Bimolecular control over polypropene stereochemical microstructure in a well-defined two-state system and a new fundamental form: stereogradient polypropene.
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Biomedical subjects
Matthew B Harney
Publications and source records attributed to Matthew B Harney.
Discrete, multiblock isotactic-atactic stereoblock polypropene microstructures of differing block architectures through programmable stereomodulated living Ziegler-Natta polymerization.
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Living Ziegler-Natta polymerization by early transition metals: synthesis and evaluation of cationic zirconium alkyl complexes bearing beta-hydrogens as models for propagating centers.
The synthesis and characterization of a series of cationic zirconium and hafnium complexes with alkyl substituents bearing beta-hydrogens of general formula {(eta(5)-C5Me5)MR[N(Et)C(Me)N(t-Bu)]}[B(C6F5)4] [M = Zr; R = Et, n-Pr, i-Pr, n-Bu, i-Bu, and 2-ethylbutyl (5a-f) and M = Hf; R = i-Bu and t-Bu (6 and 7, respectively)] is described, including several isotopically labeled derivatives. The ability of these complexes to serve as model complexes for the living Ziegler-Natta polymerization of olefins that can be effected using the initiator 2a (R = Me in 5) has been addressed. The results obtained shed additional light on the steric and electronic factors that can contribute to the living character of a Ziegler-Natta polymerization based on an early transition metal initiator.
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End-group-confined chain walking within a group 4 living polyolefin and well-defined cationic zirconium alkyl complexes for modeling this behavior.
Living polymers derived from the polymerization of 1-butene using the cationic zirconium initiator, {Cp*ZrMe[N(Et)C(Me)-N(tBu)]}[B(C6F5)4] (Cp* = eta5-C5Me5) (1), have been shown to undergo end-group-confined chain walking that is competitive with direct beta-hydride elimination and chain release at -10 degrees C. The well-defined complexes, {Cp*Zr(iBu)[N(Et)C(Me)N(tBu)]}[B(C6F5)4] (2) and {Cp*Zr(2-ethylbutyl)[N(Et)C(Me)N(tBu)]}[B(C6F5)4] (3), were prepared, and each was found to possess a strong beta-hydrogen agostic interaction that is absent in the living polymer. The isotopically single- and double-labeled derivatives, {Cp*Zr(2-d-2-methylpropyl)[N(Et)C(Me)N(tBu)]}[B(C6F5)4] (2') and {Cp*Zr(1-13C-2-d-2-methylpropyl)[N(Et)C(Me)N(tBu)]}[B(C6F5)4] (2' '), were also prepared and found to undergo isotopic label scrambling at 0 degrees C. For 2' ', the observation that after scrambling each deuterium label is located on a 13C-labeled carbon atom is consistent with the Busico mechanism for chain-end epimerization rather than the Resconi mechanism. Decomposition of 3 yielded olefinic products also consistent with chain walking prior to beta-hydride elimination and chain release. Finally, the unexpected decrease in stability of the living polymer relative to that of the model complexes reveals the importance of subtle differences in steric and electronic factors in controlling beta-hydride elimination and chain release.
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