US 9,989,847 B2  70  69  -continued  In formula (el), R 1$_{a }$ is as defined above;  Ra  is a straight  C$_{1}$-C$_{20 }$or branched or cyclic C$_{3}$ -C$_{20 }$monovalent hydrocar› bon group which may be substituted with or separated by a  heteroatom, with the proviso that the monovalent hydrocar› bon group of  Ra  has 1, 2, 3 or 4 substituent groups having 5  formula (e2).  Herein R 6  is an acid labile group.  Illustrative examples of the recurring units having for› mula (el) are given below, but not limited thereto.  (e2)  Ria  Ria  I  I  ---t CH2 - C-,-- ---f-CH2 - C -,-- OAO  OAO  ~  /yORb  YORb  y  ORb  ORb 