Chemistry Reference
In-Depth Information
help increase the stereoregularity of the product. Thus, for instance, dialkylzinc plus titanium
trichloride catalyst yields polypropylene that is 65% stereoregular. Addition of an amine, however,
to this catalytic system raises stereoregularity to 93% [
229
]. Heterogeneous catalysts, typically, form
polymers with very wide molecular weight distributions.
Many mechanisms were proposed to explain the action of heterogeneous Ziegler-Natta catalysts.
All agree that the polymerizations take place at localized active sites on the catalyst surfaces. Also,
it is now generally accepted that the reactions take place by coordinated anionic mechanisms. The
organometallic component is generally believed to activate the site on the surface by alkylating the
transition metal. Some controversy, however, still exists about the exact mechanism of catalytic
action, whether it is
monometallic
or
bimetallic
. Most of the opinion leans to the former. Also, it is
well accepted that the monomer insertion into the polymer chain takes place between the transition
metal atom and the terminal carbon of the growing polymeric chain [
230
].
Only one
bimetallic mechanism
is presented here, as an example, the one originally proposed by
Natta [
230
]. He felt that chemisorptions of the organometallic compounds to transition metal halides
take place during the reactions. Partially reduced forms of the di- and tri-chlorides of strongly
electropositive metals with a small ionic radius (aluminum, beryllium, or magnesium) facilitate
this. These chemisorptions result in formations of electron-deficient complexes between the two
metals. Such complexes contain alkyl bridges similar to those present in dimeric aluminum and
beryllium alkyls [
231
]. The polymeric growth takes place from the aluminum-carbon bond of the
bimetallic electron-deficient complexes [
232
,
234
]:
CH
2
CH
2
CH
2
Ti
Al
Ti
Al
Ti
Al
CH
2
Ti
R
R
R l
δ
δ
CH
2
CH
2
Ti
Al
R
where R represents an ethyl group. The bulk of the evidence, however, indicates that chain growth
occurs through repeated four-center insertion reactions of the monomer into the transition metal-
carbon
-bond [
230
]. It is still not established whether the base metal alkyls serve only to produce the
active centers and have no additional function. This would make the mechanism monometallic. If,
however, the active centers must be stabilized by coordination with base metals then the mechanism
is bimetallic [
234
]. The two active centers are depicted as follows:
s
R
R
X
X
X
X
M
T
M
T
R
M
n
X
X
X
X
X
monometallic site
bimetallic site
where M
T
represents the transition metal, M
n
a base metal, and a vacant site. Both types of active
centers might conceivably be present in heterogeneous Ziegler-Natta catalysts [
234
]. The exact
locations of the sites in the solid catalyst crystals are still debated. Some speculations center on
whether they are located over the whole crystal surface or only over the edges of the crystals [
235
].
Most evidence points to location at the edges. An example of the
monometallic
mechanisms is one
originally proposed by Cossee and Arlman [
236
,
237
]. This mechanism assumes that the reaction
