Polymer Formation (VCE SSCE Chemistry): Revision Notes

Polymer Formation

Introduction to polymers

Polymers are extremely large molecules made up of thousands of smaller units called monomers. The word 'polymer' comes from Greek: 'poly' meaning 'many' and 'mer' meaning 'part'. These molecules are formed when monomers join together through a process called polymerisation.

You'll find polymers everywhere in daily life - from plastic bottles and food containers to clothing and sporting equipment. Polymers are widely used in manufacturing because they are cheap to produce, versatile, and easy to work with.

Understanding the term 'plastic'

It's important to understand the difference between 'polymer' and 'plastic'. In chemistry, 'plastic' describes a property of a material, not the material itself. A material is described as plastic if it can be moulded into different shapes readily when heated.

For example, a plastic basket has plastic properties because the polymer material can be heated and reshaped. However, a polymer saucepan handle is hard and brittle - it will not melt and cannot be reshaped when heated, so it doesn't have plastic properties even though it's made from a polymer.

Polymer structure

Polymers are covalent molecular substances formed by joining many small molecules together. These small molecules are called monomers (where 'mono' means 'one'). Through polymerisation, thousands of monomer molecules bond together to create one very long polymer chain.

Addition polymerisation

What is addition polymerisation?

Addition polymerisation occurs when unsaturated molecules (containing carbon-carbon double bonds) react with themselves to form long chains. During this reaction:

• The $\text{C=C}$ double bond in each monomer breaks
• New single $\text{C-C}$ bonds form between neighbouring monomers
• All atoms from the monomers are present in the final polymer
• No other products are formed

Ethene to polyethene

The most common example of addition polymerisation is the conversion of ethene ($\text{C}_2\text{H}_4$) into polyethene. Several thousand ethene monomers typically react together to form one polyethene molecule.

In the polymerisation reaction:

1. The $\text{C=C}$ double bonds in ethene break open
2. Carbon atoms from neighbouring ethene molecules form new single $\text{C-C}$ bonds
3. A long chain of carbon atoms with hydrogen atoms attached is created

Representing polymer structures

Because polymer chains are so long, chemists use a simplified notation. Large square brackets with a subscript $n$ represent the repeating unit of the polymer. The value of $n$ indicates how many times the unit repeats - typically around $20\,000$ for a polyethene chain.

Both representations show the same molecule, but the notation on the right is much simpler to draw and understand.

This 3D model shows how the carbon atoms form a zigzag chain with hydrogen atoms bonded to each carbon.

Important characteristics of addition polymers

When ethene polymerises:

• The original unsaturated molecule (containing $\text{C=C}$) becomes a saturated polymer chain (containing only $\text{C-C}$ single bonds)
• The empirical formula of the monomer is the same as the polymer
• The name of the polymer often includes the monomer name (ethene → polyethene)

Common examples of monomer-polymer pairs:

Monomer	Polymer
Ethene	Polyethene
Propene	Polypropene
Tetrafluoroethene	Polytetrafluoroethene

Why are polymers solid at room temperature?

Polyethene is essentially an extremely long alkane molecule. Even though individual alkane molecules are gases or liquids at room temperature, polymers are solids. This is because of dispersion forces (also called London forces) between the long polymer chains.

The dispersion forces between these very long molecules are strong enough to hold them together as a solid at room temperature. The longer the chain, the stronger these intermolecular forces become.

General properties of polymers

Most polymers share these common properties:

• Lightweight compared to metals
• Non-conductors of electricity
• Durable and long-lasting
• Versatile - can be made with different properties
• Resistant to acids
• Flammable

Low-density polyethene (LDPE)

Formation and structure

LDPE was the earliest form of polyethene to be discovered (accidentally in 1933). It's produced using:

• High temperatures (around $300°\text{C}$)
• Extremely high pressures

Under these harsh conditions, the polymer forms very rapidly. This doesn't give the molecules time to arrange neatly, resulting in a structure with many branches - short side chains extending off the main polymer backbone.

Why branching matters

The presence of branches significantly affects the polymer's properties. Branched chains cannot pack closely together, leaving more space between molecules. This means:

• Dispersion forces between molecules are weaker (molecules are further apart)
• The arrangement is described as amorphous or non-crystalline (disordered)
• The material has relatively low density

Properties of LDPE

The branched structure gives LDPE these characteristics:

• Low density
• Relatively soft and flexible
• Low melting point
• Non-crystalline structure
• Opaque appearance
• Non-conductor of electricity

Common uses: Plastic bags, cling film, squeeze bottles, flexible containers

High-density polyethene (HDPE)

Formation and structure

HDPE was developed later (late 1960s) using a different production method:

• Low pressure
• Lower temperature
• Specialised Ziegler-Natta catalysts (transition metal catalysts)

These milder conditions allow the polymer to form more slowly and systematically, resulting in very few branches.

Why unbranched structure matters

Without branches, HDPE molecules can pack together much more tightly. This leads to:

• Stronger dispersion forces (molecules are closer together)
• Higher density
• More ordered arrangement with crystalline sections
• Harder, more rigid material

Properties of HDPE

The unbranched structure gives HDPE these characteristics:

• High density
• Hard and rigid
• Higher melting point than LDPE
• Crystalline sections in structure
• Opaque appearance
• Non-conductor of electricity

Common uses: Milk bottles, detergent containers, pipes, harder plastic products

Comparing LDPE and HDPE

Feature	HDPE	LDPE
Production	Catalysts control polymerisation at low pressure	High temperature and pressure, uncontrolled
Structure	Long molecules, few branches	Many branches
Packing	Molecules pack tightly	Molecules cannot pack tightly
Forces	Stronger dispersion forces	Weaker dispersion forces
Strength	Higher tensile strength	Lower tensile strength
Flexibility	Can be rigid or flexible	Flexible
Appearance	Opaque	Transparent
Density	High	Low

Crystalline vs amorphous polymers

In some polymers, the entire solid is amorphous (disordered). Amorphous polymers are generally:

• Less rigid and weaker
• Often transparent (see-through)
• More flexible

HDPE contains crystalline sections (ordered regions) while LDPE is relatively amorphous. This structural difference explains their different properties.

Other addition polymers

Addition polymerisation can occur with any unsaturated monomer (containing $\text{C=C}$ double bonds). Here are two more important examples:

Bromoethene to polybromoethene

Bromoethene polymerises to form polybromoethene, a speciality polymer used when flame-resistant properties are needed.

Notice that:

• The $\text{C=C}$ double bond breaks
• Bromine atoms alternate with hydrogen atoms along the chain
• All atoms are retained in the polymer (no byproducts)

Propene to polypropylene

Polypropylene (PP) is one of the few polymers manufactured in Australia. It has many uses including synthetic sports fields, microwave containers, and rope.

The methyl group ($\text{CH}_3$) attached to alternate carbon atoms affects the properties of the polymer compared to polyethene.

Condensation polymerisation

What is condensation polymerisation?

Condensation polymerisation is a different method of forming polymers. Instead of using monomers with double bonds, it uses monomers with functional groups at each end of the molecule. Key features include:

• Functional groups at each end of monomers react with each other
• An ester link typically forms between monomers
• A small molecule (usually water) is released as a byproduct
• This explains the name 'condensation' - water condenses out

The mechanism

The diagram shows:

1. Monomer 1 has carboxyl groups ($\text{-COOH}$) at each end
2. Monomer 2 has hydroxyl groups ($\text{-OH}$) at each end
3. When these functional groups meet, they react to form an ester linkage
4. Water ($\text{H}_2\text{O}$) is released
5. The process repeats, building a long polymer chain

The key reaction is:

Carboxyl group + Hydroxyl group → Ester group + Water

Polyethylene terephthalate (PET)

PET is one of the world's most-used condensation polymers. You'll recognise it as polyester fabric or plastic water bottles (recycling code 1).

