Polymers Revision Notes

A monomer is a small molecule that can chemically bond to other identical or different molecules to form a larger molecule. A polymer is a large molecule (macromolecule) made up of many repeating monomer units linked together in a chain or network.

Polymers have high relative molecular masses compared to monomers. The process of joining monomers to form polymers is called polymerisation.

• Monomer: small, simple molecule capable of bonding to form polymers.
• Polymer: large molecule made of many monomers linked together.
• Polymerisation: chemical process linking monomers into polymers.

Polymerisation involves repeated chemical reactions where monomers join together, often by breaking double bonds or reacting functional groups, to form long chains.

Each monomer adds to the growing polymer chain, creating a repeating pattern called the repeating unit. The polymer chain length can vary, affecting the polymer's properties.

For example, ethene (C2H4) monomers can polymerise by opening their double bonds to form poly(ethene), a long chain of –CH2–CH2– units.

• Polymerisation is a repeated chemical reaction linking monomers.
• Repeating units are the structural units that repeat along the polymer chain.
• Polymer chain length influences physical properties.

Monomers are represented by their molecular or structural formulae, showing their atoms and bonds.

Polymers are represented by repeating units enclosed in brackets with a subscript n indicating many repeats, for example, poly(ethene) is written as –[CH2–CH2]n–.

This notation emphasises that the polymer consists of many identical units derived from the monomer.

• Monomer formula shows individual molecule structure.
• Polymer formula shows repeating unit with subscript n.
• Example: Ethene monomer C2H4; poly(ethene) repeating unit –[CH2–CH2]n–.

The large size of polymers results in different physical properties compared to monomers, such as higher melting points, strength, and flexibility.

The chemical nature of the monomer and the arrangement of the polymer chain influence properties like hardness, elasticity, and chemical resistance.

Understanding the monomer structure helps predict polymer characteristics.

• Polymer size affects melting point and mechanical properties.
• Monomer type and polymer structure determine polymer behaviour.
• Polymers have properties distinct from their monomers.

Ethene (C2H4) is a simple alkene monomer with a carbon-carbon double bond.

During polymerisation, the double bond breaks, allowing each ethene molecule to link to another, forming a long chain.

The repeating unit in poly(ethene) is –CH2–CH2–, and the polymer formula is written as –[CH2–CH2]n–, where n is the number of repeating units.

• Ethene monomer: H2C=CH2
• Polymerisation breaks C=C double bond.
• Poly(ethene) repeating unit: –CH2–CH2–
• Polymer formula: –[CH2–CH2]n–

Ethene (C2H4) is an alkene with a carbon–carbon double bond (C=C). Each carbon atom is bonded to two hydrogen atoms and shares a double bond with the other carbon atom. The double bond consists of one sigma (σ) bond and one pi (π) bond, which is the site of reactivity in polymerisation.

The molecular formula of ethene is C2H4, and its structural formula is H2C=CH2. The double bond is reactive and can open to allow bonding with other molecules.

• Ethene has a carbon–carbon double bond (C=C).
• Each carbon is bonded to two hydrogens.
• The double bond is the reactive site for polymerisation.

Addition polymerisation involves the breaking of the carbon–carbon double bond in ethene monomers. The π bond breaks, allowing each carbon atom to form a new single bond with adjacent monomers, linking them into a long chain called poly(ethene).

This process does not produce any by-products because the monomers simply add together without elimination of small molecules.

The repeating unit in poly(ethene) is –CH2–CH2–, which is derived from the opened ethene monomer.

• The π bond in ethene breaks during polymerisation.
• Monomers link by forming new single bonds.
• No by-products are formed.
• The polymer chain consists of repeating –CH2–CH2– units.

Polymerisation of ethene requires specific conditions to initiate and propagate the reaction. These include elevated temperature and pressure, and often the presence of a catalyst or initiator such as a peroxide.

Typical industrial conditions are temperatures around 200°C and pressures up to 2000 atmospheres, although variations exist depending on the process (e.g., high-pressure or low-pressure polymerisation).

The catalyst or initiator generates free radicals that start the chain reaction by opening the double bond in ethene monomers.

• High temperature (e.g., ~200°C) and high pressure (up to 2000 atm).
• Use of catalysts or initiators (e.g., peroxides).
• Free radicals initiate the polymerisation chain reaction.

The polymerisation can be represented by the following equation, where n represents the number of monomer units:

n CH2=CH2 → –(CH2–CH2)–n (poly(ethene)) This shows that many ethene molecules add together to form the polymer.

The structural formula of the repeating unit in poly(ethene) is: –CH2–CH2– with single bonds only, indicating the double bonds have opened during polymerisation.

• Balanced equation: n CH2=CH2 → –(CH2–CH2)–n
• Repeating unit has single C–C bonds.
• No small molecules are lost during the reaction.

Poly(ethene) is a widely used plastic due to its flexibility, chemical resistance, and ease of processing.

