Stereoselective Ring Expansion Metathesis Polymerization with Cationic Molybdenum Alkylidyne N-Heterocyclic Carbene Complexes

Abstract

Molybdenum alkylidyne <i>N</i>-heterocyclic carbene (NHC) complexes of the type [Mo(C-<i>p</i>-C₆H₄Y)(OC(R)(CF₃)₂)₂ (L)(NHC)][B(Ar^F)₄] (Y = OMe, NO₂; R = CH₃, CF₃; L = none, pivalonitrile, tetrahydrofuran; NHC = 1,3-dimesitylimidazol-2-ylidene (IMes), 1,3-dimesityl-3,4-dihydroimidazol-2-ylidene (IMesH₂), 1,3-dimesityl-3,4-dichloroimidazol-2-ylidene (IMesCl₂), 1,3-diisopropylimidazol-2-ylidene (I<i>i</i>Pr); B(Ar^F)₄^- = tetrakis(3,5-bis(trifluoromethyl)phen-1-yl)borate) were used in the ring expansion metathesis polymerization (REMP) of cyclic olefins. With <i>cis</i>-cyclooctene (<i>c</i>COE) cyclic, low molecular weight oligomers were obtained at low monomer concentrations and the cyclic nature of the polymer was confirmed by MALDI-TOF measurements. High-molecular weight cyclic poly(<i>c</i>COE) became available at high monomer concentrations. Also, post-REMP allowed for converting low-molecular-weight cyclic poly(cCOE) into high-molecular-weight cyclic poly(cCOE). Tailored catalysts together with suitable additives offered access to the stereoselective REMP of functional norbornenes providing functional <i>cis</i>-isotactic (<i>cis-it</i>), <i>cis</i>-syndiotactic (<i>cis-st</i>) and <i>trans-it</i> poly(norbornene)s with up to 99% stereoselectivity. Mechanistic details supported by density functional theory (DFT) calculations are outlined.