NEET Study Notes for Polymers: Polymers are formed from the combination of similar or different molecules that vary in size. These are also known as macromolecules. Polymers can mainly be classified into natural and synthetic polymers based on the source of their availability

• This process of formation of molecules is known as polymerization. The degree of polymerization is the number of macromolecules that form the polymer.
• NEET Chemistry Syllabus is mainly divided into topics from Class 11th and 12th. Polymers are a part of Class 12th syllabus for Organic Chemistry.
• With an overall weightage of around 34% in Organic chemistry, candidates can expect around 2-3 questions from this unit in NEET 2022.

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Important Topics in NEET Polymers

Polymers and their classification

Based on the source of origin

• Natural Polymers- Polymers that are derived naturally from natural sources like plants and animals Biopolymers are biodegradable polymers.

Examples of Natural Polymers are:

• Semi-Synthetic Polymers-these polymers are obtained from natural sources. However, they are synthesized further by chemical processes. Examples- are cellulose acetate, cellulose nitrate.
• Synthetic Polymers- Polymers that are produced by synthesis or man-made processes are known as synthetic polymers. Examples- are nylon, PVC, polystyrene, plastic which have wide industrial applications.

Based on the structure:

The polymers that are formed by the union of small molecules are called monomers. Based on the structure of this chain of monomers, classification can be done into

• Linear Polymers- when the polymer structure is such that monomeric units form long and chain-like structures it is known as linear polymers. These are placed on top of each other and appear as a packed structure. These polymers have density, tensile strength, and high melting and boiling point due to the tightly-packed structure. Example- fiber, plastics, PVC (Polyvinyl chloride), etc.

• Branched-chain polymers- When the long chain of polymers forms branches of varying lengths, these are known as branched-chain polymers. They have a low density and low melting and boiling point due to the loosely packed structure. Example- glycogen, amylopectin, low density of polythene.

• Cross-Linked- Polymers- These are also known as three-dimensional polymers they are mainly composed of trifunctional or bifunctional monomers. The covalent bond of cross-linked polymers is strong. Examples- bakelite, melamine.

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Based on the mode of Polymerisation

Polymers can be divided into

• Addition Polymers- addition polymers are formed when there is no removal of any elements. The repeated addition of double- triple bond monomer molecules is called addition polymers. Examples are polyethylene, polypropylene.

• Condensation Polymers- These polymers are formed on the removal of water, ammonia, alcohol. A repeated condensation process takes place amongst different bifunctional or trifunctional monomeric units. Examples- terylene, Nylon 6.6 formed by condensation of adipic acid and hexamethylenediamine.

Based on Intermolecular Forces

• Elastomers- the intermolecular force of attraction between polymers chains is weak. Crosslinks amongst elastomers help are prevention chains from slipping. On joining linear polymer chains, a three-dimensional network structure is formed, these are elastomers. Elastomers have a high degree of elasticity.

• Fibers- the interaction forces are strong with Fibres possess high tensile strength, high melting point, These intermolecular forces of interaction between fibers is due to H-bonds or dipole-dipole bonds. Examples- nylon-6, 6

• Thermoplastic Polymers- The intermolecular forces between them are intermediate in comparison to fibers and elastomers. They can be used in making toys by the process of repeated heating and cooling. Example- Polyethylene, Polystyrene, PVC, etc.

• Thermosetting Polymers- the chemical properties of these polymers are enhanced by heating. On heating, these semi-fluid substances are transformed into hard substances because of the crosslinking. Examples- Bakelite, silicones, phenolics, Melamine Formaldehyde.

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Polymerisation and its methods

When molecules are combined together to form polymer chains in structures like polymer chains, three-dimensional structures, cross-linked or branch chain polymers, the process is called polymerization. These polymers consist of small molecules, which are called monomers.

Methods of Polymerisation

There are mainly four methods of polymerization- Heterogeneous, Homogeneous, addition, and condensation polymerization.

• Homogeneous - Homogenous Polymerisation can be further classified into bulk and solution polymerization.

In bulk polymerization, the monomer acts as a liquid and an initiator helps in dissolving it In the solution that is created, the chain transferring agent dissolves. In the presence of light, the solution undergoes mass and transfer of heat. This results in the formation of a heavy molecule. This process enables the formation of a pure form of polymer.

In solution Polymerisation, monomer and chain agent are made to dissolve simultaneously in a solvent. This process is used when polymers are used as a solution

• Heterogeneous- Heterogeneous techniques can be further classified into suspension and emulsion polymerization. In bulk polymerisation , excessive heat was produced due to chain agent dissolving. When this heating exceeded the specific limits, it caused explosions.

Suspension polymerization was developed to overcome this. In solution polymerization, water is made to dissolve as fine drops, Water-soluble protective colloids and surface agents were used for the prevention of coalescing. Monomer droplets are formed, which transfer heat in the water. Suspension polymerization can only be used for monomers that are insoluble in water.

