Polymers

In a nutshell

Polymers are long molecules built by joining many small monomers. At A-level the new idea is condensation polymerisation, which makes polyesters and polyamides by linking two functional groups together and losing a small molecule (water or HCl) each time.

This note covers how those polymers form, the linkages that hold them together, why their chains stick to each other, and why they can be broken down by hydrolysis when polyalkenes cannot.

Assumed knowledge: Alkenes (addition polymers), Alcohols, and esters and amides from Carboxylic acids and derivatives (3.3.9).

Core content

Two types of polymerisation

There are two ways monomers join into a polymer:

• Addition polymerisation (from 3.3.4): unsaturated monomers with a C=C bond add together, and nothing is lost. The product is a polyalkene (e.g. poly(ethene), poly(propene)).
• Condensation polymerisation (new here): two monomers join and a small molecule is eliminated each time a bond forms, usually water or hydrogen chloride.

Keep two labels separate, because the exam asks for them separately:

• Type of polymerisation = addition or condensation.
• Type of polymer = polyalkene, polyester or polyamide.

Polyesters

A polyester is a condensation polymer whose monomers are joined by ester linkages (-CO-O-).

It forms from either:

• a dicarboxylic acid + a diol (releases water), or
• a diacyl chloride + a diol (releases HCl).

The common example is Terylene (also called PET), made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol.

Typical uses of Terylene / PET: clothing fibres, drink bottles, and tyre cords.

Polyamides

A polyamide is a condensation polymer whose monomers are joined by amide linkages (-CO-NH-). When the polymer is built from amino acids the same linkage is called a peptide link.

It forms from either:

• a dicarboxylic acid + a diamine (releases water), or
• a diacyl chloride + a diamine (releases HCl), or
• amino acids (each monomer already carries both a -COOH and an -NH₂ group).

Nylon 6,6 is made from hexanedioic acid and hexane-1,6-diamine. The "6,6" means 6 carbons in each monomer, so its repeating unit contains 12 carbon atoms.

Kevlar is a polyamide made from benzene-1,4-dicarboxylic acid and benzene-1,4-diamine. Its flat rings and strong hydrogen bonding make it extremely strong for its weight.

Typical uses: nylon in clothing, ropes and carpets; Kevlar in body armour, bulletproof vests and protective clothing.

The linkages, and the equations

The ester linkage and the amide linkage are the heart of this topic. Both contain a carbonyl carbon that is δ+ (because the two attached oxygens, or the oxygen and nitrogen, are more electronegative). That polarity is what lets them be hydrolysed later.

As a general scheme, writing the equations per n repeating units:

• n dicarboxylic acid + n diol → poly(ester) + 2n H₂O
• n diacyl chloride + n diol → poly(ester) + 2n HCl
• n dicarboxylic acid + n diamine → poly(amide) + 2n H₂O
• n diacyl chloride + n diamine → poly(amide) + 2n HCl

The acid route needs an acid catalyst and only reaches equilibrium. The more reactive acyl chloride route goes to completion without a catalyst, but gives off corrosive HCl fumes.

Still don't get it? · the repeating unit and its trailing bonds

Think of the polymer as a very long train made of identical two-carriage sets clipped together. The repeating unit is one of those sets, drawn with the couplings still sticking out at each end, because that is where the next set clips on.

Now the exam version. A repeating unit is the smallest piece that repeats, drawn in square brackets with a small n outside. The two short bonds that poke out through the brackets (the "trailing bonds") are not optional decoration: they show the unit joins on to the next one, so a chain really is continuous.

Two traps follow from this:

• Draw only one repeating unit, never two or three. Examiners report students repeatedly drawing "twice the repeating unit".
• Keep the atoms exact: do not add an extra oxygen, and do not leave the trailing bonds off. A missing trailing bond or a stray O loses the mark even when the rest is right.

Intermolecular forces between the chains

A polymer's physical properties come from the forces between neighbouring chains. You must be able to name the specific force in each polymer and compare them.

