The word ‘polymer‘ is composed of two greek words; poly = many, and mer = simple unit. So, the term polymer, coined by Jöns Jacob Berzelius means a large molecule say macromolecule formed by the repetition of simple units. In general words, polymers are long-chain high molecular weight compounds obtained by the combination of a large number of simple molecules. The simple molecules taking part in the formation of polymers are called monomers and this process of polymer formation is called polymerization. Thus, polymerization is the process of the formation of a large molecule (polymer) by the combination of a large number of smaller molecules (monomer).
The monomers are the repeating units of polymers. All the covalent macromolecules which have carbon in their backbones are called organic polymers. Polyethylene, PVC, Nylon-6,6, etc. are some examples of it. An inorganic polymer is a polymer whose backbone structure is made up of elements other than carbon. Some examples of Inorganic polymers are silica, glass, silicone, phosphazenes, and so on. The similarity between the inorganic and organic polymers is that both can be made by addition and condensation methods.
The polymers possess distinctive physio-chemical characteristics and unique physical, mechanical and electrical properties. These are utilized extensively in our daily life, particularly in the area of engineering and technology.
Classification of polymers
Polymers are classified into the following categories:
Classification of polymer based on composition
Depending on whether the polymer contains the atom of only one element or of different elements in its backbones, the polymers are classified into the following groups:
The polymers which contain the atom of only one element in their backbone are called homopolymers. It is formed by the addition of identical repeating units, and can be represented as:
Silicon, sulphur, germanium, etc. are some examples of inorganic homopolymers.
Heteropolymers contain the atoms of different elements in their backbone. In other words, polymers formed by the alternation of two or more different atoms in backbone structures are called heteropolymers. Quartz, phosphazine, borazine, boron nitride, etc. are examples of heteropolymers.
Copolymers: The polymers which contain more than one type of monomers in their molecules are called copolymers. It is categorized into the following four types:
- Alternating copolymers: Repeating units are distributed alternately throughout the polymer. It can be represented as (-A-B-A-B-A-B-A-B-)
- Random copolymer: When two different monomers or repeat units in a copolymer are distributed at random throughout the polymer chain, the polymer is called a random copolymer. The distribution of monomer units along the chain is irregular. It can be illustrated as (-A-A-B-A-B-B-A-B-A-A-A)
- Block copolymer: When a sequence or block of one repeat unit follows one another along with the main polymer chain, it is called block copolymer. The monomer units forming block copolymer can be represented as (-A-A-A-B-B-B-B-A-A-A-A-B-B-).
- Graft copolymer: The main chain consists entirely of one repeat unit or one type of monomer and the branch chain consists of different types of repeat units. The side chains of different repeat units have been grafted on a linear polymer chain.
Classification of polymer based on the type of reaction
- Addition polymers: When a large number of monomers are combined together to form a large-weight polymer compound without the elimination of any small molecules like H2O, NH3, HCl, etc., the polymers formed are called addition or chain polymers. This process of formation of addition polymer is called addition polymerization. Some examples of addition polymers are sulphur nitride, polyphosphonitric chloride, PVC, polyethylene, and so on.
- Condensation polymers: The polymers formed by the condensation between monomers along with the elimination of smaller molecules are called condensation or step-growth polymers. Polyamides, borazines, silicones, etc are a few examples of condensation polymers.
Classification of polymer based on the action of heat
- Thermoplastics: The plastics or polymers which become soft on heating and can be molded and remolded into different desired shapes are called thermoplastics. It is formed by addition polymerization. The intermolecular force in thermoplastics is weak and does not possess a cross-link between different chains.
- Thermosetting plastics: The polymers which do not soft on heating and thus cannot be molded and remolded into different shapes are called thermosetting plastics. It is formed by condensation polymerization and possesses strong intermolecular forces with extensive cross-links between molecular chains.
Difference between Homopolymers and Heteropolymers
|Formed by the catenation||Formed by the alternation|
|Catenation bond is pure covalent in nature due to a lack of difference in electronegativity.||The alternation bond is polar in nature due to some differences in electronegativity.|
|Linear and long chain in structure.||Cyclic, linear, or bridge structure.|
|Thermally less stable except diamond and graphite||Thermally more stable|