Problem 75 - Chemistry and Chemical Reactivity

Question

Write an equation for the formation of the following polymers. (a) Polystyrene, from styrene \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}=\mathrm{CH}_{2}\right)\) (b) PET (polyethylene terephthalate), from ethylene glycol and terephthalic acid

Step-by-Step Solution

Answer

Polystyrene: \((\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}=\mathrm{CH}_{2}) \rightarrow (-\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CH}_{2}-)_{n}\). PET: from ethylene glycol and terephthalic acid, releasing water.

Step 1: Understand Monomers and Form Polystyrene

Polystyrene is formed from the monomer styrene, which has the chemical structure \( \mathrm{C}_{6} \mathrm{H}_{5}\mathrm{CH}=\mathrm{CH}_{2} \). The polymerization process involves opening the double bond \( \mathrm{CH}=\mathrm{CH}_{2} \) and linking these units in a chain. \[ \text{n} \cdot \left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}=\mathrm{CH}_{2}\right) \rightarrow \left(-\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CH}_{2}-\right)_{n} \] Here, \( \text{n} \) represents the repeat units in the polymer chain.

Step 2: Identify Monomers for PET Formation

PET is formed from the reaction between two monomers: ethylene glycol \( \mathrm{HOCH}_2\mathrm{CH}_2\mathrm{OH} \) and terephthalic acid \( \mathrm{HOOC}-\mathrm{C}_6\mathrm{H}_4-\mathrm{COOH} \). These monomers undergo a condensation reaction to form PET, releasing water as a byproduct.

Step 3: Write the Equation for PET Formation

In the polymerization of PET, ethylene glycol and terephthalic acid undergo a condensation reaction. Each ester linkage forms with the release of a water molecule \( \mathrm{H}_2\mathrm{O} \). The equation for the formation is: \[ \text{n} \cdot (\mathrm{HOCH}_2\mathrm{CH}_2\mathrm{OH}) + \text{n} \cdot (\mathrm{HOOC}-\mathrm{C}_6\mathrm{H}_4-\mathrm{COOH}) \rightarrow \left(-\mathrm{OCCH}_2\mathrm{CH}_2\mathrm{OCO}-\mathrm{C}_6\mathrm{H}_4-\right)_{n} + 2n \cdot \mathrm{H}_2\mathrm{O} \] This shows the formation of PET and the release of water.

Key Concepts

Polystyrene Polymerization

Polystyrene polymerization is the process of creating polystyrene by linking many styrene monomers together. To understand this, we start with styrene's structure, which is formally noted as \( \mathrm{C}_{6} \mathrm{H}_{5}\mathrm{CH}=\mathrm{CH}_{2} \). The core idea here is the polymerization reaction, where the double bond in the styrene monomer breaks open. This allows the monomers to connect with each other, forming long, repeating chains.

Polystyrene is produced by a type of polymerization known as addition polymerization, also called chain-growth polymerization. Here are the basic steps:

• Initiation: A radical (often heat or light-induced) initiates the polymerization by reacting with the styrene monomer, disrupting the double bond.
• Propagation: The active radical site adds to another styrene monomer, creating a new radical at the end of the growing chain.
• Termination: The reaction stops when radicals combine, or another process removes the radical site, forming a stable chain.

This entire process results in the polymer, polystyrene, characterized by long chains of repeating units \( \left(-\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{CH}_{2}-\right)_{n} \), where \( \text{n} \) is the number of repeat units.

PET Synthesis

The synthesis of PET, or polyethylene terephthalate, involves two main monomers: ethylene glycol \( \mathrm{HOCH}_2\mathrm{CH}_2\mathrm{OH} \) and terephthalic acid \( \mathrm{HOOC}-\mathrm{C}_6\mathrm{H}_4-\mathrm{COOH} \). These monomers undergo a reaction known as condensation polymerization. In this process, each time a bond forms between the monomers, a molecule of water is released.

The formation of PET can be broken down into steps:

• Monomer Interaction: Ethylene glycol's hydroxyl groups \( -\mathrm{OH} \) react with terephthalic acid's carboxylic groups \( -\mathrm{COOH} \).
• Release of Water: During this reaction, each link that forms ejects a water molecule \( \mathrm{H}_2\mathrm{O} \).
• Formation of Esters: The remaining structure forms an ester linkage, \( -\mathrm{OCCH}_2\mathrm{CH}_2\mathrm{OCO}-\mathrm{C}_6\mathrm{H}_4- \).

The overall reaction transforms the monomers into long chains of PET, characterized by repeating ester linkages. The byproduct, water, is significant in this reaction and must be managed during industrial production.

Condensation Reaction

Condensation reactions are a fundamental concept in polymer chemistry. They are key to forming many types of polymers, such as PET. Unlike addition polymerizations (like that of polystyrene), condensation polymerizations involve the combination of monomers where each bond formation results in the release of small molecules like water or alcohol. These reactions are essential because:

• Byproduct Formation: Every time monomers join, a small molecule, often water (\( \mathrm{H}_2\mathrm{O} \)), is released.
• Stepwise Growth: Unlike chain-growth polymerizations, the polymer grows in a stepwise manner, which can be controlled more easily.
• Diversity of Structures: They enable the creation of complex and diverse polymer structures by allowing different monomers containing varying functional groups to react together.

In summary, condensation reactions are essential for crafting polymers like PET, providing an effective method to engineer a wide range of polymeric materials with unique properties.