GB2075035A - Methacrylate Based Dental Compositions - Google Patents

Abstract

A polymerizable composition particularly for dental use, comprises at least one methacrylate monomer having 2 to 4 polymerizable double bonds, from 0 to 400% based on the monomer of inorganic particulate filler, from 0 to 5% based on the monomer or silane coupling agent, an effective catalyst-activating amount of accelerator comprising copper ion and from 0.5 to 5.0% based on the monomer of (a) an organic peroxy compound and (b) reducing agent for said peroxy compound or a mixture of (a) and (b) wherein the concentration of (b) does not exceed 40% of the total quantity of (a) and (b).

Description

SPECIFICATION Dental Compositions The invention relates in general to polymerizable compositions and particularly but not exclusively to such compositions for dental use capable of undergoing a rapid rate of cure to produce a product polymerizate having a relatively higher quantity of insoluble component.

Polymerizable dental compositions based on the use of methacrylate monomers, e.g. the reaction product of the bis glycidyl ether of bis phenol A and methacrylic acid (hereinafter designated BIS GMA), are commonly used as fillings, pit and fissure sealants and are advantageously adapted to a wide variety of dental restorative techniques. Such compositions typically include one or more methacrylate monomers and at least one component of a free radical liberating (redox) polymerization system for said monomer(s). Usually, the monomer composition so formulated includes the peroxy type catalyst (oxidant) which is later contacted with the reducing agent (reductant) shortly prior to dental use.Upon contact of the oxidant with the reductant compositions, usually for convenience provided as pastes, polymerization occurs leading to the formation of a polymerizate having the physical properties necessary to the maintenance of good structural integrity within the oral cavity, e.g. high compressive strength and high degree of insolubles. The latter condition is highly desirable to minimize leaching out of vital ingredients and consequent deterioration of the polymerizate.

Rapid and complete curing are thus highly desirable objectives. To some extent, increase in catalyst concentration enhances curing rate; however, correlating enhancement in the degree of cure does not necessarily obtain. In fact, it is often the case that large catalyst concentrations impair the degree of cure thus producing a relatively larger percentage of soluble and leachable components in the final polymerizate. Moreover, larger amounts of peroxy catalyst increase the possibility of pre polymerization of methacrylate monomer, i.e. prior to contact with reductant thereby often necessitating the use of adjuvants, often in unusually large amounts having a stabilizing or polymerization retardant effect.However, such retardant effects are inevitable manifested to some extent during monomer-oxidant-reductant contact thereby depressing the cure rate and, unavoidably the degree of cure.

A primary object of the invention is to provide polymerizable dental compositions wherein the foregoing and related disadvantages are eliminated or at least mitigated to a substantial extent.

Another object of the invention is to provide polymerizable dental compositions capable of undergoing a rapid rate of cure to produce a polymerizate having a relatively high quantity of insoluble component.

Yet another object of the invention is to provide such a composition having good structural stability within the environment of the human oral cavity.

Still another object of the invention is to provide such a composition wherein any requirements for using higher catalyst concentrations to achieve effective rate and degree of cure are obviated.

A further object of the invention is to provide such compositions having the aforedescribed beneficial characteristics despite the absence of filler and/or coupling components and the like.

A still further object of the invention is to provide such compositions wherein in accordance with a particularly effective embodiment, stability of monomer in the presence of peroxy catalyst is effectively improved.

Yet a still further object of the invention is to provide a process of utilizing such compositions to prepare a high quality polymerizate.

Other objects and advantages of the invention will become apparent hereinafter as the description proceeds.

According to the present invention, a polymerizable composition, having an accelerated curing rate and being more particularly but not exclusively for dental use, comprises at least one methacrylate monomer having 2 to 4 polymerizable double bonds, from 0 to 400% based on the monomer of inorganic particulate filler, from 0 to 5% based on the monomer of silane coupling agent, an effective catalyst-activating amount of accelerator comprising copper ion and from 0.5 to 5.0% based on the monomer of (a) free radical liberating polymerization catalyst comprising an organic peroxy compound, (b) reducing agent for said peroxy compound or a mixture of (a) and (b) wherein the concentration of (b) does not exceed 40% of the total quantity of (a) and (b).The invention also extends to a process for forming such a composition comprising contacting a methacrylate monomer having 2 to 4 polymerizable double bonds with a redox polymerization catalyst capable of initiating the polymerization of the methacrylate monomer and comprising an organic, peroxy compound and a reducing agent therefore, the said contacting being carried out in the presence of an effective catalyst activating amount of copper ion.