It is used in packaging (plastic bags, films), containers, pipes, and household goods.

Understanding its formation helps explain its molecular structure and properties.

• Poly(ethene) is flexible and chemically resistant.
• Used in packaging, pipes, and containers.
• Formed by linking ethene monomers through addition polymerisation.

Polymers are very large molecules (macromolecules) formed by the joining of many smaller molecules called monomers. Each monomer is a small molecule that can chemically bond to identical or different monomers to form a long chain or network structure.

The process of linking monomers to form polymers is called polymerisation. This results in molecules with repeating structural units derived from the monomers.

Polymers can be natural (e.g., cellulose, proteins) or synthetic (man-made). Plastics are synthetic polymers designed for various uses.

• Polymer: large molecule made of repeating units.
• Monomer: small molecule that forms polymers.
• Polymerisation: chemical process joining monomers.

Plastics are materials made from synthetic polymers. These polymers consist of long chains of repeating monomer units connected by covalent bonds.

The polymer chains can be linear, branched, or cross-linked, affecting the plastic’s properties such as flexibility, strength, and melting point.

Common plastics include poly(ethene), poly(propene), and poly(vinyl chloride), each derived from their respective monomers ethene, propene, and vinyl chloride.

• Plastics are synthetic polymers.
• Polymer chains determine plastic properties.
• Examples: poly(ethene) from ethene monomers.

The physical and chemical properties of plastics depend on the nature of the polymer chains: their length, branching, and cross-linking.

Longer polymer chains generally increase strength and melting point due to more intermolecular forces.

Branching reduces the ability of chains to pack closely, lowering density and melting point, making plastics more flexible or less crystalline.

• Chain length affects strength and melting point.
• Branching affects density and flexibility.
• Cross-linking increases rigidity and thermal resistance.

Polymers are represented by repeating units enclosed in brackets with a subscript n indicating the number of repeating units.

For example, poly(ethene) is shown as [-CH2-CH2-]n, where the ethene monomer (CH2=CH2) polymerises by opening the double bond to form single bonds linking the units.

This notation emphasises the repeating nature of the polymer structure.

• Polymer formula: repeating unit in brackets with n.
• Poly(ethene) formula: [-CH2-CH2-]n.
• Monomer ethene: CH2=CH2.

Given the polymer formula [-CH2-CH2-]n, identify the plastic and its monomer.

The repeating unit [-CH2-CH2-] corresponds to poly(ethene), a common plastic.

The monomer is ethene, CH2=CH2, which polymerises by addition polymerisation to form poly(ethene).

• Polymer: poly(ethene) [-CH2-CH2-]n.
• Monomer: ethene, CH2=CH2.
• Plastic formed: poly(ethene), used in bags and containers.

Plastics are synthetic materials composed of long chains of repeating units called polymers. Their chemical structure, density, and biodegradability vary widely, affecting how they behave in the environment and how they can be disposed of safely and effectively.

Key characteristics include: chemical composition (e.g., addition or condensation polymers), physical properties (density, melting point), and biodegradability (ability to be broken down by microorganisms).

• Polymers are large molecules made from monomers.
• Plastics differ in chemical structure and physical properties.
• Biodegradability varies among plastics, influencing environmental persistence.

Biodegradable plastics can be broken down naturally by microorganisms into water, carbon dioxide (or methane in anaerobic conditions), and biomass. This process reduces long-term environmental pollution.

Non-biodegradable plastics resist microbial degradation and can persist in the environment for hundreds of years, leading to accumulation in landfills and natural habitats.

Biodegradable plastics are often disposed of via composting or controlled landfill environments that support microbial activity, whereas non-biodegradable plastics require alternative disposal methods.

• Biodegradable plastics decompose naturally, reducing pollution.
• Non-biodegradable plastics accumulate and persist in the environment.
• Disposal methods depend on biodegradability to minimise environmental harm.

Recycling involves collecting, sorting, cleaning, melting, and remoulding plastics to produce new products. The chemical structure of the plastic determines whether it can be recycled effectively.

Thermoplastics, such as poly(ethene) and poly(propene), soften on heating and can be remoulded multiple times, making them suitable for mechanical recycling.

Thermosetting plastics, which have cross-linked polymer chains, do not melt on heating and cannot be remoulded, limiting recycling options to chemical recycling or energy recovery by incineration under controlled conditions.

• Thermoplastics can be melted and remoulded for recycling.
• Thermosetting plastics cannot be remelted due to cross-linking.
• Chemical structure dictates recycling feasibility and method.

Density differences among plastics allow separation by flotation methods during recycling. For example, poly(ethene) has a density around 0.91–0.96 g/cm³, while poly(vinyl chloride) (PVC) is denser (~1.4 g/cm³).

Sorting plastics by density improves recycling efficiency by grouping similar materials, preventing contamination and ensuring consistent melting points and properties in recycled products.

Incorrect sorting can degrade recycled plastic quality and reduce the economic viability of recycling.

