Chemistry Reference
In-Depth Information
10.4. ASYMMETRIC ALTERNATING CO - AND TERPOLYMERIZATION OF
OLEFINS WITH CO
10.4.1. Asymmetric CO /Olefi n Copolymerization
10.4.1.1. General Aspects
Over the last decades, the industrial interest in polyketones
obtained via the catalytic copolymerization of carbon monoxide (Scheme 10.12) with
one or more alkene monomers has relentlessly increased.
O
[Pd]
R
+
CO
n
R
Scheme 10.12.
These materials represent a class of low-cost thermoplastics whose synthesis, proper-
ties, and applications are still the object of intense fundamental and applied research.
At least two of these CO/olefi ns polymers were industrially produced: Carilon® from
Shell [80] and Ketonex® from BP [81]. The properties of polyketones may be both
modifi ed and improved by changing the number or the nature of the comonomers and
tuning the structure of the metal catalyst, which makes them superior to polyolefi ns,
polyamides, and polyacetals [82].
10.4.1.2. Mechanism
A detailed mechanistic study of the Pd-catalyzed CO/ethene
perfectly alternating copolymerization was fi rst reported by Drent et al. in 1991. Since
then, a large number of studies from both experimental and theoretical approaches have
been reported and have contributed to a better understanding of each step of the cata-
lytic reaction as well as to identify and obtain information on active species and resting
states in the reaction. Most of these studies were previously reviewed [83-85], and only
a brief description of the catalytic cycle, which is applicable to other alkenes, will be
given here.
As shown in Scheme 10.13, two competing cycles whose prevalence depends on the
reaction conditions and that are connected via cross terminating steps are participating
during the production of polyketones. One cycle initiates via a Pd-H species, which, by
rapid insertion of ethane, gives an alkyl species that reacts reversibly with CO to yield
an acyl complexes. This latter species then irreversibly inserts a second ethene molecule.
The propagation thus occurs through alternating CO and ethene insertions. From this
cycle, ketoesters and diketones can be produced, depending on the termination path.
The methanolysis of an acyl species yields a ketoester, whereas protonolysis of an alkyl
complex yields a diketone. The second cycle initiates by a Pd-methoxy species that
reacts with CO to form a Pd-carbomethoxy complex. During this cycle, ketoesters are
also produced, together with copolymers with diester structure that are produced via
methanolysis of a Pd-acyl. Using organic oxidants, the palladium hydride species was
the most probable initiator, and methanolysis is, therefore, the main termination mecha-
nism. Kinetic studies have shown that ethene insertion is the rate-limiting step of the
reaction [84]. Two competing chain-transfer mechanisms were proposed for the alternat-
ing copolymerization of CO and ethane [86].