The accelerator preferably comprises cupric ions. Cupric ions are most conveniently supplied in the form of copper salts with organic and inorganic acids. Particularly preferred materials for use herein include, cupric acetate, cupric acetyl acetonate, cupric chloride, and cupric sulphate. The compounds may be used singly or in admixture comprising two or more. In addition, cuprous ion may be present with the foregoing, although the cupric compounds are found to be generally more effective to give both high degree and rate of cure. Accordingly, it is particularly preferrred herein that cupric ion (Cu++), constitutes at least about 50% of the total copper ion used. However, and as will be hereinafter demonstrated, the use of cuprous salts alone provides significant improvement when compared to test runs omitting entirely the copper ion.Whether this is due to the presence of cupric ions invariably present in available cuprous salts or to the cuprous ion or to the combination has not been clearly established.

The amount of copper ion present in the monomer composition is exceedingly small generally ranging from about 5 to 100 ppm preferably 8 to 50 ppm based on total monomer. Otherwise stated, the amount of copper ion is at least that necessary to provide effective catalyst activating or accelerating effects. The term "total monomer" as used herein refers to the total monomer which would be present at the time polymerization actually occurs. Thus, in accordance with a preferred embodiment, peroxy catalyst, monomer and filler if used are included in the first past composition referred to as the "oxidant" paste and the reducing agent for said peroxy catalyst is included in the second "reductant" paste preferably including the same components as the first paste except for the peroxy catalyst. The pastes are mixed shortly prior to use e.g. dental use, whereupon polymerization is initiated.The copper ion may be added to either or both of the paste compositions with its presence in the oxidant composition being most preferred. In any event, copper ion concentration in this embodiment has reference to the total monomer present in both the oxidant and reductant pastes. On the basis of catalyst, the concentration of copper ion ranges from about 0.02 to 0.4% of the peroxy compount. Generally, it is preferred to add the copper compound to the fluid monomer system (absent filler) which is intended as the oxidant paste.

The copper compound can be added to the composition by dispensing same in the normally liquid monomer(s) component. The requisite dispersion can be achieved by adding the copper compound to the monomer as a solution in a solvent preferably a polar organic solvent of low boiling point such as ether or lower alkanol, e.g. methanol. Since the solvent is merely a carrier for the copper compound it is most desirable that it be of high volability so that generally, most if not all of the solvent is removed in subsequent handling operations, e.g. mixing, etc. The amount of solvent used is quite low, thus to provide 80 ppm (Cu++), and using only a 1% solution in methanol, the volume of solvent would be only 12.5% of monomer volume and 2.5% of a suitable paste on a weight basis.Alternatively, the copper compound may be sorbed onto the filler component, e.g. silica, should such material be used, for mixing with the monomer component(s).

In addition to the copper compound, the preferred compositions herein are as follows: Parts by weight Monomer(s) 100 Inorganic particulate filler 0--400 Silane Coupling agent 0.5-5.0 Peroxy catalyst 0.5-5.0 Reducing agent 0.3-2.0 The methacrylate monomer is selected from materials having at least two, and preferably two to four polymerizable double bonds per molecule in order that the cured composite be crosslinked and thus better suited for use in the oral cavity. The most preferred monomers are those having two polymerizable double bonds per molecule. Desirable characteristics for such monomers include low polymerization shrinkage, low exotherm during polymerization, low water sorption and the ability to cure rapidly and completely in the mouth.It is also desirable that the monomers be low in volatility and non-irritating to the tooth pulp.

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.

