Chapter 15: Introduction to Polymer Chemistry
In the 1930s a research group at the American company DuPont set out to answer a question that sounds deceptively simple: could a chemist build, in a flask, a molecule as long as the ones nature builds? Wallace Carothers, who led the group, was convinced that silk, cotton and rubber behave the way they do because they are made of ordinary small units joined end to end into very long chains — and that such chains could be assembled deliberately.
His team began linking a diacid with a diamine. Every time an acid group met an amine group, a molecule of water was pushed out and a new bond took its place. Repeated thousands of times, this quiet little reaction produced a thick molten mass. When a worker touched a glass rod to the melt and drew it upward, a fine thread came away — and the thread grew stronger the more it was stretched.
That thread was nylon. Within a few years it was being spun into stockings, and during the Second World War into parachutes and ropes. A material with no counterpart in nature had been built to order from small molecules.
The same idea now shapes the bucket in your kitchen, the pipe under your street, the tyre on your bus and the non-stick coating on your pan.
Diagram
Fig. 15.0 – A single small monomer unit shown at the left, an arrow labelled “polymerization” leading to a long repeating chain at the right, with everyday polymer objects (nylon rope, plastic bucket, tyre, water bottle) drawn around the chain.
Keep these questions in mind as you read — the chapter will give you everything you need to reason your way to each answer.
15.1 Introduction
Today, the overall development in polymer science and technology has enriched human life in a way few other advances have. The world would be a totally different place without polymers such as artificial fibres, plastics and elastomers.
Look around and the point makes itself. From the throwaway candy wrapper to the artificial heart, polymers touch our lives as does no other class of material. In short, we are living in the world of polymers.
Because so much knowledge has been built up in this field, and because the applications keep increasing in everyday life, polymer chemistry emerged as a separate branch of chemistry during the last several decades. It is no longer a corner of organic chemistry; it is a subject in its own right.
What Exactly Is a Polymer?
Chemically, polymers are complex, giant macromolecules. A macromolecule simply means a very large molecule — one built by joining a great many small molecules together with covalent bonds.
These giant molecules are made from repeating units, and those repeating units are derived from small molecules called monomers. The monomer is the raw material; the repeating unit is what that monomer becomes once it has been stitched into the chain.
The name itself tells the story. The term ‘polymer’ originates from the Greek word ‘poly’, meaning many, and ‘mer’, meaning part or unit. Interlinking of many units constitutes polymers.
The sequence from a small molecule to a giant one is worth fixing in your mind before we go further, and it is summarised in FC 15.1.
Flowchart
FC 15.1 – Building a polymer: many small monomer molecules → joined end to end by covalent bonds → repeating units linked into a long chain → giant macromolecule (polymer) of molecular mass 10³–10⁷ u
How Big Is a Polymer Molecule?
Polymers are high molecular mass macromolecules (\(10^3 - 10^7\ \text{u}\)).
To feel what that means, compare it with molecules you already know. Water has a molecular mass of about \(18\ \text{u}\) and glucose about \(180\ \text{u}\); a polymer molecule is thousands to millions of times heavier. The symbol u stands for the unified atomic mass unit, the same unit in which you have been writing atomic and molecular masses since Class 11.
This enormous size is not a curiosity — it is the reason polymers behave as they do. A chain that long can coil, tangle with its neighbours, line up beside them or be pulled straight, and these possibilities are what give plastics, fibres and rubbers their very different mechanical behaviour.
What This Chapter Covers
Both inorganic as well as organic polymers are known. Inorganic polymers such as silicones have chains built around atoms other than carbon; organic polymers have carbon-based chains. In this chapter we will study some aspects of organic polymers.
You have already met an important group of them. In Chapter 14 you learnt about carbohydrates, proteins and nucleic acids, which are important organic biopolymers playing a crucial role in the living world — polymers that living cells build for themselves.
In this chapter we will consider mainly man-made organic polymers, with reference to aspects such as their types, their preparation and their applications.
15.2 Classification of Polymers
Polymers are classified in a number of ways, on the basis of their source, chemical structures, mode of polymerization, molecular forces, type of monomers and biodegradability.
Notice that these are six different questions being asked about the same material, not six different sets of substances. A single polymer such as nylon 6,6 has an answer under every heading at once — it is synthetic, linear, made by condensation, a fibre, a homopolymer, and non-biodegradable. Keep that in mind so the classifications do not feel like a list to be memorised.
The whole scheme is collected as a tree diagram in Fig. 15.2, which you can use as a map while working through this section.
Diagram
Fig. 15.2 – Tree diagram titled “Polymers” branching into six bases: based on source or origin (natural, synthetic, semisynthetic); based on structure (linear, branched chain, cross-linked); based on intermolecular forces (elastomers, fibres, thermoplastic, thermosetting); based on mode of polymerization (addition, condensation, ring opening); no. of monomers (homopolymers, copolymers); biodegradability (biodegradable, non-biodegradable).
15.2.1 Classification of polymers on the basis of source or origin
Polymers are divided into three categories: a. natural, b. synthetic, c. semisynthetic.
a. Natural polymers
The polymers obtained from a natural source are said to be natural polymers. They are further subdivided into two types.
i. Plant polymers: These are obtained from plants. For example, cotton and linen are obtained from the cotton plant and the flax plant respectively. Natural rubber is another example of a natural polymer, which is manufactured from the latex obtained from the bark of rubber trees.
ii. Animal polymers: These are derived from animal sources. For example, wool is obtained from the hair of sheep. Silk is obtained from the silkworm.
b. Synthetic polymers
These are man-made polymers. These polymers are artificially prepared by polymerization of one monomer or copolymerization of two or more monomers.
Nylon, terylene and neoprene are synthetic polymers. These are further divided into three subtypes, namely, fibres, synthetic rubbers and plastics.
The advantage of a synthetic polymer is control. Because the chemist chooses the monomer and the reaction conditions, the properties of the product — strength, flexibility, melting behaviour, resistance to chemicals — can be adjusted to suit the job, in a way that a natural fibre harvested from a plant or an animal cannot be.
c. Semisynthetic polymers
These are derived from natural polymers. These are also called regenerated fibres.
The idea behind the word “regenerated” is worth pausing on. A natural polymer chain is not built from scratch here; an existing natural chain, usually cellulose, is chemically treated so that it dissolves, and is then reformed into a new physical shape such as a thread or a film. The chain is largely the same; its arrangement is new.
Cellulose derivatives such as cellulose acetate rayon, cellulose nitrate, viscose rayon and cuprammonium rayon are a few examples of this category.
Semisynthetic polymers are used in the preparation of non-inflammable photographic films, cinema films, varnishes, etc.
15.2.2 Classification of polymers on the basis of structure
Depending upon how the monomers are linked together, that is, the chain configuration, polymers are classified in three general types: a. linear, b. branched, and c. three dimensional cross-linked polymers. The three arrangements are drawn side by side in Fig. 15.1.
Diagram
Fig. 15.1 – Different chain configurations of polymers: (a) several long unbranched chains of beads lying parallel; (b) a chain of beads carrying side chains branching off it; (c) two or more chains joined to each other by short cross-links, forming a network.
The nature of linking the monomers depends upon the nature and number of functional groups in them.
That last sentence is the key to the whole subsection, so let us unpack it. The number of reactive sites a monomer carries is called its functionality. A monomer with two reactive sites can only grow in two directions, so it gives a chain. A monomer with three or more reactive sites can grow in a third direction as well, so it can branch or link across to a neighbouring chain.
a. Linear or straight chain polymers
When the monomer molecules are joined together in a linear arrangement, the resulting polymer is a straight chain polymer. It is obtained from bifunctional monomers or alkenes, as in Fig. 15.1(a). For example: PVC, high density polythene.
Because the chains are unbranched, they can lie close to one another. Close packing means the chains attract one another strongly over their whole length, which is why linear polymers tend to be denser, stronger and higher melting than their branched versions.
b. Branched chain polymers
The second most common arrangement is that of a branched chain. A monomer having 3 functional groups, or already having side chains, gives rise to branched chain polymers, as in Fig. 15.1(b). For example: low density polythene.
The branches act as spacers. They prevent neighbouring chains from settling close to one another, so the chains are held loosely and the material is less dense and more flexible.
c. Cross-linked polymers
The third type of arrangement is said to be cross linked or network polymers, where cross links are produced between linear chains as shown in Fig. 15.1(c). Cross linking results from polyfunctional monomers. For example, bakelite, melamine.
Here the separate chains are stitched to one another by covalent bonds. In effect the whole moulded article becomes one gigantic molecule, which cannot flow or be reshaped — a point that will explain the behaviour of thermosetting polymers in Section 15.2.4.
15.2.3 Classification of polymers on the basis of mode of polymerization
Polymerization is the fundamental process by which low molecular mass compounds are converted into high molecular weight compounds by linking together of repeating structural units with covalent bonds. This process is illustrated below.
\[\text{Low molecular mass material (possessing reactive groups)} \xrightarrow{\ \text{high temperature and/or pressure and/or catalyst}\ } \text{High molecular mass material}\]
Notice the phrase “possessing reactive groups”. A monomer must have something a reaction can grab hold of — a double bond, or a functional group such as \(\mathrm{-OH}\), \(\mathrm{-COOH}\) or \(\mathrm{-NH_2}\). A saturated molecule such as ethane has neither, which is why it can never polymerize.
There are three modes of polymerization according to the types of reactions taking place between the monomers, and they are collected in FC 15.2.
