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First published 1999
Electronic version 2001
© Scottish Consultative Council on the Curriculum 1999
This publication may be reproduced in whole or in part for educational purposes by educational establishments in Scotland provided that no profit accrues at any stage.
Acknowledgement
Learning and Teaching Scotland gratefully acknowledge this contribution to the Higher Still support programme for Chemistry.
ISBN 1 85955 836 4 w w w .LTScotl and.com
2 C H E M I S T R Y

Teachers’/lecturers’ guide
Background notes
Introduction
Kevlar
Poly(ethenol)
Poly(ethyne)
Poly(vinyl carbazole)
Biopol
Photodegradable low density poly(ethene)
Summary
Activities
Questions
25
33
41
1
12
18
20
23
3
3
4
8
C H E M I S T R Y i i i

I V C H E M I S T R Y

TEACH ERS’ /LECTURERS’ GUIDE
This publication addresses the following content statements, which relate to the
‘Recent developments in polymers’ section in Unit 2 of the Higher Chemistry course:
Kevlar is an aromatic polyamide which is extremely strong because of the way in which the rigid, linear molecules are packed together.
Kevlar has many important uses.
Poly(ethenol) is a plastic which readily dissolves in water.
Poly(ethenol) is made from another plastic by a process known as ester exchange.
The percentage of acid groups which have been removed in the production process influences the strengths of the intermolecular forces upon which the solubility depends.
Poly(ethenol) has many important uses.
Poly(ethyne) can be treated to make a polymer which conducts electricity.
The conductivity depends on delocalised electrons along the polymer chain.
A conducting polymer, similar to poly(ethyne), is used to make the membrane for high-performance loudspeakers.
Poly(vinylcarbazole) is a polymer which exhibits photoconductivity and is used in photocopiers.
Biopol is an example of a biodegradable polymer.
The structure of low density polythene can be modified during manufacture to produce a photodegradable polymer.
Included in this booklet are background notes along with suggested student activities and questions .
The background notes are introduced with a brief history of the manufacture of synthetic polymers highlighting the change from the serendipitous approach in the early days of polymer science through to the more focussed approach of designing polymers which now pertains. This is followed by detailed notes on each of the six polymers featured in the content statements. Here, an attempt has been made to
C H E M I S T R Y 1

TEACH ERS’ /LECTURERS’ GUIDE explain the properties of each polymer in terms of its structure and bonding and to relate these properties to its uses. Although the background notes are written primarily for teachers/lecturers there is no reason why students should not be able to access them.
There are four suggested activities and all are practical in nature. While it is recommended that the last one – the preparation of the conducting polymer, poly(pyrrole) – be demonstrated, students proficient in practical work should have little difficulty in carrying it out.
The questions would be suited to homework.
It is recommended that approximately three hours of contact time be spent on this aspect of Unit 2. Teachers/lecturers are reminded that they may use as little or as much of the material in this booklet as they so wish.
Much of this work was derived from the book, Salters Higher Chemistry . This in turn was influenced by the Salters Advanced Chemistry project produced by the University of York Science Education Group. We thank the Director, Dr G M Pilling and her colleagues at York as well as the publishers, Heinemann, for their agreement to this publication.
Salters Higher Chemistry (Oxford: Heinemann, 1999) covers all the content statements for Higher Chemistry:
Unit 1 Energy Matters
Unit 2 The World of Carbon
Unit 3 Chemical Reactions
Another resource to be consulted is:
Garforth, Francesca and Stancliffe, Alan, eds., Polymers : Polymer Industry
Education Centre, University of York, 1994
Further information on polymers can be obtained from:
Chemical Industry Education Centre, Department of Chemistry,
University of York, York YO1 5DD
2 C H E M I S T R Y

B A C K G R O U N D N O T E S
If an era is known by the kinds of materials its people use to build the world in which they live, then the Stone Age, the Bronze Age and the Iron Age have given way to our own Plastic Age.
The birth of plastics, or synthetic polymers, can be traced as far back as 1846 when Christian Schoenbein, a Swiss chemist, accidentally made nitrocellulose. He had spilt a mixture of nitric and sulphuric acids in his kitchen at home and had used his wife’s cotton apron to wipe up the mess.
He washed the apron through, hung it up in front of a hot s tove to dry and to his amazement watched it spontaneously ignite and disappear in a sheet of flame. He investigated further and concluded that the cellulose in the cotton apron had reacted with the acid mixture to form nitrocellulose. As well as being highly flammable he found that nitrocellulose was highly explosive under certain conditions. Predictably it was this latter property of nitrocellulose which was first exploited, in the manufacture of gun cotton, a substitute for gun powder. In 1856, howeve r, Alexander Parke, an
Englishman, discovered that by mixing nitrocellulose with camphor he could produce a material which was capable of being moulded at moderate temperatures and proved to be remarkably hard and tough. Parke called this new polymer pyroxylin but he was unable to find a market for it. The story then switches to America where in 1870 a New England firm offered a
$10,000 prize for a substitute for ivory, which they used to make billiard balls and piano keys and was becoming very expensive and scarce. John
Hyatt, a New Yorker and a printer by trade, rose to the challenge and recognising the potential of Parke’s pyroxylin he refined and improved its synthesis. He called the improved polymer celluloid, and although he didn’t win the prize he patented a method for making billiard balls. Hyatt’s discovery provided a reprieve for many elephants but caused consternation in many Western saloons when, on occasion, the new billiard balls, on striking each other, exploded!
The first synthetic fibre to come on to the market was also discovered by accident. The French chemist Louis Chardonnet was the man responsible.
While wiping up a spill of nitrocellulose from a bench he noticed long, silk like strands of material hanging from the cloth when he l ifted it from the bench. Chardonnet had inadvertently made a new polymer. This new material was developed and ‘Chardonnet silk’ was launched at the Paris
C H E M I S T R Y 3

