_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Catalysis of reactions of alkenes by transition metal complexes
The scale of consumption of polyethene and polypropene plastics in Western
countries and some ofthe uses ofthese materials are listed in Table 12.3. For their
contributions which ha ve made such a mark on the plastics industry Karl Ziegler
and Giulio Natta were jointly awarded the Nobel Prize for Chemistry in 1963.
All commercial catalysts for the production of HDPE and of PP are heterogeneous.
Homogeneous systems are applied. however, in stereospecific polymerization
of 1, 3-butadiene to give cis-1, 4-polybutadiene, which bas properties
similar to natural rubber. There ha ve also been many laboratory studies of such
homogeneous catalysts. Some ofthe most active are organolanthanides (p. 401 ).
Cyclopentadienyl titanium complexes such as Cp 2 TiCl 2 activated by aluminium
alkyls are also effective. This work has contributed information about the
mechanism of the polymerization. Even so, there is still much controversy about
this; probably different pathways are followed, maybe simultaneously, in
different systems. The most widely accepted mechanism is based on one put
forward by Cossee in 1964 (Fig. 12.10). The early commercial Ziegler-Natta
catalysts were prepared by treating TiC1 4 or TiCl 3 with an aluminium alkyl. Alkyl
transfer to titanium and if necessary reduction to Ti(III) occurs initially.
Polymerization apparently takes place at the edges of small TiCl 3 crystallites or at
defects in the TiCl 3 structure. This may be responsible for producing a
stereoregular product.
Catalysts more than a thousand times as active as the original Ziegler-Natta
systems have now been developed. In one form magnesium chloride, which has a
similar layer lattice to rx-TiCl 3 , is milled with ethyl benzoate and then treated with
TiCl 4 • The resulting solid retains about 1% titanium and ester; it is activated with
Et 3 Al. Polymerization ofpropene is carried out in the liquid phase under pressure
(55°C/20atm) and of ethene, in the gas phase. The reaction is strongly
exothermic (p. 124). Such low concentrations of these modern supported
catalysts are required that no removal of catalyst residues from the product is
necessary. Moreover such highly crystalline polypropene is produced that there is
no need to separate unwanted atactic material.
12.3. 7 Olejin metathesis
Olefin metathesis or dismutation was discovered in the laboratories of Phillips
Petroleum in the late 19 SOs. In o ne of the early experiments propene was passed
over an alumina catalyst which had been pretreated with Mo( C0) 6 • Ethene and 2-
butene (cis and trans isomers) were obtained with negligible by-product
formation. Without a catalyst, such reactions require temperatures above 700°C
and do not occur cleanly.
2MeCH=CH 2
121 'C/
====' MeCH=CHMe + H 2 C=CH 2
catalyst
Rapid development followed and the first commercial plant, the now defunct
Triolefin Process, was opened in the mid 1960s near Montreal to deal with a local
373