Polymerization

What is Polymerization?

Polymerization is a process through which a large number of monomer molecules react together to form a polymer. The macromolecules produced from a polymerization may have a linear or a branched structure. They can also assume the shape of a complex, three-dimensional network. There exist several different categories of polymerization reactions, the most notable of which being step-growth polymerization, chain-growth polymerization (both of which fall under the category of addition polymerization), and condensation polymerization. 

A polymer is a substance that is made up very large molecules that are, in turn, made up of many repeating units called monomers. Polymerization is the process through which these monomers come together to form the macromolecules that constitute polymers. An illustration detailing the polymerization of the monomer styrene into the polymer known as polystyrene is provided below. 

Polymerization of Styrene

Depending on the functional groups present in the reacting monomers, the complexity of the mechanism of the polymerization reaction may vary. The most simple polymerization reactions involve the formation of polymers from alkenes via free-radical reaction. Polyethylene, which is one of the most commercially important polymers, is prepared via such a polymerization process (the reactant monomer used here is ethylene). 

It can be noted that polymerizations involving only one type of monomer are called homopolymerizations whereas those involving more than one type of monomer are called copolymerization processes. Polymerization in the simplest form can be described as a chemical process that results in the formation of polymers. In simple words, we can basically describe Polymerization as the process of creating polymers. When polymerization occurs, the smaller molecules which are known as monomers via chemical reaction are combined to form larger molecules. A collection of these large molecules form a polymer. The term polymer in general means “large molecules” with higher molecular mass. They are also referred to as macromolecules.

Table of Content

Polymers are formed by the addition of a network of structural units or monomers as mentioned above. The interesting part is that these are reactive molecules and are usually linked to each other by covalent bonds. These monomers add together to form a long chain to form a product with specific properties. This whole process of formation of polymers is polymerization. Polythene and Nylon 66 are some examples of polymers.

Mechanism of Polymerization

Generally, polymerization consists of three steps which include initiation, propagation, and termination. As for the reaction mechanism, the process of polymerization mainly involves two different methods. These include the step-growth mechanism and chain-growth mechanism.

Step Growth Polymerization

In step-growth polymerization, the polymers are formed by the independent reaction between the functional groups of simple monomer units. In step-growth, each step may consist of a combination of two polymers having a different or same length to form a longer length molecule.

The reaction is a lengthy process and the molecular mass is increased at a very slow rate. An example of step-growth polymerization is condensation polymerization where a water molecule is evolved in the reaction when the chain is lengthened.


Condensation Polymerization

In condensation polymerization, the formation of the polymer occurs when there is a loss of some small molecules as byproducts through the reaction where molecules are joined together. The byproducts formed may be water or hydrogen chloride. Polyamide and proteins are examples of condensation polymers.

Some of the different types of condensation polymerization are given below

Polyamides

They are synthetic fibres and are called as nylons. These polymers have an amide linkage between them. Condensation polymerization of di-amines with di-carboxylic acid and also of amino acids and their lactams will create a polyamide.

  • Nylon 66: This polymer is prepared under the condition of high pressure and temperature by the condensation polymerization of hexamethylenediamine with adipic acid.
  • Nylon 6: Prepared by heating of caprolactam with water under high temperature. It is used for tyre cords, fabrics and ropes.

Polyesters

When dicarboxylic acids and diols undergo polycondensation, polyesters are formed. Prepared by heating a mixture of terephthalic acid and ethylene glycol at 460 k by using zinc acetate antimony trioxide as a catalyst. Dacron or terylene are the best-known examples for polyesters. And also they are used for glass reinforcing materials in safety helmets.

Phenol-Formaldehyde Polymer

These are the old synthetic polymers, obtained by condensation polymerization of phenol with formaldehyde in the presence of either an acid or base as a catalyst.

Phenol – Formaldehyde Polymer

Novolac on heating with formaldehyde undergoes crosslinking and forms an infusible sold mass named as Bakelite. They are used for combs, electric switches and phonograph records.

Melamine – Formaldehyde Polymer

It is formed by the condensation polymerization of melamine and formaldehyde in certain conditions. They are used for the manufacture of unbreakable crockery.

Chain Growth Polymerization

In chain-growth polymerization, the molecules of the monomers are added together to form a large chain. The monomers adding maybe the same type or different. Generally, alkenes, alkadienes and their derivatives are used. In this mode, lengthening of chains occurs as a result of the formation of either free radicals or ionic species.

Free Radical Mechanism

Many of the monomers like alkenes or dienes and their derivatives are polymerized in the presence of free radicals. In the polymerization of ethene to polythene by heating or exposing to light by using a small amount of benzoyl peroxide initiator. The phenyl free radical formed by peroxide is added to the ethene double bond and hence formed a new larger free radical.

Also Read: Chemical Bonding

It is called a chain initiation step. This newly formed radical will reacts with another molecule of ethene to form another new free radical and so on. This repeated formation of a new free radical is called chain propagation. Finally, at some stage, the polymerized product will be formed and this step is called a chain termination step. The steps are given below.

