Condensation Polymers: Polyesters

Introduction

A polyester is identified by ester linkages repeated along its chain. Combining a diol with a dicarboxylic acid makes a familiar step-growth polyester while eliminating water. The reaction illustrates how monomer composition differs from the composition of the final repeat unit.

Core explanation

An alcohol group and a carboxylic acid group can form an ester bond, –C(=O)–O–, with water as a by-product. In a bifunctional system, a diol HO–R–OH and a diacid HOOC–R′–COOH each have two reactive ends, so their coupling can continue into a chain with alternating residues. The repeat unit contains atoms from both monomers minus the atoms lost in condensation. For PET, ethylene glycol, HO–CH₂–CH₂–OH, reacts with terephthalic acid, HOOC–C₆H₄–COOH, producing repeat segments that contain –O–CH₂–CH₂–O–C(=O)–C₆H₄–C(=O)–. Industrial PET processes may use different ester derivatives and release methanol or glycol instead of water in particular stages, so identify the actual feedstock before naming a by-product. The aromatic ring helps stiffen the chain, while the ester groups confer polarity and allow hydrolysis under suitable conditions. High molar mass requires near-stoichiometric end-group balance and extensive conversion. Because esterification can be reversible, removing water or another small molecule helps shift the equilibrium toward polymer formation. PET is used in fibres and beverage containers because its chain structure can give useful strength, processability and barrier properties. Polyester chemistry does not by itself guarantee ready environmental biodegradation; rates depend on structure and conditions.

Step-by-step reasoning

Write the ester-forming reaction between one acid and one alcohol group. Replace the monofunctional reactants with a diacid and diol, leaving opposite end groups on the first oligomer. Repeat the linkage to draw the constitutional repeat unit, then account for water or the actual leaving molecule.

Visual explanation

Draw a diol as a two-ended blue bar and a diacid as a two-ended red bar. Link alternating blue and red bars with small ester symbols, and place a water symbol beside each acid–alcohol coupling.

Real-world analogy

Two-ended building blocks can join into a long alternating garland. Each clasp represents an ester linkage, while the small water molecule leaving at the clasp explains why the chain's repeat-unit formula is not simply the sum of intact monomer formulas.

Real-world example

PET appears in textile fibres and many beverage bottles. Its repeat unit joins a flexible ethylene glycol segment to a rigid terephthalate ring, giving a material that can be melt processed and drawn into strong filaments.

Why?

Two reactive ends per monomer allow repeated coupling. Removing the small by-product drives the reversible ester-forming reaction toward more bonds, which is essential when very high conversion is needed for long chains.

Common misconception

Not every polyester-forming process releases water: transesterification can release an alcohol instead. The term polyester identifies the repeated ester linkage, while the exact leaving molecule depends on the monomer route.

Worked example

Question: Ethylene glycol reacts with terephthalic acid to form one new ester bond. What leaves? Reasoning: A diacid carboxyl group supplies OH and a glycol hydroxyl supplies H as the C–O bond forms. Answer: One H₂O molecule leaves per simple acid–alcohol esterification link; continued reactions create PET chains.

Quick check

1. Which functional-group bond identifies a polyester? Answer: A repeated ester linkage, –C(=O)–O–, along the polymer chain.

Exam focus

Draw the repeat unit with open bonds at both ends and make its orientation consistent across the chain. Show a chemically plausible monomer pair and name the small by-product for the specific reaction route.

Advanced insight

Polyester hydrolysis cleaves ester linkages, but practical recyclability depends on reaction selectivity, additives, crystallinity and collection systems. Chemical recycling of PET can recover monomers or useful intermediates under appropriate processing conditions.

Summary

Diols and diacids can undergo step-growth esterification to form polyesters, releasing water in the direct acid route. PET contains alternating ethylene glycol and terephthalate residues. Its repeat unit excludes the atoms lost as small by-products, and high molar mass requires balanced groups and very high conversion.

Practice questions

1. Name the two monomer classes used in direct polyesterification. Answer: A diol and a dicarboxylic acid.

2. What linkage repeats in a polyester? Answer: The ester linkage –C(=O)–O–.

3. What small molecule leaves when a carboxylic acid reacts directly with an alcohol? Answer: Water.

4. Why can removing the by-product help polymerisation? Answer: It shifts a reversible condensation equilibrium toward ester bonds and higher functional-group conversion.