Polymerisation

Condensation polymerisation: two functional groups and a small molecule lost

Condensation polymerisation is the joining together of many monomer molecules to form one very large molecule, with the loss of a small molecule at every link that is made. That small molecule is usually water, when a carboxylic acid reacts with an alcohol or an amine, or hydrogen chloride, when an acid chloride reacts with an alcohol or an amine. Every monomer must carry two functional groups, because the chain has to be able to grow at both ends: a molecule with only one reactive group would cap the chain and stop it dead.

The two groups react to give a linkage. An alcohol (-OH) with a carboxylic acid (-COOH) gives an ester link, -COO-, and an amine (-NH2) with a carboxylic acid (-COOH) gives an amide link, -CONH-. The chain still carries a functional group at each end after each link is made, so it can react again and the chain keeps growing. This is the opposite of addition polymerisation, where the monomers must contain a C=C double bond and nothing at all is lost, so the polymer has exactly the same empirical formula as the monomer.

Condensation polymers are named after the linkage they contain. Chains joined by ester links are polyesters; chains joined by amide links are polyamides. Both are made on a very large industrial scale, and both can be hydrolysed back into smaller molecules, which is exactly why they behave so differently from poly(alkenes) when they are thrown away.

Polyesters: a diol with a dicarboxylic acid or a dioyl chloride

A polyester is made by reacting a diol with a dicarboxylic acid. The best known example is the reaction of ethane-1,2-diol, HOCH2CH2OH, with benzene-1,4-dicarboxylic acid (terephthalic acid), HOOC-C6H4-COOH. Each -OH group of the diol esterifies one -COOH group of the acid, so two ester links are formed per repeat unit and two molecules of water are lost. The product is poly(ethylene terephthalate), PET, which is also sold under the name terylene.

An acid and an alcohol reach an equilibrium and the esterification is slow on its own, so the industrial process is driven forward by removing the water as it forms. Replace the dicarboxylic acid with a dioyl chloride and the reaction changes completely: an acid chloride reacts with an alcohol rapidly at room temperature and without a catalyst, because a chloride ion is a far better leaving group than a hydroxide ion. The link formed is still an ester link, but the small molecule lost is hydrogen chloride rather than water. This is why dioyl chlorides are used when a polymer has to be made quickly and in high yield.

The mechanism is nucleophilic addition-elimination, and you can describe it in words. A lone pair on the oxygen of the alcohol attacks the delta-positive carbon of the C=O group of the acid chloride; the pi bond of the C=O breaks and its electrons move onto the oxygen, giving a tetrahedral intermediate. The chloride ion then leaves, taking the pair of electrons of the C-Cl bond with it, and the intermediate loses a proton to give the ester.

PET is a tough, strong, semi-crystalline thermoplastic whose melting point is high for its chain length, because the ester links are polar and the chains pack closely together. Its main uses are drinks bottles and polyester fibre for clothing, and because it contains ester links it can be broken back down to its monomers by hydrolysis, a route known as chemical recycling.

Polyesters from a single monomer: the hydroxycarboxylic acids

You do not need two different monomers to make a condensation polymer. A hydroxycarboxylic acid carries both an -OH and a -COOH in the same molecule, so each molecule can react with its neighbours: the -COOH of one molecule esterifies the -OH of the next, and the chain grows at both ends. 2-hydroxypropanoic acid (lactic acid, CH3CH(OH)COOH) polymerises this way to give polylactic acid, PLA, and 2-hydroxyethanoic acid (glycolic acid) gives poly(glycolic acid), PGA.

When a single monomer condenses with itself, the repeat unit is exactly one molecule of the monomer minus one molecule of water. For PLA the repeat unit is -O-CH(CH3)-CO-, so the polymer has the empirical formula C3H4O2 while the monomer has C3H6O3. Notice that the repeat unit still contains one ester link and one free bond at each end, which is what lets the next unit join on.

PLA and PGA are both polyesters, so both are broken down by hydrolysis, and that is precisely why they are used where a plastic has to disappear. PLA goes into compostable food packaging, disposable cups and 3D printing filament; PGA and its copolymers are used for dissolvable surgical stitches and for implants that slowly break down inside the body, because the ester links are hydrolysed by water in the tissue and the harmless products are absorbed.

How fast such a polymer disappears can be designed. More ester links per unit length, or a lower degree of crystallinity, lets water reach the links more easily, so the polymer hydrolyses faster. That is a real design decision in medical materials, not a fixed property of the plastic.

Polyamides: diamines, dicarboxylic acids, amino acids and proteins

A polyamide is made by reacting a diamine with a dicarboxylic acid. Hexane-1,6-diamine, H2N(CH2)6NH2, reacts with hexanedioic acid (adipic acid), HOOC(CH2)4COOH, to give nylon-6,6: each -NH2 condenses with a -COOH to form an amide link, -CONH-, and a molecule of water is lost, so two molecules of water are lost per repeat unit. The name records the carbon count of each monomer, six and six. Replace the dicarboxylic acid with hexanedioyl chloride and the reaction is fast at room temperature, losing hydrogen chloride instead of water; this is the reaction used in the nylon rope demonstration, where a solution of the diamine is poured onto a solution of the dioyl chloride and the polymer is drawn out as a continuous thread.

As with polyesters, a single monomer carrying both groups is enough. 6-aminohexanoic acid, H2N(CH2)5COOH, condenses with itself to give nylon-6, whose repeat unit is -NH-(CH2)5-CO-. That repeat unit makes the point plainly: the amide link forms between the amine end of one molecule and the acid end of the next.

Amino acids behave in exactly the same way. A 2-amino acid such as glycine, H2NCH2COOH, carries an -NH2 and a -COOH, so it condenses with another amino acid to give a peptide link, which is simply an amide link. Condensing many amino acids gives a polypeptide, and a protein is a polyamide whose repeat unit is decided by the sequence of amino acids in the chain. The chemistry of nylon and the chemistry of a protein are the same reaction.

