Maharashtra Board Class 12 Chemistry Chapter 15 Introduction To Polymer Chemistry PDF Download

Introduction To Polymer Chemistry

Today the overall development in polymer science and technology has enriched human life. The world would be at totally different place without polymers such as artificial fibres, plastics and elastomers. From the throwaway candy wrapper to the artificial heart, polymers touch our lives as does no other class of material. In short we are living in the world of polymers. Polymer chemistry emerged as a separate branch of chemistry during the last several decades due to the voluminous knowledge built up in this field and the ever increasing applications in everyday life.

Chemically polymers are complex, giant macromolecules made from the repeating units which are derived from small molecules called monomers. The term polymer originates from Greek word poly meaning many and mer meaning part or unit. Interlinking of many units constitutes polymers.

Polymers are high molecular mass macromolecules \(10^3 - 10^7 \text{ u}\).

Both inorganic as well as organic polymers are known. In this chapter we will study some aspects of organic polymers. You have learnt in Chapter 14 about carbohydrates, proteins and nucleic acids which are important organic biopolymers playing crucial role in living world.

In this chapter we will consider mainly man made organic polymers with reference to aspects such as types, preparation and applications.

Teacher's Note

Polymers are all around us in daily life. For example, plastic bags, water bottles, and clothes are all made from polymers. Understanding polymers helps us know what we use every day.

Exam Trick

Remember: Polymer = Poly (many) + Mer (part). Just like many bricks make a wall, many monomers join together to make a polymer.

Points to Remember

Polymers are giant molecules made by joining many small units called monomers.
The word polymer comes from Greek words meaning many parts.
Polymers have very high molecular mass from 10 to the power 3 to 10 to the power 7.
Both natural and man-made polymers exist in the world.

Classification Of Polymers

Polymers are classified in number of ways on the basis of their source, chemical structures, mode of polymerization, molecular forces, type of monomers and biodegradability.

Classification Of Polymers On The Basis Of Source Or Origin

Polymers are divided into three categories: Natural, Synthetic, and Semisynthetic.

a. Natural Polymers

The polymers obtained from natural source are said to be natural polymers. They are further subdivided into two types.

i. Plant polymers: These are obtained from plants. For example, cotton and linen are obtained from cotton plant and flax plant respectively. Natural rubber is another example of natural polymer which is manufactured from the latex obtained from bark of rubber trees.

ii. Animal polymers: These are derived from animal sources. For example, wool is obtained from hair of sheep. Silk is obtained from silkworm.

b. Synthetic Polymers

These are man-made polymers. These polymers are artificially prepared by polymerization of one monomer or copolymerization of two or more monomers. Nylon, terylene, neoprene are synthetic polymers. These are further divided into three subtypes, namely, fibres, synthetic rubbers and plastics.

c. Semisynthetic Polymers

These are derived from natural polymers. These are also called regenerated fibres. Cellulose derivatives such as cellulose acetate rayon, cellulose nitrate, viscose rayon, cuprammonium rayon are a few examples of this category.

Semisynthetic polymers are used in preparation of non-inflammable photographic films, cinema films, varnishes, etc.

Teacher's Note

Natural polymers come from nature like rubber from trees. Synthetic polymers are man-made like plastic. India produces a lot of synthetic polymers in factories.

Exam Trick

Remember three types: Natural (from nature), Synthetic (man-made), and Semisynthetic (from natural but changed). Think Natural = Nature, Synthetic = Science made.

Points to Remember

Natural polymers come from plants and animals in nature.
Synthetic polymers are made by humans in factories.
Plant polymers include cotton and natural rubber.
Animal polymers include wool and silk.
Semisynthetic polymers are made by changing natural polymers.

Classification Of Polymers On The Basis Of Structure

Depending upon how the monomers are linked together, that is, the chain configuration, polymers are classified in three general types: linear, branched and three dimensional cross-linked polymers. The nature of linking the monomers depends upon the nature and number of functional groups in them.

a. Linear Or Straight Chain Polymers

When the monomer molecules are joined together in a linear arrangement the resulting polymer is straight chain polymer. It is obtained from bifunctional monomers or alkenes. For example: PVC, high density polythene.

b. Branched Chain Polymers

The second most common arrangement is that of branched chain. Monomer having 3 functional groups or already having side chains give rise to branched chain polymers. For example: low density polythene.

c. Cross-linked Polymers

Third type of arrangement is said to be cross linked or network polymers where cross links are produced between linear chains. Cross linking results from polyfunctional monomers. For example, bakelite, melamine.

