What Is a Polymer? Long Chains Built From Repeating Units

The properties of a polymer come mostly from the length and arrangement of the chains rather than from the chemistry of the individual units. Long chains tangle with each other, and separating tangled chains requires far more force than separating small molecules, which is why polymers are tough and why the same chemistry in short chains gives a liquid. Chains can slide past each other, which gives flexibility, unless they are chemically linked at intervals, which prevents sliding and gives a rigid or elastic material depending on how many links there are. Chains can pack into ordered crystalline regions in some places and stay disordered in others, and the proportion of each controls stiffness, transparency and melting behaviour. Small changes to any of these produce materials with completely different properties from identical starting units, which is the central fact of the subject.

Living things build almost everything structural from polymers:

• •Cellulose, chains of glucose, which is the main structural material of plants and the most abundant organic polymer on Earth
• •Starch and glycogen, also chains of glucose differently linked, which store energy rather than providing structure
• •Proteins, chains of amino acids, whose folded shapes do nearly all the chemical work in a cell
• •Nucleic acids, chains carrying genetic information in the sequence of their units
• •Chitin, which builds insect exoskeletons and fungal cell walls
• •Natural rubber, collected as latex, which is elastic because its chains coil and recover

Industrial polymers were developed largely in the twentieth century and transformed material culture within a few decades. Several were discovered accidentally, including the first fully synthetic plastic and one of the most widely used non-stick materials. Two broad families exist and the distinction matters practically. Thermoplastics soften on heating and can be melted and reformed repeatedly, which makes them mouldable and recyclable in principle, and includes most familiar plastics. Thermosets form permanent chemical links during manufacture and cannot be melted again without destroying them, which makes them stable and essentially unrecyclable, and includes the materials used where heat resistance matters. Elastomers, including rubbers, are lightly linked chains that stretch and recover. The properties can be tuned by additives, which is why two objects made from the same named plastic can behave very differently.

Two general routes produce these molecules and the difference shows up in the product. Addition polymerisation joins units that contain a double bond, which opens and links to the next unit, adding them one at a time with nothing left over, and it produces the common packaging plastics. Condensation polymerisation joins units by a reaction that expels a small molecule, usually water, at each link, and it produces polyesters, nylons and, in living things, proteins and nucleic acids. Controlling the reaction determines the result, since the average chain length, the spread of lengths around that average and whether chains branch all follow from the conditions and from any catalyst used. The development of catalysts allowing precise control over chain structure in the mid twentieth century was what turned polymer chemistry from a source of curiosities into an industry, since it permitted properties to be specified rather than discovered.

The durability that makes these materials useful is the source of the difficulty with them, and the chemistry explains why the problem is hard. Long tangled chains of carbon are not something most organisms have enzymes for, since nothing in evolutionary history presented them, so degradation is slow and proceeds mainly by physical fragmentation into smaller pieces rather than by chemical breakdown, which produces the small particles now found essentially everywhere. Recycling is limited by several factors at once, since mixed types cannot be melted together usefully, additives and contamination degrade the result, and each melting cycle shortens the chains and therefore the properties, so recycled material is generally worse than new. Chemical recycling that breaks chains back into their units addresses the quality problem and is energy-intensive. Biodegradable alternatives exist and mostly require industrial composting conditions that are not widely available.

Properties come from chain length and arrangement rather than from the units, since long chains tangle and resist separation while short ones give a liquid. Cellulose, starch, proteins and nucleic acids are all polymers. Thermoplastics remelt and thermosets do not, which decides what can be recycled. Nothing evolved enzymes for these chains, so they fragment physically instead of breaking down.