syn-Mo(CHCMe(2)Ph)(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(CHCMe(2)Ph)(NAd)[OCH(CF3)(2)](2). 2a reacts with (o-(trimethylsilyl)phenyl)acetylene (o-TMSPA) solely via formation of an alpha-substituted metallacyclobutene intermediate (alpha 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 degrees C). Other versions of the Mo(CHCMe(2)Ph)(NAd)[OCH(CF3)(2)](2)(base) catalyst are either inactive (base = pyridine) or unstable (base = 2-(3-pentyl)pyridine). Mo(CHCMe(2)Ph)(NAr’)(OC6F5)(2)(quinuclidine) (7; Ar’ = 2,6-Me(2)C(6)H(3)) 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 similar to 25 double bonds, is air-sensitive and has a significantly red-shifted lambda(max). 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 a 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 lambda(max).