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

Abstract

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 (T g) 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.

Using Redox‐Switchable Polymerization Catalysis to Synthesize a Chemically Recyclable Thermoplastic Elastomer

“…This polymer was interesting because it features end‐blocks that are expected to form stereocomplexes, a well‐known property displayed by mixtures of PLLA and PDLA [61] . Notably, the accessibility of L‐THF‐D is enabled using the unidirectional, redox‐switchable polymerization method employed as opposed to conventional bidirectional, telechelic techniques used to synthesize triblock copolymers containing PLLA end‐blocks [37–41,62–66] . All copolymers were isolated on a multi‐gram scale and were confirmed to be triblock copolymers as determined by 1 H NMR, SEC, and DOSY spectroscopy (See Figure S22–50).…”

Using Redox‐Switchable Polymerization Catalysis to Synthesize a Chemically Recyclable Thermoplastic Elastomer

“…This polymer was interesting because it features end‐blocks that are expected to form stereocomplexes, a well‐known property displayed by mixtures of PLLA and PDLA [61] . Notably, the accessibility of L‐THF‐D is enabled using the unidirectional, redox‐switchable polymerization method employed as opposed to conventional bidirectional, telechelic techniques used to synthesize triblock copolymers containing PLLA end‐blocks [37–41,62–66] . All copolymers were isolated on a multi‐gram scale and were confirmed to be triblock copolymers as determined by 1 H NMR, SEC, and DOSY spectroscopy (See Figure S22–50).…”

High Molar Mass Polycarbonates as Closed-Loop Recyclable Thermoplastics

“…CO 2 and epoxide ring-opening copolymerization (ROCOP) is a front-runner carbon dioxide utilization technology. − Most polymerizations apply catalysts to produce hydroxyl telechelic oligocarbonates (1 < M n < 20 kg mol –1 , where M n is the number average molecular weight), or CO 2 -polyols, which are important as surfactants and in production of polyurethane foams, elastomers, coatings, sealants, and adhesives. − Some excellent catalysts include Co­(III) or Al­(III) complexes tethered to ionic cocatalysts (often bis­(triphenylphosphine)­iminium halides, e.g., PPNCl); dimeric/dinuclear complexes, e.g., Zn­(II)­Zn­(II), Zn­(II)­Mg­(II), Co­(II)­Mg­(II), Ln­(III)­Zn­(II), which operate without any cocatalyst; or borane complexes tethered to/used with ionic cocatalysts (ammonium or phosphonium halides). − Such CO 2 -polyol catalysts need to be applied with a very large-excess (10–100 equiv vs catalyst) of a protic chain transfer agent (CTA), often a diol, which controls the oligomer chain length and hydroxyl chain end-groups . Chain transfer reactions are alcoholysis processes, whereby all alcohol groups react rapidly with the growing polymer chains, moving them rapidly on/off the catalyst; these processes generally occur faster than propagation. ,, Most catalysts also react with residual water, either in the apparatus or monomers, to ring-open epoxides and generate diols in situ, which are also bifunctional CTAs.…”

“…In 2022, Frey and Floudas investigated, both experimentally and computationally, the chain dynamics of moderate molar mass PCHC (5 < M n < 33 kg mol –1 ) produced using a [( R , R -salcy)­CoCl]/PPNCl catalyst system, estimating an M e of 16 kg mol –1 . It is also relevant to note that PCHC has been successfully applied as a “rigid” block in various phase separated copolymers ( T g of PCHC ∼ 110–120 °C), producing adhesives, elastomers, and plastics, depending on the comonomers and overall molar mass values. ,,,− Beyond CHO as comonomer in ROCOP, the teams of Rieger and Greiner independently focused on related 6-membered ring, biobased poly­(limonene carbonate), developing routes to high molar mass and terpolymers with PCHC. − Wu and co-workers reported poly­(cyclopentene carbonate) (PCPC) with molar masses up to 84 kg mol –1 , but the analyzed sample ( M n = 20 kg mol –1 , T g = 70 °C) displayed moderate tensile strength (20 MPa) and no yield point …”