Copolymerization process and optical copolymer produced therefrom

A process for preparing a copolymer suitable for use in an optical element which comprises the step of radically polymerizing a composition which comprises at least 45% by weight, of the weight of the polymerizable composition, of at least one first monomer of represented by the formula (I):

• ##STR5## wherein a and b are integers of from 0 to 4, R.sub.1 and R.sub.2 represent H or CH.sub.3, and R.sub.3 represents ##STR6## and - 40weight percent, of weight of the polymerizable composition, of at least one second monomer represented by any one of the formulas (II), (III) or (IV):
• ##STR7## wherein R.sub.4 represents H or CH.sub.3, X represents C.sub.1 -C.sub.4 alkyl or halogen other than fluorine, c represents an integer of from 0 to 5, and d represents an integer of from 0 to 5;
• in the presence of 0.005 to 1.0 weight percent, based on said polymerizable composition, of an organic chain transfer agent having a chain transfer coefficient at 50.degree.-60.degree. C. greater than 0.5 with acrylic monomer or styrenic monomer;
• and 0.05-2.0% by weight based on the weight of the polymerizable composition, of a radical polymerization initiator.