The formation involves:

• 1,4-benzenedicarboxylic acid (also called terephthalic acid) - has two carboxyl groups
• Ethane-1,2-diol (also called ethylene glycol) - has two hydroxyl groups
• These react repeatedly to form long chains linked by ester groups
• Water molecules are released at each linkage

Polylactic acid (PLA)

Polylactic acid demonstrates that condensation polymers don't always need two different monomers. PLA forms from just one monomer - lactic acid - which has different functional groups at each end.

The polymerisation proceeds as follows:

1. One end of lactic acid has a carboxyl group ($\text{-COOH}$)
2. The other end has a hydroxyl group ($\text{-OH}$)
3. These groups react between neighbouring molecules
4. Ester linkages form, releasing water
5. The process continues, building the polymer chain

PLA is becoming increasingly popular because it's biodegradable - it breaks down naturally in the environment much faster than traditional plastics.

Comparing addition and condensation polymerisation

Feature	Addition polymerisation	Condensation polymerisation
Monomer requirement	Contains $\text{C=C}$ double bond	Functional groups at each end
Bond change	Double bond breaks	Functional groups react
Polymer structure	Only contains $\text{C-C}$ single bonds	Contains atoms other than carbon (e.g., oxygen in ester links)
Byproducts	No byproducts formed	Small molecules released (usually water)
Example structure	$\text{-(CH}_2\text{-CH}_2\text{)}_n\text{-}$	Complex structure with ester/amide links

Natural polymers

While humans build factories to manufacture synthetic polymers, living organisms have been creating natural polymers for millions of years. Many biological molecules are polymers formed through condensation polymerisation within the organism itself.

Examples of natural polymers

Important natural polymers include:

• Proteins - made from amino acid monomers
• Cellulose - found in plant cell walls
• Starch - energy storage in plants
• Silk - protein fibres produced by insects
• Chitin - found in fungal cell walls and exoskeletons

Chitin

Chitin is a particularly interesting natural polymer. It's found in:

• Cell walls of fungi
• Exoskeletons of crustaceans (like crabs and lobsters)
• Exoskeletons of insects

The chemical structure shows that chitin is built from repeating monomer units. The condensation polymerisation reaction occurs naturally within the organism, allowing it to build its protective structures.

All these natural polymers are formed through condensation reactions, demonstrating that this type of polymerisation is fundamental to life itself.

History of polymers

Natural polymers like wool, cellulose, and proteins have existed for millions of years. However, synthetic polymers have only been widely used for about 100 years.

Early developments

The story of synthetic polymers began with rubber. Native Americans were already playing with crude rubber balls when European settlers arrived in North America - these balls were made from rubber tree sap.

The first completely synthetic polymer was Bakelite, created by Belgian-American chemist Leo Baekeland in 1909. He reacted phenol with formaldehyde to form a hard material. Bakelite is still used today for bowling balls and saucepan handles.

Timeline of significant polymer developments

Year	Polymer	Significance
1869	Celluloid (cellulose nitrate)	First commercial plastic; used for billiard balls, photographic film, and table-tennis balls
1907	Bakelite (phenol formaldehyde)	Used for light switches and saucepan handles
1927	Nylon	Created a shopping frenzy when used to make stockings in 1939
1927	PVC (polyvinyl chloride)	Low flammability and low electrical conductivity
1933	Perspex (polymethyl methacrylate)	Transparency enabled it to replace glass during World War II
1937	Polyurethane	Invented in Germany by Professor Otto Bayer; first used to replace rubber
1938	Teflon (polytetrafluoroethene)	Extremely difficult to handle due to its lack of 'stickiness'
1951	Polypropene (PP)	Second-most used polymer in the world
1972	Kevlar	Very strong and lightweight polymer; flameproof
1980	Polyacetylene	Conductive polymer
1990	Polylactic acid	Biodegradable polymer

Polymers in modern life

The rapid development of polymer technology is visible in many areas. Compare these two images of cyclists:

Commercial use of polyethene flourished during World War II, when it replaced much heavier components in planes and ships, demonstrating the strategic importance of polymer development.