Copolymer

• Copolymers are very long chained molecules of polymers that are used in making plastics, car tyres etc.

• They are a type of polymers that have high molecular weight and are prepared by combinations of two or more simple compounds. These simple compounds are the structural units of a copolymer and are called monomers. 

• The structural units of monomers may be present in random order or in alternating manner.

• Copolymers are produced from many species of monomers by a process called copolymerization. This method is used to add certain desirable properties to plastics.

• A copolymer is different from a homopolymer in which only one type of monomer is used. The difference between the two can be illustrated as below:

• Copolymers are commercially very important. Few examples of copolymers are nitrile rubber, polyethylene-vinyl acetate (PEVA), and acrylonitrile butadiene styrene (ABS).

Classification of Copolymers

Based on the structure, copolymers can be categorized into two types; 

1. Linear- These contain a single chain of monomers.

2. Branched- These have polymeric side chains or branches.

What are the Different Types of Linear Copolymers?

Linear copolymers are further classified into different types such as periodic, block, alternating, gradient and statistical copolymers. The classification is based on the kind of arrangement of structural units or monomers on the main chain.

Suppose there are two monomers ‘A’ and ‘B’. These monomers A and B can be arranged in many ways so as to form a copolymer. These are as follows: 

Alternating Copolymers 

• It consists of a single main chain of monomers.

• The two monomers A and B are arranged in an alternating manner and the copolymer formed is called alternating copolymer. The following illustration will explain this arrangement.

• An important fact about this arrangement is that the two monomers are in the ratio of 1:1. 

• The general formula for this arrangement can be written as (-A-B) n.

• Copolymer is made through copolymerization processes.

• Example: Nylon 6,6 is an alternating copolymer, consisting of units of hexamethylene diamine and adipic acid that are alternating in nature.

Periodic Copolymers

• In this type of polymers there is a repeating sequence of monomers which are arranged in a single chain.

• The following illustration explains this arrangement.

Block Copolymers

• All of one type of monomer units is placed in one part of the chain followed by the second type of monomers.

• It appears as if one homopolymer chain is attached to another type of homopolymer chain at one end through covalent bonds.

• There is an intermediate unit called junction block where two homopolymer chains are linked.

• Block copolymers may be di block or tri block copolymers depending on whether there are two homopolymer blocks or three homopolymer blocks used.  

• The following illustration explains the arrangement.

                                               BLOCK COPOLYMER

• There are not many commercially available block- copolymers as they are not easy and cheap to make. 

• One example of such a polymer is SBS rubber, also referred to as acrylonitrile butadiene styrene, which is used in soles of shoes and in treads of tyres.

Statistical Copolymer

• In this type of copolymer there are two or more monomers arranged in a sequence as per some statistical rule. Sometimes the likelihood of discovering a said type monomer residue at a specific location in the chain is akin to the mole fraction of the residue of the monomer in the same chain, then in this instance, the polymer is then said to be a random copolymer.

• These copolymers are made through a free radical polymerization method.

• The ratio of the monomers that are used is not 1:1 but depends on some variables like the properties of monomers, polymerization conditions and conversion of polymerization.

• There is blending of properties of individual homopolymers to produce commercially desirable properties in random copolymer.

•  Rubber made from the copolymers of styrene and butadiene is an example of statistical polymer.

• The following illustration explains the arrangement.

                                                          RANDOM COPOLYMER

Gradient and Stereoblock Copolymers

Gradient Copolymers - These are also single chain polymers in which the compositions of monomers change gradually along the main chain. 

Stereoblock Copolymers - Sometimes in a polymer, the tenacity of the monomers varies with different units or blocks. If this happens, the polymer is called stereo block copolymer.

                                       A Stereo Block Vinyl Copolymer

What is a Branched Copolymer?

• These are polymers that have branches or side chains attached to the main chain. The side chains may or may not have the same monomers as the main chain.

• During polymerization, the side reactions cause the formation of branches.

• In a branched polymer the side chains must have at least one complete monomer unit.

• LDPE or Low density polyethylene is the most common example of branched copolymer. It is used in textiles, utensils, packaging materials, containers etc.

• Depending on the length of side chains, these polymers may be long or short branched polymers. Long branched polymers may have star, comb or brush like structures.

• The side chains may further branch but they do not connect to other polymer chains.

• In a star copolymer, there is a central core to which many polymeric chains are attached. Star copolymers may have different structures depending on the way side chains are attached to the multifunctional central core. The following illustration is an example of star copolymer.

Graft Copolymer

• The following illustration explains this arrangement.

• This is a branched copolymer where the main chain and side chains have different structures.

• The main chain and the side chains may be made up of the same monomers (homopolymers) or different monomers (copolymers).

• If an A-B di block copolymer forms the main chain with A-B alternating copolymer side chains, the resulting branched copolymer is called a graft copolymer.

• Example of a graft copolymer is high impact polystyrene or HIPS . If a free-radical polymerization of styrene is done in the presence of polybutadiene we get polystyrene chains growing out in either direction from the places where there were double bonds, with a one-carbon rearrangement. So there is a polystyrene backbone with polybutadiene side chains grafted on the backbone. Polystyrene gives material strength and polybutadiene branches impart resilience and make the product less brittle.