| http://www.w3.org/ns/prov#value | - In a compound of formula (I) the bonding of the divalent phenylene group may be ortho, meta or para, and the oxa or thia group is preferably adjacent the phenylene or when X is other than phenylene then X1 may be --CH2 --O--CH2 -- or CH2 --S--CH2 --.
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