Cambridge International AS & A Level Chemistry 9701 · A Level topic 35

Polymerisation (A Level)

What this chapter covers35.1–35.3

Plastic bottles, the fibres of a fleece, nylon rope, the dissolvable stitches used in surgery and the proteins in every cell are all polymers — very large molecules built from many small repeating units. At AS Level you met addition polymers such as poly(ethene), formed when alkene molecules add together through their C=C bonds. This chapter introduces the second great family: condensation polymers, in which monomers join through reactions of functional groups and a small molecule is lost at each link.

The two condensation polymers in the syllabus are polyesters and polyamides. Their linkages are the ester and amide groups of chapters 33 and 34, formed in exactly the same way, but repeated thousands of times along a chain. Because those linkages can be hydrolysed, polyesters and polyamides can be broken down again — a property that addition polymers lack, and one with large consequences for the environment.

What topic 35 asks you to do

35.1 Condensation polymerisation — describe the formation of polyesters (a diol with a dicarboxylic acid or dioyl chloride; a hydroxycarboxylic acid) and polyamides (a diamine with a dicarboxylic acid or dioyl chloride; an aminocarboxylic acid; amino acids); deduce the repeat unit from given monomers; identify the monomers in a given section of polymer.

35.2 Predicting the type of polymerisation — predict the type of polymerisation for given monomers, and deduce it from a section of polymer.

35.3 Degradable polymers — recognise that poly(alkenes) are chemically inert and difficult to biodegrade, that some polymers are degraded by light, and that polyesters and polyamides are biodegradable by acidic and alkaline hydrolysis.

What you are assumed to know already

Addition polymerisation of alkenes; repeat units of polyalkenes (topic 14).
Esters and their hydrolysis (topic 19); acyl chlorides (topic 33).
Amides, amino acids and peptide bonds (topic 34).

Addition and condensation compared35.1

In addition polymerisation the monomer contains a C=C double bond. The π bond breaks and the monomers join end to end; nothing else is formed, and the backbone of the polymer consists entirely of C–C single bonds. In condensation polymerisation each monomer has two reactive functional groups. A group on one monomer reacts with a group on the next, forming a covalent link and eliminating a small molecule — water or hydrogen chloride. Because every monomer has two reactive groups, the chain can keep growing in both directions.

Figure 35.1 Addition and condensation polymerisation. In a condensation polymer the linkages (ester or amide) are part of the backbone, and a small molecule is lost as each link forms.

A repeat unit is the smallest section of the chain that, repeated, gives the whole polymer. It is drawn in square brackets with a subscript n, and with continuation (trailing) bonds passing through the brackets to show where it joins its neighbours. When a question asks for the linkage to be "displayed", every bond in the ester or amide group must be shown, including C=O and N–H.

Polyesters35.1.1

From a diol and a dicarboxylic acid

A diol has an –OH group at each end; a dicarboxylic acid has a –COOH group at each end. Each –OH reacts with a –COOH to form an ester link, –O–CO–, and water is lost. The best-known example, poly(ethylene terephthalate) (PET), used for drinks bottles and polyester fibres, is made from ethane-1,2-diol and benzene-1,4-dicarboxylic acid:

n HOCH₂CH₂OH + n HOOC–C₆H₄–COOH → [–OCH₂CH₂O–CO–C₆H₄–CO–]_n + 2n H₂O

If the dioyl chloride (ClOC–C₆H₄–COCl) is used instead of the acid, the reaction is faster and HCl is lost instead of water — the same difference as between making an ester from an acid and from an acyl chloride.

From a hydroxycarboxylic acid

A single monomer with an –OH group at one end and a –COOH group at the other can polymerise on its own. 2-Hydroxypropanoic acid (lactic acid) gives poly(lactic acid), a biodegradable polyester used in packaging and dissolvable surgical stitches:

n HOCH(CH₃)COOH → [–O–CH(CH₃)–CO–]_n + n H₂O

Figure 35.2 Polyesters from two monomers and from one. The ester link forms between –OH and –COOH (or –COCl).

