Triblock polyester thermoplastic elastomers with semi-aromatic polymer end blocks by ring-opening copolymerization

Abstract

Thermoplastic elastomers benefit from high elasticity and straightforward (re)processability; they are widely used across a multitude of sectors. Currently, the majority derive from oil, do not degrade or undergo chemical recycling. Here a new series of ABA triblock polyesters are synthesized and show high-performances as degradable thermoplastic elastomers; their composition is poly(cyclohexene-<i>alt</i>-phthalate)-<i>b</i>-poly(ε-decalactone)-<i>b</i>-poly(cyclohexene-<i>alt</i>-phthalate) {PE-PDL-PE}. The synthesis is accomplished using a zinc(ii)/magnesium(ii) catalyst, in a one-pot procedure where ε-decalactone ring-opening polymerization yielding dihydroxyl telechelic poly(ε-decalatone) (PDL, soft-block) occurs first and, then, addition of phthalic anhydride/cyclohexene oxide ring-opening copolymerization delivers semi-aromatic polyester (PE, hard-block) end-blocks. The block compositions are straightforward to control, from the initial monomer stoichiometry, and conversions are high (85-98%). Two series of polyesters are prepared: (1) TBPE-1 to TBPE-5 feature an equivalent hard-block volume fraction (<i>f</i> _hard = 0.4) and variable molar masses 40-100 kg mol^-1; (2) TBPE-5 to TBPE-9 feature equivalent molar masses (∼100 kg mol^-1) and variable hard-block volume fractions (0.12 < <i>f</i> _hard < 0.4). Polymers are characterized using spectroscopies, size-exclusion chromatography (SEC), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA). They are amorphous, with two glass transition temperatures (∼-51 °C for PDL; +138 °C for PE), and block phase separation is confirmed using small angle X-ray scattering (SAXS). Tensile mechanical performances reveal thermoplastic elastomers (<i>f</i> _hard < 0.4 and <i>N</i> > 1300) with linear stress-strain relationships, high ultimate tensile strengths (<i>σ</i> _b = 1-5 MPa), very high elongations at break (<i>ε</i> _b = 1000-1900%) and excellent elastic recoveries (98%). There is a wide operating temperature range (-51 to +138 °C), an operable processing temperature range (+100 to +200 °C) and excellent thermal stability (<i>T</i> _d,5% ∼ 300 °C). The polymers are stable in aqueous environments, at room temperature, but are hydrolyzed upon gentle heating (60 °C) and treatment with an organic acid (<i>para</i>-toluene sulfonic acid) or a common lipase (Novozyme® 51032). The new block polyesters show significant potential as sustainable thermoplastic elastomers with better properties than well-known styrenic block copolymers or polylactide-derived elastomers. The straightforward synthesis allows for other commercially available and/or bio-derived lactones, epoxides and anhydrides to be developed in the future.