Highly Active Ethene Polymerization Catalysts with Unusual Imine Ligands - Late Transition Metal Polymerization Catalysis - page 60

pressive amount of attention. Not surprising, these novel families of catalysts ini-
tiated an intense research activity in academic and industrial laboratories due to
their anticipated short-term industrial application, evidenced by a steep increase
in patenting activity concerning homo- and copolymerizations of ethylene and 1-
olefins [9]. It not only became feasible to prepare known polyolefinic materials un-
der even more simple conditions, but also new materials were synthesized with a
molecular precision characteristic of metallocene single site catalysts. Sterically de-
manding diimine palladium complexes played a key role in the development of
this field. In Fig. 3.1, the structure of a typical example of such a diimine catalyst
precursor is shown. Here an acyl derivative is depicted, a catalyst that may also be
used for the perfectly alternating copolymerization of 1-olefins and carbon monox-
ide [10]. The formation of polyketones exemplifies the high tolerance of the late
transition metal catalysts toward functional groups, or, put differently, the higher
affinity of the Lewis acid late transition metal centers for olefinic entities in com-
parison to heteroatom donors.
Not only palladium, but many more non-metallocene late (and early) transition
metal catalysts for the coordination polymerization of ethene and 1-olefins were
reported [11]. Among the most significant findings in this area are the disclosures
of novel highly active and versatile catalysts based on (i) bidentate diimine [N,N]
nickel and palladium complexes [12], (ii) tridentate 2,6-bis(imino)pyridyl [N,N,N]
iron and cobalt complexes [13], and (iii) bidentate salicyl imine [N,O] nickel com-
plexes [14].
Remarkably high activities for the polymerization of ethene similar to those of
active Ziegler-Natta systems have been reported and the physical properties of the
polyolefins produced can be tailored by the choice of the metal center and the
substitution pattern of the ligand backbone [12-14]. Some of these catalytic sys-
tems are not only compatible with polar monomers, but also copolymerization of
Fig. 3.1 Molecular structure of
the acetonitrile adduct of mono
cationic bis(2,6-diisopropylphe-
nyl)-2,3-dimethyl-2,3-diazabuta-
diene palladium acyl (tetrakis
(3,5-bistrifluoromethanephe-
nyl)borate as non-coordinating
anion not shown). Selected bond
distances (pm): Pd(1)-C(29) =
196.1(6), Pd(1)-N(1)=215.6(4),
Pd(1)-N(2) =204.4(4), Pd(1)-
N(3)=201.0(5). Selected angles
( ): N(1)-Pd(1)-N(2) =76(6),
C(29)-Pd(1)-N(3) =98.8(2),
N(1)-Pd(1)-N(3) =95.5(2),
N(2)-Pd(1)-C(29)=98.1(2),
Pd(1)-C(29)-O(1)=121.1(5).