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Herein RA is each independently hydrogen or methyl. R 21 is a single bond or methylene. R 22 is hydrogen or a C 1 -C 4 15 alkyl group, the alkyl group being preferably straight or branched. X 1 is a single bond, ether bond, ester bond, amide bond, ----C(=O)---0-R 23 -, phenylene, -Ph-C(=O)› O-R24-, or -Ph-R 25 ---0----C(=O)-R 26 -, wherein Ph is phenylene, R 23 is a C 1 -C$_{10 }$ alkylene group which may be 20 straight, branched or cyclic, and contain an ether bond or ester bond, R 24 , R 25 and R 26 are each independently a single bond or a C 1 -C 6 straight or branched alkylene group. 

The subscript m is an integer of 1 to 5, n is an integer of 0 to 4, and lsm+ns5. It is preferred that n be an integer of 1 to 3 and m be an integer of 1 to 3 because the inclusion of hydroxyl group ensures more efficient generation of second› ary electrons, leading to a higher sensitivity. 

Examples of the monomer from which recurring units (al) are derived are shown below, but not limited thereto. Herein RA is as defined above. 

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