Polymerisation

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Cambridge International AS & A Level Chemistry (9701)

Polymerisation

The big idea: In addition polymerisation the π bond of each alkene monomer opens and the monomers join into one long saturated chain, with nothing else produced. The same strong, non-polar C–C and C–H bonds that make poly(alkene)s useful also make them hard to get rid of.

Summary — What This Chapter Covers

Addition polymerisation of ethene to poly(ethene) and chloroethene to poly(chloroethene), PVC.
Drawing the repeat unit from a monomer.
Working back from a polymer section to its monomer(s).
Disposal problems: non-biodegradable, and harmful products when burnt.

1. Addition Polymerisation

20.1

Monomers to polymer

In addition polymerisation many unsaturated monomer molecules join together to form a single long-chain molecule, the polymer, and no other product is formed. The C=C π bond in each monomer breaks and the electrons form new C–C σ bonds to neighbouring monomers. The polymer is saturated: it is a very long alkane-like chain.

Monomer	Polymer	Repeat unit
ethene, CH₂=CH₂	poly(ethene)	–[CH₂–CH₂]–
chloroethene, CH₂=CHCl	poly(chloroethene), PVC	–[CH₂–CHCl]–
propene, CH₂=CHCH₃	poly(propene)	–[CH₂–CH(CH₃)]–

General equation n CH₂=CHCl → –[CH₂–CHCl]_n–

Repeat unit from a monomer

Draw the two carbons of the C=C as a single bond, keep every substituent where it was, and add a bond sticking out of each end (through the brackets). Only the two C=C carbons go in the backbone; any other carbons are side groups.

Repeat unit of PVC: the bond lines pass through the brackets, showing the chain continues.

Monomer from a polymer

Mark off the backbone two carbons at a time. Each two-carbon unit, with its side groups, came from one monomer: put back a C=C between those two carbons. If the two-carbon units differ along the chain, there was more than one monomer.

Worked Example

A polymer section is –CH₂–CH(CH₃)–CH₂–CH(CH₃)–CH₂–CH(CH₃)–. Identify the monomer. A sample of this polymer has M_r = 2.10 × 10⁵; how many monomer units are in each chain?

Practice Question

A polymer section reads –CH₂–CH₂–CH₂–CHCl–CH₂–CH₂–CH₂–CHCl–. Identify the monomers.

20.1

Disposal of poly(alkene)s

Non-biodegradable.

Memorise

Addition polymer

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Polymerisation

Polymerisation

Monomers to polymer

Repeat unit from a monomer

Monomer from a polymer

Disposal of poly(alkene)s

Addition polymer

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