B A C K G R O U N D N O T E S
Exposition in 1891. It was subsequently called rayon because it was so shiny that it appeared to give off rays of light.
Strictly speaking, nitrocellulose and rayon are not true synthetic polymers but are simply modifications of the natural polymer cellulose. The first truly synthetic polymer was made in 1907 by the Belgian –American chemist, Leo
Baekeland. He mixed phenol and methanal and from the reaction mixture obtained a resinous material which he called Bakelite. Some ten years later the first synthetic rubber was made by a group of German chemists. Their efforts were in response to a severe shortage of raw materials as a result of blockading during the First World War.
Up until the 1920s there was little understanding of the molecular structure of these new materials. It was generally assumed that the small molecules from which they were made simply aggregated together into larger units rather than joining together to make larger molecules. It was the German chemist
Hermann Staudinger who first recognised that polymers were made up of very large molecules. Another chemist who contributed g reatly to the understanding of polymers as giant molecules was the American Wallace
Carothers.
The development of a comprehensive understanding of the structure and properties of polymers and the advancement in scientific equipment and chemical techniques marked the start of a revolution in polymer chemistry.
The ‘hit or miss’ approach to polymer synthesis was largely superseded and became much more rigorous and focussed. Over the past seventy years or so thousands of polymers have been made and used in such diverse items as artificial joints, compact discs, computers, contact lenses, clothing, dentures, furnishings, glues, motor vehicles, packaging, paints, photographic film, wallpaper and toys.
Polymer chemistry has developed to such a sophisticated st ate that many of our newer polymers were first designed to meet predetermined specifications and then synthesised. In what follows we’ll look at just a few of these new designer polymers.
Kevlar is the trade name of one of the first aromatic pol yamides to be made.
It was patented by DuPont in 1965 but it was several years before it was fully commercialised. As a result of its remarkable combination of properties – particularly its strength – it is used today in a wide variety of applications.
4 C H E M I S T R Y

B A C K G R O U N D N O T E S
Kevlar is derived from the aromatic diacid benzene -1,4-dicarboxylic acid and the aromatic diamine benzene -1,4-diamine (or 1,4-diaminobenzene) by the condensation polymerisation reaction illustrated in Figure 1.
Figure 1
We can see that the acidic carboxyl groups (-COOH) on the diacid monomers react with the basic amino groups ( -NH
2
) on the diamine monomers. At each reaction point a water molecule is eliminated and an amide link ( -CONH-) is formed. The same amide groups link the monomer units together in nylon.
They are also present in proteins – condensation polymers derived from amino acid monomer units – where they are more usually described as peptide links.
The individual polymer chains in Kevlar are very strong and extremely stiff.
The reason for this will become clear when we look closely at Figure 2, which shows how the atoms are actually arranged in a single molecule of
Kevlar.
Figure 2
The molecule is essentially flat and this is mainly due to electron delocalisation, which is not confine d just to the benzene rings but extends over the whole length of the molecule. Electron delocalisation also strengthens the covalent bonds and prevents any rotation about individual
C H E M I S T R Y 5

B A C K G R O U N D N O T E S bonds within the chain. All these factors make the Kevlar molecule str ong, stiff and rod-like. In other words, it doesn’t readily flex, stretch or twist.
The linear polymer chains line up parallel to one another, forming a sheet of molecules. This packing arrangement is illustrated in Figure 3.
Figure 3
The amide groups (-CONH-) are polar and it is through them that hydrogen bonds are set up between the molecules. Although they are considerably weaker than covalent bonds, the hydrogen bonds keep the polymer chains in alignment. This arrangement imparts even greater strength to Kevlar.
In the Kevlar fibre itself the flat sheets of molecules stack together around the fibre axis. This arrangement is shown in Figure 4.
6 C H E M I S T R Y

B A C K G R O U N D N O T E S
Figure 4
It is the very high degree of molecular alignment within the fibre that is the largest contributing factor in Kevlar’s exceptional strength. However, like most polymer fibres, Kevlar suffers from fibrillation, i.e. from ‘split ends’ – just like in hair – which arises because the polymer chains are bound to one another by relatively weak hydrogen bonds.
As well as being very strong, Kevlar is insoluble in water and this created a problem which almost threatened its commercial production. The difficulty was that it precipitated out of solution long before polymer chains of any great length could be formed. A solvent had to be found which would prevent this happening. Concentrated sulphuric acid proved to be suitable but the chemical engineers were none too keen to use such a corrosive and dangerous chemical in the large quantities that wo uld be needed. There was no alternative, however, and in the early 1970s the first plant set up for the full-scale manufacture of Kevlar used concentrated sulphuric acid as a solvent.
Kevlar has a low density because the atoms which make it up – carbon, hydrogen, nitrogen and oxygen – all have small relative atomic masses, i.e. they are light. It is also resistant to heat and this can be attributed to the close packing of the molecular sheets and the strength of the hydrogen bonding between the polymer chains within each sheet. Large amounts of energy are required to break down the structure and Kevlar only begins to decompose and char when heated to temperatures in excess of 400°C. The unique structure of Kevlar also makes it resistant to fire, to chem ical attack and to abrasion.
C H E M I S T R Y 7

B A C K G R O U N D N O T E S
One of the first uses of Kevlar was in car tyres. It replaced steel reinforcement and made tyres much lighter and far stronger. It is also used to make ropes and cables which, weight for weight, are about twenty times stronger than those made from steel. It has found use in the construction of aircraft, space vehicles, boats and even skis, where the combination of strength and light weight is important. Its resistance to heat and fire has made Kevlar an ideal component in t he clothing worn by firefighters. It is also used in body armour – such as bullet-proof vests – where it offers protection against handgun fire, shotgun pellets and knife slashes. Kevlar is also used in brake pads, clutch linings and the protective suits of motorcyclists where high resistance to abrasion is essential.
Poly(ethenol) – formerly known as polyvinyl alcohol – is an addition polymer and unlike most other synthetic polymers it is soluble in water. Part of its structure is illustrated in Figure 5.
Figure 5
H
C
H
OH H
C
H
C
H
OH H
C
H
C
H
OH
C
H
At first glance it looks as if poly(ethenol) could be prepared from the unsaturated monomer ethenol (vinyl alcohol):
H OH
C C
H H
(Ethenol is like ethanol but it contains a carbon–carbon double bond)
H OH
C C H
H
C C
O
H H H H
Ethenol, however, is highly unstable and the molecule rapidly rearranges into its more stable isomer, ethanal:
8 C H E M I S T R Y

B A C K G R O U N D N O T E S
As a result of this instability it is impossible to prepare poly(ethenol) directly from ethenol. In practice, it is made from another polymer called poly(ethenyl ethanoate). The latter is manufactured by the addition polymerisation of ethenyl ethanoate, which is an unsaturated ester ( see Figure
6).
Figure 6
Poly(ethenyl ethanoate) is then treated with methanol (CH
3
OH), and is transformed into poly(ethenol) by a process called ‘ester exchange’, which is outlined in Figure 7.
Figure 7
C H E M I S T R Y 9

