Chemistry Reference
In-Depth Information
1
Nickel Polymerization Catalysts with Ylide Steering Ligands
Aleksander Ostoja Starzewski
Abstract
Ylide ligands in nickel catalysts possess a remarkable steering power. Bis(ylide)-
nickel complexes provide nonionic organometallic catalyst architectures, which ef-
ficiently suppress chain transfer and termination reactions and instead promote
chain growth in ethylene polymerizations. Pronounced ligand effects are observed
concerning catalyst activity, molecular weight and branching behavior. The catalyst
activities without co-catalysts are remarkable in unpolar and even in highly polar
solvents. The polar group tolerance makes these catalysts suitable for a variety of
polar monomers. Polyacetylene can now be prepared in a controlled fashion in
stabilizing highly polar matrices and, in this way, becomes easily processible and
even water soluble - ready for high tech applications.
1.1
Introduction
1.1.1
Ylides and Ylidic Bond Systems
Charge distributions and energy levels of phosphorus ylides can be described as the
result of specific donor-acceptor interactions between phosphanes and carbenes, re-
sulting in highly polar dative phosphorus-carbon bonds with varying degrees of
-
contribution. Similar orbital interactions are also operative with other donor atoms
such as nitrogen, arsenic, sulfur or iodine in complexes with carbenes (i.e. in nitro-
gen, arsine, sulfur ylides), with nitrenes (i.e. in phosphine-imines, arsine-imines,
sulfimines), and with oxygen (i.e. in phosphine-oxides, sulfoxides). As a conse-
quence all of these ylidic bond systems exhibit dramatically lower first ionization
potentials than normal
-bonds: trimethylphosphine-methylene 6.8 eV versus ethyl-
ene 10.5 eV, trimethylphosphine-imine 8.3 eV versus azomethine 12.4 eV, trimethyl-
phosphine oxide 9.9 eV versus formaldehyde 14.4 eV [1-7].
The extreme bond system of phosphorus-carbon ylides, indicated in Eq. (1), is
reflected in outstandingly low first ionization potentials (Tab. 1.1).
