Polymers

13.1Synthetic polymers

Think of a necklace made of hundreds of identical beads. A polymer is a giant molecule built in the same way: a very large number of small units joined one after another by covalent bonds. The small molecules that join together are monomers, and the reaction that joins them is polymerisation. Plastic bags, water pipes, nylon clothes and polyester bottles are made in factories, so they are synthetic polymers. Starch, cellulose, proteins and natural rubber are natural polymers.

Polymer

A compound of high molecular mass whose molecules are made of a large number of simple repeating units joined by covalent bonds.

Monomer

A simple molecule of low molecular mass that can join with other monomers to form a polymer, for example ethene, $\mathrm{CH}{}_2=\mathrm{CH}{}_2$.

Polymerisation

The reaction by which monomers are converted into a polymer.

Repeat unit

The smallest part of a polymer chain that repeats again and again. It is written in brackets with a subscript $n$.

Degree of polymerisation, $n$

The number of repeat units in one polymer chain; usually several hundred to many thousands.

The formation of polyethene (polythene) from ethene (ethylene) is the simplest example. The double bond of each ethene molecule opens and the molecule links to its neighbours.

Nothing is lost in this reaction, so the molar mass of the polymer is $n$ times the molar mass of the repeat unit:

Homopolymers, copolymers and terpolymers

Homopolymer

A polymer made from only one kind of monomer, for example polythene (from ethene) and PVC (from chloroethene).

Copolymer

A polymer made from two different monomers, for example nylon-6,6 (from a diamine and a dicarboxylic acid) and terylene (from a diol and a dicarboxylic acid).

Terpolymer

A polymer made from three different monomers, for example ABS plastic (used for helmets and toys), made from acrylonitrile, butadiene and styrene.

13.2Addition polymersOften examined

Polymers are classified by the way they are made into two main types: addition polymers and condensation polymers.

Addition polymers are formed by combining alkene monomers (molecules with a $\mathrm{C}=\mathrm{C}$ double bond), one after another, to give just one huge molecule. The weaker $\pi$ bond of each double bond breaks, and its electrons form new $\sigma$ bonds with the neighbouring monomers. Nothing is lost: monomers simply add to each other.

• The monomer must contain a $\mathrm{C}=\mathrm{C}$ double bond: an alkene or a substituted alkene such as $\mathrm{CH}{}_2=\mathrm{CHCl}$.
• No small molecule is eliminated, so the polymer has the same empirical formula as the monomer, and its molar mass is exactly $n$ times the monomer's molar mass.
• The backbone (main chain) is made only of carbon atoms, the ones that originally formed the double bonds.
• The polymer is saturated. Its chain has only strong, non-polar $\mathrm{C}-\mathrm{C}$ and $\mathrm{C}-\mathrm{H}$ bonds, which is why poly(alkenes) are very unreactive.
• The reaction is a fast chain reaction, started by a catalyst or initiator such as a peroxide or an acid.

Conditions and catalysts

With a peroxide initiator, ethene is polymerised at a very high pressure (about $1000\mathrm{atm}$) and about $200{}^{\circ }\mathrm{C}$. With Ziegler catalysts, a mixture of a trialkylaluminium $\mathrm{AlR}{}_3$ (for example triethylaluminium) and titanium tetrachloride $\mathrm{TiCl}{}_4$ in an inert solvent, much lower temperatures (about $60{}^{\circ }\mathrm{C}$) and pressures (a few atmospheres) can be used. Ziegler catalysts give straight, unbranched chains that pack closely, so the polythene is denser and stronger.

How the chain grows

The book does not give the mechanism, but it shows why nothing is lost. A peroxide initiator gives free radicals, species with an unpaired electron (shown by a dot).

Polythene (polyethene)

Polythene, $-[\mathrm{CH}{}_2-\mathrm{CH}{}_2]{}_n-$, is the simplest addition polymer and the most widely used plastic. The conditions decide how its chains are arranged.