B A C K G R O U N D N O T E S
The reaction looks quite complicated but if we focus on the groups of atoms which are in bold print we will be able to follow what is happening a bit more clearly. The carboxylic acid part ( CH
3
CO) of the ester side chains on poly(ethenyl ethanoate) is removed and replaced by the H atom from the -O H group in methanol. So as well as poly(ethenol), methyl ethanoate is also formed in the exchange process. By carefully controlling the reaction temperature and the reaction time a variety of poly(ethenol)s can be made.
For example, if the reaction temperature is high and the reaction time is long then virtually all of the acid groups in poly(ethenyl ethanoate) will be exchanged and the resulting poly(ethenol) will contain a high percentage of
-OH groups. On the other hand, if the reaction tempe rature is low and the reaction time is short relatively few acid groups will be replaced and a poly(ethenol) with a low percentage of -OH groups will be formed.
In poly(ethenol), both hydrogen bonds and van der Waals’ bonds operate between the long polymer chains. It is the polar -OH groups attached to poly(ethenol)’s backbone of carbon atoms which allow the hydrogen bonds to be set up. In ‘pure’ poly(ethenol) – poly(ethenol) with a very high percentage of -OH groups – the hydrogen bonding will be extens ive (see
Figure 8) and since hydrogen bonds are relatively strong the chains will be held together quite tightly.
Figure 8
1 0 C H E M I S T R Y

B A C K G R O U N D N O T E S
As the number of -OH groups in poly(ethenol) decreases, the number of acid groups will increase and as a result there will be fewe r hydrogen bonds but more van der Waals’ bonds operating between the chains. Since van der
Waals’ bonds are weaker than hydrogen bonds, the forces of attraction between adjacent polymer chains will weaken and less energy will be needed to break them. We would therefore predict that the temperature at which poly(ethenol) softens and melts will fall as the percentage of -OH groups in its structure decreases. And indeed, this is the case.
It is the presence of the -OH groups in poly(ethenol) which accounts for its solubility. The -OH groups are polar and they can form hydrogen bonds with the polar water molecules. The energy released in making these hydrogen bonds is sufficient to overcome the attraction between the polymer chains, thus separating them and allowing them to dissolve. As was mentioned earlier, the percentage of -OH groups in poly(ethenol) can vary. As a consequence we would predict that its solubility in water will also vary. We could further argue that as the number of -OH groups on the polymer chains decreases the solubility would also decrease since there would be fewer opportunities for hydrogen bonds to be set up between the chains and the water molecules. The data in Table 1 clearly indicate that poly(ethenol)’s solubility in water does in fact depend on the percentage of -OH groups present.
Table 1
Percentage of -OH groups
100 – 99
Solubility insoluble
98 – 97
96 – 90 soluble in hot water soluble in warm water below 90 soluble in cold water
However, the data also show that t he variation is not in the direction we had predicted. For example, we reasoned that ‘pure’ poly(ethenol) with its very high percentage of -OH groups would have the greatest solubility. But it is insoluble, even in hot water! The explanation for this u nexpected trend lies in the strength of the bonding between the polymer chains. In ‘pure’ poly(ethenol) the interchain bonding will be at its strongest due to the extensive network of hydrogen bonds (see Figure 8). Far too much energy would be needed to break these bonds, preventing the polymer from dissolving. But as the number of -OH groups decreases the solubility increases. Fewer -OH groups imply fewer hydrogen bonds and therefore less
C H E M I S T R Y 1 1

B A C K G R O U N D N O T E S attraction between the molecules. The water molecules are abl e to break these bonds more readily, thus separating the chains and allowing them to dissolve. The solubility of poly(ethenol), however, reaches a maximum and then decreases until eventually the polymer becomes insoluble once more.
Poly(ethenol) has long been used in adhesives, paper coatings and for sizing
(stiffening) textiles. But in recent applications its variable water solubility has been more fully exploited. It is used, for example, in making hospital laundry bags. The dirty laundry is placed in the bag and because poly(ethenol) is impermeable to bacteria it provides an effective barrier to isolate any infected material. When the bag is placed in the washing machine it quickly dissolves and releases the dirty laundry. Poly(ethenol) with a hi gh percentage of -OH groups has to be used because it must be soluble only in hot water. Were it soluble in cold water, any damp laundry would dissolve the bag and expose hospital staff to the risk of infection. Threads of poly(ethenol) are used in surgery for internal suturing or stitching. The type of poly(ethenol) the surgeon uses will depend on how long the stitches have to remain in place before they dissolve. Poly(ethenol) has also found application as a medium in which to deliver medicines within a patient.
Pellets of the polymer impregnated with the medicine are implanted in the body. As the polymer slowly dissolves the medicine is released in a continuous and controlled manner. Poly(ethenol) also finds use as a coating for seeds. When the seed is sown the coating absorbs water from the soil and provides a protected environment in which the seed can germinate. The poly(ethenol) will slowly dissolve and disintegrate by which time the seedling will be firmly established.
One of the great advantages of poly(ethenol) is its biodegradability. It is the presence of the -OH groups in its structure that make this possible. The degradation of the large poly(ethenol) molecules into smaller, harmless products is catalysed by enzymes.
A field of research that has been developing in recent years is that of conducting polymers. Because of their simple molecular covalent structure, organic polymers are normally electrical insulators and are widely used as such. To have polymers which are able to conduct electricity is therefore a remarkable phenomenon.
1 2 C H E M I S T R Y

B A C K G R O U N D N O T E S
Poly(ethyne) is a conducting polymer and it is manufactured from the monomer unit ethyne, which has the following structural formula:
H C C H
Ethyne is the simplest member of the family of hydrocarbons called alkynes, which are characterised by the presence of a carbon –carbon triple bond. It is a linear molecule and it undergoes addition polymerisation (see Figure 9) to form poly(ethyne).
Figure 9
Notice that the backbone of the polymer contains alternating carbon –carbon single and double bonds. The way the polymer structure has been drawn suggests that the backbone of carbon atoms is linear but, in fact, this is not the case. To get the true picture we have to dig ress a little and consider a topic in chemistry known as ‘geometric isomerism’. We know that isomers are compounds which have the same molecular formula but their atoms are arranged in different ways and we are already familiar with one type of isomerism, namely structural isomerism. Here the atoms in the isomers are linked together in a different order or sequence. Butane and methylpropane, for example, are structural isomers and they are presented in Figure 10.
C H E M I S T R Y 1 3

