Addition Polymers (HSC SSCE Chemistry): Revision Notes

Addition Polymers

What are addition polymers?

Addition polymers are large molecules formed by joining together many small monomer units without the loss of any atoms. This process is called addition polymerisation, which is a type of addition reaction where carbon-carbon double bonds are broken to allow monomers to link together.

For addition polymerisation to occur, the monomer must contain a double bond between two carbon atoms (C=C). When this double bond breaks, each carbon atom has an available electron to form new single bonds with adjacent monomers. The resulting polymer chains can contain anywhere from $100$ to over $100,000$ monomer units.

General mechanism of addition polymerisation

The general process shows how alkene monomers join together. Each coloured substituent group (shown as circles in the diagram above) can represent hydrogen atoms or different functional groups. The double bond opens up, and new single bonds form between adjacent monomers to create a long chain.

The general equation for addition polymerisation can be written as:

$n(\text{CH}_2 = \text{CH-X}) \rightarrow [\text{—CH}_2\text{—CH(X)—}]_n$

where $X$ represents hydrogen or a substituent group (such as chlorine, a benzene ring, or an alkyl group), and $n$ is the number of repeating units.

Polyethylene: polymers from ethylene

Polyethylene (also called polyethene) is one of the most common and important addition polymers. It forms when ethylene (ethene) monomers join together through addition polymerisation.

Formation of polyethylene

The polymerisation reaction can be represented as:

$n(\text{CH}_2 = \text{CH}_2) \rightarrow \text{—CH}_2\text{—CH}_2\text{—CH}_2\text{—CH}_2\text{—CH}_2\text{—CH}_2\text{—}$

During polymerisation:

1. One bond in each C=C double bond breaks under the influence of a catalyst
2. This occurs at high temperature and pressure
3. Each carbon atom now has one unpaired electron available for bonding
4. New covalent bonds form between carbon atoms on adjacent ethylene molecules
5. This creates long chains of repeating —CH₂—CH₂— units

The polymer structure is often written in abbreviated form:

where $n$ represents the number of monomer units in the polymer chain.

Two types of polyethylene: LDPE and HDPE

There are two main types of polyethylene with different properties due to differences in their molecular structure: low-density polyethylene (LDPE) and high-density polyethylene (HDPE).

Low-density polyethylene (LDPE)

Production conditions:

• Gas phase process
• High temperature: $300°\text{C}$
• Very high pressure: $1000$–$3000$ atmospheres
• Requires an initiator to start the reaction

Structure:

The significant chain branching distinguishes LDPE from HDPE. Some hydrogen atoms are replaced by alkyl groups, and these branches prevent polymer chains from packing closely together. This creates much empty space between chains, resulting in an amorphous (disordered) structure with weaker dispersion forces between chains due to greater separation.

Properties:

• Low density ($0.92\text{ g mL}^{-1}$)
• Low melting point ($80°\text{C}$)
• Soft and flexible
• Transparent
• Low tensile strength ($15\text{ MPa}$)
• $40$–$55\%$ crystalline regions
• Impermeable to water vapour
• Unreactive towards acids and bases

Common uses:

• Plastic bags and shopping bags
• Plastic food wraps and cling film
• Soft toys
• Wire and cable insulation
• Lamination film for juice and milk cartons

High-density polyethylene (HDPE)

Production conditions:

• Ziegler-Natta process
• Lower temperature: $60°\text{C}$
• Much lower pressure: only a few atmospheres
• Uses a catalyst mixture (titanium(III) chloride and trialkylaluminium compound)

Structure:

HDPE consists of unbranched (linear) polymer chains that pack closely together in an orderly arrangement. This creates less empty space and a crystalline (ordered) structure with stronger dispersion forces between chains due to closer packing.

