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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. 

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