The past two decades have witnessed dramatic advances in the development of well-defined transition-metal complexes that can function as catalysts for the stereoselective Ziegler-

A variety of amphiphilic quaternary dimethylammonium compounds bearing n-alkyl and oxyethylene groups have been designed and synthesized as antimicrobial additives for use in self-decontaminating surfaces. The effectiveness of these additives stems from a unique ability to self-concentrate at the air-polymer interface without the incorporation of exotic perfluorinated or polymeric functionalities. X-ray photoelectron spectroscopy analysis reveals surface enrichment as high as 18-fold, providing a 7-log reduction of both Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria. The migration to the surface is a consequence of the hydrophobicity of the additive within the hydrophilic polyurethane resin, over which an unprecedented level of control can be exerted by altering the lengths of the n-alkyl and oxyethylene groups. Thus, for the first time, specific surface and bulk coating concentrations can be achieved as desired using a single class of antimicrobial additives.

(4b)], which were characterized in solution. Compounds 4a and 4b were evaluated as initiators for the stereospecific living Ziegler-Natta polymerization of 1-hexene. These results reveal that, while an extremely high level of stereoselectivity can be achieved to produce isotactic poly(1-hexene) in a living fashion, the rate constant for polymerization, k p , using either 4a or 4b, is~60 times less than that of the analogous zirconium initiators. Finally, upon substoichiometric activation of 3a with [PhNHMe 2 ][B(C 6 F 5 ) 4 ] in a 2:1 ratio, degenerative transfer living Ziegler-Natta polymerization of 1-hexene can be accomplished to produce atactic poly(1-hexene).

The recent development of transition-metal complexes that can function as catalysts for the stereoselective Ziegler-Natta polymerization of propene has provided a wealth of new polypropene-based materials through extensive manipulations of the ligand environment about the metal center.[1] The currently practiced "one catalyst-one material" strategy, however, has significant disadvantages and practical limitations for fine-tuning physical properties of the polypropene material through minor adjustments about a given microstructure, or for accessing a completely different microstructure altogether. Thus, not only is it a labor-intensive synthetic undertaking to prepare a large variety of catalyst structural variants that may, or may not, yield a desired microstructure but even after several decades of effort the "rational design" of new catalysts that can produce a specific polypropene microstructure is still out of reach; and even more so for non-metallocene-based systems. [2] Dynamic unimolecular processes that are competitive with propagation, such as site isomerization in structurally constrained C 1 -symmetric ansa-bridged metallocenes, [3] conformational flexibility in unconstrained "oscillating" metallocenes, [4] and "chain-end epimerization" [5] or ligand-sphere rearrangements in non-metallocenes, [6] can give rise to polypropene materials that display promising technologically desirable properties as the result of varying degrees and patterns of stereoerror incorporation, such as in the case of elastomeric polypropene. [3][4][5][6] Owing to the intrinsically low energy barriers associated with these unimolecular processes, however, to date, the only means available by which to exert some level of external control in order to access, to a significant degree, a wider range of microstructures for a given catalyst has been to capitalize on the bimolecular nature of propagation (olefin complexation) by varying propene pressure, and hence the rate of propagation (v p ) versus that of

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.