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Exemplary monomers from which the repeat units of for› mula (2) are derived include monomers of formula (2a). 

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Herein 

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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 

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include the following. 

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Suitable aromatic hydrocarbon groups represented by R 2 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. 

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