The three steps followed by a free radical mechanism

Free Radical Mechanism

Polymerization Chemical Reaction

When we talk about polymerization chemical reaction we are basically referring to a polymerization reaction of organic monomers. These monomers are in a solution which further consists of particles that will be coated with the formed polymer deposited on the particle surface. This leads to the formation of a coating layer. The reaction includes either monomer adsorption polymerization and emulsion polymerization.

Preparation of Polymers

Polyethene

We have two types of polyethylenes and they are given below:

1. Low-Density Polyethene

This type of polymers is obtained by the polymerization of ethene under the condition of high pressure of 1000 to 2000 atmospheres at 350 to 520 k temperature in the presence of dioxygen or peroxide initiator as a catalyst in a very small amount.

It is formed through the free radical addition and H-atom abstraction having a highly branched structure. It is chemically inert in nature and tough but flexible. It is a poor conductor of electricity. LDP is used for the manufacture of toys, squeeze bottles and flexible pipes.

2. High-Density Polyethene

Prepared by the polymerization addition of ethene in the presence of a catalyst like triethyl aluminium and titanium tetrachloride. The process takes place in a hydrocarbon solvent, in a condition of low pressure of 3 to 4 atmospheres and 343 k temperature.

Like the LDP it is chemically inert but comparatively tougher and harder. It is used for the manufacture of buckets, dustbins, pipes, etc.

Polytetrafluoroethylene

It is also known as Teflon and is manufactured by heating of tetrafluoroethylene with a free radical at high pressure. Teflon is chemically inert and less corrosive due to the resistive property against the corrosive agents. It is used for gaskets and not stick surface coated utensils.

Polytetrafluroethene

Polyacrylonitrile

This polymer is formed by the addition polymerization of acrylonitrile in the presence of a peroxide catalyst. It is used as a substitute for wool in making of commercial fibres as Acrilan.

Polyacrylonitrile

Anionic Polymerization

It is an addition polymerization that involves the polymerization of monomers that are initiated with anions. This polymerization will be initiated by the transfer of electrons from the ion to the monomer.

The initiators used may be weakly nucleophilic if the monomer is highly electrophilic. In the propagation, the complete consumption of monomer occurs and this will be faster even at low temperature. Generally, vinyl monomers are polymerized by this method. It is very sensitive to the solvent used in the reaction. This method is used for the production of synthetic polydiene rubbers, SBR and thermoplastic styrene elastomers.

Example reaction for anionic polymerization

Classification of Polymerization

Polymers are classified into different categories based on several factors such as source, structures, mode of polymerization, molecular forces and growth of polymers. Let’s discuss them in detail below.

Based on Source

They are again divided into three subcategories:

  1. Natural polymers: They are found naturally in plants and animals. Resins, starch and rubber are examples of this.
  2. Semi-synthetic polymers: This is a modified version of natural rubber. That means natural rubbers are treated with chemicals to make them semi-synthetic. Cellulose acetate and cellulose nitrate are the examples come under this subcategory.
  3. Synthetic polymers: Polymers which are completely man-made are called synthetic polymers. Polythene, Nylon 66, synthetic rubber are the widely used synthetic polymers

Based on the Structure of Polymers

There are three different types.

Structure of Polymers

  1. Linear polymers: Consists of a long and straight-chain of monomers. PVC is a linear polymer
  2. Branched polymers: They are linear polymers containing some branches. Low-density Polythene is an example.
  3. Network or cross-linked polymer: Polymers having cross-linked bonds with each other is called cross-linked or network polymer. Generally, they are formed from bi-functional or tri-functional monomers. Bakelite and melamine are examples of this type of polymer.

Based on Mode of Polymerization

They are divided into two subcategories:

  1. Addition Polymers: Polymers formed by the repeated addition of monomers by possessing the double or triple bonds. If the addition is of the same species they are called homopolymers and if the addition is of different monomers they are called copolymers. Examples are polythene and Buna-s respectively.
  2. Condensation polymers: These polymers are formed by repeated condensation of tri or bifunctional monomeric units. In this reaction elimination of some small molecules like water and hydrogen chloride etc will take place. Terylene and Nylon 6,6 are examples.

Based on Forces Between Molecules

They are again classified into four subgroups.

  1. Elastomers: Polymers that are rubber-like solids and having elastic properties. Here the polymer bonds are held together by weak intermolecular forces and that allows these polymers to stretch. The cross-links present in the polymer between the chains helps to retrace the original position after the removal of the applied force. Examples are Buna-s and Buna-s
  2. Fibres: They are polymers having strong intermolecular forces like hydrogen bonding. Due to this strong force molecules are kept closer, that is they are closely packed. Because of this property, they are crystalline in nature. Polyamide and polyesters are examples.
  3. Thermoplastic polymers: These are the liner or slightly changed to branched polymers that can be softened on continues heating and hardening on cooling.Their intermolecular force lying in between the fibres and elastomers. Polyvinyls, polystyrene etc are examples of thermoplastic polymers.
  4. Thermosetting polymers: Polymers comes under the category of heavily branched or cross-linked, which can mould on heating and can’t regain the original shape. So these cannot be reused. Bakelite is an example.

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