B A C K G R O U N D N O T E S
In butane the four carbon atoms are l inked in a continuous chain whereas in methylpropane the sequence of atoms is different – three carbon atoms are in a chain and the fourth is attached to the middle atom of the chain.
Geometric isomerism is just another type of isomerism. In this case, however, the atoms in the isomers are linked in the same order but their spatial arrangements are different. To illustrate geometric isomerism, let’s look at but-2-ene:
H
3
C HC CH CH
3
It contains a carbon–carbon double bond and Figure 11 shows the spatial arrangement of the bonds around each carbon atom.
Figure 11
This arrangement is completely flat and all the bond angles around each carbon atom are 120°. Furthermore, the bonds are fixed in relation to one another. This implies that it is impossible to rotate one end of an alkene molecule around the double bond while the other end is fixed. It is for this reason that some alkenes can exhibit geometric isomerism. Returning to our example, we find that there are two geometric isomers of but -2-ene. One is referred to as the cis isomer and the other is the trans isomer, and their structures are outlined in Figure 12.
Figure 12
H
3
C CH
3
H CH
3
H cis
C C
-but-2-ene
H H
3
C
C C trans -but-2-ene
H
1 4 C H E M I S T R Y

B A C K G R O U N D N O T E S
‘ Cis ’ means ‘on the same side’ and we can see that in cis -but-2-ene both methyl groups lie on the same side of the double bond. ‘
Trans
’ means ‘on opposite sides’ and so in trans -but-2-ene the methyl groups are on opposite sides of the double bond. It is important to reme mber that isomers – whether they be structural or geometric – are different compounds with distinctive physical properties. For example, cis -but-2-ene melts at –139°C and boils at
4°C while the trans isomer melts at –106°C and boils at 1°C.
Let’s now apply our new-found knowledge to poly(ethyne) with its alternating carbon–carbon single and double bonds (see Figure 13).
Figure 13
H H H H H H H H H H H H
C C C C C C C C C C C C
At one extreme we could have a cis arrangement around every double bond.
This form would be called cis -poly(ethyne) and would have the shape illustrated in Figure 14.
Figure 14 cis -poly(ethyne)
Trans -poly(ethyne) would lie at the other extreme and its shape is shown in
Figure 15.
Figure 15 trans -poly(ethyne)
C H E M I S T R Y 1 5

B A C K G R O U N D N O T E S
Here, we have a trans arrangement around every double bond. In addition to cis -poly(ethyne) and trans -poly(ethyne), there will be countless forms in which there are both cis and trans arrangements. Figure 16 shows the s hape of just one of these mixed forms.
Figure 16
But a poly(ethyne) sample containing molecules with so many different shapes would find little use. What was needed was a method of preparing poly(ethyne) in which all the molecules were identical in sh ape – either all cis or all trans , for example. Fortunately in the early 1950s, catalysts had been developed which controlled the way monomers link to each other as they polymerise, thus ensuring that polymer molecules of the same shape were consistently produced. These catalysts are known as Ziegler –Natta catalysts, so-called after their discoverers, Karl Ziegler and Giulio Natta. Their work revolutionised polymer chemistry, opening up almost countless possibilities for chemists to design polymers to su it any purpose. In fact, it was chemists in Natta’s research group who first prepared poly(ethyne) in 1955. The polymer they made consisted of a mixture of both cis -poly(ethyne) and trans poly(ethyne). They didn’t develop it further and it was another t wenty years before any significant advance was made on the poly(ethyne) front. A group in Tokyo headed by Hideki Shirakawa and Sakuji Ikeda made poly(ethyne) by blowing ethyne gas onto the surface of a solution of a Ziegler –Natta catalyst held at low temperatures. Only the cis -poly(ethyne) formed but as it warmed up it converted entirely into the trans isomer. This happens because of the instability of the cis isomer. If we look back at Figure 14 we can see that some of the hydrogen atoms in cis -poly(ethyne) are very close together and it is the close proximity of these hydrogen atoms and the consequent repulsion between them which makes it unstable. The cis -poly(ethyne) molecule undergoes a rearrangement by rotation about the carbon –carbon single bonds into the more stable trans isomer (see Figure 15) where the hydrogen atoms are further apart and repulsion between them is minimised. The Japanese team found that trans -poly(ethyne) conducted electricity only very poorly but
1 6 C H E M I S T R Y

B A C K G R O U N D N O T E S a few years later it was discovered that if certain chemicals, like iodine for example, were introduced into the trans -poly(ethyne) sample its conductivity dramatically increased and matched that of some metals. The added chemical is known as a dopant.
So why does doped trans -poly(ethyne) conduct electricity? If we look back at the structure of trans -poly(ethyne) (Figure 15) we can see that each carbon atom bonds to three other atoms and in so doing uses up three of its four outer electrons. The remaining electron is free and it along with one from each of the other atoms in the chain occupies a molecular orbital which extends along the carbon backbone of the molecule. These electrons are delocalised and therefore free to conduct electricity. They are represented by the dotted line in Figure 17.
Figure 17
While individual trans -poly(ethyne) molecules are good conductors, a collection of them – as you would get in a fibre or a film – will conduct electricity very poorly. This is not unexpected since the delocalised electrons are unable to ‘hop’ from one molecule to another. But we now know that trans -poly(ethyne) doped with iodine is an excellent conductor. The iodine is capable of accepting electrons from the poly(ethyne) molecules and in so doing is reduced to iodide ions:
I2 + 2e

2I–
The reaction is reversible and the iodide ions, on oxidation, can push electrons onto adjacent poly(ethyne) molecules:
2I–

I2 + 2e
Essentially the iodine molecules ‘bridge’ the gap between the polymer molecules and provide a pathway for delocalised electrons to move from one polymer molecule to another. In this way the bulk sample, and not just individual molecules within it, conducts electricity.
C H E M I S T R Y 1 7

B A C K G R O U N D N O T E S
Most conducting polymers, including doped poly(ethyne), are not stable in air and as they react with oxygen and/or water their conductivity rapidly falls.
As a result of its instability no applications of poly(ethyne) yet exist. But once a stable form has been developed it is likely to have a variety of applications, particularly in the el ectronics industry. Of the few air -stable conducting polymers that do exist, one (doped poly(pyrrole)) has already found use as a membrane in high -performance loudspeakers.
One of the more unusual polymers commercially exploited in recent years is poly(vinyl carbazole). When it is doped with a substance called fluorenone it becomes photoconductive , which means that it conducts electricity when it is exposed to light. Its monomer unit is vinyl carbazole, which has the structure:
The branch containing the benzene rings and the nitrogen atom is the
‘carbazole’ part. Since vinyl carbazole contains a carbon –carbon double bond it undergoes addition polymerisation to form poly(vinyl carbazole).
The reaction is outlined in Figure 18.
Figure 18
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B A C K G R O U N D N O T E S
As a result of its extraordinary photoconducting property, doped poly(vinyl carbazole) has found use in photocopiers, replacing selenium which is toxic.
The various steps involved in making a photocopy and poly(vinyl carbazole)’s key role in the process are illustrated in Figure 19.
Figure 19
The drum in the photocopier has a very thin
‘three-layered’ film (10 –5 m) attached to its surface. Sandwiched between the outer and inner layers is the doped poly(vinyl carbazole).
In the first stage of the photocopying process the film’s outer layer is given a positive electrical charge.
The document to be photocopied is placed on the glass plate. It is then exposed to a light which scans across its surface.
When the light strikes dark areas on the document most of it is absorbed but when it strikes light areas it is reflected through a series of mirrors and lenses onto the surface of the drum. The light which reaches the drum ‘triggers’ off electrical conductivity in the doped poly(vinyl carbazole).
Some of the electrons in the exposed areas are thus freed and they neutralise the positive charges on the film.
The other areas which were not exposed to light retain their positive charge. In this way an image of the document is projected onto the drum.
C H E M I S T R Y 1 9

