1.10.6 Addition Polymerisation

Addition polymerisation

• Addition polymerisationis one of the most important addition reactions of alkenes which form the basis of the plastics industry
• Addition polymerisation is the reaction in which manymonomerscontaining at least one C=C double bond form long chains ofpolymersas the only product
  • Just like in other addition reactions of alkenes, the π-bond in each C-C bond breaks and then the monomers link together to form new C-C single bonds

• Apolymeris a long-chain molecule that is made up of many repeating units
• The small, reactive molecules that join together to form the polymer are calledmonomers
• A polymerisation reaction can be represented by ageneral formulaor by usingdisplayed formulae
  • Eg. poly(ethene) and poly(chloroethene) (also known asPVC) are polymers made up of the ethene and chloroethene monomers respectively and are commonly used in making plastics

The general formulae of the addition polymerisation of ethene (1) and chloroethene (2)

The displayed formulae of the addition polymerisation of ethene (1) and chloroethene (2)

• Just like any otheradditionreaction of alkenes, addition polymerisation gives onlyoneproduct

Deducing repeat units

• Arepeat unitis the smallest group of atoms that when connected one after the other make up the polymer chain
  • It is represented bysquare bracketsin the displayed and general formula

• Inpoly(alkenes)(such as poly(ethene)) andsubstituted poly(alkenes)(such as PVC) made ofone type of monomerthe 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

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)