Chemistry Reference
In-Depth Information
frequently precedes the insertion process and this polymerization is therefore
often called
coordination polymerization
. The most important group of initiators
in this category are Ziegler
Natta catalysts.
A different group consisting generally of supported oxides of transition metal ele-
ments from Groups V
VII of the periodic table is used to produce linear polyethylene.
11.5.1
Ziegler
Natta Catalysts
Ziegler
Natta catalysts consist of a combination of alkyls or hydrides of Group
I
VIII metals. The most generally efficient
catalyst combinations are those in which an aluminum alkyl derivative is inter-
acted with titanium, vanadium, chromium, or zirconium salts. The most important
application of these catalysts is in the polymerization of olefins and conjugated
dienes. Not every catalyst combination is equally effective in such polymeriza-
tions. As a general rule, Ziegler
III metals with salts of the Group IV
Natta combinations that will polymerize 1-
olefins will also polymerize ethylene, but the reverse is not true.
The Ziegler
Natta catalysts that are used most industrially are solids that are
suspended in the reaction medium. The polymerization reaction that is favored
depends on the catalyst components and on their state of aggregation and other
details of their preparation. Heterogeneous catalytic systems appear to be neces-
sary for the production of isotactic polyolefins, but soluble catalysts have been
used for the synthesis of polyethylene and syndiotactic polypropylene.
Although there are two metals in the Ziegler
Natta catalyst, the weight of
current evidence indicates that polymerization takes place at
the transition
metal
carbon bond. The mechanism is illustrated here with reference to polymer-
ization by TiCl
3
/
Al(CH
2
CH
3
)
3
catalyst complex. The normal geometry for Ti
atoms is octahedral, and the catalyst site, as shown in 11-21, is believed to be a
coordinately unsaturated Ti bonded to four Cl's (which in turn are bridged to two
other Ti's) and to an alkyl group, derived from the aluminum alkyl component.
vacant d−orbital
Cl
CH
Et
T
i
Al
Et
Cl
Cl
Cl
11-21
