To realize excellent heat resistance and strength by specifying reduced viscosity in a chloroform soln. of a polymer of an α-substd. acrolein dimer.

The polymer of an α-substd. acrolein dimer, when dissolved in chloroform, has a reduced viscosity of 1-3 dl/g in a 1 wt.% chloroform soln. at 35°C and has a structure represented by the formula. In the formula R1 is a type of org. group selected from the group consisting hydrogen, a 1-6 C alkyl group, a cycloalkyl group, a phenyl group and a benzyl group. Preferable is a polymer of methacrolein dimer which has a methyl group as R1 in the formula. The polymer of an α-substd. acrolein dimer of the present invention is generally obtd. by dissolving an α-substd. acrolein dimer in an appropriate org. solvent, adding about 1.5-10 mol.% of a Lewis acid as a catalyst to the reaction mixture, subjecting it to reaction by agitating it at -20-70°C for several hr, then depositing the polymer in a solvent such as acetone.