Metallocene Polymerization: Metallocene polymerization is catalyzed by metallocenes. It allows to make polymers of very high molecular weights in comparision to Ziegler Natta catalyst. Metallocene polymerization is also good for making polymers of very specific tacticities. A metallocene is a positively charged metal ion sandwiched between two negatively charged cyclopentadienyl anions. Cyclopentadienyl anoin is made from cyclopentadiene.
In Cyclopentadienyl most of the carbon atoms has one hydrogen atom but one carbon atom has two hydrogen atoms. One of those two hydrogen atoms are acidic which separates very easily. So, the carbon atom is left with only one hydrogen atom with an extra pair of electrons.
The ring in anionic form is very stable.These cyclopentadienyl ions have a charge of –1.When a cation like Fe with a +2 charge comes along , two of the anions forms an iron sandwich called as ferrocene. When a metal with a bigger charge is involved, like zirconium with a +4 charge, the Zirconium will bond to two chloride ions to balance the charge, -1 charge on each to give a neutral compound.
In Zirconocences extra chlorine ligands can not be adjusted in between the cyclopentadienyl rings. To make room for the chlorines, the rings tilts with respect to each other. This tilting happens whenever a metallocene has more ligands than just the two cp rings.
In bis- Chlorozirconocene each cp ring has aromatic ring fused to it. The two-ring system fused to a phenyl ring is called an indenyl ligand. There is an ethylene bridge that links the top and bottom cp rings. These two features make this compound a great catalyst for making isotactic polymers. The bulky ligands pointed in opposite directions guide the incoming monomers so that they can only react when pointed in the right direction to give isotactic polymers. The ethylene bridge holds the two indenyl rings in place.
To make Zirconocene complex catalyze a polymerization, a co-initiator methyl allumoxane (MAO) is added to it. The chlorines of zirconocenes are labile that means they like to fall off of the zirconocene.MAO replace them with some of its methyl groups. The methyl groups can fall off too. When one of them falls off a complex is formed. The positively charged zirconium is stabilized because the electrons from the carbon-hydrogen bond are shared with the zirconium to form a α-agostic associatio.
Zirconium still lacking in electrons. The bonding is satisfied by the olefin monomer. In Propylene, carbon-carbon double bond is having electrons to share, so it shares a pair with the zirconium to satisfy the bonding. The precise nature of the complex between the zirconium and the propylene is complicated.
This compexation stabilizes the zirconium but not for long. When this complex forms, it rearrange itself into a new form. The electrons in the zirconium-methyl carbon bond shift to form a bond between the methyl carbon and one of the propylene carbons.
The electron pair that had been forming the alkene-metal complex shifts to form an outright bond between the zirconium and one of the propylene carbons. As can be seen in the picture, this happens through a four membered transition state. Also zirconium ends up just like it started, lacking a ligand, but with an agostic association with a C-H bond from the propylene monomer.
Another propylene monomer react just like the first one.
The propylene coordinates with the zirconium , then the electrons shuffle.
When second propylene monomer has added to the chain, the methyl groups are always on the same side of polymer chain which leads to an isotactic polymer.
The propylene monomer always approaches the catalyst with its methyl group pointed away from the indenyl ligand.
If the methyl group were pointed towards the indenyl ligand, the two would bump into each other keeping the propylene from getting close enough to the zirconium to form a complex. So, only when the methyl group is pointed away from the indenyl ligand, the complex of ziroconium with propylene is formed.
When the second monomer is added it approaches from the other side and its methyl group away from indenyl ring. The methyl group is pointed up rather than down. This is so because the second propylene is adding from the opposite side as the first,it must be pointed in the opposite direction if the methyl groups are to end up on the same side of the polymer chain.
Structure Property Relationship: PP is a linear polymer with little or no branching. Methyl group in the chain leads to increase in melting point and chain stiffening. The tertiary carbon atom provides a site for oxidation so that the polymer is less stable than PE in the presence of oxygen. Methyl group leads to products of different tacticity. Commercial polymers are usually about 90-95% isotactic.
Tacticity:
Isotactic
Syndiotactic
Atactic
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