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Addition And Condensation Polymers

Learn and apply Addition And Condensation Polymers in the published Chemistry course sequence.

  • GCE A-Level H1 Chemistry 8873-2027
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Addition and Condensation Polymers (H1): Orientation

Polymer questions become systematic when you identify the monomer functionality, preserve every substituent and check how the chain continues through the repeat-unit brackets.

H1 8873 scope
  • Recognise the syllabus size threshold for a macromolecule.
  • Distinguish addition from condensation polymerisation and translate between monomers and repeat units.
  • Protein chemistry, material properties and sustainability are developed in the next H1 lessons.

Definitions (Must Know)

  • A monomer is a small molecule that can join repeatedly to form a polymer.
  • A polymer is a macromolecule built from monomers, with average relative molecular mass of at least 1000 or at least 100 repeat units in 8873.
  • An addition polymer forms when unsaturated monomers join without eliminating a small molecule.
  • A condensation polymer forms when bifunctional monomers join and eliminate a small molecule.
  • A repeat unit is the smallest structural segment that repeats along the polymer chain.

Detailed Explanations

A. Addition repeat units

Locate the two alkene carbons, replace the double bond by a single bond and keep every attached group on its original carbon. Draw continuation bonds through the brackets and place n outside.

B. Recovering an addition monomer

Select one backbone repeat, remove the continuation bonds and restore a double bond between the two carbons that came from the alkene. Do not move substituents while doing this.

A diol and dicarboxylic acid form a polyester containing -COO⁻ links. A diamine and dicarboxylic acid form a polyamide containing -CONH⁻ links. Each monomer needs two reactive ends so chain growth can continue.

D. Average size

Estimate the number of repeat units using

average number of repeat units = (average Mᵣ of polymer)/(Mᵣ of repeat unit).

Compare the result with the syllabus threshold rather than assuming every long-looking molecule qualifies.

Worked Examples

Modelled example 1

Recover Chloroethene from Its Repeat Unit

Core

Problem

A repeat unit is [-CH₂-CHCl-]ₙ. Identify the monomer and polymerisation type.
Study the worked solution

  1. Isolate one backbone repeat

    Method

    Remove the continuation bonds and keep chlorine attached to its original carbon.

    Reason

    Recovering the monomer changes the backbone bond but does not move substituents.

    Working

    Backbone fragment: -CH₂-CHCl⁻.

  2. Restore the alkene

    Method

    Form a double bond between the two backbone carbons.

    Reason

    An addition-polymer repeat unit is reversed by restoring the alkene bond that opened during polymerisation.

    Working

    Monomer: CH₂ = CHCl, chloroethene.

  3. Classify the process

    Method

    Name addition polymerisation.

    Reason

    The alkene monomers join without elimination of a small molecule.

    Working

    Chloroethene → poly(chloroethene) by addition polymerisation.

Guided practice 2

Test the Macromolecule Threshold

About 5 min

Problem

A polymer has average relative molecular mass 12 000 and repeat-unit relative mass 120. Estimate its average number of repeat units and interpret the result.

Try this before viewing the solution

Required operation
Average repeat units

Hints

Hint 1: ratio
The whole-chain mass is repeat count multiplied by repeat-unit mass.
Hint 2: threshold
Compare the result with “at least 100 repeat units” and also note the average mass.
View solution step by step

  1. Calculate the count

    Method

    Divide average polymer Mᵣ by repeat-unit Mᵣ.

    Reason

    The ratio estimates how many repeat units contribute to the average chain mass.

    Working

    12000/120 = 100 repeat units.

  2. Interpret the result

    Method

    State that the sample meets the syllabus polymer scale.

    Reason

    It has at least 100 repeat units and its average Mᵣ also exceeds 1000.

    Working

    The sample satisfies both stated scale criteria.

Common misconception 3

A Repeat Unit Must Continue

Find and correct the mistake

Learner drawing

A learner draws the repeat unit of poly(ethene) as [CH₃-CH₃]ₙ with no bonds passing through the brackets. Correct the drawing and explain both errors.

