Benzonorbornadiene (1), 2,3-dicarbomethoxynorbornadiene (2), and 2,3-bis(trifluoromethyl)norbornadiene (5) can be polymerized by Mo(CH-t-Bu)(NAr)(O-t-Bu)2 (Mo(CH-t-Bu)) in a well-behaved living manner to give essentially monodisperse homopolymers. Poly-2 and (especially) poly-5 are highly trans and are believed to be tactic. The rate of polymerization of 2 is approximately 30 times faster than that of 5 at room temperature, a factor of approximately 10 of which can be ascribed to the lower reactivity of 5 relative to 2, and the remainder to the lower reactivity of living poly-5 relative to living poly-2. 7-lsopropylidene-2,3-dicarbomethoxynorbornadiene is not polymerized at all by Mo(CH-t-Bu), although it does react with Mo(CH-t-Bu) to give an isolable “first insertion product” (4) about 500 times more slowly than 2 reacts with Mo(CH-t-Bu).An X-ray structure of 4 shows it to be a pseudo-tetrahedral species containing a syn alkylidene ligand (substituent pointing toward the imido nitrogen atom), one face of which is blocked by the isopropylidene group and the other face of which is blocked by a carbomethoxy group.(Crystal data for 4 are a = 13.64 (1) Å, b = 17.414 (5) Å, c = 17.64 (1) Å, V = 4130 (5) Å3,β= 99.84 (6)°, space group = P2l/n, Z = A, Mr= 735.85, p = 1.183 g/cm3, μ = 3.48 cm-1, R1 = 0.047, and R2 = 0.064.) 5 reacts with W(CH-Z-Bu) to give an isolable square-pyramidal metallacycle at a rate that is second order overall and for which ΔS⋆ = -40 eu. (Crystal data for 6 are a = 12.268 (8) Å, b = 18.436 (3) Å, c = 16.08 (1) Å β = 94.63 (3)°, V = 3624 (5) Å3, space group = P21/n, Z = 4, Mr = 803.62, p = 1.473 g/cm3, μ = 33.12 cm-1, R1 = 0.067, and R2 = 0.074.) The imido group is in an apical position in this metallacycle with the tert-butyl group in the WC3 ring pointing toward the imido nitrogen atom and the monomer away from it (a trans metallacyclic ring). This metallacycle rearranges to give the gfirst insertion product the in a first-order manner for which ΔS⋆ = -3 eu. © 1990, American Chemical Society. All rights reserved.
