Polymer Chemistry: 5 Types of Classification of Polymers
What is a Polymer?
Polymers are defined as macromolecules having high molecular mass (10^3-10^7 u) substances in which each molecule consists of a very large number of the simple repeating structural unit (monomers) joined together through covalent bonds in a regular fashion.
The simple and reactive molecules from which the repeating structural units are derived are called monomers and the process by which these simple molecules (monomers) are converted into the polymer is called polymerization.
For Example:
Classification of Polymers
Polymers are classified in many ways:
1. Classification based upon the source of availability
Depending upon the source from which they are obtained, polymers are broadly divided into the following three classes:
1. Natural Polymers
Polymers that are directly found in nature that is in animals and plants are called natural polymers.
For example:
(a) Proteins which make up our body.
(b) Nucleic acids which control heredity at the molecular level.
(c) Cellulose which provides food, clothing, and shelter.
(d) Rubber is used for making various articles of daily use.
2. Semi-synthetic Polymers
As the name suggests, the polymers which are mostly derived from naturally occurring polymers by chemical modification are called semi-synthetic polymers.
For Example:
(a) Cellulose on acetylation with acetic anhydride in presence of concentrated sulphuric acid gives cellulose diacetate which is used for making threads of acetate rayon and other materials like films, glasses, etc.
(b) Vulcanised rubber which is superior to natural rubber is used for making tires.
(c) Gun cotton which is cellulose trinitrate is used for making explosives and smokeless powders.
3. Synthetic Polymers
Polymers that are artificially prepared in the laboratory are called synthetic polymers. They are also known as man-made polymers.
For Example:
(a) Fibres (nylon, polyester)
(b) Plastics (polythene, polypropene)
(c) Rubbers (neoprene, polystyrene)
2. Classification based upon the structure
Based on structure, polymers are divided into three types:
1. Linear Polymers
Polymers in which the monomers are joined together to form long straight chains of polymer molecules are called linear polymers. The various polymeric chains are then stacked over one another to give a well-packed structure as shown in the figure.
Properties of Linear Polymers
(a) Linear polymers have a high melting point.
(b) Linear polymers have high densities.
(c) Linear polymers have high tensile strength.
For Example:
High-density polythene, polyvinyl chloride (PVC), nylons, Polyesters, etc.
2. Branched Chain Polymers
Polymers in which the monomer units not only combine to produce the linear chain (called the main chain) but also form branches of different lengths along the main chains are called branched-chain polymers.
In these polymers, the branches do not pack well, as a result, branched-chain polymers have lower melting points, densities, and tensile strength as compared to linear polymers.
For example:
Low-density Polythene, amylopectin, glycogen, etc.
3. Cross-linked or 3-D network polymers
The polymers in which monomer units are cross-linked together to form a three-dimensional (3-D) network are called cross-linked or 3-D network polymers.
In these polymers, the initially formed linear polymer chains are joined together to form a 3-D network structure. Only two cross-links per polymer chain are required to join together all the long-chain polymer molecules to form a giant molecule.
Properties of cross-linked polymers
(a) Cross-linked polymers are hard.
(b) Cross-linked polymers are rigid.
(c) Cross-linked polymers are brittle.
For Example:
Bakelite, Urea-formaldehyde polymer, melamine-formaldehyde polymer, etc.
3. Classification based upon the mode of polymerization
Based on the mode of polymerization, polymers are classified into two sub-groups:
1. Addition Polymers
Polymers that are formed by the repeated addition of a large number of same or different monomers having double or triple bonds are called addition polymers. The process by which addition polymers are formed is called addition polymerization.
Addition polymers are further classified into two types that are addition homopolymers and addition copolymers.
(a) Addition Homopolymers
Addition polymers that are formed by the addition polymerization of only one type of monomer unit are called addition homopolymers.
For Example:
(b) Addition copolymers
Addition polymers which are formed by repeated addition of two or more types of monomer units are called addition copolymers.
For Example:
2. Condensation Polymers
Polymers which are formed by repeated condensation reaction between two bifunctional or trifunctional monomer units usually with the elimination of small molecules like water, alcohol, ammonia, carbon dioxide, hydrogen chloride are called condensation polymers. The process by which condensation polymers are formed is called condensation polymerization.