B A C K G R O U N D N O T E S
The toner particles in the cartridge are negatively charged and as the drum rotates they are attracted only onto those areas of its surface that have a positive charge. The toner powder sticks to these areas and the drum then has an exact copy of the document held on its surface.
A sheet of paper is passed through rollers and as the drum rotates the toner powder is transferred onto the paper. A charge underneath the paper attracts the toner from the drum.
The final stage in the photocopying process occurs as the paper passes between two hot rollers, fusing the image onto the paper.
Synthetic polymers are highly stable and although it is this durability which makes them so useful it has created a problem in their disposal. Was te polymers are normally disposed of in landfill sites but because they don’t readily break down or degrade in the environment, these landfill sites are rapidly reaching capacity. With applications of polymers becoming increasingly important in our modern society, the problem of disposal will grow ever more severe. Recycling and incineration of polymers are possible solutions but another tactic in dealing with the plastic industry’s environmental problems is to make biodegradable polymers. Biopolymers are such a group – they are natural polymers and many have properties similar to synthetic polymers. Unlike synthetic polymers, however, they can be broken down by microorganisms present in the environment, i.e. they are biodegradable. Biopolymers have bee n the subject of intense research over the past few decades and one which showed a great deal of promise was p oly(3h ydroxy b utanoate) or PHB for short.
It was as long ago as 1925 that it was discovered that certain bacteria were able to synthesise PHB. These bacteria use it as an energy store rather like
2 0 C H E M I S T R Y

B A C K G R O U N D N O T E S plants use starch and animals use fats. This phenomenon was regarded merely as a scientific curiosity up until the early 1960s when the chemical industry first showed interest in producing PHB commer cially. This is achieved by inoculating a fermentation mixture with the PHB -making bacterium Alcaligenes eutrophus . The bacterial cells grow and multiply until the phosphate nutrient in the fermentation mixture is used up. At this point glucose is introduced and it ferments to form ethanoic acid, which the bacterium uses to synthesise PHB. Pairs of ethanoic acid molecules (C
2 units) are coupled together and converted into 3 -hydroxybutanoic acid (a C
4 unit), which then undergoes polymerisation. The PHB is finally harvested by breaking open the bacterial cells and extracting the polymer.
We know that PHB is a biopolymer but what can we deduce about its nature?
Its monomer, 3-hydroxybutanoic acid, has the following structure:
It contains both a hydroxyl group and a carboxyl group and it is through these that the monomers are able to link together, as outlined in Figure 20.
Figure 20
At each reaction point a water molecule is eliminated and an ester linkage
(-COO-) is formed therefore not only is PHB a condensation polymer it is also a polyester.
C H E M I S T R Y 2 1

B A C K G R O U N D N O T E S
In practice a modified form of PHB is made in industry. The change was achieved by adding small amounts of propanoic acid to the fermenting glucose. As well as making 3-hydroxybutanoic acid the bacterium also makes
3-hydroxypentanoic acid by coupling together ethanoic acid (a C
2
unit) and propanoic acid (a C
3
unit) molecules. The two monomers then condense in random order to form the polymer PHB-V . The ‘V’ derives from pentanoic acid’s traditional name of v aleric acid. Part of the structure of a PHB -V molecule is drawn in Figure 21.
Figure 21
We can see that this part of the chain has been derived from two
3-hydroxybutanoic acid monomers and two 3 -hydroxypentanoic acid monomers. A whole range of PHB-V polymers are possible depending on the amounts of propanoic acid introduced into the fermentation mixture during manufacture.
Although strong covalent bonds hold the atoms together inside the PHB -V molecules, only weak van der Waals’ bonds and, to a limited extent, hydrogen bonds operate between the polymer molecules. Very little energy is needed to break the intermolecular bonds and this explains why PHB -V polymers soften and melt at relatively low temperatures – they are thermoplastics. As the 3-hydroxypentanoic acid content of PHB-V increases so too does the flexibility and toughness of the polymer and it was for these reasons that the modification to PHB was made.
PHB-V was launched on the market in 1990 under the trade name Biopol – so-called because it is a biopol ymer. It found use as a packaging material for cosmetics and motor oils although its main applications were thought likely to be medical – for surgical stitches and in the controlled release of medicines into the body, for example. Applications of Biopol in the field of orthopaedics were also under investigation. Here, for example, it could have been used to hold broken bones in place and while the healing process took place the Biopol would slowly degrade.
2 2 C H E M I S T R Y

B A C K G R O U N D N O T E S
Unfortunately, Biopol has not proved to be commercially successful.
Production ceased early in 1999 despite the major advantages it seemed to have over traditional synthetic polymers – its biodegradability and the fact that its main feedstock (glucose) is renewable. The cruci al reason for its demise was the high production costs – it is about 15 times more expensive to make Biopol than to make poly(ethene), for example. Another reason has been a recent shift in attitude among manufacturers away from biodegradable to recyclable polymers. But there are situations where it is inappropriate to recycle polymers and so the current halt in the commercial production of biopolymers is likely to prove a temporary one.
Another approach in dealing with polymer waste is to take existing polymers and redesign them in such a way that they become degradable. The degradation process, however, has to be relatively slow because polymers must have a reasonable lifetime – shoppers, for example, would not appreciate carrying their purchases in bags which would rapidly degrade from the moment of their production. It would be even better to have polymers in which the degradation process was triggered off, for example by exposure to ultraviolet light.
Low density poly(ethene)* (LDPE) is just one of a number of synthetic polymers which have been redesigned into photodegradable polymers.
During the manufacture of LDPE carbonyl groups (C=O) are incorporated into the backbone of the poly(ethene) molecules. Pa rt of such a modified polymer chain is shown in Figure 22.
Figure 22
* Poly(ethene) is manufactured in three main forms and LDPE is one of these. The poly(ethene) molecules in LDPE have long hydrocarbon branches attached to their backbone and they prevent close packing of the molecules. It is this more open structure of LDPE which leads to its lower density.
C H E M I S T R Y 2 3

