Polymer Repeat Units (OCR A Level Chemistry)

Repeat units for addition polymers

• A repeat unit is the smallest group of atoms that when connected one after the other make up the polymer chain
  • It is represented by square brackets in the displayed and general formula

• In poly(alkenes) (such as poly(ethene)) and substituted poly(alkenes) (such as PVC) made of one type of monomer the repeating unit is the same as the monomer except that the C=C double bond is changed to a C-C single bond

The repeating units of poly(ethene) and poly(chloroethene) are similar to their monomer except that the C=C bond has changed into a C-C bond

Answers:

Answer 1:

When ethenol (CH(OH)=CH₂) is polymerised, the C-C double bond opens to produce a repeating unit of CH(OH)-CH₂. This gives the polymer poly(ethenol)

 

Answer 2:

• • • To find the monomer, first the repeating unit should be deduced. Repeating units have only 2 carbons in the polymer main chain

 

• • • Since the repeating unit is now found, it can be concluded that the monomer is prop-2-enoic acid

Answer 3:

• • • Again, the repeating unit only has 2 carbons in the polymer chain which in this case are two carbon atoms that each contain one OH group
    • Thus, when ethene-1,2-diol (CH(OH)=CH(OH)) is polymerised, the C=C double bond opens to produce a repeating unit of CH(OH)-CH(OH) which gives the polymer poly(ethene-1,2-diol)

 

Repeat units for condensation polymers

• Remember we can tell the type of polymerisation by identifying the linking between the monomers
• If a chan of carbon atoms is present, the polymer is an addition polymer
• If there is an ester link, the polymer is a polyester (formed by condensation polymerisation)
• If there is an amide link, the polymer is a polyamide (formed by condensation polymerisation)

This polymer structure shows an ester functional group linking monomers together

An amide link - also known as a peptide link - is the key functional group in a polyamide

Answer: