One-Pot Synthesis of CO₂-Based Polylactide-b-Poly(ether carbonate)-b-Polylactide Triblock Copolymers and Their Mechanical Properties

The synthesis of CO₂-based triblock copolymers made of a central poly(ether carbonate) blocks flanked by two external polylactide blocks is described. Such triblock copolymers are obtained in one-pot by first CO₂/epoxides copolymerization using a difunctional onium salt initiator in the presence of triethylborane, followed by ring-opening polymerization (ROP) of lactide (LA) assisted by diphenylurea (UPh), a hydrogen-bonding activator. Upon varying the pressure of CO₂ during its copolymerization with propylene oxide (PO), the T_g of the central block could be fine-tuned allowing the synthesis of poly(propylene oxide-co-propylene carbonate) (PPPC) with various carbonate versus ether contents. These PPPC samples are then mechanically characterized; as their content in ether linkages progressively increased, a transition from brittle-to-ductile-to-elastomeric behavior could be seen. Upon appending two external enantiomeric poly(d-lactide) (PDLA) blocks to the central PPPC block, the generated PDLA-b-PPPC-b-PDLA triblock copolymers exhibit an impressive increase of their ductility compared to that of the parent statistical PPPC copolymers. When benchmarked against commercial polyolefins the latter CO₂-based triblock copolymers outperform the mechanical performances of high density polyethylene (HDPE) and low density polyethylene (LDPE) by all accounts, their elasticity modulus, tensile strength, toughness and elongation at break being indeed superior to those of the polyolefins tested.