Addition polymers and repeating units explained

Organic chemistry • Synthetic and natural polymers

Flashcards

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How does the pi bond behave during addition polymerisation?

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The pi bond breaks and its electrons form new sigma bonds to neighbouring monomer units, allowing chain growth.

Key concepts

What you'll likely be quizzed about

Definition of addition polymerisation

Addition polymerisation requires monomer molecules with a carbon–carbon double bond. The pi bond opens and allows each monomer to link head-to-tail with neighbours, forming a long saturated carbon chain. No small molecules (for example water or HCl) form as by-products in pure addition polymerisation.

Drawing polymer formation from an alkene

A diagram begins with the alkene monomer drawn showing the C=C and any side groups. An arrow labeled 'polymerisation' or conditions (heat, pressure, catalyst or initiator) points to a chain with single C–C bonds. The broken double bonds convert into single bonds that connect successive monomer units. Brackets with a subscript n indicate that the central unit repeats many times.

Deriving the repeating unit

The repeating unit is the monomer skeleton after the C=C bond converts into two single bonds to neighbours. Represent the repeating unit in square brackets with linking bonds shown at the left and right, and place an 'n' after the brackets to denote polymer length. The side groups on the monomer appear unchanged in the repeating unit.

Common examples and notation

Ethene forms poly(ethene) with repeating unit [-CH2-CH2-]n. Propene forms poly(propene) with repeating unit [-CH2-CH(CH3)-]n. Chloroethene (vinyl chloride) forms poly(chloroethene) with repeating unit [-CH2-CH(Cl)-]n. Styrene forms poly(styrene) with repeating unit [-CH2-CH(Ph)-]n, where Ph denotes a phenyl ring attached to the CH group.

Limiting factors affecting chain length

Chain length depends on monomer concentration, temperature, pressure, and the type and amount of initiator or catalyst. Termination steps such as radical combination or disproportionation stop chain growth and therefore limit molecular weight. Impurities and chain-transfer agents also reduce average chain length.

Key notes

Important points to keep in mind

Monomer must contain a C=C double bond for addition polymerisation.

The repeating unit equals the monomer structure with the double bond opened and shown inside brackets with 'n'.

Draw the monomer, break the double bond in diagrams, then show new single bonds linking units.

Side groups on the monomer remain attached to the same carbon atoms in the repeating unit.

Brackets and a subscript n denote repetition; dashes show linkage points to the chain.

Chain length depends on monomer concentration, initiator/catalyst, temperature and termination events.

Termination (combination or disproportionation) stops chain growth and limits molecular weight.

Head-to-tail addition gives a regular repeating pattern; head-to-head or tail-to-tail gives irregular placement of substituents.

No small molecules form as by-products in pure addition polymerisation.

Impurities and chain-transfer agents reduce average polymer chain length.