• Polyamides form hydrogen bonds between the δ+ hydrogen of an N-H group on one chain and the δ- oxygen of a C=O group on a neighbouring chain. This is the strongest of the three, so polyamides have higher melting points than polyesters.
• Polyesters have permanent dipole-dipole forces between the polar C=O groups on neighbouring chains.
• Polyalkenes are non-polar, so only van der Waals forces act between their chains.

Still don't get it? · melting a polyamide (what actually breaks)

Imagine two ropes lying side by side, stitched to each other every few centimetres with loose thread. To pull the ropes apart you only snap the thread between them, not the ropes themselves.

The chains are the ropes; the hydrogen bonds between chains are the thread. To melt the polymer you supply enough energy to overcome the forces between the chains, you do not break the covalent bonds along the chain.

So when you explain why a polyamide melts higher than a polyester, say the hydrogen bonds between chains are stronger than the permanent dipole-dipole forces between chains, so more energy is needed. Writing about "breaking the amide bonds" or "breaking the carbon chain" is the classic lost mark.

Making nylon 6,6 (skills opportunity)

Making nylon 6,6 is an AQA practical skills opportunity (not one of the numbered required practicals).

In the demonstration ("the nylon rope trick"), hexane-1,6-diamine dissolved in water is layered with hexanedioyl dichloride (the diacyl chloride) dissolved in an organic solvent. The two solutions do not mix, and nylon forms as a film at the boundary between the layers. The film can be picked up and pulled out as a continuous thread. Using the diacyl chloride rather than the diacid means the reaction happens quickly at room temperature.

Hydrolysis and biodegradability

This is the spec's key "explain" point: why polyesters and polyamides can be hydrolysed but polyalkenes cannot.

• Polyesters and polyamides contain polar C=O linkages with a δ+ carbonyl carbon. This carbon is attacked by a nucleophile (water, or OH^- from alkali), so the linkage is hydrolysed and the polymer breaks down. They are therefore biodegradable.
• Polyalkenes have only non-polar C-C and C-H bonds. There is no δ+ carbon for a nucleophile to attack, so they cannot be hydrolysed and are non-biodegradable.

Hydrolysis needs hot aqueous acid (e.g. concentrated HCl or H₂SO₄) or hot aqueous alkali (e.g. NaOH). Cold water or a weak acid is not enough. The products are the original monomers, but in a salt form that depends on the conditions:

• Acid hydrolysis of a polyamide gives the dicarboxylic acid and the -NH₃⁺ salt of the diamine.
• Alkaline hydrolysis of a polyamide gives the carboxylate salt (-COO^-) and the free diamine.

Disposal of polymers

There is no single best method; each has trade-offs.

Using biodegradable polymers (polyesters and polyamides, which hydrolyse) in place of inert polyalkenes helps waste break down faster in the environment.

Worked examples

1. Draw the repeating unit from the monomers (polyester).

Given benzene-1,4-dicarboxylic acid and ethane-1,2-diol:

1. Line the two monomers up, acid next to alcohol.
2. Form an ester link: the -OH of the acid and the H of the alcohol's -OH leave as H₂O, and the remaining O joins the carbonyl carbon.
3. Join one acid to one diol, then put the result in brackets with n and draw the trailing bonds through the brackets.
4. Balance: two ester links form per repeat unit, so 2n H₂O is released. (See the Terylene figure above.)

2. Deduce the monomers from a section of polymer.

Given a section of polymer chain:

1. Find the linkages: -CO-O- means a polyester; -CO-NH- means a polyamide.
2. Break each linkage and add back the small molecule that was lost: put -OH back on each carbonyl carbon (regenerating -COOH) and put an H back on each O (regenerating -OH) or on each N (regenerating -NH₂).
3. Read off the two monomers (a dicarboxylic acid and a diol, or a dicarboxylic acid and a diamine).

