Direct copolymerization of L‐lactic acid with γ‐butyrolactone in the absence of catalysts

The copolymerization of L ‐lactic acid (LA) with γ‐butyrolactone (BL), performed without catalyst at 200°C under nitrogen in order to obtain a relatively low‐molecular‐weight copolymer, proceeds via direct condensation between linear LA and linear 4‐hydroxybutyric acid produced by hydrolysis of the cyclic BL. The copolymers were characterized with respect to the copolymerizability ( 13 C NMR spectroscopy), composition ( 1 H NMR spectroscopy), molecular weight (terminal group analysis and gel permeation chromatography), and crystallinity (differential scanning calorimetry). The copolymerizability of BL is low, e. g., the contents of BL units in the copolymers were found to be 12, 16, and 19 mol‐% for initial BL contents in the monomer feed of 30, 50, and 70 mol‐%, respectively, in which copoly(LA/BL) is composed of blocks of lactyl units separated by single γ‐oxybutyryl unit.