Synthetic And Naturally Occurring Polymers
Übersicht
In the spring of 1933 two chemists at a works laboratory in Cheshire squeezed ethene gas to close to two thousand times atmospheric pressure, hoping to force it to react with something else. They opened the vessel and found neither of the products they had planned for. Clinging to the inside was a white, waxy solid that nobody had asked for and nobody could immediately identify. It took two more years to work out how to make it on purpose, and the trick turned out to be a leak: a trace of oxygen was starting the reaction. Within a decade that accident was insulating the cables of wartime radar sets, and within thirty years it had rebuilt the shopping bag, the bucket, the bottle and the water pipe.
This lesson is about how a gas with one double bond becomes a solid you can stand on. You will see exactly what happens to that double bond, learn to draw the repeat unit of a polymer from any alkene you are handed, and find out why the same monomer can be turned into a floppy carrier bag or a rigid drainpipe depending only on the temperature, the pressure and the catalyst used. Then comes the part the world is still arguing about: what to do with the waste, why microbes will not touch most of it, and whether a bag grown from maize is the answer or only part of one.
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. 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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