Copolymers of L-Lactide and ε-Caprolactone: Synthesis, Structural Analysis, and Mechanical Properties

The copolymers of L-lactide and ε-caprolactone are synthesized in the presence of tin(II) ethylhexanoate at 130°C for 72 h. The composition and structure of the products are determined by IR and ¹Н NMR spectroscopy, gel permeation chromatography, and elemental analysis. The molar ratio [monomer] : [initiator] = 4000 : 1 allowed the synthesis of the polymers with an number-average molecular weight of 47 000–66 000 g/mol with the polydispersity index not above 1.6. It was found that during transition from the glassy to highly elastic state, an increase in the content of ε-caprolactone units in the copolymer from 14 to 74 mol % leads to a decrease in the elastic modulus from 1379 to 65 MPa and a decrease in the maximum stress from 33 to 4 MPa. The copolymers containing 27 and 37 mol % of ε-caprolactone units possess plasticity and sufficient strength, which enables their use in medicine and the packaging industry.