Polymers are macromolecules which are formed by certain repeating units. These repeating units are called monomers. Monomers have some functional groups which react with each other to form a long polymeric chain. The chemical bonds between monomer units are usually covalent bonds which are formed due to addition between some unsaturated monomer units or condensation between two different functional groups such as -COOH, -NH2, -OH etc.
The process of formation of polymers from monomer units is called a polymerization process. Polymerization can be because of the addition reaction between unsaturated monomer units or condensation between two different monomers with the elimination of certain small molecules such as water, carbon dioxide, etc.
“Polyethylene terephthalate is a condensation polymer of ethylene glycol and terephthalic acid.” The by-product of the reaction to create this compound is water so it is an example of condensation or step-growth polymerization.
On the basis of mechanism, there are two types of polymerization; step-growth and chain-growth polymerization. Chain growth polymerization is also called as addition polymerization in which monomer units are bonded with each other through their multiple bonds. For example polymerization of ethylene leads to the formation of polyethylene polymer which is also known as polythene.
This polymerization is a three-step process. The first step is the formation of free radicals in the presence of some initiator like peroxide. We know that a free-radical is a chemical species with a free electron.
R-O-O-R → 2 R-O.
These free radicals involve initiation steps and form new free radicals with monomer units which begin the polymer chain. The double bond of monomer units breaks and attach with free radical to form new free radicals.
In acidic medium, protonation of terephthalic acid occurs that reacts with ethylene glycol to form an intermediate which rearranges and with the transfer of OH- ions, forms polyethene terephthalate polymer.
Overall both monomer units are joined together with ester linkage therefore commonly called as polyester. It is a chemical compound that is used to make plastic and fibre. It is formed by polycondensation of terephthalic acid and ethylene glycol.
Polyethylene terephthalate which is also abbreviated as PET / PETE is mainly used to manufacture the packaging material for food products such as fruit and drinks containers. It is lightweight, transparent and also available in some colour. It is a member of the ester family so also called as polyester. It is a recyclable thermoplastic polymer with good strength, ductility, stiffness and hardness, therefore, can be processed through vacuum forming, injection moulding, compression moulding and blow moulding.
PET / PETE can also be recycled back to its original elements as well as into polyester fibres. These polyester fibres are used in the manufacturing of synthetic carpets, synthetic clothing and other textile products. PET fibres are wrinkle-free and less expensive therefore often mixed with natural fibres. It is also used for the manufacture of microwavable trays and for the packaging of microwavable meals, in containers for cosmetic products and pharmaceutical products.
A list of uses of polyethylene terephthalate is given below.