“Additional Polymerisation, monomers join together without the loss of atoms from the molecule. ” (Reference 1) Additional polymerisation is the process in which many monomers bond together to produce large chains called ‘Polymers’. Polyethene is an example of this. It is made of many ethene monomers linked together as one, commonly known as polythene. In additional polymerisation, all the monomers are alkenes (contain a carbon-carbon double bond). It is respectively this double bond that breaks and splits to form bonds with other ethene molecules.
(Reference 3) There are certain conditions that need to be met for the reaction to be carried out efficiently. These are: Temperature: About 200 C Pressure: About 2000 atmospheres Initiator: A small amount of oxygen as an impurity This process starts with a new radical being donated, (a species with an unpaired electron. ) In this case, it is donated from the oxygen, it is created by breaking the oxygen-oxygen bond, in such a way that the two electrons involved end up on the separated pieces of the original molecule, this is known as a ‘homolytic cleavage’.
Here is an example of a common inhibitor: Benzoyl Peroxide (Reference 4 ) The radical now combines with the spilt carbon-carbon double bond. The new radical continues to react with more alkenes and hence the chain grows. Eventually two free radicals collide producing a final molecule, as this has a random timing the chain lengths can vary from 2000 molecules to 20000. However, a chain can also combine with another growing chain if they were to rub together causing a bond to form between two carbons. E. g. (Reference 4 ).
The greater the temperature and pressure, the greater the degree of branching. This means that the chains are unable to get close together and form many intermolecular bonds with each other therefore it has a low density. Polyethene has a long chain structure with no cross-links to other chains. The chains forms bonds and join to one another to produce one large chain. For polythene a critical length of 100 molecules has to be reached before the chain becomes stronger, this great length improves the polymers tensile strength.
It does this because when long chains are tangled they are close enough to form intermolecular bonds; therefore, it is hard for an individual molecule to move due to the bonding in between chains. However, Polyethene is a thermoplastic and has no cross-links which means that the intermolecular forces are weak and can be easily broken by heating. This allows it to be melted and remoulded into shape. Polypropene is also a thermosetting polymer, however it has three structural forms which all have different physical properties.
If you look at the isotactic and syndiotactic structures, the chains can fit closely together due to the orderly -CH3 groups. This means that there will be regular strong intermolecular forces. This makes the structure stronger and more rigid. However the atactic form has an unorganised – CH3 group and therefore the chains cannot fit closely together. This produces a soft and very flexible polymer chain, having a low density and softening temperature. Serendipity has been the only way of improving the production of Polyethene.
It all started March 24 1933 when an attempt to produce a ketone with a smaller pressure led to the discovery of a whit waxy solid which was later on discovered to be a polymer of Ethene. This process was repeated which led to many unfortunate outcomes such as many explosions. It was researched and found to be unlike plastics of the current day. In 1935, the experiment was carried out in a vessel and it was found to have difficulty reaching the desired pressure, even though they continued to add more and more ethene. An assumption was made that there was a leak and it was abandoned, however they produced a product.
After a decision was made to mass-produce it, they discovered that it was the leak in the vessel that enabled a product to be made, as the oxygen was allowed to react. We now know that Oxygen is needed as a radical to carry out the process. They also discovered after many explosions that it was due to the temperature at a high level that produced the polymer to be so reactive. They tackled this problem by continuously adding new cold ethene. When the first polymer was founded in1846 it was discovered to be a nitrated cellulose.
This caused problems, as it was extremely flammable. As the process had to be carried out at very high pressures and temperatures the flammability increased and caused explosions. This proved to be very hard to control. These high pressures made the polymer very expensive to produce and this reduced its commercial viability. As you know cost is of great concern in the chemistry department therefore a new solution had to be found which was cheaper; polymers were in high demand as they had been discovered to be very useful. To bring the cost down catalysts were derived.
Zeiger and Natta did much work on this problem and came up with ‘Zieger-Natta’ catalysts. These used triethylaluminium catalysts. These produced longer, less branched chains which meant the chains could get closer together and form more intermolecular bonds which were stronger. With Ziegler-natta catalysts the polymer is produced at much lower pressures than the previous method, they are sensitive to oxygen; oxygen is used to polymerise monomers (see page 1). Polymers produced with Ziegler-natta catalysts have higher melting points than those produced by the old high-pressure method.
This makes these polymers much more commercially useful than the previous high-pressure polymers. The catalysts allowed the polymer to grow outward from the catalsyst surface. E. g. Another catalyst metallocene causes a problem, as it is difficult to prepare. These catalysts allow the chemists to control the polymer’s molecular mass as well as its structure. However scientists still don’t have total control of the reaction. These catalysts only work on certain polymers up to a certain size of monomer. The catalyst may be poisoned causing the chain formation to stop if it becomes damaged.