The previous sections asked what a structure does. This one runs the question backwards: given a material you need, what monomers make it — and given a polymer you no longer want, what happens to it.
Reading a polymer backwards
The disconnection is mechanical once you know which kind you are looking at.
- Only carbons in the backbone? It came from an alkene. Find the repeat unit, put the double bond back, and you have the monomer.
- An ester in the backbone? Cut the acyl C–O bond — the one between the carbonyl carbon and the oxygen — putting OH on the acid side and H on the alcohol side. What comes back is a diacid and a diol. Cutting the other C–O bond, the one to the CH2, strips the oxygen out of the alcohol fragment and leaves you with no diol at all.
- An amide in the backbone? Cut every C–N bond. What comes back is a diacid and a diamine.
The rule that makes this reliable is the one from the first section: every monomer must have two reactive sites. If a disconnection hands you a fragment with only one, you have cut in the wrong place.
Choosing monomers for a property
| You need | Build in | Because |
|---|---|---|
| High strength | Aromatic rings, para-substituted | Rigid, straight chains that align |
| Hydrolytic resistance | Amides rather than esters | Amides are the least reactive acyl derivative |
| Flexibility | Long flexible spacers, or branching | Lowers Tg, prevents packing |
| Transparency | Amorphous structure: atactic or bulky | No crystalline regions to scatter light |
| A shape that will not melt | A trifunctional monomer | Three sites gives a network |
| Degradability | Ester bonds in the backbone | They hydrolyze, and enzymes can reach them |
Notice that the last two rows point in opposite directions from the second. A property that makes a material durable is the same property that makes it persist in the environment, and you cannot have one without the other — which is the honest version of the recycling problem.
What happens at the end
Three fates, and the chemistry decides which is available.
- Mechanical recycling. Melt and remold. Only works for thermoplastics, and each cycle shortens the chains a little, so the material degrades with reuse.
- Chemical recycling. Depolymerize back to monomers and rebuild. Doing it by hydrolysis needs a backbone that can be cleaved, so it works for PET and for nylon and not for polyethylene — a C–C backbone has no bond that hydrolysis can select. Polyethylene can be cracked thermally back to a mixture of olefins and naphtha, but that is pyrolysis: it destroys the chain indiscriminately and returns feedstock, not monomer.
- Degradation. Polyesters such as PLA hydrolyze, and given the right conditions organisms can finish the job. Polyolefins essentially do not, because there is nothing in a saturated hydrocarbon chain for water or an enzyme to attack.
Biodegradable by design
PLA — polylactic acid — is the clearest example of designing for the end. Lactic acid carries both an OH and a COOH, so it is its own diol-and-diacid and polymerizes with itself. The resulting polyester hydrolyzes back to lactic acid, which is a normal metabolite.
The qualifier matters, though: PLA composts efficiently in an industrial facility, which runs hot and wet with the right microbes, and negligibly in a garden heap or in seawater — in seawater it shows essentially no measurable breakdown over years. "Biodegradable" is a claim about conditions as much as about structure.
What carries forward
Disconnect a polymer by finding the backbone functional group and cutting it, checking that every fragment has two reactive sites. Choose monomers by the property you need, knowing that rigidity, durability and persistence are the same property seen three ways. And when a question asks whether something can be recycled chemically, the answer is whether its backbone has a bond that hydrolysis can find.