Catalyst composition for the preparation of polyesters, and related processes

The catalyst compn., an improved method of prepn. of a polyester on the two-stage process of ester interchange and polycondensation, comprises the combination of (a) a titanium-based compd. or zirconium-based compd. and (b) a lanthanide series compd. or hafnium, and optionally, a phosphate-forming compd. Thus, poly(butylene terephthalate) was prepd. by a two-stage polymn. of 1,4-butanediol and di-Me terephthalate in an ester interchange reaction process in the presence of 0.08 phr titanium tetrabutoxide and in a polycondensation stage of 0.14 phr hafnium tetrakis(acetylacetonate).