prov:value
| - As examples there are mentioned those having the following catechol, salicylic acid, phthalic acid, 2,3-naphthalenediol and 2,3-naphthalenedicarboxylic acid units, or substituent groups on these benzene rings: ##STR54## wherein X represents hydrogen, e.g. Cl or Or, methyl, ethyl, methoxy, ethoxy or phenyl, and k is 0 to 2.
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