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It:Ziegler-Natta

2006-12-07T11:34:03Z

Pyc05: /* Introduction */

== Introduction ==

A Ziegler – Natta catalyst is a method of producing unbranched, stereoregular vinyl polymers, such as linerar unbranched polyethylene and isotactic polypropylene. Ziegler – Natta catalysts are typically based on titanium chlorides and organometallic trialkyl aluminium based co-catalyst.

The catalyst was discovered by a German Chemist Karl Ziegler (1898-1973) in 1953. He produced a tougher polymer with higher melting point than polyethylene which was produced at that time using a resin catalyst. The catalyst was modified by an Italian chemist Giulio Natta (1903-1979) in 1953, a new type of plastic was produced which he called ‘isotactic’ polymers. [http://nobelprize.org/nobel_prizes/chemistry/laureates/1963/ziegler-bio.html Karl Ziegler] and [http://nobelprize.org/nobel_prizes/chemistry/laureates/1963/natta-bio.html Giulio Natta] were awarded the Nobel Prize in Chemistry in 1963 for the discovery and its uses to produce stereoregular polymers.

The type of Ziegler-Natta catalysts used in the synthesis could be:
*Homogeneous - the aluminium cocatalyst is not required in the polymerization
*Heterogeneous - the cocatalyst is required in the polymerisation as shown in the isotactic polymerization

Different types of Ziegler-Natta catalysts used leads to stereospecific polymerization of alkenes, e.g. isotactic polypropylene (or polypropene) from titanium-based catalyst, syndiotatic polypropylene from vanadium-based catalyst4 .

The structure of syndiotactic, isotactic and atactic polypropylene:

[[Image:Polypropene.png]]

== Preparation of Catalyst ==

Most of the time the catalyst and co-catalysts pair are TiCl3 and Al(C2H5)2Cl, or TiCl4 with Al(C2H5)3.

[[Image:no1.bmp]]

In this case, the TiCl3 and Al(C2H5)2Cl system will be discussed. TiCl3 can arrange itself into a number of crystal structures. The one that is interested in is called α- TiCl3.

[[Image:no2.bmp]]

As it can be seen, there are six chlorine atoms coordinated to each titanium atom, with an octahedral geometry. In the interior of the crystal, each titanium is surrounded by six chlorines, but on the surface, a titanium atom is surrounded on one side by five chlorine atoms, and the other side by empty space.

[[Image:no3.bmp]]

Titanium has six empty orbitals, one 4s and five 3-d orbitals, in their outermost electron shells. The titanium atom on the surface of the crystal has an empty orbital.

[[Image:no4.bmp]]

Al(C2H5)2Cl , the co-catalyst, donates one of its ethyl groups to the impoverished titanium, but in the process, one of the chlorines leaves. Therefore, there is still an empty orbital on the
titanium.

[[Image:No5.JPG]]

The aluminium stays coordinated, though not covalently bonded, to the CH2 carbon atom of the ethyl group it just donated to the titanium. It also coordinates itself to one of the chlorine atoms adjacent to the titanium.

So then a vinyl monomer like propylene comes along. The two electrons in the π-system of the carbon-carbon double bond can be used to fill the empty orbital of the titanium. The propylene and the titanium form a complex.

[[Image:No6.JPG]]

===Alkene-metal complexes ===
A carbon-carbon double bond, is made up of a σ bond and a π bond.

[[Image:No7.JPG]]

The π bond consists of two π- orbitals. One is the π-bonding orbital shown in blue and the other is the π-antibonding orbital, shown in red. The π-antibonding orbital is too high in energy, so majority of the time it stays empty.

[[Image:No8.JPG]]

The green lobes are empty orbital and the pink lobes are one of the filled orbitals. The empty orbital is going to share a of pair electrons with the alkene’s π-bonding orbital.

[[Image:No9.JPG]]

== The Polymerization ==
==='''Isotactic polymerization'''===

It is believed that the first electron pairs shift is that pair from the carbon-carbon π-bond that is complexed with the titanium. It’s going to shift to form simple titanium-carbon bond. Then the electrons from the bond between the carbon of the ethyl group and the titanium will shift to form a bond between the ethyl group and the methyl-substitued carbon of the propylene monomer.

Next a migration will occur. The atoms rearrange themselves to form a slightly different structure as below:

[[Image:No10.JPG]]

The aluminium is complexed with one of the carbon atoms from the propylene monomer. As it is shown, titanium is once again with an empty orbital.

So when another propylene molecule comes along, the whole process will starts all over and the polymerization continues and resulting:

[[Image:No11.JPG]]

This will react with more propylene molecules and the polymer chain extends. All the methyl groups on the growing polymer are on the same side of the chain. With this mechanism, isotactic polypropylene is produce.

[[Image:No12.JPG]]

=== '''Syndiotactic polymerization''' ===
The mentioned catalyst system gives isotactic polymers, there are other systems tha can give syndiotactic polymers. The one that is focused on now is based on vanadium rather than titanium. That system is VCl4/Al(C2H5)2Cl. This complex will act similar to the titanium system. First the propylene will attack the vanadium and then the electrons will shift just like with titanium before and propylene is inserted between the metal and the ethyl group.

