7a. The formation of POLYMERS and the USES of PLASTICS - Macromolecules

Reactions of alkenes (4) polymerisation/polymerization

(See also section 11. More on other types of polymers)

The formation of big polymer molecules called polyalkenes from small molecules called alkenes

Alkenes are made by cracking some of the fractions obtained by the fractional distillation of crude oil

The diagram below shows the general equation for the formation of a plastic polymer from alkenes.

There are many different alkenes, so there are many different polymers with a range of physical properties eg strength.

• When catalysed and heated under pressure, unsaturated alkenes link together when 'half' of the double bond opens. The spare bond on each carbon atom of the double bond are used to join up the molecules.

  • The general equation for polyalkene formation is shown in the diagram above.

  • The essential molecular linking is equivalent to ...

  • ... C=C + C=C + C=C ... etc.  ===>  ... C-C-C-C-C-C ... etc.

• The original small molecule is called the monomer and the long molecule is called the polymer, which is the sort of molecule most plastics consist of.

  • The polymer is now a saturated molecule but has the same C:H ratio as the original alkene.

• So lots of small molecules join up to form a big long molecule in a process called addition polymerisation and the polymers are named as poly(name of original alkene), i.e. poly(alkene) and several examples are shown below.

• Poly(ethene)

• A 'picture' of a section of a very long chain poly(ethene) polymer molecule, essentially consisting of hundreds or thousands of -CH₂-CH₂- ethene units all joined together.

• Above is a 'ball and stick' type of model and below is a 'space-filling' model.

• Although it looks straight, in reality, the long molecule will be all twisted-jumbled up like in the diagram on the right above.

• Poly(ethene) from ethene is a cheap but very useful plastic used for plastic bags and bottles (old or commercial names: polyethylene, polythene and polyethene). It is an example of a thermoplastic, because it can be heated to soften it, reshape it e.g. in an injection mould system, and on cooling retains its new shape - bottle, bowl, toy etc.

• Poly(propene) from propene is stronger and more hard wearing than polythene and is used for making crates, fibres and ropes (old or commercial names: polypropylene, polyprene and polypropene).

• 

• Poly(chloroethene), PVC, made from chloroethene (old names vinyl chloride and polyvinyl chloride from which PVC is derived)

  • Poly(chloroethene) is much tougher than poly(ethene), very hard wearing with good heat stability, so is used for covering electrical wiring and plugs.

  • poly(chloroethene), PVC

  • 

  • It is also replacing metals for use as gas and water drain pipes and has found a use as artificial leather and readily dyed to bright colours!

• Polystyrene is made from 'styrene^* (another alkene monomer) and is used in a gas expanded form for packaging and insulation - 'polystyrene foam' or 'expanded polystyrene'.

  • New modern systematic name: poly(phenylethene), a polymer made from polymerising phenylethene monomer.

• Polymers have many useful applications and new uses and new polymers are being developed all the time

  • There are lots of examples! - those listed are on many GCSE syllabuses

  • Many metals have become quite expensive so things made from plastics derived from oil and quite cheap at the moment, but prices will rise as cheap oil gets scarcer.

  • New packaging materials which are light and stretchable.

  • Waterproof coatings for fabrics

  • Dental polymers for tooth fillings

  • Wound dressings made from hydrogels - polymers that hold water and keep wounds moist.

  • Smart materials (including shape memory polymers) eg 'memory foam' is a polymer that gets softer when it gets warmer so you can make a mattress which adopts your body shape for comfortable sleeping!

  • Elastic fibres that are very stretchable to make tight fitting clothing eg Lycra fibres for tights and sports clothing.

• Many polymers are not biodegradable, so they are not broken down by microbes and this can lead to problems with waste disposal - see section 7c below.

  • Because of lack of biodegradability, you get environmental problems with landfill sites and street litter and it may take years before the plastic materials break down - ie decompose!

  • However, all is not lost and chemistry and technology can come to the 'rescue' and lots of new improved polymers for existing applications as well as new polymers for new applications are constantly being developed.

  • eg plastic bags are being made from polymers derived from cornstarch making biodegradable plastics that break down more easily via microorganisms in the environment.

  • AND, wherever possible, recycle and reuse! (see section below).

• Extra little note on natural polymers

  • Starch and cellulose are natural polymers but are not used as 'material polymers' commercially.

  • DNA is actually a polymer, but of rather a delicate constitution!

  • Rubber has been used for centuries as a natural elastic polymer, but it has been replaced by synthetic polymers like neoprene.