AnimationHow polyesters form

A carboxylic acid and an alcohol form an ester; a dicarboxylic acid and a diol form a polyester such as PET.

Polyamides35.1.2

From a diamine and a dicarboxylic acid or dioyl chloride

Each –NH₂ group reacts with a –COOH (or –COCl) group to form an amide link, –NH–CO–. Nylon-6,6 is made from hexane-1,6-diamine and hexanedioic acid, or, more readily in the laboratory, hexanedioyl chloride:

n H₂N(CH₂)₆NH₂ + n ClOC(CH₂)₄COCl → [–NH(CH₂)₆NH–CO(CH₂)₄CO–]_n + 2n HCl

The "6,6" records the six carbon atoms in each monomer. Aromatic polyamides such as Kevlar, made from benzene-1,4-dioyl chloride and benzene-1,4-diamine, are exceptionally strong, because their flat, rigid chains line up and are held together by many hydrogen bonds between N–H and C=O groups on neighbouring chains.

From an aminocarboxylic acid

A monomer with –NH₂ at one end and –COOH (or –COCl) at the other polymerises on its own. 6-Aminohexanoic acid gives nylon-6:

n H₂N(CH₂)₅COOH → [–NH(CH₂)₅CO–]_n + n H₂O

From amino acids

Proteins are polyamides formed from amino acids; the amide links are the peptide bonds of chapter 34. The repeat unit of a polypeptide made from one amino acid is –NH–CH(R)–CO–. Two different amino acids can also form a copolymer, for example alanine with 4-aminobutanoic acid.

Figure 35.3 Three routes to polyamides. The amide link forms between –NH₂ and –COOH (or –COCl).

Drawing repeat units that score

Show the link correctly: an ester is –CO–O–, an amide –CO–NH–. "–COO–NH–" is not a linkage.
Include every carbon of each monomer; count them against the monomer formula.
Every carbon has four bonds — no trivalent carbon atoms.
Draw exactly the number of repeat units asked for, with continuation bonds; for a dipeptide show terminal –NH₂ and –COOH instead.
In skeletal formulae a continuation bond can look like a methyl group; structural or displayed formulae avoid the ambiguity.

From monomers to repeat unit, and back35.1.3, 35.1.4

Monomers → repeat unit. Remove H from one reacting group and OH (or Cl) from the other, join what remains through the new link, and put brackets around one complete unit containing one residue of each monomer.

Polymer → monomers. Find each ester or amide link and break it between the carbonyl carbon and the O or N. Add –OH to the carbonyl carbon (giving –COOH) and –H to the oxygen or nitrogen (giving –OH or –NH₂). The fragments that repeat are the monomers — two different ones alternating if the polymer was made from two monomers, or one if it was made from a hydroxy- or amino-acid.

Figure 35.4 Working back from a polymer section to its monomers. This polyester is made from two different hydroxycarboxylic acids.

Worked example 35.1 · A polyester with a given number of carbon atoms

Problem	Suggest a diol and a dicarboxylic acid that give a polyester with six carbon atoms in its repeat unit, and draw the repeat unit.
Reasoning	The repeat unit contains all the carbon atoms of one diol and one diacid. Any pair whose carbons total six will do, for example a C₂ diol with a C₄ diacid.
Answer	Ethane-1,2-diol, HOCH₂CH₂OH, and butanedioic acid, HOOCCH₂CH₂COOH. Repeat unit: [–OCH₂CH₂O–CO–CH₂CH₂–CO–]_n.
Check	Name the monomers correctly: ethane-1,2-diol, not "diethanol"; butanedioic acid, not "dibutanoic acid".

Loading the model…

Quick check 35.1

Give the repeat unit of the polyester from propane-1,3-diol and benzene-1,4-dicarboxylic acid.
answer
[–O(CH₂)₃O–CO–C₆H₄–CO–]_n
What small molecule is lost when hexane-1,6-diamine reacts with hexanedioyl chloride?
answer
HCl.
Draw the monomer of the polyamide [–NH(CH₂)₃CO–]_n.
answer
H₂N(CH₂)₃COOH, 4-aminobutanoic acid.
Why does a condensation monomer need two reactive groups?
answer
So that after one group has reacted, the other can react with a further monomer and the chain keeps growing.