B A C K G R O U N D N O T E S
When the modified LDPE is exposed to strong sunlight, the carbonyl groups absorb some of its ultraviolet radiation and trap the energy associ ated with it.
This energy is used to break the carbon –carbon bonds in the vicinity of the carbonyl groups and as a consequence the polymer chain breaks down into shorter fragments. Continued exposure to sunlight causes the polymer to disintegrate into increasingly smaller fragments, which can then biodegrade.
By varying the number of carbonyl groups in the polymer chains the rate of degradation on exposure to sunlight can be carefully controlled.
Photodegradable LDPE is already used as can rings round s ix-pack drinks and as a gardening mulch. However, other applications are likely to come on stream – for example, as carrier bags, as packaging material and as protective films around young plants.
Take away synthetic polymers and we r evert to the Iron Age
2 4 C H E M I S T R Y

S U M M A R Y
Although the Plastic Age commenced in 1846 with the accidental synthesis of nitrocellulose it was some eighty years before it really took off. It was during the 1920s that chemists developed a comprehensive understandin g of the structure and properties of polymers and this marked the start of a revolution in polymer chemistry which to this day continues unabated.
Polymer chemistry has progressed to such a sophisticated level that nowadays many polymers are chemically tailored to have specific properties that meet specific needs. Some of these new designer polymers are described here.
Kevlar is one of these new polymers and its main attribute is its strength – weight for weight, Kevlar is about twenty times stronger tha n steel. Kevlar is a polyamide made by the condensation reaction between diamine and dicarboxylic acid monomers:
Kevlar molecules are rigid and rod-like and they line up parallel to one another, forming flat sheets. Hydrogen bonds operate between the polymer chains, keeping them aligned with each other and the flat molecular sheets stack together around the fibre axis:
C H E M I S T R Y 2 5

S U M M A R Y
Hydrogen-bonded molecular sheet Stacked sheets
It is the high degree of molecular alignment within the fibre which gives
Kevlar its exceptional strength. As well as being very strong, Kevlar is resistant to heat, to fire, to chemical attack and to abrasion and with such a unique combination of properties it finds many specialist applications. It is used to reinforce tyres and in making brake pads, clutch linings, ropes and cables. It is also used in the construction of aircraft, space vehicles and boats. It is a major component in the protective clothing worn by firefighters and motorcyclists and it finds use in bullet -proof vests.
Unlike the vast majority of synthetic polymers, poly(ethenol) is soluble in water. It is an addition polymer and has the following structure:
H OH H OH H OH
C C C C C C
H H H H H H
2 6 C H E M I S T R Y

S U M M A R Y
Although it appears as if poly(ethenol) is made from the unsaturated monomer ethenol,
H
C
H
OH
C
H this is impossible since ethenol is highly unstable. In practice, poly(ethenol) is made by treating another polymer, called poly(ethenyl ethanoate), with methanol in a process known as ‘ester exchange’:
In the ‘exchange’ process, the acid part (CH
3
CO-) of the ester side chains on poly(ethenyl ethanoate) is removed and replaced by the H atom from the -OH group in methanol. By careful control of reaction time and temperature a variety of poly(ethenol)s can be made. For example, if the reaction temperature is high and the reaction time is long then virtually all of the acid groups will be exchanged and a poly(ethenol) containing a very high percentage of -OH groups will be formed.
Both hydrogen bonds and van der Waals’ bonds operate between adjacent poly(ethenol) molecules and the numbers of each type of bond will vary according to the percentage of -OH groups present. In a poly(ethenol) with a
C H E M I S T R Y 2 7

S U M M A R Y high percentage of -OH groups the hydrogen bonding will be much more extensive than the van der Waals’ bonding and since hydrogen bonds are stronger than van der Waals’ bonds the polymer chains will be held together more tightly:
It is the polar -OH groups in the poly(ethenol)s that account for their solubility and, contrary to what one might have expected, water solubility increases as the percentage of -OH groups decreases. In practice, poly(ethenol)s with a very high content of -OH groups (99–100%) are insoluble. In such cases the hydrogen bonding is so extensive that far too much energy would be needed to break these bonds, thus preventing the molecules from separating and dissolving. But as the number of -OH groups decreases there will be fewer hydrogen bonds and as a result less ener gy will be required to break them, thus allowing the molecules to separate and dissolve. Poly(ethenol)s with an -OH group content of just below 90% are soluble even in cold water.
Poly(ethenol) is not a new polymer and has long been used in adhesives, paper coatings and in sizing (stiffening) textiles. Recent applications, however, are different and in these the variable water solubility of the poly(ethenol)s has been exploited. Hospital laundry bags, for example, are made from a poly(ethenol) which is soluble only in hot water. Dirty linen is safely contained in the bag until it is placed in the washing machine where the bag dissolves and the laundry is released into the wash. Threads of
2 8 C H E M I S T R Y

S U M M A R Y poly(ethenol) are used in surgery for internal stitching – the type of poly(ethenol) employed depends on how long the stitches have to remain in place before they dissolve. Poly(ethenol) has found use as a medium in which to deliver medicines into a patient’s body in a continuous and controlled manner. It is also used as a coating for seeds.
One of the most exciting developments in recent years has been that of conducting polymers. The first to be made was poly(ethyne) which is an addition polymer:
In poly(ethyne) each carbon atom uses three of its four outer el ectrons to bond with three other atoms. The remaining electron along with one from each of the other carbon atoms in the chain are delocalised and free to conduct electricity. Although these electrons are delocalised within a molecule they can’t ‘jump’ from one molecule to another. As a result, a collection of poly(ethyne) molecules – as you would get in a fibre or film – will conduct electricity only very poorly. However, if certain chemicals, like iodine for example, are introduced into the sample the conductivity dramatically increases and matches that of a metal. These added chemicals are referred to as ‘dopants’ and they ‘bridge’ the gap between the polymer chains, allowing the delocalised electrons to move from one molecule to another. In this way a bulk sample of poly(ethyne) conducts, and not just individual molecules within it.
Doped poly(ethyne) is unstable in air and as yet it has no practical applications. However, once this problem has been surmounted then it is likely to have wide application in electronic circuitry. Of the few air -stable conducting polymers that do exist, one of them, doped poly(pyrrole), finds use as a membrane in high-performance loudspeakers.
C H E M I S T R Y 2 9

