245  US 9,285,679 B2  246  -continued  Particularly preferred repeating units having a lactone  group include the following repeating units. By selecting an  optimal lactone group, the pattern profile and the iso/dense  bias are improved. In the formulae, each of Rx and R repre› sents H, CH$_{3}$ ,  CH$_{2}$OH or  CFy  [Chem. 143]  j:  ~  0  0  [Chem. 144]  Rx  ~  0~  0  Rx  ~  0 