Condensation polymers are further classified into two types that is condensation homopolymers and condensation copolymers.
(a) Condensation homopolymers
Condensation polymers that are formed by the condensation polymerization of only one type of monomer unit are called condensation homopolymers.
For example:
(b) Condensation copolymers
Condensation polymers which are formed by repeated condensation of two or more types of monomer units are called condensation copolymers.
For example:
4. Classification based upon molecular forces
The polymer's mechanical properties like tensile strength, elasticity, toughness, etc depend upon the intermolecular forces of attraction such as van der Waals forces, hydrogen bonds, and dipole-dipole interactions between the adjacent polymer chains.
Depending upon the magnitude of these intermolecular forces, polymers have been divided into the following four categories:
1. Elastomers
Polymers in which the intermolecular forces of attraction between the polymer chains are the weakest are called elastomers.
Elastomers are amorphous polymers that have a high degree of elasticity that is they can stretch out over ten times of their normal length and return to their original position when the force is withdrawn.
The reason behind the elastic nature of Elastomers
Elastomers consist of randomly coiled molecular chains of irregular shapes having a few cross-links. When a force is applied, these randomly coiled chains straighten out and the polymer is stretched.
Since the van der Waals forces of attraction between the polymer chains are very less and very weak, these cannot maintain this stretched form. Therefore, as soon as the force is withdrawn, the polymer returns to its original randomly coiled state.
Thus, weak van der Waals forces of attraction permit the polymer chains to be stretched, the cross-links help the polymer to come back to the original position when the force is withdrawn.
For Example:
Buna-S, Buna-N, neoprene, etc.
2. Fibres
Polymers in which the intermolecular forces of attraction are the strongest are called fibers. Due to strong intermolecular forces of attraction like H-bonding or dipole-dipole interactions, fibers have high tensile strength and least elasticity.
Also, the molecules of these polymers are long, thin, and thread-like and hence can be easily packed. As a result, they have high melting points and low solubility.
For example:
(a) Polyamides - Nylon 6,6
(b) Polyesters - terylene, dacron, etc
(c) Polyacrylonitrile - orlon, acrilon, etc.
3. Thermoplastics Polymers
Polymers in which the intermolecular forces of attraction are in between those of elastomers and fibers are called thermoplastics.
These are linear or slightly branched long-chain molecules capable of repeatedly softening on heating and hardening on cooling without any change in chemical composition and mechanical properties of the plastic.
Due to this property, thermoplastics are used to make daily use of items like toys, beads, buckets, telephone, television cases, etc.
For Example:
Polythene, polypropene, polystyrene, polyvinyl chloride, Teflon, polyvinyl acetate, nylon 6, nylon 6,6, etc.
4. Thermosetting Polymers
Thermosetting polymers can be defined as semi-fluid substances with low molecular masses which when heated in a mold, undergo a permanent change in chemical composition to give a hard, infusible, and insoluble mass.
This happens due to extensive cross-linking between different polymer chains to give a 3-D network solid. In other words, we can say that thermosetting polymer can be heated only once when it undergoes a permanent change and sets into a solid which cannot be re-melted and re-worked.
For Example:
Phenol-formaldehyde (bakelite), Urea-formaldehyde, melamine-formaldehyde, etc.
5. Classification based upon the type of mechanism involved during the growth of the polymerization chain.
Polymers can also be classified based on the type of mechanism involved during the growth of the polymerization chain. This includes:
1. Chain growth Polymers
Chain growth polymers are formed by repeated addition of monomer molecules to the growing chain carrying a reactive intermediate such as a free radical, carbocation, or a carbanion.
For Example:
Polyethylene, Polybutadiene, Teflon, polyvinyl chloride (PVC), etc.
2. Step growth Polymers
Step growth polymers are formed by stepwise condensation reactions which take place in several steps. The condensation process may or may not be accompanied by the elimination of smaller molecules such as water.
For example:
Nylon 6,6, dacron, Bakelite, etc.
