The rate constants of cross reactions in the copolyconclensation of bis(3-hydroxypropyl)terephthalate (BHPT) and bis(2-hydroxyethyl)terephthalate (BHET), k(12) and k(21), were determined with the results obtained from H-1 NMR spectroscopy analysis. BHPT and BHET were polymerized and copolymerized at 260, 270, and 280degreesC with titanium tetrabutoxide as a catalyst. With the adoption of second-order kinetics to the polycondensation, k(11) and k(22) were calculated. The monomer reactivity ratios of BHPT were much larger than those of BHET, indicating the block nature of the copolycondensation, but the difference between the reactivity ratios was lowest at the highest polycondensation temperature of 280degreesC. This indicated that the probability of randomization was increased. (C) 2003 Wiley Periodicals, Inc.