• Density differences enable plastic separation by flotation.
• Proper sorting is essential for effective recycling.
• Contamination reduces recycled plastic quality.

Landfill disposal leads to long-term environmental issues for non-biodegradable plastics, including soil and water pollution and wildlife harm.

Incineration reduces plastic volume and can generate energy but may release toxic gases such as dioxins and hydrogen chloride, especially from PVC, requiring gas scrubbing and pollution control.

Recycling conserves resources and reduces landfill use but requires energy and infrastructure; it is most effective when plastics are properly sorted and cleaned.

• Landfill disposal causes long-term pollution for non-biodegradable plastics.
• Incineration can release toxic gases; requires pollution control.
• Recycling conserves resources but depends on sorting and processing.

Plastics are synthetic polymers that are generally non-biodegradable, meaning they do not decompose easily by natural biological processes. When disposed of in landfills, plastics can remain intact for hundreds of years.

The slow degradation of plastics in landfills leads to long-term accumulation of waste, occupying large areas of land and potentially causing land degradation.

Some plastics may leach harmful additives or monomers into the soil and groundwater over time, contaminating ecosystems and possibly entering the food chain.

• Plastics are resistant to microbial attack and sunlight, slowing decomposition.
• Landfills accumulate plastic waste, reducing available land and harming soil quality.
• Leaching of toxic substances from plastics can pollute soil and groundwater.

Plastic waste often enters oceans through rivers, wind, or direct dumping. In marine environments, plastics break down into smaller pieces called microplastics but do not fully biodegrade.

Marine animals may ingest plastic debris or become entangled, causing injury or death. For example, sea turtles mistake plastic bags for jellyfish and birds ingest plastic fragments.

Plastic pollution disrupts marine ecosystems by affecting food chains and habitats, and microplastics can accumulate in organisms, potentially transferring toxins through the food web.

• Plastics fragment into microplastics but do not biodegrade in oceans.
• Ingestion and entanglement harm marine wildlife.
• Microplastics can bioaccumulate and disrupt marine ecosystems.

Incineration is sometimes used to reduce plastic waste volume by burning it at high temperatures. However, burning plastics releases toxic gases and pollutants into the atmosphere.

Common toxic gases emitted include hydrogen chloride (HCl) from PVC plastics, carbon monoxide (CO), dioxins, and other volatile organic compounds (VOCs). These gases contribute to air pollution and pose health risks such as respiratory problems and cancer.

The general combustion reaction for a hydrocarbon-based plastic (e.g., polyethene, C2H4)n is: (C2H4)n + 3n O2 → 2n CO2 + 2n H2O (balanced for complete combustion). Incomplete combustion produces CO and other toxic substances.

• Incineration reduces plastic volume but releases harmful gases.
• PVC produces hydrogen chloride gas, which is corrosive and toxic.
• Incomplete combustion emits carbon monoxide and dioxins, harmful to health and environment.

Complete combustion of polyethene (C2H4)n can be represented by the balanced equation: (C2H4)n + 3n O2 → 2n CO2 + 2n H2O

If oxygen supply is insufficient, incomplete combustion occurs, producing carbon monoxide (CO) and soot (carbon particles): (C2H4)n + (2.5)n O2 → 2n CO + 2n H2O

Burning PVC (polyvinyl chloride) releases hydrogen chloride gas: (C2H3Cl)n + (3.5)n O2 → 2n CO2 + n H2O + n HCl

• Balanced equations show products of plastic combustion.
• Incomplete combustion produces toxic carbon monoxide.
• PVC combustion releases corrosive hydrogen chloride gas.

Landfill accumulation of plastics leads to persistent environmental contamination and land use issues.

Ocean pollution by plastics threatens biodiversity and disrupts marine food chains.

Toxic gases from incineration contribute to air pollution, acid rain, and respiratory illnesses in humans and animals.

• Plastics persist in environment causing long-term harm.
• Marine ecosystems are endangered by plastic debris.
• Incineration gases cause air pollution and health risks.

Addition polymers form by the repeated addition of unsaturated monomer molecules, typically alkenes, containing carbon–carbon double bonds (C=C). During polymerisation, the double bonds open up, allowing monomers to link together in a long chain without the loss of any atoms.

The repeating unit in an addition polymer corresponds to the monomer structure with the double bond replaced by single bonds connecting to adjacent units.

For example, ethene (CH2=CH2) polymerises to form poly(ethene) with the repeating unit –CH2–CH2–. The polymer chain is represented as: –[CH2–CH2]n–, where n is the number of repeating units.

• Addition polymerisation involves breaking C=C double bonds.
• No atoms are lost; the polymer contains only atoms from the monomers.
• Repeating unit is the monomer structure minus the double bond, shown in square brackets with subscript n.

To identify the repeating unit in an addition polymer, locate the alkene monomer and remove the double bond, showing the single bonds linking the units.

For example, propene (CH2=CH–CH3) forms poly(propene) with repeating unit –CH2–CH(CH3)–.

