Skip to main content
ZIEGLER –NATTA AND STEREOCHEMICAL CONTROL IN POLYMERIZATION
Anand S. Burange, Wilson College, Mumbai
asgburange@gmail.com

Introduction
The traditional route for the synthesis of LDPE (Low Density Polyethylene) requires very high pressure i.e. nearly 20,000 psi and high temperature (500 oC). These stringent reaction conditions have safety issues. But these severe conditions can be overcome by Ziegler-Natta catalyst where polymerization takes place at comparatively lower reaction temperature and pressure. The catalyst is also useful to synthesize HDPE which is composed of long linear chain that pack closely together to form rigid polymer.
There are various Ziegler-Natta catalysts reported so far. The most commonly used, is the combination of TiCl4 and triethylaluminium (Et3Al). The catalyst must be handled with care because it catches fire on contact with air.
Before 1953, isotactic and syndiotactic addition polymers were unknown.  Karl Ziegler, Germany and Giulio Natta, Italy independently discovered these catalysts (TiCl4 and R3Al) and their applications in polymerization reactions. Because of their extensive and pioneering work in polymerization, both were awarded with Nobel Prize in 1963.

Stereochemistry in Polymers
In case of polymerization of unsymmetrical alkenes say propene; during the polymerization the initiating radical attacks on alkene to form stable radical and propogation of reaction starts. It leads to the formation of one asymmetric carbon or stereogenic centre in polymer shown below (Figure 1).



Figure 1. Radical polymerization: Formation of asymmetric carbon 


The change and the sequence of absolute configuration, affect the packaging and density of the polymer. On the basis of absolute configuration (R or S) of stereocentre in the polymer; they are classified into three types enlisted below:
1.      Isotactic polymer: All the asymmetric carbon atoms have same absolute configurations. For example in case of polypropylene all methyl groups are arranged on the same side of the chain.

2.      Syndiotactic polymer: Alternate absolute configurations of stereocentres. They have alternate arrangement of methyl groups in polypropylene.

3.      Atactic polymer: Random absolute configurations. Random arrangement of methyl groups in polypropylene.
(Refer Figure 2)



Figure 2. Types of polymer on the basis of arrangement of absolute configuration


What is true catalyst for the polymerization process? TiCl4 and Et3Al react with each other and there is alkyl group migration from Al to Ti in exchange of chloride to form TiCl3-Et which serves as true catalyst in Ziegler Natta catalysis.
The actual catalyst is solid where one ligand (-Cl-) is shared by two Ti atoms in a crystal therefore the actual oxidation state of Ti is +3  not +4.
Recently, zirconium complexes are used as modified Ziegler Natta catalyst since they are soluble in reactions solvents thus imparts more selectivity (Homogeneous Catalysis).

References:
1.       Organic Chemistry, Clayden
2.       Principles of Organic Synthesis, R.O.C. Normon Coxon
3.       Organic Chemistry, I. L. Finar
4.       Organic Chemistry, Solomon and Fryhle
5.       Organich Chemistry, Janice S. Smith

Comments

Post a Comment

Popular posts from this blog

BAEYER STRAIN THEORY AND STRAIN Anand S. Burange, Wilson College, Mumbai asgburange@gmail.com. Small rings like cyclopropane and cyclobutane have larger values of heat of combustion per –CH 2 - group compared to other larger rings. The answer to this observation lies in a theory proposed by Adolf Von Baeyer from University of Munich in 1885. His theory is also known as Baeyer’s Strain Theory but all the aspects discussed in theory are not completely accepted as some of them were proven wrong in the course of history. Now a days this theory, is modernized and then discussed for the better understanding of students/researchers. Baeyer’s Strain Theory Assumptions: Baeyer in his theory, he did few arguments where he failed to convince many aspects while some of them are well accepted. Few important arguments from his theory are as follows. # Argument 1 In cyclopropane and cyclobutane, all carbon atoms are SP 3 hybridized therefore expected bond angle (C-C-C bond angle) s...
1 comment
Read more

CYCLOHEXANE STEREOCHEMISTRY

CYCLOHEXANE STEREOCHEMISTRY Anand S. Burange, Wilson College, Mumbai asgburange@gmail.com Cyclohexane is an industrially important molecule mainly used for the production of epsilon caprolactum and adipic acid which are precursors to Nylon. Cyclohexane has a molecular formula C 6 H 12 and has a cyclic structure. At room temperature it exists as a liquid with boiling point 80.74 o C. Generally, representation of cyclohexane in a line structure form is shown as a hexagon but practically it is not the case. Cyclohexane has six C-C bonds and due to cyclic structure there is a restricted bond rotation. Rotations of bonds lead to the various conformations. The most important conformations of cyclohexane are enlisted below. Conformations of Cyclohexane 1. Boat 2. Chair 3. Twist boat 4. Twist Chair Boat Conformation In a boat conformation, four carbon atoms lie in a plane while remaining two carbons atoms (C1 and C4) are out of plane (above or below) but ...
Post a Comment
Read more

LOSCHMIDT'S BENZENE

While taking group photographs in conferences, seminars, workshops, etc.   Indian chemists and researchers prefer to say ‘benzene’....instead old one ‘cheese’ to make happy faces. But what do we know about benzene? As a chemist do we really know the struggle faced by the researchers to derive its structure? Who discovered benzene? Who synthesized benzene for the first time?   I hope this blog will be quite useful for those who are willing to know the answers of the above mentioned questions. Benzene What we know today about benzene? Benzene is a colourless organic flammable liquid with characteristic odour which boils and freezes at 80.1 and 5.5 °C respectively. It is immiscible with water and has low density than it (water). The molecular formula of benzene is C 6 H 6 where all the C-C and C-H bonds are of equal length. Benzene is a planar aromatic molecule where all carbon atoms are SP 2 hybridized. The aromatic nature can be evidenced from its NMR spectrum. I...
6 comments
Read more