Polyvinyl chloride (PVC)

PVC, or poly(chloroethene), is obtained by the addition polymerisation of vinyl chloride (chloroethene), $\mathrm{CH}{}_2=\mathrm{CHCl}$. The book gives the conditions as $13\mathrm{atm}$ and $40$ to $60{}^{\circ }\mathrm{C}$, with a small amount of a peroxide as initiator. The molecular formula of vinyl chloride is $\mathrm{C}{}_2\mathrm{H}{}_3\mathrm{Cl}$, so PVC is $(\mathrm{C}{}_2\mathrm{H}{}_3\mathrm{Cl}){}_n$.

Vinyl chloride itself is obtained from acetylene (ethyne) by adding hydrogen chloride in the presence of mercury(II) chloride, $\mathrm{HgCl}{}_2$, as catalyst:

Properties of PVC

Uses of PVC

• Construction, about 70% of the PVC used in Pakistan: water and drainage pipes, fittings, window and door profiles, corrugated roofing sheets and floor coverings.
• Electrical: insulation of cables and wires.
• Consumer goods: artificial leather, footwear, raincoats, packaging and, in the past, gramophone records.
• Medical devices, made of flexible PVC: blood and drip bags, tubing and gloves.

13.2Condensation polymersOften examined

Condensation polymers are formed by the combination of monomers with the elimination of a simple molecule such as water ($\mathrm{H}{}_2\mathrm{O}$), ammonia ($\mathrm{NH}{}_3$), hydrogen chloride ($\mathrm{HCl}$) or an alcohol ($\mathrm{ROH}$) at every link. The two main types are polyesters and polyamides.

For a chain to grow, every monomer needs two functional groups, one at each end: one end reacts with the monomer in front, the other with the monomer behind. The two groups can be on two different monomers (a diol with a dicarboxylic acid, or a diamine with a dicarboxylic acid) or on the same monomer (a hydroxycarboxylic acid or an amino acid).

The general equation for a polyester shows why $(2n-1)$ water molecules form: $2n$ monomer molecules give a chain with $2n-1$ links, and one water molecule is lost at each link.

Amides from acyl chlorides and ammonia or amines

The amide link \ce{\bond{-}CO-NH\bond{-}} of nylon and Kevlar is easiest to understand from a simple reaction. An acyl chloride, \ce{R-COCl}, reacts with ammonia or with an amine at room temperature to give an amide and hydrogen chloride. It is a condensation reaction, because a small molecule (\ce{HCl}) is eliminated.

Why it is fast at room temperature. In an acyl chloride the carbonyl carbon is bonded to two electronegative atoms, \ce{O} and \ce{Cl}, so it carries a large partial positive charge. Ammonia and amines have a lone pair on nitrogen, so they are good nucleophiles. The reaction is vigorous and gives white fumes of \ce{HCl}, or a white solid (an ammonium salt) when ammonia or the amine is in excess, because the base neutralises the \ce{HCl}.

With ammonia: a primary amide

With a primary amine: an N-substituted amide

13.2 (a)Polyamides: nylon-6,6, Kevlar and polypeptidesOften examined

A polyamide is a condensation polymer whose repeat units are joined by amide links, \ce{\bond{-}CO-NH\bond{-}}. Polyamides can be made from a variety of reactants: a diamine with a dicarboxylic acid (or its dioyl chloride), or amino acids.

Nylon-6,6

Nylon-6,6 is made from a diamine and a dicarboxylic acid. A hydroxyl group (\ce{-OH}) from the acid's \ce{-COOH} group combines with a hydrogen atom from one of the diamine's \ce{-NH2} groups to form water, and an amide link forms between the two molecules. Because both molecules have two functional groups, one at each end, the process happens again and again, and a long polyamide chain is built.

It is obtained by heating adipic acid (hexanedioic acid) with hexamethylenediamine (hexane-1,6-diamine) under nitrogen at about 200 to 280\,^\circ\mathrm{C}. Nitrogen keeps out the oxygen of the air, which would oxidise the hot monomers. The name nylon-6,6 comes from the starting materials: each has six carbon atoms.

Properties of nylon-6,6

Neighbouring nylon chains are held together by hydrogen bonds between the \ce{C=O} oxygen of one chain and the \ce{N-H} hydrogen of the next. These strong intermolecular forces explain most of its useful properties.

