Living Polymerization of (o-(Trimethylsilyl)phenyl)acetylene by Molybdenum Imido Alkylidene Complexes

syn-Mo(CHCMe2Ph)(NAd)[OCH(CF3)2]2(2,4-lutidine) (2a; Ad = 1-adamantyl) is a distorted trigonal bipyramid in which 2,4-lutidine occupies an axial position, a structure that results from addition of 2,4-lutidine to the CNO face of unstable pseudotetrahedral syn-Mo(CHCMe2Ph)(NAd)[OCH(CF3)2]2. 2a reacts with (o-(trimethylsilyl)phenyl)acetylene (o-TMSPA) solely via formation of an α-substituted metallacyclobutene intermediate (α addition) that opens to give a single rotamer of a disubstituted alkylidene complex. o-TMSPA is smoothly polymerized at a rate k 2a [2a]0[o-TMSPA] when [2a] < 1 mM with a propagation rate constant k 2a = 0.30 s-1 M-1. Additional studies confirmed that the disubstituted alkylidene propagating species is essentially base-free (K 2a = 62 M-1) and that the propagating species is stable under catalytic conditions (25 °C). Other versions of the Mo(CHCMe2Ph)(NAd)[OCH(CF3)2]2(base) catalyst are either inactive (base = pyridine) or unstable (base = 2-(3-pentyl)pyridine). Mo(CHCMe2Ph)(NAr‘)(OC6F5)2(quinuclidine) (7; Ar‘ = 2,6-Me2C6H3) will also react smoothly with (o-(trimethylsilyl)phenyl)acetylene to give poly(o-TMSPA) with K 7 = 1380 M-1 and k 7 = 0.23 s-1 M-1. Low-polydispersity polyenes containing up to 150 equiv of o-TMSPA can be obtained readily using either catalyst. The thermodynamically most stable form of poly(o-TMSPA), which contains ∼25 double bonds, is air-sensitive and has a significantly red-shifted λmax. o-t-BuPA also can be polymerized to give highly conjugated polyenes, but o-i-PrPA, o-MePA, and phenylacetylene itself add to initiator 2a with decreasing α regiospecificity (73%, 60%, and 56%, respectively). A lack of regiospecificity we propose leads to polymers that do not have a pure head-to-tail structure, have a lower degree of conjugation, and have a progressively more blue-shifted λmax.