Past-paper practice · Set 35A · Polyesters and polyamides: repeat units and monomers

Examination questions on this part of the unit. Try each one on paper before opening the answer.

Question 35A.1[2]

Answer and marking guidance

Repeat unit –CO–CH₂CH₂–CO–NH–(CH₂)₆–NH– with trailing bonds at each end: M1 the amide linkage displayed correctly (C=O and N–H shown) ✔; M2 the rest of the repeat unit correct, including the trailing bonds ✔. Examiner insight: this was answered well.

Question 35A.2[5]

Answer and marking guidance

(c) the two hydroxycarboxylic acids: HOCH₂COOH ✔ and HOCH(CH₃)COOH ✔. (d) poly(serine) polypeptide: M1 the peptide linkage displayed, with a saturated carbon on each side ✔; M2 the rest of the structure — two repeat units, each carrying the –CH₂OH side chain — with continuation bonds ✔. (e) addition polymers do not hydrolyse, or condensation polymers can be hydrolysed ✔. Examiner insight: (c) was answered correctly by most; some omitted the hydrogen atoms of both –OH groups and drew only fragments, and ethanoic and propanoic acids were common wrong answers. (d) discriminated well; common errors were a trivalent carbon or an incorrect linkage such as –COO–NH–. In (e) most stated correctly that condensation polymers can be hydrolysed.

Question 35A.3[5]

Answer and marking guidance

(i) any structure containing one –COOH (or –COCl) group and one –NH₂ group in the same molecule, e.g. H₂NCH₂COOH ✔. (ii) HOCH₂CH₂OH ✔, ethane-1,2-diol ✔; HO₂CCO₂H or ClOCCOCl ✔, ethanedioic acid or ethanedioyl chloride ✔ (the repeat unit –OCH₂CH₂O–COCO– has four carbon atoms). Examiner insight: (i) was usually correct. (ii) was done poorly and few gained full credit: ethanoic acid and ethanol were regularly seen, and where the structures were right the names were often wrong, for example "diethanoic acid" and "diethanol".

Question 35A.4[4]

Parts (b)(i)–(ii) of this question are in chapter 34.

Answer and marking guidance

(iv) the middle amide group displayed ✔; the rest of the structure correct: –NH–CH(CH₃)–CO–NH–(CH₂)₃–CO– with trailing bonds ✔. (v) condensation ✔. (vi) C is biodegradable / easily hydrolysed ✔. Examiner insight: in (iv) common errors were an incorrect linkage such as –COO–NH–, a trivalent carbon, or a missing carbon atom. (v) was usually known; "addition" was the common error. (vi) was well known.

Question 35A.5[4]

Answer and marking guidance

(a) all three correct for [1]: diol + dicarboxylic acid — condensation; dioyl chloride + diol — condensation; the two alkenes — addition ✔. (b)(i) amide links displayed correctly (–CO–NH–) ✔; three alanine residues only, each with its CH₃ side chain ✔; one repeat unit, –NH–CH(CH₃)–CO–, identified, with continuation bonds ✔. Examiner insight: (a) was answered correctly by most. (b)(i) discriminated well; common errors were not identifying the repeat unit, a trivalent carbon, a section with only one or two monomer residues, and an incorrect –COO–NH– linkage.

Question 35A.6[3]

Answer and marking guidance

(i) a dicarboxylic acid or a dioyl chloride, e.g. hexanedioic acid or hexanedioyl chloride ✔. (ii) a section with two residues of each monomer and trailing bonds, the repeat unit identified, and the amide link displayed with C=O ✔✔ (two points 1 mark, four points 2 marks). Examiner insight: (i) discriminated well. Those who did not name a specific compound in (i) found (ii) very difficult; when a suitable dioic acid had been given, full credit was often awarded.

