Catalytic polymerization of isoprene using an ultrahigh active iron(ii) complex with an ortho-modified 1,10-phenanthroline ligand, access to a thermal plastic rubber†

Bidentate N,N-ligands have played a vital role in the iron-mediated polymerization of 1,3-diene over the past few decades. In this work, iron complexes chelated with a 1,10-phenanthroline ligand modified with an electronic donating (thio)ether, diethylamine substituents at the 2-position, were synthesized and characterized. The presence of the donor promoted the catalytic activity by one-fold for isoprene polymerization, reaching the most active iron system (2.60 × 10⁷ g mol⁻¹ h⁻¹) ever found so far. An elevated 3,4 selectivity of 69.9% with moderate syndiotacticity (rr: 60.2%) was achieved at −40 °C. The activity was heteroatom-dependent, with the catalyst bearing ether (1.20 × 10⁷ g mol⁻¹ h⁻¹ –0.96 × 10⁷ g mol⁻¹ h⁻¹) performing more actively than thiophenyl ether (Fe5, 0.89 × 10⁷ g mol⁻¹ h⁻¹) and diethylamine (Fe6, 0.67 × 10⁷ g mol⁻¹ h⁻¹) analogues. Polyisoprenes synthesized under various temperatures were pressed and molded to afford a new type of material with integrated excellent strength (breaking strength, 15.6 MPa) and toughness (elongation up to 589%). The thermal plasticity granted a good reprocessability with satisfactory property recovery after three cycles.