Cycloheptyl-Fused Iminopyridine-Iron Catalysts for Isoprene Polymerization with Enhanced Activity and Thermal Stability

In this study, a set of six cycloheptyl-fused iminopyridine-ligated iron complexes were prepared and investigated for isoprene polymerization under various reaction conditions. These complexes have been prepared either through a one-pot process or via ligand synthesis prior to complex formation. Both methods resulted in structurally equivalent iron complexes in good yields, as confirmed by X-ray diffraction, FTIR spectra, elemental analysis, and ESI mass spectrometry. Upon activation with MAO, all complexes were active catalysts for isoprene polymerization. Systematic variations in the steric hindrance of ligand ortho-substituent and reaction conditions exhibited significant effects on polymerization activity and polymer properties. The catalyst with less steric hindrance showed remarkable activity and produced high molecular weight polyisoprene but was less selective. Conversely, sterically encumbered catalysts were less active but more selective in both regio- and stereoselectivity. Particularly, Fe1 exhibited high polymerization activities, achieving as high as 1.2 × 108 g (mol of cat.)−1 h–1 over a period of less than 1 min, with complete monomer conversion. Moreover, this catalyst demonstrated high thermal stability, achieving good to excellent monomer conversions over a wide temperature range of −25 to 100 °C, and the resultant polyisoprenes consistently maintained the molecular weight level of approximately 105 g mol–1 across all reaction temperatures. Notably, the polyisoprene predominantly consisted of 1,4 and 3,4 configurations in the range of 55–71 and 29–45 mol %, respectively, while 1,4 units showed remarkably high cis stereoselectivity (cis-1,4/trans-1,4: >99/1).