Highly Active Ethene Polymerization Catalysts with Unusual Imine Ligands - Late Transition Metal Polymerization Catalysis - page 84

a more crowded 5-coordinate transition state for transfer than a 4-coordinate for
insertion [6 j, 34].
It is interesting to notice that the molecular weight of the polymer produced by
complex 31 is quite comparable to the one obtained by 133 , but the branching is
different. Complex 31 yields a polyethene with significantly less branching and a
substantially higher glass transition temperature ( T g ). Both carry the diisopropyl
phenyl imine moiety. Obviously, the chain end isomerization and olefin insertion
into the secondary nickel alkyl is slower for 31 . Catalysis with complex 37 where
one N-hetaryl substituent is the parent phenyl group yields a higher branched
polyethylene very similar to 133 , but with half the molecular weight. This shows
clearly that the properties of the polyolefin production can be tailored with the li-
gand frame of the catalyst.
3.3.2
Diiminepyridine Iron and Cobalt Complexes with Peripheral N-Heteroaromatic
Substituents
The same general polymerization set up as described for the diimine complexes
was used here. The polymerization time was reduced to one hour. In cases that
no solid polymer was formed, the organic layer was evaporated to yield an oily
residue. Tab. 3.3 summarizes the polymerization results together with relevant
properties of the polymers.
From these data, the following general trends can be deduced.
(i) All the precatalysts 53 - 62a containing N-pyrrolyl, -indolyl, -carbazolyl substit-
uents showed remarkably high activities, similar to those of alkyl N-aryl sub-
stituted catalysts [13].
(ii) Iron(II) catalysts 53a - 62a are in general more active by a factor of 10-100 in
comparison to cobalt(II) complexes 53b - 61b . The low activity of iron(III)
complexes 54 c , 55c suggests that only Fe(II) and not Fe(III) species are ac-
tive in catalysis.
(iii) Increasing the steric bulk of the peripheral N-azolyl groups leads to poly-
mers with higher molecular weight, and a decrease in activity. The highest
activity for the homopolymerization of ethene is shown by the 2,5-dimethyl-
pyrrolyl complex 54a . It is interesting to note that also in the case of alkyl N-
aryl catalysts the analogous 2,4,6-trimethylphenyl Fe(II) complex is the most
active complex [12-15]. In the cobalt series the most productive complex is
57b with 2-methyl-5-phenyl-pyrrolyl substituents.
(iv) The molecular weights of the polymers obtained are in general lower than
those of the polymers derived from alkyl N-aryl catalysts [12, 13] but the poly-
dispersities are smaller. The cobalt complexes yield mainly oils of low molec-
ular weight, one extreme case is complex 61b which yields highly branched
oligomers with a chains of less than eighteen carbons (GC analysis).
(v) Oligomers produced with the N-pyrrole-derived complexes differ substantially
from the oligomers obtained from the N-aryl-based systems. While the latter