How Polymers Form Long Molecular Chains

A monomer is a relatively small molecule able to link with others. Polymerisation joins many monomers through covalent bonds, creating a chain or network. The repeating part of the final structure is called a repeat unit.

In addition polymerisation, monomers containing carbon-carbon double bonds open those bonds and connect into a long chain. Ethene monomers can form polyethene, while propene monomers form polypropene. No small molecule is removed during this type of reaction.

Condensation polymerisation usually joins monomers with two reactive functional groups. Each new link can release a small molecule such as water. Polyesters, polyamides and many biological polymers form through reactions in this broad family. The exact monomers and functional groups determine which links form and what by-products appear.

Long chains can tangle like cooked noodles, making them harder to pull apart. Strong attractions between neighbouring chains can increase strength or melting temperature. Weaker attractions allow chains to slide more easily, producing softer or more flexible materials.

Branching changes how closely chains can pack. Highly branched polymers may be less dense and more flexible than straighter versions made from the same monomer. Cross-links connect separate chains and can create rigid networks or elastic materials that return towards their original shape.

Additives also change behaviour. Plasticisers increase flexibility, pigments add colour and stabilisers slow damage from light or heat. A product labelled as one polymer may therefore contain a carefully engineered mixture rather than a lonely pure substance waiting nobly in a bottle.

Living organisms make many essential polymers. Proteins are chains of amino acids, DNA is built from nucleotides and cellulose contains repeating sugar units. Their sequences and shapes allow them to store information, form structures and carry out chemical work.

Manufactured polymers are useful because they can be light, durable and easy to shape. Those same features can create waste problems when products are used briefly but break down slowly. Recycling depends on polymer type, contamination, collection systems and whether the material can be processed without losing too much quality.

To compare polymers, examine:

• •The monomers used.
• •The type of link between units.
• •Chain length and branching.
• •Cross-links between chains.
• •Additives mixed into the material.

Labels on plastic products often blur two separate questions: where the material came from and what happens to it afterwards. A bio-based polymer is made from plant sources rather than fossil fuels. Polylactic acid, often shortened to PLA, is commonly produced from fermented plant sugars or starch. Polyethene can also be made from ethanol derived from sugar cane. That plant-made polyethene is chemically the same as the fossil version, so it behaves identically and is no more biodegradable.

Biodegradable means microorganisms can break a material down into simpler substances, but the conditions matter greatly. PLA typically breaks down well in industrial composting facilities, which hold waste warm, moist and well aerated for weeks. In a cold garden heap or in seawater, the process can be very slow. Some biodegradable plastics can also contaminate recycling streams if they are mixed with conventional plastics. When you meet a green label, ask which claim is being made and under what conditions it is true.

Polymers fall into two broad groups depending on what happens when you heat them. Thermoplastics are made of chains held together mainly by forces between molecules. Heating weakens those forces, so the chains can slide past one another and the material softens and melts. It can then be reshaped and cooled into a new form, which is why most mechanical recycling targets thermoplastics such as polyethene, polypropene and the polyester used in drinks bottles. The same chains can, in principle, be melted several times, although some quality is lost each round.

Thermosets behave differently. While they are being made, or cured, covalent cross-links form between the chains, locking them into one large network. Heating cannot make the chains slide because they are bonded together, so a thermoset stays hard until it eventually chars. Bakelite, developed by Leo Baekeland in 1907, was an early example and was popular for electrical fittings and radio cases because it did not soften near heat. Two-part epoxy glue works the same way: mixing the parts starts the cross-linking reaction that sets it permanently.

Today it seems obvious that polymers are giant molecules, but in the early twentieth century many chemists rejected the idea. Rubber, cellulose and proteins behaved strangely in solution, and a popular view held that they were clusters of small molecules stuck together by weak attractions, more like a loose crowd than a chain. From around 1920 the German chemist Hermann Staudinger argued that they were genuine macromolecules, with thousands of atoms joined by ordinary covalent bonds. Many colleagues found this hard to accept, because molecules of such size seemed implausible to them.

Evidence built up from several directions. Staudinger showed that adding hydrogen to rubber, which should have broken up a cluster held together by its double bonds, left a substance that still behaved like a giant molecule, and measurements of how polymer solutions flowed pointed the same way. In the 1930s Wallace Carothers at the DuPont company deliberately built long chains from known small molecules using condensation reactions, producing nylon and showing that chain molecules could be designed on purpose. Staudinger received the Nobel Prize in Chemistry in 1953, by which time his idea underpinned a fast-growing plastics industry.

Chain length changes a polymer almost as much as its chemistry. The repeat unit of polyethene is two carbon atoms with their four hydrogens, which has a relative mass of 28. Hydrocarbon chains built from the same links but only twenty to forty carbon atoms long make up paraffin wax, which is soft and crumbles easily. Grow the chains to thousands of units and you get the tough plastic of bags and bottles. A polyethene chain with a molar mass of 280,000 grams per mole contains about 10,000 repeat units, because 280,000 divided by 28 is 10,000. Longer chains tangle more and resist being pulled apart.

At the extreme, ultra-high molecular weight polyethene has chains with molar masses in the millions. It is so tough and slippery that it is used for the bearing surfaces of artificial hip and knee joints, and when its chains are drawn out and aligned into fibres it makes ropes and protective materials that are remarkably strong for their weight. One complication is that a real sample never contains chains of a single length. Polymerisation stops at random points, so chemists describe an average molar mass and a measure of spread called dispersity. Two samples with the same average can behave differently if one contains many short chains.

Polymers are large molecules created by joining many monomers into chains or networks. The chemistry of the repeat units and the arrangement of the chains control properties such as strength, flexibility and heat resistance. Follow the path from monomer to structure to property, and polymer chemistry becomes a useful design story rather than a catalogue of plastic names.