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Polymers are high molecular mass macromolecules composed of repeating structural units derived from monomers. Polymers have a high molecular mass $\left(10^{3}-10^{7} \mathrm{u}\right)$. In a polymer, various monomer units are joined by strong covalent bonds. Polymers can be natural as well as synthetic. Polythene, rubber, and nylon 6,6 are examples of polymers.

Monomers are simple, reactive molecules that combine with each other in large numbers through covalent bonds to give rise to polymers. For example, ethene, propene, styrene, vinyl chloride.

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Natural polymers are polymers that are found in nature. They are formed by plants and animals. Examples include protein, cellulose, starch, etc.

Synthetic polymers are polymers made by human beings. Examples include plastic (polythene), synthetic fibres (nylon 6, 6), synthetic rubbers (Buna - S).

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The functionality of a monomer is the number of binding sites that is/are present in that monomer.

For example, the functionality of monomers such as ethene and propene is one and that of 1,3-butadiene and adipic acid is two.

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Polymerization is the process of forming high molecular mass $\left(10^{3}-10^{7} \mathrm{u}\right)$ macromolecules, which consist of repeating structural units derived from monomers. In a polymer, various monomer units are joined by strong covalent bonds.

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$(\mathrm{NH}-\mathrm{CHR}-\mathrm{CO})_{n}$ is a homopolymer because it is obtained from a single monomer unit, $\mathrm{NH_2}-\mathrm{CHR}-\mathrm{COOH}$.

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Elastic properties in elastomers:

Elastomers possess elastic properties because the intermolecular forces are the weakest. As a result, they can be readily stretched by applying small stress and regain their original shape when the stress is removed. The elasticity can be further increased by introducing some cross - links in the polymer chains.

Natural rubber is the most popular example of elastomers. A few more examples are buna-S, buna- $\mathrm{N}$ and neoprene.

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Addition polymerization is the process of repeated addition of monomers, possessing double or triple bonds to form polymers. For example, polythene is formed by addition polymerization of ethene.

$\left.n \mathrm{CH_2}=\mathrm{CH_2} \longrightarrow (\mathrm{CH_2}-\mathrm{CH_2}\right)_{n}$

Ethene Polyethene

Condensation polymerization is the process of formation of polymers by repeated condensation reactions between two different bi-functional or tri-functional monomers. A small molecule such as water or hydrochloric acid is eliminated in each condensation. For example, nylon 6, 6 is formed by condensation polymerization of hexamethylenediamine and adipic acid.

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The process of forming polymers from two or more different monomeric units is called copolymerization. Multiple units of each monomer are present in a copolymer. The process of forming polymer Buna-S from 1, 3-butadiene and styrene is an example of copolymerization

Nylon 6, 6 is also a copolymer formed by hexamethylenediamine and adipic acid.

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Mechanism of free radical polymerization of ethene:

Step I. Chain initiation step

Step II. Chain propogation step

Step III. Chain terminating step

For termination of the long chain, these free radicals can combine in different ways to form polythene.

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Thermoplastic polymers are linear (slightly branched) long chain polymers, which can be repeatedly softened and hardened on heating. Hence, they can be modified again and again. Examples include polythene, polystyrene.

Thermosetting polymers are cross-linked or heavily branched polymers which get hardened during the molding process. These plastics cannot be softened again on heating. Examples of thermosetting plastics include bakelite, urea-formaldehyde resins.

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(i) Vinyl chloride $\left(\mathrm{CH_2}=\mathrm{CHCl}\right)$

(ii) Tetrafluoroethylene $\left(\mathrm{CF_2}=\mathrm{CF_2}\right)$

(iii) Formaldehyde $(\mathrm{HCHO})$ and phenol $\left(\mathrm{C_6} \mathrm{H_5} \mathrm{OH}\right)$

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One common initiator used in free radical addition polymerization is benzoyl peroxide. Its structure is given below.

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Natural rubber is a linear cis-polyisoprene in which the double bonds are present between $C_{2}$ and $C_{3}$ of the isoprene units.

Because of this cis-configuration, intermolecular interactions between the various strands of isoprene are quite weak. As a result, various strands in natural rubber are arranged randomly. Hence, it shows elasticity.

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Natural rubber though useful has some problems associated with its use. These limitations are discussed below:

1. Natural rubber is quite soft and sticky at room temperature. At elevated temperatures ( $>335 \mathrm{~K}$ ), it becomes even softer. At low temperatures ( $<283 \mathrm{~K}$ ), it becomes brittle. Thus, to maintain its elasticity, natural rubber is generally used in the temperature range of $283 \mathrm{~K}-335 \mathrm{~K}$.

2. It has the capacity to absorb large amounts of water.

3. It has low tensile strength and low resistance to abrasion.

4. It is soluble in non-polar solvents.

5. It is easily attacked by oxidizing agents.

Vulcanization of natural rubber is done to improve upon all these properties. In this process, a mixture of raw rubber with sulphur and appropriate additive is heated at a temperature range between $373 \mathrm{~K}$ and $415 \mathrm{~K}$.

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The monomeric repeating unit of nylon 6 is $\left[\mathrm{NH}-\left(\mathrm{CH_2}\right)_{5}-\mathrm{CO}\right]$, which is derived from Caprolactam.

The monomeric repeating unit of nylon 6,6 is $\left[\mathrm{NH}-\left(\mathrm{CH _2}\right) _{6}-\mathrm{NH}-\mathrm{CO}-\left(\mathrm{CH _2}\right) _{4}-\mathrm{CO}\right]$, which is derived from hexamethylene diamine and adipic acid.

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	Polymer	Monomer	Structure of monomer
i	Buna-S	1,3 -butadiene	$\mathrm{CH_2}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH_2}$
		Styrene	$\mathrm{C_6} \mathrm{H_5} \mathrm{CH}=\mathrm{CH} 2$
ii	Buna-N	1, 3-butadiene	$\mathrm{CH_2}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH_2}$
		Acrylonitrile	$\mathrm{CH_2}=\mathrm{CH}-\mathrm{CN}$
iii	Neoprene	Chloroprene	$\mathrm{CH _2}=\stackrel{\substack{\mathrm{O} \\ |}}{\mathrm{C}}-\mathrm{CH}=\mathrm{CH _2}$
iv	Dacron	Ethylene glycol	$\mathrm{HOH_2} \mathrm{C}-\mathrm{CH_2} \mathrm{OH}$
		Terephthalic acid

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(i) The monomers of the given polymeric structure are decanoic acid $\left[\mathrm{HOOC}-\left(\mathrm{CH _2}\right) _{8}-\mathrm{COOH}\right]$ and hexamethylene diamine $\left[\mathrm{H _2} \mathrm{~N}\left(\mathrm{CH _2}\right) _{6} \mathrm{NH _2}\right]$.

(ii) The monomers of the given polymeric structure are

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The condensation polymerisation of ethylene glycol and terephthalic acid leads to the formation of dacron.

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A polymer that can be decomposed by bacteria is called a biodegradable polymer.

Poly- $\beta$-hydroxybutyrate-CO- $\beta$ - hydroxyvalerate (PHBV) is a biodegradable aliphatic polyester.