One of the most important commercial application of free radical reaction is with ethylene and vinyl compounds;   CH₂=CHX. X can be any group, the most important being Cl and Ph. Others include −COOCH₃ and −OCOCH₃. CH₂=CHCl and CH₂=CHPh are commercially important because it requires only one or, the most, two simple reactions to prepare them from − acetylene and ethylene − the primary products of petrochemicals. So they are the cheaper chemicals.

You must not consider all factors - chemical and physical - that exist in a chemical reaction. Here we are dealing with a very long linear radical. (Linear here does not mean alignment in a straight line, it means a normal alkane). So at every carbon of the molecule there is a 109� change in direction. Statistical thermodynamics favoured a coiled conformation.

Of course it is a dynamic coil with every part twisting and turning all the time, but overall it maintains the shape of the conformation of a coil. So on some occasion the free radical will be at the surface of the coil, but on other occasion in may be embedded in the interior of the coil (show by the red dot in the diagram).

When it is on the surface of the coil it is fully exposed to the ethylene in the solution. They knocked and another unit of ethylene is added to the chain. However when it is embedded in the coil the neighbourhood is different. It now depends on how soluble the molecule and the ethylene is soluble in the solvent used. If the molecule is very soluble in the solvent it will accommodate more solvent molecules in the coil resulting in a coil with a larger diameter, and the ethylene will be swimming in and out like a fish in a coral reef. On the other hand if the large molecule is less soluble in the solvent, it will tend to exclude the solvent from the interior of the coil. Consequently there will be less ethylene present in the interior of the coil.

The free radical is very active and will not stay single for long. So if it cannot find an ethylene molecule it will attack a proton nearby. This is just like a group of stranded people running out of food and eating one of their own.

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If you have a model you will see that the proton facing the free radical is attached to the fourth carbon from it.

This is known as a chain transfer reaction. (What else can we call it?). This passing of the free radical to and flo can continue until it collide with another species it can react, most likely the ethylene molecule. The propagation reaction will then continue until the radical again execute a chain transfer or gets terminated.

So the final polyethylene is not exactly a linear (on normal alkane). t has side branches of butyl groups attached to it.

How does the side-chain affect the property?

We can prepare polyethylene without side-chains using other (non-free radical) chemistry, and this polyethylene has a much, much higher tensile strength than those prepared by free radical chemistry.

Now matter how big the molecule is it cannot be more than a few inches long when stretched linearly. So the tensile strength of plastic depends of the power of the molecule to hold on to each other. It is not difficult to agree that if the molecule can have a regular pattern it will be easier to arrange them so that there will be more intermolecular attraction between them. Just imagine stacking party chairs of the same type as compare with those from different manufacturers. So if we have a linear (no side groups) it will be easier to stack them against each other maximising the amount of inter-molecular attraction. With the presence of irregular side groups it is very difficult to arrange the molecules orderly next to each other. So the side-butyl groups hinder inter-molecular attraction. The different parts of the molecule will then interact with each other; intra-molecular interaction.

Since the difference between the two is due to its ability to compact themselves it is not surprising that they have different density. The perfectly linear polyethylene will have a higher density and those prepared from free radical chemistry a lower density. So the next time we come across LDPE (Low Density Polyethylene) and HDPE (High Density Polyethylene) you should know the difference. Now it is possible to prepare another grade of PE which has less regularity than HDPE but more than LDPE. This is known as LLDPE (Linear Low Density PE). It still has side-groups but less.

Because polyethylene is the cheapest plastic to be produced it is used when possible. Usually for general applications like wrappings, plastic bags, containers for ordinary conditions (everyday use). It is the highest volume of plastic produced in the world, way above the others in quantity.

POLYPROPYLENE (PP)

Without much thought we would assume that propylene can also be polymerised by free radical chemistry. But if we examine the chemistry we will realise that the propylene free radical can extract a proton from propylene to give an very stable allylic free radical.

