Polyvinyl Chloride (PVC)

Key Concepts

• Polyvinyl choride is abbreviated to PVC.
• Poly(chloroethylene) and poly(chloroethene) are other names for polyvinyl chloride.
• Polyvinyl chloride is produced in an addition polymerisation reaction using the chloroethene (vinyl chloride) monomer.
• This polymerisation reaction proceeds by a free-radical mechanism.
• Polyvinyl chloride is a white, rigid, quite brittle, solid.
• Additives are used to modify the properties of polyvinyl chloride to make it more useful.
• Polyvinyl chloride is a thermoplastic, it softens when heat and pressure is applied, so it can be moulded into different shapes.

Addition Polymerisation Reaction

The monomer, chloroethene (or vinyl chloride), has a boiling point of -14^oC (259K) so it is a gas at room temperature and pressure. Under pressure, the gaseous chloroethene molecules are forced closer together to form a liquid. The polymerisation reaction is carried out a pressure of 13 atm (1317 kPa) to keep the monomer in the liquid phase.

The polymerisation reaction is highly exothermic

CH₂=CHCl → -[-CH₂-CHCl-]_n-     ΔH = -96 kJ mol^-1

By Le Chatelier's Principle, increasing the temperature at which the reaction occurs would favour the reactant, monomer, side of the equation. The addition polymerisation reaction is therefore carried out at very mild temperatures, the liquid chloroethene monomer being dispersed in water at 52-77^oC (325-350 K).

Polyvinyl chloride precipitates out as a white solid as it forms because it is insoluble in water and insoluble in chloroethene.

Reaction Mechanism

The addition polymerisation of chloroethene to produce poly(chloroethene) proceeds by a free-radical mechanism.
A free radical is a molecule that has no charge, but, is highly reactive because it has an unpaired valence electron.
In the industrial preparation of polyvinyl chloride, a free radical is used to initiate the chain reaction. This source of free radicals is called the initiator.
Organic peroxides with the general formula R-O-O-R are often used as initiators because they can split into free radicals at elevated temperatures:

The alkoxy free radical combines with a molecule of chloroethene to form a new free radical:

This new free radical can then combine with another molecule of chloroethene:

This new free radical can react with another chloroethene molecule to form a new free radical, etc, etc until a very large polyvinyl chloride molecule has been built.
The polymerisation stops when two free radicals react with each other.

Structure of PVC

1. All the chloroethene molecules combine so that the chlorine atoms are all on the same side of the carbon backbone of the polymer chains:

This structure is known as isotactic polyvinyl chloride.

The regular arrangement of the chlorine atoms in this structure allows the polymer chains to pack together closely and maximises the intermolecular forces between the chains. Close packing reduces the flexibility of the material, so isotactic polyvinyl chloride is quite rigid, and because the intermolecular forces between polymer chains is maximised, it is also strong. Isotactic polyvinyl chloride is said to be highly crystalline.
Although we commonly like to draw the structure of polyvinyl chloride as the isotactic structure because it is easy to see the repeating vinyl chloride units, in fact, when vinyl chloride monomers are polymerised very little of the resulting polyvinyl chloride is in the isotactic form.

2. The chloroethene molecules combine so that the chlorine atoms alternate between being above the plane of the carbon backbone and being below it:

This structure is known as syndiotactic polyvinyl chloride.

The regular arrangement of the chlorine atoms in this structure allows the polymer chains to pack together closely and be held in place by intermolecular forces between the chains. Close packing reduces the flexibility of the material, so syndiotactic polyvinyl chloride is quite rigid, and the action of intermolecular forces between polymer chains makes it quite strong. Syndiotactic polyvinyl chloride, like isotactic polyvinyl chloride, is said to be highly crystalline.
Very little of the polyvinyl chloride produced by the addition polymerisation of chloroethene is syndiotactic polyvinyl chloride.

3. The chloroethene molecules combine so that chlorine atoms are randomly oriented along the chains, with some above and some below the plane of the carbon backbone:

This structure is known as atactic polyvinyl chloride.

The large chlorine atoms sticking out at random along the chains prevent the polymer chains from packing together closely. Atactic polyvinyl chloride is not crystalline, rather, it is said to be amorphous. Non-crystalline or amorphous polymers are expected to be softer and more flexible, but atactic polyvinyl chloride is actually quite rigid. This is because chlorine is more electronegative than carbon so each chlorine atom takes on a partial negative change, Cl^δ-, while the carbon atom takes on a partial positive charge, C^δ+. The attraction of the C^δ+ to the Cl^δ- result in dipole-dipole interactions which impart strength and rigidity to the polymer.

Most of the polyvinyl chloride produced by the addition polymerisation of chloroethene is atactic polyvinyl chloride.

Properties and Uses of PVC

Polyvinyl chloride (PVC) is a linear polymer, and, like most linear polymers, the application of heat and pressure will cause it soften and take on new shapes. These linear polymers are said to be thermoplastic. PVC is a thermoplastic. PVC is used to make electrical cable coverings, hoses, pipes, guttering, floor tiles, shoes, leather-look clothing, storage containers, and credit cards.