59  US 9,323,150 B2  60  different from each other. Further, the plurality of present  substituents (Rb$_{2}$ )  may be bonded to each other to thereby  form a ring.  -continued  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  isomer is mainly used, the optical purity thereof is preferably  90% ee or higher, more preferably 95% ee or higher.  Particular examples of the repeating units each having a  lactone structure contained in the resin are shown below,  which in no way limit the scope of the present invention. In  the following formulae, Rx represents  H, CH$_{3}$ ,  CH$_{2}$0H  or  CF$_{3}$ •  Rx  Rx  ,:to  ~  clo  P  0  Rx  ~  ~  0  Rx  ~  Llt  0  0  Rx  ~  H02C  Rx  ~  HO  0 