The invention relates to a composition comprising a titanium or zirconium alkoxide or aryloxide, wherein the alkoxy group in the titanium or zirconium alkoxide is a group of formula R—O− wherein R is an alkyl group having 1 to 4 carbon atoms and the aryloxy group in the titanium or zirconium aryloxide is a group of formula Ar—O− wherein Ar is an aryl group having 6 to 12 carbon atoms, and wherein the composition additionally comprises 0.1 to 50 wt. % of an organic carbonate, based on the total weight of the composition.