LIVING RING-OPENING METATHESIS POLYMERIZATION OF 2,3-DIFUNCTIONALIZED 7-OXANORBORNENES AND 7-OXANORBORNADIENES BY MO(CHCME2R)(N-2,6-C6H3-ISO-PR2)(O-TERT-BU)2 AND MO(CHCME2R)(N-2,6-C6H3-ISO-PR2)(OCME2CF3)2

7-Oxabenzonorbornadiene (9), 2,3-endo-cis-diacetoxy-7-oxanorbornene, and 2,3-endo-cis-(isopropylidenedioxy)-7-oxanorbornene can be polymerized by Mo(CHCMe2R)(NAr)(O-t-Bu)2 (Ar = 2,6-C6H3-i-Pr2; R = CH3, C6H5) ([Mo]-(CHCMe2R)) to give polymers with narrow molecular weight distributions (PDI < 1.17). However, 2,3-bis(trifluoromethyl)-7-oxanorbornadiene (1), 2,3-dicarbomethoxy-7-oxanorbornadiene (4), and 2,3-trans-dicyano-7-oxanorborn-5-ene (12) form stable metallacycles upon addition to [Mo](CHCMe2R). The rate of reaction of 1 with [Mo](CHCMe2R) has been determined qualitatively to be comparable to that of norbornene in competition experiments, a result that implies that the lone pair on the oxygen in the 7-position is accelerating metallacycle formation. The metallacycle that results from addition of 1 to [Mo](CHCMe2Ph) is square pyramidal with the imido ligand in the apical position. (Crystal data for 2b: a = 12.420 (3) angstrom, b = 15.466 (4) angstrom, c = 20.856 (6) angstrom, V = 3852 (2) angstrom 3, beta = 105.97 (2)degrees, space group = P2(1)/n, Z = 4, M(r) = 779.76, rho = 1.345 g/cm3, mu = 3.9 cm-1, R1 = 0.049, R2 = 0.068.) The MoC3 ring is planar with the CMe2Ph group pointed toward the imido ligand and the C-7 frame of the monomer pointed away from it (a trans metallacyclic ring). The oxygen in the 7-position of the norbornene moiety is located 3.332 (4) angstrom from the metal. Electronic and steric factors that influence first-order breakup of the metallacycle have been studied for monomers 1 and 4. The more reactive catalyst, Mo(CHCMe2Ph)(NAr)(OCMe2CF3)2, can polymerize 1, 4, and 12 to give polymers with narrow molecular weight distributions (PDI < 1.15).