Allylic onium salts with different hetero-atoms and various substituent groups at the allylic double bond were shown to be very efficient initiators for cationic polymn. When attacked by a radical, they become radical cations, which are highly unstable species, and undergo fragmentation into smaller radical cations called onium radical cations. The reaction mechanism involves radical formation, addn. and fragmentation. Factors affecting the polymn. efficiency are discussed theor. using semi-empirical quantum mech. techniques. The type of radical species, substituent group at the allylic side, the heteroatom at the onium side, and the onium group itself are analyzed sep. For this purpose, the geometries of different onium radical cations to be fragmented are optimized and the strength of the C-heteroatom bond to be broken and the size of the radical cations after fragmentation are considered.