Development of a H-bond donor-Lewis basic anion bifunctional organocatalyst for ring-opening polymerizations

A bifunctional H-bond donor-Lewis basic anion (HBD-anion) organocatalyst manifold was proposed for ring-opening polymerizations (ROPs) of cyclic esters. A series of squaramide-phenolates (Sq-PhO) as representative HBD-anion catalysts were designed, in which the Lewis basic phenolate anion, in place of neutral bases of the Takemoto-type catalyst, activated the initiator/chain-end, while the squaramide activated the monomer by H-bonding, cooperatively. The squaramide-phenolate was generated in situ by deprotonating the phenolic hydroxyl of the counterpart squaramide-phenol with (super) strong bases. An optimal N-benzyl-N′-p-hydroxylbenzyl squaramide (Sq3-p) and the DBU base co-catalyst transformed lactides, lactones, and cyclic carbonates into the corresponding polyesters in a short time, with high conversion and a narrow dispersity (Đ = 1.06-1.19). The bifunctional activation mechanism was proposed and validated. Both the H-bond donors and basic anions are various and tunable, which is hopefully accessible to extend this catalytic tool in wider scope of bifunctional organocatalysis.