Amidinate Based Catalysts for the Stereospecific and Living Ziegler-Natta Polymerization of Alpha-Olefins
Keaton, Richard J
Sita, Lawrence R
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The living Ziegler-Natta polymerization of alpha-olefins has been accomplished with a series of precatalysts based on cyclopentadienyl and pentamethylcyclopentadienyl zirconium amidinates (Cp and Cp*ZA's, respectively) upon activation by a borate cocatalyst at 10 °C in chlorobenzene. For the latter, the symmetry of the precatalyst determines the polymer microstructure: C(1)-symmetry gives isotactic polymer, while C(s)-symmetry gives nearly atactic material. The living behavior has been proven through kinetic analyses, narrow molecular weight distribution polymers, formation of telechelic polymers, and synthesis of well defined block copolymers. Aside from simple straight chain alpha-olefins, non-conjugated dienes and vinylcyclohexane have also been polymerized in a living fashion with this series of precatalysts. Characterization of several catalytically active derivatives through solution NMR studies and single crystal X-ray analyses were successful. In the solid state, the initiator appears as a Zr-Me cation that is involved in a doubly methyl bridged dimeric structure. The presence of ether is sufficient to break up the dimer affording a monomeric species. Also, substantially increasing the steric hindrance of the amidinate ligand yields a monomeric structure. The Zr-Me cations undergo rapid methyl group exchange as evidenced through a crossover experiment between C(1)- and C(s)-symmetric initiators. Similarly, the methyl cations can engage in methyl-polymer group exchange, thus providing a new method toward stereoblock copolymer production. Insertion of cyclopentene is also successful into the Zr-Me bond, though further propagation steps do not occur. The product of initiation is the previously unobserved cis-1,2-product, which upon warming quantitatively isomerizes to the cis-1,3-product. The former features a beta-hydrogen agostic interaction with a low J value of 87.7 Hz. Upon isomerization, two beta-agostic hydrogens are present, with J values of 97.5 and 107.2 Hz. Oligomeric polymers were prepared from low ratios of monomer to initiator. Extensive NMR studies showed a 9 : 1 selectivity for the enantiofacial selectivity of the initiation step and perfect stereospecificity thereafter. Quenching the polymerization after extremely long times, or performing the polymerization at higher temperatures, afforded evidence that beta-hydride elimination was a rare, yet active, path, and that chain-walking occurs along the alkyl chain of the last inserted monomer unit.