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6.6.2Organic Chemistry and Analysis

Polymer Repeat Units — OCR A Level Chemistry A H432 Revision Notes & Topical Questions

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📚 OCR A Level Chemistry A H432 Hub / Topical Revision
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Section 1 of 5Addition Polymer Repeat Units

6.6.2 Polymer Repeat Units

In addition polymerisation the C=C double bond opens, and the repeat unit has the same atoms as the monomer.

Poly(ethene)
  • The monomer CH₂=CH₂ gives the repeat unit –CH₂–CH₂–, shown with a bond extending through each bracket end.
Poly(chloroethene)
  • The monomer CH₂=CHCl gives the repeat unit –CH₂–CHCl–, with the chlorine on one of the two backbone carbons.
Poly(propene)
  • The monomer CH₂=CHCH₃ gives the repeat unit –CH₂–CH(CH₃)–, with the methyl group as a side group.
Rule
  • Only the two carbons of the former double bond lie in the backbone, and all other groups hang off as side groups.

Section 2 of 5Condensation Polymer Repeat Units

Condensation Polymer Repeat Units
Nylon-6,6
  • The repeat unit is –NH(CH₂)₆NHCO(CH₂)₄CO–, containing one diamine part and one diacid part.
PET
  • The repeat unit is –OCH₂CH₂OOC–C₆H₄–CO–, with an ester link at each end of the benzene ring part.
Include both
  • The repeat unit must contain a piece of each monomer, and the bonds at both ends are drawn extending outward.
Small molecule
  • The H₂O or HCl lost on joining is not part of the repeat unit.

Section 3 of 5Deducing Monomers

1
For an addition polymer, select one repeat unit and add a double bond between the two backbone carbons.
2
Then change the bonds at the two ends of the repeat unit into the remaining substituents, such as H atoms.
3
For a polyester or polyamide, find each ester or amide link and cut through it between the carbonyl carbon and the O or N.
4
Add –OH to the carbonyl carbon end (or Cl for a diacyl chloride) and –H to the O or N end.
5
Name the monomers that result, checking that each has a functional group at both ends.

Section 4 of 5Worked Example

Question

  • A sample of poly(chloroethene) has an average relative molecular mass of 125 000.
  • Calculate the number of repeat units in an average chain.

  1. The repeat unit is –CH₂CHCl– with formula C₂H₃Cl.
  2. Mr of C₂H₃Cl = (2 × 12.0) + (3 × 1.0) + 35.5 = 62.5.
  3. Number of repeat units = 125 000 ÷ 62.5 = 2000.

Answer There are about 2000 repeat units per chain.

Section 5 of 5Exam Checklist & Tips

6.6.2 – Key Points for the Exam

  • Repeat unit The smallest section of a polymer chain that repeats along its length.
  • Monomer The small molecule from which the polymer is made.
  • Side group An atom or group attached to the backbone.
  • Backbone The main chain of atoms in a polymer.

Common Mistakes

  • Drawing the C=C double bond in the repeat unit of an addition polymer.
  • Omitting the bonds extending from both ends of the repeat unit.
  • Including the lost water molecule inside a condensation repeat unit.
  • Cutting the polymer chain at the wrong bond when finding the monomers.

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