The most widely used technique is emulsion polymerization, In emulsion polymerization, monomers are dissolved as fine droplets in an aqueous phase. These monomers are emulsified by buffers, protective colloids, and surface agents. This reduces the surface tension and micelles are formed by surfactants. Monomers are emulsified by the inner hydrogen phase of the micelles.

The other two common polymerization techniques of Polymerisation are Addition Polymerisation and Condensation Polymerisation.

• Addition Polymerisation- the combining of two or more monomers of varying species to form a polymer is known as addition polymerization. When the monomers belong to the same group, the polymer formed is called a homopolymer. Example- Polythene.

When monomers that belong to different groups are used, the polymer formed is called a copolymer. Example- Buna-S, Buna-N.

• Condensation Polymerisation- In condensation polymerization, water, alcohol, HCl are removed and during the process of formation of polymers, monomers condense. The process takes place between bi function and tri function monomers. Examples- Nylon.

Degree of Polymerisation

The chain of monomers form Polymers, The number of monomers that form chain-like structures of polymers is known as the degree of polymerization. The calculation of the degree of polymerization is similar to the calculation of weighted averages.

Components of Polymer and atomic number are used for the calculation of the degree of polymerization.

Example- Consider a polymer, AB2-AB2 with polymerization degree, if we assume the atomic mass of A and B- 10 and 20 respectively. Thus the molecular weight is

the Molecular weight of AB2 and AB2 = 10×2 + 20×4

Thus, 20+80 = 100g mol-1

If the molecular mass of polymer AB2-AB2 is 1,00,000

Degree of Polymerisation is calculated by DP = M/Mo

where DP is the degree of polymerization

M is the molecular mass of the polymer and M₀ is the molecular mass of the monomers,

Therefore, degree of polymerization is 100000/100= 1000

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Some Important Polymers

Bakelite

Monomers Phenol and Formaldehyde from a polymer called Bakelite. Bakelite is a thermosetting polymer.

Preparation of Bakelite

On taking excess phenol the reaction medium becomes acidic, a condensation reaction takes place which gives rise to an acidic product. Whereas on taking excess of formaldehyde, the condensation reaction occurs in a basic medium. The product formed is Resol, Bakelite is formed when in the presence of Phenol (a cross-linking agent), Novolac undergoes cross-linking.

Bakelite can also be formed by heating formaldehyde and phenol in presence of either one of the catalysts- hydrochloric acid, zinc chloride, ammonia.

Properties of Bakelite

• Due to its property of getting molded quickly, Bakelite finds a large number of applications commercially,
• The product formed on molding Bakelite is heat resistant.
• Bakelite is also resistant to electric current due to its low electrical conductivity.

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Rubbers

Rubber is an elastomer that can be produced artificially like synthetic rubber can be manufactured using various chemical processes and it can also be obtained from natural sources.

Natural Rubber

• On suspension of solid particles, in latex natural rubber, is formed. In natural rubbers, isoprene monomers form a chain-like structure.
• Natural rubbers can be formed by methods like Rubber taping, Mastification, Calendering, and Vulcanization.

Synthetic Rubber

• Petroleum and Natural gas are used for the production of Synthetic rubber. It can be produced by the polymerisation of derivatives of 1, 3- butadiene
• The copolymerization of 1, 3- butadiene with unsaturated monomers also forms synthetic rubber.

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Copolymerisation

Polymers are formed from monomers. When a polymer is formed from monomers of different species, copolymers if formed. This process of polymerization is termed copolymerization Examples- Butadiene styrene copolymer, styrene isoprene styrene, nitrile rubber, etc.

• Copolymers can be broadly classified into linear and branched polymers. Linear copolymers can be further classified into alternating, statistical, and block copolymers whereas branched copolymers can be further classified into grafted and star-shaped copolymers.
• Copolymers have wide applications. graft copolymers are used as emulsifiers and for the separation of gas and liquids by acting as a membrane.
• Block Polymers are used for sport goos, hose tubing, and automotive components.

NEET Sample MCQs on Polymers

Question: What amongst the following is a property of polymer?

1. Less Hardness
2. High tensile strength
3. High-temperature stability
4. Highly elastic

Answer: Highly elastic

Question: Which is a thermosetting polymer?

1. Neoprene
2. Polystyrene
3. Bakelite
4. Terylene

Answer: Bakelite

Question: Name the natural polymer from the following

1. Lexan
2. Silk
3. Kevlar
4. Bakelite

Answer: Silk

Question: Polymers can be classified into— based on the process of their formation

1. Addition polymers
2. Copolymers
3. Condensation polymers
4. Addition and condensation polymers

Answer: Addition and condensation polymers

Question: Which monomers are used in addition to polymers?

1. Saturated compounds
2. Bifunctional saturated compounds
3. Trifunctional saturated compounds
4. Unsaturated compounds

Answer: unsaturated compounds

Question: What is the first step in the formation of polymers?

1. Hydrolysis of monomers
2. condensation reaction between monomers
3. Monomer conversion to monomer ions by protons
4. Monomer’s coordination reaction

Answer: Condensation reaction between monomers

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