Pick up a clear drinks bottle made of PET. You can ask three questions about it. What monomers was it made from? If you wanted a different material, such as a stiffer fiber or a clear sheet, which monomers would you choose instead? And once the bottle is empty, what can happen to it? This section answers all three, and each answer comes from the same place: the bonds in the polymer’s backbone.
Reading a polymer backwards
Finding the monomers is retrosynthesis applied to a chain. You look for the bonds that polymerization made and undo them on paper. Each of those cuts is a disconnection: a bond you break in your head to see what two pieces could have formed it.
Start with PET. Walk along its chain and you pass two esters in every repeat unit. Each ester was made by acyl substitution: an alcohol oxygen attacked a carbonyl carbon and an OH left (Nucleophilic acyl substitution). So the bond that formed is the one from the carbonyl carbon to that oxygen, the acyl C–O bond, and that is the bond to cut.
After each cut, give the carbonyl carbon back its OH and give the oxygen back its H. The piece holding the ring and its two carbonyl carbons becomes HO2C–C6H4–CO2H, terephthalic acid, a diacid. The piece holding the two oxygens and the CH2CH2 becomes HO–CH2CH2–OH, ethylene glycol, a diol.
An ester has a second C–O single bond, from that oxygen to the CH2. Cutting there is the usual mistake. Both oxygens then stay with the acid piece, and the CH2CH2 piece comes away with no oxygen at either end. A piece with nothing to react with could never have been a monomer.
The same reasoning covers addition polymers, polyesters and polyamides; what sits in the backbone tells you which bond to cut.
- Only carbons in the backbone? The polymer came from an alkene. Find the repeat unit, put the double bond back, and you have the monomer.
- An ester in the backbone? Cut every acyl C–O bond, as for PET. Out comes a diacid and a diol. (If each repeat unit holds only one ester, as in PLA, out comes a single hydroxy acid.)
- An amide in the backbone? Cut every amide C–N bond, the one from the carbonyl carbon to the nitrogen, not the one from the nitrogen to the CH2. Out comes a diacid and a diamine. (If each repeat unit holds only one amide, as in nylon 6, out comes a single amino acid, here 6-aminohexanoic acid.)
One check makes this reliable, and it is the rule from What a polymer is: every monomer in a chain has two reactive sites, one for each neighbor it joins. If a disconnection hands you a piece with only one, you have cut in the wrong place.
Given. A repeat unit –[NH–(CH2)6–NH–C(=O)–C6H4–C(=O)]–, with the ring joined at para positions. The drawing below shows a stretch of this chain.
Step 1 — what is in the backbone? Draw enough chain to show two full linkages, then walk along it and name every group you pass. Twice you pass an N bonded to a C=O, so the backbone holds amides. This is a polyamide, made from a diacid and a diamine.
Step 2 — cut every amide C–N bond. Put OH on the carbonyl side and H on the nitrogen side. Out come H2N(CH2)6NH2 and HO2C–C6H4–CO2H.
Step 3 — check that each piece has two sites. The diamine has two NH2 groups and the diacid has two CO2H groups. Both pass, so the cuts were in the right place.
Step 4 — name it. Hexamethylenediamine plus terephthalic acid gives nylon 6,T. It is an aromatic nylon with only one aromatic partner. That is why it is not Kevlar: in Kevlar, both monomers are aromatic and para.
Step 5 — predict the material. Each repeat unit has one rigid ring, and amide hydrogen bonds hold neighboring chains together. So nylon 6,T should melt higher and be stiffer than nylon 6,6. Its flexible (CH2)6 run, which Kevlar lacks, keeps it less rigid than Kevlar.
The trap in step 2. Cutting the other C–N bond, the one from the nitrogen to the CH2, hands you a piece with an NH2 at one end and nothing at the other. Whenever a piece comes back with a single site, you cut the wrong bond.
Step 5 needs one caution. Do not stretch the prediction to “so it can be melted and molded.” Pure nylon 6,T melts near 370 °C (its Tm, the melting point of its crystalline regions), and amides start to decompose below that. The polymer would break down before it flowed.
The commercial 6T nylons get around this by being copolymers, chains built from two kinds of repeat unit. Some of the terephthalic acid is swapped for adipic acid (giving 6T/66) or for isophthalic acid, the meta isomer (giving 6T/6I). The mixed units stop the chains from packing so regularly, which brings Tm down into a range where the polymer can be melted safely. A meta ring also puts a bend in the chain.
Kevlar would melt higher still, well above the point where it decomposes, so it is never melted at all. It is dissolved in concentrated sulfuric acid and spun into fiber from that solution.
Choosing monomers for a property
Now run the question the other way. Suppose you need a clear, rigid sheet, such as a safety visor. A polymer turns cloudy when its crystalline regions are large enough to scatter light at their boundaries with the tangled material around them (Structure and properties). The safe route to a clear sheet is an amorphous one, with no crystalline regions at all.
What keeps the chains from packing is irregularity: a random, atactic arrangement of side groups, or an awkwardly shaped backbone. Bulk alone is not enough, as syndiotactic polystyrene shows in Structure and properties. The stiffness then has to come from somewhere other than packing. It comes from a glass transition temperature, Tg, well above room temperature, which keeps the tangled chains frozen in use. PMMA is atactic, and its bulky side groups raise its Tg. Polycarbonate gets its high Tg from the rigid, awkwardly shaped bisphenol A units in its backbone. Both are clear as glass.