Uses. Nylon-6,6 was developed as a synthetic fibre for the production of stockings and other clothing, and it was introduced to the public at the New York World's Fair in 1939. It is also used to make fibres for clothing and carpets, filaments for fishing lines and ropes, bristles for brushes, and moulded objects such as gears and bearings.

Kevlar

Kevlar is another polyamide. Its monomers are benzene-1,4-dicarboxylic acid (benzene-1,4-dioic acid, terephthalic acid) and benzene-1,4-diamine (para-phenylenediamine). In each monomer the two functional groups are on opposite (1,4) carbon atoms of a benzene ring, so Kevlar is an aromatic polyamide.

In industry the acid is replaced by its more reactive acyl chloride, benzene-1,4-dioyl chloride (terephthaloyl chloride). The reaction then takes place at low temperature and \ce{HCl} is eliminated instead of water (Concept assessment exercise 13.2 below).

Why Kevlar is so strong

Kevlar chains are straight and rigid because of the flat benzene rings. The chains line up parallel to each other, and every \ce{C=O} and \ce{N-H} group forms a hydrogen bond with the neighbouring chain. The chains form flat, tightly packed sheets, so a force must break a huge number of bonds at once.

Polypeptides and proteins

A polypeptide is produced by the polymerisation of amino acid molecules. An amino acid has an amine group (\ce{-NH2}) and a carboxylic acid group (\ce{-COOH}) in the same molecule. The \ce{-COOH} of one amino acid reacts with the \ce{-NH2} of the next, a water molecule is eliminated, and an amide link forms. In proteins this link is called a peptide link. Proteins are natural polyamides.

13.2 (b)Polyesters: terylene and polylactic acidOften examined

A polyester is a condensation polymer whose repeat units are joined by ester links, \ce{\bond{-}CO-O\bond{-}}. Polyesters are produced by the condensation of diols with dicarboxylic acids (or dioyl chlorides), or from hydroxycarboxylic acids. Each ester link forms exactly as in esterification: the \ce{-OH} of the acid and the \ce{H} of the alcohol leave as water.

Terylene (PET)

The most common polyester is terylene, also known as polyethylene terephthalate (PET). It is made from the monomers ethane-1,2-diol (ethylene glycol) and benzene-1,4-dicarboxylic acid (terephthalic acid), heated with an acid catalyst. One molecule of water is formed for each ester group formed.

The book lists its main uses as plastic bottles, clothing and carpets.

Polyesters from a hydroxycarboxylic acid

A molecule that has both an \ce{-OH} and a \ce{-COOH} group can form a polyester by itself. For example, polylactic acid (PLA) is made from the monomer 2-hydroxypropanoic acid (lactic acid): the \ce{-OH} of one molecule reacts with the \ce{-COOH} of the next.

13.2Addition and condensation polymerisation comparedOften examined

This comparison is the most frequently asked question of the unit. It appeared three times in the 2026 papers: Set B Q.2 (x), Set D Q.2 (xi) and the old-syllabus paper Q.4.

Predicting the type of polymerisation

• One monomer with a \ce{C=C} double bond: addition polymerisation.
• One monomer with two different groups that react with each other (\ce{-OH} and \ce{-COOH}, or \ce{-NH2} and \ce{-COOH}): condensation polymerisation, giving a polyester or a polyamide.
• Two monomers, each with two identical functional groups (a diol or a diamine with a dicarboxylic acid or a dioyl chloride): condensation polymerisation.

Monomers and repeat units from a polymer segmentOften examined

Exam questions often show a section of a polymer chain and ask for the repeat unit, the monomer or monomers, and the type of polymer. Work in this order.

13.4Biodegradable and non-biodegradable polymersOften examined

Biodegradable polymers

Polymers that can be decomposed by the action of microorganisms (bacteria and fungi) into simple substances such as carbon dioxide and water. Examples: polyamides and polyesters, which have hydrolysable amide or ester groups that enzymes, acids and alkalis can break easily.

Non-biodegradable polymers

Polymers that cannot be decomposed by the action of microorganisms, for example polyethylene, polypropylene, polyvinyl chloride and polyethylene terephthalate (PET).

Polymers formed by addition polymerisation are unreactive (inert) because they are non-polar and saturated. This makes them difficult to break down and biodegrade. Some can be recycled to form new polymers for a different purpose.