3. Model "explain" answer: why nylon 6,6 can be hydrolysed but poly(ethene) cannot. (4 linked points)

1. Nylon 6,6 is a polyamide and contains polar amide linkages with a δ+ carbonyl carbon.
2. This δ+ carbon is attacked by a nucleophile such as water or the hydroxide ion, so the amide link is hydrolysed.
3. Poly(ethene) has only non-polar C-C and C-H bonds, so there is no δ+ carbon to be attacked.
4. Therefore poly(ethene) cannot be hydrolysed and is non-biodegradable, whereas nylon 6,6 can and is biodegradable.

Common exam mistakes

• Drawing more than one repeating unit. The question asks for one; students often draw two or three, or "twice the repeating unit", and lose the mark.
• Leaving off the trailing bonds that extend through the square brackets, or adding an extra oxygen to the linkage. Both are reported repeatedly and both lose the mark.
• When asked for a displayed formula, not showing all bonds, especially the O-H bond of a -COOH or -OH group (drawing C-OH as if it were just "OH").
• Explaining hydrolysis by writing about the type of polymerisation instead of the real reason: polyalkenes are non-polar so are not attacked by hydroxide ions/nucleophiles and therefore are not hydrolysed.
• Saying polyalkenes resist hydrolysis only because their bonds are strong; the mark needs non-polar bonds (no δ+ carbon to attack).
• Not naming the specific intermolecular force in each polymer, or calling the force in polyamides "dipole-dipole" when it is hydrogen bonding.
• Explaining melting or strength by "breaking the amide/ester bonds" or "breaking the carbon chain"; you overcome the forces between chains, you do not break the covalent chain.
• Writing "van der Waals bonds" or "forces between atoms"; AQA penalises both. Write "van der Waals forces between the chains".
• Giving hydrolysis conditions as just "water"; it needs hot acid or hot alkali (water, acidified water and weak acids are rejected).
• Building a polyamide from an amino carboxylic acid when the question asks for a dicarboxylic acid + a diamine (and vice versa).
• Naming slips: the "e" is required in diols and diamines (ethane-1,2-diol, hexane-1,6-diamine), and for hydrogen bonding the H must be on an electronegative atom (N, O or F).

Key definitions

• Condensation polymerisation: the joining of many monomers into a polymer with the elimination of a small molecule (such as water or hydrogen chloride) each time a linkage forms.
• Repeating unit: the smallest group of atoms that, repeated, makes up the polymer, drawn in brackets with a trailing bond through each bracket.
• Ester linkage: the -CO-O- group that joins the monomers in a polyester.
• Amide (peptide) linkage: the -CO-NH- group that joins the monomers in a polyamide.
• Hydrolysis: the breaking of a bond by reaction with water (here of an ester or amide linkage, using hot acid or hot alkali).
• Biodegradable: able to be broken down (polyesters and polyamides can be hydrolysed and are therefore biodegradable).

Specification

• I can state that condensation polymers form between dicarboxylic acids and diols, dicarboxylic acids and diamines, and from amino acids.
• I can identify the repeating unit and the linkage in a polyester (ester link) and in a polyamide (Terylene, nylon 6,6, Kevlar).
• I can state typical uses of these polymers.
• I can draw the repeating unit from the monomer structure(s).
• I can draw the repeating unit from a section of the polymer chain.
• I can draw the monomer(s) from a section of the polymer.
• I can explain the intermolecular forces between molecules of condensation polymers (hydrogen bonding in polyamides, permanent dipole-dipole in polyesters).
• I can state that polyalkenes are chemically inert and non-biodegradable, while polyesters and polyamides can be hydrolysed and are biodegradable.
• I can explain why polyesters and polyamides can be hydrolysed but polyalkenes cannot.
• I can give the advantages and disadvantages of landfill, incineration and recycling as methods of polymer disposal.

Related notes

• Alkenes (addition polymers)
• Alcohols
• Carboxylic acids and derivatives (3.3.9)
• Amino acids, proteins and DNA (3.3.13)
• Organic synthesis (3.3.14)