Achieving isopropenyl-enriched polyisoprene: unraveling the role of bidentate vs. tridentate iron precatalysts through ligand framework design

The coordination sphere and steric variations in iron catalysts present a fascinating strategy for adjusting monomer regio- and stereoselective enchainment, leading to the development of novel polymer structures in isoprene polymerization. This study investigates a range of iron complexes with variations in the coordination spheres (bidentate and tridentate) and steric/electronic properties of side arms to evaluate their impact on isoprene polymerization. X-ray analysis revealed that the tridentate Fe-NMe2 complex has a dinuclear structure with a μ₂-O bridge, where each iron center is monoligated in an octahedral geometry. In contrast, the bidentate Fe-NHPh crystallizes as an ion pair, consisting of a trisligated [L₃Fe]²⁺ cation and counteranions [FeCl₄]^- and [Cl]^-. Toward isoprene polymerization, iron complexes with tridentate ligands bearing NMe₂ or NⁱPr₂ side arms exhibited high catalytic activity, whereas those with NH₂ or OH side arms showed significantly lower activity. In addition, both bidentate iron complexes were also highly active precatalysts, with activities reaching up to 1.8 × 10⁵ g (IP) per mol (Fe) per h. Variations in the ligand framework led to significant differences in polymer molecular weights, ranging from 37.5 × 10³ g mol^-1 to 272.8 × 10³ g mol^-1, with narrow to broad dispersities. Of significant note, tridentate iron complexes were particularly effective for 3,4 monomer addition (up to 63%), resulting in isopropenyl-enriched polyisoprene. In contrast, bidentate complexes showed a nearly equal preference for both 1,4 and 3,4 additions (1,4/3,4 ≈ 50/50). Polyisoprenes with a high number of isopropenyl pendant groups are highly sought after for post-functionalization and the production of high-performance resins.