Trans-1,4 selective polymerization of 1,3-butadiene with symmetry pincer chromium complexes activated by MMAO

Tridentate chromium complexes (Cr1–Cr7) incorporated with symmetrical pincer ligand bis(arylimino)pyridine and bis(pyrzaolyl)pyridine have been synthesized and characterized by elemental analyis, FT-IR as well as ESI-MS. X-ray diffraction reveals solids-state structures of Cr2, Cr4 and Cr6 all adopt pseudo-octahedral coordination environment with respect to metal center. All complexes have been tested in stereoregulated polymerization of butadiene under various polymerization conditions. The trans-1,4 and cis-1,4 enchainment of resultant polymer are found to be dependent on the structure of ligand and amount of activator used. Under the optimized condition, free ortho-substitutes Cr catalysts Cr1, Cr3, Cr4 and Cr6 are capable of initiating high trans-1,4 selectivity (trans-1,4: 89.2%–92.0%) with good polymer yields (71.5%–78.0%), while counterparts with ortho-positioned alkyl groups Cr2, Cr5 and Cr7 display mixed selectivities with moderate polymer yields. The sterical effect of ligand and amount of MMAO on the catalytic performance, in particular, the stereoselectivity and polymer yield, has been also elucidated by conjugated diene polymerization mechanism.