Question

Baked-on paints used for automobiles and many appliances are often based on alkyds, such as can be made from terephthalic acid and glycerol. Sketch a section of the resultant polyester polymer that would be obtained if two glycerols reacted with two terephthalic acids, using the \(-\mathrm{OH}\) on the first and third carbon of glycerol. Note that the glycerol can actually be esterified at any of the three alcohol groups, providing cross-linking to form a very strong surface.

Short answer

The polymer has ester linkages between terephthalic acid and glycerol, involving the 1st and 3rd hydroxyls of glycerol.

Step-by-step solution

Analyze Reactants

Understand that terephthalic acid contains one carboxylic group on each benzene ring, and glycerol contains three hydroxyl groups. The problem specifies using the two hydroxyl groups on the first and third carbon of glycerol.

Identify Functional Groups

Glycerol has \(- ext{OH}\) groups available for esterification. Terephthalic acid has carboxyl \(- ext{COOH}\) groups that will react with glycerol's hydrogen atoms from the hydroxyl groups to form ester links.

Formation of Ester Linkages

Combine glycerol and terephthalic acid via esterification, which involves the reaction between an acid (from terephthalic acid) and an alcohol (from glycerol) to form an ester with the release of water.

Sketch the Polymer

Sketch the polymer structure by first placing glycerol in the center with terephthalic acid on either side. Show a repeated pattern: benzene ring (from terephthalic acid) - ester link - glycerol - ester link - benzene ring. Indicate ester linkages on the first and third carbon of glycerol.

Cross-linking Potential

Acknowledge that the third hydroxyl group on each glycerol can also potentially react, allowing for the formation of a cross-linked network, although typically esterification will proceed between specified hydroxyl groups initially.

Key concepts

Alkyds

Alkyds are versatile polymers widely used in coatings like baked-on paints. They are known for their excellent adhesion and glossy finish. Derived from polyesters cross-linked into a three-dimensional structure, alkyds are often used in automotive paints and appliances.

Because they can form strong bonds, alkyds provide durable, protective surfaces. They are formed through a reaction process involving polyols (like glycerol) and polybasic acids (such as terephthalic acid). This creates a polymer network capable of resisting environmental wear and tear.

Esterification

Esterification is a key chemical process where an acid reacts with an alcohol to form an ester and water. In the context of polyester polymer formation, terephthalic acid and glycerol undergo esterification to form ester bonds.

This reaction is crucial for creating the structure of alkyds:

• Carboxylic groups ( ext{-COOH}) from acids react with hydroxyl groups ( ext{-OH}) from alcohols.
• Water is released as a byproduct, indicating the formation of ester linkages.

Through esterification, strong and durable polymer chains are formed, providing rigidity and stability to the final product.

Terephthalic acid

Terephthalic acid is an essential compound in creating polyester polymers. It contains a benzene ring with two carboxylic acid groups, making it a key ingredient in producing esters.

The acid groups are highly reactive and facilitate the formation of ester linkages when paired with alcohols like glycerol. This reactivity is pivotal for constructing polymers used in resistant coatings and fibers.

Glycerol

Glycerol is a triol, meaning it has three hydroxyl ( ext{-OH}) groups, which make it highly versatile in chemical reactions like esterification. These functional groups allow glycerol to form ester bonds with acids like terephthalic acid.

This flexibility is advantageous in producing complex polymers, as additional cross-linking can occur through its third hydroxyl group, resulting in enhanced polymer strength and durability. Its role in creating flexible and durable polymers makes it a staple in polyester manufacturing.

Cross-linking

Cross-linking is a process that enhances the strength and stability of polymers by creating bonds between polymer chains. This occurs when glycerol’s third hydroxyl ( ext{-OH}) group reacts beyond the initial esterification points.

These additional bonds form a network of interconnected polymer chains, leading to:

• Increased thermal stability.
• Improved mechanical properties.
• Resistance to solvents and wear.

This cross-linked network is ideal for creating durable coatings that can withstand significant environmental stresses.