Why poly(alkenes) are chemically inert

• Their chains contain only strong \ce{C-C} and \ce{C-H} single bonds (bond energies about 350 and 410\,\mathrm{kJ\,mol^{-1}}).
• These bonds are almost non-polar, so there is no \delta+ or \delta- site for water, acids, alkalis or nucleophiles to attack.
• They are saturated: the double bonds were used up in polymerisation, so addition reactions are no longer possible.
• They have no functional groups (no ester or amide links) that the enzymes of bacteria and fungi can recognise and break.
• They are hydrophobic (water-repelling), so water and microorganisms cannot get inside them.

As a result, these polymers remain intact for hundreds of years and harm the environment. Pieces of them can also enter our food chain.

Degradation by light

Some polymers can be degraded by ultraviolet (UV) light, for example polyethylene, polypropylene and polystyrene. UV light carries enough energy to break bonds in the polymer chains and form free radicals. The chains become shorter and the material weakens, becomes brittle and crumbles.

Photodegradable plastics are made by building a few light-absorbing carbonyl (\ce{C=O}) groups into the chains, or by adding light-sensitive additives. They break down in sunlight, but only into small fragments, and not at all when buried in a landfill away from light.

Long-term effects of non-biodegradable polymers

Non-biodegradable polymers accumulate in the environment because they do not decompose naturally. In the presence of sunlight, water and air they slowly produce highly toxic pollutants, which enter water and cause water pollution.

• Accumulation. Plastic waste piles up in landfills, streets and fields and blocks drains and sewers, which causes urban flooding.
• Toxic pollutants. As plastics weather, they release additives such as plasticisers, dyes and heavy-metal stabilisers into soil and water.
• Microplastics. Sunlight and waves break plastics into tiny pieces that are eaten by fish and shellfish and pass up the food chain to humans.
• Harm to animals. Turtles, seabirds and cattle swallow plastic bags or get trapped in nets and rings, and starve or drown.
• Air pollution when burnt. Open burning gives carbon monoxide, soot and, from PVC, hydrogen chloride and dioxins, which are toxic and can cause cancer.

Managing polymer waste

13.5Polymer hydrolysisOften examined

Condensation polymers can be broken down by hydrolysis, the reverse of condensation: water adds across each ester or amide link and the chain splits back into its monomers. Pure water reacts far too slowly, so the reaction is carried out under acidic or basic conditions. This is why polyesters and polyamides are biodegradable by acid and alkaline hydrolysis.

Hydrolysis of one link

Why acids and alkalis speed up hydrolysis

In acid, \ce{H+} bonds to the oxygen of the \ce{C=O} group. This makes the carbonyl carbon much more positive, so a water molecule (a weak nucleophile) can attack it easily. In neutral water there is no \ce{H+} to activate the carbonyl group, so hydrolysis is extremely slow. In alkali the attacking species is \ce{OH-}, a much stronger nucleophile than water, and the acid formed is removed as its salt, so the reaction goes to completion. This is why polyesters react readily with alkalis but only slowly with acids.

13.6Polymers in artificial organs

An artificial organ is a biological device or tissue created by scientists to replace, replicate or augment a functional, naturally occurring organ. Artificial organs can be used as a substitute for organ donors and as a medical education tool. There are three main types, depending on the materials used to create them.

Mechanical artificial organs

Made entirely of non-living materials such as polymers (plastics) and metals. Examples: artificial heart valves, hip and knee joints, the total artificial heart, and the dialysis machine that does the work of the kidneys.

Biomechanical (bio-artificial) organs

Contain both living cells and non-living materials, for example living liver or pancreas cells held inside a polymer capsule or scaffold, or skin grown on a polymer mesh for burn patients.

Biological organs

Made entirely of living tissue. Natural organs such as the heart, lungs, liver, brain and kidneys are specialised structures of different tissues working together in organ systems (digestive, respiratory, circulatory and nervous). In bioengineering, biological artificial organs are grown from a patient's own cells, often on a biodegradable polymer scaffold that later dissolves.

