Polymer Basics - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 91

polystyrene. However, the use of organometallic catalysts enables the
formation of atactic, isotactic, or syndiotactic polystyrene depending on the
organometallic catalyst.
The use of organometallic catalysts dates back to the 1950s and remains
an active research area to this day. Prior to the 1950s, all polyethylene was
made by a high pressure, high temperature radical process. Because of the
high temperatures and the indiscriminant nature of radical reactions, the
polyethylene prepared by this process is not perfectly linear, but rather has
some branch points. This prevents the polyethylene chains from packing as
tightly and the resultant polymer has lower density and crystallinity. It is
called low density polyethylene (LDPE). In the early 1950s, Karl Ziegler
found that if an aluminum alkyl compound (now often referred to as the
cocatalyst) were added to a titanium or vanadium compound (the catalyst),
ethylene could be polymerized at low pressures. Much research has been
done and it remains an active research subject to this day. A search of U.S.
patents reveals that since 1975 more than 22,000 U.S. patents were issued
containing the word “Ziegler.” Shortly after Ziegler's work, Guilio Natta,
working independently, applied these catalysts to the polymerization of
propylene and was able to make isotactic polypropylene. In 1963, Ziegler
and Natta were awarded the Nobel Prize in chemistry. The catalyst systems
are referred to as Ziegler - Natta catalysts and most polyethylene and
polypropylene produced today is made with Ziegler - Natta catalysts. For a
variety of reasons, polyethylene is the largest volume plastic in the world.
Polypropylene and polyvinyl chloride have similar manufactured volumes
and are second and third after polyethylene. Because of the large volumes
of polyethylene and polypropylene produced, there has been and continues
to be considerable development of these systems and many variants have
been discovered. A typical Ziegler - Natta catalyst system might comprise
a titanium tetrachloride catalyst and a triethyl aluminum cocatalyst. Often,
the catalyst is supported, for example on magnesium chloride. Sometimes,
donors such as esters are added to control the stereoregularity. Recognize that
because there are no groups on the polyethylene homopolymer chain, tac-
ticity of polyethylene is not a factor, but this is important for polypropylene
manufacture.
Ziegler - Natta catalysts are very important, have been known for more
than 60 years, and have been the subject of many studies. There are sev-
eral different active catalytic sites. Because the different active sites have
different reaction kinetics, there is a wider molecular weight distribution than
found in many other types of polymerizations. However the details of the
active catalytic sites are not precisely known. This can be because they are
heterogeneous and therefore more difficult to study by spectroscopic means.
In the case of an olefin polymerization using TiCl 4 supported on MgCl 2 and