Unsymmetrical Phosphinoquinoline Iron(II) Complexes with Enhanced Thermal Stability and High Activity for Isoprene Polymerization

Polyisoprene is the most important composition of natural rubber. The green catalytic polymerization of isoprene by earth-abundant metal iron is a promising next-generation manufacturing process in the rubber industry. We now show that a series of unsymmetrical phosphinoquinoline Fe(II) precatalysts can efficiently catalyze isoprene polymerization, producing polyisoprene elastomers with controlled microstructures. The structures of these designed Fe(II) complexes were well-defined by NMR spectra and X-ray single crystal diffraction analysis. Activation with only 5 equiv of dMAO (dry methylaluminoxane) enabled the phosphinoquinoline Fe(II) complexes to catalyze isoprene polymerization with remarkable activity (up to 3431 kgPI·molFe–1·h–1), yielding polyisoprene with a predominantly cis-1,4/3,4 mixed microstructure (ca. 1:1 ratio). The bidentate N,P-ligands, containing strongly coordinating phosphorus atoms, effectively stabilized the iron active centers, offering high monomer conversion and polyisoprene with high molecular weight at approximately 105 Da across a broad temperature range (−10 to 100 °C). Control over polymerization activity and polyisoprene microstructure was achieved by modifying the ligand structures. Aryl-substituted catalysts exhibited high activity (409 kgPI·molFe–1·h–1) and excellent cis-1,4 stereoselectivity (cis-1,4/trans-1,4 > 97:1), highlighting the synergistic effects of ligand electronic and steric properties on catalytic performance and the isoprene coordination–insertion polymerization mechanism.