Repeat Unit

What is Repeat Unit?

The smallest section of a polymer chain which repeats along its length, drawn in brackets with a subscript n and with open continuation bonds passing through the brackets at both ends.

This definition is part of the Organic Chemistry chapter in Cambridge IGCSE Chemistry.

Concepts related to Repeat Unit

• monomer
• continuation bonds
• linkage

Common mistakes with Repeat Unit

• “The repeat unit of poly(ethene) is [CH2CH2]n.” Supplement 11.8.6 why it fails Written that way, the unit looks like a small closed molecule. A repeat unit is a section cut out of a much longer chain. correct Draw open continuation bonds passing through the brackets on both sides, to show that the chain carries on. Taught in Lesson 11.8C.
• 14. “The repeat unit of poly(ethene) is [CH2CH2]n.” Supplement 11.8.6 defect Missing continuation bonds. why Without bonds crossing the brackets, the unit reads as a small closed molecule rather than a section cut from a long chain. repair Draw a bond running out of each bracket: –[–CH2–CH2–]n–. transfer Draw the repeat unit of poly(chloroethene) and mark both continuation bonds.
• 15. “The repeat unit of poly(ethene) is –[–CH2=CH2–]n–.” Supplement 11.8.7 defect The double bond kept after polymerisation. why Opening the C=C is the reaction. If it is still there, nothing has happened — and each backbone carbon would need five bonds: 2 hydrogens, a double bond counting 2, and one continuation bond. repair Change the double line to a single line. Count the bonds on one backbone carbon: it must total four. transfer Take the repeat unit –[–CH2–CHCl–]n– back to its monomer and say what you had to add.
• 16. “The repeat unit of poly(propene) is –[–CH3–CH2–]n–.” Supplement 11.8.7 defect A substituent moved onto the wrong backbone carbon. why In propene the CH3 hangs off the second double-bond carbon. Putting it into the backbone gives a carbon with five bonds. repair –[–CH2–CH(CH3)–]n–. Mark the two double-bond carbons before redrawing, and move nothing else. transfer Deduce the repeat unit of the polymer made from CH2=CHF, keeping the F on its own carbon.