US 10,915,021 B2  55  56  -continued  CcJ)  I  RIS  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.  -continued 