of methacrylate monomer compositions used in dental repair and restoration - Google Patents

Abstract

A polymerizable composition is described which comprises at least one methacrylate monomer having 2-4 polymerizable olefinic bonds and at least one free-radical liberating polymerization catalyst capable when activated of initiating polymerization of said monomer or monomers, the catalyst being present in amounts sufficient to achieve a predetermined note and/or degree of polymerization, the said monomer or monomers having been treated with a sulphonic acid type cation-exchange resin in free acid form. This composition is stable against premature activation of the catalyst and consequent premature polymerization.

Description

The present invention relates to/compositions adapted for dental restorative and repair applications, particularly to such compositions having good stability on standing under varying conditions over extended time periods.

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.

Generally, such compositions are used according to a prescribed chronology whereby contacting of polymerization 13 catalyst 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 polymerization upon such contacting to produce a solid mass polymerizate within the oral cavity. To facilitate HO 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 compositions containing respectively, monomer/catalyst, - la 51153 monomer/activator, etc. The compositions as made available commercially are separately packaged.

Despite the precaution exercised by manufacturers, it is found in practice that undesired polymerization of the monomer composition on standing nevertheless occurs fairly rapidly and to a disturbing degree prior to the time of contacting same with the activator composition. Such undesired polymerization is noted despite the inclusion of various inhibitors in the monomer compositions to react with adventitious free radicals that might induce polymerization. Polymerizates formed from such de-stabilized monomer compositions are invariably of inferior 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 try to be prohibitive. To/avoid the premature polymerization problem, it is common practice for the dentist to refrigerate the components 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 tha use of rather high loadings of filler, e.g., silica, to achieve adequate storeageabllity. However, inorganic fillers of the type - 2 Si ί S3 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 counteracted, apparently by use of siliane coupling or keying agents which purportedly 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 supressing undesired 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 representative of relevant technology on the subject, it is apparent that effective implementation of the remedial process may be difficult, requiring a careful and precise balancing of numerous factors which in and of themselves may present further problems.

According to the present invention a polymerizable composition comprises at least one methacrylate monomer having 2 to 4 polymerizable olefinic double bonds and at least one free radical-liberating polymerization catalyst capable when activated of initiating polymerization of the said monomer or monomers, the catalyst being present in amounts sufficient to achieve a predetermined rate and/or degree of polymerization, the said monomer or monomers having been treated with a sulphonic acid type cationexchange resin in free acid form, the treatment being believed to stabilize the composition against premature activation of the catalyst and consequent premature polymerization.

According to a further aspect of the invention the above defined treatment of the monomer or monomers with a sulphonic acid type cation exchange resin in free acid form is carried out prior to contact of the monomer or monomers with the catalyst.

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 interaction which might occur - 3 51153 between and amongst materials employed as co-ingredients in the monomer composition and 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 admixed with catalyst particularly of the peroxide and hydroperoxide 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 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 sulphonic 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.

The invention also extends to a process for the preparation of a polymerizable composition as defined above comprising treating a methacrylate monomer composition containing at least one methacrylate monomer having 2 to 4 polymerizable olefinic double bonds with a sulphonic acid type cation exchange resin in free acid form, and to a dental process of repair and/or restoration which comprises intimately mixing such a composition with an activator material capable of initiating and/or - 4 51153 accelerating the free radical producing reaction of the catalyst to provide a mass of paste-like consistency, the amount of activator and catalyst being sufficient to achieve a relatively rapid rate of polymerization resulting in the formation of a solid mass polymerizate, and placing the composition and activator mixture, while of paste like consistency, in contact with the area designated for dental repair or restoration.

As touched upon previously, the stability problem is especially acute with monomer compositions containing BIS-GMA, this substance being found, surprisingly, to undergo polymerization after but one day at room temperature even in the presence of (e.g. 2% by weight based upon monomer) cumene hydroperoxide, a thermally stable catalyst. Conversely, commercial inhibitor-containing methyl methacrylate containing from 1-2% by weight benzoyl peroxide undergoes polymerization after a few days at 25°C, 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 73°C and for cumene hydroperoxide and t-butyl hydroperoxide, 160°C and 170°C, respectively. BIS-GMA is accordingly somewhat anomalous in that it undergoes fairly rapid premature polymerization even with thermally stable catalyst material.

The 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 exists with monomers such as methyl methacrylate. In the former case, treatment with the ion-exchange resin should preferably be before contact of monomer and catalyst.

