(Co)polymerization of (-)-Menthide and β-Butyrolactone with Yttrium-bis(phenolates): Tuning Material Properties of Sustainable Polyesters

Pyridine-based heteroaromatic yttrium-bis(phenolate) complexes were investigated in ring-opening polymerization of racemic β-butyrolactone (rac-BBL) and (−)-menthide. In addition, AB and BAB copolymers from poly((−)-menthide) (PM, block A) and poly(3-hydroxybutyrate) (PHB, block B) were synthesized via sequential addition. BAB copolymers were accessible through introduction of a bifunctional pyrazine initiator enabling chain growth in two directions. Block copolymers were analyzed using thermal analysis, X-ray diffraction, small-angle X-ray scattering, NMR analysis and stress–strain experiments. These measurements confirmed a microphase separation within the semicrystalline BAB materials. Regarding the mechanical behavior, two hot-moldable PHB-based (co)polymers were investigated. The syndiotactic PHB is less brittle than natural PHB, resulting in a lower Young's modulus. Incorporating PM in the structure, the elastic modulus is further lowered with increasing elongation at break.