Polyester Synthesis at Moderate Temperatures via the Direct Polycondensation of Dicarboxylic Acids and Diols Catalyzed by Rare-Earth Perfluoroalkanesulfonates and Bis(perfluoroalkanesulfonyl)imides

Recently, we reported that scandium trifluoromethanesulfonate [Sc(OTf)3] and scandium trifluoromethanesulfonimide could catalyze polycondensations of dicarboxylic acids and diols. Although these polycondensations were one-step and occurred at the moderate temperature of 35 °C, they required large amounts of the catalysts (ca. 1 mol %) and long reaction times (>100 h). Therefore, scandium catalysts having stronger electron-withdrawing ligands (scandium perfluorooctanesulfonate, scandium nonafluorobutanesulfonate, and scandium bis(nonafluorobutanesulfonyl)imide [Sc(NNf2)3]) were synthesized, and their catalytic efficiencies were tested. We have now demonstrated and report herein that scandium catalysts with the stronger electron-withdrawing ligands could catalyze the polycondensation of adipic acid and 3-methyl-1,5-pentanediol under conditions of a moderate temperature, a short time period, and a lesser amount of catalyst. Other rare-earth-metal catalysts containing ligands with strong electron-withdrawing abilities were also synthesized, and their catalytic efficiencies were tested using the aforementioned polycondensation. As a result, thulium bis(nonafluorobutanesulfonyl)imide [Tm(NNf2)3] was a more effective catalyst than was Sc(OTf)3, Sc(NNf2)3, thulium trifluoromethanesulfonate [Tm(OTf)3], or thulium trifluoromethanesulfonimide [Tm(NTf2)3]. Our work now makes it possible to synthesize aliphatic polyesters of M n > 1.0 × 104 at a moderate temperature (60 °C), within a short period of time (6 h), and using a small amount of catalyst (0.05−0.1 mol %).