US 10,871,711 B2  101  102  -continued  (a2)  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.  OH  OH  OH  I  OH  OH  I I  I I  OH  I  1•’,’  ;tR’ ,OH  OH  lXOH  CCI  -continued  I  & A  #  OH  ~  I  I  ~  #0  I  o  I ✓,:::;  ~  0  0  OH  I  I  ✓,p  I  A  0~  OH  0  I  1/ I  I  #  I  I 