The sulphonic acid cation-exchange resins are well known and commercially available in a wide variety of forms. Particularly preferred - 5 51153 herein are the sulphonated, crosslinked, polystyrene resins such as that available commercially as DOWEX 50 W-X8 in the form of beads (1.9 meq/ml, H+ form wet - 5.1 meq/ml dry). DOWEX is a Registered Trade Mark. Other suitable sulphonic acid cation-exchange resins include DOWEX 50W-X2, DOWEX 50W-X4 and DOWEX 50W-X10 (Dow Chemical Co.) as well as Amerlite IR 120,Anberlyst 15 (Rohm & Haas) and Rexyn 101(H) (Fisher Chem. Co. - characterized as 4.6 meq./ml - dry basis). Amberlite, “Amberlyst and Rexyn are Registered Trade Marks.

Cation exchange resins are used in the present invention to treat the monomer preferably prior to contact thereof with the catalyst, i.e. as a pretreatment. According to this method, contaminants removable 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 - 5a.

Si 1 53 the methacrylate monomer. Such contacting may be by conventional 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 maintained until contaminant is at least substantially removed, this being determined by storage stability tests For example, in a laboratory run a solution mixture of 8 parts of cation exchange resin with 52 parts of a BIS-GMA containing monomer composition provided a substantially contaminant-free monomer product after 24 hours standing with agitation. Alternatively, 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 this type, BIS-GMA is particularly preferred and in preferred embodiments constitutes at least about 50# by weight of the total monomer composition. The commercial BIS-GMA available from Freeman Chemical Co. under the trademark NUPOL Is an example of materials useful herein. - 6.51153 Methacrylate monomers particularly useful in this invention are those represented by the following general formulae: [(M- A - 0)n- Ar]2 -B I (M-A)mCRp MgR' (M - A - 0C0)2Ar II (M - A - 0C0 - NH)2R3 III IV CH, - M I CH - M' I 0¾ - M VI wherein M is methacryloyloxy, i.e. CHg = 0(011^)000-} M' is methacryloyloxy or liydroxyl} A is alkylene having 1-3 carbon atoms, such as methylene, propylene, isopropylene, liydroxyalkylcne having 1-3 carbon atoms, such as Ixydroxymcthylene, 2-hydroxypropylene or acetoxyalkylene having 3-5 carbon atoms in the alkylene group such as 2-acetoxypropylene, 3-aeotoxyamylene etc.jn is 1-4 preferably 1 or 2;m is 2 or 3 and p is 1 or 2 with the proviso the sum of m and pis 4; R is hydrogen, methyl, ethyl or -A-M wherein A and M are previously described} Ar is phenylene, e.g., o-phenylene, m-phenylene or p-phenylene, alkyl substituted phenylene, e.g., tolylene or 5-t-buty.l-m-phcnylene or cycloaliphatic having 6 to 10 carbonatoms such as 1,3-cyclohexylene} jj is X X wherein R. and Rr are Independently hydrogen, / X 4 j K5 ilkyl. e.g. C to 0 , or substituted alkyl; and R' is alkylene 4having 2 to 12 carbon atoms such as ethylene, dodecylene, etc. o 2 or -R 40-R ·) OR- wherein R is alkylene having 2 or 3 carbon atoms such as ethylene, propylene or isopropylene and x is - 7 51153 zero to 55 and r3 ]_2 phenylene, tolylene, methylcne-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 arc readily provided by conventional synthetic routes, for example, by reacting a phenolic compound such as diphenolic acid, phloroglucinol or bisphenol A with glycidyl methacryiate χη the presence of various tertiary amines and/or phosphines or by rt-actinr methacrylic acid with an epoxide-containing compound such as the dirlycidyl ether of a bisphenol. Some of these monomers also are made by reacting appropriate alcohols with methacrylic acid, methacrylyl chloride or methacrylic anhydride.

Illustrative monomers having these formulae include: HO OH C } CH2OCOC(CH3) = CHgk; . 0-CH2-CH(0H) CHgOCOC (0¾ )=0¾ 01^= C (01^)000(0¾ )^0000(0¾ )=0¾.

