Abstract: Poly(butylene terephthalate) copolyester advantageous of use as sheath materials in bicomponent fibers are prepared by a process for preparing a high-molecular weight, linear copolyester by condensing 40-85 molar percent of terephthalic acid optionally in the form of a dialkyl ester, half of which component can optionally be replaced by another dicarboxylic acid also optionally in the dialkyl ester form, and 60-15 molar percent of a blend of dimethyl adipate, dimethyl glutarate and dimethyl succinate with an alkanediol of 2-6 carbon atoms in its carbon chain, at an elevated temperature in the presence of a conventional catalyst, in a first condensation stage which is an interesterification or esterification stage and a second condensation stage which is polycondensation stage, whereby a copolyester melt is produce. The process comprises, before or during the first condensation stage, adding 0.005-0.1 molar percent, based on the total acid component, of a C.sub.6 -aryl or C.sub.

Abstract: Poly(butylene terephthalate) copolyester advantageous of use as sheath materials in bicomponent fibers are prepared by a process for preparing a high-molecular weight, linear copolyester by condensing 40-85 molar percent of terephthalic acid optionally in the form of a dialkyl ester, half of which component can optionally be replaced by another dicarboxylic acid also optionally in the dialkyl ester form, and 60-15 molar percent of a blend of dimethyl adipate, dimethyl glutarate and dimethyl succinate with an alkanediol of 2-6 carbon atoms in its carbon chain, at an elevated temperature in the presence of a conventional catalyst, in a first condensation stage which is an interesterification or esterification stage and a second condensation stage which is a polycondensation stage, whereby a copolyester melt is produce. The process comprises, before or during the first condensation stage, adding 0.005-0.1 molar percent, based on the total acid component, of a C.sub.6 -aryl or C.sub.