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.
Example
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:
$$
n CH_2=CH_2 \rightarrow (-CH_2-CH_2-)_n
$$
Example
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.
Common Mistake
- 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.
- Ensure that the repeating unit includes all atoms and bonds necessary to reconstruct the polymer.
Example
Polypropene
- The monomer propene ($CH_2=CHCH_3$)polymerizes to form polypropene.
- A section of the polymer chain might look like this: $$
-CH_2-CH(CH_3)-CH_2-CH(CH_3)-
$$ - The repeating unit is:$$
[-CH_2-CH(CH_3)-]_n
$$
Atom Economy in Polymerization
What Is Atom Economy?
Definition
Atom economy
Atom economy measures how efficiently the atoms in the reactants are incorporated into the desired product.
It is calculated using the formula:
$$
\text{Atom Economy} = \left( \frac{\text{Mass of Desired Product}}{\text{Mass of All Reactants}} \right) \times 100\%
$$
Why Addition Polymerization Has 100% Atom Economy
- In addition polymerization, no atoms are wasted.
- All the atoms in the monomers are incorporated into the polymer.
- For example: $$
n CH_2=CH_2 \rightarrow (-CH_2-CH_2-)_n
$$ - This makes addition polymerization highly efficient in terms of atom economy.
Tip
Reactions with high atom economy are more sustainable and environmentally friendly, as they minimize waste.
Note
- While addition polymerization has 100% atom economy, this is not true for all polymerization reactions.
- For example, condensation polymerization releases a small molecule, such as water or HCl, as a byproduct.
