Transition Metal-Catalyzed Polymerization in Aqueous Systems - Late Transition Metal Polymerization Catalysis - page 236

7.2.2.1 Alternating Olefin/Carbon Monoxide Copolymerization
Since the aforementioned pioneering discovery of Reppe and Magin, catalytic ole-
fin-CO copolymerization (in non-aqueous media) has found continuous, wide-
spread interest. Carbon monoxide is an exceptionally cheap starting material, and
the aliphatic alternating polyketones obtained possess attractive materials proper-
ties [31-33].
A major breakthrough has been the finding that catalysts based on cationic pal-
ladium(II) complexes with bidentate ligands, most often diphosphines, exhibit sub-
stantially increased activities in ethylene-carbon monoxide copolymerization by
comparison to previously known nickel(II) and palladium(II) systems [34-38].
Methanol is typically used as a reaction medium, demonstrating the stability of
these catalysts towards protic media.

C. Incorporation of propy-
lene decreases the melting point, enabling processing of

An alternating ethylene-CO copolymer melts at 257

the materials (e.g.

6 mol% propene: T m 220 C) [35]. Semicrystalline ethylene/propylene/carbon mon-

oxide terpolymers have been commercialized by Shell as a new engineering ther-
moplastic termed Carilon TM since 1995, and BP has developed similar materials
termed Ketonex [39, 40]. However, it has recently been announced that this busi-
ness is to be abandoned for undisclosed reasons [41]. The polyketones have a
good resistance to hydrocarbon solvents and good abrasion properties [42]. As a
drawback, the presence of a large number of carbonyl groups renders the poly-
mers sensitive to UV light, furthermore undesired condensation-crosslinking can
occur at the high temperatures required for processing.
In 1994, Sen and Jiang reported aqueous ethylene/CO copolymerization by cat-
ionic palladium(II) catalysts with water-soluble bidentate sulfonated nitrogen- or
phosphine-based ligands, such as 1 .At50 C and 35 bar each of ethylene and CO,

moderate activities of up to ca. 10 2 TO h -1 were observed (with a catalyst prepared
in situ from [Pd II (NCCH 3 ) 4 ](BF 4 ) 2 /bidentate water-soluble ligand).
An alternating propylene-CO copolymer with a molecular weight of 1.4 10 4 g

mol -1 (vs. polystyrene standards) was also prepared, albeit at lower rates [43]. A
few years later, Sheldon et al. reported significantly higher activities of up to
1.5

10 4 TO h -1 in aqueous ethylene/CO copolymerization, using a very similar cat-
alyst system comprising the same ligand 1 . These higher activities, comparable to
those observed with 1,3-bis(diphenylphosphino)propane (dppp) as the non-sulfo-
nated model for 1 in non-aqueous polymerization in methanol, were ascribed to a
higher purity of the sulfonated phosphine ligand and the addition of a Brønsted
acid, increasing catalyst stability. Molecular weights of the copolymer were found
to be similar to materials obtained in methanol as a non-aqueous reaction medium
under otherwise comparable conditions, with up to M w 6

10 4 g mol -1 , M w / M n ca. 2.