│ │
CH2 CH2
│ │
This shows how a radical is formed within a chain.
│ │
CH2 CH2
│ │
∙CH2 + ∙CH2−CH2−CH2−CH2− → CH −CH2−CH2−CH2−CH2−
│ │
CH2 CH2
│ │
An attack by another radical can then lead to branching on the original chain.
Stage 3: Termination
R−CH2−CH2−CH2−CH2∙ + R∙ → R−CH2−CH2−CH2−CH2−R
In this stage the radicals combine which stops the chain growing.
Once the three stages are complete the ldpe has been formed. The extensive branching and irregular structure lead to a low density.
Poly(ethene) can exit as high and low density poly(ethene), hdpe and ldpe. The differences in their structure lead to different physical properties. The polymer chains in hdpe are much less branched than ldpe, which allow them to pack much closer together which leads to a more regular, crystalline structure. This causes a higher density and tensile strength. This also increases the instantaneous dipole-induced dipole attractions between chains, which increases its strength and boiling point. The structure of hdpe lead to it having a higher softening temperature, tensile strength, density and it is also hard and strong. In ldpe the opposite is true. The extensive side branching leads to an irregular, amorphous structure as the chains cannot pack as closely together. This increases space between chains, which decreases the density. As the chains are not as close together the intermolecular forces are weaker. This results in ldpe having a lower boiling point, being weaker and more flexible. Due to the irregular arrangement of the chains, ldpe's tensile strength is lower, thus it increases its elongation at fracture, 600% (figure taken from Chemical Storylines). In linear low density poly(ethene), lldpe, a compromise is achieved. The chains do not pack as regularly as in hdpe, so it has a lower density, but the presence of short chains allows for sufficient crystalline regions for the polymer to withstand tearing forces. Lldpe has properties somewhere between ldpe and hdpe.
Poly(propene) can exist in three different forms, isotactic, syndiotactic and atactic, each with different physical and molecular properties. In isotactic poly(propene) the orientation of the methyl groups is always on the same side of the chain. This results in a more crystalline structure. In syndiotactic poly(propene) the methyl groups alternate regularly along the chain. This is less crystalline than isotactic, but more crystalline than atactic. In atactic poly(propene) the methyl groups are randomly distributed on both sides of the chain. This has the most amorphous structure. Due to the regular arrangement of methyl groups in isotactic and syndiotactic the chains can pack closer together than the atactic form. This results in them having stronger intermolecular forces and these materials are stronger and more rigid. As the chains can pack closest in isotactic, then syndiotactic, then atactic, isotactic has the highest density, followed by syndiotactic, then atactic. The same is true for strength and boiling point.
Today chemists have a great degree of control over polymerisation reactions. However, this was not always the case. When these polymerisation reactions first took place the results were very much down to chance. Serendipity played an important role.
Take the discovery of poly(ethene). Gibson and Fawcett were trying to react ethene with benzadehyde to form a ketone. However, due to faulty equipment oxygen leaked into the reaction causing the polymerisation of ethene. When ethene is polymerised the temperature, pressure and amount of oxygen have to be carefully controlled or it will explode, it was just lucky that in this reaction there was the correct temperature, pressure and amount of oxygen for poly(ethene) to be made. This lucky breakthrough lead to more research into the polymerisation of ethene and was the first step to the control we have over today over the reaction.
In 1953 Karl Zieger was investigating the reactions of organometallic reagents. One reaction studied was trialkylaluminium, (C2H5)Al with ethene. This reaction occasionally produced poly(ethene), but was more crystalline and had a higher melting point than ldpe. However, the most exciting thing was that the reaction took place at much lower pressures than require to produce ldpe. One experiment yielded no polymer. This was tracked down to dirty apparatus in which traces of a nickel compound were present. From this Zieger decided to investigate adding other metal ions to the trialkylaluminium and ethene reaction. They found that some metal ions inhibited the reaction, like nickel did, but they discovered that titanium(IV) chloride and zirconium(IV) chloride cause the ethene to readily polymerise. The ethene formed was hdpe. So by using a dirty vessel luck played the vital first step into how hdpe was discovered.
The catalysts discovered by Zieger still had problems and it was down to another accident to solve them. In 1975 a student was working with trimethylaluminium and titanocen to polymerise ethene. Little was expected to happen, but the student did not flush the apparatus with an inert gas as the catalysts are oxygen sensitive and a large quantity of poly(ethene) was formed. It was then discovered that water in the air from the apparatus cause the polymerisation. It was then established that using a 1:1 mixture of trimethylaluminium and water the polymerisation of ethene could be raised by a million fold. This fortunate break through paved the way for more investigation and a new catalyst was discovered methyl alumoxane, MAO. This reacts with the metallocene to produce a catalyst suitable for large-scale polymerisation. This new catalyst was then tried in the polymerisation of propene. It was found that by changing the molecular structure of this catalyst it could affect the structure of the poly(propene) formed. So from a “lazy” research student came isotactic, atactic and syndiotactic poly(propene).
Serendipity played an important role into the level of control we have over polymerisation today.
The first polymerisation reaction of ethene could not be totally controlled by chemist due to the fact work was at a purely experimental stage. The chemists were not sure what temperature and pressure they reaction should take place at so explosions occurred and the poly(ethene) produced would vary in quantity and physical and molecular properties. Explosions would occur due to the exothermic deposites of ethene. At this stage chemists could not control the amount of branching that the poly(ethene) had because the reaction had to be carried out at high temperatures and pressures. This led to the poly(ethene) all being of ldpe.
When polymerising propene the chemists were not in complete control as the catalyst could become poisoned or damaged so that the chain would stop growing. Also, a secondary catalyst particle could result in a side-branch growing from the chain.
References:
Salters Advance Chemistry, Chemical Ideas
Salters Advance Chemistry, Chemical Storylines
Article 1 From Accident to Design: the progress of poly(ethene)
Article 2 Shaping Up: the story of poly(propene)
Summary
Today addition polymerisation reactions such as ethene to poly(ethene) are precisely controlled. The smallest change to the process will greatly affect the polymer produced. Serendipity has played a vital role in chemists gaining more control over polymerising reactions. The polymerisation reactions are catalysed by metallocenes. Different metallocenes used result in different properties for the polymer.
