How are polymers made from small molecules, and what are the two main types of polymerisation?
Addition polymerisation of alkenes and condensation polymerisation, the structures of the polymers formed, the differences between the two types, and the uses and environmental impact of polymers including biodegradability and disposal.
A CCEA Life and Health Sciences answer on polymers: addition polymerisation of alkenes and condensation polymerisation, the structures formed, the differences between the two types, and the uses and environmental impact of polymers.
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What this dot point is asking
CCEA wants you to describe addition polymerisation of alkenes and condensation polymerisation, draw and recognise the structures of the polymers formed, explain the differences between the two types, and discuss the uses and the environmental impact of polymers, including biodegradability and disposal. It builds on the alkene addition chemistry and functional-group knowledge from earlier dot points, and connects to biological polymers such as proteins.
Addition polymerisation
The classic example is poly(ethene) from ethene: the double bond in each ethene molecule opens and the carbons link to form a long, saturated hydrocarbon chain, with the ethene unit as the repeating unit. Other addition polymers are made the same way from substituted alkenes, for example poly(chloroethene) (PVC) and poly(propene). Addition polymers have a continuous carbon backbone of strong single bonds, which makes them chemically unreactive and durable. To draw the repeating unit, you take the monomer, change the double bond to a single bond, and show the unit in brackets with bonds extending out of each side.
Condensation polymerisation
The need for two functional groups is what lets each monomer join at both ends to build a long chain. The repeating linkage is an ester (in polyesters) or an amide (in polyamides and proteins). This is the key difference from addition polymerisation: condensation loses a small molecule and forms a polar linkage, whereas addition loses nothing and forms a continuous carbon chain. Biological polymers such as proteins, made from amino acid monomers with the loss of water, are condensation polymers, which links this chemistry directly to the biology of the qualification.
Uses, environmental impact and disposal
Polymers are extremely useful: poly(ethene) for bags and bottles, PVC for pipes and insulation, polyesters for fabrics and packaging. But addition polymers create environmental problems because they are not biodegradable: their strong, non-polar carbon-to-carbon and carbon-to-hydrogen bonds cannot be broken by microorganisms or their enzymes, so the polymers persist for a very long time. Disposal options each have drawbacks: landfill takes up space and the polymers do not rot; incineration recovers energy but can release toxic gases (for example from PVC) and carbon dioxide; and recycling saves resources but requires sorting and cleaning and the quality can fall. These problems drive research into biodegradable polymers (which microorganisms can break down) and into reducing single-use plastics. Condensation polymers such as polyesters can sometimes be broken down by hydrolysis of their ester linkages, so some are more degradable than addition polymers.
Examples in context
Example 1. Plastic waste in the environment. Because poly(ethene) and similar addition polymers do not biodegrade, plastic bags and bottles accumulate in landfill and the oceans for decades. This persistence, a direct consequence of their unreactive carbon backbone, is the driver behind plastic-reduction policies and biodegradable alternatives, linking organic chemistry to environmental and public-health concerns.
Example 2. Proteins as natural condensation polymers. Proteins are condensation polymers of amino acids, formed with the loss of water and joined by peptide (amide) linkages. This is the same chemistry as making a polyester, showing that the body builds its key molecules by condensation polymerisation and connecting the organic chemistry unit to the biology of the Human Body Systems and Genetics units.
Try this
Q1. State what type of monomer is needed for addition polymerisation. [1 mark]
- Cue. An unsaturated monomer (an alkene with a carbon-to-carbon double bond).
Q2. State the small molecule usually lost in condensation polymerisation and one example of a condensation polymer. [2 marks]
- Cue. Water; a polyester (or a protein/polypeptide).
Q3. Explain why poly(ethene) is not biodegradable. [2 marks]
- Cue. Its strong, non-polar carbon-to-carbon and carbon-to-hydrogen bonds cannot be broken by microorganisms or their enzymes.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA A2 26 marksDescribe addition polymerisation, using the formation of poly(ethene) from ethene as an example, and explain why poly(ethene) is not biodegradable.Show worked answer →
Describe the process and the structure, then explain the lack of biodegradability.
