Chemistry Reference
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non-aqueous polymerization in methylene chloride, are observed. However, this
high stability is due to an “encapsulation” of the water-insoluble catalyst in the
growing hydrophobic polymer, which protects the catalyst from access of water.
Aqueous
solutions
of water-soluble derivatives of
5
with sulfonate-substituted dii-
mine ligands are inactive for ethylene polymerization [62]. Mechanistic studies re-
vealed that a complex of type
5
is stable in a water-containing solution, which
means that neither the Pd-Me moiety nor the diimine ligand are hydrolyzed.
However, decomposition occurred instantaneously upon addition of ethylene
monomer [61].
Regarding the possible undesired binding of water as a ligand [cf. Eq. (2)], in
this cationic palladium system water coordinates weakly relative to ethylene, and
does not block coordination sites for the monomer [63, 65].
Whereas traditional polymerization in methylene chloride under similar reac-
tion conditions (temperature, ethylene pressure) affords a highly viscous liquid
polyethylene (
M
w
3
10
4
g mol
-1
, ca. 100 branches/1000 carbon atoms,
T
g
-70
C),
polymerization by
5a
in aqueous suspension yields a rubbery amorphous solid of
significantly higher molecular weight and lower branching [Eq. (5)]. Rather than a
conceivable direct interaction of water with the catalyst, this effect appears to be
related to the different phases encountered by the catalyst during polymerization
(heterogeneous aqueous suspension of polymer encapsulating the catalyst vs.
homogeneous solution) [61]. Polymerization of 1-olefins by catalysts of type
5
in
aqueous emulsion has been claimed to afford stable lattices [64].
In view of many potential applications, the synthesis of a largely linear polyethy-
lene with some degree of crystallinity in aqueous emulsion would be of interest.
Recently, Mecking et al. and Spitz et al. independently reported nickel(II)-cata-
lyzed polymerization of ethylene to linear material in aqueous emulsion [65, 66].
Neutral nickel(II) complexes
6
and
7
(Scheme 7.7) based on known bidentate P-
O-ligands [67-70] were found to be suitable catalyst precursors.
Using water-soluble catalyst precursors
6a
, stable latexes of low-molecular-weight
polyethylene could be obtained [65, 71, 72]. For example, a dispersion of polyethylene
of
M
w
3
10
3
g mol
-1
,
M
w
/
M
n
2 to 3 was obtained with 10
3
TO h
-1
under moderate
reaction conditions (70
C, 50 bar ethylene pressure). The catalysts are stable in the
aqueous polymerization for several hours. With
7
significantly higher activities of up
to ca. 3
10
4
TO h
-1
were observed in aqueous emulsion; however, the latexes pre-
pared with these lipophilic catalyst precursors were reported to be colloidally un-
stable [66, 73]. By comparison to traditional polymerization in non-aqueous organic
media such as toluene, catalyst activities and polymer molecular weights are reduced
in the aqueous polymerizations. The lower activities and molecular weights can be
related to a lower rate of chain growth in the aqueous polymerization, caused by an
insufficient local ethylene concentration at the catalytically active centers [71]. To im-
prove catalyst performance, good catalyst activities at limited ethylene concentra-
tions can be expected to be advantageous.
In this context, a systematic understanding of the relationship between catalyst
structure and the effect of ethylene concentration on the chain growth rate is of
interest. To date, however, there is no concise picture: for several neutral nickel
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