wherein R1 to R4 are a hydrogen atom or a linear or branched hydrocarbon group having 1 to 20 carbon atoms, in which R3 and R4 may integrally form a ring; X is a halogen atom; M is an element selected from the group consisting of those belonging to 3-Group, 4-Group, 6-Group (exclusive of chromium), 13-Group, 14-Group and 15-Group of the Periodic Table; m and n are numbers satisfying the relationsh