Page 128 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN002C-85 May 17, 2001 20:35

476 Catalysis, Homogeneous

FIGURE 45 Simplified flow-scheme of SHOP.

in an excess of monomers. Four types of reaction can be properties of the polymer. Isotactic PP is the well-known
distinguished for organic monomers: semicrystalline material that we all know, while atactic
PP is a rubber-like material. The degree of isotacticity de-
1. radical polymerization; termines the “melting point” of the polymer. Isotactic PP
2. anionic polymerization; with less than one mistake in any of its 200 insertions
◦
3. coordination polymerization (e.g., Ziegler-Natta melts at 166 C.
polymerization); and The catalyst and the concomitant technology have un-
4. cationic polymerization. dergone drastic changes over the years. The titanium cat-
alysts can be prepared by the interaction of TiCl 4 and
Transition metal complexes play the key role in coordina- alkylaluminum compounds in a hydrocarbon solvent. This
tion polymerization. reaction can be carried out with numerous variations to
give a broad range of catalysts. It is a heterogeneous high-
2. Introduction to Ziegler-Natta Polymerization surface TiCl 3 material of which the active site is titanium
in an unknown valence state. It is quite likely that alkyltita-
The titanium catalyst for the stereoselective polymeriza-
nium groups at the surface are responsible for the coordi-
tion of propene to isotactic polymer was discovered by
nation polymerization. Since the 1980s, titanium catalysts
Ziegler and Natta in the mid-1950s. Soon after its discov-
supported on magnesium salts have been used.
ery it was commercially exploited and even today isotactic
polypropylene is one of the most important commodity
polymers. Polymers with side-arms, polypropylene (PP) 3. The Cossee-Arlman Mechanism
being the simplest one containing methyl groups, may The mechanism proposed for the solid titanium chloride
possess stereoregularity in the arrangement of these side- catalysts is essentially the same for all catalysts and it
arms. For example, if all methyl groups in PP are ori- is usually referred to as the Cossee-Arlman mechanism.
ented toward the same direction in the stretched polymer
Titanium is hexa-coordinated in the TiCl 3 catalysts or sup-
chain, we say this polypropene is isotactic (see Fig. 47).
ported analogs by three bridging chlorides, one terminal
If the methyl groups are alternatingly facing to the front
chloride, and one terminal chloride that is replaced by an
and the back, we say the polymer has a syndiotactic mi-
alkyl group by the alkylating agent (Et 2 AlCl or Et 3 Al),
crostructure. Random organization of the methyl groups and a vacancy that is available for propene coordination
(not shown) is found in atactic polymers. The stereoreg-
ularity of a polymer has an enormous influence on the

FIGURE 46 Example of new catalyst for ethene oligomerization. FIGURE 47 Microstructures of polypropylene.