41  US 9,989,847 B2  42  -continued  -continued  The above examples correspond to those units of formula  (2) wherein  za  is a single bond. Where  za  is other than a  single bond, a combination with a similar acid labile group  is possible. Thus examples of the recurring units of formula  (2) wherein  za  is other than a single bond are as illustrated  above.  In formula (3), YL is hydrogen, or YL is a polar group  having one or more structures selected from the group  consisting of hydroxyl, cyano, carbonyl, carboxyl, ether  bond, ester hand, sulfonate (sulfonic acid ester) bond, car- bonate bond, lactone ring, sultone ring, and carboxylic  anhydride.  Illustrative examples of the recurring units of formula (3)  are given below, but not limited thereto.  (~) (~)  /  Fo  /  Fo  )  ~  if  (~)(~)  /  Fo  /  Fo  (~)(~)  /  Fo  /  Fo  HO  JO  HO  (~)(~)  /  Fo  /  Fo  0  0  &op  0  (~)(~)  /  Fo  /  Fo  0  0  0  fY  0  (~) (~)  /  Fo  /  Fo  0  0  Ol  Gl  0  0  (~)  /  Fo  0  0  I  0  (~)  /  Fo  0  (~)(~)  /  Fo  /  Fo  0  0  (~)  /  Fo  0  0  0l  0  (~)  /  Fo  0  0 