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The acid labile group R $^{2 }$in recurring units (al) may be a group having the general formula (A-3)-25. 

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Herein R 25 - 1 is each independently hydrogen or a straight, branched or cyclic C$_{1 }$-C$_{6 }$ alkyl group. When m25 is 2 or more, two R 25 - $^{1 }$taken together may form a non-aromatic ring of 2 to 8 carbon atoms. The circle denotes a link between carbons CA and CB, selected from among ethylene, propylene, butylene and pentylene. R 25 - 2 is a C$_{1}$-C$_{4 }$ alkyl, alkoxy, alkanoyl or alkoxycarbonyl, hydroxyl, nitro, CcC$_{10 }$ aryl, halogen or cyano group. R is as defined above. R 25 - 1 is not hydrogen when the circle is ethylene or propylene. The subscripts m25 and n25 are an integer of 1 to 4. 

Examples of the monomer from which recurring units (al) having an acid labile group of formula (A-3)-25 substituted thereon are derived are shown below. 

