In this unit, we will discuss chapter polymers class 12 of Chemistry. Here we have given detailed notes on Polymers, Classification of Polymers for the students, especially of Class 12. We have also provided with a List of Polymers and their monomers that are prescribed on the NCERT Book of Class 12.
Polymers: Full Chapter Notes
They are divided into three categories:
Classification Of Polymers
On the basis of Source from which they are obtained:
These are obtained from plants and animals. Examples of natural polymers are proteins (polymer of amino acids), cellulose, starch (polymer of glucose) some resins and rubber (polymer of isoprene,2-methyl-1,3-butadiene). Enzymes are biocatalysts which are proteins and thus, these are also polymers.
These are obtained from natural polymers. Eg: Cellulose derivatives such as cellulose acetate (rayon) and cellulose nitrate. Cellulose acetate is used in making threads, films and sunglasses.
These are man-made polymers that are used in day-to-day life as well as in industry. Nylon 6,6, Buna- S and polythene are some examples.
On the basis of Structure of Polymers
These consist of long and straight chains. They have high densities, high m.p. along with high tensile strength and are thus well-packed structures. Examples, PVC, HDP or high-density polyethene, etc.
They have linear chains along with some branches. Due to their irregular packed structure, they have low tensile strength, low densities and lower m.p. as compared to a linear polymer, e.g., glycogen, low-density polyethene.
They are hard and rigid and are obtained from bi-functional and tri-functional monomers and possess strong covalent bonds between the linear polymer chains. , for e.g. melamine, bakelite, etc.
Based On the Mode of Polymerisation:
The repeated addition of monomer species having double or triple bonds forms these polymers.
These are further classified into:
Homopolymers: They are formed by the polymerisation of a single monomeric species, e.g., polythene formation from ethene.
Copolymers: These polymers are formed by the polymerisation of two different monomers, e.g., the formation of Buna-S from styrene and 1,3-butadiene, etc.
The repeated condensation reaction between different bi-functional or tri-functional monomeric units forms such polymers along with the elimination of molecules such as alcohol, water, etc. For example, the condensation of hexamethylenediamine with adipic acid forms nylon 6, 6. Eg: Terylene (dacron), nylon 6, etc.
On the basis of Molecular Forces
They possess elastic properties and are rubber-like solids that are held together by the weakest I.M. forces. Eg: neoprene, Buna-N, Buna-S, etc.
They contain linear or slightly branched chain molecules that have the capability of repeatedly softening on heating and hardening on cooling. Their intermolecular forces of attraction are intermediate to the elastomers and fibres. Examples are polyvinyls, polythene, etc.
They possess high tensile strength and high modulus and also have strong intermolecular forces like H-bonding which lead to close packing of chains and thus impart crystalline nature. Examples are nylon 6, 6, terylene, etc.
They are heavily branched or cross-linked molecules which undergoes extensive cross-linking and become infusible on heating in moulds and cannot be used again. Some examples are bakelite, urea-formaldelyde resins, etc.
Order of Intermolecular forces:
Elastomers < Thermoplastic < Fibres < Thermosetting.
Types of Polymerisation Reactions
Chain Growth Polymerisation or Addition Polymerisation:
Addition of molecules of the same or different monomers like alkenes, alkadienes and their derivatives together on a large scale to form a polymer. The most common mode is the free radical governed addition or chain growth polymerisation.
Free radical mechanism
It includes three steps, 1. chain initiating step, 2. chain propagating step and 3. chain terminating step.
Chain initiating step: The homolysis of benzoyl peroxide forms a phenyl free radical which adds to the double bond of ethene and results in the formation of a new and larger free radical. This step is called chain initiating step.
Chain propagating step: The radical formed thus reacts with another molecule of ethene to form another bigger sized radical. The process is repeated continuously resulting in the propagation of the reaction in the forward direction.
Chain terminating step: Ultimately, the product radical thus formed combines with another radical to form the polymerised product and terminates the reaction.
Polymers And Their Monomers List Class 12
Preparation of some important addition polymers
Polythene: In the preparation of polythene from ethene, benzoyl peroxide acts as an initiator or free radical generator.
Polythene is of two types :
Low-density polythene (LDP):
Under high pressure of 1000-2000 atm at a temp. of 350-570 K in the presence of trace amount of dioxygen or peroxide initiator, the polymerization of ethene leads to the formation of LDP. Are chemically inert and tough but flexible. And also a poor conductor of electricity. Some common uses are- manufacture of toys and flexible pipes.
High-density polythene (HDP):
Heating of ethene in the presence of a triethylaluminium and titanium tetrachloride (Ziegler-Natta catalyst) catalyst under a pressure of 6-7 atm and a temp of 333-343 K produces HDP. Due to close packing, it has high density. It is also chemically inert and more tough and hard. Some common uses are manufacturing buckets, dustbins, bottles, pipes, etc.
Heating of tetrafluoroethylene with a persulphate catalyst or free radical at high pressures produces Teflon. It is chemically inert and resistant to the attack of corrosive reagents. Some uses are making oil seals and gaskets, non – stick surface coated utensils.
