You also need to work backwards: given a polymer or its repeating unit, find the monomer.Method:
1. Identify the repeating unit. For an addition polymer it has two carbon atoms in the main chain.
2. The extension bonds on each side are removed.
3. Put a double bond (C=C) between the two main-chain carbons. Keep all the side groups where they are.Worked example 1: the repeating unit –[CH2–CH(CH3)]–. Removing the extension bonds and adding a C=C between the two backbone carbons gives CH2=CHCH3, propene.Worked example 2: a section of chain –CH2–CHCl–CH2–CHCl–. The pattern CH2–CHCl repeats, so the repeating unit is –CH2–CHCl– and the monomer is CH2=CHCl, chloroethene.Worked example 3: –[CH(CH3)–CH(CH3)]–. Each backbone carbon has an H and a CH3, so the monomer is CH3CH=CHCH3, but-2-ene. Take care not to read it as a four-carbon backbone: the CH3 groups are side groups.Likewise –[CF2–CF2]– comes from CF2=CF2, tetrafluoroethene.A check: the monomer must have exactly the same atoms as the repeating unit, just rearranged with a C=C. Count the atoms of each element in both; if they differ, the repeating unit has been chosen wrongly.
1. Identify the repeating unit. For an addition polymer it has two carbon atoms in the main chain.
2. The extension bonds on each side are removed.
3. Put a double bond (C=C) between the two main-chain carbons. Keep all the side groups where they are.Worked example 1: the repeating unit –[CH2–CH(CH3)]–. Removing the extension bonds and adding a C=C between the two backbone carbons gives CH2=CHCH3, propene.Worked example 2: a section of chain –CH2–CHCl–CH2–CHCl–. The pattern CH2–CHCl repeats, so the repeating unit is –CH2–CHCl– and the monomer is CH2=CHCl, chloroethene.Worked example 3: –[CH(CH3)–CH(CH3)]–. Each backbone carbon has an H and a CH3, so the monomer is CH3CH=CHCH3, but-2-ene. Take care not to read it as a four-carbon backbone: the CH3 groups are side groups.Likewise –[CF2–CF2]– comes from CF2=CF2, tetrafluoroethene.A check: the monomer must have exactly the same atoms as the repeating unit, just rearranged with a C=C. Count the atoms of each element in both; if they differ, the repeating unit has been chosen wrongly.