Structural formulae clearly show the arrangement of atoms in the repeating unit, which is essential for understanding polymer properties.

• Repeating unit is derived by replacing the C=C bond with single bonds linking monomers.
• Use structural formulae to show side groups attached to the main chain.
• Example: poly(propene) repeating unit is –CH2–CH(CH3)–.

Condensation polymers form by a reaction between two different monomers, each with two reactive functional groups, resulting in the elimination of a small molecule such as water.

Common condensation polymers include polyamides and polyesters.

Polyamides form from a dicarboxylic acid (–COOH groups) and a diamine (–NH2 groups). The amide link (–CONH–) forms with the elimination of water: one –COOH and one –NH2 group react to form –CONH– plus H2O.

• Condensation polymerisation involves elimination of small molecules (e.g., H2O).
• Monomers have two reactive end groups allowing chain growth.
• Polyamides contain amide linkages (–CONH–).
• Polyesters contain ester linkages (–COO–).

The repeating unit in a condensation polymer includes the functional groups from both monomers minus the eliminated small molecule.

For example, nylon-6,6 is a polyamide formed from hexanedioic acid (HOOC–(CH2)4–COOH) and hexane-1,6-diamine (H2N–(CH2)6–NH2). The repeating unit contains the –CONH– linkage connecting the two monomer fragments.

Similarly, polyethylene terephthalate (PET) is a polyester formed from benzene-1,4-dicarboxylic acid and ethane-1,2-diol, with repeating units containing ester linkages (–COO–).

• Repeating unit shows both monomer parts connected by the condensation linkage.
• Small molecule (usually water) is not shown in the polymer structure.
• Example: nylon-6,6 repeating unit: –[OOC–(CH2)4–CO–NH–(CH2)6–NH]–.

Addition polymers form from one type of monomer with double bonds, without loss of atoms, resulting in simple repeating units.

Condensation polymers form from two different monomers with functional groups, with elimination of small molecules, resulting in more complex repeating units containing functional linkages.

Identifying the repeating unit requires recognising the monomer structures and the type of polymerisation involved.

• Addition polymerisation: one monomer, no atoms lost, simple repeating unit.
• Condensation polymerisation: two monomers, small molecule lost, repeating unit includes functional linkages.
• Structural formulae help distinguish polymer types and repeating units.

Addition polymerisation is a chemical process where many small alkene molecules (monomers) join together to form a very large molecule called a polymer. This occurs by opening the carbon–carbon double bonds (C=C) in the alkene monomers, allowing them to link in a long chain without producing any by-products.

Alkenes are hydrocarbons with the general formula CₙH₂ₙ and contain at least one carbon–carbon double bond. The double bond is reactive and is the site where polymerisation occurs.

The polymer formed is called an addition polymer because the monomers add together without the loss of any atoms or molecules. The polymer consists of repeating units derived from the original alkene monomer.

• Alkenes have a carbon–carbon double bond (C=C).
• Addition polymerisation involves breaking the double bond and linking monomers.
• No atoms are lost; the polymer is a chain of repeating units from the monomer.

The repeating unit in an addition polymer corresponds to the alkene monomer with the double bond replaced by single bonds to adjacent units in the chain.

For example, ethene (C₂H₄) polymerises to form poly(ethene). The double bond in ethene opens up, and many ethene units join to form a long chain. The repeating unit is –CH₂–CH₂–.

The repeating unit is usually shown in square brackets with a subscript 'n' to indicate many repeats: [–CH₂–CH₂–]ₙ. This notation emphasises the polymer's structure as a long chain of identical units.

• The double bond in the alkene becomes a single bond linking repeating units.
• Repeating units are shown in square brackets with subscript n.
• Example: Ethene monomer C₂H₄ → repeating unit –CH₂–CH₂–.

The general equation for addition polymerisation shows many alkene monomers joining to form a polymer. For ethene, the equation is:

n CH₂=CH₂ → –(CH₂–CH₂)–ₙ (poly(ethene)) where n is a large number.

This equation is balanced because the number of carbon and hydrogen atoms is the same on both sides; the double bonds are broken and replaced by single bonds linking the units in the polymer chain.

• Polymerisation involves many monomers (n) linking to form a polymer.
• No atoms are lost; the equation is balanced.
• Example: n CH₂=CH₂ → –(CH₂–CH₂)–ₙ.

To find the repeating unit from an alkene: identify the carbon atoms involved in the double bond and write the structure with the double bond replaced by single bonds linking to adjacent units.

For example, propene (CH₂=CH–CH₃) polymerises to form poly(propene). The repeating unit is –CH₂–CH(CH₃)–.

Structural formula: the double bond is removed, and the carbons are linked in a chain with the side group (CH₃) attached to the second carbon.

• Remove the double bond and link carbons with single bonds.
• Include any side groups attached to the carbons.
• Example: Propene monomer → repeating unit –CH₂–CH(CH₃)–.

