58. Addition Polymers & Polymerisation
1. The Basics of Polymerisation
Monomers: Small, individual molecules (like ethene) that join together to form a long chain.
Polymers: The long-chain molecule formed when many monomers join together.
The Process: Alkenes are unsaturated because they contain a carbon-carbon double bond (C=C). Under high pressure and in the presence of a catalyst, this double bond breaks into a single bond, allowing the carbons to bond with adjacent monomer molecules.
2. Drawing the Equations
To represent the reaction efficiently, we use a single monomer and a "repeating unit" rather than drawing the entire chain.
On the Left (Monomer):
Draw the C=C double bond in the center.
Draw the four attached groups pointing straight up and down (not at angles).
Place a large 'n' in front of the molecule to represent a large number of monomers.
On the Right (Repeating Unit):
Draw the same structure but change the double bond to a single bond (C-C).
Draw "empty" bonds extending out to the left and right, passing through large square brackets.
Place the 'n' at the bottom-right of the brackets.
3. Naming Polymers
Naming is straightforward: place the word "poly" in front of the monomer name and put the monomer name in brackets.
Ethene becomes Poly(ethene).
Butene becomes Poly(butene).
Chloroethene becomes Poly(chloroethene).
4. Complex Monomers (e.g., Butene)
When dealing with longer chains like butene (C4H8):
Identify the C=C double bond.
Rearrange the other atoms into four distinct groups attached to those two carbons.
For larger side groups (like a C2H5 group in butene), simplify them into shorthand before drawing them pointing straight up or down.
Important: Ensure the bond is drawn from carbon to carbon, not to a hydrogen atom in a side group.
5. Reaction Conditions
Addition polymerisation reactions generally require:
High pressure
A catalyst