Properties required of a biomedical polymer

• Biocompatible: causes no inflammation, allergy or rejection by the immune system.
• Non-toxic: no harmful monomers, additives or breakdown products leak out.
• Chemically stable in body fluids, or biodegradable at a controlled rate into harmless products.
• Suitable mechanical properties: strength, flexibility, elasticity and wear resistance that match the tissue it replaces.
• Blood compatible: blood must not clot on its surface.
• Can be sterilised and shaped easily, and lasts for many years.

Challenges

• Rejection and inflammation caused by the immune system.
• Infection that starts on the surface of the implant.
• Blood clots forming on artificial surfaces, which can cause strokes.
• Wear and fatigue: tiny particles from worn joints damage nearby bone, so the joint may need replacing.
• Limited life, high cost and years of testing before approval.
• Copying complex organ functions, such as the chemical work of the liver or kidney, is very difficult.

13.7Importance of chemical industries in Pakistan

The chemical industry is a major contributor to Pakistan's economy. Almost every other industry depends on chemicals, from the fertiliser used on farms to the dyes used in textile mills.

• It accounts for about 4.5% of the country's total exports and 12% of its total imports, so it strongly affects the balance of trade.
• It is a key factor in the growth of forward-oriented industries, which use chemicals to make finished goods: the automobile industry, textiles and leather goods, shoes, furniture, and food and beverages.
• It also supplies backward-oriented industries, such as the makers of surfactants, which are essential for oil refiners and extractors.
• Home production of fertilisers, cement, soda ash, PVC and other chemicals saves foreign exchange and supports agriculture and construction.
• It has grown rapidly because of rising domestic demand, better availability of raw materials, improved government policies, increased foreign investment, advances in technology and increased regional integration. It also creates jobs.

Therefore the chemical industry influences international trade and increases the country's economic competitiveness.

Raw materials available in Pakistan

The main raw materials are fossil fuels (coal, natural gas and oil), air, water, salt, limestone, sulphur, and specialised raw materials such as phosphates and fluorides. Pakistan also has mineral deposits of coal, copper, gold, chromite, rock salt, bauxite and many other minerals, and various precious and semi-precious minerals are mined.

Main raw materials and products named in the book

Soapstone

One of the main raw materials of Pakistan's chemical industry; 85% of it is used in other industries such as textiles, paper, soap, detergents, leather and food.

Polyvinyl chloride (PVC)

Mainly used in Pakistan for pipes, fittings, cables, profiles and footwear, and also for medical devices, packaging materials and consumer products. The construction industry accounts for about 70% of total consumption.

Resin

A synthetic organic polymer used to make plastics, adhesives, paints and coatings. Its production is based mainly on petrochemical raw materials such as petroleum, ethylene and propylene.

Soda ash

The common name of sodium carbonate, \ce{Na2CO3}, used to make glass, detergents, paper and textiles. In Pakistan it is made from local rock salt and limestone.

Hydrogen peroxide

\ce{H2O2}, used for bleaching, disinfecting and oxygenating. Its production in Pakistan is largely based on imported raw materials: anthraquinone and hydrogen gas.

The real process runs in several steps using ammonia and carbon dioxide; only the overall change is shown.

Formula sheet

Key terms

Polymer

A high molecular mass compound whose molecules are made of a large number of simple repeating units.

Monomer

A small molecule that joins with others of its kind (or with another monomer) to form a polymer.

Polymerisation

The reaction by which monomers are converted into a polymer.

Repeat unit

The smallest section of a polymer chain that repeats; written in brackets with subscript n.

Degree of polymerisation

The number n of repeat units in one polymer chain.

Homopolymer

A polymer made from only one kind of monomer, for example polythene.

Copolymer

A polymer made from two different monomers, for example nylon-6,6.

Terpolymer

A polymer made from three different monomers, for example ABS plastic.

Addition polymer

A polymer formed when alkene monomers add together with no loss of any atoms, for example PVC.

Condensation polymer

A polymer formed when monomers join with the elimination of a small molecule such as \ce{H2O} or \ce{HCl}, for example nylon-6,6.

Ziegler catalyst

A mixture of a trialkylaluminium \ce{AlR3} and \ce{TiCl4} that polymerises alkenes at low temperature and pressure.

Free radical

An atom or group with an unpaired electron; peroxide radicals start addition polymerisation.

Plasticiser

A small-molecule additive that pushes polymer chains apart and makes a plastic such as PVC soft and flexible.

