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

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 degrees C, which is similar to 10 degrees C higher than poly(epsilon-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 Tm 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-8DLMe 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 similar to 65 degrees C and T-m2 similar to 160 degrees C. Semi-crystalline random copolymers with Tm up to 164 degrees 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.