Kevlar is a polyamide made from benzene-1,4-diamine and benzene-1,4-dicarbonyl chloride. Both monomers are rigid and flat, so the chains are straight rods rather than tangles, and the amide links of neighbouring chains hydrogen bond to each other over long distances. It is that combination of rigid chains and extensive hydrogen bonding, not any covalent cross-linking, that makes Kevlar strong enough for bullet-resistant vests and for the cords of tyres.

Deducing the repeat unit, and identifying the monomers

To deduce the repeat unit from a monomer or a pair of monomers, work out which atoms are lost. The -OH of the alcohol (or the -H of the amine) and the -OH of the carboxylic acid leave together as water; the -H of the alcohol or amine and the Cl of an acid chloride leave together as hydrogen chloride. What remains is the repeat unit, and it must be drawn with a bond at each end, because that is where the next unit joins on.

To go the other way, from a section of a polymer to its monomers, find the linkage first. If the linkage is an ester, the polymer is a polyester and its monomers are an alcohol and an acid, so add the elements of water across each ester link: the -CO- side takes back an -OH to become a -COOH and the -O- side takes back an H to become an -OH. If the linkage is an amide, the polymer is a polyamide, and adding water across -CO-NH- gives a -COOH on one side and an -NH2 on the other.

How many different monomers a section contains is decided by the pattern of the linkages. If every ester link in the chain is identical and the whole chain can be built from one molecule carrying both an -OH and a -COOH, the polymer came from a single hydroxycarboxylic acid or amino acid. If the chain alternates between two different fragments, for example an aromatic ring carrying two -CO- groups alternating with a two-carbon unit carrying two -O- groups, the polymer came from a dicarboxylic acid and a diol.

Remember that a repeat unit is the smallest fragment that repeats. Copying out the whole visible section, or leaving out the two bonds that join the repeat units together, both lose the mark.

Predicting the type of polymerisation

To predict which polymerisation a monomer will undergo, look at the functional groups, not at the size of the molecule. If the only reactive feature is a C=C double bond, the monomer undergoes addition polymerisation, because the double bond opens and joins the monomers together with nothing lost. If the monomer carries two functional groups that can condense, such as an -OH with a -COOH, an -NH2 with a -COOH, or an acid chloride with an -OH or an -NH2, it undergoes condensation polymerisation.

A single monomer can undergo condensation polymerisation provided it carries both of the reacting groups in the same molecule, which is why 2-hydroxypropanoic acid and 6-aminohexanoic acid are condensation monomers rather than addition monomers. A pair of monomers undergoes condensation when between them they provide the two reacting groups, as in a diol with a dicarboxylic acid, or a diamine with a dioyl chloride.

The trap is a molecule carrying both a C=C bond and a second functional group. Propenoic acid, CH2=CHCOOH, has a double bond and a carboxylic acid group, but a single -COOH has nothing to condense with, so propenoic acid polymerises by addition through the C=C bond, leaving the -COOH groups hanging from the chain as pendants. That polymer, poly(propenoic acid), is a real material: as its sodium salt it is the superabsorbent that swells inside a disposable nappy.

To work the other way, from a section of a polymer to the type of polymerisation, look for the tell-tale groups. A backbone of carbon atoms only, with substituent groups hanging from it and no polar link inside the chain, must have come from addition polymerisation of an alkene. A chain interrupted at regular intervals by -COO- or -CONH- links, with a heteroatom and a C=O inside the backbone, must have come from condensation polymerisation.

Degradable polymers: why poly(alkenes) persist and polyesters do not

A polymer is biodegradable if microorganisms, or the water, acids and alkalis in the environment, can break its chains down into small molecules that living things can use. Poly(alkenes) such as poly(ethene), poly(propene) and PVC are built on a backbone of C-C and C-H single bonds, which are strong, non-polar and chemically unreactive. There is no polar group for water to attack and no link that an enzyme can hydrolyse, and the long, tangled, hydrophobic chains offer a microorganism nothing to work on. A poly(alkene) can therefore sit in landfill or in the sea for decades, which is the whole reason for the recycling and waste-reduction effort.

Polyesters and polyamides are different. Both contain a polar carbonyl group in the backbone, and both the ester link and the amide link can be attacked by water. Acid hydrolysis is catalysed by H+ ions and alkaline hydrolysis by OH- ions, and the alkaline reaction is faster because the hydroxide ion is a much better nucleophile than a water molecule. Microorganisms supply the same chemistry with enzymes: esterases attack polyester links and proteases attack polyamide links, which is why a polyester or a polyamide buried in soil or in compost will break down while a poly(alkene) will not.

The hydrolysis cuts the chain into shorter and shorter fragments until the products are the original monomers or their ions. From a polyester you get the diol and the dicarboxylic acid, or the salt of the acid in alkali. From a polyamide you get the diamine and the dicarboxylic acid, or their salts in alkali. Those small, water-soluble molecules can then be metabolised, which is what makes the polymer genuinely biodegradable rather than merely brittle.

Some polymers are designed to be broken down by light. If ethene is copolymerised with a small amount of carbon monoxide, ketone carbonyl groups are built into the chain at intervals. A C=O group absorbs ultraviolet light, and the energy it absorbs breaks the chain at that point. Sunlight therefore chops the chain into short fragments, the plastic becomes brittle and falls apart, and those fragments are then small enough for microorganisms to finish off. This is how photodegradable mulch film and photodegradable carrier bags work, and the limitation is worth remembering: light alone only fragments the plastic, so unless the fragments are themselves biodegradable the plastic has been broken up rather than removed.