3.4.5 Addition Polymers

Formation of Addition Polymers

What are Addition Polymers?

Addition polymers are created through addition polymerisation, where small unsaturated molecules called monomers (often alkenes) join together to form a larger molecule called a polymer. In this reaction, the double bonds in the monomers break, allowing them to link together without producing any by-products.

Making Poly(phenylethene) from Phenylethene

Polymerisation of phenylethene involves breaking the $C=C$ bond, resulting in the formation of poly(phenylethene). The presence of a bulky benzene ring makes it less tightly packed, leading to weaker intermolecular forces and greater flexibility compared to poly(ethene).

Structure of Addition Polymers

Repeating Units

The repeating unit of a polymer is the smallest section that repeats throughout the polymer chain. It is derived from the original monomer but without the double bond.

Drawing Repeating Units from a Monomer

1. Identify the structure of the monomer.
2. Remove the double bond and ensure the molecule is single-bonded.
3. Enclose the repeating unit in brackets and add subscript 'n' to denote repetition.

Drawing the Monomer from a Repeating Unit

To identify the monomer:

4. Locate the repeating unit.
5. Remove the terminal bonds and add a double bond between the carbon atoms.

Naming Addition Polymers

IUPAC nomenclature dictates that polymers are named by prefixing the monomer's name with 'poly'. For instance:

• Ethene becomes poly(ethene).
• Propene becomes poly(propene).
• Chloroethene becomes poly(chloroethene) (commonly PVC).

Properties of Addition Polymers

Why are Addition Polymers Unreactive?

Addition polymers are generally unreactive due to:

• Saturation: The carbon chain consists entirely of single bonds ($C-C$), which are stable.
• Non-polar Nature: Most polymers have non-polar carbon chains, reducing their chemical reactivity.

Intermolecular Forces in Polyalkenes

1. Van der Waals Forces: These are the primary intermolecular forces between non-polar polymer chains. The longer and less branched a polymer, the stronger these forces.
2. Permanent Dipole-Dipole Interactions: In polymers like PVC, where there are polar groups, additional dipole-dipole forces contribute to rigidity.

Impact on Physical Properties:

• High melting and boiling points due to strong intermolecular forces.
• Flexibility and rigidity depend on the chain length and branching.

Common Addition Polymers and Their Uses

Poly(ethene) (PE)

• Monomer: Ethene ($CH₂=CH₂$)
• Properties: Lightweight, flexible.
• Uses: Plastic bags, film wrapping (Low-Density Polyethene - LDPE), and kitchenware (High-Density Polyethene - HDPE).

Poly(phenylethene) (Polystyrene)

• Monomer: Phenylethene (styrene).
• Properties: Lightweight, good insulator, brittle when solid.
• Uses: Packaging materials, insulation, disposable containers.

Poly(chloroethene) (PVC)

• Monomer: Chloroethene ($CH₂=CHCl$)
• Properties: Durable, resistant to water, electrical insulator.
• Modification: Adding plasticisers makes PVC flexible.
• Uses:
  • Without plasticiser: Drain pipes, window frames.
  • With plasticiser: Electrical cable insulation, raincoats, flexible flooring.

Historical and Industrial Context

The understanding and production of polymers have advanced significantly over time:

• Initially, natural polymers like rubber and cellulose were used.
• The development of synthetic polymers (like PVC and poly(ethene)) has transformed industries from packaging to construction.
• Research continues into making polymers more sustainable, including biodegradable options and recycling processes.

Drawing Skills for Exams

Drawing a Repeating Unit from a Monomer

• Step 1: Identify the monomer structure.
• Step 2: Remove the double bond and connect the carbons.
• Step 3: Enclose the repeating unit in square brackets and add 'n'.

Drawing a Polymer Section from a Monomer

• Extend the repeating unit into a chain.
• Show multiple repeats clearly.

Converting Polymer Sections Back to Monomers

• Remove terminal bonds.
• Reintroduce a double bond to create the original monomer structure.