US 2020/0209747 Al  30  Jul. 2, 2020  [0102]  In a further embodiment, recurring units (d)  derived from an onium salt having a polymerizable unsatu› rated bond may be incorporated in the base polymer. The  preferred recurring units ( d) are recurring units having the  following formulae (dl), (d2) and (d3). These units are  simply referred to as recurring units (dl), (d2) and (d3),  which may be used alone or in combination of two or more  types.  [0103]  In formulae (dl) to (d3),  RA  is each independently  hydrogen or methyl. Z 1  is a single bond, phenylene,  ---O-Z 11 -, -C(=0)-0-Z 11 - or -C(=0)-NH› Z11-, wherein Z 11  is a C$_{1}$-C$_{6 }$ alkanediyl group, C$_{2}$-C$_{6 }$ -continued  (d3)  (d2)  R23  I  +s-R24  I  R2s  (dl) 