| http://www.w3.org/ns/prov#value | - c) compounds of formula: ##STR2## wherein R2 is as defined in a) and at least one R2 is alkyl and R4 is a divalent radical of the formula: ##STR3## wherein R5 is hydrogen or lower alkyl; the molar ratio of a:b:c being n:(n+1):2, wherein n has a value from 1 to 20.
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