Polymerisation
Joining small molecules into chains thousands of units long — the reaction that turns a gas into a solid and underlies every synthetic plastic.
A polymer is one molecule repeated. Polymerisation links a small unit — ethylene, vinyl chloride — end to end into a chain of thousands, and the resulting substance behaves nothing like the gas it was made from. Ethylene is a gas at room temperature; polyethylene is a solid that can be a milk bottle or a bulletproof fibre depending on how the chains are arranged.
That is the striking part. The properties come less from the chemistry of the unit than from the length of the chains, how much they branch, and how neatly they pack — which means one feedstock and one reaction can produce materials with completely different uses by changing the conditions rather than the ingredients.
Processing
Control of chain length and branching is the whole of the industrial art, and catalysts are how it is exercised. Early polyethylene needed extreme pressure and produced heavily branched chains that packed badly and gave a soft, low-density material. Catalysts that let the reaction run at modest pressure produce straight chains that pack closely, and the same polymer becomes rigid and much stronger.
The reaction is strongly exothermic, so the engineering problem at scale is removing heat fast enough — a runaway polymerisation is a serious industrial hazard rather than a spoiled batch.
Environmental impact
The durability that makes a polymer useful is the same property that makes its disposal a problem. A chain built to resist water, acid and sunlight resists them in a landfill and in the ocean too.
Mechanical recycling shortens the chains and degrades the material with each cycle, so a plastic is generally recycled into something less demanding rather than into itself. Chemical recycling that breaks chains back to monomer avoids that and costs considerably more energy.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
produces
- Polyethylene — material · from ethylene; chain length and branching decide whether it is a milk bottle or a fibre
- Polyvinyl chloride — material · from vinyl chloride, itself made from the chlorine of chlor-alkali electrolysis
- Polypropylene — material · polymerised from propylene with a Ziegler-Natta catalyst, which is what controls the arrangement of the side groups and so whether the product is a structural plastic or a gum
- Polyethylene terephthalate — material · a condensation polymerisation rather than an addition one — the units join and release water, which is why the reaction runs in reverse when the polymer is processed wet
- Nylon — material · condensation polymerisation of a diamine with a diacid, which is where the two numbers in nylon 6,6 come from
- Polystyrene — material · straightforward addition polymerisation of styrene
- Polytetrafluoroethylene — material · found by accident when a cylinder of tetrafluoroethylene polymerised itself, and made deliberately the same way since
- Polycarbonate — material · from bisphenol A and a carbonate source — and the bisphenol A is the material's standing regulatory problem
- ABS — material · styrene and acrylonitrile polymerised in the presence of polybutadiene rubber, so the rubber phase is grafted in rather than blended
- PMMA — material · free-radical polymerisation of methyl methacrylate, cast between glass for optical sheet or in bulk for moulding granules
- POM — material · formaldehyde polymerised and then end-capped — the capping is the invention, because an uncapped chain unzips from its ends
- PEEK — material · a step-growth polymerisation between aromatic monomers at high temperature in a polar solvent
- PLA — material · ring-opening polymerisation of lactide, itself made by fermenting plant sugar to lactic acid
- Aramid fibre — material · condensation polymerisation into a rigid-rod polymer that is then spun from a liquid crystalline solution in sulfuric acid
- Styrene-butadiene rubber — material · emulsion or solution copolymerisation of styrene with butadiene
- EPDM — material · ethylene and propylene copolymerised with a Ziegler-Natta catalyst, plus a few per cent of a diene to give vulcanisation something to work with
- Neoprene — material · emulsion polymerisation of chloroprene
- Butyl rubber — material · cationic polymerisation at around −95 °C, one of very few industrial polymerisations run that cold
- Polyurethane — material · a diisocyanate and a polyol reacting as they are mixed, so the polymer and the finished part are made in the same moment
- Epoxy resin — material · the resin is made first and the network second — cure is a polymerisation carried out by the user rather than the manufacturer
- Phenolic resin — material · phenol condensed with formaldehyde, releasing water
- Plastic — material · the reaction the whole class is defined by — long chains built from small repeating units
- Silicone rubber — material · and it is not organic chemistry at all: the backbone is silicon and oxygen, which is why the temperature range is what it is
takes as input
- Chlorine — element · as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil
- Fluorine — element · as tetrafluoroethylene, which is where a substantial share of industrial fluorine chemistry ends up
- Ethylene — compound · into polyethylene, and via ethylene dichloride into PVC — the two highest-tonnage plastics between them
- Propylene — compound · into polypropylene, but only with a catalyst that controls the side-group geometry; without one the product is a useless gum
- Styrene — compound · into polystyrene, into ABS with acrylonitrile and butadiene, and into SBR with butadiene
Sources
- Material WorldOur own writing