Structure 2.4.5 — Addition Polymers

Addition polymers are formed by the reaction of alkene monomers. The double bond in each monomer breaks, and the monomers join together to form long chains called polymers.

Mechanism: The process involves:

• Breaking of the carbon-carbon double bond (\( \text{C=C} \)) in alkenes
• Formation of new single bonds to link monomers into long chains

General representation of addition polymerization:

\( n \, \text{CH}_2=\text{CHR} \rightarrow [-\text{CH}_2-\text{CHR}-]_n \)

Here, \( R \) represents a variable side group like H, Cl, CH₃, etc.

Repeating Unit: The smallest unit in the polymer that repeats. It is written using square brackets with a subscript \( n \), indicating many units.

Examples of Addition Polymerization:

Monomer	Polymer	Repeating Unit	Diagram
Ethene \( \text{CH}_2=\text{CH}_2 \)	Polyethene	\( [-\text{CH}_2-\text{CH}_2-]_n \)
Propene \( \text{CH}_2=\text{CHCH}_3 \)	Polypropene	\( [-\text{CH}_2-\text{CH(CH}_3)-]_n \)
Chloroethene (vinyl chloride) \( \text{CH}_2=\text{CHCl} \)	Polyvinyl chloride (PVC)	\( [-\text{CH}_2-\text{CHCl}-]_n \)
Tetrafluoroethene \( \text{CF}_2=\text{CF}_2 \)	Polytetrafluoroethene (PTFE or Teflon)	\( [-\text{CF}_2-\text{CF}_2-]_n \)

Important Notes:

• Only alkenes (or substituted alkenes) can undergo addition polymerization.
• No small molecules (like water or HCl) are lost during addition polymerization.
• Polymers are usually named by adding the prefix “poly-” to the name of the monomer.

Deducing Monomers from Polymers:

• Look for the repeating unit in the polymer chain.
• Reverse the polymerization step by identifying where double bonds would have been.
• Use brackets to isolate the repeating unit and then insert a double bond between the carbons.

Example: From polymer to monomer (PVC)

Polymer repeating unit: \( [-\text{CH}_2-\text{CHCl}-]_n \)
Monomer: \( \text{CH}_2=\text{CHCl} \)

Factors affecting properties of polymers:

Properties of polymers: softness, flexibility, melting point, tensile strength etc.

• The average length of polymer chain: As the chain length of a polymer increases the melting point and tensile strength increases.
• Branching of chains: The greater the branching in a polymer chain the less the melting point and tensile strength of that polymer and also lower density
• Intermolecular forces between chains: the higher the intermolecular forces between chains, the higher the melting point.
• Cross-links between chains: the stronger the bonding between two chains the higher the melting point, rigidity and hardness.