S2.4.5 Addition polymers

Polymerization Mechanism, Repeating Units, and Atom Economy

Imagine you’re at a craft workshop, stringing beads onto a necklace. Each bead represents a small molecule, or monomer, and the necklace represents a polymer—a long chain of repeating units.

How do these beads join together to form such lengthy chains?

This is the essence of polymerization, a process that underpins the creation of materials like plastics, synthetic fibers, and even biological macromolecules such as DNA.

How Do Addition Polymers Form?

What makes alkenes like ethene ( $C_{2} H_{4}$ ) 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 ( $C_{2} H_{4}$ ) 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:

$n C H_{2} = C H_{2} \to (- C H_{2} - C H_{2} -)_{n}$

Example

Poly(chloroethene)

Poly(chloroethene), commonly known as PVC, is formed from the monomer chloroethene ( $C_{2} H_{3} C l$ ). The polymerization reaction is:

$n, C H_{2} = C H C l \to (- C H_{2} - C H C l -)_{n}$

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 ( $C H_{2} = C H C H_{3}$ )polymerizes to form polypropene. A section of the polymer chain might look like this:

$- C H_{2} - C H (C H_{3}) - C H_{2} - C H (C H_{3}) -$

The repeating unit is:

$[- C H_{2} - C H (C H_{3}) -]_{n}$

Self review

Identify the repeating unit of a polymer formed from butene ( $C H_{2} = C H C H_{2} C H_{3}$ ).

Hint: Focus on the structure of the monomer and how the double bond opens up.

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:

$Atom Economy = (\frac{Mass of Desired Product}{Mass of All Reactants}) \times 100 %$

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:

$n C H_{2} = C H_{2} \to (- C H_{2} - C H_{2} -)_{n}$

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.

Theory of Knowledge

Atom economy aligns with the principles of green chemistry, which emphasize reducing waste and conserving resources.

How does the concept of atom economy influence decision-making in industrial processes?

Applications and Implications

Real-World Applications of Addition Polymers

Addition polymers are everywhere in modern life:

Polyethene: Used in packaging, bottles, and plastic bags.
Polypropene: Found in ropes, carpets, and reusable containers.
PVC: Used in construction materials and medical devices.

However, the widespread use of polymers raises environmental concerns. Many addition polymers, such as polyethene and PVC, are non-biodegradable, leading to long-term waste accumulation.

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.

Reflection Questions

Self review

Why does addition polymerization have 100% atom economy, while condensation polymerization does not?
How might the properties of a polymer differ from its monomer? Consider factors like melting point, flexibility, and chemical reactivity.

Theory of Knowledge

What are the ethical implications of using non-biodegradable polymers in single-use products? How can science address these challenges?

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