[[Image:No13.JPG]]

However, noted that when the second propylene adds to the chain, the chain changes position again. It’s back in the position where it started. The methyl groups of the first monomer in blue and the second monomer in red, they are on the opposite sides of the polymer chain. When the polymer chain is in one position the propylene monomer can only add so that the methyl group is on one side of the chain. When the chain is in the other position, propylene only adds the methyl group on the opposite side. Due to this switches positions with each propylene monomer added, the methyl groups are on alternating sides of the chain, producing a syndiotactic polymer.

[[Image:No14.JPG]]

'''Example of Titanium catalyst during polymerisation'''

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<inlineContents>HEADER CSD ENTRY TEWGUX
COMPND UNNAMED
AUTHOR GENERATED BY CONQUEST
CRYST1 17.034 11.534 19.534 90.00 108.49 90.00 P 21/n 4
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CONECT 1 2 10 11 12 13 14
CONECT 2 1
CONECT 3 0
CONECT 4 31
CONECT 5 31
CONECT 6 35
CONECT 7 35
CONECT 8 36
CONECT 9 36
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CONECT 11 1 16 17 32
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CONECT 13 1 22 26
CONECT 14 1 27
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CONECT 16 11 15 42 43
CONECT 17 11 18 44 45
CONECT 18 12 17 46 47
CONECT 19 12 20 48 49
CONECT 20 10 19 50 51
CONECT 21 10 22 52 53
CONECT 22 13 21 23
CONECT 23 22 24 54
CONECT 24 23 25 55
CONECT 25 24 26 56
CONECT 26 13 25 57
CONECT 27 14 28 29 30
CONECT 28 27 58 59 60
CONECT 29 27 61 62 63
CONECT 30 27 64 65 66
CONECT 31 4 5 67 68
CONECT 32 11 33 34 69
CONECT 33 32 70 71 72
CONECT 34 32 73 74 75
CONECT 35 6 7 76 77
CONECT 36 8 9 78 79
CONECT 37 12 38 39 80
CONECT 38 37 81 82 83
CONECT 39 37 84 85 86
CONECT 40 15
CONECT 41 15
CONECT 42 16
CONECT 43 16
CONECT 44 17
CONECT 45 17
CONECT 46 18
CONECT 47 18
CONECT 48 19
CONECT 49 19
CONECT 50 20
CONECT 51 20
CONECT 52 21
CONECT 53 21
CONECT 54 23
CONECT 55 24
CONECT 56 25
CONECT 57 26
CONECT 58 28
CONECT 59 28
CONECT 60 28
CONECT 61 29
CONECT 62 29
CONECT 63 29
CONECT 64 30
CONECT 65 30
CONECT 66 30
CONECT 67 31
CONECT 68 31
CONECT 69 32
CONECT 70 33
CONECT 71 33
CONECT 72 33
CONECT 73 34
CONECT 74 34
CONECT 75 34
CONECT 76 35
CONECT 77 35
CONECT 78 36
CONECT 79 36
CONECT 80 37
CONECT 81 38
CONECT 82 38
CONECT 83 38
CONECT 84 39
CONECT 85 39
CONECT 86 39
MASTER 0 0 0 0 0 0 0 0 86 0 86 0
END</inlineContents>
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== Limitation ==

Ziegler-Natta doesn’t work for some kinds of monomers. For example, poly(vinylchoride) cannot be produced by Ziegler-Natta polymerization. This is because when the catalyst and the co-catalyst come together to form the initiating complex, radicals are being produced during the intermediate steps of the reaction, which will initiate free radical polymerization.

== Reference ==
1. Potapov, A.G. / Bukatov, G.D. / Zakharov, V.A., Journal of Molecular Catalysis. A, Chemical, Mar 2006

2. http://www.hyle.org/journal/issues/5/cerruti.htm

3. http://www.pslc.ws/mactest/ziegler.htm

4. H. Hagen, J. Boersma and G. Koten, ''Chem. Soc. Rev.'', 2002, '''31''', 357

2006-11-30T11:43:24Z

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=== Wiki Templates ===

[[Template:DOI]] and [[Template:Doi-inline]] are providea as (protected) templates for your use. Many other templates exist, often to be found on e.g. Wikipedia pages. You may decide one of these is of particular use, or of interest. If so, you can install it on the wiki here for you and others to use. Add below a line that looks like {{template-name|parameter}}, save, and click on the red text to create the new template. If you prefer the task of adding useful templates to that of adding information about molecules, then you will be given full credit for performing this valuable service for others!
--[[User:Rzepa|Rzepa]] 14:41, 20 October 2006 (BST)

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[[Template:Chem-Data]]

[[Template:Chembox supplement]] - to be linked to from the supplementary section of the table in the template above, for usage see [[Template_talk:chembox_supplement|here]]

[[Template:NFPA_704]] - for notes on how to use, see [[Template_talk:NFPA_704|here]]

[[R & S Phrases]]