Abstract: An aminotriazine prepolymer is provided which gives resin with toughness and favorable transparency without use of filler agent by mixing 2-hydroxyethyl methacrylate (781 g: 6 mole), melamine (126 g: 1 mole), 95% by weight of p-formaldehyde (189 g: 6 mole as formaldehyde), p-toluene sulfonate (3.0 g), hydroquinone monomethyl ether (0.2 g) in a two liter separation flask with a stirring device, a thermometer, an air-inlet tube, and a cooling tube for removing the distillate. The mixture is heated while air is blown in. After the melamine and paraformaldehyde are dissolved in 2-hydroxyethyl methacrylate, at the temperature of 80.degree. C. to 100.degree. C., the reaction is continued in the temperature range of 105.degree. C. to 115.degree. C. until the quantity of extracted water is 108 g (6 mole). The triazine ring structure-containing methacrylate prepolymer thus obtained has a viscosity of 2,000 cP at 20.degree. C., a bromine number of 92.1 (Br g/100 g), and a Gardner color degree of not greater than 5.

Abstract: An aminotriazine prepolymer is provided which gives resin with toughness and favorable trans-Darency without use of filler agent by mixing 2-hydroxyethyl methacrylate (781 g: 6 mole), melamine (126 g: 1 mole), 95% by weight of p-formaldehyde (189 g: 6 mole as formaldehyde), p-toluene sulfonate (3.0 g), hydroquinone monomethyl ether (0.2 g) in a two liter separation flask with a stirring device, a thermometer, an air-inlet tube, and a cooling tube for removing the distillate. The mixture is heated while air is blown in. After the melamine and paraformaldehyde are dissolved in 2-hydroxyethyl methacrylate, at the temperature of 80.degree. C. to 100.degree. C., the reaction is continued in the temperature range of 105.degree. C. to 115.degree. C. until the quantity of extracted water is 108 g (6 mole). The triazine ring structure-containing methacrylate prepolymer thus obtained has a viscosity of 2,000 cP at 20.degree. C., a bromine number of 92.1 (Br g/100 g), and a Gardner color degree of not greater than 5.