An addition polymer comes from an alkene, and the mechanism is the electrophilic or radical addition you already know, repeated. Nothing is lost, so the repeat unit has exactly the same formula as the monomer — which is the quickest way to recognize one.
The chain reaction, in three stages
Radical polymerization is the commonest, and it is a chain reaction with the three stages any chain reaction has:
- Initiation. A peroxide or azo compound breaks homolytically to give two radicals, one of which adds to a monomer.
- Propagation. The radical adds to the next alkene, generating a new radical at the end of a longer chain. This repeats thousands of times.
- Termination. Two radicals meet — either combining, or one abstracting a hydrogen from the other to give an alkene and an alkane.
Because termination needs two radicals to find each other in a solution where they are very dilute, propagation runs a long way first. That is why chains are long.
Head-to-tail, and why
Take a monosubstituted alkene, CH2=CHX. The growing radical can add to either carbon, and it adds to the CH2 end — because that leaves the radical on the substituted carbon, which is the more stable of the two.
Do that repeatedly and every substituent ends up on alternate carbons, which is called head-to-tail linking. It is the same regiochemical argument as Markovnikov's rule, reached by asking which intermediate is more stable rather than by quoting a rule.
The polymers worth recognizing
| Monomer | Polymer | Where you meet it |
|---|---|---|
| Ethylene, CH2=CH2 | Polyethylene | Bags, bottles, pipe |
| Propylene, CH2=CHCH3 | Polypropylene | Rope, containers, carpet |
| Vinyl chloride, CH2=CHCl | PVC | Pipe, flooring, insulation |
| Styrene, CH2=CHPh | Polystyrene | Packaging, foam cups |
| Tetrafluoroethylene, CF2=CF2 | PTFE (Teflon) | Non-stick coatings |
| Methyl methacrylate | PMMA (Plexiglas) | Transparent sheet |
Every one of these is drawn the same way. Take the monomer, break the π bond, and write the repeat unit in brackets with a bond out of each side: –[CH2–CHX]–n.
Tacticity: the same polymer, three materials
In a monosubstituted polymer such as polypropylene, every substituted carbon in the chain has a definite orientation — the substituent points to one side or the other — so the arrangement along the chain matters. (These are not stereocenters in the CIP sense: the two chain directions are near-identical, which is exactly why tacticity gets its own vocabulary instead of R and S. Polyethylene and PTFE have no such centers at all, and no tacticity.) Three patterns have names:
- Isotactic — all substituents on the same side. Packs well, crystalline, strong.
- Syndiotactic — substituents alternating regularly. Also packs well.
- Atactic — random. Cannot pack, so the material is amorphous and soft.
Branching, and the two polyethylenes
A growing radical sometimes abstracts a hydrogen from its own chain a few carbons back, and the chain then continues from there — leaving a branch. Radical polymerization of ethylene at high pressure does this constantly, giving low-density polyethylene: branched, poorly packed, flexible, the stuff of plastic bags.
A Ziegler–Natta catalyst suppresses it, giving high-density polyethylene: linear, well packed, rigid, the stuff of milk bottles and pipe. Identical repeat unit, identical formula, different architecture, and genuinely different materials.
What carries forward
Initiation, propagation, termination; head-to-tail regiochemistry from the more stable radical; and two structural variables — tacticity and branching — that change the material without changing the chemistry. When a question gives you an alkene and asks for the polymer, break the π bond and bracket the repeat unit; when it gives you two materials with the same formula, the answer is architecture.