The Development of Poly(ethene) andPoly(propene).
The Development of Poly(ethene) and Poly(propene)
Addition polymerisation involves the joining together of monomers; usually compounds containing C=C double bonds such as alkenes, to form saturated long-chain polymers1[LE1][LE2]:
The reaction occurs when catalyst such as the Zeigler-Natta catalyst, used to make high-density poly(ethene), is present.
Another type of poly(ethene), low-density poly(ethene) was Discovered in 1933 by Eric Fawcett and Reginald Gibson. In their experiment, the polymerisation was free radical polymerisation. This type of polymerisation has three stages. First is initiation, started off by an initiator molecule. Here, this is benzoyl peroxide[LE3]:
This molecule splits into two initiator fragments, each having an unpaired electron, meaning that these molecules are free radicals2. These, combined with the oxygen that leaked into Fawcett and Gibson's experiment, provided the free radicals required to catalyse the mechanism.
Electrons in The C=C double bond are easily attacked by the free radicals, forming a bond between the initiator fragment and one of the C=C atoms. The remaining electron attaches to the other carbon atom. The process now starts again because a new free radical has been formed3[LE4].
The next stage is propagation, where the new radical reacts with another ethene monomer, as with the initiator fragment. This causes the addition of more monomers and the growth of a chain[LE5]:
Termination, the last stage of the mechanism occurs when all the radicals are used up. One of the ways that this happens is through a radical exchange leaving a free radical within a polymer chain, meaning that another radical can attack at this site, causing a branch in the chain[LE6].
The higher the temperature and pressure of the mechanism, the greater the degree of branching. Because the branched chains cannot fit closely together, the plastic has a low density, and it therefore known as Low Density Poly(ethene), ldpe.
The structures of polymers like poly(ethene) affect their physical properties. Ldpe such as that above is made up of branched chains. These branches prevent the separate chains getting close together, reducing the intermolecular forces and causing a low softening temperature tensile strength.
In the hdpe produced by Zeigler-Natta catalysts, the chains are almost unbranched due to their formation process. This means that they can lie closer together, creating greater crystallinity and stronger intermolecular forces. These cause the polymer to have a greater tensile strength, density, relative molecular mass and softening temperature.
In the other ...
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The structures of polymers like poly(ethene) affect their physical properties. Ldpe such as that above is made up of branched chains. These branches prevent the separate chains getting close together, reducing the intermolecular forces and causing a low softening temperature tensile strength.
In the hdpe produced by Zeigler-Natta catalysts, the chains are almost unbranched due to their formation process. This means that they can lie closer together, creating greater crystallinity and stronger intermolecular forces. These cause the polymer to have a greater tensile strength, density, relative molecular mass and softening temperature.
In the other form of polymer, linear low-density poly(ethene), the polymer molecules don't pack together as closely as those in hdpe, but the presence of short branches allow the formation of sufficient crystalline regions for the material's tensile strength to be greater than that of ldpe.
Three forms of poly(propene) can be produced, all with different structures due to the orientation of the methyl groups[LE7]:
The carbon chains in the isotactic and syndiotactic forms can fit closer together than in the atactic form due to their regular structure. The molecules coil together into a helical shape and pack together to form a strong crystalline polymer4. This closer proximity increases their intermolecular forces and consequently the product's strength. In the atactic form, the random arrangement of the -CH3 groups means that the chains cannot fit closely together, giving the Polymer an amorphous structure5, which is soft and flexible.
Serendipity has played a large part in the development of polymers, but also influenced the amount of control chemists have over the polymerisation process. At first, scientists didn't have control of the high pressure, high temperature polymerisation due to this element of lucky chance. As time has gone by, control has become greater and the reliance on serendipity has largely disappeared.
Explosions plagued the production of the first polymers because scientists didn't have this control. The risk of explosions increased if the temperature became too high, so using the constant addition of cold ethene they kept the reactant cold.
In one of Zeigler's experiments, he found a trace of nickel left on the apparatus from a previous experiment had prevented a polymer from forming. The subsequent investigation led to the development of the Zeigler-Natta catalysts, which allowed increased, but not total control of the polymerisation process.
