Identifying addition polymers and their monomers
Organic chemistry • Synthetic and natural polymers
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Definition of addition polymer and addition polymerisation
Addition polymerisation is a process in which identical unsaturated monomers join together to form a long chain (polymer) with no small molecules produced. The monomer must contain a carbon–carbon double bond (C=C). The C=C bond opens and each carbon forms two new single bonds to neighbouring monomer units, creating a continuous carbon backbone. The polymer repeat unit corresponds to the monomer after the C=C bond breaks and the two carbons gain extra single bonds. The overall composition of the polymer chain equals many repeated copies of this modified monomer unit.
Recognising monomers from diagrams
Monomers for addition polymers show a clear C=C bond in structural formulae, for example CH2=CH2, CH2=CHCl or CH2=C(CH3)2. Presence of the C=C functional group is the primary indicator that the monomer can undergo addition polymerisation. Skeletal or displayed structures that include a double bond between two carbon atoms identify potential addition monomers. Absence of a C=C bond means the monomer cannot form an addition polymer by the simple opening-of-double-bond mechanism.
Recognising polymers from diagrams
Addition polymers appear as long chains of carbon atoms or as a repeat unit shown in brackets with an subscript ‘n’, for example [-CH2-CH2-]n for poly(ethene). The repeat unit matches the monomer after removal of the double bond and addition of single bonds to connect adjacent units. Polymer diagrams often omit terminal groups and focus on the repeating segment. A bracketed repeat unit with repeating single bonds and the label n indicates a polymer rather than a discrete molecule.
Cause → effect: C=C in monomer → polymer repeat unit
The presence of a C=C bond in the monomer causes the carbon atoms to be available for further bonding. During addition polymerisation the C=C bond breaks and each carbon forms new bonds to neighbouring monomer carbons, so the effect is formation of single-bonded carbon–carbon chains as the polymer backbone. The chemical cause (breaking of the double bond) explains the visual effect in diagrams (disappearance of C=C in the repeat unit and appearance of single C–C links). No small molecules form because atoms from the monomer stay within the polymer chain.
Limiting factors and distinguishing from condensation polymers
Addition polymerisation requires monomers with C=C bonds and proceeds without elimination of small molecules. If a polymer-forming reaction produces small molecules such as water, the polymerisation is condensation type and not addition. Some natural polymers and many condensation polymers (for example proteins and polyesters) do not show C=C in their monomers and produce small molecules during formation. Presence of C=C and absence of by-product formation together identify addition polymer systems.
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