20  US 8,877,424 B2  19  -continued  -continued  The monomer having formula  (1)  may be prepared by any  well-known methods. Two exemplary methods are illustrated  below although the synthesis route is not limited thereto. The  first method proceeds through steps I and II according to the  reaction scheme shown below.  RI  RI  ~o  ~o  ~)O  0  0  RI  ~o  ~)O  0  RI  ~o  ~ob  06  0  rb  RI  ~o  0  0k  OXo  RI  ~o  0  b  RI  Oy_  ~o  0  Orf) 