Polymer

A polymer is a substance made of very large molecules called macromolecules. These massive structures are built from many repeating subunits called monomers.

Because of their enormous molecular mass, polymers possess unique physical properties. They often show great toughness, high elasticity, or viscoelasticity. They can form organized semicrystalline structures or messy, disordered amorphous structures. Polymers are essential to both the natural world and modern technology. They exist as both natural biopolymers and man-made synthetic materials.

Creating these large molecules requires a process called polymerization. This process combines small monomers into long chains or networks through covalent chemical bonds. In chain polymerization, monomers are added to a growing chain one at a time. An example of this is the radical polymerization of styrene. In step-growth polymerization, smaller chains can combine directly with one another. This method includes polycondensation, where a small molecule is lost during each step. For instance, making PET polyester involves losing two water molecules during the reaction.

Polymers are categorized by their composition into homopolymers and copolymers. A homopolymer consists of only one type of repeating unit. Polystyrene is a common homopolymer made of styrene-based units.

A copolymer contains two or more different types of repeat units. If a polymer has three different types, it is called a terpolymer. For example, styrene-butadiene rubber is a random copolymer. These different combinations allow scientists to create materials with specific, custom properties.

The history of polymer science involves a shift in how we understand molecules. The term "polymer" was coined in 1833 by Jöns Jacob Berzelius. His original definition focused on substances with related molecular formulae. However, the modern concept emerged much later in 1920. Hermann Staudinger proposed that polymers were large structures held by covalent bonds. He spent the next decade gathering experimental evidence to prove this hypothesis. His work laid the foundation for the modern field of polymer science.

Natural polymers, or biopolymers, are fundamental to all life on Earth. There are three main classes: polysaccharides, polypeptides, and polynucleotides.

In living cells, enzymes manage the synthesis of these molecules. For example, DNA polymerase catalyzes the formation of DNA. Proteins are also polymeric, built from amino acids through transcription and translation. Other natural examples include cellulose in wood, silk, and natural rubber. Even in space, polymers exist, such as the amino acid polymer hemoglycin found in meteorites.

Synthetic polymers are man-made materials used in massive quantities. In 2015, the world produced more than 330 million tons of these polymers. Common examples include polyethylene, polypropylene, and polyvinyl chloride. Many of these use a backbone of carbon atoms. However, other elements like silicon can form polymers like silicone.

Humans also modify natural polymers to create new products. Heating natural rubber with sulfur creates vulcanized rubber. Reacting cellulose with nitric acid produces nitrocellulose.

The physical behavior of a polymer depends on its microscopic structure. Polymers can be linear, meaning they consist of a single unbranched chain.

They can also be branched, with side chains extending from the main backbone.

Some polymers feature cross-linking, where chains are tied together. Wide-meshed cross-linking often creates elastomers, while close-meshed links create hard thermosets.

These structural details determine if a material is a flexible plastic or a rigid solid.