Addition Polymerisation: Building Giants from Tiny Molecules
The Basic Building Blocks: Monomers and Polymers
Imagine you have a single LEGO brick. It's small and not very useful on its own. But if you connect hundreds or thousands of these identical bricks together, you can build something huge and complex, like a spaceship or a castle. This is the core idea behind addition polymerisation.
In chemistry, the small LEGO brick is called a monomer (from the Greek words mono, meaning "one," and meros, meaning "part"). The large, finished structure is called a polymer (poly meaning "many"). The process of connecting all these monomers is called polymerisation.
For addition polymerisation to work, the monomer must have a special, reactive feature: a carbon-carbon double bond. This is written as $C=C$. This double bond is relatively weak and can be "opened up," creating two free bonds that are eager to connect to other carbon atoms.
$ n \, \text{Monomer} \rightarrow \text{Polymer} $
Where $n$ represents a very large number of monomer units, often in the thousands or millions.
The Three-Step Chain Reaction Mechanism
The process of addition polymerisation is a chain reaction, much like a line of falling dominoes. It occurs in three distinct stages:
1. Initiation: This is the step that starts the entire reaction. A small amount of a highly reactive substance called an initiator is added. This initiator breaks apart to form a reactive species, often a free radical[2] (a molecule with an unpaired electron). This free radical then attacks the double bond of a monomer molecule. It "opens" the double bond and attaches itself, creating a new, larger reactive species. The domino has been tipped.
2. Propagation: This is the rapid, repetitive growth stage. The new, larger radical now attacks another monomer molecule, adding it to the chain. This process repeats over and over again, thousands of times, at a very high speed. The polymer chain grows longer and longer with each addition. This is the core of the chain reaction.
3. Termination: Eventually, the chain reaction must stop. This happens during termination. Two growing polymer chains with radical ends can meet and combine their unpaired electrons, forming a single, stable polymer molecule. Alternatively, the radical end of one chain might be transferred to another molecule, stopping its growth. The reaction is complete, and a giant polymer molecule has been formed.
Common Polymers and Their Monomers
Many of the most common plastics we use every day are created through addition polymerisation. By changing the type of monomer, chemists can create polymers with a wide range of properties, from flexible and transparent to rigid and tough.
| Polymer Name (Common Brand Names) | Monomer Structure | Properties and Uses |
|---|---|---|
| Polyethylene (PE) (e.g., plastic bags, bottles) | Ethene: $ H_2C=CH_2 $ | Flexible, chemically resistant, inexpensive. Used for packaging, containers, and toys. |
| Polypropylene (PP) (e.g., food containers, car parts) | Propene: $ H_2C=CHCH_3 $ | Tough, resistant to fatigue. Used for hinged caps, automotive parts, and textiles. |
| Polyvinyl Chloride (PVC) (e.g., pipes, window frames) | Vinyl Chloride: $ H_2C=CHCl $ | Can be rigid or flexible. Durable, weather-resistant. Used for plumbing, cables, and siding. |
| Polystyrene (PS) (e.g., disposable cutlery, foam packaging) | Styrene: $ H_2C=CHC_6H_5 $ | Hard, brittle, and transparent. When foamed, it becomes Styrofoam®, a lightweight insulator. |
| Polytetrafluoroethylene (PTFE) (e.g., Teflon® non-stick pans) | Tetrafluoroethene: $ F_2C=CF_2 $ | Extremely slippery, very chemically inert, and heat-resistant. Used for non-stick coatings and seals. |
From Lab to Life: The Production of Polyethylene
Let's follow the journey of the world's most common plastic, polyethylene, from a gaseous monomer to a solid material. The monomer, ethene ($C_2H_4$), is derived from petroleum or natural gas. In a massive chemical reactor, ethene gas is subjected to high pressure and temperature in the presence of a special initiator (a catalyst).
The initiator starts the chain reaction. Ethene molecules rapidly add to the growing chains. The reaction conditions (pressure, temperature, type of catalyst) can be controlled to produce different types of polyethylene. For example, high-pressure conditions create a low-density, branched polymer (LDPE) that is flexible, perfect for plastic bags and squeeze bottles. Lower pressure with a different catalyst creates a high-density, linear polymer (HDPE) that is more rigid, ideal for milk jugs and detergent bottles.
Once the polymerisation is complete, the resulting polymer is melted, extruded into long strands, and chopped into small pellets. These pellets are then sold to manufacturers who melt them again and mold them into the final products we use every day. This entire process, from gas to pellet, produces nothing but the polymer, perfectly illustrating the definition of addition polymerisation.
Important Questions
What is the main difference between addition polymerisation and condensation polymerisation?
The key difference lies in the byproducts. In addition polymerisation, the monomer molecules simply add together, and no other products are formed. The polymer is the sole output. In condensation polymerisation, monomers join together, but a small molecule, like water or hydrochloric acid, is eliminated as a byproduct for each bond formed between monomers. Nylon and polyester are made by condensation polymerisation.
Can a polymer be broken back down into its monomers?
Generally, it is very difficult to reverse addition polymerisation under normal conditions. The process is not easily reversible because the carbon-carbon single bonds in the polymer backbone are very strong and stable. However, through a process called depolymerisation using high heat (pyrolysis) or specific chemical treatments, some polymers can be broken down, though not always perfectly back to the original monomers. This is a major area of research for plastic recycling.
Why are some plastics flexible while others are rigid?
The flexibility of a plastic depends on the structure of the polymer chains and the forces between them. If the chains are long, straight, and can pack closely together (like in HDPE), the strong intermolecular forces make the material rigid. If the chains are branched (like in LDPE), they cannot pack tightly, making the material more flexible. Also, adding small molecules called plasticizers can get between the polymer chains, pushing them apart and making the material more flexible, as is done with PVC.
Conclusion
Addition polymerisation is a remarkably efficient and powerful chemical reaction that transforms simple, small molecules into the giant macromolecules that form the basis of the modern plastics industry. By understanding the roles of monomers, initiators, and the three-step mechanism of initiation, propagation, and termination, we can see how materials with such diverse properties are created from a common principle. From the packaging that preserves our food to the pipes that deliver our water, the products of addition polymerisation are integral to daily life. As we continue to use these materials, the challenges of sustainability and recycling highlight the importance of understanding their chemical origins.
Footnote
[1] Carbon-Carbon Double Bond ($C=C$): A covalent bond between two carbon atoms where two pairs of electrons are shared. This bond is a site of high reactivity, which allows it to be broken and transformed into two single bonds during addition reactions.
[2] Free Radical: An atom or molecule that has one or more unpaired electrons, making it highly reactive and unstable. Free radicals are often generated by the decomposition of initiators like benzoyl peroxide and are crucial for starting the addition polymerisation chain reaction.