Chemistry A2 · Polymerisation. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
In condensation polymerisation, monomers with two functional groups join end to end. Each time a link forms, a small molecule is lost: H2O from a carboxylic acid, or HCl from an acyl chloride.
A polyester has ester links, –CO–O–. It forms from a diol with a dicarboxylic acid or dioyl chloride, or from one hydroxycarboxylic acid. A polyamide has amide links, –CO–NH–. It forms from a diamine with a dicarboxylic acid or dioyl chloride, from one aminocarboxylic acid, or from amino acids (giving polypeptides and proteins).
To find the monomers from a chain, break every ester or amide link at the C–O or C–N bond. Add –OH to each C=O and –H to each O or N.
Students subtract only one water molecule for a repeat unit made from two monomers. Each repeat unit contains two links, so two molecules of H2O (or HCl) are lost for every repeat unit.
Answer: –O–(CH2)3–O–CO–(CH2)2–CO–, Mr 158 (a polyester)
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