| http://www.w3.org/ns/prov#value | - A composition comprising at least two compounds of formula (II): ##STR00005## wherein R.sub.1 and R.sub.2 are each independentlyC.sub.1-C.sub.4 alkyl, and n is an integer .gtoreq.0 and wherein the average molar value of n for the total of the compounds of formula (II) in said composition is in the range of 1 to 2.2.
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