22-02-2013, 04:38 PM
Polymeric Materials
[attachment=51514]
Introduction
Polymers are ubiquitous; from the natural world (DNA, proteins, natural rubber,
cellulose, silk) to man made (Nylon, polyethylene, polyvinylcholride, superglue
etc.)
Metal complexes are of interest in polymerisation because:
1. They can catalyse polymerisation
2. Control polymer chain length and structure
3. Polymers containing metals can have useful properties (magnetic, electrical,
optical, catalytic)
The topics this course will cover
1. The synthesis and mechanism of organic polymers using metal complex
catalysts:
Ziegler- Natta catalysis: used to prepare polyethylene and polypropylene.
millions of tons a year.
Ring opening polymerisation catalysis: used to prepare ‘designer’ functional
polymers and polymers with high mechanical strength.
Polymerisation of alkenes
In 1955 Zeigler discovered that heterogeneous mixtures of TiCl4 and AlEt3
catalysed the polymerisation of ethene to linear polyethylene at room temperature
and pressure.
Later Natta subsequently developed this and similar systems and demonstrated
that propene could be polymerised stereospecifically to isotactic polypropylene.
Why early metal polymerisation catalysts work
1. Early metal complexes are electrophilic. Rapid binding of alkene increases kp
2. Group 4 complexes are d0. Back-bonding from metal to alkene is weak
lowering the activation energy for insertion, kp not reduced.
3. For metallocenes kp >>> kt leads to every metallocene being catalytically active
So called single-site catalysis leads to low polydispersity if ki is > kp
Many monomers can be polymerised by metallocenes but they are very sensitive
to water (need lot of MAO) and are intolerant to some monomer polar functional
groups.
[attachment=51514]
Introduction
Polymers are ubiquitous; from the natural world (DNA, proteins, natural rubber,
cellulose, silk) to man made (Nylon, polyethylene, polyvinylcholride, superglue
etc.)
Metal complexes are of interest in polymerisation because:
1. They can catalyse polymerisation
2. Control polymer chain length and structure
3. Polymers containing metals can have useful properties (magnetic, electrical,
optical, catalytic)
The topics this course will cover
1. The synthesis and mechanism of organic polymers using metal complex
catalysts:
Ziegler- Natta catalysis: used to prepare polyethylene and polypropylene.
millions of tons a year.
Ring opening polymerisation catalysis: used to prepare ‘designer’ functional
polymers and polymers with high mechanical strength.
Polymerisation of alkenes
In 1955 Zeigler discovered that heterogeneous mixtures of TiCl4 and AlEt3
catalysed the polymerisation of ethene to linear polyethylene at room temperature
and pressure.
Later Natta subsequently developed this and similar systems and demonstrated
that propene could be polymerised stereospecifically to isotactic polypropylene.
Why early metal polymerisation catalysts work
1. Early metal complexes are electrophilic. Rapid binding of alkene increases kp
2. Group 4 complexes are d0. Back-bonding from metal to alkene is weak
lowering the activation energy for insertion, kp not reduced.
3. For metallocenes kp >>> kt leads to every metallocene being catalytically active
So called single-site catalysis leads to low polydispersity if ki is > kp
Many monomers can be polymerised by metallocenes but they are very sensitive
to water (need lot of MAO) and are intolerant to some monomer polar functional
groups.