Question

Polycarbonates are a class of thermoplastic polymers that are used in the plastic lenses of eyeglasses and in the shells of bicycle helmets. A polycarbonate is made from the reaction of bisphenol A (BPA) with phosgene \(\left(\mathrm{COCl}_{2}\right)\) : polycarbonate \(+2 n \mathrm{HCl}\) Phenol \(\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\right)\) is used to terminate the polymer (stop its growth). a. Draw the structure of the polycarbonate chain formed from the above reaction. b. Is this reaction a condensation or an addition polymerization?

Short answer

a. The structure of the polycarbonate chain formed from the reaction is: \( [\text{BPA}-\mathrm{O} - \mathrm{C}(\mathrm{O} - \mathrm{Cl}) - \mathrm{R}_{n}]- \mathrm{Phenol}\). b. This reaction is a condensation polymerization.

Step-by-step solution

Bisphenol A

Bisphenol A (\(\mathrm{C}_{15}\mathrm{H}_{16}\mathrm{O}_{2}\)) has the following structure: \[ \mathrm{H}_{2} \mathrm{C}= \mathrm{C} - \mathrm{C}=\mathrm{C}- \mathrm{H}_{2} \mathrm{O} - \mathrm{C} - (\mathrm{O} - \mathrm{C} - \mathrm{H})_{4} \]

Phosgene

Phosgene (\(\mathrm{COCl}_{2}\)) has the following structure: \[ \mathrm{C}(\equiv \mathrm{O})(\mathrm{Cl})_{2} \] ***Step 2: Draw the structure of the polycarbonate chain***

BPA and Phosgene reaction

The reaction occurs between the oxygen atom of bisphenol A and one carbon of the phosgene. The complete polycarbonate structure can be represented as: \[ [\text{BPA}-\mathrm{O} - \mathrm{C}(\mathrm{O} - \mathrm{Cl}) - \mathrm{R}_{n}]- \mathrm{Phenol} \] ***Step 3: Determine the type of polymerization***

Polymerization Type

As the reaction between BPA and phosgene produces \(\mathrm{HCl}\) (hydrogen chloride) as a byproduct, the reaction is a condensation polymerization.

Answer

a. The structure of the polycarbonate chain formed from the reaction is: \( [\text{BPA}-\mathrm{O} - \mathrm{C}(\mathrm{O} - \mathrm{Cl}) - \mathrm{R}_{n}]- \mathrm{Phenol}\). b. This reaction is a condensation polymerization.

Key concepts

Condensation Polymerization

Condensation polymerization is a process where monomers join together, with the simultaneous elimination of small molecules as byproducts. In the case of polycarbonate polymerization, bisphenol A (BPA) reacts with phosgene (COCl₂) to form the polymer chain.

During this reaction, hydrochloric acid (HCl) is released. This byproduct distinguishes condensation polymerization from addition polymerization, where no small molecules are lost.

• Monomers like BPA and phosgene are key players.
• Byproducts need to be managed, such as removing HCl efficiently.

Condensation polymerization is significant in producing a variety of useful polymers, including polycarbonates.

Bisphenol A (BPA)

Bisphenol A, or BPA, is a crucial monomer in making polycarbonates. It is an organic compound with the formula C₁₅H₁₆O₂. Its structure consists of two phenol functional groups, often depicted with hydroxyl (OH) groups.

These hydroxyl groups are reactive, allowing BPA to easily interact with phosgene to form long polymer chains.

• BPA provides strength and clarity to polymers.
• It is readily available and cost-effective.

BPA's role in forming reliable, durable plastics is essential across various industries like eyewear and safety equipment.

Phosgene (COCl₂)

Phosgene is a reactive carbonyl compound with the formula COCl₂. It acts as a linking agent in the formation of polycarbonate polymers.

Phosgene consists of one carbon atom doubly bonded to an oxygen atom and singly bonded to two chlorine atoms. This structure allows it to react swiftly with the hydroxyl groups in BPA.

• Highly efficient in forming polymer chains.
• Requires careful handling due to toxicity.

The use of phosgene is crucial for creating strong bonds within polycarbonate structures.

Phenol (C₆H₅OH)

Phenol is a simple aromatic compound often used to terminate polymer chains. It has a structure defined by a benzene ring bonded to a hydroxyl group (OH).

In the context of polycarbonate formation, phenol is introduced to halt the polymer growth, defining the chain length and properties.

• Helps in controlling polymer molecular weight.
• Ensures uniformity in polymer products.

Phenol’s ability to stop polymerization at the right moment makes it essential for precise manufacturing of polycarbonates.

Polycarbonate Structure

Polycarbonate polymers have a repeating structural unit formed by the reaction of BPA and phosgene. The general structure is often characterized by the presence of carbonate groups (–O–C(=O)–O–) linked together.

These polymers are noted for their durability, transparency, and heat resistance, making them ideal for applications such as optical lenses and helmet shells.

• Resilient under stress, providing safety.
• Maintains clarity over time, ideal for lenses.

The polycarbonate structure provides a balance between strength and flexibility, essential for various industrial applications.