Animated Solution for Chemistry - Organic Chemistry: Monomer units of dacron polymer are

Unraveling the Polyester

Dacron, also commercially known as Terylene, is one of the most ubiquitous synthetic polymers in our modern world. From the wrinkle-resistant shirts in your closet to the durable plastic bottles in your refrigerator, this remarkable material is everywhere. But what exactly is it made of? To understand Dacron, we must dive into the fascinating world of condensation polymerization.

Unlike addition polymers (like polyethylene), which are formed by simply stringing together monomers with double bonds, condensation polymers are formed through a step-growth process. This means that when two monomer units join together, they undergo a chemical reaction that kicks out a small byproduct molecule—most commonly water ($H_{2}O$).

For a polymer chain to grow continuously, the monomers involved must be at least bi-functional. This means they need to have reactive functional groups on both ends of their molecular structure. If they only had one reactive group, the reaction would stop after forming a single, small molecule.

The term "polyester" is a massive clue to the structure of Dacron. It tells us that the polymer chain is held together by multiple ester linkages ($-COO-$).

In organic chemistry, an ester is typically formed by the reaction between a carboxylic acid ($-COOH$) and an alcohol ($-OH$). Therefore, to create a long chain of esters, we need a monomer with two carboxylic acid groups (a dicarboxylic acid) and another monomer with two alcohol groups (a diol).

Let's break down the repeating unit of Dacron to identify its parent molecules.

The acid component of Dacron features a benzene ring with two carbonyl groups attached at opposite ends (the para positions). This specific molecule is terephthalic acid, formally known as benzene-1,4-dicarboxylic acid.

The alcohol component is a simple two-carbon chain with a hydroxyl group on each carbon. This is ethylene glycol, formally known as ethane-1,2-diol.

When these two monomers are heated together, the hydroxyl group ($-OH$) from the terephthalic acid and the hydrogen atom ($-H$) from the ethylene glycol combine to form water ($H_{2}O$), which is eliminated. The remaining fragments bond together to form the ester linkage, creating the long, durable chains of Dacron.

It is crucial to pay attention to the exact isomers used in polymer chemistry. For instance, if we were to use phthalic acid (benzene-1,2-dicarboxylic acid) instead of terephthalic acid, the resulting polymer would not be Dacron. Because the functional groups are adjacent (ortho) rather than opposite (para), the polymer chains would cross-link extensively, forming a rigid, thermosetting plastic known as Glyptal, which is commonly used in paints and lacquers.

Understanding these subtle structural differences is key to mastering the chemistry of polymers!