Properties:

• Higher density ($0.96\text{ g mL}^{-1}$)
• Higher melting point ($135°\text{C}$)
• Denser and tougher
• More rigid and less flexible
• Opaque (not transparent)
• Greater tensile strength ($29\text{ MPa}$)
• $70$–$80\%$ crystalline regions
• Impermeable to water vapour
• Unreactive towards acids and bases

Common uses:

• Detergent bottles and containers
• Milk and water jugs
• Fuel tanks for vehicles
• Bottle caps
• Food storage containers
• Kitchen utensils
• 3D printer filament
• Medical equipment and plastic surgery applications

Comparing LDPE and HDPE properties

Property	LDPE	HDPE
Melting point	$80°\text{C}$	$135°\text{C}$
Density	$0.92\text{ g mL}^{-1}$	$0.96\text{ g mL}^{-1}$
Tensile strength	$15\text{ MPa}$	$29\text{ MPa}$
Crystalline regions	$40$–$55\%$	$70$–$80\%$
Structure	Branched, amorphous	Linear, crystalline
Bonding	Weak dispersion forces	Strong dispersion forces
Flexibility	Soft, flexible	Rigid, tough

Understanding crystalline and amorphous structures

Polymer chains can be arranged in two different ways that fundamentally affect their properties:

Crystalline regions:

Polymer chains packed closely together in regular, ordered patterns create stronger intermolecular forces between adjacent chains. The chains cannot move easily past each other, resulting in stronger, more rigid, less flexible material.

Amorphous regions:

Polymer chains arranged randomly with greater separation between chains have weaker intermolecular forces. The chains can move more easily past each other, resulting in softer, more flexible material.

Other addition polymers

Many different addition polymers can be made by substituting hydrogen atoms in ethylene with different functional groups. Ethylene can be converted into various useful monomers, which then undergo addition polymerisation.

Naming polymers

Polymers are named by adding the prefix 'poly' before the monomer name:

• Ethylene → polyethylene
• Styrene → polystyrene
• Propylene → polypropylene

Polytetrafluoroethylene (PTFE or Teflon)

Monomer: Tetrafluoroethylene (CF₂=CF₂)

In this monomer, all hydrogen atoms have been replaced by fluorine atoms.

Structure:

The polymer contains repeating —CF₂—CF₂— units with all hydrogen atoms replaced by fluorine.

Manufacturing:

• Not made by simple substitution
• Complex process involving chloroform, fluorospar, and sulfuric acid
• Some steps require temperatures around $700°\text{C}$

Properties:

PTFE exhibits remarkable properties due to its unique structure:

• Very strong and tough (due to polar C—F bonds and dipole-dipole interactions)
• Non-stick surface (has third-lowest coefficient of friction of any solid)
• Resistant to dispersion forces
• Chemically inert
• Excellent electrical insulator

Uses:

• Non-stick coating on cookware and frying pans
• High-grade electrical insulation for aerospace and computer wiring ($50\%$ of production)
• Pipe thread sealant
• Stain-resistant treatment for carpets and fabrics
• Artificial body parts in medical applications

Poly(vinyl chloride) (PVC)

Monomer: Vinyl chloride (chloroethene): CH₂=CH—Cl

Polymerisation equation:

$n(\text{CH}_2 = \text{CH—Cl}) \rightarrow [\text{—CH}_2\text{—CH(Cl)—}]_n$

Structure:

• Chlorine atoms orientated randomly along the polymer chain
• Mainly amorphous structure (like LDPE)
• Chlorine atoms stick out from the chain
• Large size of chlorine prevents close packing

Bonding:

Chlorine is highly electronegative, which creates polar C—Cl bonds (Cl slightly negative, C slightly positive). Dipole-dipole interactions between chains add to dispersion forces, resulting in stronger intermolecular forces than polyethylene.

