Chemistry Reference
In-Depth Information
The resulting macromolecules are still reactive toward additional diazoalkanes. The above
step-growth polymerization reactions can also yield block copolymers:
F
F
3
B
n
m
R
Formation of polymethylene by this reaction is not practical for commercial utilization.
Colloidal gold and fine copper powder also catalyze diazoalkane polymerizations. The reaction
appears to precede by formation of alkylidine or carbene species that are bound to the surfaces of
metals [
31
-
33
]. The initiations are completed by additions of diazoalkanes to the bound carbenes
followed by liberation of nitrogen. Termination may take place by chain transfer, perhaps to a
monomer, or to the solvent [
31
-
33
].
Many different diazoalkanes lend themselves to these polymerization reactions. Polypentylidine,
polyhexylidine, polyheptilidine, and polyoctylidine form with a gold complex catalyst, AuCl
3
-
pyridine [
34
].
An entirely different route to preparation of macroparaffins is through a high-pressure reaction
between hydrogen and carbon monoxide. Transition metals, like finely divided ruthenium, catalyze
this reaction. At pressures of about 200 atm and temperatures below 140
C, polymethylene of
molecular weight as high as 100,000 forms [
35
]:
Ru
H
2
+
CO
n
6.2 Polypropylene
Propylene monomer, like ethylene, is obtained from petroleum sources. Free-radical polymeri-
zations of propylene and other
-olefins are completely controlled by chain transferring [
36
].
They are, therefore, polymerized by coordination polymerization. At present, mainly isotactic
polypropylene is being used in large commercial quantities. Also, there is some utilization of atactic
polypropylene as well. Syndiotactic polypropylene, on the other hand, is still mainly a laboratory
curiosity.
The polypropylene that was originally described by Natta contained less than 50% of isotactic
fractions. The remainder was atactic material. Some stereoblocks composed of isotactic and atactic
polypropylenes were also formed. This type of product forms when
a
-olefins are polymerized in inert
hydrocarbons with catalysts prepared by reducing high valence metal compounds, like TiCl
4,
with
organometallic compounds like Al(C
2
H
5
) prepared by reducing high valence metal compounds, like
TiCl
4
, with organometallic compounds like Al(C
2
H
5
)
3
.
Later heterogeneous highly crystalline catalysts based on transition metals (valence 3 or less) like
TiCl
2
, TiCl
3
, ZrCl
3
, and VCl
3
were developed that yielded stereospecific polypropylene. The metal
halides were combined with selected metal alkyls. Only those alkyls were picked that would not
destroy the crystalline lattice of the transition metal salts in the process of the reaction. The resultant
catalysts yielded crystalline polypropylenes with high fractions of the isotactic material.
The products, however, also contained some low molecular weight fractions, some amorphous and
stereoblock materials, that still required costly purification and separations to obtain relatively
pure isotactic polypropylene. The atactic polymer is a wax-like substance that lacks toughness.
Also, presence of amorphous materials, or very low molecular weight compounds, causes tackiness
a
