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

Molybdenum alkylidyne N -heterocyclic carbene (NHC) complexes of the type [Mo(C- p -C 6 H 4 Y)(OC(R)(CF 3 ) 2 ) 2 (L)(NHC)][B(Ar F ) 4 ] (Y = OMe, NO 2 ; R = CH 3 , CF 3 ; L = none, pivalonitrile, tetrahydrofuran; NHC = 1,3-dimesitylimidazol-2-ylidene (IMes), 1,3-dimesityl-3,4-dihydroimidazol-2-ylidene (IMesH 2 ), 1,3-dimesityl-3,4-dichloroimidazol-2-ylidene (IMesCl 2 ), 1,3-diisopropylimidazol-2-ylidene (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 cis -cyclooctene ( c 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( c 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 cis -isotactic ( cis-it ), cis -syndiotactic ( cis-st ) and trans-it poly(norbornene)s with up to 99% stereoselectivity. Mechanistic details supported by density functional theory (DFT) calculations are outlined.