Polymers | Definition, Classification, Examples, and Uses

Polymers, derived from the Greek words "poly," meaning many, and "mer," meaning part or unit, represent a cornerstone of modern materials science and engineering. Polymers are prevalent in our daily lives; from the plastic bottles we use to the synthetic fibers in our clothing.

Definition of Polymers

Polymers are large molecules composed of repeating structural units called monomers. These monomers are covalently bonded together to form long chains or networks.

Classification of Polymers with Examples

Polymers can be classified in various ways based on different criteria such as their source, structure, chemical composition, and physical properties.

Based on Source

Polymers can be classified based on their source into: natural polymers, synthetic polymers.

1. Natural Polymers

These are polymers that occur naturally in living organisms or the environment. Natural polymers often exhibit biocompatibility and biodegradability. Natural polymers examples are proteins, cellulose, starch, rubber (polyisoprene), and DNA.

2. Synthetic Polymers

These are man-made polymers synthesized through chemical processes. Synthetic polymers are widely used in various industries due to their versatility and customizable properties. Synthetic polymers examples are  polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), and polyethylene terephthalate (PET).

Based on Structure

Polymers can be classified based on their structure into: linear polymers, branched polymers, cross-linked polymers, and network polymers.

1. Linear Polymers

Linear polymers consist of long chains of monomer units linked end-to-end without any significant branching or cross-linking (shown in figure (a)). Examples include high-density polyethylene (HDPE) and polyethylene glycol (PEG).

2. Branched Polymers

Branched polymers have side chains or branches attached to the main polymer chain (shown in figure (b)). These branches can affect properties such as flexibility and crystallinity. Examples include low-density polyethylene (LDPE) and polypropylene (PP).

3. Cross-linked Polymers

Cross-linked polymers have covalent bonds between polymer chains, forming a three-dimensional network structure (shown in figure (c)). Cross-linking enhances properties such as mechanical strength and thermal stability. Examples include cross-linked polyethylene and epoxy resins.

4. Network polymers

Multifunctional monomers forming three or more active covalent bonds make three-dimensional networks (shown in figure (d)) and are termed network polymers. Actually, a polymer that is highly crosslinked may also be classified as a network polymer. These materials have distinctive mechanical and thermal properties. Examples include polyurethanes and phenol-formaldehyde.

Based on Chemical Composition

Polymers can be classified based on their composition into: Homopolymers and Copolymers

1. Homopolymers

Homopolymers consist of repeating units of a single type of monomer. Homopolymers examples are polyethylene (made from ethylene monomers), polypropylene (made from propylene monomers), and polyvinyl chloride (made from vinyl chloride monomers).

2. Copolymers

Copolymers consist of repeating units of two or more different types of monomers. Copolymers can be further classified into: random copolymers, alternating copolymers, block copolymers, and graft copolymers.

Random Copolymers: Random copolymers have monomer units arranged in a random sequence along the polymer chain (shown in figure (a)). Random copolymers examples include styrene-butadiene copolymer (SBR) and acrylonitrile butadiene styrene (ABS).

AABABBBBAABABAAB . . .

Alternating Copolymers: Alternating copolymers have monomer units arranged in a regular alternating sequence along the polymer chain (shown in figure (b)). Alternating copolymers Examples include polyethylene-alt-propylene (PEP) and polyamides (nylons).

ABABABABABAB . . .

Block Copolymers: Block copolymers have blocks of different monomer units arranged in a regular pattern along the polymer chain (shown in figure (c)). Block copolymers examples include styrene-butadiene-styrene (SBS) and polyethylene oxide-polypropylene oxide (PEO-PPO).

AAAAA—BBBBB— . . .

Graft copolymers: Appendages of one type of monomer are grafted to the long chain of another (shown in figure (d)). Graft copolymers examples include PE (Polyethylene)-graft-PEO (Polyethylene Oxide) and PP (Polypropylene)-graft-MA (Maleic Anhydride).

               AAAAAAAAAAAAAAAAAAAAA

               B                   B

               B                   B

               B                   B

Based on Physical Properties

Polymers can be classified based on their physical properties into: Plastics and Elastomers.

Thermoplastics are polymers that soften and become pliable when heated and solidify upon cooling. They can be reshaped and remolded multiple times without undergoing chemical changes. This behavior is due to the weak intermolecular forces between polymer chains, allowing them to slide past each other when heated. Thermoplastics examples include Polyethylene (PE), Polypropylene (PP), Polystyrene (PS), and Polyvinyl chloride (PVC).

Thermosetting polymers, also known as thermosets, are polymers that undergo irreversible chemical crosslinking reactions when heated, resulting in a hardened, three-dimensional network structure. Once set, thermosets cannot be remelted or reshaped by heating without undergoing decomposition.

Most thermosetting plastics consist of a network of carbon atoms covalently bonded to form a rigid solid. Sometimes nitrogen, oxygen, sulfur, or other atoms are also covalently bonded into a thermoset network structure. 

Thermosetting examples include Epoxy resins, Phenolic resins, Polyurethane (certain types), Melamine formaldehyde, and Unsaturated polyester resins.

2. Elastomers

They are polymers that possess the ability to undergo large elastic deformations under stress and return to their original shape once the stress is removed.

This elasticity is due to the long, coiled polymer chains within the material, which can be stretched out when a force is applied and then return to their original coiled state when the force is removed.

Elastomers examples include Natural Rubber, Synthetic Rubber (Styrene Butadiene Rubber, Polybutadiene Rubber, Polyisoprene Rubber, Nitrile Rubber), and Polyurethane (PU).

Note that:

• Most polymeric materials are non-crystalline, but some consist of mixtures of crystalline and non-crystalline regions.

• Nylons are highly crystalline polymeric materials because of the regular symmetrical structure of their main polymer chains.

• The high strength of the nylons is partly due to the hydrogen bonding between the molecular chains.

Polymers uses in daily life

Polymers have a wide range of uses including packaging, textiles, construction materials, automotive parts, electronics, medical devices,  household products, and personal care products.

Plastic Packaging:
One of the most prevalent uses of polymers is in packaging materials such as plastic bags, bottles, containers, and films.

Clothing and Textiles:
Synthetic polymers like polyester, nylon, and spandex are widely used in clothing and textiles due to their durability, elasticity, and versatility.