Aluminum porphyrin complexes via delicate ligand design: emerging efficient catalysts for high molecular weight poly(propylene carbonate)

Abstract

Due to the deep concern over residual, toxic cobalt or chromium from catalysts in biodegradable poly(propylene carbonate) (PPC), bifunctional aluminum porphyrin complexes with quaternary ammonium salts anchored on the ligand framework were prepared, and a delicate design of the porphyrin ligand was obtained. An optimized catalyst was complex 6b, which had two para-bromine benzenes and two quaternary ammonium cations linked to benzene via a six-methylene spacer in the meso-position of the porphyrin framework and NO₃⁻ as axial ligand and quaternary ammonium anion, showed TOF of 560 h⁻¹ at 80 °C and 3 MPa to yield PPC with 94% carbonate linkage and number average molecular weight of 96 kg mol⁻¹. The PPC selectivity reached 93%, which was the highest record in this copolymerization for aluminum porphyrin complexes. The soil tolerant bifunctional aluminum porphyrin complexes are becoming increasingly competitive catalysts, since they can be left with the plastics without any extra separation.