- Addition polymerization (or chain growth polymerization)
- Condensation polymerization (or step growth polymerization)
- Ring opening polymerization
Flowchart
FC 15.2 – Modes of polymerization: monomer → branch 1: addition (chain growth) — monomer has a double bond, no small molecule lost, repeating unit same composition as monomer; branch 2: condensation (step growth) — polyfunctional monomers, small molecule such as H₂O/HCl/CH₃OH/NH₃ eliminated, repeating unit differs in composition from monomer; branch 3: ring opening — cyclic monomer opened by acid or base, single monomer units added, repeating unit same composition as monomer
a. Addition polymerization
Addition polymerization is a process of formation of polymers by addition of monomers without loss of any small molecules. The repeating unit of an addition polymer has the same elemental composition as that of the original monomer.
Compounds containing a double bond undergo addition polymerization. This makes chemical sense: the \(\pi\) bond of the double bond is the weaker of the two bonds between the carbon atoms, and it can be broken to supply two new single bonds — one to the unit in front and one to the unit behind — without anything being thrown away.
It is also referred to as vinyl polymerization, since the majority of monomers are from the vinyl category. For example: vinyl chloride (\(\mathrm{CH_2}\!=\!\mathrm{CHCl}\)), acrylonitrile (\(\mathrm{CH_2}\!=\!\mathrm{CHCN}\)). Formation of polyethylene from ethylene is a well known example of addition polymerization.
Addition polymerization produces high molecular mass polymeric materials without formation of any intermediate low molecular mass polymeric materials. In other words, a chain either stays as a small radical or shoots up to full length very quickly; there is no slow build-up of medium-sized molecules in between. This is exactly why the process is also called chain growth polymerization.
The free radical mechanism
Free radical mechanism is most common in addition polymerisation. It is also called a chain reaction, which involves three distinct steps: chain initiation, chain propagation and chain termination. These three steps are set out in FC 15.3.
Flowchart
FC 15.3 – Free radical addition polymerization: initiator (e.g. acetyl peroxide) → homolysis → free radical R• → initiation: R• adds to CH₂=CHY → new radical → propagation: radical adds monomer after monomer, chain grows rapidly → termination: two growing chain radicals combine → polymer
A free radical is a species carrying a single unpaired electron, shown as a dot beside the atom that carries it. Because that electron is unpaired, the radical is extremely reactive and will attack a double bond to pair it up — and in doing so it creates a fresh radical, which attacks the next molecule. That is what makes the process self-sustaining.
Step 1: Chain initiation: The chain reaction is initiated by a free radical. An initiator (catalyst) such as benzoyl peroxide, acetyl peroxide, tert-butyl peroxide, etc. can be used to produce a free radical.
All three of these initiators contain a weak oxygen–oxygen single bond, which breaks evenly on gentle heating so that each oxygen keeps one electron. For example, acetyl peroxide generates a methyl radical as shown below and in Fig. 15.8.
\[\mathrm{CH_3COO\!-\!OOCCH_3} \rightarrow 2\ \mathrm{CH_3CO\overset{\bullet}{O}}\]
\[\mathrm{CH_3CO\overset{\bullet}{O}} \xrightarrow{\ -\,\mathrm{CO_2}\ } \overset{\bullet}{\mathrm{C}}\mathrm{H_3}\ \text{(methyl radical)}\]
Diagram
Fig. 15.8 – Structural formula of acetyl peroxide CH₃–CO–O–O–CO–CH₃ with fish-hook (single-headed) arrows on the O–O bond showing homolysis into two CH₃–CO–O• radicals, and a second arrow showing loss of CO₂ to leave •CH₃.
The free radical (say \(\overset{\bullet}{\mathrm{R}}\)) so formed attaches itself to the olefin (vinyl monomer) and produces a new radical, made up of two parts, namely, the attached radical and the monomer unit.
\[\overset{\bullet}{\mathrm{R}} + \mathrm{CH_2}\!=\!\mathrm{CHY} \rightarrow \mathrm{R\!-\!CH_2\!-\!\overset{\bullet}{C}HY}\]
Here \(\mathrm{R}\) is the fragment supplied by the initiator (free radical), \(\mathrm{CH_2}\!=\!\mathrm{CHY}\) is the vinyl monomer, and \(\mathrm{R\!-\!CH_2\!-\!\overset{\bullet}{C}HY}\) is the new radical. \(\mathrm{Y}\) stands for whatever group the particular vinyl monomer carries — \(\mathrm{Cl}\) in vinyl chloride, \(\mathrm{CN}\) in acrylonitrile.
Step 2: Chain propagation: The new radical formed in the initiation step reacts with another molecule of vinyl monomer, forming another still bigger sized radical, which in turn reacts with another monomer molecule. The repetition of this sequence takes place very rapidly. It is called chain propagation.
\[\mathrm{R\!-\!CH_2\!-\!\overset{\bullet}{C}HY} + n\,\mathrm{CH_2}\!=\!\mathrm{CHY} \rightarrow \mathrm{R\!-\!(CH_2\!-\!CHY)_n\!-\!CH_2\!-\!\overset{\bullet}{C}HY}\]
This step is very rapid and leads to a high molecular mass radical. Observe that the radical site is never destroyed during propagation — it simply moves to the far end of the growing chain, ready for the next monomer.
Step 3: Chain termination: Ultimately, at some stage, termination of the growing chain takes place. It may occur by several processes. One mode of termination is by combination of two growing chain radicals.
\[2\ \mathrm{R\!-\!(CH_2\!-\!CHY)_n\!-\!CH_2\!-\!\overset{\bullet}{C}HY} \rightarrow \mathrm{R\!-\!(CH_2CHY)_{n+1}\!-\!(CHYCH_2)_{n+1}\!-\!R}\quad \text{(polymer)}\]
When the two unpaired electrons pair up with each other, no radical is left behind, and growth of that particular molecule stops for good.
b. Condensation polymerization
Consider the formation of terylene, a polyester polymer, from ethylene glycol and terephthalic acid.
\[n\,\mathrm{HO\!-\!CH_2\!-\!CH_2\!-\!OH} + n\,\mathrm{HOOC\!-\!C_6H_4\!-\!COOH} \xrightarrow{\ -\,n\mathrm{H_2O}\ } \mathrm{HO\!-\![CH_2\!-\!CH_2\!-\!O\!-\!CO\!-\!C_6H_4\!-\!CO]_n\!-\!OH}\]
In this reaction an alcoholic \(\mathrm{OH}\) group in ethylene glycol condenses with a carboxyl group in terephthalic acid by eliminating a water molecule to form an ester linkage.
Both monomers here are bifunctional — ethylene glycol carries two \(\mathrm{-OH}\) groups and terephthalic acid carries two \(\mathrm{-COOH}\) groups — so each molecule can react at both ends and the chain keeps extending in both directions.
Diagram
Fig. 15.9 – The ester link in terylene: draw one –O–CO– unit joining an ethylene glycol residue to a terephthalic acid residue, circle the linkage and label it “ester link”, with the eliminated H₂O shown leaving.
The process of formation of polymers from polyfunctional monomers with the elimination of some small molecules such as water, hydrochloric acid, methanol, ammonia is called condensation polymerization.
In this type of polymerization the chain growth occurs by a series of condensation steps. Therefore it is also referred to as step growth polymerization. This process is continued until a high molecular mass polymer is obtained.
The word “step” is exact. Two monomers first join to give a dimer; two dimers may then join to give a tetramer, and so on. Unlike chain growth, the mixture here really does pass through medium-sized molecules on its way to the final polymer, and the average chain length climbs gradually.
Because a small molecule is expelled at every linkage, the repeating unit of the polymer must be lighter than the monomers that made it.
\[2n\,\mathrm{CH_2}\!=\!\mathrm{CH\!-\!CH_3} \rightarrow \mathrm{H_3C\!-\![CH_2\!-\!CH(CH_3)]_n\!-\![CH(CH_3)\!-\!CH_2]_n\!-\!CH_3}\]
\[n\,\mathrm{HO\!-\!(CH_2)_x\!-\!OH} + n\,\mathrm{HOOC\!-\!(CH_2)_y\!-\!COOH} \rightarrow \mathrm{HO\!-\![(CH_2)_x\!-\!O\!-\!CO\!-\!(CH_2)_y\!-\!CO]_n\!-\!OH}\]
c. Ring opening polymerization
The third type of polymerization is ring opening polymerization. Cyclic compounds like lactams, cyclic ethers, lactones, etc. polymerize by ring opening polymerization. Strong acid or base catalyze this reaction. For example: polymerization of \(\varepsilon\)-caprolactam. (For more details see Section 15.3.5 (b).)
\[\varepsilon\text{-caprolactam} \xrightarrow{\ \text{acid or base}\ } \mathrm{[NH\!-\!(CH_2)_5\!-\!CO]_n}\]
Diagram
Fig. 15.10 – ε-caprolactam drawn as a seven-membered ring containing the –NH–CO– group, with the ring carbons labelled α, β, γ, δ, ε; an arrow shows the ring opening at the N–CO bond to give the open repeating unit –[NH–(CH₂)₅–CO]–.
A ring is a strained, closed arrangement; opening it releases that strain and leaves a molecule with a reactive end at each side, which can then be joined to the next opened ring.
Elemental composition of the repeating unit in the polymer resulting from ring opening polymerization is the same as that of the monomers, as in the case of addition polymerization. Nothing is expelled here either — the ring is merely unfastened, not shortened.
Addition polymerizations are often very rapid. But ring opening polymerization proceeds by addition of a single monomer unit (but never of larger units) to the growing chain molecules. In this sense, ring opening polymerization is a step growth polymerization similar to condensation polymerization.
So ring opening polymerization sits between the other two: it resembles addition polymerization in composition, and condensation polymerization in the gradual, step-by-step way the chain grows.