Examiner's overall observation · Condensation polymers

Answered well: drawing the repeat unit of a polyamide from a named diamine and dicarboxylic acid; identifying the two hydroxy-acid monomers of a polyester section; knowing that a condensation polymer can be hydrolysed.

Found difficult: choosing two monomers that give a polyester with a stated number of carbon atoms, and naming them; drawing a polymer section with the requested number of residues and one repeat unit clearly identified; the second monomer for a polyamide when only one was given.

Recurring errors: incorrect linkages such as –COO–NH–; a trivalent carbon or a missing carbon atom; too few monomer residues; not identifying the repeat unit; fragments without the H atoms of the –OH groups; ethanoic and propanoic acids as monomers; "diethanoic acid" and "diethanol" as names; "addition" as the type of polymerisation for a polyamide.

What successful answers did: counted atoms in the monomers against the repeat unit, displayed the full ester or amide link, and marked the repeat unit clearly with its continuation bonds.

Predicting the type of polymerisation35.2.1, 35.2.2

The type of polymerisation is decided by the functional groups of the monomers.

Monomers containing a C=C double bond (and no pair of reacting groups) undergo addition polymerisation, giving a polyalkene. Two different alkenes give an addition copolymer, in which both units appear in the chain.
Monomers with two reactive groups that can react with each other — –OH with –COOH or –COCl, or –NH₂ with –COOH or –COCl — undergo condensation polymerisation, giving a polyester or a polyamide.

Figure 35.5 Deciding the type of polymerisation from the monomers' functional groups.

Working from a polymer section, look at the backbone. A chain made only of carbon atoms (C–C bonds), with side groups, is an addition polymer; the monomer is found by putting back a C=C bond between each pair of backbone carbons in the repeat unit. A chain containing ester or amide links is a condensation polymer.

Table 35.1 Recognising the two types of polymerisation.
	addition	condensation
functional group in monomer	C=C	two of –OH, –COOH, –COCl, –NH₂
number of products	one — the polymer	two — the polymer and H₂O or HCl
backbone	C–C only	contains –CO–O– or –CO–NH– links
repeat unit	two carbon atoms of the original C=C	one residue of each monomer
examples	poly(ethene), poly(chloroethene), poly(phenylethene)	PET, poly(lactic acid), nylon-6,6, nylon-6, Kevlar, proteins

AnimationWhat type of polymer is it?

Classify each structure as an addition polymer, a polyester or a polyamide.

AnimationWhich polymer forms from these monomers?

Match monomers to the polymers they form.

Some monomers contain both a C=C bond and a pair of groups that can condense. Such a monomer can form either an addition polymer or a condensation polymer, depending on the conditions. Fumaric acid, HOOC–CH=CH–COOH, forms the addition polymer [–CH(COOH)–CH(COOH)–]_n, or a polyester with a diol in which the C=C bond survives in every repeat unit. Likewise CH₂=CHCH(NH₂)COOH gives an addition polymer through its C=C bond, or a polyamide through its –NH₂ and –COOH groups.

Worked example 35.2 · Addition or condensation?

Problem	For each pair, name the type of polymerisation: (a) HO(CH₂)₃OH and ClOC(CH₂)₂COCl; (b) CH₂=CHCl and CH₂=CHCN; (c) H₂N(CH₂)₄NH₂ and HOOC–C₆H₄–COOH.
Reasoning	Look for C=C, then for pairs of groups that react together.
Answer	(a) condensation — a polyester, losing HCl; (b) addition — a copolymer; (c) condensation — a polyamide, losing H₂O.
Check	Every condensation pair has one "acid" monomer (–COOH or –COCl) and one "–OH or –NH₂" monomer, or a single monomer carrying one of each.

Degradable polymers35.3.1–35.3.3

Why polyalkenes persist

The backbone of a polyalkene consists only of C–C bonds, with C–H (and sometimes C–Cl) bonds to the side. C–C and C–H bonds are strong and almost non-polar, so there is no δ+ site for water, acids, alkalis or enzymes to attack. Polyalkenes are therefore chemically inert: this makes them useful for containers and packaging, but it also means they biodegrade extremely slowly and accumulate in landfill and in the oceans. Their strength and inertness — not "strong C=C bonds", which a polyalkene no longer contains — is the reason.

