Condensation Polymerisation: The Step-by-Step Assembly
The Essential Ingredients and Mechanism
Imagine building a chain of paper clips, but every time you connect two clips, a small bead falls off. That’s the core idea of condensation polymerisation. It requires monomers that have at least two reactive functional groups, ready to form a bond with their neighbors. The most common reactions are between an alcohol group ($-OH$) and a carboxylic acid group ($-COOH$), or between an amine group ($-NH_2$) and a carboxylic acid group.
The magic—and the “condensation” part—happens when these groups react. The bond forms between the two monomers, but atoms from each group (usually hydrogen H and oxygen O) are kicked out, combining to create a small, stable molecule. Most often, this small molecule is water ($H_2O$).
This step repeats over and over. The dimers (two linked monomers) can react with other monomers or with each other, steadily growing the chain. This is why it’s also called step-growth polymerisation; the polymer grows stepwise, not in a rapid chain reaction.
Classic Examples in Chemistry and Nature
Let's look at two of the most famous synthetic polymers and one vital natural polymer created through condensation.
1. Making Polyester (Terylene/PET): This is the reaction between a dicarboxylic acid and a diol (an alcohol with two $-OH$ groups). A common example is ethylene glycol (diol) reacting with terephthalic acid (dicarboxylic acid). Each time an ester bond ($-COO-$) forms, a molecule of water is eliminated. The polymer, polyethylene terephthalate (PET), is used in plastic bottles and clothing fibers.
2. Making Nylon-6,6: This is created from hexamethylenediamine (a diamine) and adipic acid (a dicarboxylic acid). They react to form an amide bond ($-CONH-$), which is the same linkage found in proteins. With each amide bond formed, a molecule of water is released. The resulting strong, flexible fibers are used in textiles, ropes, and carpets.
3. Proteins (Nature’s Polymers): In living cells, amino acids act as monomers. Each amino acid has an amine group ($-NH_2$) and a carboxylic acid group ($-COOH$). During protein synthesis, these groups react, forming a peptide bond (an amide bond) and eliminating a water molecule. This biological condensation polymerisation is the foundation of life's machinery.
Condensation vs. Addition Polymerisation
It's crucial to distinguish condensation polymerisation from its counterpart, addition polymerisation. The table below highlights the key differences, which help in identifying and understanding polymer types.
| Feature | Condensation Polymerisation | Addition Polymerisation |
|---|---|---|
| Monomers | Two different types or one type with two different functional groups (e.g., $-COOH$ and $-OH$). | Usually one type of monomer with a carbon-carbon double bond (e.g., $CH_2=CH_2$). |
| By-product | Always produces a small molecule (e.g., $H_2O$, $CH_3OH$). | No by-product is eliminated; monomers add directly. |
| Growth Type | Step-growth: Molecules of all sizes react. | Chain-growth: Only the active chain end adds new monomers rapidly. |
| Example Polymers | Polyester, nylon, proteins, starch. | Polyethylene, polypropylene, PVC, polystyrene. |
From Lab to Life: Everyday Applications
Condensation polymers are all around us, from the clothes we wear to the food we eat. Let's trace the journey of a condensation polymer from a chemical reaction to a practical product.
Case Study: The PET Bottle. The journey starts with the monomers ethylene glycol and terephthalic acid. Under controlled heat and pressure, they undergo condensation polymerisation. The long polyester chains (PET) produced are then melted and molded into small pellets. These pellets are shipped to bottle manufacturing plants, where they are reheated and blown into the familiar shape of a plastic bottle. The polymer's strength, clarity, and ability to act as a barrier to gases make it perfect for packaging drinks. This entire process is possible because of that simple, repeated condensation step that links monomers and releases water.
Other applications include:
- Nylon in Gear: The toughness of nylon, derived from its strong intermolecular forces (hydrogen bonds between amide groups), makes it ideal for mechanical parts like gears in machinery and bristles in toothbrushes.
- Kevlar for Protection: This super-strong polymer, made from condensation of specific monomers, is used in bulletproof vests and helmets due to its exceptional strength-to-weight ratio.
- Bakelite in Electronics: One of the first plastics, Bakelite is a condensation polymer used in electrical switches and handles because it is a good insulator and heat-resistant.
Important Questions
Q1: Is the small molecule eliminated always water?
No, water is the most common by-product, but it's not the only one. For example, in the production of polycarbonates, the small molecule eliminated can be phenol. In the formation of certain polyurethanes, no small molecule is eliminated in the final step, but the initial reactions leading to the prepolymer often involve condensations.
Q2: Can a condensation polymer be made from just one type of monomer?
Yes, absolutely. This happens when a single monomer contains two different functional groups that can react with each other. A classic example is the formation of nylon-6 from a monomer called caprolactam. Upon heating, it rearranges and links up, eliminating water in the process. Another example is the formation of proteins from amino acids, where each amino acid monomer has both an amine and a carboxyl group.
Q3: Why is condensation polymerisation important for recycling?
It presents both a challenge and an opportunity. The challenge is that condensation polymers like PET can break down (hydrolyze) under certain conditions, potentially limiting their reuse. However, this chemical sensitivity is also the key to chemical recycling. PET can be depolymerized—the reverse of condensation—by reacting it with water or other chemicals under heat, breaking it back down into its original monomers. These pure monomers can then be repolymerized into new, high-quality plastic, creating a true circular loop, unlike traditional mechanical recycling which often downgrades the material.
Conclusion
Condensation polymerisation is a elegant and versatile chemical dance where molecules link hands, letting go of a small partner (like a water molecule) with each new connection. From the synthetic fibers in our backpacks and the plastic bottles we drink from, to the very proteins that build our muscles, this stepwise assembly process is a cornerstone of both materials science and biology. Understanding the principle of small molecule elimination not only helps us classify polymers but also opens doors to designing new materials and developing sustainable recycling methods for the future.
Footnote
1 Monomer: A small, simple molecule that can join with other similar molecules to form a polymer. It is the basic building block of a polymer.
2 Polymer: A large molecule (macromolecule) composed of many repeated subunits (monomers).
3 Polyester: A category of polymers formed by condensation reactions where the ester functional group ($-COO-$) is the key linkage in the main chain.
4 PET (Polyethylene Terephthalate): A specific, common type of polyester used in fibers for clothing and containers for liquids and foods.
5 Functional Group: A specific group of atoms within a molecule that determines the characteristic chemical reactions of that molecule (e.g., $-OH$, $-COOH$, $-NH_2$).