Methacrylate monomers particularly useful in this invention are those represented by the following general formulae: F (N- A - O) - Ar ] - B ( - A - OCO)2Ar (M - A) CR n 2 in I II III N (N - A - OCO - NH)2R M2R' (M - A - OCO - NH)2Rj IV V

wherein M is methacryloyloxy, i.e. CH2=C(CH3)CO0-; M' is methacryloyloxy or hydroxyl;A is alkylene having 1-3 carbon atoms, such as methylene, propylene, isopropylene hydrocyalkylene having 1-3 carbon atoms, such as hydroxymethylene, 2-hydroxypropylene or acetoxyalkylene having 3-5 carbon atoms in the alkylene group such as 2-acetoxpropylene, 3-acetoxyamylene etc.; n is 1 to 4 preferably 1 or 2; m is 2 or 3 and p is 1 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 previously described; Ar is phenylene, e.g. o-phenylene, mphenylene or p-phenylene, alkyl substituted phenylene, e.g. tolylene or 5-t-butyl-m-phenylene or cycloaliphatic having 6 to 10 carbon atoms such as 1 ,3-cyclohexylene; B is

wherein R4 and R5 are independently hydrogen, alkyl, e.g.C, to C4, or substituted alkyl and R' is alkylene having 2 to 1 2 carbon atoms such as ethylene, dodecylene etc., or R2(OR2)x OR2- wherein R2 is alkylene having 2 or 3 carbon atoms such as ethylene, propylene or isopropylene and x is zero to 5; 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 or by reacting methacrylic acid with an epoxide containing compound such as the diglycidyl 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:

C f CH2OCOC(CH3) =

C}{2- C(CH3)COO(CH2)40CoC(Cli3)=CH2i CH,= C (CH3 ) COOCH2CH20CH2CH20CH2Cil20COC (CH2 ) =CH2;

Monomers having the formulae I, II, III and IV are preferred in the practice of this invention. Of these monomers I, II and lil 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; (MR4OAr)2C(CH3)2 wherein R4 is isopropylene; (MR5OAr)2 and (MR5O)2 wherein R5 is 2-hydroxypropylene; MARES wherein Re is hydroxycyclopentyl or hydroxycyclohexyl, and A is 2-hydroxyethylene; and M2R8 wherein R8 is:

Preparative details for many of those monomers are given in U.S. Patent Nos. 3,066,1 12; 3,721,644; 3,730,947; 3,770,881 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, mixture 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.

The inorganic particulate filler employed in the compositions of this invention include fused silica, quartz, crystaline silica, amorphous silica, soda glass beads, glass rods, ceramic oxides, particulate silicate glass, radiopaque glasses (barium and strantium glasses), and synthetic minerals such as betaeucryptic (LiAISi04) 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. Also available as a filler are colloidal or submicron silicas coated with a polymer. Small amounts of pigments to allow matching of the composition to various shades of teeth can be included. Suitable pigments include iron oxide black, cadmium yellows and oranges, fluorescent zinc oxides, titanium dioxide, etc.The filler particles would be generally smaller than about 50 microns in diameter and preferably smaller than 30 microns. It will be noted that the filler is an optional ingredient, unfilled formulations being employed where the dental composition is intended for use as a coating, margin sealant for amalgam restorations or adhesive.

The silane coupling agents or keying agents are materials that contain at least one polymerizable double bond to react with the methacrylate monomers. Examples of suitable coupling agents are vinyl trichlorosilane, tris (acetoxy) vinyl silane, 1 -N(vinylbenzylamineothyl) aminopropyl trimethoxysilane-3, or 3-methacryloxypropyl trimethoxy silane. The last named material is preferred for use with methacrylate monomers because of the similarity in reactivity of the double bonds.

Peroxy catalysts useful herein and capable of initiating polymerization of the methacrylate monomer(s) are well known in the art for such use and include, without limitation, conventional peroxy as well as hydroperoxy compounds such as cumene hydroperoxide, p-methane hydroperoxide, diisopropyl benzene hydroperoxide and t-butyl hydroperoxides. Hydroperoxides are the preferred species of organic peroxy polymerization catalyst with cumene hydroperoxide (CHP) being particularly preferred. If desired, peroxide stabilizers such as ascorbic acid, maleic acid and the like may be included in small amounts.

