The preliminary polymerization may be carried out without solvent.The conditions for the preliminary polymerization are preferably controlled to a limiting viscosity (??) of the preliminary polymerization product (measured in decalin at 135?? C.) of 0.1 dL/g or more and an amount of the preliminary polymerization catalyst of from 1 to 10,000 g, and particularly from 10 to 1,000 g, per 1 mmol of th