Polymers are built from small monomer molecules joined into chains by one of two reactions: addition, where a double bond opens and the units link end to end with nothing lost, or condensation, where each join releases a small molecule such as water. Cross-links between chains then turn a meltable plastic into a permanent network, which is why the choice of reaction and the amount of cross-linking decide whether you get polyethylene, Bakelite or hardened rubber.
Part of Plastics and Polymers
Polymer chemistry describes how small monomer molecules react to form long chains and networks, and how the type of linkage sets a plastic’s behaviour.
Two Reactions Build Almost Every Plastic
The general account of chain-building is in Polymerization; this page applies it to plastics.
Addition polymerisation. The monomer has a carbon-carbon double bond. One of the two bonds opens and attaches to the next monomer, and the process repeats. Ethylene becomes polyethylene this way: n CH₂=CH₂ gives a chain of –CH₂–CH₂– units. Nothing is lost, so the chain’s mass is the sum of the monomers’ masses. The thermoplastics in Plastics Manufacturing (polyethylene, polypropylene, polystyrene, PVC) are addition polymers.
Condensation polymerisation. The monomers carry reactive groups, and each time two join they release a small molecule, usually water. Phenol and formaldehyde join this way to make Bakelite, and the water that is released is the reason a thick phenolic part can blister if it is cured too fast.
Chain Length And Cross-Links Do Different Jobs
Two features of the structure explain most of what a plastic does.
- Chain length. Longer chains tangle more, so the material is tougher and more viscous when molten. Every reprocessing cycle and every overheating breaks some chains and shortens them, which is why remelted plastic is weaker.
- Cross-links. Chemical bridges between chains stop them sliding. A few bridges make an elastic solid: sulfur bridges turn rubber into vulcanised rubber. Many bridges make a hard, rigid network: the phenol and formaldehyde network of Bakelite, or casein hardened with formaldehyde. See Sulfur Vulcanization for the rubber case.
Cross-linking is a one-way step. That is the entire chemical difference between thermoplastics and thermosets.
Where The Monomers Come From
| Plastic | Monomer source | Reaction |
|---|---|---|
| Polyethylene, polypropylene, polystyrene, PVC | Petroleum or natural gas, by cracking | Addition |
| Bakelite | Phenol (coal tar) and formaldehyde (from methanol) | Condensation |
| Casein plastic | Milk protein, already a natural polymer | Cross-linked with formaldehyde |
| Celluloid | Cellulose, already a natural polymer | Chemically modified, then plasticised |
Two of the four need no monomer at all. Casein and cellulose are natural polymers that are modified rather than built, which is why they are the easy entries for a settlement.
Monomers Are More Hazardous Than The Finished Plastic
A finished polymer is mostly inert. The monomers and the by-products of making it are not: phenol burns skin, formaldehyde irritates the lungs and is a carcinogen, and the water or acid released during condensation can be hot and corrosive. Plan for the reagents, not the product.
Reactions that release heat and gas
Condensation reactions give off water that boils into steam in a hot mass, and phenol and formaldehyde react exothermically. Never run a polymerisation in a sealed vessel, keep vapour outdoors or under strong ventilation, and keep skin clear of every monomer.
Common Mistakes
- Confusing a natural polymer with a synthetic one — casein and cellulose are polymers already; the work is modifying them.
- Expecting a thermoset to be reversible — cross-linking cannot be undone by heat.
- Judging toxicity from the finished plastic — the feedstock is the hazard.
What’s Next
- Learn next: Chain Structure — how linear, branched and cross-linked chains behave
- Combine with: Bakelite — a condensation polymer a settlement can make