63  US 9,360,753 B2  64  -continued  The acid labile group R 2  in recurring units (al) may be a  group having the general formula (A-3)-23.  Herein R 23 - $^{1 }$is hydrogen, a  C$_{1 }$-C$_{4 }$ alkyl, alkoxy, alkanoyl or  alkoxycarbonyl,  CcC$_{10 }$ aryl, halogen or cyano group, and  m23 is an integer of 1 to 4.  Examples of the monomer from which recurring units (al)  having an acid labile group of formula (A-3)-23 substituted  thereon are derived are shown below.  ==to  0  ==to  0  ==to  0  ==to  ==to  OH  0  &6  ==to  ==to  oA,  0  &6  ==to  0  (A-3)-23  X  #  #  -continued 