Chem 421: Introduction to Polymer Chemistry
Coordination (Ziegler-Natta) Polymerization
Early work:
Insertion of aluminum alkyls into olefins was studied by Ziegler:
Important discovery: R3Al + Lewis acids:
Another important discovery: tacticity control:
Results:
- Nobel Prize in Chemistry for Zeigler and Natta (1963)
- Multibillion $ industry
Overall Scheme of Coordination Polymerization
- Limited to ethylene and other a-olefins like propylene. (Actually, it
is the only good way to polymerize these monomers.)
- Produces linear polymer, with very few branches (e.g., high density
polyethylene, HDPE).
- Capable of producing homo-tactic polymers.
- Most commercial initiators are insoluble complexes or supported on
insoluble carriers.
- Very complex mechanism, still poorly understood for the heterogeneous
systems.
- Termination is almost exclusively by chain transfer.
- Modern "high mileage" initiators produce up to 1000's of kg per
g initiator.
- Initiators are often called "catalysts" even though they are
consumed by the process. Many chains are started per molecule of initiator.
Mechanism of Coordination Polymerization
The mechanism is poorly understood because it takes place on the surface of
an insoluble particle, a difficult situation to probe experimentally. The
mechanism shown below is one of several models proposed to at least partially
explain the action of the Ziegler-Natta systems, but it is only an approximation
of the more complex process that actually occurs.
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