15.2.4 Classification of polymers on the basis of intermolecular forces
Mechanical properties of polymers such as tensile strength, toughness and elasticity differ widely depending upon the intermolecular forces. Polymers are classified into various categories on the basis of intermolecular forces as follows.
Before going into the four classes, fix the underlying idea. Within a chain the atoms are held by strong covalent bonds, and those are much the same in every polymer. What differs from one polymer to the next is how strongly one chain grips its neighbours. If that grip is weak, the chains slide past one another and the material stretches; if it is strong, they hold their positions and the material resists being pulled apart.
a. Elastomers
Elasticity is a property by which a substance gets stretched by external force and restores its original shape on release of that force.
Elastomers, the elastic polymers, have weak van der Waals type of intermolecular forces which permit the polymer to be stretched. A few crosslinks between the chains help the stretched polymer to retract to its original position on removal of the applied force.
Both halves of that sentence matter. Weak forces alone would let the chains slide apart permanently, like chewing gum being pulled; the occasional cross-link acts as an anchor that pulls the chains back to where they started.
For example: vulcanized rubber, buna-S, buna-N, neoprene, etc.
b. Fibres
Polymeric solids which form threads are called fibres.
The fibres possess high tensile strength, which is a property to have resistance to breaking under tension. This characteristic is due to the strong intermolecular forces like hydrogen bonding and strong dipole-dipole forces.
Due to these strong intermolecular forces the fibres are crystalline in nature. “Crystalline” here means the chains lie neatly side by side in ordered regions rather than in a random tangle, and ordered chains grip one another along their whole length — which is exactly what a thread needs if it is not to snap.
For example: polyamides (nylon 6, 6), polyesters (terylene).
c. Thermoplastic polymers
Plasticity is a property of being easily shaped or moulded. Thermoplastic polymers are capable of repeated softening on heating and hardening on cooling.
These polymers possess moderately strong intermolecular forces that are intermediate between elastomers and fibres. For example: polythene, polystyrene, polyvinyls, etc.
Heating supplies enough energy to overcome those moderate attractions, so the chains can slide and the solid softens; cooling lets the attractions take hold again. Since only intermolecular forces are being broken and remade — never covalent bonds — the cycle can be repeated.
d. Thermosetting polymers
Thermosetting polymers are rigid polymers. During their formation they have the property of being shaped on heating; but they get hardened while hot.
Once hardened these become infusible; they cannot be softened by heating and therefore cannot be remoulded. This characteristic is the result of extensive cross linking by covalent bonds formed in the moulds during the hardening/setting process while hot.
For example: bakelite, urea formaldehyde resin, etc.
Diagram
Fig. 15.11 – Four labelled photographs of everyday articles, one per class: a heap of coloured rubber bands (elastomer), bundles of coloured nylon rope (fibre), plastic water bottles (thermoplastic), black moulded bakelite machine parts (thermosetting).
15.2.5 Classification of polymers on the basis of type of different monomers
Polymers are divided into two classes:
a. Homopolymers
The polymers which have only one type of repeating unit are called homopolymers. Usually they are formed from a single monomer.
In some cases the repeating unit is formed by condensation of two distinct monomers. This is the point students most often trip over: what decides the answer is the number of kinds of repeating unit in the finished chain, not the number of kinds of starting material.
For example: polythene, polypropene, Nylon 6, polyacrylonitrile, Nylon 6, 6.
b. Copolymers
The polymers which have two or more types of repeating units are called copolymers. They are formed by polymerization of two or more different types of monomers in the presence of each other.
The different monomer units are randomly sequenced in the copolymer. For example: Buna-S, Buna-N.
15.2.6 Classification of polymers on the basis of biodegradability
Most of the synthetic polymers are not affected by microbes. These are called non-biodegradable polymers.
These, in the form of waste material, stay in the environment for a very long time and pose pollution hazards. The very inertness that made the material attractive — it does not rot, rust or dissolve — is what makes its waste so persistent.
Most natural fibres, in contrast, are biodegradable.
In an attempt to tackle the environmental problem, scientists have developed bio-degradable synthetic polymers. More details will be described in Section 15.5.
15.3 Some Important Polymers
15.3.1 Rubber
Elastomers are popularly known as rubbers. For example, balloons, shoesoles, tyres, surgeon’s gloves, garden hose, etc. are made from elastomeric material or rubber.
Natural rubber
The monomer of natural rubber is isoprene (2-methyl-1,3-butadiene).
\[\mathrm{CH_2}\!=\!\mathrm{C(CH_3)\!-\!CH}\!=\!\mathrm{CH_2}\qquad \text{(isoprene)}\]
Isoprene is a diene — it carries two carbon–carbon double bonds — and this is what allows it to keep a double bond in the finished polymer, as you will see in a moment.
Natural rubber is a high molecular mass linear polymer of isoprene. Its molecular mass varies from 130, 000 u to 340, 000 u (that is, the number of monomer units varies from 2000 to 5000).
Diagram
Fig. 15.12 – Structure of natural rubber: a stretch of the cis-polyisoprene chain showing two C=C double bonds with both –CH₂– groups on the same side of each double bond, methyl groups marked, and wavy line ends indicating chain continuation.
The reaction involved in the formation of natural rubber by the process of addition polymerization is as follows.
\[n\,\mathrm{CH_2}\!=\!\mathrm{C(CH_3)\!-\!CH}\!=\!\mathrm{CH_2} \xrightarrow{\ \text{polymerization}\ } \mathrm{[CH_2\!-\!C(CH_3)}\!=\!\mathrm{CH\!-\!CH_2]_n}\]
\[\text{(isoprene)}\qquad\qquad \text{(polyisoprene / rubber)}\]
Look closely at the product. The two double bonds of isoprene are used to make the new links to the neighbouring units, but a fresh double bond appears in the middle of each repeating unit. This surviving double bond is not a detail — it fixes the shape of the chain, and later it is also the site that sulfur attacks during vulcanization.
Properties of natural rubber
The polyisoprene molecule has the cis configuration of the \(\mathrm{C}\!=\!\mathrm{C}\) double bond. It consists of various chains held together by weak van der Waals forces and has a coiled structure.
It can be stretched like a spring and exhibits elastic property.
The two properties are connected. A double bond cannot rotate, so the cis arrangement puts a permanent kink at every repeating unit; a chain full of kinks cannot lie straight against its neighbour and instead coils up. Coiled chains that touch each other only at scattered points attract each other weakly — and a coiled chain, like a spring, can be pulled out and will spring back.
Vulcanization of rubber
To improve the physical properties of natural rubber, a process of vulcanization is carried out. In 1839 Charles Goodyear, an American inventor, invented the process of vulcanization.
The process by which a network of cross links is introduced into an elastomer is called vulcanization.
The profound effect of vulcanization enhances the properties like tensile strength, stiffness, elasticity, toughness, etc. of natural rubber. Sulfur vulcanization is the most frequently used process. Sulfur forms crosslinks between polyisoprene chains, which results in improved properties of natural rubber.
How vulcanization is carried out
Vulcanizing is carried out by heating raw rubber with sulfur powder in the presence of some organic compounds called accelerators at about \(150\ ^{0}\text{C}\). (The most common accelerator is ZBX or zincbutyl xanthate.)
By increasing the amount of sulfur, the rubber can be hardened. For example, when the amount of sulfur is raised to 40-45 %, a non-elastic hard material known as ebonite is obtained. The trend is worth noting: more sulfur means more cross links, and more cross links mean less freedom for the chains to slide — so the material moves from soft, to elastic, to rigid.
One or more sulfur atoms cross-link two polyisoprene chains. Cross-linking takes place by opening of a double bond and produces three dimensional vulcanised rubber. This is where the double bond that survived polymerization earns its place: it provides the reactive site at which the sulfur bridge is formed. The probable 3-D structure of vulcanized rubber is shown in Fig. 15.13.
Diagram
Fig. 15.13 – Probable 3-D structure of vulcanized rubber: three wavy polyisoprene chains lying roughly parallel, joined to one another by short vertical –S–S–S– sulfur bridges attached at carbon atoms bearing CH₃ groups.
15.3.2 Polythene
Polythene is the simplest and most commonly used hydrocarbon thermoplastic and has the following structure.
\[\mathrm{[CH_2 - CH_2]_n}\]
The IUPAC name of polyethylene is polythene. Polythene is of two kinds, namely low density polythene (LDP) and high density polythene (HDP).
Both are made from the same monomer, ethene. What separates them is not the monomer but the reaction conditions, which decide whether the chains come out branched or straight.
a. Low density polyethylene (LDP)
LDP is obtained by polymerization of ethylene under high pressure (1000 - 2000 atm) and temperature (350 - 570 K) in presence of traces of \(\mathrm{O_2}\) or peroxide as initiator.
\[\mathrm{CH_2}\!=\!\mathrm{CH_2} \xrightarrow[\text{1000 - 2000 atm}]{\ \text{traces of O}_2\text{, peroxide at 350 - 370 K}\ } \text{LDP}\]
The mechanism of this reaction involves free radical addition and H-atom abstraction. The latter results in branching. As a result the chains are loosely held and the polymer has low density.
Where the branches come from
In the H-atom abstraction process involved in the formation of LDP, the terminal carbon radical abstracts an H-atom from an internal carbon atom, in the form of an internal carbon radical. The termination step in addition polymerization gives rise to branching of these internal carbons.
Follow what this does. The radical was at the end of the chain, where further growth would simply lengthen it; after abstraction the radical sits in the middle of the chain, and growth from that point sticks out sideways as a branch. Branches hold neighbouring chains apart, packing is poor, and the density falls — exactly the structure–property link you met in Section 15.2.2.
