Mo [OCMe(CF3)2]2 (Mo-I) catalyzes the cyclodimerization of 1,1-disubstituted silacyclopent-3-enes, RR'SiCeH* (R = R' = Ph (1); R = Me, R' = Ph (2); R = R' = Me (3)), in aromatic solvent to mixtures of cis,cis-, cis,trans-, and trans,transdisilacyclodeca-3,8-dienes. Acyclic metal alkylidene end-capped oligomers are also evident (by NMR) in the equilibrium reaction mixtures, cis,cis• 1,1,6,6-Tetraphenyl-1,6-disilacyclodeca-3,8-diene (6a) crystallizes from the reaction medium at room temperature: an X-ray structure determination (monoclinic, I2/a, a = 25.058(2) A, b = 6.6731(6) A, c = 16.769(1) ,ß = 101.961-(9)°, V = 2743.1 A3, Z = 4, final R = 0.0409, Rw = 0.0351 based on 2184 reflections) shows an "armchair" conformation which is also consistent with low-temperature NMR data for 6a. In the absence of solvent, Mo-I polymerizes 1 and 2 to poly(silapentenes), containing 55% and 75% trans double bonds, respectively. By comparison, the conventional initiator WCle/Me4Sn affords >90% c¿s-poly-1 and ca.80% cis-poly-2, respectively. The molecular weight distributions of all the polymers tend to the most probable value of 2.0, consistent with substantial secondary metathesis reactions. The polymers may also be degraded back to an equilibrium mixture of the 5-and 10-membered rings upon treatment with Mo-I in aromatic solvent. "Living" polymers degrade similarly upon dissolution in benzene or toluene. Mo-I also catalyzes the cyclization and cyclodimerization of diallylsilicon reagents (with evolution of ethylene) to similar equilibrium mixtures of 5-and 10-membered rings in aromatic solvent; polymers were obtained in the absence of solvent. Significantly, poly-1-3 prepared in this manner possess a substantially higher trans content than those obtained from 1-3. Degradation of high trons-poly-1 by Mo-I in toluene gives trans,trans-1,1,6,6-tetraphenyl-l,6-disilacyclodeca-3,8-diene (6c), whose crystal structure has been determined (triclinic, , a = 7.
