Coordination Polymerisation

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Coordination Polymerisation — Note

Coordination Polymerisation

1. Introduction

Coordination polymerisation, also known as Ziegler–Natta polymerisation, is a type of chain-growth polymerisation using transition metal coordination catalysts to polymerise unsaturated monomers such as alkenes. It allows precise control of stereochemistry (isotactic, syndiotactic, atactic).

  • Named after Karl Ziegler and Giulio Natta (1950s).
  • Used industrially for polyethylene, polypropylene, and stereoregular polymers.

2. Suitable Monomers

  • ?-olefins: CH2=CHR (R = H, CH3, C2H5, etc.)
  • Examples: Ethene, Propene, 1-Butene, Styrene

3. Catalysts

  • Ziegler catalysts: TiCl4 or TiCl3 + Al(C2H5)3
  • Metallocenes: Cp2TiCl2 + MAO (methylaluminoxane), for precise stereocontrol

4. Mechanism of Coordination Polymerisation

A. Initiation

Formation of the active catalyst site; transition metal forms a coordination complex with monomer.

TiCl4 + Al(C2H5)3 → Active Ti–Al site

B. Propagation

Monomer coordinates to the metal center; insertion into metal–carbon bond occurs. Chain grows regiospecifically and stereospecifically.

-Ti-CH2-CH2-R + CH2=CH2 → -Ti-CH2-CH2-CH2-CH2-R

C. Termination

Rare in industrial systems; chain remains attached to metal until quenched.

-Ti-(CH2CH2)n-R + H2O → HO-(CH2CH2)n-R + Ti-OH

5. Features

  • Produces stereoregular polymers (crystalline, high-strength).
  • High activity catalysts ? rapid polymerisation.
  • Can polymerise ?-olefins not suitable for radical polymerisation.
  • Low polydispersity compared to radical methods.

6. Industrial Applications

Monomer Polymer Application
Ethene (CH2=CH2) Polyethylene (HDPE, LLDPE) Bottles, pipes, films
Propene (CH2=CH–CH3) Polypropylene (isotactic, syndiotactic) Packaging, textiles, automotive parts
Styrene Syndiotactic polystyrene High-strength engineering plastics
1-Butene, 1-Hexene Co-monomers in LLDPE Flexible films, packaging

7. Advantages

  • High stereospecificity ? isotactic or syndiotactic polymers.
  • Polymerises ?-olefins with high molecular weight efficiently.
  • Produces crystalline, high-strength polymers.
  • Industrial catalysts are highly active.

8. Limitations

  • Strictly anhydrous conditions; moisture poisons catalysts.
  • High cost of some metallocene catalysts.
  • Complex catalyst preparation for stereocontrol.

9. Summary

Coordination polymerisation provides controlled polymer architecture and stereochemistry, enabling production of high-performance polyolefins like HDPE, LLDPE, and isotactic polypropylene. It is the backbone of modern industrial polyolefin production due to efficiency, stereospecificity, and versatility.

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