Polymerization

What is Polymerization?

Before understanding the polymerization, let us understand what polymers are.

• The word polymers are formed from two Greek words, poly and mirrors, where poly means many and mirrors mean parts). So, polymers mean many parts.

• The polymer is a compound of high molecular weight formed by the combination of several molecules. The small units that constitute the repeating units of a polymer are called the monomer units.

• So, the process by which these small units (simple molecules or monomers) transform into a polymer is called the process of polymerization or simply polymerization.

• Let’s look at a polymerization example of ethylene:

Here, the ethylene molecules are considered monomer units.

• Polymers are giant molecules, so they are called the macromolecules. 

Let’s understand the types of polymerization:

Types of Polymerization

We generally use two major methods for preparing polymers (methods of polymerization). They are:

1. Addition polymerization or chain-growth polymerization

2. Condensation polymerization/ step-growth polymerization

1. Addition Polymerization 

• The polymers that are formed when the monomeric units can add to each other successively are known as chain-growth polymers. 

• This process of addition polymerization involves the addition of monomer units of the growing chain by a chain mechanism. That’s why this process is known as a chain-growth mechanism.

• The chain-growth process involves the formation of some active intermediate species, which may be free radical, cation or anion. So, the methods of polymerization for these species are:

1. Radical polymerization

2. Cationic polymerization

3. Anionic polymerization

Let’s see polymerization examples for these three types:

a. Free Radical Addition Polymerization

• In this process, unsaturated compounds like alkenes or dienes are polymerized.

• A radical initiator is added to the alkene (also known as a monomer) to convert it into radical. 

• The initiator breaks into radicals and adds to the alkene monomer, converting it into a radical.

• This alkene radical reacts with another monomer, and this process keeps on propagating the chain, and the chain formation continues endlessly.

a. Cationic Polymerization 

• In this case, the initiator is an electrophile (BF3 or Al3Cl3)  that adds to an alkene causing it to become a cation.

• The cation formed in the initiation process reacts with the second monomer forms a new cation, and this process continues forever.

• The different stages of polymerization are as follows:

[Image will be Uploaded Soon]

b. Anionic Polymerization

• In this process, the initiator is a nucleophile that reacts with the alkene to form a propagating site, i.e., an anion.

•  However, the attack of a nucleophile isn’t an easy reaction because alkenes are electron-rich species.

• So, we will use an alkene with an electron-withdrawing substituent attached to it, besides considering a strong nucleophile (Sodium Amide or butyllithium) here.

Now, let us see the step-by-step anionic polymerization using butyllithium nucleophile:

[Image will be Uploaded Soon]

Let’s see another example using Sodium Amide nucleophile:

[Image will be Uploaded Soon]

Here, R is an electron-withdrawing group.

Some common examples of alkene that undergo anionic polymerization are:

1. Vinyl Chloride

2. Acrylonitrile

3. Methyl Methacrylate

4. Styrene

Now, let’s discuss the second type of polymerization:

2. Condensation Polymerization

• In the condensation or step-growth polymerization, a stepwise intermolecular condensation takes place through a series of independent reactions. 

• Each reaction involves a condensation process involving the release of a simple molecule like NH3 or H2O, or HCl, etc.

• This reaction occurs when monomer molecules have more than one similar or dissimilar functional groups.

• Let’s illustrate the step-growth polymerization most simply by taking monomers M and N:

Step 1: 

Step 2:  

Step 3:          

This stepwise process of chain growth goes on infinitely. We can represent the same process in another way. Let’s see:

Step 1: 

Step 2:  

• The condensation polymers like dacron, bakelite, and nylon are formed by this polymerization process.

Molecular Mass of Polymers

During the formation of polymers, different macromolecules have varying degrees of polymerization (different chain lengths). This means molecular masses of the individual macromolecules in a particular polymer are different.

Let’s suppose that a polymer sample has the following:

N1 molecules have molecular mass M1 each

N2 molecules have molecular mass M2 each

N3 molecules have molecular mass M3 each,……and so on.

Then,

The total mass of all N1 molecules = N1M1

The total mass of all N2 molecules = N2M2

The total mass of all N3 molecules = N3M3..., and so on.

Now, the total mass of all the molecules = N1M1 +  N2M2 + N3M3 +....

                                                             = Σ NiMi

Total number of all the molecules = Σ Ni

Hence, the number-average molecular mass is given by

So, the formula can be re-written as: