Polymers Lecture 1 PDF

Polymers: Lecture 1 Unit-3a Polymer: A polymer is a long molecule formed by joining together of thousands of small molecules by chemical bonds. A polymer is called a macromolecule because of its large size. Polymerization: The chemical process leading to the formation of polymer from its monomer is...

Polymers: Lecture 1 Unit-3a Polymer: A polymer is a long molecule formed by joining together of thousands of small molecules by chemical bonds. A polymer is called a macromolecule because of its large size. Polymerization: The chemical process leading to the formation of polymer from its monomer is called polymerization. Monomers: The small molecules which combine to form large molecules or polymer are known as monomers. For eg: n(CH2=CH2) (CH2-CH2) ethene (monomer) polyethene or polyethylene (polymer) n is the degree of polymerization. Degree of polymerization (DP): It is the number of repeat units or monomer units in a polymer. DP in cellulose is 250 and in vinyl polymers are 400. It is the long chain that gives the polymer its unique properties. Ethane, CH3-CH3, is a gas molecule at room temperature. Because of their small size, ethane molecules are very mobile and can run almost anywhere they want without interacting with other molecules. Now, if we double the chain length or the total number of carbons to four, we get butane, CH3-CH2-CH2-CH3. It is a liquid fuel. In liquids, atoms or molecules can no longer act as independent units. Because of their larger size, butane molecules are less mobile than ethane molecules. Their lowered mobility allows them to run into or interact with one another more frequently. When the chain length increases 6 fold, as in paraffin, CH3(CH2CH2)10CH3, we get a waxy substance. In this case, the solid-like property of paraffin is a reflection of the entanglement of its long molecules when they move. If we keep increasing the number of repeating carbon units to, say, 2000, i.e., CH3(CH2CH2)2000CH3, we have a polyethylene polymer, which is a very strong, brittle solid. The polymer molecules have become so long and so entangled that their movement becomes almost completely restricted. Functionality: It is the number of bonding or reactive sites in a monomer. Reactive sites can be double bond, a triple bond, –NH2, -OH, -COOH, -SH etc. For a molecule to behave as a monomer, minimum two functional groups must be present. Hence molecules like acetic acid, benzoic acid, ethyl alcohol, aniline, methyl isocyanate etc cannot act as monomer since they are mono-functional. Vinyl monomer is bifunctional because the double bond in it is considered as a site for two free valencies. When the double bond is broken, two single bonds become available for combination. CH2=CHX …CH2-CHX… The double bond is the vital feature that allows these monomers to form the long polymer chains. The highlighted areas show the side groups on these monomer molecules. These groups give the polymer chain some of its properties. In the following examples, the functional groups are highlighted in yellow. Each molecule has two functional groups. Prepared by Dr Arunima Nayak and Dr Brij Bhushan Polymers: Lecture 1 Unit-3a It is the presence of two functional groups that allow such molecules to behave as monomer giving them ability to form chains and polymerize. Classify the following on the basis of their functionality: a. Ethylene glycol: Its chemical formula is: Functional groups present are 2 hydroxyl groups. Hence it is bifunctional. b. Vinyl chloride. Its chemical formula is CH2=CHCl Number of double bonds is one. Hence it is bifunctional. c. Lactic acid: Chemical formula is Functional groups present are 1hydroxyl group and 1 carboxylic acid group. Hence it is bifunctional. Significance of functionality: 1. When the functionality of monomer is two, linear or straight chain polymer molecule is formed. Example of bifunctional monomer is vinyl monomers, adipic acid, ethylene glycol, amino acid, hexamethylene diamine etc. 2. When the functionality of monomer is three, a three dimensional network polymer is formed. Example is phenol, melamine etc. 3. When a trifunctional monomer is mixed with a bifunctional monomer, a branched chain polymer is formed. 4. When a bifunctional monomer is mixed with a trifunctional monomer, a three dimensional network polymer is formed. Prepared by Dr Arunima Nayak and Dr Brij Bhushan