What are polymerization reactions?

The method of formation of macromolecules from the combination of repeating structural units is identified as polymerization. The macromolecule produced in the method is known as polymer, whereas the repeating structural units are identified as monomers.

Overview of polymerization reactions

The polymerization reactions are the method of chemical combination of repeating structural units termed monomers to generate macromolecules identified as polymers. It can be assumed that polymers are genuinely a huge chain of molecules or an arrangement of molecules. Initially, the name polymerization was employed as a characteristic term for such methods that led to the development of products with identical empirical formulas but variable molecular weight from a single substance. Those single substances included in the technique were coined as monomers, and the product developed was the multiple of these monomers.

But later on, it was found that upon suitable treatment, the compounds with great molecular weight produced a large number of monomers, but they didn't require having the same empirical formula as the parent monomer. This finding led to the foundation of the modern definition of the terms "polymer" and "polymerization". Ideally, Carothers described polymerization as "the intermolecular compounds holding the ability to progress infinitely". Yet, functionally, the method of polymerization does not continue infinitely. Later developing to a specific limit relying upon several situations, the chain ends, and so termination is an essential step in polymerization.

What are the reaction mechanisms of polymerization reactions?

If we speak about the reaction mechanism of the polymerization reactions, then there are two kinds of reaction mechanisms followed by the polymerization reactions. They are:

• Step growth or condensation polymerization
• Addition polymerization or chain-growth polymerization

Step growth or condensation polymerization

In step-growth polymerization, two monomers are coupled with the elimination of small molecules, usually water or alcohol. This kind of polymerization happens if the monomer molecules hold more molecules than the functional group. The polymer's average molecular weight produced by condensation polymerization decreases when a reactive group gets combined with a monomer. Hence the density and average crosslink molecular weight help to estimate the functionality of each monomer. For example, terylene is created by step-growth polymerization of ethylene glycol and terephthalic acid.

Addition polymerization or chain-growth polymerization

Polymers that are created when the monomers join successively are identified as chain-growth polymers, and the method is recognized as chain-growth polymerization. There are three different methods of chain-growth polymerization:

• Free radical polymerization 
• Cationic polymerization 
• Anionic polymerization

Free radical polymerization 

Free-radical polymerization is performed by alkenes and dienes in the appearance of initiators like benzoyl peroxide, tertiary butyl peroxide, and acetyl peroxide. For instance, the polymerization of ethane to polythene is inducted by a small quantity of benzoyl peroxide in the presence of heat or light. The process is completed in three steps:

Chain initiation step

By the title itself, it is obvious that initiation is the first step of the polymerization reaction. Throughout this stage, an active center is produced for the production of the polymer chain. The step of initiation further includes two sub-steps.

• Radicals (usually one or two in number) are produced from the initiating molecules.
• The generated radicals get shifted from the initiating molecules to the units of monomers present.

Chain propagation step

Propagating indicates rising in numbers or expansion in a solid body. Therefore, it is obvious that throughout the method of polymerization, this step includes the extension of the chain length. The method of propagation begins with the attack of the radical initiator on the monomer and extends till the termination, such that the propagation proceeds till all the monomers last.

Chain termination step

Due to the great reactivity of radicals in the polymerization reaction, the chain termination is defined. In this step, the free radicals join to produce polythene.

Cationic polymerization

In cationic polymerization, the initiator is an electrophile that supplements the alkene, making it become a cation. The initiator employed is most often BF_₃ or AlCl_₃. Cationic polymerization is performed in three steps: initiation, propagation, and termination. Some of the alkenes which experience cationic polymerization are propylene, isobutylene, vinyl chloride, and styrene.

Anionic polymerization

In anionic polymerization, the initiator is a nucleophile that reacts with the alkene to produce an anion. The chain extension for each chain would be started simultaneously when catalyst and monomers are mixed. Some popular initiators are sodium amide or butyl lithium. The nucleophile must include an electron-withdrawing group to make it strong. An example is the polymerization of acrylonitrile in the appearance of butyllithium.

Classification of polymers based on origin

On the basis of the origin, the polymers are classified into the following three categories:

• Natural polymers
• Synthetic polymers
• Semi-synthetic polymers

Natural polymers

Natural polymers happen naturally in the world around us and are derived from animals and plants. Examples comprise proteins, cellulose, starch, and rubber.

Synthetic polymers

Synthetic polymers are man-made polymers with huge industrial applications. Plastic is one such polymer. Other examples are nylon and polyether. A catalyst like titanium trichloride is known as the Ziegler-Natta catalyst. The use of this catalyst in polymer synthesis is that they promote the formation of only specific isomers.

Semi-synthetic polymers

Semi-synthetic polymers are polymers in which the primary products or components used are naturally found, but chemicals are utilized to transform their characteristics. Some examples are cellulose nitrate and cellulose acetate.

