What is the Difference Between Ionic and Free Radical Polymerization

Ionic polymerization and free radical polymerization are two methods of polymerization. Polymerization is the process of linking many small molecules (monomers) to form a single large molecule (polymer). Polymers are abundantly found around us. For example, plastics, rubber, and even DNA are polymers.

What is the difference between ionic and free radical polymerization? In ionic polymerization, the active species is ions, whereas in free radical polymerization, the active species is free radicals.

Key Areas Covered

1. What is Ionic Polymerization
      – Definition, Features 
2. What is Free Radical Polymerization
      – Definition, Features 
3. Similarities Between Ionic and Free Radical Polymerization
      – Outline of Common Features
4. Difference Between Ionic and Free Radical Polymerization
      – Comparison of Key Differences
5. FAQ: Ionic and Free Radical Polymerization
      – Answers to Frequently Asked Questions

Key Terms

Ionic Polymerization, Free Radical Polymerization

What is Ionic Polymerization

Ionic polymerization is a type of chain-growth polymerization where charged particles (ions) participate in the linking of monomers into longer polymer chains. Here, the positively charged species called cations or negatively charged species called anions act as the active species that drive the polymerization reaction. Accordingly, there are two types of polymerization: cationic polymerization and anionic polymerization.

One advantage of ionic polymerization is the controlled chain growth. Ionic species offer more control over the polymer structure compared to free radicals.  Hence, polymers with a more uniform size and predictable properties are formed.

One disadvantage of ionic polymerization is sensitivity. Ionic polymerization is highly sensitive to impurities, especially water. Even a small amount of water can deactivate the charged species, causing the reaction to stop occurring further. This sensitivity requires very clean and controlled reaction conditions.

There are many applications of ionic polymerization. Polystyrene and nitrile rubber are products of ionic polymerization.

What is Free Radical Polymerization

Free radical polymerization is where polymerization happens, relying on highly reactive molecules called free radicals. Therefore, the active species of this type of polymerization are free radicals. These free radicals have an unpaired electron in their outer shell. This presence makes them highly reactive in stable bond formation.

Figure 1: Radical Polymerization of Ethylene

Free radical polymerization starts with a catalyst, often a peroxide or azo compound, breaking apart due to heat, light, or pressure, generating free radicals. These free radicals then attack the double bonds in monomers, which are the building blocks of polymers. Next is the propagation process. Free radical formed from the initiation step gets an electron from a monomer double bond to form a new bond. This creates a new radical on the other end of the attached monomer. This new radical searches for another monomer with which to create a bond. This process continues, extending the polymer chain. The last step is the chain termination step. Here, the reaction of forming the polymer comes to an end. Two free radicals can collide and combine, deactivating each other and ending chain growth. Alternatively, a free radical might react with another molecule or a chain end, halting further propagation.

Plastics used to make bottles, rubber in rubber bands, and fibers in clothes are examples of polymers made from free radical polymerization.

Similarities Between Ionic and Free Radical Polymerization

1. Both involve a chain-growth mechanism.
2. In both processes, covalent bonds are formed between the monomer units.
3. Both can utilize similar types of monomers, particularly unsaturated monomers containing double bonds (alkenes) that can be easily broken and reformed to create the polymer backbone.

Difference Between Ionic and Free Radical Polymerization

Definition

• Ionic polymerization is a type of chain-growth polymerization where charged particles (ions) participate in the linking of monomers into longer polymer chains. On the other hand, free radical polymerization is a type of polymerization that rely on highly reactive molecules called free radicals. 

Active Species

• In ionic polymerization, the active species is either cations or anions. In free radical polymerization, the active species are free radicals.

Monomers

• In free radical polymerization, a wider range of monomers can be polymerized, but in ionic polymerization, there is more limitations in compatible monomers.

Control

• In free radical polymerization, less control is there over the polymerization process (due to the randomness of free radical reactions). However, ionic polymerization has much more control over the polymerization process.

Sensitivity

• Free radical polymerization is less sensitive to impurities like water or oxygen. Ionic polymerization is highly sensitive to impurities like water and other species that can deactivate the charged species.

Industrial Scale

• Ionic polymerization is more difficult to carry out on an industrial scale than free radical polymerization.

Conclusion

Ionic and free radical polymerization are two main mechanisms for creating polymers. Ionic polymerization uses charged ions (cations or anions) for controlled chain growth. Free radical polymerization uses free radicals to make the polymer. Polymers made from each type of polymerization have a wider use in various fields across various industries.

FAQ: Ionic and Free Radical Polymerization

1. What is the major difference between free-radical and cationic polymerization mechanisms?

• The main difference between the two polymerizations lies in the reactive species that carry the chain. Free-radical polymerization uses free radicals with an unpaired electron while cationic polymerization uses positively charged species called cations.

2. What are the advantages of ionic polymerization?

• The advantages of ionic polymerization include having narrow molecular weight distribution, having controlled tacticity, controlled polymerization, having lower reaction temperatures, and obtaining highly regular polymers.

3. What are the applications of ionic polymerization?

• Ionic polymerization is seen in making block polymers, high-performance polymers, functional polymers, and in advanced materials research. For example, ionic polymerization is seen in applications such as organic electronics and photovoltaics, biocompatible materials for medical devices, and nanocomposites with unique properties.

4. What are the advantages of free radical polymerization?

• Some of the advantages of free radical polymerization are simplicity and versatility, cost effectiveness, fast reaction rates, ability to form a broad range of products and its tolerance to impurities.

5. What are the applications of free radical polymerization?

• Free radical polymerization is used in making polymers in the industry due to its simplicity, versatility, and cost effectiveness. It is used to make bulk polymers, such as polyethylene, polypropylene, polystyrene, polyvinyl chloride and elastomers, and fibers. It is also used in making polymers in adhesives and coatings. In addition, it has biomedical applications.

Reference:

1. “Ionic Polymerization.” Science Direct.
2. “Free Radical Polymerization.” Science Direct.

Image Courtesy:

1. “RadicalPolymerization ethylene” By V8rik at English Wikipedia (CC BY-SA 3.0) via Commons Wikimedia