Addition polymerisation: many small unsaturated monomers (alkenes) add together to form one long polymer chain, with no other product formed. The carbon-to-carbon double bond in each monomer opens, and the monomers join end to end.
Poly(ethene): many ethene molecules join. The double bond in each ethene opens and the carbons link to form a long saturated chain, the repeating unit being the ethene unit. No small molecule is lost.
Why it is not biodegradable: the polymer chain is made of strong, non-polar carbon-to-carbon and carbon-to-hydrogen single bonds, like an alkane. Microorganisms and their enzymes cannot break these bonds, so the polymer is not broken down naturally and persists in the environment.
Markers reward many monomers adding with no other product, the double bond opening to give a saturated chain, and the unreactive carbon-carbon and carbon-hydrogen bonds explaining the lack of biodegradability.
CCEA A2 25 marksCompare addition polymerisation with condensation polymerisation, referring to the type of monomer used and whether a small molecule is produced.Show worked answer →
A compare answer needs matched points for both types.
Type of monomer: addition polymerisation uses unsaturated monomers (alkenes) that contain a carbon-to-carbon double bond. Condensation polymerisation uses monomers with two reactive functional groups (for example a dicarboxylic acid and a diol, or amino acids), so they can join at both ends.
Small molecule produced: in addition polymerisation no small molecule is lost; all the atoms of the monomers end up in the polymer. In condensation polymerisation a small molecule (usually water) is lost each time a bond forms between two monomers.
Resulting bond and structure: condensation polymers contain ester or amide linkages between the monomer units, whereas addition polymers have a continuous carbon backbone.
Markers reward unsaturated alkene monomers versus monomers with two functional groups, no small molecule lost versus a small molecule (water) lost, ideally with the ester or amide linkage point.
Related dot points
- The classification of organic compounds by functional group, homologous series and general formulae, IUPAC nomenclature, the different ways of representing organic molecules, and the meaning of structural isomerism.
A CCEA Life and Health Sciences answer on organic classification: functional groups, homologous series and general formulae, IUPAC nomenclature, the ways of representing organic molecules, and an introduction to structural isomerism.
- The characteristic reactions of alkanes (combustion and substitution), alkenes (addition), alcohols (oxidation, combustion and dehydration) and carboxylic acids, and the reaction types of combustion, substitution, addition and oxidation.
A CCEA Life and Health Sciences answer on organic reactions: the characteristic reactions of alkanes, alkenes, alcohols and carboxylic acids, and the reaction types of combustion, substitution, addition and oxidation.
- Structural isomerism (chain, position and functional group isomers), stereoisomerism including cis-trans (E-Z) isomerism in alkenes, the conditions needed for each, and why isomers can have different properties.
A CCEA Life and Health Sciences answer on isomerism: structural isomerism (chain, position and functional group), stereoisomerism including cis-trans (E-Z) isomerism in alkenes, the conditions for each, and why isomers differ in properties.
- The principles and uses of instrumental methods for identifying organic compounds, including mass spectrometry, infrared spectroscopy and chromatography, and how data from these methods are interpreted to determine structure.
A CCEA Life and Health Sciences answer on instrumental analysis: the principles and uses of mass spectrometry, infrared spectroscopy and chromatography, and how their data are interpreted to identify and determine the structure of organic compounds.
- The structure of DNA and RNA, the gene as a sequence of bases coding for a protein, the genetic code, and the stages of protein synthesis (transcription and translation).
A CCEA Life and Health Sciences answer on DNA, genes and protein synthesis: the structure of DNA and RNA, the gene and the genetic code, and the stages of transcription and translation that make a protein from a gene.
Sources & how we know this
- CCEA GCE Life and Health Sciences specification — CCEA (2016)