Diagram
Fig. 15.14 – Intramolecular H-atom abstraction in LDP formation: a chain with the radical dot on the terminal carbon, a curved arrow showing an H atom transferring from an internal carbon to the chain end, the resulting internal carbon radical, and a final structure showing a new branch R growing from that internal carbon.
Properties of LDP: LDP films are extremely flexible, but tough, chemically inert and moisture resistant. It is a poor conductor of electricity, with melting point \(110\ ^{0}\text{C}\).
Uses of LDP: LDP is mainly used in the preparation of pipes for agriculture, irrigation and domestic water line connections, as well as insulation for electric cables. It is also used in submarine cable insulation. It is used in producing extruded films and sheets, mainly for packaging and household uses like the preparation of squeeze bottles, attractive containers, etc.
b. High density polyethylene (HDP)
It is essentially a linear polymer with high density due to close packing.
\[\mathrm{CH_2}\!=\!\mathrm{CH_2} \xrightarrow[\text{6 - 7 atm, catalyst}]{\ \text{333 K - 343 K}\ } \text{HDP}\]
HDP is obtained by polymerization of ethene in the presence of the Zieglar-Natta catalyst, which is a combination of triethyl aluminium with titanium tetrachloride, at a temperature of 333 K to 343 K and a pressure of 6-7 atm.
Notice how much milder these conditions are than those for LDP. The catalyst does the work that extreme pressure was doing before, and it also holds each incoming monomer in a fixed orientation, so the chain grows straight instead of branching.
Properties of HDP: HDP is crystalline, with melting point in the range of \(144 - 150\ ^{0}\text{C}\). It is much stiffer than LDP and has high tensile strength and hardness. It is more resistant to chemicals than LDP.
Uses of HDP: HDP is used in the manufacture of toys and other household articles like buckets, dustbins, bottles, pipes, etc. It is used to prepare laboratory wares and other objects where high tensile strength and stiffness is required.
15.3.3 Teflon
Chemically, teflon is polytetrafluoroethylene. The monomer used in the preparation of teflon is tetrafluoroethylene, (\(\mathrm{CF_2}\!=\!\mathrm{CF_2}\)), which is a gas at room temperature.
Tetrafluoroethylene is polymerized by using free radical initiators such as hydrogen peroxide or ammonium persulphate at high pressure.
\[n\,\mathrm{CF_2}\!=\!\mathrm{CF_2} \xrightarrow[\text{peroxide}]{\ \text{polymerization}\ } \mathrm{[CF_2 - CF_2]_n}\]
\[\text{(tetrafluoroethene)}\qquad\qquad \text{(teflon)}\]
Properties: Teflon is tough, chemically inert and resistant to heat and attack by corrosive reagents.
The \(\mathrm{C}\!-\!\mathrm{F}\) bond is very difficult to break, and it remains unaffected by corrosive alkali and organic solvents.
There is a clear structure–property link here. Fluorine is the most electronegative element, and the small fluorine atoms pack tightly around the carbon backbone. The resulting \(\mathrm{C}\!-\!\mathrm{F}\) bonds are exceptionally strong, and the sheath of fluorine atoms physically shields the carbon chain, so reagents cannot reach it. A chain that nothing can attack is a chain that nothing sticks to either.
Uses: Teflon is used in making non-stick cookware, oil seals, gaskets, etc.
15.3.4 Polyacrylonitrile
Polyacrylonitrile is prepared by addition polymerization of acrylonitrile by using a peroxide initiator.
\[n\,\mathrm{CH_2}\!=\!\mathrm{CHCN} \xrightarrow[\text{peroxide}]{\ \text{polymerization}\ } \mathrm{[CH_2 - CH(CN)]_n}\]
\[\text{(acrylonitrile)}\qquad\qquad \text{(polyacrylonitrile)}\]
Polyacrylonitrile resembles wool and is used as a wool substitute and for making orlon or acrilan.
The resemblance is not accidental. The \(\mathrm{-CN}\) group is strongly polar, so neighbouring chains attract one another through strong dipole-dipole forces — much as the polar groups in the protein chains of wool do — and it is these forces that give the fibre its warmth-holding bulk and strength.
15.3.5 Polyamide polymers
Polyamide polymers are generally known as nylons. Nylon is the generic name of the synthetic linear polyamides obtained by the condensation polymerization between dicarboxylic acids and diamines, the self condensation of an amino acid, or by the ring opening polymerization of lactams.
Polyamides contain \(\mathrm{-CO\!-\!NH-}\) groups as the inter unit linkages. Two important polyamide polymers are nylon 6, 6 and nylon 6.
The \(\mathrm{-CO\!-\!NH-}\) group is the amide linkage you met in Chapter 13, and it is the same linkage that joins amino acids in proteins. Its importance here is that the \(\mathrm{N}\!-\!\mathrm{H}\) of one chain can hydrogen bond to the \(\mathrm{C}\!=\!\mathrm{O}\) of the next — the strong intermolecular attraction that makes every nylon a fibre.
a. Nylon 6,6
The monomers adipic acid and hexamethylendiamine on mixing form nylon salt, which upon condensation polymerization under conditions of high temperature and pressure gives the polyamide fibre nylon 6,6.
\[n\,\mathrm{HOOC\!-\!(CH_2)_4\!-\!COOH} + n\,\mathrm{H_2N\!-\!(CH_2)_6\!-\!NH_2}\]
\[\text{(adipic acid)}\qquad\qquad \text{(hexamethylene diamine)}\]
\[\downarrow\]
\[n\left[\mathrm{{}^{-}O\!-\!CO\!-\!(CH_2)_4\!-\!CO\!-\!O^{-}}\quad \mathrm{{}^{+}H_3N\!-\!(CH_2)_6\!-\!NH_3^{+}}\right] \qquad \text{(nylon salt)}\]
\[\xrightarrow[\ \text{high pressure},\ -\,n\mathrm{H_2O}\ ]{\ 553\ \text{K}\ } \mathrm{[CO\!-\!(CH_2)_4\!-\!CO\!-\!NH\!-\!(CH_2)_6\!-\!NH]_n} \qquad \text{(nylon 6,6)}\]
The salt-forming step is worth a moment. An acid and an amine first simply transfer a proton to give an ionic salt, which brings the two monomers together in exactly a 1 : 1 ratio; on heating, this salt loses water and closes the amide linkage. Getting the ratio right matters, because in step growth polymerization an excess of either monomer would cap the chain ends and stop the chain from growing long.
The numerals 6,6 in the name of this polymer stand for the number of carbon atoms in the two bifunctional monomers, namely, adipic acid and hexamethylenediamine.
Nylon 6,6 is a high molecular mass (12000-50000 u) linear condensation polymer. It possesses high tensile strength. It does not soak in water. It is used for making sheets, bristles for brushes, surgical sutures, textile fabrics, etc.
b. Nylon 6
When epsilon (\(\varepsilon\))-caprolactam is heated with water at high temperature, it undergoes ring opening polymerization to give the polyamide polymer called nylon 6.
\[\varepsilon\text{-caprolactam} \xrightarrow[\text{533 - 543 K}]{\ \mathrm{H_2O}\ } \mathrm{[NH\!-\!(CH_2)_5\!-\!CO]_n} \qquad \text{(nylon 6)}\]
The name nylon 6 is given on the basis of six carbon atoms present in the monomer unit. Here only one monomer is involved, so only one number appears in the name — compare this with the two numbers of nylon 6,6.
Due to its high tensile strength and lustre, nylon 6 fibres are used for the manufacture of tyre cords, fabrics and ropes.
15.3.6 Polyesters
The polyester polymers have an ester linkage joining the repeating units.
Commercially the most important polyester fibre is ‘terylene’ (also called dacron). It is obtained by condensation polymerization of ethylene glycol and terephthalic acid in the presence of a catalyst at high temperature. You met this reaction in Section 15.2.3 as the standard example of condensation polymerization; here are the industrial conditions under which it is actually carried out.
\[n\,\mathrm{HO\!-\!CH_2\!-\!CH_2\!-\!OH} + n\,\mathrm{H\!-\!O\!-\!CO\!-\!C_6H_4\!-\!CO\!-\!O\!-\!H}\]
\[\text{(ethyleneglycol)}\qquad\qquad \text{(terephthalic acid)}\]
\[\xrightarrow[\ 420 - 460\ \text{K},\ -\,n\mathrm{H_2O}\ ]{\ \text{zinc acetate - antimony trioxide catalyst}\ } \mathrm{[O\!-\!CH_2\!-\!CH_2\!-\!O\!-\!CO\!-\!C_6H_4\!-\!CO]_n} \qquad \text{(terylene or dacron)}\]
Terylene has a relatively high melting point (\(265\ ^{0}\text{C}\)) and is resistant to chemicals and water.
The rigid benzene ring built into every repeating unit is what raises the melting point — a stiff, flat unit lets the chains stack in an ordered way and resists the disorder that melting requires.
It is used for making wrinkle free fabrics by blending with cotton (terycot) and wool (terywool), and also as glass reinforcing material in safety helmets.
PET is the most common thermoplastic, which is another trade name of the polyester polyethyleneterephthalate. It is used for making many articles like bottles, jams and packaging containers.
Polycarbonates are also a kind of polyester polymers. These are high melting thermosetting resins.
15.3.8 Buna-S rubber
Buna-S is an elastomer which is a copolymer of styrene with butadiene (Fig. 15.5). Its trade name is SBR (styrene-butadiene rubber).
