Biobased Thermoplastic Elastomers Derived from Citronellyl Glycidyl Ether, CO2, and Polylactide

Biodegradability and biobased feedstocks are key requirements for sustainable materials. This work presents the synthesis of PLLA-b-PCitroGEC di- and triblock copolymers [PCitroGEC: poly(citronellyl glycidyl ether carbonate)] as degradable thermoplastic elastomers (TPEs), sourced from biorenewable feedstocks. l,l-Lactide (LLA) is produced by the fermentation of corn or sugar on a large scale, while citronellol can be extracted from rose or lemon grass. A key feature of the current TPE structures is their low glass temperature (Tg) of the highly flexible polycarbonate midblock, based on PCitroGEC. The latter was synthesized by catalytic copolymerization of CO2 and citronellyl glycidyl ether, using (R,R)-(salcy)-Co(III)Cl (CoSalenCl) and bis(triphenylphosphine)iminium chloride ([PPN]Cl) as a catalyst system. The resulting PCitroGEC macroinitiators (11,000 to 26,000 g·mol–1, DMF) were used in a DBU-catalyzed ring opening polymerization of LLA, resulting in a series of PLLA-b-PCitroGEC triblock copolymer structures. Molar masses range between 18,000 and 41,000 g·mol–1, with the molar fraction of the "soft" PCitroGEC block varied between 22 and 60 mol %. Glass temperatures of the block copolymers were studied with a combination of temperature-modulated differential scanning calorimetry and dielectric spectroscopy techniques. Small-angle X-ray scattering (SAXS) confirmed nanophase separation for all synthesized TPEs. SAXS was further employed to construct the PLLA-b-PCitroGEC-b-PLLA phase diagram. It comprises classical phases (spheres, cylinders, and lamellae). Tensile testing illustrated elastic properties for all TPEs with elongation at break up to 600% and almost no plastic deformation for copolymers with a PCitroGEC content above 31 mol %. Cyclic tensile tests confirmed the elastic recovery properties of the TPEs. Furthermore, the materials exhibited low E-moduli of 0.15–1.0 MPa, rendering PLLA-b-PCitroGEC-b-PLLA triblock copolymers suitable for potential use in soft tissue engineering.