To find the alkene monomer from a polymer's repeating unit, identify the repeating unit's carbon skeleton and side groups, then reintroduce the double bond between the two carbons that were originally double bonded.

For example, given the repeating unit –CH₂–CH(CH₃)–, the monomer is propene (CH₂=CH–CH₃).

This process involves recognising that the polymer chain is formed by opening the double bond and linking the monomers, so reversing this gives the original alkene.

• Identify the carbon chain and side groups in the repeating unit.
• Reintroduce the double bond between the two carbons formerly linked by a single bond.
• Example: Repeating unit –CH₂–CH(CH₃)– corresponds to propene monomer CH₂=CH–CH₃.

Condensation polymerisation is a process where monomers with two functional groups react to form long-chain polymers, with the elimination of small molecules such as water. Unlike addition polymerisation, condensation polymerisation involves the formation of covalent bonds between monomers with the loss of a small molecule.

Polyamides and polyesters are two important classes of condensation polymers. Polyamides are formed from dicarboxylic acids and diamines, while polyesters are formed from dicarboxylic acids and diols (compounds with two alcohol groups).

• Condensation polymerisation involves elimination of small molecules (usually water).
• Monomers have two reactive functional groups.
• Polyamides and polyesters are examples of condensation polymers.

Polyamides are formed by the reaction between a dicarboxylic acid (containing two –COOH groups) and a diamine (containing two –NH2 groups). Each carboxyl group reacts with an amine group to form an amide linkage (–CONH–) and eliminates a molecule of water.

The general reaction can be represented as: dicarboxylic acid + diamine → polyamide + water.

The repeating unit of the polyamide contains the amide bond linking the two monomer units. The polymer chain is formed by repeated amide linkages between the monomers.

• Amide linkage formed: –CONH–
• Water is eliminated during bond formation.
• Example: Nylon-6,6 is formed from hexanedioic acid and hexane-1,6-diamine.

Polyesters are formed by the reaction between a dicarboxylic acid and a diol (a molecule with two –OH groups). Each carboxyl group reacts with a hydroxyl group to form an ester linkage (–COO–) and eliminates a molecule of water.

The general reaction is: dicarboxylic acid + diol → polyester + water.

The repeating unit of the polyester contains the ester bond linking the monomers. The polymer chain consists of alternating units connected by ester linkages.

• Ester linkage formed: –COO–
• Water is eliminated during bond formation.
• Example: Polyethylene terephthalate (PET) is formed from benzene-1,4-dicarboxylic acid and ethane-1,2-diol.

To determine the repeating unit of a condensation polymer, identify the functional groups in the monomers and the bonds formed between them.

For polyamides, the amide bond forms between –COOH and –NH2 groups; for polyesters, the ester bond forms between –COOH and –OH groups.

Balanced equations for the polymerisation show the monomers reacting with elimination of water molecules. Each bond formed corresponds to one water molecule lost per repeating unit formed.

• Identify monomer functional groups.
• Draw structural formulae showing bond formation and water elimination.
• Write balanced equations including state symbols if relevant.
• Example: Formation of nylon-6,6 from hexanedioic acid and hexane-1,6-diamine:
• HOOC–(CH2)4–COOH + H2N–(CH2)6–NH2 → –[–OC–(CH2)4–CO–NH–(CH2)6–NH–]– + H2O

Hexanedioic acid (HOOC–(CH2)4–COOH) reacts with hexane-1,6-diamine (H2N–(CH2)6–NH2) to form nylon-6,6.

Each carboxyl group reacts with an amine group to form an amide bond (–CONH–) and eliminate water.

The repeating unit of nylon-6,6 is: –[–OC–(CH2)4–CO–NH–(CH2)6–NH–]–, showing the amide linkages between the monomers.

• Balanced equation:
• n HOOC–(CH2)4–COOH + n H2N–(CH2)6–NH2 → [–OC–(CH2)4–CO–NH–(CH2)6–NH–]n + 2n H2O
• Water molecules are eliminated during polymerisation.
• The polymer chain consists of repeating amide linkages.

Polymerisation is the chemical process by which small molecules called monomers join together to form large molecules known as polymers. Two main types of polymerisation are addition polymerisation and condensation polymerisation. Understanding the differences between these processes is essential for recognising how various polymers are formed and their chemical characteristics.

• Polymers are large molecules made from repeating units called monomers.
• Addition and condensation polymerisation are distinct mechanisms of polymer formation.

Addition polymerisation involves the joining of monomers that contain carbon–carbon double bonds (unsaturated monomers), typically alkenes. During the reaction, the double bonds open up and link together to form a long chain polymer. Importantly, no small molecules are lost or formed as by-products in this process.

The general mechanism is a chain reaction initiated by free radicals, heat, or catalysts, causing the monomers to add sequentially.

Example: Polymerisation of ethene to poly(ethene) (polyethylene).

