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Polymerization experiments carried out using Pd(dpm)
2
also showed that nor-
bornene was polymerized quite effectively in the absence of AlEt
3
at a 4000 : 1 : 9
monomer: Pd : B(C
6
F
5
)
3
ratio (run 6, Tab. 4.2). The polymer was soluble enough at
50
Cin
ortho
-dichlorobenzene to obtain a
1
H NMR spectrum. No olefinic reso-
nances were observed, confirming the formation of addition poly(norbornene).
However, the solubility difficulties encountered prevented further characterization.
Our search for a more active system led us to bis(pentafluorophenyl)nickel com-
plexes originally reported by Klabunde and co-workers in the 1980s [56]. One of
the more interesting complexes reported was (
6
-toluene)Ni(C
6
F
5
)
2
(Fig. 4.26). Tol-
uene can be readily replaced by a number of neutral electron donors including xy-
lene, mesitylene, THF, PEt
3
, and norbornadiene. In fact, Klabunde noted that for-
mation of (norbornadiene)Ni(C
6
F
5
)
2
was accompanied by intractable polymer. Kla-
bunde speculated that vinyl addition polymerization occurred with possible cross-
linking. Unfortunately, the insolubility of the norbornadiene polymer prevented
further analysis.
Thus, we synthesized and tested this toluene complex and found that it does
indeed effect the polymerization of norbornene-type monomers. Polymerization of
norbornene-type monomers is not restricted to nickel complexes containing C
6
F
5
ligands. We have found that the electron-withdrawing tris(2,4,6-trifluoromethyl-
phenyl) ligand [57] is also quite effective in polymerizing both norbornene and 5-
triethoxysilylnorbornene, for example. At a 4000 : 1 monomer to nickel ratio,
Ni[2,4,6-tris(trifluoromethyl)phenyl]
2
(1,2-dimethoxyethane) (Fig. 4.27) gave 37%
conversion into polymer from an 80 : 20 norbornene : 5-triethoxysilylnorbornene
monomer mixture.
Thus, we discovered the unusual activation of nickel toward the polymerization
of norbornene-type monomers by C
6
F
5
transfer from B(C
6
F
5
)
3
to nickel [58], a re-
action pathway that is typically a decomposition route for transition metal cata-
lysts [59]. This discovery led to the development of a class of neutral, single-site
nickel complexes containing electron-withdrawing group such as C
6
F
5
that are ef-
fective for the polymerization of norbornene-type monomers.
The most direct proof of the initiation mechanism would come from elucida-
tion of the head group structure of the polymer produced using (
6
-toluene)-
Fig. 4.26
Neutral bis(pentafluorophenyl)nickel initiator for the polymeriza-
tion of norbornenes.
Fig. 4.27
Ni[2,4,6-tris(trifluoromethyl)phenyl]
2
(1,2-dimethoxy-
ethane).
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