\( \mathrm{~A}\) polyester has the repeating unit \(-\left(\mathrm{OCH}_{2} \mathrm{C}_{6} \mathrm{H}_{4} \mathrm{CH}_{2} \mathrm{OOCC}_{6} \mathrm{H}_{4} \mathrm{CO}\right)_{n}\)-. Identify the monomers of the polyester.

Polymerization is a process where small molecules, known as monomers, join together to form long chains or networks known as polymers. Imagine it like building a train by connecting individual carriages one by one; the monomers are the carriages and the entire train represents the polymer. This process can occur via various mechanisms, but for the polyester we're discussing, a specific type called step-growth polymerization is relevant.

During step-growth polymerization, monomers with two or more reactive end groups combine to form dimers, trimers, and eventually long polymer chains. The reaction continues until a large network is formed, giving us materials with unique properties like the polyester used in clothing and containers. In step-growth, the polymerization can involve either a condensation reaction, which releases a small molecule like water, or an addition reaction, where no byproducts are formed.

In order to identify the monomers that constitute a polymer, one needs to be a little bit like a detective. This involves understanding the structure of the polymer's repeating unit. For the provided polyester, each repeating unit contains segments that suggest its origins from two different types of monomers: a diol (a molecule with two alcohol groups, -OH) and a dicarboxylic acid (a molecule with two carboxylic acid groups, -COOH).

Diols and dicarboxylic acids are frequently used to create polyesters. By identifying the specific structural features of the repeating unit, we can backtrack to determine the exact monomers. For instance, the presence of -OCH2- groups indicates part of a diol, specifically ethylene glycol, while the CO- groups are part of a dicarboxylic acid, which in this case is terephthalic acid. Recognizing these parts within the complex structure is crucial to understanding how the polymer is formed.

A condensation reaction is a type of chemical reaction where two molecules combine to form a larger molecule, simultaneously releasing a smaller molecule, most commonly water. This is precisely what happens during the formation of the polyester mentioned in the exercise. The condensation reaction occurs between the hydroxyl groups (-OH) of the diol and the carboxyl groups (-COOH) of the dicarboxylic acid.

In the simplified form, the reaction can be viewed as: Diol HO-R-OH + Dicarboxylic acid HOOC-R'-COOH → Polyester -O-R-OOC-R'-CO- + 2 H2O. Every connection of a diol and a dicarboxylic acid releases one molecule of water, hence the repeat units are joined by what are essentially dehydration bonds—establishing a larger, stable polyester chain. Understanding this reaction is fundamental in grasping how many synthetic materials, such as plastics and fibers, are made in industrial chemistry.