Living Polymerization of Isoprene and Synthesis of Polar Functionalized Copolymers Catalyzed by Robust Cobalt Complexes Bearing Imidazolidin-2-imine and Phosphine Ligands

Developing polydienes assembled by diversified monomer microstructures through copolymerization has been a long task in polymer synthesis catalysis, and there remains a demand for high-performance transition metal catalysts. Here, a series of well-defined cobalt complexes (Co1-Co5) bearing strong electron-donating imidazolidin-2-imine and phosphine ligands were successfully synthesized and structurally characterized through single-crystal X-ray diffraction analysis. The Co complexes are highly active and thermally stable for isoprene polymerization in the presence of a small amount of Et₂AlCl cocatalyst. It affords ultrahigh-molecular-weight polyisoprenes (M_n of up to ∼1.2 million g·mol^-1) with cis-1,4/3,4 sequences accompanied by a narrow polydispersity index (PDI = 1.01-1.55), featuring a typical "living" character. Most importantly, Co1-Co5 catalyzed the direct copolymerization of isoprene with a freshly synthesized polar-functionalized conjugated diene monomer, IP_COOMe, without using any protective masking reagent, showing their strong polarity tolerance. Polar functionalized copolymers with high molecular weight and reasonable polar monomer incorporation (2.8-11.1 mol %) are produced to demonstrate enhanced hydrophilicity compared to the typical polyisoprene-based hydrocarbon materials. These Co catalysts are promising candidates for preparing unique copolymers from different diene monomers, and the electron-donating strategy also provides a new pathway for the rational design and convenient synthesis of organometallic catalysts in the future.