CHp= C (Cll3) COOCHgCHgOCIIgCHgOCHgCHgOCOC (CHg )=CHg J - 8 51153 CHgsC (CH3) COO~CHj,CH( 0H)CH>-0- gCH(OII)CH2OCOC(CH3)=CHt bci^CH(0H)0C0C(CH3)=CH2 Cll, C (CHj )CO0-CH2CH20C0NH-^q' NHCOOCH CH OCOC(CH )=CH 2 2 3 2 9¾ C H, (CUj) COO-Cl^CH-OCONH-C^CHgC-g-C-CHgNHCOOCH-CHg-OCO-C (CH3)=CH^ 1 lift CH CH3 CH .10 IS Monomers having the formulae I, XI, XII and IV are preferred In the practice of this invention. Of these monomers, I, II and 111 arc particulai’ly preferred, monomers IV being employed more often in admixture with or 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; (MR/|0Ar)2C(ClIj)2 wherein l^is isopropylene; (MHJ0Ar)2and(MR^0)2Ar wherein / is 2-hydroxypropylene; MA R M wherein R is liydroxycyclopentyl or £ bydroxyeyc]ohexyl, and A is 2-h.ydrox.vethylene; and wherein R i S : c*2 or to - 9 S3 Generally these monomers are commercially available or readily prepared. Preparative details for many of these monomers are given in U.S. 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 U.S. Patent No. 3,539,526. All of the aforementioned patents are herewith incorporated by reference in their entirety.

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 characteristics 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 comor thereabouts position comprises a l:l/mixture of BIS-GMA and HMDMA, this system having excellent stability at both room and elevated temperature (37°C). - 10 51153 Free radical liberating catalysts useful herein include, generally, organic compounds which, when activated, liberate free radical species capable of initiating polymerization of the aforedescribed monomers to form solid MAS polymerizates having, in particular, in the case of filler5 containing systems good compressive strength, of the order of at least 25,000 to 30,000 p.s.i.(1757750 to 2109300 gm/cm2), and preferably at least 35,000 to 40,000 p.s.i. (2460850 to 2812400 gm/cm2). Preferred materials are the organic peroxides and hydroperoxides and of these, benzoyl peroxide, cumene hydroperoxide, t-butyl hydroperoxide, p-methane hydroperoxide and diisopropyl benzene hydroperoxide are especially preferred. The catalyst is generally present in amounts of from 0.5 to 5%, preferably 1 to 4% and most preferably 1 to 3% by weight of total monomer. A particular advantage of 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 storageability 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. - 11 51153 which may be The inorganic particulate fillers Employed in the compositions of this invention include fused silica, quartz, crystalline silica, amorphous silica, soda glass beads, barium glass and other radiopaque glasses, glass rods,’ceramic oxides, particulate silicate glass and synthetic minerals such as beta-eucryptite (LiAlSiO^), having a negative coefficient 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, fluoresand cent zinc oxides,/titanium dioxide.. The filler particles would be generally smaller than about 5θ microns in diameter and preferably smaller than 30 microns. Unfilled compositions are particularly useful where the dental composition Is intended for use as a cotaing, margin sealant for margin restorations or adhesive.

As mentioned, keying or coupling agents likewise optional for use herein, are particularly beneficial when using a silica filler, since they tend to at least partly counteract the polymerization inhibiting effects of the filler thereby restoring an approximately equivalent measure of polymerizability. The beneficial effects of the keying agents which are silane compounds are most evident with respect to the compressive strength characterizing the final dental polymerizate.

The silane coupling agents or keying agents are materials that contain at least one polymerizable double bond - 12 51153 to react with the methacrylate monomers. Examples of suitable coupling agents are gamma-methacryloxypropyl trimethoxy silane, vinyl vinyl trlchlorosilane, tris(2-methoxyethoxy)/silane, tris (acetoxy) vinyl silane, 1 - N-(vlnylbenzylaminoethyl) amino5 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 if used, composition containing the filler,/there being no requirement 10 for prior hydrolysis according to, for example the acid and alkaline hydrolysis techniques described in U.S. 3,066,112 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 .3% to6% coupling based on the weight of the monomer by weight of the/agent/is deposited on or reacted with the filler and thereafter blended with the monomer composition. Apparently, tho 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 substituted thioureas, e.g. acetyl thiourea, Ν,Ν-dimethyl pnra-toluldlnc and para-toluencsulphonic acid. Polymerization and consequent hardening occurs rapidly expeditiously bul· over a suffieiput interval enabling the dentlst/to - 13 511S3 or restoration treat the area designated for dental repair/with 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 have been found to be formulated as herein described / 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 indicating good stability for extended periods at temperature on-the order of 37°C.

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 high contaminant monomer and relatively stable catalyst material. The importance of the catalyst in this respect is not totally discounted.