  • Silk has been used as a clothing and decorative fabric for thousands of years, and, like sheeps wool, is essentially a protein polymer material. Synthetic man-made polymers like nylon and terylene where developed and designed to try and mimic its useful clothing properties, though nylon's excellent properties eg strength have meant it has found many and diverse applications.

1. Polymers or plastics cannot be easily broken down by micro-organisms i.e. most, at the moment, most are NOT biodegradable (non-biodegradable) which leads to waste disposal and other environmental problems eg

   • 'Non-rotting' litter around the environment.

   • Land-fill sites are getting full and recycling isn't as easy as it may seem. (see 3.below)

   • Incineration i.e. using waste plastic as fuel must be very efficient to avoid any other pollution problem. (see 2.)

   • Even our lakes, seas and oceans are carrying waste plastic materials causing harmful effects on wildlife.

2. Can we burn waste plastic?

   • When plastic materials burn they can produce highly toxic gases such as carbon monoxide, hydrogen cyanide and hydrogen chloride (particularly from PVC and other plastics containing chlorine and nitrogen).

   • The toxic fumes cause deaths in house fires and controversial problems with alleged inefficient waste incinerators as they will definitely cause environmental problems if burned on waste tips!

3. How can we recycle plastics? What ways are used to recycle plastics?

   • It is difficult to recycle plastics because of separation into the various types of polymer and their different physical properties. BUT this should not prevent us from trying and it would be beneficial to prolong the life of the finite crude oil reserves AND reduce pollution and space in land-fill sites.

   • There are problems in trying to sort out the different plastics into useful categories, they are not easy separate, you can't just use any old mixture of polymers.

     • At the moment, a lot may have to be sorted by hand - lack of automation makes the sorting more costly.

   • However, people are coming up with ideas. A company in Scarborough, England, is collecting waste plastic. This is shredded and compressed into porous pads and used for good 'underground' drainage layers for footpaths, golf greens and sand bunkers etc. and has a good working life because the material isn't biodegradable!

   • Clear soft drink bottles are made from PET (polyethene/polyethylene terephthalate) which can be recycled as fibre-fill for pillows and carpets.

     • This save 90% of the energy costs compared to the original manufacturing process.

     • Energy costs are a big economic recycling factor, its not just about making naturally occurring resources like oil last longer.

     • However, it takes about 20,000 drinks bottles to make a tonne of recycled PET.

   • New plastics are being developed which are more biodegradable or can be recycled, so will the paper bag and cardboard package make a comeback? (in Ireland you have to bring your own bag or buy one, and not necessarily a plastic one!), this isn't a recycling process BUT it does reduce environmental pollution..

   • In 1988 Australia issued bank notes made from recycled poly(propene). These plastic notes apparently have the advantage of being more difficult to forge and they last longer.

     • Waste poly(propene)/polypropylene can be ground up and recycled to make pipes, compost bins and flower pots

   • Other ideas include making more durable plastic bags that can be used many times for shopping.

   • Ideally some recycled thermoplastics and scrap material from a plastic product manufacturing process, can be heated and remoulded in the same process or another product.

   • It is possible in some cases to break the plastic material down with heat (a sort of 'cracking') into smaller organic hydrocarbon molecules that can act as chemical feedstock like oil, from which you can make other valuable products including plastics.

     • Less green, but useful purpose, is to use scrap plastic as a fuel, but complete incineration is not always easy to be efficient.

   • All of these will slow down the rate at which valuable oil-petroleum deposits are depleted - the latter are finite, so we should make the best use of them.

4. See also section 4. Pollution Carbon monoxide etc.

Multiple Choice Quizzes and Worksheets

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (easier-foundation-level)

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (harder-higher-level)

KS4 Science GCSE/IGCSE m/c QUIZ on other aspects of Organic Chemistry

and 3 linked easy Oil Products gap-fill quiz worksheets

ALSO gap-fill ('word-fill') exercises originally written for ...

... AQA GCSE Science Useful products from crude oil AND Oil, Hydrocarbons & Cracking etc.

... OCR 21st C GCSE Science Worksheet gap-fill C1.1c Air pollutants etc ...

... Edexcel 360 GCSE Science Crude Oil and its Fractional distillation etc ...

... each set are interlinked, so clicking on one of the above leads to a sequence of several quizzes

Advanced Level Organic Chemistry revision notes

Notes information to help revise KS4 Science Additional Science Triple Award Separate Sciences GCSE/IGCSE/O level Chemistry Revision-Information Study Notes for revising for AQA GCSE Science, Edexcel GCSE Science/IGCSE Chemistry & OCR 21st Century Science, OCR Gateway Science WJEC/CBAC GCSE science-chemistry CCEA/CEA GCSE science-chemistry (and courses equal to US grades 8, 9, 10)