The copolymer is usually obtained from 75 parts of butadiene and 25 parts of styrene subjected to addition polymerization by the action of sodium. It is vulcanized with sulfur.
\[\mathrm{CH_2}\!=\!\mathrm{CH\!-\!CH}\!=\!\mathrm{CH_2} + \mathrm{C_6H_5CH}\!=\!\mathrm{CH_2} \xrightarrow{\ \mathrm{Na}\ } \text{SBR (Buna-S)}\]
\[\text{1, 3 - butadiene (75 \%)}\qquad \text{styrene (25 \%)}\]
Diagram
Fig. 15.5 – Formation of SBR (Buna - S): 1,3-butadiene (75 %) and styrene (25 %) above an arrow, leading to a chain in which several –CH₂–CH=CH–CH₂– butadiene units are randomly interrupted by a –CH₂–CH(C₆H₅)– styrene unit, with the phenyl group labelled.
Because the two kinds of repeating unit are randomly sequenced along the chain, Buna-S is a copolymer, as you established in Section 15.2.5. The bulky phenyl rings from styrene sit at irregular intervals, and they are the reason the material resists being scuffed away.
Buna-S is superior to natural rubber with regard to mechanical strength and has abrasion resistance. Hence it is used in the tyre industry.
15.3.9 Neoprene
Neoprene, a synthetic rubber, is an addition polymer of chloroprene (2-chloro-1,3-butadiene). Chloroprene polymerizes rapidly in the presence of oxygen.
\[n\,\mathrm{CH_2}\!=\!\mathrm{C(Cl)\!-\!CH}\!=\!\mathrm{CH_2} \xrightarrow{\ \text{polymerization}\ } \mathrm{[CH_2\!-\!C(Cl)}\!=\!\mathrm{CH\!-\!CH_2]_n}\]
\[\text{(chloroprene)}\qquad\qquad \text{(neoprene)}\]
Compare this monomer with isoprene: the only change is that the methyl group has been replaced by a chlorine atom. That single substitution is responsible for everything that follows, because the polar \(\mathrm{C}\!-\!\mathrm{Cl}\) bond makes the chain unattractive to non-polar liquids such as petroleum and vegetable oils, which therefore cannot swell it.
Vulcanization of neoprene takes place in the presence of magnesium oxide. The reactions involved can be represented in Fig. 15.6.
Diagram
Fig. 15.6 – Neoprene and vulcanization: (a) polymerization of chloroprene to neoprene showing the repeating unit –CH₂–C(Cl)=CH–CH₂–; (b) two neoprene chains each carrying Cl, with MgO added, arrow labelled “vulcanization”, giving the cross-linked product in which the two chains are joined through oxygen bridges in place of the chlorine atoms.
Neoprene is particularly resistant to petroleum, vegetable oils, light as well as heat. Neoprene is used in making hose pipes for the transport of gasoline and making gaskets. It is used for manufacturing insulator cable, jackets, belts for power transmission and conveying.
15.3.10 Viscose rayon
Viscose rayon is a semisynthetic fibre which is regenerated cellulose. Cellulose in the form of wood pulp is transformed into viscose rayon.
Cellulose is a linear polymer of glucose units and has the molecular formula \(\mathrm{(C_6H_{10}O_5)_n}\) — the natural polymer you studied in Chapter 14. A modified representation of the molecular formula of cellulose, Cell-OH, is used in the reactions involved in viscose formation, as shown in Fig. 15.7. Writing it this way keeps the equations readable and, more importantly, focuses attention on the one \(\mathrm{-OH}\) group that actually takes part in the chemistry.
\[\mathrm{Cell\!-\!OH} + \mathrm{NaOH} \rightarrow \mathrm{Cell\!-\!O^{-}Na^{+}} + \mathrm{H_2O}\]
\[\text{(cellulose pulp wood)}\qquad \text{(alkali cellulose)}\]
\[\mathrm{Cell\!-\!O^{-}Na^{+}} + \mathrm{CS_2} \rightarrow \mathrm{Cell\!-\!O\!-\!CS\!-\!S^{-}Na^{+}}\]
\[\text{(cellulose xanthate)}\]
\[\mathrm{Cell\!-\!O\!-\!CS\!-\!S^{-}Na^{+}} + \mathrm{H_2O} \rightarrow \mathrm{Cell\!-\!O\!-\!CS\!-\!SH} + \mathrm{NaOH}\]
\[\mathrm{Cell\!-\!O\!-\!CS\!-\!SH} + \mathrm{H_2O} \rightarrow \mathrm{Cell\!-\!OH} + \mathrm{CS_2}\]
\[\text{(viscose rayon — regenerated cellulose filaments)}\]
Diagram
Fig. 15.7 – Formation of viscose rayon: the four equations above written in structural form, with the C=S double bond shown explicitly in the xanthate group and each product labelled (alkali cellulose, cellulose xanthate, regenerated cellulose filaments).
Follow the process as a whole. Cellulose in the form of wood pulp is treated with concentrated \(\mathrm{NaOH}\) solution to get fluffy alkali cellulose. It is then converted to xanthate by treating with carbon disulphide. On mixing with dilute \(\mathrm{NaOH}\) it gives a viscose solution, which is extruded through the spinnerets of a spinning machine into an acid bath, where regenerated cellulose fibres precipitate. The whole route is summarised in FC 15.5.
Flowchart
FC 15.5 – Formation of viscose rayon: wood pulp (cellulose, Cell–OH) → treat with concentrated NaOH → fluffy alkali cellulose (Cell–O⁻Na⁺) → treat with CS₂ → cellulose xanthate → mix with dilute NaOH → viscose solution → extrude through spinnerets into acid bath → regenerated cellulose filaments precipitate → viscose rayon fibre
Note what has and has not changed. The last equation returns Cell-OH — the same cellulose the process started from. Chemically nothing new has been made; the chain has merely been dissolved and then reformed as a continuous filament instead of a short plant fibre. That is exactly what “regenerated” means, and it is why viscose rayon is classed as semisynthetic rather than synthetic.
15.4 Molecular Mass and Degree of Polymerization of Polymers
A polymer is usually a complex mixture of molecules of different molecular masses. Hence, the molecular mass of a polymer is an average of the molecular masses of the constituent molecules.
This is unlike anything you have met so far. Every molecule of glucose in a sample has a mass of exactly \(180\ \text{u}\), but in a polymer sample the chains stop growing at different moments, so one chain may carry 900 repeating units and the next 4000. Only an average can describe such a mixture.
Molecular mass of a polymer depends upon the degree of polymerization (DP). DP is the number of monomer units in a polymer molecule.
Relating DP to Molecular Mass
Since the chain is simply the repeating unit written over and over, its molecular mass follows directly from the DP:
\[\text{Molecular mass of polymer} = \text{DP} \times \text{molecular mass of the repeating unit}\]
so that
\[\text{DP} = \frac{\text{molecular mass of polymer}}{\text{molecular mass of the repeating unit}}\]
Be careful about which mass goes in the denominator. For an addition polymer the repeating unit and the monomer have the same composition, so either may be used. For a condensation polymer a small molecule was expelled at every linkage, so the repeating unit is lighter than the monomers and it is the repeating unit that must be used.
Why Molecular Mass Has to Be Controlled
Most of the mechanical properties of polymers depend upon their molecular mass.
Low molecular mass polymers are liable to be brittle and have low mechanical strength. Short chains have little length over which to attract their neighbours, and they cannot entangle with one another, so the solid gives way easily.
If a polymer is allowed to attain very high molecular mass, it becomes tough and unmanageable. Here the chains are so long and so thoroughly entangled that the melt will barely flow, and the material can no longer be spun into fibre, drawn into film or forced into a mould.
Both these ends are undesirable. A polymer must possess a molecular mass more than a certain minimum value in order to exhibit the properties needed for a particular application. This minimum molecular mass corresponds to the critical degree of polymerization. But the polymerization process has to be controlled after a certain stage.
For polymers containing hydrogen bonding, the critical degree of polymerization is lower than for those containing weak intermolecular forces.
The reason is worth spelling out, because it ties this section back to Section 15.2.4. Useful strength comes from the total attraction between neighbouring chains. If those attractions are strong, as with hydrogen bonds, a fairly short chain already grips its neighbours firmly enough; if they are weak, the chain must be made much longer to accumulate the same total attraction. Strong intermolecular forces therefore lower the chain length at which a polymer becomes useful.
Diagram
Fig. 15.15 – Schematic graph with degree of polymerization on the x-axis and two curves plotted against it: mechanical strength rising steeply and then levelling off, and ease of processing (flow/mouldability) falling as DP increases; mark the critical degree of polymerization on the x-axis where the strength curve reaches the useful level, and shade the workable range between the two.
- Complete the following table:
| Condensation polymers | Repeating unit | Name of monomer | Formula of monomer | Uses |
|---|---|---|---|---|
| 1. Nylon 6 | ||||
| 2. Nylon 6, 6 | ||||
| 3. Terylene | ||||
| 4. Melamine |
15.5 Biodegradable Polymers
Inspite of a large number of useful applications, polymers are blamed for creating environmental pollution. To strike the golden mean, certain new biodegradable synthetic polymers have been developed.
The phrase “golden mean” is well chosen. Giving up polymers altogether is not realistic, and accepting permanent waste is not acceptable either; the way out is to design a polymer that performs while in use and then breaks down once discarded.
Aliphatic polyesters and polyamides with a large proportion of polar linkages are one of the important classes of biodegradable polymers. Some important examples are discussed below.
Why some polymers rot and others do not
Disposed natural wastes are usually attacked by soil microbes and get degraded to humus.
But most synthetic polymers and plastics cannot be degraded by microbes and stay in the environment for a very long period of time, posing pollution problems.
To overcome this problem, biodegradable polymers are being developed. These polymers contain functional groups similar to those in biopolymers such as proteins. Aliphatic polyesters are also an important class of biodegradable polymers.