S U M M A R Y
Incidentally, the poly(ethyne) molecule illustrated above is referred to a s trans -poly(ethyne). ‘ Trans
’ means ‘on opposite sides’ and we can see that the two hydrogen atoms attached to each double bond lie on opposite sides of the double bond.
One of the more unusual polymers commercially exploited in recent years is poly(vinyl carbazole) . It is an addition polymer and has the following structure:
When it is doped it becomes photoconductive, i.e. it conducts electricity when exposed to light. Because of its photoconducting property it is widely used in photocopying machines and laser printers, where it has replaced selenium which is toxic.
Biopol is a natural polyester and its major advantage over traditional synthetic polymers is its biodegradability. Biopol is produced not in the conventional manner but by fermentation of mixtures containing both ethanoic and propanoic acids by the bacterium Alcaligenes eutrophus . From these acids the bacterium first synthesises the monomers 3 -hydroxybutanoic acid and 3-hydroxypentanoic acid, and then polymerises the latter into
Biopol:
3 0 C H E M I S T R Y

S U M M A R Y
Biopol was launched onto the market in 1990 but it didn’t prove to be commercially successful and production ceased early in 1999. It found use as a packaging material for cosmetics and motor oils although its main applications were thought likely to be medical – for surgical stitches and in the controlled release of medicines into the body, for example. The major reason for Biopol’s demise was its high production costs. Another reason was a shift in attitude among manufacturers away from biodegradabl e polymers to recyclable polymers.
One approach in dealing with the growing problem of polymer waste is to take existing polymers and redesign them in such a way that they become degradable. Low density poly(ethene) (LDPE) is one such polymer and it has been redesigned so that it degrades on exposure to ultraviolet light. During the manufacture of photodegradable LDPE carbonyl groups (C=O) are introduced into the backbone of the poly(ethene) molecules:
On exposure to strong sunlight, the carbonyl group s absorb some of the ultraviolet radiation and use the energy to break carbon –carbon bonds in the backbone. In this way the long-chain molecules are broken down into much smaller fragments, which can biodegrade more readily.
Photodegradable LDPE is currently used as a garden mulch and as can rings in six-pack drinks and it is likely to find extensive use as a packaging material.
C H E M I S T R Y 3 1

S U M M A R Y
3 2 C H E M I S T R Y

A C T I V I T I E S
Activity 1 – making ‘slime’
In this activity you will use an aqueous solution of poly(ethenol) to make a novel polymeric material called ‘ slime ’. You will investigate some of the properties of ‘slime’ and relate these to its structure.
Introduction
‘Slime’ is made by mixing solutions of sodium borate and poly(ethenol).
You will recall that the structure of pol y(ethenol) resembles that of poly(ethene) but unlike poly(ethene) it has a hydroxyl group attached to every second carbon atom in its backbone:
For clarity, the carbon and hydrogen atoms in the backbone have not been drawn.
On mixing poly(ethenol) with sodium borate, the borate ions, B(OH)
4
–
, form hydrogen bonds with the -OH groups and so cross-link adjacent polymer chains.
Cross-linking also occurs in thermosetting plastics such as polyester resins.
The cross-links in ‘slime’, however, are much easie r to break than those in thermosets because hydrogen bonds are weaker than covalent bonds.
C H E M I S T R Y 3 3

A C T I V I T I E S
Requirements
100 cm 3 beaker or polystyrene cup flat wooden stick (for stirring), e.g. a lollipop stick measuring cylinders (10 cm 3 and 50 cm 3 ) poly(ethenol) solution sodium borate solution food colouring or fluorescein
Care
Wear eye protection and gloves
Once you have made the ‘slime’ do not remove it from the lab
Procedure
(a) Measure 25 cm 3 of poly(ethenol) solution into the beaker and to it add a few drops of food colouring or fluorescein dye.
(b) Measure out 5 cm 3 of sodium borate solution and add it with stirring to the poly(ethenol) solution. Continue to stir the mixture vigorously until it sets to a gel.
(c)
Remove the ‘slime’ from the beaker and carry out the following tests:
• roll it into a sausage shape and pull it slowly from either end
• roll it into a sausage shape again but this time pull it abruptly
• roll it into a ball and place it on a flat surface for a short time
• roll it into a ball again and drop it on the floor – does it bounce to any extent?
• add a small piece of the ‘slime’ to water and stir – does it dissolve?
(d)
From your observations write a brief note on the properties of ‘slime’ and try to explain these properties in terms of its structure.
Activity 2 – altering ‘slime’
Design and plan an investigation to find out what effect varying the proportion of sodium borate has on the properties of ‘slime’.
Present the plan to your teacher/lecturer and once you get the go -ahead carry out the investigation.
Write a brief report on your investigation and explain any differences you find in the properties of the samples in terms of the structure of ‘slime’.
3 4 C H E M I S T R Y

A C T I V I T I E S
Activity 3 – solubility of poly(ethenol)
Design and plan an investigation to find out the rate at which poly(ethenol) film dissolves in water at different temperatures.
Present the plan to your teacher/lecturer and once you get the go -ahead carry out the investigation.
Write a brief report on your investigation and explain any differences you find in terms of the structure of poly(ethenol).
Activity 4 – preparation and properties of poly(pyrrole)
Your teacher/lecturer will demonstrate the preparation of doped poly(pyrrole) , which is a conducting polymer like doped poly(eth yne).
Pyrrole is an unsaturated nitrogen -containing organic compound. Its full and abbreviated structural formulae are:
It polymerises to form poly(pyrrole):
C H E M I S T R Y 3 5

A C T I V I T I E S
As in poly(ethyne) some of the electrons in poly(pyrrole) are delocalised and are free to move along the whole length of the polymer chain. They are represented by the dotted lines in the following diagram:
Only when the poly(pyrrole) is doped is it able to conduct electricity. The dopant bridges the gaps between the polymer molecules and allows the sample as a whole to conduct.
Write a brief report on the preparation and properties of doped poly(pyrrole).
3 6 C H E M I S T R Y

