Polymerisation and Natural Polymers

In this section, we will explore polymerisation, the process by which small molecules, known as monomers, join to form larger molecules called polymers. We will cover both addition polymerisation and condensation polymerisation, as well as natural polymers found in living organisms.

Addition Polymerisation

What is Addition Polymerisation?

Addition polymerisation is a type of polymerisation in which unsaturated monomers (those with double bonds) join to form a polymer. During this process, the double bond in the monomer is broken, and the individual monomers are added to each other without the loss of any small molecules. This results in the formation of a long chain polymer.

Ethene Monomers (Unsaturated)

The most common example of addition polymerisation is the polymerisation of ethene (C₂H₄), a simple unsaturated molecule. Ethene molecules contain a double bond between the two carbon atoms, making them unsaturated. In polymerisation, these double bonds open up, allowing the ethene monomers to bond together, forming a long chain of poly(ethene).

Poly(ethene), also known as polyethylene, is a saturated polymer because it only contains single bonds between carbon atoms in the polymer chain. Polyethylene is widely used in everyday items, such as plastic bags, bottles, and packaging materials.

Poly(ethene) Polymers (Saturated)

The polymerisation of ethene can be represented by the following equation:

$$n\text{C}_2\text{H}_4 \rightarrow (\text{C}_2\text{H}_4)_n$$

Where n represents the number of monomers (ethene molecules) that join together to form the polymer. The polymer formed is called poly(ethene), a commonly used plastic in various applications.

Poly(propene)

Similarly, propene (C₃H₆) undergoes addition polymerisation to form poly(propene). The structure of poly(propene) is like poly(ethene), but the monomer is slightly larger, giving the polymer different properties. Poly(propene) is stronger and more rigid than poly(ethene), and it is often used in packaging, ropes, and textiles.

The polymerisation of propene can be written as:

$$n\text{C}_3\text{H}_6 \rightarrow (\text{C}_3\text{H}_6)_n$$

Condensation Polymerisation

What is Condensation Polymerisation?

Condensation polymerisation involves the joining of monomers with the elimination of a small molecule, usually water or an alcohol. This type of polymerisation occurs between two different monomers, each containing two or more functional groups. Unlike addition polymerisation, condensation polymerisation produces a polymer and a small by-product.

Amino Acids and Proteins

Amino acids are the building blocks of proteins. Each amino acid contains an amino group (-NH₂) and a carboxyl group (-COOH). When two amino acids undergo condensation polymerisation, they form a peptide bond between the amino group of one amino acid and the carboxyl group of another, releasing a molecule of water. Long chains of amino acids form proteins, which are essential for many biological functions.

The general process can be shown as:

$$\text{Amino Acid}_1 + \text{Amino Acid}_2 \rightarrow \text{Dipeptide} + \text{Water}$$ 

Proteins are used in numerous biological processes, such as catalysis (enzymes), structure (collagen), and transport (haemoglobin).

DNA (Deoxyribonucleic Acid)

DNA is a naturally occurring polymer made up of monomers called nucleotides. Each nucleotide consists of a sugar, a phosphate group, and a nitrogenous base. DNA molecules are formed by condensation polymerisation between nucleotides, creating long chains of nucleotides. These chains twist into the well-known double helix structure.

DNA carries genetic information and plays a vital role in inheritance and protein synthesis in all living organisms.

Other Naturally Occurring Polymers

In addition to proteins and DNA, there are several other important naturally occurring polymers:

Starch

Starch is a polysaccharide made up of long chains of glucose monomers. It is a carbohydrate found in plants and is used as an energy storage molecule. The glucose molecules in starch are joined together by glycosidic bonds formed during condensation polymerisation.

Cellulose

Cellulose is another polysaccharide, made up of glucose units, but with a different type of bond. It is found in the cell walls of plants and provides structural support. Like starch, cellulose is a polymer made through condensation polymerisation, but the bonds between glucose units are different, making cellulose strong and rigid.

Summary

In summary, polymerisation is the process of creating large molecules (polymers) from smaller units (monomers). Addition polymerisation occurs when unsaturated monomers, such as ethene and propene, join together to form saturated polymers like poly(ethene) and poly(propene). In condensation polymerisation, monomers such as amino acids, nucleotides, and sugars combine to form important naturally occurring polymers like proteins, DNA, starch, and cellulose. These polymers are crucial in both synthetic materials and biological systems.