• Monomers have C=C double bonds (e.g., ethene, CH2=CH2).
• No by-products are formed; the entire monomer becomes part of the polymer.
• The polymer consists of repeating units derived from the monomer.
• Reaction conditions often include heat or catalysts to initiate free radical formation.

The polymerisation of ethene can be represented as:

• n CH2=CH2 → –(CH2–CH2)–n

Condensation polymerisation involves monomers with two different functional groups, such as dicarboxylic acids and diols or diamines. When these monomers react, they form polymer chains by linking functional groups together, and a small molecule, often water, is released as a by-product.

This process typically involves stepwise reactions between functional groups, forming covalent bonds and eliminating small molecules.

Example: Formation of polyesters from a dicarboxylic acid and a diol, or polyamides from a dicarboxylic acid and a diamine.

• Monomers have two functional groups (e.g., –COOH, –OH, –NH2).
• Small molecules such as water (H2O) are released during polymer formation.
• Polymer chains form by condensation reactions between functional groups.
• Reaction conditions may include heat and catalysts to facilitate bond formation.

Consider the reaction between ethane-1,2-diol (HO–CH2–CH2–OH) and ethane-1,2-dioic acid (HOOC–CH2–COOH). Each time an –OH group from the diol reacts with a –COOH group from the acid, an ester linkage forms, and a water molecule is eliminated.

The repeating unit of the polyester can be represented as:

• –[O–CH2–CH2–O–CO–CH2–CO]–
• Balanced equation for one ester linkage formation:
• HO–CH2–CH2–OH + HOOC–CH2–COOH → –O–CH2–CH2–O–CO–CH2–CO– + H2O

The main distinctions between the two polymerisation types can be summarised as follows:

• Addition polymerisation involves unsaturated monomers and produces no by-products.
• Condensation polymerisation involves monomers with functional groups and releases small molecules such as water.
• Addition polymerisation proceeds via free radical or ionic mechanisms opening double bonds.
• Condensation polymerisation proceeds via stepwise reactions forming covalent bonds between functional groups.
• Polymers from addition polymerisation are typically polyalkenes; those from condensation polymerisation include polyesters and polyamides.

Nylon and PET are synthetic polymers widely used in textiles, packaging, and engineering materials. Nylon belongs to the polyamide family, characterised by amide linkages (-CONH-) between monomers. PET (polyethylene terephthalate) is a polyester, characterised by ester linkages (-COO-) connecting its repeating units. Both are formed by condensation polymerisation, where small molecules such as water are eliminated during polymer formation.

• Nylon is a polyamide formed from diamines and dicarboxylic acids.
• PET is a polyester formed from a diol and a dicarboxylic acid.
• Both polymers have repeating units linked by specific functional groups.
• Condensation polymerisation involves elimination of small molecules like water.

Nylon is synthesised by the condensation reaction between a diamine and a dicarboxylic acid. For example, nylon-6,6 is formed from hexane-1,6-diamine and hexanedioic acid (adipic acid). Each monomer has two reactive ends: the diamine has two -NH2 groups, and the dicarboxylic acid has two -COOH groups.

During polymerisation, an amide bond (-CONH-) forms between the -COOH group of the acid and the -NH2 group of the amine, releasing a molecule of water (H2O). This reaction repeats to form a long chain polymer with repeating amide linkages.

The repeating unit of nylon-6,6 can be represented as: [-NH-(CH2)6-NH-CO-(CH2)4-CO-]n, where n is the number of repeating units. The amide bonds provide strong hydrogen bonding between polymer chains, giving nylon high tensile strength and durability.

• Nylon-6,6 monomers: hexane-1,6-diamine and hexanedioic acid.
• Amide bond formation releases water: condensation polymerisation.
• Repeating unit contains -CONH- linkages.
• Strong hydrogen bonding between chains increases strength.

PET is formed by condensation polymerisation between ethane-1,2-diol (ethylene glycol) and benzene-1,4-dicarboxylic acid (terephthalic acid). Each monomer has two reactive groups: the diol has two -OH groups, and the dicarboxylic acid has two -COOH groups.

The polymer forms by ester linkages (-COO-) between the acid and alcohol groups, with water eliminated during each bond formation. The repeating unit contains an ester functional group connecting the aromatic ring of terephthalic acid and the ethylene glycol unit.

The repeating unit of PET can be represented as: [-O-CH2-CH2-O-CO-C6H4-CO-]n. The aromatic rings provide rigidity, while the ester linkages allow some flexibility and resistance to moisture.

• PET monomers: ethane-1,2-diol and benzene-1,4-dicarboxylic acid.
• Ester bond formation releases water: condensation polymerisation.
• Repeating unit contains -COO- ester linkages.
• Aromatic rings contribute to polymer rigidity and thermal stability.

