US 9,869,931 B2  62  61  -continued  X  /2  X  X  X  HO  X  HS  X  "’-o  "’-s  X  -continued  The acid labile group R $^{2 }$in recurring units (al) may be a  group having the general formula (A-3)-25.  (A-3)-25  Herein R 25 - 1  is each independently hydrogen or a straight,  branched or cyclic  C$_{1 }$-C$_{6 }$ alkyl group. When m25 is 2 or  more, two R 25 - 1  taken together may form a non-aromatic  ring of 2 to 8 carbon atoms. The circle denotes a link  between carbons  CA  and  CB,  selected from among ethylene,  propylene, butylene and pentylene. R 25 - 2  is a C$_{1 }$-C$_{4 }$ alkyl,  alkoxy, alkanoyl or alkoxycarbonyl, hydroxyl, nitro.  CcC$_{10 }$ aryl, halogen or cyano group. R is as defined above. R 25 - $^{1 }$is  not hydrogen when the circle is ethylene or propylene. The  subscripts m25 and n25 are an integer of 1 to 4.  Examples of the monomer from which recurring units  (al) having an acid labile group of formula (A-3)-25 sub› stituted thereon are derived are shown below. 