1465897 Polyurethane-methacrylate dental filling compositions IMPERIAL CHEMICAL INDUSTRIES Ltd 18 April 1974 [24 April 1973 (2)] 19270/73 and 19277/73 Headings C3P and C3R A dental filling composition comprises (1) a polymerizable material comprising a polymerizable prepolymer which contains at least two polymerizable ethylenically unsaturated groups and is the reaction product of a urethane prepolymer and an ester of acrylic or methacrylic acid with a hydroxy alkanol of at least 2 carbon atoms, the urethane prepolymer being the reaction product of a di-isocyanate of the structure OCN-R 1 -NCO and a diol of the structure HO-R 2 -OH, wherein R 1  is a divalent hydrocarbyl group and R 2  is the residue of a condensate of an alkylene oxide with a compound containing two phenolic groups, (2) 60-90% by weight of the composition of an inert, inorganic particulate filler of which at least 50% of the particles have a maximum dimension not greater than 500 microns and a Knoop hardness of at least 100, and (3) if necessary or desirable, sufficient liquid ethylenically unsaturated monomer, copolymerizable with the polymerizable prepolymer, to ensure that the composition has a fluid, paste-like consistency. According to Example 1, a condensate obtained by reacting 2 : 2-bis-(phydroxyphenyl) propane and propylene oxide was heated to melt the condensate and stirred under reduced pressure to degas the condensate. 500 parts of 4 : 4-diisocyanatodiphenyl methane were similarly heated, stirred and degassed separately. The condensate and the diisocyanate were then mixed together and methyl methacrylate which had been dried over calcium hydride was added to the mixture followed by dibutyl tin dilaurate (catalyst). The mixture was cooled by means of an ice-bath to control the exotherm produced by the urethane prepolymer formation reaction so that the temperature did not rise above 55‹ C. and stirred. 2-(Hydroxyethyl) methacrylate which had previously been distilled and degassed was then added and the resultant mixture was stirred to form the ethylenically unsaturated polymerizable prepolymer. The mixture of polymerizable prepolymer and methyl methacrylate prepared above were mixed with glass spheres, 80% by weight of the spheres having a diameter between 4 and 44 microns then stirred, further methyl methacrylate was added to aid mixing, and there was then added a catalyst solution formed by dissolving benzil (photosensitive catalyst) in dimethylaminoethyl methacrylate (reducing agent) and methyl methacrylate. After 15 minutes the methyl methacrylate which had been added to aid stirring was removed by evaporation. The paste-like mixture was then exposed in a mould to radiation from a blue lamp for 15 minutes to cure the mixture. In Example 2 the filler employed was hammer milled glass fibres of size 10 microns by 200 microns. Before mixing with the other components of the composition, the glass fibres were surface coated with γ-methacryloxypropyltrimethoxysilane (coupling agent). In Example 3 the filler was silane coated quartz powder. The reaction product of 4,4<SP>1</SP>-diisocyanatodiphenyl methane with the condensation product of 2,2- bis-(p-hydroxydiphenol) propane and propylene oxide, and hydroxyethyl methacrylate were dissolved in ethylene glycol dimethacrylate in the absence of blue light and dimethylaminoethyl methacrylate followed by camphorquinone (photosensitive catalyst) were added to the solution. Borosilicate glass powder of particle size less than 53 microns coated with 1% by weight of silane were stirred into the mixture and the resulting fluid paste was filled into PTFE moulds and irradiated with light to cure. In Examples 5 and 6 the photosensitive catalysts were fluorenone and α-naphthol, respectively. In Examples 8 and 11, the fillers were glass platelets and silicon gel spheres, respectively. In Examples 15 to 22 the prepolymer of Example 4 was polymerized with the following additional monomers, respectively: methyl methacrylate, methacrylic acid, hydroxyethyl methacrylate, vinyl toluene, styrene, triethylene glycol dimethacrylate, ethylene glycol dimethacrylate and allyl methacrylate. In comparative Example 23 the urethane prepolymer was prepared by reacting 2,4-toluene diisocyanate, phenyl isocyanate, anisidene diisocyanate or 4,41-diisocyanatodiphenyl methane with 2,2 - propane bis - [3 - (4 - phenoxy)- - 1,2 - hydroxy propane - 1 - methacrylate] in the presence of dibutyl tin dilaurate.