The condensation polymerisation reactions for nylon-6,6 and PET can be summarised as follows:

For nylon-6,6: Hexane-1,6-diamine + hexanedioic acid → nylon-6,6 + water

For PET: Ethane-1,2-diol + benzene-1,4-dicarboxylic acid → PET + water

• Nylon-6,6 formation:
• HOOC-(CH2)4-COOH + H2N-(CH2)6-NH2 → [-NH-(CH2)6-NH-CO-(CH2)4-CO-]n + nH2O
• PET formation:
• HOOC-C6H4-COOH + HO-CH2-CH2-OH → [-O-CH2-CH2-O-CO-C6H4-CO-]n + nH2O
• Structural diagrams should show repeating units with amide or ester linkages clearly labelled.

The amide bonds in nylon allow strong hydrogen bonding between polymer chains, resulting in high tensile strength, abrasion resistance, and durability. Nylon fibres are used in textiles, ropes, and engineering plastics.

PET’s ester linkages and aromatic rings give it good strength, chemical resistance, and moisture barrier properties. PET is widely used in plastic bottles, food packaging, and synthetic fibres.

Understanding the chemical structure helps explain why nylon is tougher and more heat resistant, while PET is more flexible and resistant to moisture.

• Nylon’s amide bonds enable strong intermolecular hydrogen bonding.
• PET’s ester bonds and aromatic rings provide strength and moisture resistance.
• Nylon is used in textiles and engineering plastics.
• PET is used in packaging and synthetic fibres.

Polyethylene terephthalate (PET) is a condensation polymer formed from two monomers: terephthalic acid (a dicarboxylic acid) and ethane-1,2-diol (also called ethylene glycol, a diol).

The repeating unit of PET contains ester linkages formed by the reaction between the carboxyl groups (-COOH) of terephthalic acid and the hydroxyl groups (-OH) of ethane-1,2-diol.

The structural formula of the repeating unit can be represented as: -[O-CH2-CH2-O-CO-C6H4-CO]- where C6H4 represents the benzene ring from terephthalic acid.

• Monomers: terephthalic acid (HOOC-C6H4-COOH) and ethane-1,2-diol (HO-CH2-CH2-OH).
• Polymer formed by condensation polymerisation with ester linkages.
• Repeating unit contains ester functional groups (-COO-).

PET can be chemically broken down into its original monomers by hydrolysis of the ester bonds. This process is called depolymerisation.

Hydrolysis can be carried out under acidic or alkaline conditions, often requiring heat and sometimes catalysts to speed the reaction.

In acidic hydrolysis, PET reacts with water in the presence of dilute acid (e.g., dilute sulfuric acid) to yield terephthalic acid and ethane-1,2-diol: the ester bonds are cleaved, regenerating the monomers.

• Acidic hydrolysis: PET + water + acid catalyst → terephthalic acid + ethane-1,2-diol.
• Alkaline hydrolysis (saponification) can also be used, producing the salt of terephthalic acid and ethane-1,2-diol.
• Heat is required to break the strong ester bonds in PET.

The recovered monomers, terephthalic acid and ethane-1,2-diol, can be purified and then re-polymerised by condensation polymerisation to form PET again.

During re-polymerisation, the hydroxyl groups of ethane-1,2-diol react with the carboxyl groups of terephthalic acid, eliminating water molecules and forming ester bonds.

This process regenerates PET with properties similar to the original polymer, allowing the production of new plastic products from recycled material.

• Re-polymerisation involves condensation reaction forming ester linkages.
• Water is eliminated during polymer formation.
• Recycled PET retains the same chemical structure and properties as virgin PET.

Recycling PET by depolymerisation and re-polymerisation reduces the need for raw materials derived from fossil fuels, conserving resources.

It decreases plastic waste accumulation in landfills and the environment, mitigating pollution problems.

Chemical recycling allows PET to be recycled multiple times without significant loss of quality, unlike mechanical recycling which can degrade polymer properties over time.

• Conserves raw materials and reduces fossil fuel use.
• Reduces plastic waste and environmental pollution.
• Maintains polymer quality through chemical recycling.
• Supports sustainable plastic use and waste management.

Amino acids are the monomer units that make up proteins. Each amino acid has a central carbon atom (called the α-carbon) bonded to four different groups: an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom (H), and a variable side chain denoted as R. The R group differs among amino acids and determines their chemical properties and behaviour.

The general structural formula of an amino acid can be represented as:

H₂N–CH(R)–COOH or more explicitly as: R–CH(NH₂)–COOH. In skeletal form, the central carbon is bonded to R, NH₂, COOH, and H. This structure allows amino acids to act as both acids and bases (amphoteric).

• Central carbon atom (α-carbon) bonded to four groups: amino (-NH₂), carboxyl (-COOH), hydrogen (H), and side chain (R).
• R group varies among amino acids, influencing polarity, charge, and reactivity.
• Amino acids exist in zwitterion form at physiological pH, with NH₃⁺ and COO⁻ groups.

Proteins are polymers formed by linking amino acid monomers through peptide bonds. A peptide bond is a type of amide bond formed by a condensation reaction between the amino group of one amino acid and the carboxyl group of another, releasing a molecule of water.

