Poly(lactic acid) (PLA) as a biodegradable and bio-derived polyester has limited application due to its brittleness. In an effort to increase the overall flexibility via insertion of soft polymeric segments into PLA, a catalytic system to polymerize tetrahydrofuran (THF) in the presence of epoxides was developed using bis(imino)pyridine iron complexes. The cationic Fe(III) catalyst that was previously reported to be active for epoxide polymerization was found to form elastomeric materials when copolymerizing epoxide and THF. Incorporation of THF into the polyether was controlled by varying the monomer ratio such that the resulting polyether obtained was predominately poly(THF). The redox-switchable characteristics of the iron-based catalysts were used to synthesize the triblock copolymers expected to have thermoplastic behavior. Redox-switchable polymerization was achieved by reducing the catalyst to an Fe(II) species, which arrested the THF/epoxide copolymerization and activated the complex for lactide polymerization. The orthogonal reactivity demonstrated by the iron-based catalyst enabled the synthesis of A-B-A' and A-B-C triblock copolymers featuring "stiff" end-blocks of crystalline PLA and a "soft" mid-block of elastomeric poly(THF-<i>co</i>-epoxide). Thermal and mechanical properties of the triblock copolymers demonstrated improved flexibility compared to PLA and behavior that makes them suitable as thermoplastic elastomers. In addition to providing access to these new thermoplastic elastomers, the redox-switchable polymerization method used to access these materials provides distinct advantages compared to more traditional routes to synthesize thermoplastic elastomers. These advantages will be highlighted throughout the talk.

Synthesis of thermoplastic elastomeric triblock copolymers derived from lactic acid and THF using redox switchable polymerization

ACS Spring Meeting 2021

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