CYCLOHEXANE STEREOCHEMISTRY

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