A C T I V I T I E S
Activity 1 – making slime
The following solutions are required to make ‘slime’:
4% borax, sodium tetraborate decahydrate (Na
2
B
4
O
7
.10H
2
O)
4% poly(ethenol) (polyvinyl alcohol)
It is recommended that poly(ethenol) of high molecular mass (85000 –
146000) be used. It should also be highly hydrolysed (~98% hydrolysed).
4% poly(ethenol) solution can be prepared in the following way:
4 g of poly(ethenol) powder ( care – avoid breathing the powder) is added to
100 cm 3 of water by slowly sprinkling it onto the surface of the water with vigorous stirring. This must be done carefully to avoid lumps forming.
Continue to stir the mixture at room temperature for about ten minutes to allow the slow process of swelling to begin. Heat the mixture slowly and with constant stirring to about 90°C. Be careful not to overheat. Maintain the temperature of the mixture around 90°C and continue to stir it until a clear, almost colourless viscous solution is formed.
The poly(ethenol) solution can be kept for several months.
Guar gum (a natural polysaccharide) can be used as a substitute for poly(ethenol).
Activity 2 – altering ‘slime’
The same poly(ethenol) and sodium borate solutions that were used in
Activity 1 can be used here.
Different ‘slime’ samples can be prepared by adding varying volumes
(between 1 cm 3 and 10 cm 3 ) of sodium borate solution to 25 cm 3 portions of poly(ethenol).
Activity 3 – solubility of poly(ethenol)
A hot-water soluble variety of poly(ethenol) film is suitable for this investigation.
C H E M I S T R Y 3 7

A C T I V I T I E S
Activity 4 – preparation and properties of poly(pyrrole)
Requirements copper foil (approx. 2 cm x 5 cm) nickel foil (approx 2 cm x 5 cm)
12 V DC supply pyrrole
0.1 mol l
–1
sodium
4-methylbenzenesulphonate
(sodium salt of p -toluenesulphonic ammeter to read to 35 mA variable resistor (0 – 5000

) emery paper
‘Brasso’ polish acid) propanone connecting wires beaker conical flask
Hazards
Both pyrrole and sodium 4-methylbenzenesulphonate are irritants. In addition, pyrrole is harmful and flammable.
Propanone is highly flammable.
Procedure
1. Clean the copper foil with emery paper and rinse it with distilled water.
2. Clean the nickel foil with ‘Brasso’ and rinse it first with distilled water and then with propanone. Leave it to dry. It is important that the nickel surface be very clean and smooth.
3. Preferably in a fume cupboard, drip 0.34 g of pyrrole into a conical flask and to this add 100 cm 3 of sodium methylbenzenesulphonate solution.
Swirl the contents of the flask until the pyrrole has dissolved.
4. Transfer this solution to a beaker and set up an electrolysis circuit to include the ammeter and variable resistor. Make the nickel the positive electrode and the copper the negative electrode.
5. Adjust the variable resistor to give a current of about 30 mA and electrolyse the solution for about 45 minutes. The nickel electrode should turn black.
6. Switch off the current, remove the nickel electrode and wash it with distilled water.
7. Carefully peel off the poly(pyrrole) film from the nickel electrode.
3 8 C H E M I S T R Y

A C T I V I T I E S
8. Allow the film to dry and test its electrical conductivity. Do not apply too high a voltage otherwise the poly(pyrrole) may get hot and decompose
(about 12 V should suffice).
Chemicals required for the activities can be obtained from:
Sigma-Aldrich Co. Ltd
The Old Brickyard
New Road
Gillingham
Dorset
SP8 4XT
C H E M I S T R Y 3 9

A C T I V I T I E S
4 0 C H E M I S T R Y

Q U E S T I O N S
1. Copy and complete the following table giving the polymer type (addition or condensation), the main properties and some uses or potential uses for each polymer. When you draw the table expand t he last two columns to give you more room to write in the ‘properties’ and ‘uses’.
Polymer type Main properties Uses
Polymer
Kevlar
Poly(ethenol)
Doped poly(ethyne)
Poly (vinyl carbazole)
Biopol
Photodegradable low density poly(ethene)
2. Part of the structure of a Kevlar molecule and that of a related polymer molecule called Nomex are illustrated below:
Kevlar
Nomex
C H E M I S T R Y 4 1

Q U E S T I O N S
(a) Kevlar and Nomex are aromatic polyamides.
Why can they be described as ‘ aromatic ’?
(b) Draw the structural formulae of the two monomers from which Kevlar is made.
(c) Draw the structural formulae of the two monomers from which Nomex is made.
(d) One of the reasons why Kevlar is so strong is because of the close packing of the polymer molecules.
Name the type of bonds that operate between the polymer chains.
(e) Nomex is not as strong as Kevlar.
Suggest a reason for this.
(f) Fibre B is another aromatic polyamide. It has a structure almost identical to that of Kevlar but is made fro m a single monomer rather than two monomers. The monomer used to make Fibre B is called
4-aminobenzoic acid and it has the structure:
(i) Draw part of the structure of Fibre B showing three monomer units linked together.
(ii) Similarly, a polymer with a structure very like that of Nomex can be derived from a single monomer.
Draw the structural formula of this monomer and name it.
3. Part of the structure of a poly(ethenol) molecule is illustrated below:
4 2 C H E M I S T R Y

Q U E S T I O N S
(a) Draw the structure of the repeating unit in poly(ethenol) and hence draw the structure of the monomer unit from which it appears to be made.
(b) Explain why poly(ethenol) is not derived from the monomer unit whose structure you’ve just drawn in part (a).
(c) Outline briefly how poly(ethenol) is made.
(d) In terms of intermolecular bonding, explain why a poly(ethenol) in which the percentage of -OH groups is 95% is soluble in water yet
‘pure’ poly(ethenol) (100% -OH groups) is insoluble.
4. Part of the structure of trans -poly(ethyne) is shown below:
(a) Draw the structural formula of its monomer and name the homologous series to which it belongs.
(b) Explain why trans -poly(ethyne) does not conduct electricity but when it is doped with iodine it does.
(c) Draw part of the structure of a poly(ethyne) molecule in which the arrangements around the carbon–carbon double bonds are in a ‘ trans , cis , trans , trans , cis , cis ’ sequence.
5. Explain why poly(vinylcarbazole) finds use in photocopiers.
6. Biopol is a polyester made by the bacterium Alcaligenes eutrophus from
3-hydroxybutanoic and 3-hydroxypentanoic acids.
(a) Why is Biopol so named?
(b) Draw the structural formulae of the two monomers used to make
Biopol.
(c) Draw the structure of part of the Biopol molecule made by polymerising three monomer units in the order
H I S T O R Y 4 3

Q U E S T I O N S
3-hydroxybutanoic acid, 3-hydroxypentanoic acid and
3-hydroxybutanoic acid.
(d) Why can Biopol be described as an ‘environmentally friendly’ polymer?
7. The structure of low density poly(ethene) (LDPE) can be modified to make it photodegradable.
(a) Name the functional group which is introduced into LDPE’s backbone to make it photodegradable.
(b) Explain why the introduction of this functional group makes LDPE photodegradable.
(c) Suggest some situations in which the use of photodegradable LDPE would be unsuitable.
(d) Explain what would happen to photodegradable LDPE if it were disposed of in landfill sites.
4 4 C H E M I S T R Y