Enhancing Thermal Stability in Aminopyridine Iron(II)-Catalyzed Polymerization of Conjugated Dienes

A library of well-defined aminopyridine iron(II) precatalysts with a tertiary amine moiety have been prepared and serve as highly efficient precatalysts for polymerization of isoprene and other biobased conjugated dienes, delivering the corresponding polyolefin elastomers. These iron complexes were synthesized and further verified by ATR-IR, NMR, HR-MS spectroscopy, and elemental analysis. In addition, single-crystal diffraction analysis reveals that the structures of representative complexes Fe-Me, Fe-Et, and Fe-iPr all displayed chloride-bridged centrosymmetric binuclear geometry. Upon activation with MAO, the aminopyridine iron(II) complexes formed homogeneous Ziegler–Natta catalyst systems, which possessed excellent activities for the polymerization of isoprene. Interestingly, the activity of precatalysts increased as the steric hindrance was enhanced (Fe-iPr > Fe-Bn > Fe-Et), and the best activity was achieved by the Fe-iPr complex (2.34 × 106 gpolymer mol–1 h–1). In addition, the high activity that could almost be maintained even at 100 °C indicated that the Fe-iPr catalyst showed excellent thermal stability. The polymerizations of biobased myrcene and β-farnesene have been achieved smoothly by the Fe-iPr precatalyst, delivering high-molecular weight (≤4.9 × 105 g/mol) "green rubber".