Ring Opening Metathesis Polymerization of Cyclic Olefins by PMe3–Supported (Arylimido)Niobium–Alkylidene Catalysts

Abstract

Ring opening metathesis polymerization (ROMP) is known as an efficient method for synthesis of functional polymers; metal-alkylidene (carbene) species play a role as catalysts. (Arylimido)niobium(V)-alkylidene catalysts display superior capability as the catalysts, and this report summarizes results in ROMP of norbornene, norbornadiene, tetracyclododecene using the catalysts containing fluorinated alkoxide ligand, Nb(CHSiMe₃)(NAr)[OC(CF₃)₃](PMe₃)₂ [Ar = 2,6-F₂C₆H₃ (1), C₆F₅ (2)], and the perhalophenoxide ligands, Nb(CHR)(NAr)(OC₆F₅)(PMe₃)₂ [R = SiMe₃, Ar = 2,6 —Cl₂C₆H₃ (3); R = CMe₂Ph, C₆F₅ (4)] and Nb(CHSiMe₃)(NAr)(OC₆Cl₅)(PMe₃)₂ [Ar = 2,6-F₂C₆H₃ (5), C₆F₅ (6)]. Among the perhalophenoxide catalysts (3-6), the pentafluorophenylimido catalysts (4, 6) showed rather low catalytic activity compared to the corresponding 2,6-dihalophenylimido catalysts (3, 5), whereas the perfluorinated alkoxide catalysts (1,2) exhibited the opposite trend. The ROMP of low strained cis-cyclooctene (COE) using the pentachlorophenoxide catalysts (5 and 6) proceeded in a controlled manner even at 100°C upon addition of PEt₃ (10 equiv), showing linear correlations between the M_n values (in the resultant polymer) and the polymer yields (TON values). The M_n values were controlled in the ROMPs of COE using catalysts 5 and 6 at 80°C by additions of allyltrimethylsilane to afford the end-functionalized oligomers.