The strategy behind that sentence is the heart of this section. Soil microbes carry enzymes that have evolved to cut particular linkages — chiefly the ester and amide linkages found in fats, carbohydrates and proteins. A polymer chain built from those same linkages presents the microbe with a bond it already knows how to break; a chain of plain \(\mathrm{C}\!-\!\mathrm{C}\) bonds presents it with nothing it can recognise. The degradation route is summarised in FC 15.6.
Flowchart
FC 15.6 – Microbial degradation of an aliphatic polyester: buried polymer chain → microbial enzymes recognise the polar ester linkage → hydrolysis of –CO–O– links (water added across the bond) → chain cut into short hydroxy acid fragments → fragments absorbed and metabolised by microbes → CO₂, H₂O and humus; branch: chain of plain C–C bonds (e.g. polythene) → no linkage the enzyme can attack → polymer persists in the environment
15.5.1 PHBV
PHBV is a copolymer of two bifunctional \(\beta\)-hydroxy carboxylic acids, namely, \(\beta\)-hydroxybutyric acid (3-hydroxybutanoic acid) and \(\beta\)-hydroxyvaleric acid (3-hydroxypentanoic acid).
Each monomer is bifunctional in an unusual way: it carries one \(\mathrm{-OH}\) group and one \(\mathrm{-COOH}\) group in the same molecule, so it can react at both ends without needing a partner of a different kind.
The hydroxyl group of one monomer forms an ester link by reacting with the carboxyl group of the other.
\[n\,\mathrm{HO\!-\!CH(CH_3)\!-\!CH_2\!-\!COOH} + n\,\mathrm{HO\!-\!CH(CH_2CH_3)\!-\!CH_2\!-\!COOH}\]
\[\text{(}\beta\text{-hydroxy butyric acid)}\qquad\qquad \text{(}\beta\text{-hydroxy valeric acid)}\]
\[\xrightarrow{\ -\,n\mathrm{H_2O}\ } \mathrm{[O\!-\!CH(CH_3)\!-\!CH_2\!-\!CO\!-\!O\!-\!CH(CH_2CH_3)\!-\!CH_2\!-\!CO]_n} \qquad \text{(PHBV)}\]
Diagram
Fig. 15.16 – Repeating unit of PHBV: the two residues drawn side by side, the β-hydroxybutyrate unit bearing a –CH₃ group and the β-hydroxyvalerate unit bearing a –CH₂CH₃ group, with each –CO–O– ester link circled and labelled “ester link” and the eliminated H₂O shown leaving.
Thus PHBV is an aliphatic polyester with the name poly \(\beta\)-hydroxy butyrate - co - \(\beta\)-hydroxy valerate (PHBV). PHBV is degraded by microbes in the environment.
Note that the chain is aliphatic — there is no rigid benzene ring of the kind that stiffens terylene — and that an ester linkage recurs every few atoms. Both features make the chain easy for water and enzymes to reach, which is precisely why it degrades.
15.5.2 Nylon 2 - nylon 6
Nylon 2 - nylon 6 is a polyamide copolymer of two amino acids, namely, glycine and \(\varepsilon\)-amino caproic acid. It is a biodegradable polymer.
Since both monomers are amino acids, each carries an \(\mathrm{-NH_2}\) group at one end and a \(\mathrm{-COOH}\) group at the other, and the chain that results is joined by \(\mathrm{-CO\!-\!NH-}\) amide linkages — the same linkage that joins amino acids in a protein.
15.6 Commercially Important Polymers
Apart from the polymers already discussed in this chapter, many more polymers are used extensively. Structures and applications of some of them are given in Table 15.1.
Read the table as a set of worked examples rather than a list to be learnt by heart. For each entry, look first at the monomer and ask whether it carries a double bond or a pair of functional groups; that single observation tells you whether the polymer was made by addition or by condensation, and the repeating unit in the third column will confirm your answer.
Table 15.1: Commercially important polymers
| Trade name | Monomer | Polymer structure | Applications |
|---|---|---|---|
| Perspex/acrylic glass | methyl methacrylate, \(\mathrm{CH_2}\!=\!\mathrm{C(CH_3)COOMe}\) | \(\mathrm{[CH_2\!-\!C(CH_3)(COOMe)]_n}\) | lenses, paint, security barrier, LCD screen, shatter resistant glass |
| Buna N | Butadiene and acrylonitrile | \(\mathrm{[H_2C\!-\!CH}\!=\!\mathrm{CH\!-\!CH_2\!-\!CH_2\!-\!CH(CN)]_n}\) | adhesives, rubber belts, shoe soles, O-rings, gaskets |
| PVC (polyvinyl chloride) | vinyl chloride, \(\mathrm{CH_2}\!=\!\mathrm{CHCl}\) | \(\mathrm{[CH_2\!-\!CH(Cl)]_n}\) | water pipes, rain coats, flooring |
| Polyacrylamide | acrylamide, \(\mathrm{CH_2}\!=\!\mathrm{CH\!-\!CONH_2}\) | \(\mathrm{[CH_2\!-\!CH(CONH_2)]_n}\) | Polyacrylamide gel used in electrophoresis |
| Urea-formaldehyde resin | a. urea b. formaldehyde | \(\mathrm{[NH\!-\!CO\!-\!NH\!-\!CH_2]_n}\) | unbreakable dinner ware, decorative laminates |
| Glyptal | a. ethyleneglycol b. phthalic acid | \(\mathrm{[O\!-\!CH_2\!-\!CH_2\!-\!OOC\!-\!C_6H_4\!-\!CO]_n}\) | paints and lacquers |
| Polycarbonate | a. bisphenol b. phosgene | \(\mathrm{[CO\!-\!O\!-\!C_6H_4\!-\!C(CH_3)_2\!-\!C_6H_4\!-\!O]_n}\) | electrical and telecommunication hardware, food grade plastic containers |
| Thermocol (made from airfilled thin walled beads of polystyrene) | Styrene, \(\mathrm{C_6H_5CH}\!=\!\mathrm{CH_2}\) | \(\mathrm{[CH_2\!-\!CH(C_6H_5)]_n}\) | non-biodegradable styrene can leach when heated. Therefore it is banned. |
Two of these repeating units are easier to follow as drawn structures than as condensed formulae, so they are shown in Fig. 15.17.
Diagram
Fig. 15.17 – Repeating units of glyptal and polycarbonate: (a) glyptal, showing an ethylene glycol residue –O–CH₂–CH₂–O– joined through two ester groups to an ortho-disubstituted benzene ring from phthalic acid; (b) polycarbonate, showing the bisphenol unit —C₆H₄–C(CH₃)₂–C₆H₄— joined to the next unit through an –O–CO–O– carbonate group, with that group circled and labelled.
15.7 Summary
A polymer is a giant macromolecule built by interlinking a large number of small repeating units, which are derived from small molecules called monomers. The word comes from Greek poly (many) and mer (part or unit). Polymers are high molecular mass macromolecules (\(10^3 - 10^7\ \text{u}\)).
Polymers are classified on six different bases — source, structure, mode of polymerization, intermolecular forces, type of monomers, and biodegradability. Every polymer has an answer under all six headings at once.
On the basis of source: natural polymers come from plants (cotton, linen, natural rubber) or animals (wool, silk); synthetic polymers are man-made (nylon, terylene, neoprene) and include fibres, synthetic rubbers and plastics; semisynthetic polymers, also called regenerated fibres, are made by chemically modifying natural polymers (cellulose acetate rayon, cellulose nitrate, viscose rayon, cuprammonium rayon).
On the basis of structure: linear polymers come from bifunctional monomers or alkenes (PVC, HDP); branched chain polymers from monomers with three functional groups or existing side chains (LDP); cross-linked or network polymers from polyfunctional monomers (bakelite, melamine).
Polymerization converts low molecular mass compounds possessing reactive groups into high molecular mass compounds by linking repeating structural units with covalent bonds, using high temperature and/or pressure and/or a catalyst.
Addition (chain growth) polymerization joins monomers containing a double bond with no loss of any small molecule, so the repeating unit has the same elemental composition as the monomer. It is also called vinyl polymerization.
The free radical mechanism of addition polymerization has three steps — chain initiation (an initiator such as benzoyl peroxide, acetyl peroxide or tert-butyl peroxide supplies a free radical, which adds to the vinyl monomer), chain propagation (the radical adds monomer after monomer very rapidly), and chain termination (two growing chain radicals combine to give the polymer).
\[\overset{\bullet}{\mathrm{R}} + \mathrm{CH_2}\!=\!\mathrm{CHY} \rightarrow \mathrm{R\!-\!CH_2\!-\!\overset{\bullet}{C}HY}\]
Condensation (step growth) polymerization joins polyfunctional monomers with the elimination of a small molecule such as water, hydrochloric acid, methanol or ammonia. The repeating unit therefore does not have the same elemental composition as the monomer. Terylene from ethylene glycol and terephthalic acid is the standard example.
Ring opening polymerization opens cyclic monomers such as lactams, cyclic ethers and lactones using a strong acid or base. No small molecule is lost, so the repeating unit matches the monomer in composition, but growth occurs one monomer unit at a time, making it a step growth process. Example: \(\varepsilon\)-caprolactam giving nylon 6.
On the basis of intermolecular forces: elastomers have weak van der Waals forces with a few crosslinks and are elastic (vulcanized rubber, buna-S, buna-N, neoprene); fibres have strong hydrogen bonding and dipole-dipole forces, are crystalline and have high tensile strength (nylon 6, 6, terylene); thermoplastics have moderately strong forces and soften on heating and harden on cooling repeatedly (polythene, polystyrene, polyvinyls); thermosetting polymers are rigid and infusible because of extensive covalent cross linking formed in the mould (bakelite, urea formaldehyde resin).