Classification of polymers based on the structure

On the basis of the structure, the polymers are classified into the following two categories:

• Crystalline polymers
• Amorphous polymers

Crystalline polymers

Polymeric chains are neatly organized to create long chains and long order derivatives. Crystallinity in the composition of polymers accounts for higher strength and rigidity. There is a very limited chance of the development of a perfectly crystallized polymer. Mostly semi-crystallized polymers are produced. The maximum crystalline order received can be around 95%.

Amorphous polymers

Polymeric chains are randomly stacked and do not have any set pattern. The molecules are randomly jumbled in amorphous polymers, which provide elasticity and flexibility to the composition. The amorphous nature relies on the functional group that is joined to the monomer. A bigger functional group raises disorientation and therefore enhances the amorphous nature of the polymer.

Classification of polymers based on their physical properties

On the basis of their physical properties, the polymers are classified into the following three categories:

• Thermoplastic polymers
• Thermosetting polymers
• Elastomers

Thermoplastic polymers

The thermoplastic polymer is hard at room temperature but can be softened and molded upon warming. These can be crystalline, amorphous, or both. Polymers soften on warming as polymer chains are bonded by weak Van der Waal forces. When they are heated, the molecular chains get sufficient energy to defeat these Van der Waal forces, and they become viscous. Once the heat is extracted, the secondary forces come into play again, thus creating these polymers reusable. Some of the examples of thermoplastics are polypropylene, polyvinyl chloride, and nylon.

Thermosetting polymers

Thermosetting polymers once hardened cannot be remolded again. Hence, these are not reusable. Polymer chains are held together by a cross-linked covalent bond. On warming, these bonds are destroyed and cannot be repaired on cooling. They are produced from amorphous crystals only. Some of the examples of thermosetting polymers are epoxy resin, melamine-formaldehyde, and polyester resin.

Elastomers

Elastomers are plastics that expand and then return to their initial shapes. These are amorphous crystals and randomly oriented. On warming, they can get deformed without any perpetual impairment to their composition. Some examples are natural rubber, silicone elastomers, and polyisoprene.

Classification of polymers based on types of monomers involved

On the basis of the types of monomers involved, the polymers are classified into the following two categories:

• Homopolymer
• Copolymer

Homopolymer

Homopolymers have only one repeating unit or monomer.

Copolymer

Copolymers have more than one monomer that polymerizes to create the larger polymer. The order of repetition is the same every time and does not vary.

Common Mistakes

Students get confused between the polymer and monomer. A monomer is a single unit of the polymer which is formed when multiple numbers of monomers are joined together through the chemical reaction.

Students get confused between the thermoplastic polymer and thermosetting polymer. The addition of polymerization usually makes the thermoplastic polymer, and it is expensive, while the thermosetting polymer is usually produced by condensation polymerization, and it is cheap.

Context and Applications

The topic of the polymerization reaction is very much significant in the several professional exams and courses for undergraduate, diploma level, graduate, and postgraduate. For example:

• Bachelor of Technology in Mechanical Engineering
• Master of Technology in Material Science Engineering
• Bachelor of Science in Chemistry
• Master of Science in Chemistry
• Bachelor of Science in Physics

Related Concepts

• Plasma polymerization
• Polymer physics
• Polymerization of isobutylene
• Polymer characterization
• Polystyrene

Practice Problems

Q1: Among the four options, which of the following can not support additional polymerization?

(a) Propylene

(b) Vinyl benzene

(c) Ethane

(d) Ethene

Correct option: (c)

Explanation: Polymers created when the monomers join successively are identified as chain-growth polymers, and the method is recognized as additional polymerization. Propylene, vinyl benzene, and ethene are the components that support additional polymerization.

Q2: Which of the following is employed in non-stick vessels?

(a) LDP

(b) HDP

(c) Orlon

(d) Teflon

Correct option: (d)

Explanation: Non-stick vessels are generally coated with poly-tetra-fluoro-ethylene (PTFE). The polytetrafluoroethylene is also commonly named Teflon. It is non-toxic material and safe to use.

Q3: Which of the following is not a kind of additional polymerization?

(a) Anionic polymerization

(b) Cationic polymerization

(c) Polycondensation polymerization

(d) Free radical polymerization

Correct option: (c)

Explanation: The additional polymerization is described as a process in which polymers are created when the monomers join successively as chain-growth polymers. The different methods of additional polymerization are free-radical polymerization, cationic polymerization, and anionic polymerization.

Q4: Which of the following is not a proper initiator for free radical addition polymerization reaction?

(a) Benzoquinone

(b) Benzoyl peroxide

(c) Acetyl peroxide

(d) Tert-Butyl peroxide

Correct option: (a)

Explanation: Free-radical polymerization is performed by alkenes and dienes in the appearance of initiators like benzoyl peroxide, tertiary butyl peroxide, and acetyl peroxide. For instance, the polymerization of ethane to polythene is inducted by a small quantity of benzoyl peroxide in the presence of heat or light.

Q5: Polymerization of vinyl cyanide with peroxide catalyst creates:

(a) HDP

(b) PVC

(c) PET

(d) PAN

Correct option: (d)

Explanation: Polyacrylonitrile (PAN) is a polymerized product formed by the combination of vinyl cyanide and peroxide catalyst.