PURPOSE: To produce a high-quality transparent plastic, by prepolymerizing a monomer which, when homopolymerized, forms a linear polymer, in the presence of a specified chain transfer agent, adding a crosslinking polyfunctional monomer, effecting the main polymerization to thereby lower volume shrinkage during the main polymerization.

CONSTITUTION: A monomer which forms a linear polymer when homopolymerized (e.g., methyl methacrylate) is mixed with below 10wt% chain transfer agent which is an α-methyl-styrene dimer of the formula, and then the mixture is prepolymerized, during which time the system is allowed to shrink fully. Then, a crosslinking polyfunctional monomer (e.g., ethylene glycol dimethacrylate) is added, and the produced polymer syrup is cooled, deaerated, poured into a desired casting mold and polymerized by heating in a drying oven to form a transparent plastic (light transmittance in the visible region ≥70%).