Addition polymerisation
Many alkene monomers (each with a C=C double bond) join into one long chain, with no atoms lost and no small molecule eliminated; the C=C opens to a single bond linking the units. Examples: ethene → poly(ethene); propene → poly(propene); chloroethene → poly(chloroethene) (PVC). To draw the repeat unit, open the C=C to C–C, bracket the unit with a bond passing through each side, and add subscript . A full definition must state that no small molecule is produced.
Plastics — disposal problems
Most synthetic polymers are non-biodegradable — microorganisms cannot break them down, so they persist for hundreds of years. Problems: they fill limited landfill space; burning them releases toxic gases (HCl from PVC, CO from incomplete combustion); as litter they harm wildlife, and microplastics enter food chains. Most plastics come from crude oil. State non-biodegradable plus a specific consequence — "harms the environment" alone scores zero.
(Extended) Condensation polymerisation
Monomers that each carry two functional groups react, and a small molecule (water or HCl) is eliminated at every bond — unlike addition (needs a C=C, loses nothing). Nylon (a polyamide) forms from a diamine + dicarboxylic acid, making an amide linkage –CONH– and losing water. Polyester forms from a diol + dicarboxylic acid, making an ester linkage –COO– and losing water. A full answer must name the two functional groups and the molecule lost.
Drawn from real examiner reports.
Repeat unit must show open bonds
A repeat unit is not a molecule — it must show it continues into the chain. Two common losses: drawing the monomer with its C=C still intact inside the brackets (that is the monomer, not the repeat unit); or closing the brackets with no bond passing through each side, so it floats free. Also add the subscript and keep every carbon at exactly four bonds.
W22 Paper 31 Q7(a)(i); W22 Paper 41 Q6(c)
(Extended) Addition vs condensation mixed up
Keep the two mechanisms apart. Addition: monomer needs a C=C, no small molecule is produced, backbone is all C–C (e.g. poly(ethene)). Condensation: monomers need two functional groups, a small molecule (water or HCl) is eliminated, product has a linkage group (nylon, polyester). Saying addition "produces water", or condensation "needs a C=C", reverses the two.
Disposal answers must be specific
"Plastics harm the environment / cause pollution" restates the question and scores zero. Give a mechanism: they are non-biodegradable (not broken down by microorganisms) so persist; or burning releases a named toxic gas (HCl, CO); or litter and microplastics harm wildlife. For "state two problems" give two distinct points — reworded ideas earn one mark.
Monomer vs repeat unit
The monomer and the repeat unit differ by the double bond. The monomer of poly(ethene) is (a C=C alkene); the repeat unit is with a single C–C bond and bonds through the brackets. They share the molecular formula (no atoms lost) but are drawn differently.
Monomer must be an alkene
Only a molecule with a C=C double bond can add-polymerise. Ethane (, an alkane) cannot — it is saturated. The monomer for poly(ethene) is ethene (), an alkene. Watch the ending: -ane means saturated (no reaction), -ene means a C=C is present (polymerises).
(Extended) Amide vs ester linkage swapped
Nylon and polyester get their linkages swapped. Nylon is a polyamide with the amide bond –CONH– (N for Nylon, N in –NH–). Polyester has the ester bond –COO– (the name tells you). Write –CONH– showing N bonded to H, not –CON–. And do not call nylon an addition polymer; both are condensation polymers.
Monomer to repeat unit
Convert a monomer to a repeat unit: (1) find the C=C; (2) make it a single C–C; (3) bracket the unit with a bond through each side; (4) add subscript . Keep each carbon at four bonds. To reverse, remove the brackets and rejoin the end carbons with a C=C.
Deduce monomer from repeat unit
Given a repeat unit, deduce the monomer: remove the brackets and open bonds, then place a C=C between the end carbons — that is the monomer alkene. Its molecular formula equals the repeat-unit formula (no atoms lost). Use the systematic name, e.g. poly(chloroethene) for PVC.
Repeat-unit Mr equals monomer Mr
No atoms are lost, so one repeat unit has the same as the monomer. For poly(ethene) the repeat unit is : , equal to ethene. Always use the repeat-unit formula, never the alkane .
Polymer: A very long-chain molecule built by joining many small repeat units together. The small starting molecules are called monomers.
Addition polymerisation: A reaction in which many alkene monomers join together through their double bonds to form a single long-chain polymer, with no atoms lost and no small molecule produced.
Condensation polymerisation (Extended): A reaction in which monomers join by reacting at two different functional groups; a small molecule (water, , or hydrogen chloride, ) is eliminated at each bond formed.
Non-biodegradable: A material that is not broken down by microorganisms in the environment — most synthetic plastics fall into this category.
Define "addition polymerisation". What type of monomer is required?
The repeat unit of poly(propene) has the molecular formula .
Calculate the relative formula mass () of the repeat unit.
(: C = 12, H = 1)