Reducing agents useful herein generally include any material capable of reacting with the peroxy compound to form polymerization initiating, free radical species as is known in the art. Particularly useful herein is a substituted thiourea having the formula:

wherein X is H or Y and Y is alkyl having 1 to 8 carbon atoms, such as methyl, butyl, octyl; cycloalkyl having 5 or 6 carbon atoms such as cyclopentyl, cyclohexyl; chloro, hydroxy or mercapto substituted alkyl having 1 to 8 carbon atoms such as chloroethyl, mercapto-ethyl, hydroxymethyl and chlorooctyl; alkenyl having 3 to 4 carbon atoms, such as allyl or methallyl; aryl having 6 to 8 carbons, such as phenyl or xylyl, and chloro, hydroxy-, methoxy-, or sulphonyl substituted phenyl such as chlorophenyl, phenylsulphonyl, hydroxyphenyl and methoxyphenyl; acyl having 2 to 8 carbon atoms such as acetyl, butyryl, octanoyl; chloro or methoxy substituted acyl, such as chloroacetyl, chlorobenzoyl, chlorotoluoyl and methoxybenzoyl; aralkyl having 7 to 8 carbon atoms, such as benzyl, or chloro or methyl substituted aralkyl such as methoxybenzyl; and Z is NH2, NHX or NX2. Examples of illustrative compounds suitable for use in the practice of this invention are methyl thiorea, isopropyl thiorea, butyl thiourea, octyl thiourea, benzyl thiourea, acetyl thiourea, benzoyl thiorea, octanoyl thiorea, cyclohexyl thiorea, allyl thiourea, 1,1,3-triphenyl thiourea, 1,1,3-trimethyl thiourea, 2,4-xylyl thiourea, ptolylsulphonyl thiourea, 1 -octyl-3-phenyl thiourea, o-methoxyphenyl thiourea, m-hydroxyphenyl thiourea, 1,1 -diallyl thiourea, 1 ,3-diallyl thiourea, 2-methallyl thiourea, o-methoxybenzyl thiourea, 1 - (hydroxymethyl)-3-methyl thiourea, 1,1-dibutyl thiourea, 1,3-dibutyl thiourea, 1-(p chloro phenyl)-3methyl thiourea, 1 butyl-3-butyryl thiourea, 1-acetyl-3-phenyl thiourea, 1 methyl-3-(p-vinylphenyl) thiourea, 1 -methyl-3-o tolyl thiourea, 1 -methyl-3-pentyl thiourea, 3-methyl-i, 1 -diphenyl thiourea and 1-acetyl 3-(2 metcaptoethyl) thiourea. While any of the aforementioned thioureas can be employed in the practice of this invention, preferred are the monosubstituted thioureas, that is, those having the aforementioned formula wherein X is H and Z is NH2. Particularly preferred are phenyl thiourea, acetyl thiourea and allyl thiourea.Preferably, the composition contains about 0.5 to about 1% by weight of reducing agent.

The following Examples are illustrative of embodiments of the invention. All parts and percentages are by weight.

Examples 1-14 Radiopaque reductant pastes having the following compositions are prepared: Composition % by Weight Ingredient A B C D E F G H Nupol 10.27 10.24 10.27 10.27 10.27 10.27 10.27 10.27 2HMDMA 10.27 10.24 10.27 10.27 10.27 10.27 10.27 10.27 3A-174 1.03 1.02 1.03 1.03 1.03 1.03 1.03 1.03 Acetyl Thiourea 0.43 0.43 0.43 0.43 0.43 0.43 0.43 0.43 Ascorbic acid - 0.07 - - - - - - 4Cu++ - - .00022 .00044 .0011 .0022 .0033 .0044 5lmsiIA-10 41.86 41.86 41.86 41.86 41.86 41.86 41.86 41.86 eCorning 7724 36.14 36.14 36.14 36.14 36.14 36.14 36.14 36,14 Total 100 100 100 100 100 100 100 100 1-Bis (GMA) 2-1,6 hexanedioldimethacrylate 3-z-methacryloxy propyl trimethoxy silane eln the form cupric acetate added from methanol solution to monomer mixture 5--Amorphous silica (Illinois Minerals) 6-Barium Glass (Corning) "Nupol", "Imsil" and "Corning" are trade marks.

