Biocompatible and low-cost pyridinium halides catalysts promoted ring-opening polymerizations of cyclic esters in bulk

Polyesters produced by ring-opening polymerization (ROP) of cyclic monomers using organocatalysts were well developed in academia. Industrially viable ROPs were polymerizations at elevated temperatures in the bulk, thus desirable features of useful organocatalysts would be thermal stable, reasonably active in fast polymerization but mild enough to avoid transesterification. More importantly, the polyesters containing residue organocatalyst should met biosafety regulations. In these regards, series of pyridinium halides readily prepared by one step from mass-produced pyridines and hydrohalic acids were evaluated in ROPs of L-lactide (LLA), trimethylene carbonate (TMC), δ-valerolactone, and ε-caprolactone in the bulk. An optimal catalyst 4-(N,N-dimethylamino)pyridine hydrochloride (DMAP·HCl) was examined in the catalytic performances in ROPs of LLA, TMC, and diblock copolymerization affording PTMC-b-PLLA. Bulk ROP of LLA at 140 °C produced PLLAs by near quantitative conversions with precise molecular weights (M_n,NMR = 3.3–16.6 kg mol⁻¹) and narrow dispersities (Đ = 1.13–1.17). Kinetics data, chain extension experiments, and MALDI-ToF MS analysis all supported the controlled/living nature of the ROPs. A bifunctional activation mechanism in which pyridinium activated the monomer and halide activated the initiator/chain end was proposed and validated by ¹H NMR and ¹³C NMR titrations. Poly(L-lactide) samples prepared by bulk ROPs of LLA that containing residue catalyst DMAP·HCl were tested by MTT assay in L929 mouse fibroblasts in vitro. High level of relative growth rate (RGR 93.6–96.8%) revealed favorable biosafety of the sample PLLAs.