Condensation Polymerisation: Polyesters, Polyamides and Proteins – Polymers and Polymerisation (Senior 6 Chemistry) | Ganzaa

Polymers and Polymerisation

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Senior 6 · Chemistry

Polymers and Polymerisation

4 lessons

Lesson 2 of 4

Condensation Polymerisation: Polyesters, Polyamides and Proteins

25 min

A bottle and a shirt from the same polymer

Read the label inside a school shirt: it often says "65 % polyester, 35 % cotton". The polyester in that shirt is usually PET, poly(ethylene terephthalate), and a clear plastic water bottle is made of the same polymer. Yet PET cannot be made from an alkene. Its chain contains oxygen atoms, joined in groups called ester links, and each link was made by pulling a molecule of water out of two monomers. This is the second way to build a polymer.

Definition

Condensation polymerisation: polymerisation in which monomers join and a small molecule, usually water or hydrogen chloride, is eliminated at each new link. Example: nylon-6,6 from hexanedioic acid and hexane-1,6-diamine, with loss of water.

Bifunctional monomer: a monomer with two reactive functional groups, one at each end, so it can link on both sides. Example: ethane-1,2-diol, $\mathrm{HOCH_2CH_2OH}$ .

Ester link: $\mathrm{-CO{-}O-}$ , formed from a carboxylic acid group and an alcohol group. A polymer held by ester links is a polyester.

Amide link: $\mathrm{-CO{-}NH-}$ , formed from a carboxylic acid (or acyl chloride) group and an amine group. A polymer held by amide links is a polyamide; in proteins the same link is called a peptide link.

What makes a condensation polymer

Each monomer has two functional groups. Either two different monomers each carry two identical groups (a diacid with a diol, or a diacid with a diamine), or one monomer carries two different groups (a hydroxy acid, or an amino acid).
At each link a small molecule is lost, so the atom economy is less than 100 % and the repeat unit has fewer atoms than the monomers it came from.
The links put O or N atoms into the main chain. These links are polar and can be split again by water (hydrolysis).
The chain grows step by step: any two molecules with suitable ends, small or large, can react. This is called step-growth polymerisation, unlike the chain-growth of the radical mechanism in Lesson 1.

Figure 2.1 How the two links form. The circled OH (from the acid) and H (from the alcohol or amine) leave as $\mathrm{H_2O}$ . The coloured blocks stand for the rest of each monomer. In a polymer, the same thing happens at both ends of every monomer.

Polyesters

PET (also sold as Terylene or Dacron) is made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol, heated with a catalyst. Written exactly, with the end groups:

$\begin{aligned} &n\,\mathrm{HOOC{-}C_6H_4{-}COOH} + n\,\mathrm{HOCH_2CH_2OH} \\ &\qquad \longrightarrow \mathrm{HO{-}\bigl[CO{-}C_6H_4{-}CO{-}O{-}CH_2CH_2{-}O\bigr]_{\mathit{n}}{-}H} + (2n-1)\,\mathrm{H_2O} \end{aligned}$

Each repeat unit contains one acid part and one diol part, and two ester links are made per repeat unit. When $n$ is large, $2n - 1 \approx 2n$ , so exam answers often write " $+ 2n\,\mathrm{H_2O}$ " and leave out the end groups. The repeat unit is

$\mathrm{\bigl[{-}CO{-}C_6H_4{-}CO{-}O{-}CH_2CH_2{-}O{-}\bigr]_{\mathit{n}}}$

A single monomer with an acid group at one end and an alcohol group at the other also gives a polyester. 2-Hydroxypropanoic acid (lactic acid, made by fermenting sugar from maize or cassava starch) gives poly(lactic acid), PLA, with repeat unit $\mathrm{-O{-}CH(CH_3){-}CO-}$ . PLA is used for compostable cups and in 3-D printing (Lesson 4).

Polyamides

Nylon-6,6 is made from hexane-1,6-diamine and hexanedioic acid. The two numbers give the number of carbon atoms in the diamine and in the diacid:

$\begin{aligned} &n\,\mathrm{HOOC(CH_2)_4COOH} + n\,\mathrm{H_2N(CH_2)_6NH_2} \\ &\qquad \longrightarrow \mathrm{HO{-}\bigl[CO(CH_2)_4CO{-}NH(CH_2)_6NH\bigr]_{\mathit{n}}{-}H} + (2n-1)\,\mathrm{H_2O} \end{aligned}$

In the laboratory the acid is replaced by its more reactive acyl chloride, hexanedioyl dichloride, $\mathrm{ClOC(CH_2)_4COCl}$ . Then hydrogen chloride is eliminated, and the reaction is fast at room temperature. If the dichloride dissolved in cyclohexane is floated on an aqueous solution of the diamine, nylon forms as a film at the boundary and can be pulled out as a continuous thread: the "nylon rope trick".

