PURPOSE: To provide a novel cationic polymn. initiator being a compd. containing an onium cation and a non-nucleophilic metal complex anion and useful for forming a high mol.wt. polymer.

CONSTITUTION: A novel cation polymn. initiator contains a compd. such as trimethyloxonium having constitution represented by formula I [wherein Y is a cation selected from a group consisting of oxonium, sulfonium, selenonium, iodonium, diazonium, pyrylium, carbenium and acylium, R is a group represented by formula II (wherein R1 is H, halogen, alkyl or aryl) or a group represented by formula III, M is an element selected from group 3-15 elements, Z is 1-4 and a, b and n are an integer forming Z ≤ n and a×b=z]. This initiator is homogenously brought into contact with a cation polymerizable monomer such as styrene oxide to obtain a high mol.wt. polymer.