Teacher's Note

Think of polymer chains like roads. Linear chains are straight roads. Branched chains have side roads. Cross-linked chains are roads that connect to each other making a network.

Exam Trick

Linear = straight line, Branched = has side branches like a tree, Cross-linked = chains holding hands with each other. Remember the shapes to identify them quickly.

Points to Remember

Linear polymers are straight chains of monomers joined in a line.
Branched polymers have side branches coming out from the main chain.
Cross-linked polymers have bridges connecting different chains together.
The type of polymer depends on the functional groups in the monomer.
Each type has different strength and properties.

Classification Of Polymers On The Basis Of Mode Of Polymerization

Polymerization is the fundamental process by which low molecular mass compounds are converted into high molecular weight compounds by linking together of repeating structural units with covalent bonds. There are three modes of polymerization according to the types of reactions taking place between the monomers: Addition polymerization, Condensation polymerization, and Ring opening polymerization.

a. Addition Polymerization

Addition polymerization is a process of formation of polymers by addition of monomers without loss of any small molecules. The repeating unit of an addition polymer has the same elemental composition as that of original monomer.

Compounds containing double bond undergo addition polymerization. It is also referred as vinyl polymerization, since majority of monomers are from vinyl category. For example: vinyl chloride \(\text{CH}_2 = \text{CHCl}\), acrylonitrile \(\text{CH}_2 = \text{CHCN}\). Formation of polyethylene from ethylene is well known example of addition polymerization.

Free radical mechanism is most common in addition polymerisation. It is also called chain reaction which involves three distinct steps: chain initiation, chain propagation and chain termination.

Step 1: Chain initiation: The chain reaction is initiated by a free radical. An initiator (catalyst) such as benzoyl peroxide, acetyl peroxide, tert-butyl peroxide, etc. can be used to produce free radical.

Step 2: Chain propagation: The new radical formed in the initiation step reacts with another molecule of vinyl monomer, forming another still bigger sized radical, which in turn reacts with another monomer molecule. The repetition of this sequence takes place very rapidly. It is called chain propagation.

Step 3: Chain termination: Ultimately, at some stage, termination of the growing chain takes place. It may occur by several processes. One mode of termination is by combination of two growing chain radicals.

Teacher's Note

Addition polymerization is like joining beads on a string without losing anything. Each bead adds to the string. Plastic bags are made this way from ethylene monomer.

Exam Trick

Remember: Addition = no loss of atoms. In addition polymerization, all atoms in monomers become part of the polymer. Nothing is thrown away.

Points to Remember

Addition polymerization joins monomers without losing any small molecules.
Double bonds in monomers are broken during this process.
Free radical mechanism has three steps: initiation, propagation, and termination.
Polyethylene is an important example of addition polymerization.
The repeating unit has the same composition as the monomer.

b. Condensation Polymerization

Consider the formation of terylene, a polyester polymer, from ethylene glycol and terephthalic acid. In this reaction an alcoholic OH group in ethylene glycol condenses with a carboxyl group in terephthalic acid by eliminating a water molecule to form an ester linkage.

The process of formation of polymers from polyfunctional monomers with the elimination of some small molecules such as water, hydrochloric acid, methanol, ammonia is called condensation polymerization.

In this type of polymerization the chain growth occurs by a series of condensation steps. Therefore it is also referred to as step growth polymerization.

Teacher's Note

Condensation polymerization is like joining things and losing something. For example, when you join two pieces of cloth with water-soluble glue, the water goes away. Nylon is made this way.

Exam Trick

Remember: Condensation = loss of atoms. Small molecules like water are released when condensation polymerization happens. Think: Condense = squeeze out water.

Points to Remember

Condensation polymerization loses small molecules like water or ammonia.
Monomers have different functional groups that react with each other.
This process also called step growth polymerization.
Nylon and terylene are important examples.
The repeating unit has different composition than the monomer.

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