73  US 9,360,753 B2  74  -continued  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 substituted  thereon are derived are shown below.  (A-3)-25  The acid labile group R $^{2 }$in recurring units (al) may be a  group having the general formula (A-3)-25.  X  X  X  X  HO  HS  ~  O  X 