OCR A-Level Chemistry AS Breadth in chemistry (01), November 2021: Question 20

✅ Correct Answer & Breakdown

The correct option is A.

• Look at the repeat unit's main carbon chain containing two carbon atoms.
• Re-form the carbon-carbon double bond (C=C) between these two backbone carbons.
• Remove the extension bonds breaking out of the square brackets.
• This reveals the monomer structure: CH₂=CH-CH₂OH (prop-2-en-1-ol), which matches option A.

💡 Key Knowledge

• Addition Polymerisation: Monomers must contain a C=C double bond which opens up during polymerisation to form a long saturated carbon backbone.
• The Reverse Process: To find the monomer from a repeat unit, convert the central single bond ( -C-C- ) back into a double bond ( C=C ).
• Do not alter functional groups attached to the side chains (like the -CH₂OH group here), as they do not take part in addition polymerisation.

🧠 Exam Technique

• Systematically check each substituent on the left and right carbon atoms of the repeat unit and ensure they transfer directly onto the carbon atoms of the double bond in your mental structure.
• Cross out incorrect options by checking for extra methyl groups or shifted double bonds that do not match the connectivity of the given repeat unit.

❌ Common Errors

• Incorrect C=C placement: Students sometimes confuse addition polymers with condensation polymers or misidentify which carbon atoms form the backbone.
• Altering side chains: Mistakenly changing or removing functional groups like the -CH₂OH alcohol group.
• Adding extra carbons: Selecting options with extra carbon units (like a propene or butene backbone) due to miscounting the atoms inside the repeat unit brackets.