Crystalline aliphatic polyesters from eight‐membered cyclic (di)esters

Abstract Ring‐opening polymerization (ROP) of lactones or cyclic (di)esters is a powerful method to produce well‐defined, high‐molecular‐weight (bio)degradable aliphatic polyesters. While the ROP of lactones of various ring sizes has been extensively studied, the ROP of the simplest eight‐membered lactone, 7‐heptanolactone (7‐HL), has not been reported using metal‐based catalysts. Accordingly, this contribution reports the ROP of 7‐HL via metal‐catalyzed coordinative‐insertion polymerization to the corresponding high‐molecular‐weight polyester, poly(7‐hydroxyheptanoate) (P7HHp). The resulting P7HHp is a semi‐crystalline material, with a T m of 68 °C, which is ~10 °C higher than poly( ε ‐caprolactone) derived from the seven‐membered lactone. Mechanical testing showed that P7HHp is a hard and tough plastic, with elongation at break >670%. P7HHp‐based polyesters with higher T m values have been achieved through stereoselective copolymerization of 7‐HL with an eight‐membered cyclic diester, racemic dimethyl diolide ( rac ‐8DL Me ), known to lead to high T m poly(3‐hydroxyburtyrate) (P3HB). Notably, catalyst's strong kinetic preference for polymerizing rac ‐8DL Me over 7‐HL in the 1/1 comonomer mixture rendered the formation of di‐block copolymer P3HB‐ b ‐P7HHp, showing two crystalline domains with T m1 ~ 65 °C and T m2 ~ 160 °C. Semi‐crystalline random copolymers with T m up to 164 °C have also been obtained by adjusting copolymerization conditions. Mechanical testing showed that P3HB‐ b ‐P7HHp can synergistically combine the high modulus of isotactic P3HB with the high ductility of P7HHp.