Polymerisation Ziegler-Natta

Carbometalation, an important reaction of RTi(TV) compounds ia which RTi adds to a C=C or CM2 multiple bond and results ia a net R—H addition, is iavolved ia Ziegler-Natta polymerisation as follows ... 

In attempts to understand more fully the mechanism of Ziegler-Natta polymerisations chemists came to develop what have become known as metallocene catalysts for polymerisation. In due course it was found possible to... 

Following the considerable commercial success of Ziegler-Natta polymerisation systems which made possible high density polyethylene, polypropylene, ethylene-propylene rubbers and a number of speciality materials, a considerable... 

Polymers for these conductive systems may be synthesised by a variety of means including Ziegler-Natta polymerisation or nucleophilic displacement reactions. The molecules tend to be rigid because of the need for them to possess extended conjugation. This lack of free rotation about carbon-carbon bonds within the molecule imposes a high energy barrier to solvation, thus making these molecules difficult to dissolve. This lack of solubility in turn... 

Synthesis of strictly alternating copolymers can be achieved via various polymerisation techniques including poly-condensation or Ziegler-Natta polymerisations [123, 124]. 

The observed 1,3-shift together with the high selectivity suggest a possible mechanism for the Ziegler-Natta polymerisation of alkenes to add to the mechanisms already put forward (17, 18, 19), as illustrated in equation (5), P = growing polymer. 

Polymerisation reaction, particularly of olefins and dienes catalysed by organometallic compounds fall under the category of coordination polymerisation, or Ziegler-Natta polymerisation. 

The three phases that are present in the Ziegler-Natta polymerisation are (i) the monomer (ii) the solvent and (iii) the catalyst. Reactions take place at certain points on the surface of catalyst particles. The polymer molecule grows as the monomer units join the chain where earlier monomer is attached to the catalyst particle. The precise nature of the action of catalyst is not yet known. However, the first step in the polymerisation process proposed is the formation of a monomer-catalyst complex between the organometallic compound and the monomer. 

Very litde is reported regarding the effect of ultrasound on Ziegler-Natta polymerisation. The first report was by Mertes [97] who obtained a more uniform poly(ethene) in the presence of ultrasound. It was suggested this was as the result of a better dispersion of the catalyst and the prevention of catalyst deactivation (sweeping clean) in the presence of ultrasound. 

Tab. 5.24 Effect of ultrasound on Ziegler-Natta polymerisation of styrene.

Cyclopentadienyl compounds (i.e. metallocenes) (Fig. 5), which have at least one direct metal-carbon bond to the C5H5 ligand, were first synthesised in the 1950s [79,80]. Since then, reactions of cyclopentadienyl reagents have been applied for almost every element [123]. The main application of metallocenes is their use as catalysts in the polymerisation of olefins by Ziegler-Natta polymerisation processes. As many metallocene compounds are volatile and thermally stable, they are also suitable for use as precursors in MOCVD [124-127]. Although cyclopentadienyl compounds have attracted considerable interest as precursors in CVD depositions they are sometimes too reactive [128]. However, high reactivity and thermal stability make cy-... 

It may be interesting that chain termination with hydrogen, which is utilised in Ziegler-Natta polymerisations, does not operate in polymerisation systems with Phillips catalysts no influence of hydrogen to control the molecular weight of polyethylene in the latter case was achieved [37],... 

Keii, T., Doi, Y. and Soga, K., Living Polymer Systems - Ziegler-Natta polymerisation , in Encyclopedia of Polymer Science and Engineering, Wiley-Interscience, John Wiley Sons, New York, 1989, Vol. 18, pp. 437 145. 

An important extension of Ziegler-Natta polymerisation is the copolymerisation of styrene, butadiene and... 

A173. T. Keii, Kinetics of Ziegler-Natta Polymerisation. Chapman Hall, London, and Kodansha, Tokyo, 1972. 262 pp. 

One of the earliest examples of the use of zirconium in catalysis is the area of Ziegler-Natta polymerisation, the catalysts employed are typically based on a family of Cp2ZrCl2 metallocene complexes." With judicious choice of the cyclopentadienyl ligand either atactic (bridged Cp-derived systems),... 

Whilst the polymerisation of propene is not truly sustainable it illustrates the importance zirconium catalysts have in the area of polymer science. The reader is referred to the following references for a comprehensive review of zirconium in Ziegler-Natta polymerisation. This chapter will focus on the use of zirconium in sustainable polymer application (polylactide or polycarbonates), its use in the homogeneous catalysis for the activation of carbon dioxide and other small molecules and finally detailing some examples of the use of zirconia in heterogeneous catalysis. 

Polypropylene obtained by Ziegler-Natta polymerisation is isotactic and highly crystalline (T , 170 °C) and thus is endowed with very interesting mechanical properties. 

Here again we may draw parallels with Ziegler-Natta polymerisations. Scheme 14. In the syndiotactic polymeHsation of propylene [76] the catalyst is... 

---