Reaction of precursors of "Cp2Ti=CH2" (Cp = η5-C5H5) with cyclobutene yields polybutadiene. The preparation of the titanacyclobutane 3 by the addition of 3,3-dimethylcyclopropene to "Cp2Ti=CH2" is described. When phosphines PMeR2 (R = Me, Ph) are added to 3, phosphine adducts of the isomeric α-substituted titanium-carbene compound are obtained. Reaction with AlMe2Cl affords a heterobimetallic complex. Poly(dimethylpropenamer) is produced from reaction of 3 with excess olefin. The observed reactivity is consistent with productive cleavage of the metal-containing ring in 3 to "Cp2Ti=CHC(CH3)2CHCH2."
Titanacyclobutanes 7 and 11 have been prepared from the reaction of norbornene with precursors of "Cp2Ti=CH2" and with metallacycle 3, respectively. On thermolysis, both 7 and 11 produce products consistent with the intermediacy of α-substituted titanocene-carbene compounds. Thermolysis at 65°C in the presence of excess norbornene affords ring-opened polynorbornene with a cis to trans ratio of 38:62. The polymerizations are highly efficient in catalyst and yield polymers of controlled molecular weight. Polymer prepared from 11 shows polydispersities as low as 1.07 based on gel permeation chromatography versus polystyrene standards. The molecular weight distributions of polymers obtained with 7 are always broader than those obtained with 11. The kinetics of both polymerizations are zero-order in monomer with ΔG‡338 = 24 kcal/mol, ΔH‡ = 27 kcal/mol, and ΔS‡ = 9 e.u. Polymerization with 7 shows an induction period corresponding to a first-order decay of 7 with t1/2 = 80 min at 65°C, whereas the polymerization with 11 is linear throughout. Simultaneous incorporation of d2-norbornene into the polymer by all active sites confirms that this system is best described as a living polymer. The polymerization mechanism is discussed as an olefin metathesis with rate-limiting cleavage of trisubstituted titanacyclobutanes affording high energy carbene intermediates.
Reaction of endo-dicyclopentadiene with "Cp2Ti=CH2" and 3 affords titanacyclobutanes 15 and 16 with exclusive addition to the bicyclic double bond. The X-ray diffraction study of 15 indicates a puckered metal-containing ring. Polymerization of endo-dicyclopentadiene occurs with cleavage of the bicyclic ring system. Preliminary results demonstrate that cyclopentene can be polymerized with 3 at high monomer concentrations. The polymerization of norbornadiene to a linear ring-opened polymer is described. Reaction of these catalysts with 1,4-epoxy-1,4-dihydronapthalene occurs at the double bond and yields oligomers.
Analysis of the microstructure of polymers prepared from methyl-norbornenes indicates that the double bonds in these polymers are predominantly trans. The catalysts exhibit a moderate head-to-tail selectivity. The preferred tacticity of dyads at trans double bonds is racemic while that at cis double bonds is meso.</p
