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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). 

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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. 

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(e2) 

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-continued 

