A preorganized dual H-bond donor promotes benzoic acid active in the polymerization of δ-valerolactone

Ring-opening polymerization (ROP) of lactones catalyzed by a (super)strong Brønsted acid offers a valuable approach to important biodegradable aliphatic polyesters. However, the need for a mild acidic catalyst in ROP has been long sought after but unmet. Here, we describe a truly weak Brønsted acid, benzoic acid, in combination with a dual H-bond donor (dHBD), which catalyzes the ROP of δ-valerolactone (VL) in solution at room temperature. A unique preorganized sulfonyl guanidine type of dHBD, 3-amino-1,2,4-benzothiadiazine-1,1-dioxide (ABTD), proves optimal to work with benzoic acid as a cocatalyst, promoting benzoic acid activity in the ROP of VL. Poly-δ-valerolactones of predictable molecular weights (from 2.13 to 9.33 kg mol^-1) and narrow dispersities (D ≤ 1.16) are synthesized. The controlled character of the ROP is verified by NMR, SEC, and MALDI-ToF MS measurements. NMR titration experiments imply that ABTD preferentially binds with benzoic acid, and the benzoic acid/ABTD pair protonates the VL monomer. Weak benzoic acid is not weak for the first time in the efficient cationic ROP of VL by the protonation mechanism.