Cations derived from elements other than C and

6 CATIONS DERIVED FROM ELEMENTS OTHER THAN C, Si AND O 

To date these have not been very important as active centres of polymerizations, mainly because of the limited number of conventional monomer types suitable for building such a centre. Nitrogen cations are an exception. Cyclic imines polymerize on centres [148, 149] 

Centres of this kind might even gain technical importance in the future because N-substituted lactam derivatives can only be polymerized by a cationic process. 

Many centres with a formal positive charge on nitrogen have already been described, especially with polymerizations of more or less exotic monomers. 4,5-Dihydro-l,3-oxazine is transformed to polymer by the overall reaction 

A fair amount of data on the structure of centres suitable for the polymerization of phosphazenes, phosphonites [153] and sulphur-containing monomers [154] can be found in the literature. 

5-Dihydro-l,3-oxazine is transformed to polymer by the overall reaction 

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