However, to the extent 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 affecting the catalyst do not in many, if not most, cases lend themselves to precise chemical identification, it is not necessarily luiown 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 - 14 5113¾ dramatic example ol* a commercial material discovered by the present applicant to contain such de-stab Hissing 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 quantities, i.e. quantities sufficient to initiate a rate of polymerization which produces significant polymer within a relatively short time. By short time in 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, insofar 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 period.

Considering also that the polymerizable 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 population, 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 understood having reference thereto. Thus, treatment ofa preformed mixture of catalyst and monomer, in accordance with one embodi- 15 51,153 ment of tlie invention, prior to appreciable free radical formation, fiim/ily 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 methacrylate (MMA) in the presence of organic peroxide and hydroperoxide catalysts is compared as follows; Portions of MMA (Rohm and Haas product containing lOppm · methj'l ether of hydroquinone,MEHQ) are mixed with catalyst to provide 2/ solutions of benzoyl peroxide (BP), cumene hydroperoxide (CUP ), and t-butyl hydroperoxide (TBH) respectively. The solutions are allowed to stand at ambient temperature (20-25°C).

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 20-25°C. The results obtained are tabulated as follows; TABLE I Gel Time (days) Catalyst Monomer BP TBII CHP (a) BIS GMA - 3 2 MMA 3 (a) (a) (a) no gelling after 9 days when experiment terminated Surprisingly, BIS-GMA appears to be more stable with a less thermally stable catalyst, i.e. BP as' compared with either TBH or CHP. As previously Indicated, both TBH nr CHP are recommended for use al. temperatures significantly higher than for BP and would be expected , Μ|„.π TgH or CHP Ls dissolved lit the monomers, to give more stability against polymerization than BP. - 16 51153 The data for MMA are more in line with what would normally be expected, underscoring the rather anomalous stability aspect of 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 illustrative of embodiments of the invention. All parts and percentages are by weight.

EXAMPLE I To 52 parts of a homogeneous 1:1 mixture of BIS-GMA, (HUPOL 46-4005 batch no. 124,793) and hexamethylene dimethacrylato (HMDMA), 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 fox· 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 respectively. 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/ CUP solution. The greatly improved stability obtained for the BIS-GMA/HMDMA monomer composition in this example is even more surprising considering the high (4#) concentration of CHP.

EXAMPLE 2 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 - 17 51153 part:; of amorphou:; silica. The composition is stable (no gel formation observed) for over one year at room temperature.

EXAMPLE 3 The aged product obtained from Example 2 is mixed with an equal portion of a similar composition but omitting tho CHP and adding 2>j acetyl thiourea as a reductant. Λ rapid and complete cure is effected.

Other monomer compositions tested as described in the foregoing examples with similar stabilization results are as follows vrith parts indicated parenthetically: EIS-GMA (71) HMDMA (29) BIS-GMA (71)1PEGDMA (29) BIS-GMA (50)2tegdma (50) BIS-GMA (71)2tegdma (29) polyethylene glycol dimethacrylate triethylene glycol dimethacrylate

Claims (10)

CLAIMS.

1. A polymerizable composition comprising at least one methacrylate monomer having 2 to 4 polymerizable olefinic double bonds and at least one free radical-liberating polymerization catalyst capable when activated of initiating polymerization of the said monomer or monomers, the catalyst being present in amounts sufficient to achieve a predetermined rate and/or degree of polymerization, the said monomer or monomers having been treated with a sulphonic acid type cation-exchange resin in free acid form.

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

3. A composition as claimed in Claim 1 or Claim 2 wherein at least 50 weight percent of the methacrylate monomer contains at least one aromatic ring in its central portion.

4. A composition as claimed in Claim 1 or Claim 2 wherein at least 50 weight percent of the methacrylate monomer is the reaction product of bisphenol A and glycidyl methacrylate.

5. A composition as claimed in Claim 4 containing up to 50 weight percent of the monomer of hexamethylene dimethacrylate.

6. A composition as claimed in Claim 5 wherein the monomer comprises an approximate 1:1 mixture of the reaction product of bisphenol A and glycidyl methacrylate and of hexamethylene dimethacrylate.

7. A composition as claimed in any of the preceding claims containing - 19 51153 up to about 400% by weight based on the weight of the monomer of particulate inorganic filler.

8. A composition as claimed in Claim 7 wherein the filler is a siliceous material. 5

9. A composition as claimed in Claim 8 wherein the filler is silica.

10. A composition as claimed in any of the preceding claims containing up to 5 weight percent of the monomer of silane coupling agent containing at least one polymerizable double bond.