Amidinium pyridinolate ion pair organocatalyst for ring-opening polymerizations of cyclic esters

Abstract

Ion pair organocatalyst composed of a positively charged hydrogen-bond donor (HBD⁺) and a nucleophilic anion (A⁻) was proposed for ring-opening polymerizations (ROPs) of cyclic esters. The HBD⁺/A⁻ ion pair catalyst was readily accessible by mixing stoichiometric of super strong nitrogen bases (i.e. DBU, TBD, and MTBD) with 2-, 3-, and 4-pyridinols (4-PyOH). Nine ion pair catalysts were evaluated in bulk ROPs of L-lactide (LLA) by ratios of 25 : 1 : 1 of the monomer to the initiator and to the catalyst at 140 °C, in which the DBUH⁺/4-PyO⁻ pair showed optimal performances in producing polylactide (PLLA) by high conversion (95 %) in 15 min. Under the catalysis of DBUH⁺/4-PyO⁻ ion pair, PLLA of predicted molecular weights (M_n,NMR = 3.3–23.9 kg·mol⁻¹) and narrow dispersity (Ð = 1.10–1.25) was prepared; ROPs of trimethylene carbonate (TMC), δ-valerolactone, and ε-caprolactone, as well as diblock-copolymers of TMC and LLA were all successful in the bulk. Bifunctional catalytic mechanism of the ion pair via activation of the monomer with the cationic HBD⁺ and activation of the initiator/chain-end with anion A⁻ was suggested and validated by NMR titrations. The design principle of H-bonding cation and nucleophilic anion ion pair organocatalyst featured easy access and multiple combinations paved avenues to wider scope of catalytic polymerizations.