Synthesis and properties of new polyesters having tetrahydrofuran rings in their main chains

New polyesters 6a–6c consisting of 2,4‐linked tetrahydrofuran rings were synthesized by bulk polycondensation of methyl trans‐ and cis‐4‐hydroxytetrahydrofuran‐2‐carboxylates (5a and 5b) and a stereoisomeric mixture of methyl 4‐hydroxy‐5‐methyltetrahydrofuran‐2‐carboxylate (5c) at high temperature. These monomers were prepared from methyl glycolate or methyl lactate and diethyl maleate through a four‐step reaction sequence. The polycondensation was carried out without solvent at different temperatures ranging from 150 to 220°C. Titanium isopropoxide was most effective among the catalysts examined, giving polyesters with number‐average molecular weights up to 2 × 104. Polyester 6a consisting of trans‐2,4‐linked tetrahydrofuran rings was soluble in trifluoroacetic acid and a mixed solvent of chloroform and methanol (10/1, v/v). Polyester 6b composed of cis‐2,4‐linked tetrahydrofuran rings was soluble in dimethyl sulfoxide and dimethylformamide in addition to the two solvents for 6a. Polyester 6c with 5‐methyl‐substituted tetrahydrofuran rings was composed of a mixture of stereoisomeric units and thus was soluble in a variety of solvents including chloroform, tetrahydrofuran, acetonitrile, dimethyl sulfoxide, and dimethylformamide. The glass transition temperatures of 6a, 6b, and 6c determined by DSC were 109, 88, and 66°C. These polyesters were found to be very slowly hydrolyzed in a neutral phosphate buffer solution at ambient temperature.