prov:value
| - Examples of the dicarboxylic acid unit (a) include ##STR1## wherein X represents hydrogen, halogen, e.g. Cl or Br, an alkyl group, e.g. methyl, ethyl, propyl, isopropyl, butyl or t-butyl, an alkoxy group, e.g. methoxy, ethoxy, propoxy or buthoxy, or phenyl group, and k is 0 to 2, ##STR2## and the like.
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