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In the formulae, R$^{18 }$is a C$_{1}$-C$_{20 }$monovalent hydrocarbon group which may contain a heteroatom. Suitable monova- lent hydrocarbon groups are as exemplified above for R $^{11 }$to Rl7_ 

Illustrative, non-limiting examples of the sulfonium, cat› ion of formula (d4) are given below. 

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