PURPOSE: To obtain a complex having ability for increasing molecular weight without stopping polymerization completely by using an organic ether and a Lewis acid.

CONSTITUTION: The organic ether (preferable a 4-12C tertiary ether and the Lewis acid (such as BCl3, AlCl3, SbF5 and ZnCl2, for example), are led into a system at random and mixed at a temperature of ≤ the decomposition temperature of the resulting complex (preferably ≤-10°C) in the presence or absence of solvent so that the complex can be prepared. This complex has ability for increasing its molecular weight preferably to ≥1000 to 100000 by adding at least one olefinic monomer (such as styrene or isobutylene, for example), and continuing the addition of the monomer.