AnimationCommon addition polymers

Poly(ethene), poly(chloroethene), poly(phenylethene) and poly(tetrafluoroethene): their structures and uses.

Photodegradable polymers

Some polymers are designed to break down in sunlight. Absorption of ultraviolet light breaks bonds in the chain, which fragments into smaller pieces. Such photodegradable polymers are useful for items that are likely to be discarded outdoors, although the process needs light and so does not occur once the plastic is buried.

Why polyesters and polyamides biodegrade

Ester and amide links contain polar C=O and C–O or C–N bonds, with an electron-deficient carbonyl carbon. They can be hydrolysed — by acids, by alkalis, or by enzymes in living organisms — breaking the chain back into its monomers (or their salts). Condensation polymers are therefore biodegradable, and can be recycled chemically by hydrolysing them to monomers that are polymerised again.

Acid hydrolysis of a polyamide gives the dicarboxylic acid and the diammonium salt of the diamine.
Alkaline hydrolysis of a polyester gives the diol and the salt of the dicarboxylic acid.

Figure 35.6 Why polyalkenes persist while polyesters and polyamides can be broken down by hydrolysis.

AnimationAddition or condensation polymer?

Sort each statement: does it describe addition polymers or condensation polymers?

A complete comparison needs both halves

To explain why a condensation polymer biodegrades more readily than a polyalkene, state that the ester or amide links can be hydrolysed, and that the non-polar C–C bonds of the polyalkene cannot. "Bacteria break it down" or "it decomposes in acid" is not enough — name hydrolysis.

Quick check 35.2

Name the type of polymerisation that produces a polymer whose backbone contains only carbon atoms.
answer
Addition.
Draw the monomer of [–CH₂–CH(CN)–]_n.
answer
CH₂=CHCN (propenenitrile).
Give two processes by which some polymers degrade in the environment.
answer
Hydrolysis (by acid, alkali or enzymes); the action of (UV) light.
Explain why nylon is biodegradable but poly(propene) is not.
answer
Nylon's amide links are polar and can be hydrolysed; poly(propene) has only non-polar C–C and C–H bonds, which cannot be hydrolysed.

Past-paper practice · Set 35B · Types of polymerisation and degradable polymers

Examination questions on this part of the unit. Try each one on paper before opening the answer.

Question 35B.1[5]

Answer and marking guidance

(a) CO₂ and H₂O ✔. (b)(i) the addition repeat unit –CH(CO₂H)–CH(CO₂H)– with trailing bonds ✔. (ii) M1 an ester group formed between the diol and a –COOH group (or between fumaric acid and an –OH) ✔; M2 the rest of the repeat unit, –OCH₂CH₂O–CO–CH=CH–CO–, including the C=C and the trailing bonds ✔. (iii) the C–C bonds of polyalkenes are non-polar and cannot be hydrolysed, whereas polyesters can be broken down by hydrolysis ✔. Examiner insight: in (a) many knew of oxidative cleavage to ethanedioic acid but did not oxidise it further to CO₂ and water. (b)(i) was generally well answered. In (b)(ii) errors included omitting the C=C bond or drawing it as C–C, omitting the H atoms on the C=C, and missing a carbon of the ethane-1,2-diol. (b)(iii) was well known.

Question 35B.2[4]

Part (a) of the question introduces compound H, CH₂=CHCH(NH₂)COOH; only its formula is needed here.

Answer and marking guidance

(i) addition repeat unit: –CH₂–CH[CH(NH₂)COOH]– with trailing bonds ✔. (ii) condensation polymer: two repeat units of –NH–CH(CH=CH₂)–CO– with the amide linkage displayed correctly ✔ and the rest of the structure correct ✔. (iii) condensation polymers can be hydrolysed ✔. Examiner insight: the addition repeat unit was usually correct. Most gave a correct amide link in (ii), but care was needed to make the rest of the structure correct. Some good answers were seen to (iii).

