Study of the THF formation during the TPA-based synthesis of PBT

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A model study is performed on the tetrahydrofuran (THF) formation, the major side reaction during the synthesis of poly(butylene terephthalate) (PBT). For this purpose, metachlorobenzoic acid (3-ClBA) was used as a model for terephthalic acid (TPA). When TPA is used for the production of PBT, as a cheaper alternative for dimethyl terephthalate (DMT), the amount of THF formed doubles. Consequently, the THF formation needs to be suppressed during this process, in order to preserve an economic advantage over the DMT-based synthesis route. It was found that only two reactions cause the formation of THF. The first source is the acid-catalyzed dehydration of the monomer, 1,4-butanediol (BD), which is the most important reason why more THF is formed during the TPA-based process. For the first stage of the DMT-based synthesis of PBT, this reaction proved to be negligible. The backbiting of the hydroxybutyl end groups of the polymer is the second side reaction that produces THF. Contrary to the dehydration of BD, this reaction is not affected by the presence of TPA. Besides this, a kinetic study revealed the activation energies of the different (side) reactions of the TPA-based model and showed the influence of temperature on the formation of THF. As Ti(OBu)4, the catalyst commercially used for the synthesis of PBT, has also no influence on the studied side reactions, it could be concluded that fast conversion of the carboxylic acid groups, followed by fast polycondensation of the formed hydroxybutyl end groups, is essential to suppress the THF formation.