AbstractA process that uses terephthalic acid and excess 1,4‐butanediol for the production of polybutylene terephthalate was investigated and the reaction conditions were maximized for fast reaction and minimal loss of butanediol by dehydration to tetrahydrofuran. For best results a mixed catalyst was selected: butylstannoic acid and titanium tetrabutoxide (0.05 and 0.025 mole %, respectively, versus terephthalic acid) with a starting material ratio of butanediol: terephthalic acid of 1.7 at 210°C. The mixture cleared at 2.5 hr with a loss of 6% of butanediol. HPLC analysis showed that the clearing point corresponds to the disappearance of terephthalic acid and monohydroxybutyl terephthalate from the reaction mixture. Thus the polycondensation conditions can be applied before the clearing point with the result that the reaction is faster and tetrahydrofuran formation is minimized. Tetrahydrofuran is formed from butanediol in the reaction mixture by acid catalysis. Excess butanediol results in the formation of larger amounts. End group cyclization of terephthalate esters accounts for the slow tetrahydrofuran formation in the absence of excess butanediol.