Radiopaque oxidant pastes having the following compositions prepared.

Composition % by weight Ingredient I J Nupol 9.94 9.78 HMDMA 9.94 9.78 All 74 0.99 0.98 Naleic acid 0.41 Ascorbic acid 0.07 Cumene hydroperoxide (80%) 1.05 1.05 Imsil A-10 41.93 41.79 Corning 7724 36.06 36.22 Total 100 100 Oxidant and reductant pastes respectively are mixed in a weight ratio of 1:1 as indicated in the following Examples. Curing times are given in minutes and include time required for mixing, approximately 30 seconds. The cessation of the curing reaction is determined by the material no longer being indentable by a spatula. It can also be done by measuring the percent of polymerizate insoluble in methanol upon completion of the curing reaction.Samples of the polymer-forming mass are take at timed intervals during the curing reaction; cessation of the latter is indicated by the point at which percent insolubles remains substantially constant. The results are summarized as follows: PPM Cu++ Example Oxidant Reductant in Reductant Cure Temp No.Paste Paste Paste Minutes OC 1 J C 2.2 3.5-4.3 21 2 " D 4.4 2.8-3.2 23 3 " E 11.0 1.8-2.2 23 4 ,, F 22.0 1.7-2.2 20.5 5 ,, G 33.0 1.5-1.8 21.5 6 ,, H 44.0 1.3-1.8 23 7 " A none 4.0--5.0 27 8 I C 2.2 3.3-3.8 21 9 ,, D 4.4 2.7-3.5 22.5 10 ,, E 11.0 1.8-2.2 23 11 ,, F 22.0 1.7-2.0 20.5 12 ,, G 33.0 1.3-1.7 21 13 ,, H 44.0 1.25-1.6 23 14 " A none 3.3 4.0 21 In each of the runs, curing is carried out in air, i.e. in an open vessel. Increased concentration of Cu++ provides more rapid cures. In comparison to Examples 7 and 14, which omit the copper accelerator, the data establish marked improvement in curing rate for the present invention.

Similar combinations (1:1) of reductant paste B with I and J produce comparable results.

Examples 15-17 The procedure of the foregoing examples is repeated but using a Nupol:HMDMA (71 :29) by weight monomer blend in each of the oxidant and reductant compositions.

Filler is omitted altogether in each of the examples. Copper ions are provided by the following compounds: Example No. Accelerator 15 cupric acetate 1 6 cupric chloride 17 cupric acetonyl acetonate (no solvent) In each case, improved rate of curing is achieved when compared to identical control runs omitting the copper compound.

Examples 18-20 The procedure of Examples 1 5-1 7 is repeated but using the following materials as accelerators: Example No. Accelerator (weight ratios) 1 8 cupric acetate/cupric chloride (50:50) 1 9 cupric acetate/cuprous chloride (50:50) 20 cupric chloride/cuprous chloride (50:50) The results are similar to those of the preceding examples.

Examples 21-34 Each of examples 11-4 is repeated with the exception that the cupric acetate is sorbed onto the filler and then added to the monomer composition. Similar improvement is obtained in curing rate by comparison with controi runs omitting the copper compound.

