Heterogeneous Organochromium Catalysts for Stereoselective Isoprene Polymerization

Heterogeneous organochromium catalysts have been developed via surface lithiation of traditional surface organometallic complexes to mediate stereoselective isoprene polymerization. Chemisorption of the molecular complex Cr(CH2SiMe3)4 on high surface area anatase titania nanoparticles as well as on a silica support led to the bipodal complexes Cr/TiO2 and Cr/SiO2, respectively. Subsequent reductive lithium intercalation with n-butyllithium led to the formation of lower valent Cr2+ species Cr/LTO and Cr/Li/SiO2. Alternatively, Cr(CH2SiMe3)4 was allowed to react with nBuLi-reduced anatase titania (LTO) to provide Cr/LTOInv. Cr/LTO and Cr/Li/SiO2 both polymerize isoprene with very high activities (154 and 174 kg molCr–1 h–1, respectively) to provide polyisoprene with cis-1,4 selectivity up to 82% in toluene at 50 °C. On the other hand, Cr/SiO2 provides polyisoprene with excellent trans-1,4 selectivity up to 98% (16 kg molCr–1 h–1), and Cr/TiO2 was catalytically inactive. Cr/LTOInv is less active and selective, and X-ray absorption spectroscopy (XAS) measurements of the precatalyst revealed the presence of a mixture of metallic Cr0, Cr4+, and only minor Cr2+ species. Thus, surface lithiation can promote the formation of lower valent metal centers which enable stereocontrol in isoprene polymerizations and may be extended as a general protocol to other catalytic systems.