The reaction can be represented as: NH₂–CHR–COOH + NH₂–CHR'–COOH → NH₂–CHR–CONH–CHR'–COOH + H₂O

In structural terms, the peptide bond is –CONH– linking the carbonyl carbon of one amino acid to the nitrogen of the next. Because the bond is an amide linkage, proteins are classified as natural polyamides. This polymerisation can continue to form polypeptides and ultimately functional proteins with specific sequences and three-dimensional structures.

• Peptide bond formation is a condensation reaction releasing water.
• Peptide bond: –CONH– linkage between amino acids.
• Proteins are natural polyamides due to amide linkages in their backbone.
• Sequence and nature of R groups determine protein structure and function.

The side chain (R group) attached to the α-carbon varies among the 20 common amino acids found in proteins. These side chains differ in size, polarity, charge, and chemical reactivity, which influence the folding, stability, and function of the protein.

For example, some R groups are nonpolar (e.g., –CH₃ in alanine), polar uncharged (e.g., –OH in serine), positively charged (e.g., –NH₃⁺ in lysine), or negatively charged (e.g., –COO⁻ in aspartic acid).

The chemical nature of the R groups affects intermolecular interactions such as hydrogen bonding, ionic bonding, and hydrophobic effects, which are critical for the protein's three-dimensional conformation and biological activity.

• R group determines amino acid properties: polarity, charge, size.
• Diverse R groups enable proteins to have varied structures and functions.
• Interactions between R groups stabilise protein folding and activity.

The general structure of amino acids can be summarised as:

R–CH(NH₂)–COOH, where R is the side chain. When amino acids polymerise, the repeating unit in the protein backbone is:

–NH–CHR–CO–, representing the amide (peptide) linkage between monomers. This repeating unit classifies proteins as polyamides.

• Amino acid monomer: R–CH(NH₂)–COOH
• Peptide bond formation: –CO–NH– linkage
• Proteins are natural polyamides formed by amino acid monomers linked via peptide bonds.

Amino acids are the monomers that make up proteins. Each amino acid contains a central carbon atom (called the alpha carbon) bonded to four different groups: an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom (H), and a variable side chain (R group) that differs among amino acids.

The general structural formula of an amino acid can be represented as:

H2N–CH(R)–COOH or in a more detailed form showing atoms: H O H O N C N C N C, where the central carbon is bonded to the amino group (N), the carboxyl group (C=O and OH), a hydrogen atom, and the R group.

• Central carbon atom (alpha carbon) bonded to four groups: amino (-NH2), carboxyl (-COOH), hydrogen (H), and side chain (R).
• Side chain (R) varies and determines the identity and properties of each amino acid.
• Contains elements carbon (C), hydrogen (H), oxygen (O), and nitrogen (N).

Proteins are formed when amino acids join together by peptide bonds through a condensation reaction. The carboxyl group (-COOH) of one amino acid reacts with the amino group (-NH2) of another, releasing a molecule of water (H2O) and forming a covalent bond called a peptide bond (-CONH-).

This linkage creates a chain of amino acids called a polypeptide, which folds into a functional protein.

The general reaction is: amino acid 1 + amino acid 2 → dipeptide + water (condensation).

• Peptide bond formation involves the carboxyl group of one amino acid and the amino group of another.
• Water is eliminated during the formation of each peptide bond.
• Proteins are natural polyamides because peptide bonds are amide linkages between amino acids.

The repeating unit in proteins can be represented as:

–NH–CH(R)–CO–, where the peptide bond links the carbonyl carbon (C=O) of one amino acid to the nitrogen (N–H) of the next.

This repeating unit contains the amide linkage characteristic of polyamides. The side chain (R) extends from the central carbon and varies between amino acids, influencing protein structure and function.

• Peptide bond is an amide bond linking amino acids.
• Proteins are polymers with repeating units of –NH–CH(R)–CO–.
• Side chains (R groups) determine protein diversity and properties.

Proteins contain the elements carbon (C), hydrogen (H), oxygen (O), and nitrogen (N). These elements are arranged in the amino acid monomers and the peptide bonds linking them.

Unlike many other organic compounds, proteins always contain nitrogen due to the amino groups in amino acids.

The presence of nitrogen distinguishes proteins from carbohydrates and lipids, which generally do not contain nitrogen.

• Proteins contain C, H, O, and N.
• Nitrogen is present due to amino groups in amino acids.
• Elemental composition is essential for protein identification.

The side chain (R group) attached to the central carbon of each amino acid varies in size, polarity, and charge. This variation affects the chemical properties of the amino acid and influences how the protein folds and functions.

Different R groups can be non-polar, polar, acidic, or basic, contributing to the protein's three-dimensional structure and biological activity.

Although the general backbone of amino acids is the same, the diversity of R groups allows proteins to perform a wide range of functions in living organisms.

• R group varies among amino acids and determines their properties.
• Side chains influence protein folding and function.
• Diversity of R groups leads to diverse protein structures.