47  US 9,221,742 B2  48  The above examples correspond to those units of formula  (2) wherein Z is a single bond. Where Z is other than a single  bond, a combination with a similar acid labile group is pos› sible. Thus examples of the recurring units of formula (2)  wherein Z 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 con› sisting of hydroxyl, cyano, carbonyl, carboxyl, ether bond,  ester bond, sulfonate (sulfonic acid ester) bond, carbonate  bond, lactone ring, sultone ring, and carboxylic anhydride.  Illustrative examples of the recurring units of formula (3)  are given below, but not limited thereto.  (~)  ✓  ro  ✓  ro  ~  (~)  ✓  ro  0  q  0  ✓  0  0  (~)  ✓  ro  0  9l  (~)  ✓  ro  0  Q  (~)  ✓  ro  0  0  0  I  (~)  ✓  ro  0  (~)  ✓  ro  0  0l  0  0  (~)  ✓  ro  0  (~)  ✓  ro  0  (~)  ✓  ro  0  (~)  ✓  ro  0  0  0  -continued 