On the basis of monomers: homopolymers have only one type of repeating unit (polythene, polypropene, nylon 6, polyacrylonitrile, nylon 6, 6); copolymers have two or more types of randomly sequenced repeating units (Buna-S, Buna-N).
On the basis of biodegradability: most synthetic polymers are not attacked by microbes and persist as pollution, while most natural fibres are biodegradable.
Natural rubber is a linear addition polymer of isoprene (2-methyl-1,3-butadiene), molecular mass 130, 000 u to 340, 000 u (2000 to 5000 units). Its cis double bonds give a coiled structure held by weak van der Waals forces, so it stretches like a spring.
Vulcanization is the process by which a network of cross links is introduced into an elastomer. Heating raw rubber with sulfur powder and accelerators (commonly ZBX, zincbutyl xanthate) at about \(150\ ^{0}\text{C}\) forms sulfur cross links at the double bonds, improving tensile strength, stiffness, elasticity and toughness; at 40-45 % sulfur the hard, non-elastic solid ebonite is obtained.
Polythene exists as LDP and HDP. LDP is made at 1000 - 2000 atm and 350 - 570 K with traces of \(\mathrm{O_2}\) or peroxide; free radical addition with H-atom abstraction causes branching, low density, flexibility and melting point \(110\ ^{0}\text{C}\). HDP is made at 333 K - 343 K and 6-7 atm using the Zieglar-Natta catalyst (triethyl aluminium with titanium tetrachloride), giving a linear, closely packed, crystalline polymer of melting point \(144 - 150\ ^{0}\text{C}\) with high tensile strength.
Teflon is polytetrafluoroethylene, made from tetrafluoroethylene (\(\mathrm{CF_2}\!=\!\mathrm{CF_2}\)) using hydrogen peroxide or ammonium persulphate at high pressure. The very strong \(\mathrm{C}\!-\!\mathrm{F}\) bond makes it tough, chemically inert and heat resistant — hence non-stick cookware, oil seals and gaskets.
Polyacrylonitrile, made by addition polymerization of acrylonitrile with a peroxide initiator, resembles wool and gives orlon or acrilan.
Polyamides (nylons) contain \(\mathrm{-CO\!-\!NH-}\) inter unit linkages. Nylon 6, 6 is a condensation polymer of adipic acid and hexamethylenediamine (through nylon salt, at 553 K and high pressure), molecular mass 12000-50000 u. Nylon 6 is made by ring opening polymerization of \(\varepsilon\)-caprolactam with water at 533 - 543 K. A lactam is a cyclic amide; a lactone is a cyclic ester.
Polyesters have ester linkages joining the repeating units. Terylene (dacron) is made from ethylene glycol and terephthalic acid at 420 - 460 K over a zinc acetate-antimony trioxide catalyst; it melts at \(265\ ^{0}\text{C}\) and gives terycot and terywool blends. PET is the same polyester as a thermoplastic used for bottles and containers. Polycarbonates are high melting thermosetting polyesters.
Bakelite, the oldest synthetic polymer, is made from phenol and formaldehyde with an acid or base catalyst in two stages to give novolac, which is then cross-linked in moulds at \(138\ ^{0}\text{C}\) to \(176\ ^{0}\text{C}\) under high pressure into a rigid, insoluble, infusible solid. Melamine-formaldehyde resin is the cross-linked thermosetting plastic of formica table tops and dinner-ware.
Buna-S (SBR) is a copolymer of 75 parts butadiene with 25 parts styrene, polymerized by the action of sodium and vulcanized with sulfur; it has better mechanical strength and abrasion resistance than natural rubber and is used in tyres. Neoprene is the addition polymer of chloroprene (2-chloro-1,3-butadiene), vulcanized with magnesium oxide, and resists petroleum, vegetable oils, light and heat.
Viscose rayon is regenerated cellulose: wood pulp is treated with concentrated \(\mathrm{NaOH}\) to give alkali cellulose, converted to the xanthate with \(\mathrm{CS_2}\), mixed with dilute \(\mathrm{NaOH}\) to give viscose solution, and extruded through spinnerets into an acid bath where cellulose filaments precipitate.
Flowchart
FC 15.7 – The chapter in one line: monomer (with reactive group) → polymerization by one of three modes (addition / condensation / ring opening) → polymer chain of a particular structure (linear / branched / cross-linked) → held by particular intermolecular forces (elastomer / fibre / thermoplastic / thermosetting) → grown to the required molecular mass (critical degree of polymerization) → useful article → disposal: biodegradable (PHBV, nylon 2-nylon 6) or non-biodegradable (polythene, teflon, thermocol)
- A polymer sample is a mixture of chains of different lengths, so its molecular mass is an average. It depends on the degree of polymerization (DP), the number of monomer units in one polymer molecule:
\[\text{DP} = \frac{\text{molecular mass of polymer}}{\text{molecular mass of the repeating unit}}\]
Low molecular mass polymers are brittle and weak; very high molecular mass polymers are tough and unmanageable. The minimum molecular mass needed for a given application corresponds to the critical degree of polymerization, which is lower for polymers with hydrogen bonding than for those with weak intermolecular forces.
Biodegradable polymers are being developed to reduce polymer pollution. They contain functional groups similar to those in biopolymers such as proteins; aliphatic polyesters and polyamides with a large proportion of polar linkages are important examples. PHBV is a copolyester of \(\beta\)-hydroxybutyric acid (3-hydroxybutanoic acid) and \(\beta\)-hydroxyvaleric acid (3-hydroxypentanoic acid); nylon 2 - nylon 6 is a polyamide copolymer of glycine and \(\varepsilon\)-amino caproic acid.
Many other polymers are commercially important, including perspex, Buna N, PVC, polyacrylamide, urea-formaldehyde resin, glyptal, polycarbonate and thermocol, each with its own monomer, repeating unit and range of applications.
15.8 Objective Questions
- Nylon fibres are —-
- Semisynthetic fibres
- Polyamide fibres
- Polyester fibres
- Cellulose fibres
- Which of the following is naturally occurring polymer?
- Telfon
- Polyethylene
- PVC
- Protein
- Silk is a kind of —- fibre
- Semisynthetic
- Synthetic
- Animal
- Vegetable
- Dacron is another name of —-
- Nylon 6
- Orlon
- Novolac
- Terylene
- Which of the following is made up of polyamides?
- Dacron
- Rayon
- Nylon
- Jute
- The number of carbon atoms present in the ring of \(\varepsilon\) - caprolactam is
- Five
- Two
- Seven
- Six
- Terylene is —-
- Polyamide fibre
- Polyester fibre
- Vegetable fibre
- Protein fibre
- PET is formed by —-
- Addition
- Condensation
- Alkylation
- Hydration
- Chemically pure cotton is —-
- Acetate rayon
- Viscose rayon
- Cellulose nitrate
- Cellulose
- Teflon is chemically inert, due to presence of ………..
- C-H bond
- C-F bond
- H- bond
- C=C bond
15.9 Check Your Concepts
Assertion: Vulcanized rubber is more elastic and tougher than raw natural rubber. Reason: Sulfur atoms introduce cross links between polyisoprene chains by opening the double bonds. Choose: (A) both assertion and reason are true and the reason explains the assertion, (B) both are true but the reason does not explain the assertion, (C) assertion true, reason false, (D) assertion false, reason true. Justify your choice in one line.
Assertion: High density polythene is stiffer and denser than low density polythene. Reason: The chains of high density polythene are extensively branched. Choose from the four options given in question 1 and justify.
Assertion: Thermosetting articles cannot be recycled by melting and remoulding. Reason: They possess extensive cross linking by covalent bonds formed in the mould. Choose from the four options given in question 1 and justify.
Why does a monomer carrying only two functional groups always give a linear polymer, while a monomer carrying three gives a branched or cross-linked one?
Nylon 6, 6 is prepared from two different monomers, yet it is called a homopolymer, whereas Buna-S is called a copolymer. Explain the difference.
Why do fibres such as nylon 6, 6 possess much higher tensile strength than an elastomer such as natural rubber? Answer in terms of the forces acting between neighbouring chains.
What happens if the proportion of sulfur used during vulcanization of rubber is raised to 40-45 %? Name the product formed and state how its elasticity compares with that of ordinary vulcanized rubber.
Complete the following reactions and name the product in each case:
- \(n\,\mathrm{CF_2}\!=\!\mathrm{CF_2} \xrightarrow{\ \text{peroxide}\ } ?\)
- \(n\,\mathrm{CH_2}\!=\!\mathrm{CHCN} \xrightarrow{\ \text{peroxide}\ } ?\)
- \(n\,\mathrm{CH_2}\!=\!\mathrm{C(Cl)\!-\!CH}\!=\!\mathrm{CH_2} \rightarrow ?\)
A polymer has the repeating unit \(\mathrm{-[NH\!-\!(CH_2)_5\!-\!CO]-}\). Name the polymer, name its monomer, state the mode of polymerization by which it is formed, and state whether any small molecule is eliminated.
Arrange the following in increasing order of the strength of the forces acting between neighbouring chains, and give one example of each: thermoplastics, elastomers, fibres.
Match the following pairs:
| Polymer | Characteristic feature |
|---|---|
| a. Neoprene | i. Vulcanized with sulfur; copolymer of butadiene and styrene |
| b. Buna-S | ii. Vulcanized with magnesium oxide |
| c. Bakelite | iii. Regenerated cellulose |
| d. Viscose rayon | iv. Cross-linked phenol-formaldehyde polymer |
| e. Nylon 6 | v. Formed by ring opening polymerization |
Teflon and polythene have backbones of the same kind, yet only teflon is used to line cookware that must resist corrosive reagents and heat. Explain the difference in terms of the bonds present in the two chains.
