# Polymerization

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## 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:

 n CH2 = CH2 → (CH2 - CH2-)nEthylene         Polyethylene

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

• 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:

2. Cationic polymerization

3. Anionic polymerization

Let’s see polymerization examples for these three types:

• 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.

 In•                          CH2 = CH2Initiator + CH2 = CH2  →  In - CH2 -CH2•      →          In-CH2-CH2-CH2-CH2• →  In-(CH2-CH2)n

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:

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:

Let’s see another example using Sodium Amide nucleophile:

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:

 Condense M + N       →      M - N      Monomer

## Step 2:

 CondenseM - N + M     →      M - N - M

## Step 3:

 CondenseM - N - M + N         →        M - N - M - N

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

## Step 1:

 Condense M + N       →       M - N       Monomer

## Step 2:

 M - N M - N     →       M - N - M - N - M - N -……. (M - N)nPolymer

• 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

 Mn$^{-}$ = $\frac{N_{i}M_{1}+N_{2}M_{2}+N_{3}M_{3}+....}{N_{1}+N_{2}+N_{3}+.....}$

So, the formula can be re-written as:

 Mn$^{-}$ = $\frac{\sum N_{i}M_{i}}{\sum N_{i}}$

Q1: Write Applications of Polymers.

Ans: Polymers are used in every vicinity of modern living. Some of the real-life applications are:

1. Grocery bags

2. Soda and water bottles

3. Textile fibers

4. Computers

5. Phones

6. Auto parts

Q2: Describe the Formation of Bakelite by a Condensation Reaction.

Ans:

• These polymers are made by the reaction of phenol with formaldehyde in the presence of a basic catalyst OH⁻.

• The reaction involves the formation of methylene bridges in ortho, para, or both, as shown below:

• The last product formed in the para-para position is a linear polymer.

• Since Bakelite is a cross-linked thermosetting polymer, the reaction between o-hydroxyphenyl polymer and p-hydroxyphenyl occurs in the following manner to form a cross-linked polymer (Bakelite):

Q3: Why is Polymerization Exothermic?

Ans: In a polymerization process, the addition of a monomer to a growing polymer chain involves the conversion of π to σ-bond with a release of high energy. That’s why polymerization is exothermic.

Q4: In a Particular Sample of Polymer, 300 Molecules have a Molecular Mass 10⁴ Each, 300 Molecules Have Mass 10³ Each, and 200 Molecules Have 10⁵ Each. Calculate the Number-average Molecular Mass.

Solution: Here, N₁ = 300, M₁ = 10⁴  , N₂ = 300, M₂ = 10³, N₃ = 200, M₃ = 10⁵, Mn⁻ = ?

Now using the formula:

Mn⁻ = (NᵢM₁ + N₂M₂ + N₃M₃ +....)/(N₁ + N₂ + N₃ +....) = ((300 x 10⁴) + (300 x 10³) + (200 x 10⁵))/(300 + 300 +200)

On solving, we get,

 Mn⁻ = 2.9 x 10⁴ or 29000