Question 35B.3[4]

Answer and marking guidance

(i) a section with two or more repeat units; correct orientation of the groups on all four rings (1,4 for the acid residue, 1,3 for the amine residue); trailing bonds; all amide links correct — two points 1 mark, all four 2 marks ✔✔. (ii) polyamide AND condensation ✔. (iii) yes, it can be hydrolysed ✔. Examiner insight: (i) was answered well, though some drew incorrect linkages or ring orientations or ignored the instruction to include two repeat units. In (ii) many placed more than one tick, and no credit could be given. Many knew that a polyamide is biodegradable; fewer knew that this is due to hydrolysis.

Question 35B.4[3]

Answer and marking guidance

(i) M1 the C–C backbone with the correct side groups (CN and CO₂CH₃ on alternate carbons) ✔; M2 continuation bonds and two repeat units: –CH₂–C(CN)(CO₂CH₃)–CH₂–C(CN)(CO₂CH₃)– ✔. (ii) addition ✔. Examiner insight: many gained credit in (i), but a significant number missed out part of the structure. Most named addition correctly.

Question 35B.5[5]

Answer and marking guidance

(c) a length of chain containing one propene and one phenylethene unit, –CH₂–CH(CH₃)–CH₂–CH(C₆H₅)– ✔, with continuation bonds ✔. (d)(i) the C–C bonds are non-polar (have no dipole), so cannot be hydrolysed ✔. (ii) hydrolysis using acid, base or enzymes ✔; the action of UV light ✔. Examiner insight: (c) often gained full credit. (d)(i) was not well known: some wrongly referred to strong C=C bonds or strong van der Waals forces. (d)(ii) was usually known, but some did not refer to hydrolysis or light and simply stated decomposition by bacteria or acid.

Question 35B.6[3]

Answer and marking guidance

(c)(ii) two repeat units of –NH–(CH₂)₅–CO–: M1 the amide link displayed ✔; M2 the rest of the structure ✔. (e) the C–C bonds of polyalkenes are non-polar and cannot be hydrolysed, while polyamides can be broken down by hydrolysis ✔. Examiner insight: most knew what two repeat units meant and the amide link was rarely wrong; candidates wisely avoided skeletal formulae. In (e) many stated that polyamides can be hydrolysed, but far fewer that polyalkenes cannot — both statements were required.

Question 35B.7[1]

Answer and marking guidance

N contains an ester linkage, which can be hydrolysed ✔. Examiner insight: it was rare to see an answer recognising that an ester bond can be hydrolysed.

Examiner's overall observation · Predicting polymerisation and degradability

Answered well: naming addition or condensation for a given set of monomers; drawing addition repeat units, including copolymers; knowing that polyesters and polyamides are biodegradable; naming hydrolysis and light as ways polymers degrade.

Found difficult: explaining why polyalkenes biodegrade slowly in terms of their structure; recognising that biodegradability comes from hydrolysis of the ester or amide link; seeing that an ester bond in an unfamiliar polymer can be hydrolysed; drawing polyester repeat units that keep a C=C bond from the monomer.

Recurring errors: "strong C=C bonds" or "strong van der Waals forces" to explain the inertness of polyalkenes; stating only that polyamides can be hydrolysed without saying polyalkenes cannot; "decomposed by bacteria or acid" without naming hydrolysis; placing more than one tick when one was asked for; leaving out part of an addition polymer's side groups; omitting the C=C or its H atoms, or a carbon of the diol, in a polyester; stopping oxidation of fumaric acid at ethanedioic acid instead of CO₂ and water.

What successful answers did: decided the type of polymerisation from the functional groups, drew every atom of each residue, and explained degradability by the presence or absence of hydrolysable polar links.

