Preparation and Overview
The process is set up and the polymerization reaction is described. The 7 thumbnail images summarize the content of the video. Click an image to see the image gallery.
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Polymerization chemistry is demonstrated by the reaction between ethylene and a Ziegler-Natta catalyst. A balloon is attached to a reaction vessel to show the ethylene being consumed during the reaction with the catalyst. Polyethylene is visible as a white opaque product of the reaction.
Since the 1950s, Ziegler-Natta catalysts have been used commercially to polymerize olefins. Classic Ziegler-Natta catalysts are formed by combinations of transition metal salts and metal alkyls. Since some catalysts of this type do not require extensive heating or pressure to react with olefins, they can be used to demonstrate polymer chemistry in the classroom or laboratory. However, since the catalyst is air sensitive, the reaction must be performed in a closed reaction vessel under a nitrogen atmosphere.
Ziegler-Natta catalysts are formed through the reaction between an unsaturated transition metal alkyl, LnM-R, and an electropositive alkyl source such as AlR3. The Ziegler-Natta catalyst used for this demonstration is a metallocene zirconium compound, Cp2-Zr-Cl2, which is combined with methyl aluminoxane, or MAO. The methyl groups from the MAO replace the chlorines on the catalyst in preparation for the first step of the polymerization reaction. Finally, one of the methyl groups comes off, leaving a positively charged zirconium center with two Cp rings and a reactive methyl group.
There are three basic steps to the polymerization reaction:
1) Initiation. As ethylene encounters the catalyst, it forms a complex involving the zirconium center and one of the methyl groups.
2) Insertion. The methyl group migrates to the ethylene and so the ethylene is inserted between the methyl group and the zirconium, forming a 3-carbon chain. Now the complex has room to react with another ethylene molecule, which will be inserted in the same way between the zirconium and the carbon chain that is being built.
3) Termination. Finally, the chain termination step occurs when a hydride, either from the growing polymer chain or from an external source, is substituted into the metal complex, displacing the newly formed polymer.
This catalysis reaction is powerful and illustrative as a demonstration due to the gaseous ethylene reacting with the solid catalyst in the presence of methyl aluminoxane, which is suspended in toluene. The resulting polymer is formed primarily at the surface of the liquid, but drifts through the reaction flask as a fine white opaque mist. The reaction is exothermic, which can be illustrated using a temperature indicator and display, such as PSL, or a thermocouple apparatus.
In the reaction set-up shown here, a reaction flask containing ethylene gas, a metallocene zirconium compound Cp2-Zr-Cl2 and a stir bar has been placed above a magnetic stirrer, and the stirrer has been turned on.
Pressurized N2 gas is introduced into a bottle of methyl aluminoxane, or MAO, by means of a needle. A second needle is inserted into the MAO bottle; this double-ended needle will form a conduit from the bottle to the reaction flask. The other end of this conduit line is inserted into the reaction flask. The bottle then is turned over to force the MAO liquid into the conduit line and subsequently into the reaction flask.
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