Name the mode of polymerization involved in each of the following, and state in each case whether the repeating unit has the same elemental composition as the monomer:
- melamine with formaldehyde
- \(\varepsilon\)-caprolactam
- vinyl chloride
What is the function of benzoyl peroxide in the preparation of an addition polymer? Predict what would happen to the reaction if it were left out altogether.
Pick the odd one out of each set and give the reason for your choice:
- Buna-S, Buna-N, neoprene, terylene
- Cotton, silk, wool, orlon
- Bakelite, melamine-formaldehyde resin, polythene, urea-formaldehyde resin
Why is viscose rayon classified as a semisynthetic fibre even though the fibre finally obtained is cellulose, the same polymer that the process started with?
Arrange the following stages of viscose rayon manufacture in the correct sequence and name the substance formed at each stage: extrusion into acid bath, treatment with carbon disulphide, treatment with concentrated sodium hydroxide, mixing with dilute sodium hydroxide.
Isoprene contains two double bonds, but the repeating unit of natural rubber contains only one. Explain where the second double bond has gone, and state what role the surviving double bond plays later.
Why is polythene not attacked by soil microbes, whereas PHBV buried in the same soil is broken down? Answer in terms of the linkages present in the two chains.
What would happen to the properties of a polyester if the flexible ethylene glycol unit in its chain were replaced by a rigid aromatic unit? Predict the effect on melting point and on the ability to be drawn into fibre, with reasons.
A polymer sample is found to be brittle and mechanically weak, although chemical analysis shows it to be the correct polymer. Suggest what has gone wrong during its manufacture, and explain how the fault would be corrected.
Identify A, B and C in the following sequence, and name the type of polymerization in the second step: ethene \(\rightarrow\) A (in presence of triethyl aluminium with titanium tetrachloride at 333 K - 343 K); phenol + formaldehyde \(\rightarrow\) B (two stages, acid or base) \(\rightarrow\) C (heated in a mould at high pressure).
Identify ‘A’ and ‘B’ in the following reaction —-
\[\mathrm{HO\!-\!CH_2\!-\!CH_2\!-\!OH} + \mathrm{H\!-\!O\!-\!CO\!-\!C_6H_4\!-\!CO\!-\!O\!-\!H} \xrightarrow[\ \Delta\ ]{\ 533\ \text{K}\ } \text{'A'}\]
\[\mathrm{H_2N\!-\!(CH_2)_6\!-\!NH_2} + \mathrm{HOOC\!-\!(CH_2)_4COOH} \xrightarrow[\ 533\ \text{K}\ ]{\ \mathrm{N_2}\ } \text{'B'}\]
- Complete the following statements
- Caprolactam is used to prepare——–
- Novolak is a copolymer of ——– and ———
- Terylene is ———-polymer of terephthalic acid and ethylene glycol.
- Benzoyl peroxide used in addtion polymerisation acts as ———-
- Polyethene consists of polymerised ———-
Draw the flow chart diagram to show classification of polymers based on type of polymerisation.
Write examples of Addition polymers and condensation polymers.
Name some chain growth polymers.
Define the terms: 1) Monomer 2) Vulcanisation 3) Synthetic fibres
What type of intermolecular force leads to high density polymer?
Give one example each of copolymer and homopolymer.
Identify Thermoplastic and Thermosetting Plastics from the following —–
- PET
- Urea formaldehyde resin
- Polythene
- Phenol formaldehyde
Write the names of classes of polymers formed according to intermolecular forces and describe briefly their structural characteristics.
Write reactions of formation of:
- Nylon 6
- Terylene
Write structure of natural rubber and neoprene rubber along with the name and structure of thier monomers.
Name the polymer type in which the following linkage is present: \(\mathrm{-CO\!-\!O-}\)
Write structural formula of the following synthetic rubbers:
- SBR rubber
- Buna-N rubber
- Neoprene rubber
- Match the following pairs:
| Name of polymer | Monomer |
|---|---|
| a. Teflon | i. \(\mathrm{CH_2}\!=\!\mathrm{CH_2}\) |
| b. PVC | ii. \(\mathrm{CF_2}\!=\!\mathrm{CF_2}\) |
| c. Polyester | iii. \(\mathrm{CH_2}\!=\!\mathrm{CHCl}\) |
| d. Polythene | iv. \(\mathrm{C_6H_5OH}\) and \(\mathrm{HCHO}\) |
| e. Bakelite | v. Dicarboxylic acid and polyhydoxyglycol |
Draw the structures of polymers formed from the following monomers: 1. Adipic acid + Hexamethylenediamine 2. \(\varepsilon\) - Aminocaproic acid + Glycine
Name and draw structure of the repeating unit in natural rubber.
Classify the following polymers as natural and synthetic polymers
- Cellulose
- Polystyrene
- Terylene
- Starch
- Protein
- Silicones
- Orlon (Polyacrylonitrle)
- Phenol-formedehyde resins
What are synthetic resins? Name some natural and synthetic resins.
Distinguish between thermosetting and thermoplastic resins. Write example of both the classes.
Write name and formula of raw material from which bakelite is made.
Identify condensation polymers and addition polymers from the following.
- \(\mathrm{-(CH_2\!-\!CH(C_6H_5)\!-)_n}\)
- \(\mathrm{-(CH_2\!-\!CH}\!=\!\mathrm{CH\!-\!CH_2)_n}\)
- \(\mathrm{-(CO(CH_2)_4\!-\!CONH(CH_2)_6NH\!-)_n}\)
- \(\mathrm{-(OCH_2\!-\!CH_2\!-\!O\!-\!CO\!-\!C_6H_4\!-\!CO\!-)_n}\)
Write the chemical reactions involved in manufacture of Nylon 6,6
Explain vulcanisation of rubber. Which vulcanizing agents are used for the following synthetic rubber:
- Neoprene
- Buna-N
Write reactions involved in the formation of — 1) Teflon 2) Bakelite
What is meant by LDP and HDP? Mention the basic difference between the same with suitable examples.
Write preparation, properties and uses of Teflon.
Classify the following polymers as straight chain, branched chain and cross linked polymers.
- \(\mathrm{-(CH_2\!-\!CH(CN)\!-)_n}\)
- \(\mathrm{-(CH_2\!-\!CH_2\!-\!CH(CH_2CH_2)\!-\!CH_2\!-\!CH_2\!-)_n}\)
- Bakelite (the cross-linked phenol-formaldehyde structure shown in the chapter)
How is polythene manufactured? Give their properties and uses.
Is synthetic rubber better than natural rubber? If so, in what respect?
Write main specialities of Buna-S, Neoprene rubber?
Write the structure of isoprene and the polymer obtained from it.
Explain in detail free radical mechanism involved during preparation of addition polymer.
Make a list of polymers used to make the following articles:
- Photographic film
- Frames of spectacles
- Fountain pens
- Moulded plastic chains
- Terywool or Terycot fabric
15.10 Practice With Numericals
(Use the atomic masses: \(\mathrm{H} = 1\ \text{u}\), \(\mathrm{C} = 12\ \text{u}\), \(\mathrm{N} = 14\ \text{u}\), \(\mathrm{O} = 16\ \text{u}\), \(\mathrm{F} = 19\ \text{u}\), \(\mathrm{Cl} = 35.5\ \text{u}\))
A sample of polyvinyl chloride has an average molecular mass of \(3.75 \times 10^{5}\ \text{u}\). Calculate its degree of polymerization.
A polythene sample has a degree of polymerization of 8000. Calculate its average molecular mass.
Calculate the molecular mass of a natural rubber molecule that contains 5000 isoprene units, and state whether the value lies within the range quoted for natural rubber.
A teflon sample has an average molecular mass of \(4.0 \times 10^{5}\ \text{u}\). Calculate the number of tetrafluoroethylene units present in one molecule of the sample.
A sample of polyacrylonitrile has an average molecular mass of \(1.06 \times 10^{5}\ \text{u}\). Calculate its degree of polymerization.
The repeating unit of nylon 6 is \(\mathrm{-[NH\!-\!(CH_2)_5\!-\!CO]-}\). Calculate the molecular mass of this repeating unit, and hence the degree of polymerization of a nylon 6 sample of average molecular mass \(22600\ \text{u}\).
The repeating unit of nylon 6, 6 is \(\mathrm{-[CO\!-\!(CH_2)_4\!-\!CO\!-\!NH\!-\!(CH_2)_6\!-\!NH]-}\). Calculate its molecular mass, and hence the degree of polymerization of a sample of average molecular mass \(45200\ \text{u}\). State whether this molecular mass lies in the range quoted for nylon 6, 6.
The repeating unit of terylene is \(\mathrm{-[O\!-\!CH_2\!-\!CH_2\!-\!O\!-\!CO\!-\!C_6H_4\!-\!CO]-}\). Calculate its molecular mass, and the degree of polymerization of a terylene sample of average molecular mass \(3.84 \times 10^{4}\ \text{u}\).
Calculate the percentage by mass of chlorine in the repeating unit of polyvinyl chloride.
Calculate the percentage by mass of fluorine in the repeating unit of teflon.
Buna-S is usually obtained from 75 parts of butadiene and 25 parts of styrene. Calculate the mass of each monomer required to prepare a 600 kg batch of Buna-S.
A polymer of ethene is found to contain 5000 monomer units per molecule. Calculate its molecular mass, and state whether such a sample would be expected to lie above or below the critical degree of polymerization needed for a useful plastic article.
A sample of natural rubber has an average molecular mass of \(1.70 \times 10^{5}\ \text{u}\). Calculate the number of isoprene units in one molecule, and compare it with the range of 2000 to 5000 units quoted for natural rubber.