41  US 9,360,753 B2  42  -continued  Herein R$^{1 }$and R$^{2 }$are as defined above.  The other acid labile group-bearing recurring units, that is,  recurring unit (a2) in formula (1) are units in which the  hydrogen atom of phenolic hydroxyl group, specifically the  hydrogen atom of hydroxyl group on hydroxystyrene or  hydroxyphenyl(meth)acrylate is substituted by an acid labile  group. Suitable monomers from which these units are derived  are exemplified below.  0  0  OJ  0  ’-R2  0  XR’  0  0  (  0~  0  R2  0  0/  0/  R2  RI  )yo  0  ~ $_{0 }$ 0  0  oy  0  0  R2  0  0/  RI  )yo  0  ~  RI  0  )yo  0  0  RI  RI  )yo  )yo  0  0  0/  R2  RI  RI  )yo  )yo  0  0  0  0  -continued 