Zeigler-Natta catalysts allow monomers to add in a regular way, but the chain stops growing if the catalyst becomes poisoned by the polymer. Alternatively, a second particle of catalyst might cause the chain to branch. This branching and variable chain length was a problem, depending on the required properties of the plastic. Zeigler-Natta catalysts only work with small hydrocarbon monomers, limiting the range of polymers that can be made with them. Both of these problems prevent total control.
The first metallocene catalysts didn't allow total control of the polymerisation process either. A metallocene is a positively charged metal ion 'sandwiched' between two negative cyclopentadienide anions, formed from cyclopentadiene as can be seen below[LE8]:6
If the metal is Zirconium, due to its greater charge, it bonds with two Cl ions. Its structure is different from ferrocene because the Cl atoms occupy space; the 'rings' become tilted, like jaws. Different transition metals cause changes in the shape of these 'jaws'. Polymerisation in the presence of this zirconium catalyst is similar to that with Zeigler-Natta catalysts; propene monomers are added to the chain next to the zirconium, causing the chain to grow out of these 'jaws'. A particular metallocene with 'jaws' of a particular shape, produces one stereo form of poly(propene).
John Ewen searched for metallocenes, one to produce each of the three forms of poly(propene) from page 3. He prepared zirconocenes with 'bridges' of atoms and bonds linking the rings. The sort of bridge, determined the orientation of the -CH3 groups along the chain and therefore the form of poly(propene[LE9]).
The zirconocene that produced the syndiotactic form was harder to solve[LE10].
It differs from the others because the bridge allows the polymer chain to 'flip' from side to side as each monomer is added, placing the CH3 groups on alternate sides of the chain. Ewen's work gave much control over the polymerisation process.
List of Sources:
. The complete A-Z of Chemistry handbook, second edition (2000), Andrew Hunt, Hodder & Stoughton
2. Free Radical Vinyl Polymerisation, page 2 of 7, http://www.psrc.usm.edu/macrog/radical.htm
3. Free Radical Vinyl Polymerisation, page 2 of 7, http://www.psrc.usm.edu/macrog/radical.htm
4. PR2, The Polyethene Story, SAC, Chemical Storylines, George Burton et al, Heinemann, 2000
5. Metallocene Catalysis Polymerisation, page 2 of 11, http://www.psrc.usm.edu/macrog/radical.htm
6. 5.5, The structure and properties of polymers: Part 1, SAC, Chemical Ideas, George Burton et al, Heinemann, 2000
The complete A-Z of Chemistry handbook, second edition (2000), Andrew Hunt, Hodder & Stoughton
2 Free Radical Vinyl Polymerisation, page 2 of 7, http://www.psrc.usm.edu/macrog/radical.htm
3 Free Radical Vinyl Polymerisation, page 2 of 7, http://www.psrc.usm.edu/macrog/radical.htm
4 The complete A-Z of Chemistry handbook, second edition (2000), Andrew Hunt, Hodder & Stoughton
5 PR2, The Polyethene Story, SAC, Chemical Storylines, George Burton et al, Heinemann, 2000.
6 Metallocene Catalysis Polymerisation, page 2 of 11, http://www.psrc.usm.edu/macrog/radical.htm
[LE1] Insert picture of polymerisation of propene from CI page 109
[LE2] insert picture of repeating until of propene from CI page 109
[LE3] insert picture of benzoyl peroxide from article page 4
[LE4] insert picture of initiation from page 2 of free radical vinyl polymerisation and the 1st stage of box 1 on page 5 of the article
[LE5] insert stage 2 of box 1 on page 5 of the article
[LE6] insert 4 pictures from page 5 of the article into a mini flow diagram
[LE7]inset pictures from article page 7. Figure 2, the three structural forms of poly(propene)
[LE8] insert picture of cyclopentadiene/cyclopentadienide ions from top of page 2 of metallocene catalysis polymerisation. Annotate: 'the cyclopentadiene acts as an acid when forming the aromatic cyclopentadienide ion'. Underneath that, inert picture from the bottom of the same page showing the sandwich formation. Loose the top annotations of both pictures.
[LE9] Inset pictures of the isotactic and atactic poly(propene)
[LE10] insert picture of the syndiotactic form of poly(propene)
