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

CONSTITUTION: A crosslinking polyfunctional monomer (e.g., diethylene glycol bisallycarbonate) is mixed with below 3wt% α-methylstyrene dimer of the formula, as a chain transfer agent, and the mixture is prepolymerized, during which time the system is allowed to shrink fully. Then, the produced polymer syrup is cooled, deaerated, poured into a desired casting mold and polymerized by heating in an air oven to form a transparent plastic (light transmittance in the visible region ≥70%).