-continued  -continued  106  US 9,052,590 B2  105  The presence of a substituent (Rb$_{2}$ )  on the portion of the  lactone structure is optional. As a preferred substituent (Rb$_{2}$ ),  there can be mentioned an alkyl group having  1  to 8 carbon  atoms, a monovalent aliphatic hydrocarbon ring group having  4  to 7 carbon atoms, an alkoxy group having  1  to 8 carbon  atoms, an alkoxycarbonyl group having  1  to 8 carbon atoms,  a carboxyl group, a halogen atom, a hydroxyl group, a cyano  group, an acid-decomposable group or the like. Of these, an  alkyl group having  1  to  4  carbon atoms, a cyano group and an  acid-decomposable group are more preferred. In the formu› lae, n$_{2 }$ is an integer of  O  to  4.  When n$_{2 }$ is 2 or greater, the  plurality of present substituents (Rb$_{2}$ )  may be identical to or  different from each other. Further, the plurality of present  substituents (Rb$_{2}$ )  may be bonded with each other to thereby  form a ring.  The repeating unit having a lactone group is generally  present in the form of optical isomers. Any of the optical  isomers may be used. It is both appropriate to use a single type  of optical isomer alone and to use a plurality of optical iso› mers in the form of a mixture. When a single type of optical  LCl-17  LCl-16  LCl-15  LCl-14  0  LCl-13  LCl-12  Ql  0  LCl-4  Ql  0  LCl-6  LCl-8  LCl-9  LCl-10  LCl-11 