Examples 35-37 The following reductant-containing and oxidant-containing compositions are prepared: Reductant Composition % by weight Ingredient K L M Nupol 9.9 9.9 9.9 HMDMA 9.9 9.9 9.9 Acetyl Thiourea 0.45 0.45 0.45 Cu++ 0.0002 0.0005 0.0010 Corning 7724a 36.25 36.25 36.25 Imsil A-10a 42.25 42.25 42.25 Colloidal Silica 1.30 1.30 1.30 aSilanized Oxidant Composition-N Ingredient Composition % by weight Nupol 9.325 HMDMA 9.325 CHP (80%) 1.05 Corning 7724a 36.25 Imsil A-10a 42.25 Colioidal Silica 1.80 aSilanized The Oxidant paste is mixed on a clean glass plate with each of the three reductant pastes in a 1:1 ratio.Curing times for each of three mixes are determined by the time required for the blended material to be resistant to indentation by a plastic spatula. The results are summarized as follows: PPM Cu++ in Example Oxidant Reductant Reductant Cure a No. Paste Paste Paste Minutes 1 N K 2 4.2-5.0 2 N L 5 2.5-3.0 3 N M 10 2.3-2.8 aat 23"C In each of the above runs, curing is carried out in air. (Shorter curing times would result under glass or in a cavity covered by a matrix strip). It is evident that increasing the concentration of Cu++ results in a shorter cure time.

Similar results are obtained when the foregoing examples are repeated but utilizing, within the ranges hereinbefore specified the following peroxy catalysts and reductant compounds: peroxy catalysts-p-methane hydroperoxide, diisopropylbenzene hydroperoxide and t-butyl hydroperoxide; reductants-ailyl thiourea, phenyl thiourea, and 3-allyl-1, l-diethylthiourea.

Claims (14)

Claims

1. A polymerizable composition comprising at least one methacrylate monomer having 2 to 4 polymerizable double bonds, from 0 to 400% based on the monomer of inorganic particulate filler, frown 0 to 5% based on the monomer of silane coupling agent, an effective catalyst-activating amount of accelerator comprising copper ion and from 0.5 to 5.0% based on the monomer of (a) free radical liberating polymerization catalyst comprising an organic peroxy compound, (b) reducing agent for said peroxy compound or a mixture of (a) and (b) wherein the concentration of (b) does not exceed 40% of the total quantity of (a) and (b).

2. A composition as claimed in Claim 1 wherein the accelerator comprises cupric ion.

3. A composition as claimed in Claim 1 or Claim 2 wherein the ratio of (a) to (b) is about 2:1.

4. A composition as claimed in any of Claims 1 to 3 the reducing agent comprises a substituted thiourea compound.

5. A composition as claimed in Claim 4 wherein the reducing agent comprises an allyl or acetylsubstituted thiourea.

6. A composition as claimed in any of the preceding claims wherein the peroxy compound comprises cumene hydroperoxide and the reducing agent is acetyl thiourea.

7. A composition as claimed in any of Claims 1 to 5 wherein the peroxy compound comprises cumene hydroperoxide and the reducing agent comprises allyl thiourea.

8. A composition as claimed in any of Claims 2 to 7 wherein the cupric ion is present as cupric acetate, cupric acetul acetonate, cupric chloride or a mixture thereof.

9. A composition as claimed in any of Claims 2 to 8 wherein the concentration of cupric ion is from 5 to 100 ppm based on the monomer.

10. A composition as claimed in any of Claims 2 to 9 wherein the cupric ion is dispersed in the monomer.

11. A composition as claimed in any of the preceding claims wherein at least about 40% of the monomer comprises the reaction product of glycidyl methacrylate and bisphenol A.

12. A composition as claimed in Claim 11 wherein up to 60% of the monomer comprises 1,6hexanedioldimethacrylate.

13. A composition as claimed in any of Claims 2 to 12 comprising up to 50% by weight based on the weight of cupric ion of cuprous ion.

14. A composition as claimed in any of the preceding claims having a paste-like consistency.

1 5. A polymerizable composition as claimed in Claim 1 and substantially as described in the Examples.

1 6. A process for forming a polymerizate comprising contacting a methacrylate monomer having 2 to 4 polymerizable double bonds with a redox polymerization catalyst capable of initiating the polymerization of the methacrylate monomer and comprising an organic, peroxy compound and a reducing agent therefor the said contacting being carried out in the presence of an effective catalyst activating amount of copper ion.

1 7. A process for forming a dental polymerizable as claimed in Claim 1 6 and substantially as described in the Examples.