A Level Chemistry Revision: Organic Chemistry – Polymers

by Kate Onissiphorou

If you’re revising for your A level organic chemistry exam, you’ll need to have a good grasp of how polymers are formed, as well as the different categories of polymers. To help you prepare, we’ve put together an overview of the key points.

Introduction to polymers

The English word ‘polymer’ originates from ancient Greek. It’s a combination of the prefix ‘pulu-’ (meaning many) and the noun ‘meros’ (meaning a share or portion). Hence, the word ‘pulomeros’ literally means having many parts. This is a good description of macromolecules, which are composed of repeating units known as monomers.

Polymers are essentially chains of repeating units that occur naturally as part of biological systems. They can exist as an amorphous solid (lacking regular structure) or as a crystalline solid (consisting of repeating patterns). Structurally, polymers can be linear, branched, or cross-linked, depending on the way the monomers interact with each other.

Biological polymers such as DNA are essential to life, while synthetic polymers have a wide range of industrial and commercial uses. The most common synthetic polymers are derived from hydrocarbons and are used in items such as plastic bags, bottles, and pipes.

Gloved hand holding white polymer beads
Polymer beads are used to make plastics

As we discuss later, polymers can be synthesised through either addition polymerisation or condensation polymerisation

Biological polymers

Polymers occur naturally and are essential for many biological processes. Some examples of biological polymers are:

  • DNA DNA, or deoxyribonucleic acid, is considered the blueprint of life, containing the codes of how an organism is assembled. It’s a double-helix structure of two polymer chains that are twisted around each other. The monomer unit of these polymer chains is nucleotides.
    The DNA double helix spiral structure

    DNA is made up of two polymer chains

  • Proteins from enzymes to muscle tissues, proteins have structural and functional roles in all biological organisms. They are polymer chains composed of monomers called amino acids.
  • Starch starches, like root crops, are complex carbohydrate polymer chains in which simple sugars are the monomers.

How are polymers synthesised?

Many commercial and industrial products, such as plastics, styrofoams, and rubbers, are all types of polymers. These polymers can be synthesised and tweaked to suit the specific application.

For example, the plastic used in electronic gadgets such as mobile phones is slightly different to the type used in grocery bags. Polymers can be synthesised through addition polymerisation or condensation.

Addition polymerisation

During addition polymerisation, unsaturated monomer units are added together to form saturated chains. The unsaturated or double-bonded monomers are alkenes. Many polymers can be derived from alkenes with the aid of catalysts, as shown in the illustration below.

The addition polymerisation process

The process involves three stages initiation, propagation, and termination.

The type of polymer that will be produced depends on the length of the chain, which is directly proportional to the number of monomers.

Condensation polymerisation

In condensation polymerisation, polymer chains are formed by linking bifunctional monomers together. The process can either be natural or synthetic.

When two monomers react in a condensation reaction, smaller molecules such as water, hydrochloric acid or ammonia are released as byproducts. 

Some examples of natural condensation polymers are DNA, proteins, and cellulose. 

Examples of synthetic condensation polymers include polyamides like nylon, and polyesters such as terylene. The illustration below shows how terylene is synthesised.

An illustration showing how terylene is synthesised.

What are the physical properties of polymers?

Polymers can exist either as an amorphous solid or a crystalline solid.

Amorphous polymers don’t have regular molecular patterns, whereas crystalline polymers do. The former has a lower melting point and can easily be reshaped under pressure. The latter has a comparatively higher melting point, greater tensile strength, and is opaque.

As the number of units in a polymer increases, the chains may branch out and interchain. This increases their tensile strength, meaning the macroscopic polymer structure can resist a large load before breaking.

For example, high-tensile ropes and fabrics can be fashioned from cotton, which is essentially cellulose.

Classification of Polymers

Polymers can be separated into four categories organic, inorganic, synthetic, and a mixture of organic and inorganic.

  • Organic polymers organic polymers occur naturally and are integral components of all living organisms. For example, biological polymers like proteins are synthesised naturally inside cells. 
  • Inorganic polymers inorganic polymers also occur naturally and can be very stable even at high temperatures. Examples of inorganic polymers include talc, borax, and clay. 
  • Synthetic polymers while synthetic polymers are man-made, they’re derived from naturally occurring substances. For example, plastics are made from alkanes and alkenes. Other examples of synthetic polymers include glass and concrete.

Polymers can also be categorised based on their unit, structure and properties.

  • Units polymers either have one type of repeating monomers (homo) or varying types of monomers (hetero).
  • Structure polymer structures may vary depending on how the chains are connected. The simplest structure is a linear arrangement. However, they can be branched and cross-linked as the molecule gets bigger. An illustration showing the four polymer structures.
  • Properties polymers can also be classified as elastomers, fibres, thermoplastic, and thermoset polymers. Thermoplastic polymers, such as PVC, can be softened and reshaped using heat. Conversely, thermoset polymers change permanently when heated, making them stronger. Meanwhile, elastomers like rubber are notoriously flexible.

For further information on organic chemistry, visit our A Level Chemistry Resources Hub.

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