Synthetic And Naturally Occurring Polymers
Übersicht
Ethene is a gas. You cannot build a bucket out of it, and at room temperature you cannot even see it. Yet almost every plastic object within reach of you right now started as a gas of that kind, in a vessel, under pressure. The step that turns one into the other is called polymerisation, and it is one of the very few reactions on this course whose product you are certainly touching as you read.
This lesson follows that product all the way from the double bond to the landfill site. You will open an alkene's double bond and build a repeat unit from it, discover how one single monomer can be turned into either cling film or a mains water pipe, learn the one structural question that decides whether a plastic can be melted down and remoulded, and work out why a material nothing can digest is both extraordinarily useful and a problem that will outlive you. It is also the topic that holds this course's most quotable mark scheme: a four-mark question on the chemistry paper that is scored entirely on getting the forces between polymer chains right, and that caps your score if you name them wrongly.
Ziele
- Alkenes can be used to make polymers such as poly(ethene) and poly(propene). In polymerisation reactions, many small molecules (monomers) join together to form very large molecules (polymers). For example: H H H H | | | | n C C → C C | | | | H H H H n ethene poly(ethene) Students should be able to recognise the molecules involved in these reactions in the forms shown in the subject content. They should be able to represent the formation of a polymer from a given alkene monomer. Further details of polymerisation are not required.
- The properties of polymers depend on what they are made from and the conditions under which they are made. For example, low-density (LD) and high-density (HD) poly(ethene) are produced using different catalysts and reaction conditions.
- Thermosoftening polymers consist of individual, tangled polymer chains. Thermosetting polymers consist of polymer chains with cross-links between them so that they do not melt when they are heated. Extension Tier students should be able to explain thermosoftening polymers in terms of intermolecular forces.
- Polymers have many useful applications and new uses are being developed. Examples include: new packaging materials, waterproof coatings for fabrics, dental polymers, wound dressings, hydrogels, and smart materials (including shape memory polymers). Students should consider the ways in which new materials are being developed and used, but will not need to recall the names of specific examples.
- Many polymers are not biodegradable, ie they are not broken down by microbes. This can lead to problems with waste disposal. Knowledge of specific named examples is not required, but students should be aware of the problems that are caused in landfill sites and in litter.
- Plastic bags are being made from polymers and cornstarch so that they break down more easily. Biodegradable plastics made from cornstarch have been developed.
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- Which type of compound can be used as a monomer to make an addition polymer? A. an alkane B. an alkene C. an alcohol D. a carboxylic acid Answer: B
- What happens to the carbon to carbon double bond of a monomer during addition polymerisation? A. It stays as a double bond in the polymer. B. It opens to become a single bond. C. It becomes a triple bond. D. It is lost as a gas. Answer: B
- Which statement about HD poly(ethene) compared with LD poly(ethene) is correct? A. It has more branched chains and a lower density. B. It has more branched chains and a higher density. C. It has fewer branched chains and a lower density. D. It has fewer branched chains and a higher density. Answer: D
- Why does a thermosetting polymer not melt when it is heated? A. Its chains are held together by weak intermolecular forces. B. Its chains are joined to each other by cross links. C. Its chains are much longer than those of other polymers. D. Its chains contain no carbon to carbon bonds. Answer: B
- Plastic bags are made from a blend of a polymer and cornstarch so that the bags: A. break down more easily B. can be stretched further C. cost more to produce D. melt at a higher temperature Answer: A
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