Polymers: addition and condensation polymerisation: WASSCE Chemistry

Plastic buckets, water tanks, nylon fishing nets, polyester school shirts, rubber and even starch and proteins are all polymers: giant molecules built from thousands of small repeating units. This lesson explains the two ways in which small molecules join to make polymers: addition and condensation polymerisation.

Learning objectives

By the end of this lesson you should be able to:

define monomer, polymer, polymerisation and degree of polymerisation
describe addition polymerisation, with examples, equations and repeat units
deduce the monomer from a polymer structure, and the polymer from a monomer
describe condensation polymerisation, with nylon and Terylene as examples
compare addition and condensation polymerisation
name natural polymers and their monomers

Key terms

Definition

A monomer is a small molecule that can join with many others like it. A polymer is a very large molecule (a macromolecule) made of many monomer units joined together. Polymerisation is the process of joining monomers into a polymer.

The degree of polymerisation, n, is the number of monomer units in one polymer molecule:

n = \frac{M_r \text{ of polymer}}{M_r \text{ of monomer}}

\begin{aligned}n &= \frac{M_r \text{ of polymer}}{M_r \text{ of monomer}}\end{aligned}

Addition polymerisation

Ethene molecules join into a long chain of poly(ethene); a dicarboxylic acid and a diol join into a polyester, releasing water at every link

Figure 1. Addition and condensation polymerisation. WASSCE Tutor diagram.

Definition

Addition polymerisation is the joining of many unsaturated monomers (containing C=C) to form a polymer without the loss of any other molecule. The polymer is the only product, and it has the same empirical formula as the monomer.

The C=C double bonds open and the monomers link up into a saturated chain:

n\,\text{CH}_2{=}\text{CH}_2 \rightarrow (\text{–CH}_2\text{–CH}_2\text{–})_n

\begin{gathered}n\,\text{CH}_2{=}\text{CH}_2 \\[6pt] \rightarrow (\text{–CH}_2\text{–CH}_2\text{–})_n\end{gathered}

The part in brackets is the repeat unit. Conditions for poly(ethene): high pressure (about 1,000–2,000 atm) and 200 °C with a trace of oxygen as initiator (low-density poly(ethene)), or low pressure with a Ziegler–Natta catalyst (high-density poly(ethene)).

Monomer	Polymer	Repeat unit	Uses
Ethene, CH₂=CH₂	Poly(ethene), polythene	–CH₂–CH₂–	Plastic bags, bottles, buckets, water tanks
Propene, CH₂=CHCH₃	Poly(propene)	–CH₂–CH(CH₃)–	Ropes, crates, chairs, carpets
Chloroethene, CH₂=CHCl	PVC, poly(chloroethene)	–CH₂–CHCl–	Water pipes, cable insulation, raincoats
Phenylethene (styrene), CH₂=CHC₆H₅	Polystyrene	–CH₂–CH(C₆H₅)–	Packaging, food boxes, insulation
Tetrafluoroethene, CF₂=CF₂	PTFE (Teflon)	–CF₂–CF₂–	Non-stick pans, plumber's tape
Methyl 2-methylpropenoate	Perspex	–	Safety glass, signs, car lights

Finding the monomer from a polymer: take the two-carbon repeat unit and put a double bond between the two carbons. For example, –CH₂–CHCl– comes from CH₂=CHCl.

Condensation polymerisation

Definition

Condensation polymerisation is the joining of monomers, each with two reactive functional groups, with the loss of a small molecule (usually water or HCl) at every link.

Nylon (a polyamide): a diamine reacts with a dicarboxylic acid (or its acid chloride). Amide links, –CO–NH–, the same as peptide bonds, join the units:

\text{dicarboxylic acid} + \text{diamine} \rightarrow \text{nylon} + \text{water}

\begin{gathered}\text{dicarboxylic acid} + \text{diamine} \\[6pt] \rightarrow \text{nylon} + \text{water}\end{gathered}

With HOOC–R–COOH and H₂N–R′–NH₂ as the monomers, the repeat unit is –OC–R–CO–NH–R′–NH–, and two molecules of water are lost for each repeat unit.

Nylon 6,6 is made from hexanedioic acid and 1,6-diaminohexane. Uses: ropes, fishing nets, clothing, tooth brushes, gears.

Terylene (a polyester): a dicarboxylic acid (benzene-1,4-dicarboxylic acid) reacts with a diol (ethane-1,2-diol). Ester links, –CO–O–, join the units. Uses: clothing fibres (often mixed with cotton), bottles (PET), sails.

Feature	Addition polymerisation	Condensation polymerisation
Monomers	Unsaturated (C=C)	Two functional groups each (–COOH, –OH, –NH₂)
By-product	None	Small molecule lost (H₂O or HCl)
Empirical formula	Same as the monomer	Different from the monomers
Links in chain	C–C bonds	Ester or amide links
Examples	Poly(ethene), PVC, polystyrene	Nylon, Terylene, proteins, starch

Natural polymers

Natural polymer	Monomer	Type
Starch, cellulose, glycogen	Glucose	Condensation
Proteins (wool, silk, enzymes)	Amino acids	Condensation (peptide links)
Natural rubber	Isoprene (2-methylbuta-1,3-diene)	Addition
DNA	Nucleotides	Condensation

Worked example

Degree of polymerisation. A sample of PVC has a relative molecular mass of 62,500. How many monomer units does each chain contain? (C = 12, H = 1, Cl = 35.5)

Mᵣ of CH₂=CHCl $= 24 + 3 + 35.5 = 62.5$
$n = 62{,}500 \div 62.5 = 1{,}000$

In Sierra Leone

Fishing communities from Tombo to Shenge use nylon nets and ropes. Nylon, a condensation polymer, is strong, light and does not rot in sea water the way older cotton or raffia nets did. That durability has a downside: lost "ghost nets" keep trapping fish and turtles for years.

Common mistake

In addition polymerisation, no small molecule is lost. If an equation shows water forming, the polymerisation is condensation, not addition.

Key points

Monomers join to form polymers; n = Mᵣ(polymer) ÷ Mᵣ(monomer).
Addition: C=C monomers join with no by-product: ethene → poly(ethene), propene → poly(propene), chloroethene → PVC, styrene → polystyrene.
Condensation: monomers with two functional groups join, losing water at each link: nylon (amide links), Terylene (ester links).
Natural polymers: starch, cellulose (glucose), proteins (amino acids), rubber (isoprene).

WASSCE examination focus

WAEC tests this

Expect to define addition and condensation polymerisation with examples, to draw repeat units and deduce monomers, to write equations for the formation of poly(ethene) and PVC, to calculate degree of polymerisation, and to name natural polymers and their monomers.

Quick check

What is the monomer of PVC?
State one difference between addition and condensation polymerisation.
Name the link that joins the units in nylon.
A poly(ethene) sample has Mᵣ = 56,000. Find n. (C = 12, H = 1)

Answers:

Chloroethene, CH₂=CHCl.
Addition polymerisation gives no by-product; condensation polymerisation loses a small molecule such as water at each link (or: addition monomers have C=C, condensation monomers have two functional groups).
The amide (peptide) link, –CO–NH–.
Mᵣ of C₂H₄ = 28; n = 56,000 ÷ 28 = 2,000.

Polymers: addition and condensation polymerisation

Learning objectives

Key terms

Addition polymerisation

Condensation polymerisation

Natural polymers

Key points

WASSCE examination focus

Quick check

Keep going with polymers and plastics

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