Addition Polymerisation of Alkenes
Polymers are large molecules built from repeating smaller units called monomers. ESAT questions test your ability to recognise the two main formation processes (addition and condensation) and to deduce the structure of either the monomer or the polymer if given the other.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Addition polymerisation involves monomers with C=C double bonds. The double bond breaks to form a long, saturated carbon chain (a polyalkene) with no other products formed.
- To find the monomer from an addition polymer, identify the two-carbon repeating unit in the backbone and reform the C=C double bond between them.
- Condensation polymerisation joins two monomers with different functional groups (e.g., a dicarboxylic acid and a diol) by eliminating a small molecule, typically water. This creates an ester or amide linkage.
- To find the monomers from a condensation polymer, 'break' the ester or amide linkage and add the components of water back: add -OH to the C=O group and H to the -O- or -N- atom.
- Proteins are natural polyamides formed by the condensation polymerisation of amino acids. The amide link is called a peptide bond.
- Biodegradable polymers (often polyesters or polyamides) can be broken down by microorganisms because the linkages are susceptible to hydrolysis. Non-biodegradable polymers like polyalkenes have a strong, unreactive C-C backbone.
Formulae
n (CH2=CHR) → -[CH2-CHR]n- General equation for the addition polymerisation of an alkene monomer.
n HOOC-X-COOH + n HO-Y-OH → -[OC-X-CO-O-Y-O]n- + 2n H2O General equation for forming a polyester from a dicarboxylic acid and a diol.
n HOOC-X-COOH + n H2N-Y-NH2 → -[OC-X-CO-NH-Y-NH]n- + 2n H2O General equation for forming a polyamide from a dicarboxylic acid and a diamine.
Definitions
- Monomer
- A small, reactive molecule that serves as the single repeating building block for a polymer chain.
- Polymer
- A large molecule (macromolecule) consisting of many repeating monomer units chemically bonded together.
- Repeating Unit
- The simplest part of the polymer's structure which, if repeated, would generate the complete polymer chain.
- Addition Polymerisation
- A reaction where monomers add to each other, breaking a C=C bond, to form a polymer as the only product.
- Condensation Polymerisation
- A reaction where monomers join together by forming a new bond and eliminating a small molecule, such as water, in the process.
Worked example
A section of a polyamide polymer has the following structure: -[NH-(CH2)2-NH-CO-(CH2)4-CO]- What are the full chemical structures of the two monomers used to produce this polymer?
- 1
First, identify the linkage type.
The -NH-CO- group is an amide linkage, so this is a condensation polymer.
- 2
To find the monomers, we must reverse the condensation reaction by hydrolysing the amide bond.
This involves breaking the C-N bond within the amide group.
- 3
Add the components of water (H and OH) back to the broken ends.
The -NH group gains an H to become an -NH2 (amine) group.
The -CO group gains an -OH to become a -COOH (carboxylic acid) group.
- 4
Breaking the bond in the repeating unit -[NH-(CH2)2-NH-CO-(CH2)4-CO]- yields two fragments:
H2N-(CH2)2-NH2 and HOOC-(CH2)4-COOH.
- 5
The two monomers are therefore a diamine and a dicarboxylic acid.
- 6
Draw the final structures:
Ethane-1,2-diamine and Hexanedioic acid.
Answer: The monomers are H2N-CH2-CH2-NH2 (ethane-1,2-diamine) and HOOC-(CH2)4-COOH (hexanedioic acid).
Common mistakes
- ×When deducing monomers from a condensation polymer diagram, it is a common mistake to simply break the link without adding back the small molecule (water). Remember to regenerate the full functional groups, e.g., turning -O- into -OH and -CO- into -COOH.
- ×Misreading the repeating unit in a complex addition polymer. Always find the continuous two-carbon chain that forms the main backbone; everything else attached to that chain is part of the monomer's side group(s).
- ×Confusing the two polymerisation types. Remember: Addition = 'Adds all' (no atoms lost), breaks a double bond. Condensation = 'Condenses out' a small molecule, joins functional groups.
No-calculator tips
- ✓When drawing a condensation polymer, physically draw the two monomers next to each other, aligning the reacting functional groups. Circle the atoms that will form water (H from the alcohol/amine, OH from the acid) and then connect the remaining parts.
- ✓To quickly identify the monomer of a polyalkene, cover up the bonds extending outwards from the repeating unit and draw a double bond between the two carbons in the backbone. That's your monomer.
- ✓Visually scan for ester (-COO-) or amide (-CONH-) links. If you see them, it's a condensation polymer. If you see a long chain of only C-C single bonds, it's likely an addition polymer.