Properties:

• Hard and rigid (despite being amorphous)
• Strong due to dipole-dipole interactions
• Can be made flexible by adding plasticisers
• Cheapest and most widely used polymer after polyethylene

Plasticisers:

Plasticisers are small molecules inserted between polymer chains that:

• Force chains further apart
• Weaken intermolecular forces between chains
• Make the polymer more flexible
• The more plasticiser added, the more flexible the polymer becomes

Uses:

• Electrical insulation for wiring
• Garden hoses
• Drainage and sewerage pipes
• Household guttering and downpipes
• Window frames

Polystyrene (PS)

Monomer: Styrene (ethenylbenzene): C₆H₅—CH=CH₂

This monomer has a phenyl group (benzene ring) substituted for one hydrogen atom in ethylene.

Polymerisation:

Structure:

• Phenyl (benzene) rings orientated randomly along the chain
• Mainly amorphous structure
• Large benzene rings stick out from the chain
• Prevents close packing of chains

Bonding:

The phenyl ring is symmetrical (no polar bonds), so there are only weak dispersion forces between chains. However, the large ring groups restrict chain movement.

Properties:

• Clear and transparent
• Hard and brittle
• Readily softened and moulded when heated
• Becomes rigid when cooled
• Can be expanded into foam

Uses:

• Car battery cases
• Tool handles
• Modern furniture
• Disposable drink cups (both foam and clear)
• Foam packing material
• Protective packaging

Polypropylene (PP)

Monomer: Propene (propylene): CH₃—CH=CH₂

Tacticity:

Polypropylene can exist in three different forms depending on the spatial arrangement of the —CH₃ (methyl) groups along the polymer chain. This property is called tacticity.

1. Atactic polypropylene:

In this form, methyl groups are randomly placed on either side of the chain. The chains cannot pack closely together, resulting in weaker intermolecular forces. This produces a soft rubbery polymer with lower melting point that is considered a waste product with limited uses: mainly roofing materials and sealants.

2. Syndiotactic polypropylene:

Here, methyl groups alternate regularly above and below the chain. This regular arrangement allows fairly close packing and intermediate intermolecular forces. The polymer is somewhat softer than isotactic form but still tough, with a clear appearance and stability to gamma radiation. Uses include packaging, medical tubing, bags and pouches.

3. Isotactic polypropylene:

In this most valuable form, all methyl groups are on the same side of the chain. The most regular arrangement allows closest packing and maximum dispersion forces between chains. The chains curl into regular spirals (not zigzag like polyethylene), producing the strongest and hardest form. This is the most commonly manufactured form with excellent resistance to stress, cracking and chemical reactions. Uses include crates, ropes, car bumpers, household goods, moulded chairs, and carpets.

Common addition polymers summary

The table below summarises the most important addition polymers, their monomers, and common uses:

Polymer	Monomer name and structure	Common uses
Polyethylene	Ethene (ethylene): CH₂=CH₂	LDPE: Plastic bags, soft toys, plastic wrap HDPE: Kitchen utensils, containers, rigid toys, rubbish bins
Polypropylene (PP)	Propene (propylene): CH₂=CH—CH₃	Car bumpers, rope and twine, household goods, moulded chairs, carpets
Poly(vinyl chloride) (PVC)	Vinyl chloride (chloroethene): CH₂=CH—Cl	Electrical insulation, garden hoses, drainage pipes, guttering
Polytetrafluoroethylene (PTFE/Teflon)	Tetrafluoroethene: F₂C=CF₂	Non-stick cookware, high-grade electrical insulation, pipe sealant
Polystyrene (PS)	Styrene (phenylethene): C₆H₅—CH=CH₂	Battery cases, tool handles, furniture, disposable cups, foam packaging
Polyacrylonitrile (acrylics)	Acrylonitrile (cyanoethene): CH₂=CH—C≡N	Wool substitute in clothing, blankets, soft furnishings, carpet
Poly(vinyl acetate) (PVA)	Vinyl acetate (ethenyl ethanoate): CH₂=CH—O—CO—CH₃	Vinyl coatings on fabrics, paint, adhesives

Copolymers

A copolymer is a polymer made from two or more different monomers. Mixing different monomers during polymerisation can create polymers with new and useful properties that neither monomer would produce alone.