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The allylic free radical being stable is less reactive. So it will take its time to chose which of the three possible reactions it will proceed with. It can add on to the CC π−bond of the propylene, it can extract a proton from propylene to regenerate another allylic free radical, or it can react with another allylic free radical to execute a termination reaction. Results showed that they all are important and consequently the product obtained are only about 10 to 20 repeating units. In physical property they are grease and lubricant. We cannot call them polymers as there as they are too small, so we have a special terms for them - oligomers. So free radical chemistry is never used to prepare polypropylene.

USA, Canada & Mexico: 2001 Polymer Production (billion lbs) Price (cent) PP HDPE LLDPE LDPE PVC PS 15.93 15.28 10.27   7.70 14.26   6.11 40 − 50 43 − 56 47 − 55 51 − 61 43 − 61 62 − 66 American Plastics Council

Since polypropylene is just as cheap as ethylene, much research was conducted and eventually we were able to prepare polypropylene using Ziegler-Natta polymerisation. The product is linear (non-side chain) and the molecules can stack properly to give a very strong plastic. So it is used for heavy-duty plastic bags, oil container, strong ropes, etc. Polypropylene is gaining importance and is competing head-to-head with HDPE. In 2001, the amount of HDPE produced in North America is about 15.3 billion lbs, while PP is about 15.9 billion lbs.

Another route for the preparation of vinyl chloride is the reaction of acetylene with HCl.

The hydrogen chloride is used to react with ethylene in the presence of oxygen (oxychlorination) to give more ethylene dichloride.

	CH2=CH2 + 2 HCl + � O2	→	CH2(Cl)−CH2Cl + H2O
The overall reaction can be written as
	2 CH2=CH2 + Cl2 + � O2	→	2 CH2=CHCl + H2O

Poly(vinyl chloride) is prepared by free radical reaction. The vinyl chloride is stirred in water containing a small amount of surfactant to suspend it as small droplets of monomer in water. The initiator is then added and the polymerisation is carried to a high degree of conversion.

Unlike ethylene which is a perfectly symmetrical molecule, vinyl chloride has two different carbon centers for the CC π−bond. So the question arises: which carbon will the free radical attack?

(A)

The impulsive answer maybe; the chloride atom is electronegative (which means electron-withdrawing) so the carbon it is attached to would find it difficult to cope with one electron less. So we would not expect the free radical to be (A).

(B)

Actually the chlorine being electronegative would do its best to possess the electron via a form of resonance shown by (B). Remember that the electronic configuration of chlorine is [Ne]: 3s², 3p⁵; 3d⁰. The greedy chlorine chose to store the extra electron in the empty d-orbitals. Of course this is against the "octet rule" for covalent bond formation as we have seen so far. But then we are not looking at a stable compound, but a relative stable free radical, a transition species. So in the free radical polymerisation of vinyl chloride, the propagating free radical is (B).

The other problem is the termination reaction of unsymmetrical alkene monomers. When the ethylene propagating free radical collide with each other with just get:

However in the case of vinyl chloride polymerisation we will end up with:

So PVC is not exactly −[CH₂−CH(Cl)]_n−, because somewhere in the molecule there will be a unit of two adjacent CH(Cl) units (known in polymer chemistry as head-to-head unit, or H-H unit). We will encounter this problem every time we polymerise unsymmetrical vinyl monomers using free radical reaction. This H-H unit has no effect on the physical property of the polymer, but its chemistry will definitely be different as compared with the other units.

So when discussing the chemical stability of polymers prepared by free radical polymerisation remember this H-H unit.

Note: If you imagine the propagating polymeric chain to be a snake, it is not difficult to understand why the carbon with the free radical is referred to as the head.

POLYSTYRENE (PS)

Styrene was discovered in 1845. Commercial production began in 1925 in Germany and the U.S. when polystyrene attracted interest. In the Second World War it was used to prepare styrene-butadiene synthetic rubber as a substitute for natural rubber since the Japanese had controlled of South East Asia.

Now styrene is commercially prepared from the dehydrogenation of ethylbenzene, which is obtained from the reaction of ethylene with benzene.

H+ �

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It can also be obtained as a co-product of propylene oxide from the POSM process.

Polystyrene is produced by free radical polymerization of styrene initiated by benzoyl peroxide. The free radical is stabilised by the benzene ring and so the propagating species is as shown in the diagram. Again somewhere in the polystyrene chain there will be a H-H unit.