PURPOSE: The titled polymerization initiator useful in the production of a high-MW polymer, containing a per ester obtained by reacting a specified compound with a benzenetricarboxylic acid trihalide in the presence of an alkali metal hydroxide as an effective component.

CONSTITUTION: A t-alkyl hydroperoxide or a t-aralkyl hydroperoxide is reacted with a benzenetricarboxylic acid trihalide at 0W40°C in the presence of 5W40wt% aqueous alkali metal hydroxide solution to obtain a per ester of the formula (wherein R is a 1W5C alkyl or phenyl), e.g., tri-t-butyl pertrimellitate. A monomer is polymerized at 70W180°C in the presence of 0.01W2wt% radical polymerization initiator having said per ester as an effective component to obtain a high-MW polymer, a block copolymer polymes, excellent in transparency and flexibility or the like.