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self under similar conditions. The catalytic activity was also sharply attenuated
with functional norbornenes.
Terpolymerizations
These nickel-based compounds with P-O ligands also proved to be capable of ter-
polymerizing norbornenes and ethylene with 1-alkenes. As in copolymerizations
with ethylene, the level of incorporation of 5-
n
-butylnorbornene in the terpoly-
mers was lower than that of norbornene. Additionally, the molecular weights ob-
tained were lower, suggesting that the incorporation of the 1-alkene facilitates
chain termination.
4.3.1.2
Palladium Catalysts
The nickel catalysts described in the preceding section are neutral in character, in
our hands cationic nickel catalysts such as those described by Brookhart et al. [20]
are ineffective in the copolymerization of norbornene and ethylene. Indeed nor-
bornene has been described as a strong catalyst poison (for both nickel and palla-
dium) by the same workers. Surprisingly we found that cationic palladium cata-
lysts, with a wide variety of chelate ligands, are very effective in the copolymeriza-
tion of ethylene and norbornenes [89]. These catalysts possess the same ability as
the above-described neutral nickel catalysts to incorporate norbornenes bearing
functional groups, but overcome two of the major limitations:
The palladium catalysts are capable of generating very high molecular weight
copolymers (up to >800000; more than an order of magnitude higher than the
capability of the nickel catalysts).
The palladium catalysts can incorporate high levels of norbornenes (up to
>90 mol% compared to the upper limit of 50 mol% achievable with the nickel
systems) resulting in
T
g
s of in excess of 200
C compared to a maximum of
around about 130
C in the case of nickel-generated copolymers.
The cationic palladium catalysts are typically prepared by reacting (cyclooctadi-
ene)palladium methyl chloride with a stoichiometric amount of the bidentate li-
gand to afford the (ligand)Pd(Me)Cl adduct. The adduct is then reacted with an
activator such as silver hexafluoroantimonate, to afford the final cationic catalyst,
(ligand)Pd(Me)
+
SbF
-
. The polymerization reactions were typically run at ambient
temperature in toluene to afford a viscous solution of the desired copolymer.
Other activators used include tris(pentafluorophenyl)borane/triethylaluminum
mixtures, NaB(C
6
H
3
(CF
3
)
2
)
4
and methaluminoxane.
Using simple chelating phosphines such as bis(diphenylphosphino)ethane as li-
gand resulted in high conversions to low molecular weight copolymers (
M
w
up to
around 10000) with high norbornene incorporation (up to around 65 mol%). The
catalysts were also capable of incorporating norbornenes bearing alkyl substitu-
ents such as 5-butylnorbornene and functional substituents such as 5-triethoxysi-
lylnorbornene. More useful, high molecular weight copolymers (
M
w
up to around
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