Chemistry in the Service to Mankind

Introduction

Applied chemistry has produced entire classes of materials that touch everyday life: polymers used in packaging, clothing and construction; dyes used to colour textiles and food; and drugs and pesticides used in medicine and agriculture. This unit surveys these four areas: polymers, dyes, drugs, and pesticides.

Polymers

A polymer is a large molecule (macromolecule) built by the repeated joining of many small, simple repeating units called monomers, through a reaction called polymerisation.

Classification of Polymers

By source:

• Natural polymers: occur in nature, e.g. cellulose, starch, natural rubber, proteins.
• Synthetic polymers: made artificially in industry, e.g. polythene, PVC, nylon, Bakelite.

By mode of polymerisation:

• Addition polymers: formed when monomer units containing a double bond simply add on to one another, with no small molecule lost, so the polymer has the same empirical formula (times n) as the monomer. Example, formation of polythene from ethene: $n\,CH_2\!=\!CH_2 \xrightarrow{\text{catalyst, pressure}} \ (-CH_2-CH_2-)_n$
• Condensation polymers: formed when monomer units (usually two different bifunctional monomers) join with the repeated elimination of a small molecule such as water or an alcohol at each step. Example, nylon-6,6 is formed from a diamine and a dicarboxylic acid with loss of water at every linkage.

By physical/mechanical behaviour:

• Elastomers: polymers with weak intermolecular forces and a coiled chain structure that can be stretched considerably and spring back to their original shape when released, e.g. natural and synthetic rubber.
• Fibres: polymers with strong intermolecular forces (often hydrogen bonding) giving high tensile strength, that can be drawn into long, thin threads, e.g. nylon, polyester.
• Thermoplastics and thermosetting plastics: thermoplastics soften on heating and can be remoulded (e.g. polythene, PVC); thermosetting plastics set permanently into a rigid, cross-linked structure on heating and cannot be remoulded (e.g. Bakelite).