Try this before viewing the solution

Carbon group in repeat
Bracket bonds

View solution step by step

  1. Correct the backbone atoms

    Method

    Use -CH₂-CH₂- rather than an ethane molecule.

    Reason

    Each carbon has two C–C bonds within the continuing polymer chain and therefore two hydrogen atoms.

    Working

    Repeat fragment: -CH₂-CH₂-.

  2. Show continuation

    Method

    Draw bonds through both brackets and place n outside.

    Reason

    The outward bonds show that the displayed fragment repeats on both sides.

    Working

    Correct repeat unit: [-CH₂-CH₂-]ₙ.

Examiner practice 4

Recover Monomers from a Polyamide Segment

4 marks

Examination question

A polyamide contains repeating -NH-R-NH-CO-R'-CO⁻ segments. Identify the two monomer classes, give their general formulae and explain why both are bifunctional. [4 marks]

Try this before viewing the solution

View solution step by step

  1. Recover the nitrogen monomer

    1 mark

    Method

    Restore amino groups to obtain a diamine, H₂N-R-NH₂.

    Reason

    Cutting each amide link returns the nitrogen end to an amino group.

    Working

    Monomer class: diamine.

  2. Recover the acid monomer

    1 mark

    Method

    Restore carboxylic-acid groups to obtain HOOC-R'-COOH.

    Reason

    The carbonyl side of each cut amide link returns to a carboxylic-acid end.

    Working

    Monomer class: dicarboxylic acid.

  3. Explain bifunctionality

    2 marks

    Method

    State that each monomer has two reactive groups and can form a link at both ends.

    Reason

    Two reactive ends allow repeated condensation to extend the chain instead of terminating after one link.

    Working

    Diamine + dicarboxylic acid → polyamide + water.

Challenge 5

Draw the Poly(propene) Repeat Unit

Minimal support

Representation transfer

Starting from propene, CH₂ = CHCH₃, draw the repeat unit of poly(propene) and explain what happens to the double bond and methyl substituent.

Try this before viewing the solution

Backbone bond
Methyl group

Hints

Hint 1: two alkene carbons
Only the two carbons of C=C become the repeating backbone.
Hint 2: complete notation
Keep CH₃ on its original carbon and show continuation bonds through both brackets.
View solution step by step

  1. Open the double bond

    Method

    Convert the two alkene carbons into a saturated backbone fragment.

    Reason

    The former pi bond supplies links between successive monomer units.

    Working

    CH₂ = CH(CH₃) → -CH₂-CH(CH₃)⁻.

  2. Complete the repeat notation

    Method

    Bracket the fragment with two continuation bonds and write n.

    Reason

    The methyl substituent stays on its original backbone carbon.

    Working

    [-CH₂-CH(CH₃)-]ₙ.

Mind Stretchers

Attempt each task before opening its hint.

Mind stretcher 1: Recovering viable polyester monomersExtension

Question. A polymer backbone contains repeated -O-R-O-CO-R'-CO⁻ segments. Identify the two monomer classes and explain why each must have two functional groups.

Show Hint

Cut each ester link and restore the alcohol and carboxylic-acid ends.

Show Answer

The monomers are a diol, HO-R-OH, and a dicarboxylic acid, HOOC-R'-COOH. Two reactive ends are required on each monomer so it can connect on both sides and extend the chain rather than terminate it.

Mind stretcher 2: Reconciling two size criteriaExtension

Question. A sample has average Mᵣ = 9600 and repeat-unit Mᵣ = 96. A second has average Mᵣ = 1200 but only 40 repeat units. Determine which syllabus criterion each sample meets.

Show Hint

The definition uses “at least 1000 or at least 100 repeat units”.

Show Answer

The first sample has 9600/96 = 100 repeat units and also exceeds Mᵣ = 1000, so it meets both criteria. The second exceeds average Mᵣ = 1000 even though it has fewer than 100 repeat units, so it still meets the alternative mass criterion.