PURPOSE: To provide the titled resin compsn. having improved heat distortion temperature, by copolymerizing two specified kinds of vinyl monomers or oligomers in the presence of a radical polymn. initiator and B2O3.

CONSTITUTION: 0.5W2pts.wt. radical polymn. initiator such as benzoyl peroxide and 10W80pts.wt. amorphous B2O3 powder having a particle size of 100μ or below, are mixed with 100pts.wt. mixture consisting of 10W60pts.wt. epoxy group-contg. vinyl monomer (A) or oligomer thereof such as glycidyl methacrylate and 90W40pts.wt. vinyl monomer (B) other than component A or oligomer thereof such as methyl methacrylate. The resulting mixture is cast into a suitable mold, and cured by heating.

EFFECT: The product shows an increase in glass transition temperature several tens of degrees higher than that of a product to which B2O3 powder is not added.