Biopolyester prepared using unsaturated betulin (betulinol) extracted from outer birch bark and dicarboxylic acid dichlorides and its thermal-induced crosslinking

Abstract

Betulin, also called betulinol, with two hydroxyl moieties extracted from white birch bark, and acid dichloride (ClCO(CH2)mCOCl) were successfully polycondensed to give the corresponding aliphatic biopolyester (ABIOS) in the presence of pyridine; in particular, in the case of sebacoyl dichloride (m = 8), derived from sebacic acid contained in castor oil of Ricinus seed, the ABIOS8 was obtained in a fairly high yield of 91%, with Mw = ∼23,800 and Mw/Mn = ∼2.1, even at room temperature. By contrast, the polymerization using other acid dichlorides with short methylene chains, i.e., m = 2, 4, 5, and 7, and long methylene chains, i.e., m = 10, resulted in relatively lower yields with a lower molecular weight, Mw. The ABIOS8 with (CH2)8 showed a high glass transition temperature (Tg ≈ 150 °C) superior to that of polylactic acid (PLA) known as a typical biopolyester. A colorless transparent film of ABIOS8 was easily fabricated by the solvent-casting method using THF or CHCl3 as solvent. Furthermore, the ABIOS8 gradually became insoluble in solvents such as THF when heated to 250 °C even under a nitrogen atmosphere. It was deduced that the CC bond in the isopropenyl moiety (H2CC(CH3)) as the third substituent within the betulin mother molecule in ABIOS8 acted as a thermally induced cross-linker.