Polyamide

A condensation polymer whose units are joined by amide links \ce{\bond{-}CO-NH\bond{-}}, for example nylon-6,6, Kevlar and proteins.

Polyester

A condensation polymer whose units are joined by ester links \ce{\bond{-}CO-O\bond{-}}, for example terylene.

Acyl chloride

A compound \ce{R-COCl}; it reacts with ammonia or amines at room temperature to form amides and \ce{HCl}.

Peptide link

The amide link \ce{\bond{-}CO-NH\bond{-}} between two amino acid units in a polypeptide or protein.

Nylon-6,6

A polyamide made from hexanedioic acid and hexane-1,6-diamine, each with six carbon atoms.

Kevlar

An aromatic polyamide made from benzene-1,4-dicarboxylic acid (or its dioyl chloride) and benzene-1,4-diamine; very strong, light and heat resistant.

Terylene (PET)

A polyester made from ethane-1,2-diol and benzene-1,4-dicarboxylic acid; used for bottles, clothing and carpets.

Polylactic acid (PLA)

A biodegradable polyester made from lactic acid (2-hydroxypropanoic acid).

Biodegradable polymer

A polymer that microorganisms can decompose, for example polyesters and polyamides.

Non-biodegradable polymer

A polymer that microorganisms cannot decompose, for example polythene, polypropene and PVC.

Photodegradable polymer

A polymer whose chains are broken by ultraviolet light, making it brittle.

Microplastics

Tiny fragments of plastic, formed when larger plastics break up, that enter water and food chains.

Hydrolysis

The splitting of a compound by reaction with water, usually catalysed by acid or alkali; it breaks ester and amide links.

Artificial organ

A device or tissue created to replace, replicate or augment a natural organ.

Biocompatible

Able to stay in the body without causing a harmful reaction or rejection.

Forward-oriented industry

An industry that uses chemicals to make finished goods, such as textiles, leather goods and automobiles.

Soda ash

Sodium carbonate, \ce{Na2CO3}, made from rock salt and limestone; used for glass, detergents and paper.

FBISE 2026 questions

Questions from the FBISE HSSC-II 2026 (1st Annual) papers that belong to this unit, answered the way an examiner expects.

Exam focus

1. Differentiate between addition and condensation polymerisation (polymers) with examples. (short)
   Asked three times in 2026: Set B Q.2 (x), Set D Q.2 (xi) and old-syllabus Q.4; also short question v and long questions iv and vii.
   Go to answer
2. Write the equation for the polymerisation of vinyl chloride to PVC and name the type of polymerisation. (short)
   2026 Set B Q.2 (ix) OR; short question i and long question i.
   Go to answer
3. Write the equations for the preparation of nylon-6,6 and Kevlar. (long)
   2026 Set D Q.4 OR (7 marks) and old-syllabus Q.4; learn both monomers of each polymer.
   Go to answer
4. Identify the monomer(s) from a polymer segment and classify the polymer. (mcq)
   2026 Set B MCQ 12 and Set D MCQ 13; long question viii and concept assessment exercises 13.1 to 13.3.
   Go to answer
5. Synthesis, properties and uses of PVC. (long)
   Long question i; PVC is named in the SLOs.
   Go to answer
6. Synthesis and applications of nylon-6,6. (long)
   Long question ii and short question viii; nylon is named in the SLOs.
   Go to answer
7. Reaction of an acyl chloride with ammonia or an amine to give an amide. (short)
   A separate SLO; short question ii and MCQ ii (\ce{HCl} is eliminated).
   Go to answer
8. Hydrolysis of polyesters and polyamides by acid and alkali; why poly(alkenes) are not biodegradable. (short)
   Short questions vii, x and xi; three SLOs on degradation.
   Go to answer
9. Environmental problems of non-biodegradable polymers and ways to reduce them. (long)
   Long questions v and x, short questions vi and xiii.
   Go to answer
10. Importance of chemical industries in Pakistan and the raw materials available. (long)
    Long question iii, short questions iii and iv, MCQ iii.
    Go to answer
11. Polymers in artificial organs: required properties and challenges. (long)
    Long question ix and short question ix.
    Go to answer