prov:value
| - In these formulae, each of X and X' independently of one another, represents halogen, especially chlorine, each of R6 and R7, independently of one another, represents identical or different, straight-chain or branched alkyl radicals having from 1 to 8 carbon atoms and/or hydrocarbon aryl radicals having from 6 to 8 carbon atoms, and each of a and b, independently of one another, respresents 2, 3 o
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