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Fibre-grade copoly(oxyethylene-b-ethylene terephthalate)s with improved optical properties RAPID COMMUNICATION Summary-The conditions of synthesis of block copoly(oxyethylene-b-ethylene terephthalate)s (PEG-b-PET) with good optical parameters and designed for the production of hydrophilic fibers are presented. For polycondensation the catalysts: antimony trioxide (Sb2O3) and titanium tetraisopropoxide [Ti(OiPr) 4 ] were used. The best optical properties of the polymers were achieved by using the mixture of Sb 2 O 3 /Ti(OiPr) 4 catalysts. The system which thermally stabilize the PEG-b-PET copolymers was selected. It was stated that the melting temperature (T m) of the copolymer was insignificantly lower than that of the PET homopolymer (∆T = 11 K) when the content of PEG 4000 comonomer amounts 30 wt. %. When the weight fraction of PEG 4000 in the copolymer does not exceed 20 wt. % a cold crystallization is observed.