Kevlar is a polyamide made from benzene-1,4-diamine and benzene-1,4-dicarbonyl dichloride; its repeat unit is $\mathrm{-NH{-}C_6H_4{-}NH{-}CO{-}C_6H_4{-}CO-}$ . The flat benzene rings make the chains straight and rigid, so they line up side by side like bundled rods, with hydrogen bonds between every amide group. Kevlar is five times stronger than steel for the same mass, which is why it is used in protective vests, helmets and cut-resistant gloves.

Proteins are natural polyamides. Their monomers are 2-amino acids, $\mathrm{H_2N{-}CHR{-}COOH}$ , and up to twenty different side chains R are used, so a protein is a condensation copolymer with a precise sequence.

The nylon rope trick: a thread of nylon is pulled up from the boundary between an aqueous solution of hexane-1,6-diamine (lower layer) and a solution of hexanedioyl dichloride in an organic solvent (upper layer). New polymer forms at the boundary as fast as the thread is drawn out. Photo: Capaccio, CC BY-SA 3.0, via Wikimedia Commons

Figure 2.2 Two nylon-6,6 chains side by side. Each N–H of one chain points at a C=O of the other, so hydrogen bonds (dotted) hold the chains together. This is why nylon fibres are strong and why nylon melts at about 265 °C, far higher than poly(ethene), whose chains are held only by London forces.

Hydrolysis: undoing the link

Because an ester or amide link was made by removing water, adding water back (with an acid or alkali catalyst, and heat) breaks it again:

Polyester + hot NaOH(aq): PET gives the salt disodium benzene-1,4-dicarboxylate and ethane-1,2-diol. This is how some factories chemically recycle PET.
Polyamide + hot HCl(aq): nylon-6,6 gives hexanedioic acid and the salt of the amine, $\mathrm{ClH_3N(CH_2)_6NH_3Cl}$ . With hot NaOH(aq) it gives sodium hexanedioate and hexane-1,6-diamine.
Proteins are hydrolysed to amino acids by enzymes in your gut, or by refluxing with 6 mol dm⁻³ HCl in the laboratory.

Addition polymers have only C–C bonds in the main chain, and these are not hydrolysed. So condensation polymers can in principle be broken down, and some (PLA, proteins) are biodegradable. In practice PET still survives for decades outdoors, because water gets into its tightly packed, water-repelling chains only very slowly.

Table 2.1: Addition and condensation polymerisation compared

Feature	Addition	Condensation
monomer	has a C=C double bond	has two functional groups (–OH, –COOH, –COCl, –NH₂)
other product	none	small molecule: $\mathrm{H_2O}$ or HCl
atom economy	100 %	less than 100 %
main chain	carbon atoms only	contains O or N in ester or amide links
growth	chain growth, through radicals (or ions)	step growth
hydrolysis	not hydrolysed	hydrolysed at the links by acid or alkali
examples	PE, PP, PVC, PTFE, polystyrene	PET, PLA, nylon-6,6, Kevlar, proteins

Worked example

Problem. (a) Write the repeat unit of nylon-6,6. (b) What mass of water is eliminated when 1.00 kg of nylon-6,6 is made from hexanedioic acid and hexane-1,6-diamine? Assume the chains are long. ( $M_{r}$ : repeat unit $\mathrm{C_{12}H_{22}N_2O_2}$ = 226.3; $\mathrm{H_2O}$ = 18.02)

Solution. (a) $\mathrm{\bigl[{-}CO(CH_2)_4CO{-}NH(CH_2)_6NH{-}\bigr]_{\mathit{n}}}$

(b) Each repeat unit contains two amide links, and each link released one water molecule. For long chains, $(2n-1) \approx 2n$ :

$n(\text{repeat units}) = \frac{1000}{226.3} = 4.419\,\mathrm{mol} \qquad n(\mathrm{H_2O}) = 2 \times 4.419 = 8.838\,\mathrm{mol}$

$m(\mathrm{H_2O}) = 8.838 \times 18.02 = 159\,\mathrm{g}$

Answer: about 159 g of water (so about 1.16 kg of monomers are needed for 1.00 kg of nylon).

Worked example

Problem. A polymer used for carpet fibre contains the section

$\mathrm{-O{-}CH_2CH_2CH_2{-}O{-}CO{-}C_6H_4{-}CO{-}O{-}CH_2CH_2CH_2{-}O-}$

(a) What type of polymer is it? (b) Name its two monomers. (c) Give the products when it is boiled with excess NaOH(aq).

