Addition Polymers
Converting alkenes into long chain macromolecules.
Key Definitions
- Monomer: Small reactive molecule containing a C=C double bond (Alkene).
- Polymer: Long chain macromolecule formed from many monomers.
- Repeating Unit: The specific arrangement of atoms that repeats 'n' times.
Mechanism Details
- Process: The π-bond of the alkene breaks, allowing carbons to form new σ-bonds with neighbors.
- Atom Economy: 100%. All atoms end up in the polymer. No waste.
Drawing Repeating Units
Examiner Strategy
- Flatten the Monomer: Draw C=C horizontally. Push attached groups (–CH₃, –Cl) strictly up or down.
- Break the Bond: Turn the double bond (=) into a single bond (–).
- Extend: Draw new single bonds extending outwards horizontally from each carbon.
- Bracket: Draw square brackets cutting through the extension bonds. Add subscript 'n'.
Propene (Monomer)
n[CH₂=CH(CH₃)]
Polypropene (Polymer)
–[CH₂–CH(CH₃)]n–
Think About It
Addition polymers like polyethene have 100% atom economy, yet they create huge environmental problems. Why is high atom economy not always "green"?
Atom economy measures waste during production, not what happens after use. Polyethene's all-carbon backbone is chemically inert — it cannot be broken down by microorganisms or hydrolysis. So while no atoms are wasted in making it, the product itself persists in the environment for centuries. This is why condensation polymers (with hydrolysable ester/amide bonds) are more biodegradable despite lower atom economy.