Polyacrylonitrile (PAN) or Acrilan or Orlon:
PAN is formed by the polymerization of acrylonitrile in presence of peroxide catalyst. It is hard, horny and high melting material. It is used as a substitute for wool in manufacturing commercial fibres as orlon which is used for making clothes, carpets and blankets or acrilan.
Condensation Polymerisation or Step Growth polymerisation
It involves repeated condensation reaction between two bi-functional monomers and the sequence of condensation goes on. This process is also called as step growth polymerisation since at each step a distinct functionalised species is produced and is independent of each other.
Best Notes On Some condensation polymers Class 12:
They possess amide linkages and are termed as nylons.
It is obtained by the condensation polymerization of adipic acid with hexamethylenediamine under high pressure and at high temperature. Common uses are making sheets, bristles for brushes and in the textile industry.
It is a polymer obtained by polymerisation of caprolactum at a high temperature with the evolution of water. It is also known as perlon. Common uses are fabrics and ropes manufacturing.
It is obtained by polymerisation of hexamethylenediamine (6 carbon atoms) and sebacoyl chloride (10 carbon atom). Common uses are textile fibre, making a parachute and heavy duty tyres.
These are formed by the condensation of dicarboxylic acids and diols. Dacron or terylene (polyesters) can be produced by heating ethylene glycol and terephthalic acid mixture at about 420 to 460 K in the presence of zinc acetateantimony trioxide catalyst. Some common examples are blending with cotton and wool fibres, glass reinforcing materials in safety helmets, etc.
Phenol – formaldehyde polymer:
The condensation reaction of phenol with formaldehyde in the presence of an acid or a base catalyst results in the formation of Phenol – formaldehyde polymer (oldest synthetic polymer). The reaction initiates with the formation of o-and/or p-hydroxymethyl phenol derivatives that further reacts with phenol to form ringed compounds joined to each other through –CH2 groups. The initial product is Novolac, a linear product, used in paints.
On heating, Novolac undergoes cross-linking with formaldehyde to form bakelite. Common uses of it are making combs, electrical switches, phonograph records and handles of various utensils.
The condensation polymerisation of melamine and formaldehyde forms Melamine-formaldehyde polymer.
A Quick Notes On Copolymerisation Of Class 12:
It is the polymerisation reaction in which two or more than two monomeric species are allowed to polymerise to form a copolymer. It can be made both by chain growth polymerisation and step growth polymerisation. For example, a copolymer can be formed by a mixture of 1, 3 – butadiene and styrene.
Rubber is also termed as elastomer.
Natural rubber may be considered as cis – 1, 4 – polyisoprene and can be obtained by the process of vulcanisation with sulphur and are held together by weak van der Waals interactions and has a coiled structure.
Vulcanisation of rubber:
Sulphur forms cross-link on vulcanisation at the reactive sites of double bonds at a temperature range between 373 – 415 K and thus the rubber gets stiffened. Natural rubber becomes soft at a high temperature (greater than 335 K) and brittle at low temperatures (less than 283 K) and shows high water absorption capacity. Rubber made with 1-3% sulphur is soft and stretchy and is used in making rubber bands and rubber made with 5% sulphur is more rigid and is used in the manufacture of tyres for automobiles, etc. It is also used in making footwear, battery boxes, balloons, toys, etc.
Preparation of Synthetic Rubbers:
It is also known as polychloroprene which can be formed by the free radical polymerisation of chloroprene and are resistant to vegetable and mineral oils. Common uses are manufacturing conveyor belts, gaskets and hoses.
Buna – N:
It results from the copolymerisation of 1, 3 – butadiene and acrylonitrile in the presence of a peroxide catalyst. Making oil seals, tank lining, etc. are its uses.
These are two examples of biodegradable polymers that we need for Polymers Class 12 syllabus of Chemistry.
Poly β-hydroxybutyrate – co-β-hydroxy valerate (PHBV):
It is obtained by the copolymerisation of 3-hydroxybutanoic acid
and 3-hydroxypentanoic acid, linked by an ester linkage.
Nylon 2–nylon 6:
It is an alternating polyamide copolymer of glycine (H2N–CH2–COOH) and amino caproic acid [H2N (CH2)5 COOH].
Polymers of Commercial Importance:
Here we have highlighted some of the important names of the monomers as well as the polymers formed from the following monomers that are to be learned for class 12 exam and have been explained in these notes.
Some important monomers:
CH2=CH2 –> Ethene
NH2 (CH2)6 NH2 –> Hexamethylene diamine
HOOC (CH2)4 COOH –> Adipic acid
CH2=CH-CH=CH2 –> 1,3- Butadiene
C6H5CH=CH2 –> Styrene
CF2=CF2 –> Tetrafluoroethene
CH2=CHCN –> Acrylonitrile
HO H2C-CH2OH –> Ethylene glycol
Some polymers and few important points:
Buna-N –> 1,3-Butadiene and Acrylonitrile. Buna-S –> 1,3-Butadiene and Styrene. Nylon 6,6 –> Hexamethylenediamine and Adipic acid. Nylon 6 –> Caprolactam. Bakelite –> Phenol and Formaldehyde. Melamine –> Melamine and Formaldehyde.