Misconceptions and how the topic is assessed35.1–35.3

Table 35.2 Misconceptions in topic 35.
misconception	why it is wrong	correct model	examination consequence
Polyalkenes are inert because of strong C=C bonds.	A polyalkene has no C=C bonds.	Non-polar C–C and C–H bonds cannot be hydrolysed.	Explanation mark lost.
A condensation polymer forms whenever two monomers are used.	Two alkenes give an addition copolymer.	The functional groups decide the type.	Wrong type of polymerisation.
The link in a polyamide is –COO–NH–.	That contains an extra O.	Amide –CO–NH–; ester –CO–O–.	Structure mark lost.
The repeat unit contains one monomer only.	With two monomers, it contains one residue of each.	–O–X–O–CO–Y–CO–, etc.	Incomplete repeat unit.
Hydrolysis products are the polymer fragments with the links still present.	Hydrolysis breaks every link.	Monomers (or their ions/salts, depending on conditions).	Wrong products.

Table 35.3 How topic 35 appears in examination questions.
question family	typical demand	what the answer needs
Repeat unit from monomers	draw one or two repeat units, link displayed	correct link; every carbon; continuation bonds
Monomers from polymer	draw the monomers of a given section	break links; add H and OH; complete –OH, –COOH, –NH₂ groups
Designing monomers	suggest monomers for a polymer with a given feature	specific named compounds; carbon count
Type of polymerisation	name or tick addition / condensation, polyester / polyamide / polyalkene	C=C → addition; paired groups → condensation
Degradability	explain biodegradation; name processes	hydrolysis of ester/amide links; polyalkenes non-polar; light

Self-test35.1–35.3

Ten questions on the whole chapter. Each gives its reason once you answer.

Loading the model…

Definitions to learn35.1–35.3

term	definition
addition polymerisation	monomers containing C=C join together; the polymer is the only product
condensation polymerisation	monomers with two reactive groups join, with the loss of a small molecule such as H₂O or HCl at each link
repeat unit	the smallest part of the polymer chain that, repeated, gives the whole chain
polyester / polyamide	a condensation polymer whose monomers are joined by ester (–CO–O–) / amide (–CO–NH–) links
copolymer	a polymer made from two or more different monomers
biodegradable	able to be broken down in the environment by natural processes, such as hydrolysis

Summary

Condensation polymers form from monomers with two reactive groups; H₂O or HCl is lost at every link.
Polyesters: diol + dicarboxylic acid (or dioyl chloride), or a hydroxycarboxylic acid alone; link –CO–O–.
Polyamides: diamine + dicarboxylic acid (or dioyl chloride), an aminocarboxylic acid alone, or amino acids; link –CO–NH–.
A repeat unit contains one residue of each monomer and is drawn with continuation bonds; monomers are recovered by breaking each link and adding back H and OH.
C=C monomers undergo addition polymerisation; monomers with paired –OH/–NH₂ and –COOH/–COCl undergo condensation.
Polyalkenes are inert and biodegrade very slowly; some polymers are photodegradable; polyesters and polyamides are hydrolysed by acids and alkalis and are biodegradable.

Examination checklist

Can I draw the repeat unit of a polyester or polyamide from any pair of monomers, or from a single monomer, with the link displayed?
Can I identify the monomers in a given section of a condensation or addition polymer?
Can I suggest and name suitable monomers for a polymer with stated features?
Can I predict the type of polymerisation, and the type of polymer, from the monomers or from a polymer section?
Can I explain why polyalkenes persist and polyesters and polyamides biodegrade, and name two degradation processes?

Knowledge organiser

idea	key facts	must-remember distinctions and common errors
Polyester	–OH + –COOH (lose H₂O) or –COCl (lose HCl); link –CO–O–	PET; poly(lactic acid)
Polyamide	–NH₂ + –COOH / –COCl; link –CO–NH–	nylon-6,6; nylon-6; Kevlar; proteins
Repeat unit	one residue of each monomer; [ ]_n; continuation bonds	no –COO–NH–; count carbons
Finding monomers	break C(=O)–O or C(=O)–N; add OH to C=O, H to O or N	complete –OH groups
Type	C=C → addition; paired groups → condensation	two alkenes → addition copolymer
Degradation	ester/amide links hydrolysed (acid, alkali, enzymes); some polymers by light	polyalkene C–C non-polar, cannot be hydrolysed