Polymerization Mechanism, Repeating Units, and Atom Economy
How Do Addition Polymers Form?
- What makes alkenes like ethene ($C_2H_4$) special? The answer lies in their double bonds.
- These bonds are highly reactive and can "open up," allowing the monomers to connect and form a polymer. This process is known as addition polymerization.
Definition
Addition polymerization
Addition polymerization is a reaction in which monomers with double or triple bonds join together without forming any by-products, creating a polymer chain.
Ethene
- Monomer: Ethene ($C_2H_4$) contains a C=C double bond.
- Polymerization: When the double bond breaks, the two carbon atoms form new single bonds with neighboring monomers.
- Polymer: The resulting polymer is polyethene (also called polyethylene), a widely used plastic.
- The reaction can be summarized as:
$$
n CH_2=CH_2 \rightarrow (-CH_2-CH_2-)_n
$$
Poly(chloroethene)
- Poly(chloroethene), commonly known as PVC, is formed from the monomer chloroethene ($C_2H_3Cl$).
- The polymerization reaction is: $$
n ,\ CH_2=CHCl \, \rightarrow \, (-CH_2-CHCl-)_n
$$ - PVC is widely used in pipes, window frames, and electrical insulation due to its strength and durability.
- When drawing the repeating unit of a polymer, students often forget to show the open bonds extending beyond the brackets.
- These bonds indicate that the chain continues indefinitely.
Representing the Repeating Unit of a Polymer
- Polymers are composed of repeating units, which are derived from their monomers.
- To represent a polymer, we use the structural formula of the repeating unit enclosed in brackets, with bonds extending through the brackets to show the continuation of the chain.
- A subscript $n$ is added to indicate that the polymer consists of many repeating units.
How to Identify the Repeating Unit
To determine the repeating unit:
- Look for the smallest segment of the polymer chain that repeats.