For every property in the table below, the method is the same: name the structural feature that produces it, then choose monomers that build it in.
| You need | Build in | Because |
|---|---|---|
| High strength | Aromatic rings, para-substituted | Rigid, straight chains line up and pack side by side |
| Hydrolytic resistance | Amides rather than esters | Amides are the least reactive acid derivative |
| Flexibility | Long flexible spacers, or branching | Spacers lower Tg; branches stop the chains packing |
| Transparency | Amorphous structure: atactic chains or an irregularly shaped backbone | No crystalline regions to scatter light |
| A shape that will not melt | A monomer with three reactive sites | Three sites give a network |
| Degradability | Ester bonds in the backbone | They hydrolyze, and enzymes can reach them |
Notice that the last row pulls against the second. An ester that lets a material break down is also a weak point while you are using it, and an amide that resists water also resists breakdown. The feature that makes a material durable is the feature that makes it persist in the environment. That trade-off sits at the heart of the plastics problem, and the choice is made when someone picks the monomer, not at the recycling plant.
What happens at the end
An empty PET bottle, like any used plastic, has three possible fates, and its chemistry decides which are open.
- Mechanical recycling. Melt it and mold it again. This works only for thermoplastics (What a polymer is); a thermoset never melts or flows. Each cycle also shortens the chains a little, so the material gets worse with reuse.
- Chemical recycling. Break the polymer back down to monomers and build it again. Doing this by hydrolysis needs a backbone bond that water can break, so it works for PET and for nylon but not for polyethylene.
- Degradation. Polyesters such as PLA hydrolyze, and under the right conditions microorganisms finish the job. Polyolefins, the polymers of simple alkenes such as polyethylene, essentially do not.
Why PET and PLA can be taken apart and polyethylene cannot comes down to one functional group. An ester’s carbonyl carbon is electron-poor, so water can attack it, and the alkoxy oxygen can leave, exactly as in acyl substitution. A saturated C–C chain has only nonpolar C–C and C–H bonds, with no electrophilic carbon and no leaving group, so water and enzymes have nothing to attack.
Polyethylene can still be broken down by heat. Heated hard enough, its C–C bonds break at random, and it gives a mixture of small alkenes and hydrocarbon oil. This is pyrolysis, or thermal cracking. It is not recycling back to monomers, because it destroys the chain without any selectivity. What comes back is a feedstock that has to be refined all over again, not the ethylene you started from.
Breaking PET back down to monomers is called depolymerization, and it needs no new reaction. It is acyl substitution run backwards. There are three ways to do it for PET, and they differ only in the nucleophile.
- Hydrolysis uses water, either with an acid catalyst or with a full equivalent of base (saponification), in which case acid is added afterwards to free the terephthalic acid. It gives terephthalic acid and ethylene glycol, the original monomers.
- Methanolysis uses methanol, so it is a transesterification: one ester’s alkoxy group swapped for another (Esters & amides). It gives dimethyl terephthalate and ethylene glycol, which is the other pair of monomers PET is made from industrially.
- Glycolysis uses excess ethylene glycol, also a transesterification. It cuts the chains down to BHET, bis(2-hydroxyethyl) terephthalate: terephthalic acid with each CO2H esterified by one ethylene glycol. BHET can be polymerized straight back to PET.
Biodegradable by design
PLA, polylactic acid, is the clearest example of a polymer designed for its end. Lactic acid, CH3CH(OH)CO2H, is a hydroxy acid: one molecule carries both the OH and the CO2H an ester needs, so it can polymerize with itself. The polyester it forms hydrolyzes back to lactic acid, which is a normal product of metabolism.
In practice PLA is made in two stages, and the reason is the conversion arithmetic from Condensation polymers. Heated on its own in the melt, lactic acid usually stops at short chains, called oligomers. The melt is too thick for the water to escape, so the equilibrium stops well short of the conversion a useful material needs.
So the short chains are heated under vacuum with a catalyst, and the OH at the end of each chain bites back on the ester two units along, releasing lactide. Lactide is the cyclic diester of two lactic acids: a six-membered ring in which each lactic acid is esterified to the other. Lactide then polymerizes by ring-opening. The growing chain end attacks a carbonyl in the ring and breaks the bond from that carbonyl carbon to the ring oxygen, so the ring opens onto the chain. This is chain growth. It loses no small molecule, and it reaches the molecular weight a fiber needs. The backbone is the same either way: a polyester of lactic acid.
“Biodegradable” needs a qualifier, though. PLA composts efficiently in an industrial facility, which runs hot and wet with the right microorganisms. In a garden heap it breaks down very little, and in seawater, studies have found almost no measurable breakdown after more than a year. Whether a polymer biodegrades depends on the conditions as much as on its structure.
What carries forward
To find a polymer’s monomers, find the functional group in its backbone and cut at the bond that polymerization made, then check that every piece has two reactive sites. To choose monomers, start from the property you need, and expect durability and persistence to come together. To decide whether a polymer can be recycled back to monomers, ask whether its backbone holds a bond that hydrolysis can pick out.