Solution. (a) The main chain contains $\mathrm{-O{-}CO-}$ groups: it is a polyester, a condensation polymer. (b) Split each ester link and add back OH to the carbonyl carbon and H to the oxygen. The two parts are $\mathrm{HO{-}CH_2CH_2CH_2{-}OH}$ , propane-1,3-diol, and $\mathrm{HOOC{-}C_6H_4{-}COOH}$ , benzene-1,4-dicarboxylic acid. (c) Alkaline hydrolysis gives propane-1,3-diol and the salt disodium benzene-1,4-dicarboxylate, $\mathrm{NaOOC{-}C_6H_4{-}COONa}$ (the acid is neutralised by the excess alkali).

Warning

Common mistakes. (1) Drawing the repeat unit of an "AA + BB" polymer with only one monomer in it: PET and nylon-6,6 repeat units each contain one piece of EACH monomer. (2) Leaving an OH or H on the ends inside the brackets: those atoms left as water. (3) Writing $\mathrm{-CO{-}O-}$ where the chain needs $\mathrm{-CO{-}NH-}$ . (4) Saying that nylon's strength comes from covalent cross-links: it comes from hydrogen bonds between separate chains.

Safety in the nylon rope trick. Hexanedioyl (or decanedioyl) dichloride is corrosive and releases HCl; hexane-1,6-diamine irritates the skin and eyes; cyclohexane is flammable. Work in a fume cupboard with gloves and goggles, no flames, and wash the nylon thread with water before touching it.

Tip

To get monomers from a condensation polymer, "cut and add water": cut every ester or amide link between C=O and O (or N), then put OH on the carbonyl carbon and H on the O or N. The pieces you get are the monomers.

Chemistry Explained: Polyesters and polyamides (A-level). Watch how the repeat units of PET and nylon-6,6 are built, how to find the monomers from a repeat unit, and how acid and alkaline hydrolysis break the links.

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Practice

State two differences between addition and condensation polymerisation.
Name the link formed and the small molecule eliminated when (a) a diol reacts with a diacid, (b) a diamine reacts with a diacyl dichloride.
Draw the repeat unit of the polyester formed from ethane-1,2-diol and hexanedioic acid.
Nylon-6,10 is made from hexane-1,6-diamine and decanedioyl dichloride, $\mathrm{ClOC(CH_2)_8COCl}$ . Write its repeat unit, and name the small molecule eliminated.
Explain how 2-hydroxypropanoic acid alone can form a polymer, and draw the repeat unit of PLA.
Give the products of boiling nylon-6,6 with dilute hydrochloric acid.
A sample of nylon-6,6 has an average $M_{r}$ of $1.5 \times 10^{4}$ . Calculate its average degree of polymerisation. ( $M_{r}$ of repeat unit = 226.3)
Kevlar and nylon-6,6 are both polyamides. Explain why Kevlar has a much higher tensile strength.

Show answersHide answers

Any two: addition monomers have C=C, condensation monomers have two functional groups; addition gives no other product, condensation eliminates $\mathrm{H_2O}$ or HCl; addition polymers have a carbon-only main chain, condensation polymers have O or N links; condensation polymers can be hydrolysed.
(a) Ester link, water. (b) Amide link, hydrogen chloride.
$\mathrm{\bigl[{-}O{-}CH_2CH_2{-}O{-}CO(CH_2)_4CO{-}\bigr]_{\mathit{n}}}$
$\mathrm{\bigl[{-}NH(CH_2)_6NH{-}CO(CH_2)_8CO{-}\bigr]_{\mathit{n}}}$ ; hydrogen chloride, HCl.
Each molecule has an –OH group and a –COOH group, so the –OH of one molecule forms an ester link with the –COOH of the next, again and again. Repeat unit: $\mathrm{\bigl[{-}O{-}CH(CH_3){-}CO{-}\bigr]_{\mathit{n}}}$ .
Hexanedioic acid, $\mathrm{HOOC(CH_2)_4COOH}$ , and hexane-1,6-diammonium chloride, $\mathrm{ClH_3N(CH_2)_6NH_3Cl}$ (the amine is protonated by the acid).
$n = \frac{1.5 \times 10^{4}}{226.3} = 66$ (2 significant figures).
Kevlar's benzene rings make its chains rigid and straight, so they pack parallel and very closely, with a hydrogen bond at almost every amide group all along the chains (plus attractions between the stacked rings). Nylon's flexible $\mathrm{(CH_2)_n}$ sections let the chains coil and pack less regularly, so fewer hydrogen bonds act along a given length.

Sources and credits

Image: Nylon-6-6.png by Capaccio, via Wikimedia Commons, licensed under CC BY-SA 3.0.
Video: polyesters and polyamides, A level Chemistry OCR A 6.2.3, Chemistry Explained, embedded from YouTube.
The text, diagrams, table, worked examples and Practice items were written and drawn by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1A bottle and a shirt from the same polymer

2What makes a condensation polymer

3Polyesters

4Polyamides

5Hydrolysis: undoing the link

A bottle and a shirt from the same polymer

What makes a condensation polymer

Polyesters

Polyamides

Hydrolysis: undoing the link