26  US 8,968,979 B2  25  Exemplary monomers from which the repeat units of for› mula (2) are derived include monomers of formula (2a).  Herein  denotes a substituted or unsubstituted aromatic hydrocarbon  group, R $^{1 }$is each independently hydrogen, methyl or trifluo› romethyl, R $^{2 }$is hydrogen or a straight, branched or cyclic  C $_{1 }$-C $_{12 }$alky 1 or aromatic hydrocarbon group, and "a" is 1 or 2.  Specifically, non-limiting examples of  include the following.  Suitable aromatic hydrocarbon groups represented by R include aryl groups such as phenyl and tolyl. Suitable alkyl  groups represented by R $^{2 }$include methyl, ethyl, propyl, iso› propyl, butyl, isobutyl, tert-butyl, cyclopentyl, and cyclo› hexyl.  Shown below are examples of the monomers of formula  (2a) from which the repeat units of formula (2) are derived.  -continued  Hfi  0  0  (to  Me  Hff  0  0  -00  Hff  0  0  (to  (2a) 