Introduction:

Polymers we extremely large molecules which are made up of repeating (that is, recurring) structural units. Polymers are made by linking altogether of many smaller units (that is, molecules) termed as monomers, the entire process being termed as polymerisation.

nA → An ≡ - A - A - A - A - A - A - A -

Monomer     Polymer

Polymerisation is stated as the method (reaction) through which numerous simple molecules (that is, monomers) join altogether to form giant molecules (that is, polymers) of high molar mass (>200,000). That is two kinds of polymerisation, addition polymerisation and condensation polymerisation.

A few typical monomers and the corresponding polymer are illustrated below:

Monomer             Polymer

Ethene                 Polythene

Vinyl chloride        Polyvinyl chloride

Amino acids          Proteins

Glucose                Starch

The polymers can be naturally-occurring example: protein, starch, rubber and cellulose or synthesized in the laboratory example - polythene, nylon 66, polyvinyl chloride and terylene. The naturally occurring polymers are known as natural polymers whereas the man-made polymers are known as synthetic polymers.

Polymerisation Processes:

Addition polymerisation:

Monomers that are unsaturated (that is, having multiple bond) can react by one other to form addition compounds, a polymer having the similar empirical formula as the monomer. Ethene and substituted ethenes, for illustration, form the addition polymer.

Fig: Addition polymerisation

Several kinds of this polymer and their uses are listed in the table shown below:

Table: Types of Polymer and uses

Condensation polymerisation:

The condensation polymerisation takes place between the monomers which have at least two functional groups which can react or repeated by one other. The polymerisation method takes place by the elimination of small molecules like H₂O or NH₃ between the two different monomer molecules each of which has at least two functional groups that can participate in the condensation. Nylon-6, 6, for illustration, is a condensation polymer made by the elimination of H₂O molecules from the hexadioic (adipic) acid and hexane -1, 6-diamine.

Fig: Condensation polymerisation

Nylon-6,6 is an illustration of a copolymer. Whenever two different monomers, example: ethene and propene, hexadioic acid and hexane -1,6-diamine, react to give a polymer the product is termed as a co-polymer.

The table illustrated below represents condensation polymers and their uses.

Plastics and Resins:

The plastics are high molar mass synthetic (that is, man-made) polymers which can be deformed and molded into different shapes, at high temperatures. The linear polymers or copolymers have only weak van der waals forces between their long chains. Such polymers and those having only a few, weak cross links between the chains are known as thermoplastics as on heating they soften and on cooling harden again. Thermoplastics can be re-softened and re-hardened over and over again; and this hardly influences the property of the plastics. They are generally soluble in the organic solvents.

Illustrations of thermoplastics are the cellulose acetate, polyvinyl chloride and polythene. Thermoplastics are usually employed in the form of molded shapes, fibres, pipe, sheets or films. They are usually very good insulators and are resistant to numerous chemicals. If the original thermoplastic is too fragile its properties can be modified by adding plasticizers example: esters of benzene dialkanoic acids.

Polymers that have highly cross linked structure can't be softened once they have hardened, that is they can merely be heat-treated once. They are known as thermosetting plastics and are insoluble in any type of solvent. The rigidity in structure on cooling is as an outcome of the chemical reactions leading to the extensive cross-linking. Illustrations of thermosetting plastics are phenol-methanal polymer, polyurethanes and alkyd resins.

Fig: Thermosetting plastics

Thermosetting plastics are employed to make articles like electric plugs and switches, telephones, wireless and television cabinets, ash trays, lavatory seats and plastic tableware. Bakelite is a good insulator, polyurethanes are employed in floor finishes and bard-wearing paints, in the form of foams and alkyd resins are employed as binding rains and in alkyd paints.

The naturally occurring resins are sticky substances, insoluble in water and which flows out from most plants when cut or secreted through plants and animals. Shellac is a natural resin oozes out via insects living on trees.

Natural Polymers:

The Polymer materials are broadly found in the living organisms where they play significant structural and physiological roles. The carbohydrates and proteins belong to this class of the natural polymers.

Carbohydrates:

Carbohydrates are large groups of compounds having the molecular formula that can be written as (C_x(H₂O)y; such a formula doesn't, though, point out the correct structural arrangement. All carbohydrates are comprised of carbon, hydrogen and oxygen; among which comprises sugars, starches and celluloses.

Carbohydrates can be categorized as illustrated below:

Fig: Classification of Carbohydrates

The Sugars are crystalline substances having sweet taste and are soluble in water. They are usually categorized into the monosaccharides and the disaccharides. Non-sugars are complex molecules having relatively bigger molar mass than the simple sugars.

1)  Monosaccharides: These are the carbohydrates having six or less carbon atoms per molecule. Monosaccharides sugars can't be hydrolyzed to the smaller sugar molecules; they are the simplest unit of carbohydrates. The two most significant monosaccharide are glucose, which in an aldose (as it includes an aldehyde,-CHO group) and fructose that is ketoses (as it includes a keto, >C=0. group). They both encompass a molecular formula of C₆H₁₂O₆, however this symbolizes many isomers.

Fig: Glucose and Fructose

2) Disaccharides: These are the carbohydrates having twelve carbon atoms per molecule and having the molecular formula C₁₂H₂₂O₁₁. They are made by the elimination of water molecule from two C₆ monosaccharide molecules that is, 2C₆H₁₂O₆-H₂O = C₁₂H₂₂O₁₁. Whenever hydrolyzed, the disaccharides splitted into two C₆ monosaccharides. There are the two classes of disaccharides, the reducing sugars and the non-reducing sugars. The reducing sugar is a disaccharide that turns Fehling's solution from blue to red; whereas a non-reducing sugar consists of no effect on Fehling's solution. Sucrose (that is, a non-reducing sugar) and maltose (that is, a reducing sugar) are the most significant disaccharide.

Sucrose, which is obtained from the sugar cane, is a colourless crystalline solid having a sweet taste. The sugar that we use for our tea is sucrose. On hydrolysis with dilute acids, sucrose provides equivalent amounts of glucose and fructose.

C₆H₁₁O₅ - O - C₆H₁₁O₅ → H⁺ → C₆H₁₂O₆ + C₆H₁₂O₆

            Sucrose                          glucose     fructose

3) Polysaccharides: These are the high molar mass polymers of monosaccharides. They are built up from lots of C₆ monosaccharides linked altogether in long-chains having water molecule being removed between each and every pair of the C₆ molecules. Polysaccharides encompass a general formula:

(C₆H₁₀O₅)n i.e. nC₆H₁₂O₆ - H₂O

Here n = very large number

Significant polysaccharides comprise starch (n ≈ 330) and cellulose (n ≈ 600). On hydrolysis, the polysaccharides splitted up to the disaccharides and/or monosaccharides. Starch takes place as white granules in nearly all plants example: rice, barley, maize, wheat and potatoes. This is utilized by plants as a reserve food supply and it gives a very significant component of animal's diet as the source of energy. Cellulose is the major constituent of the cell-walls of plants example: cotton, jute, flax and is very broadly distributed.

Protein:

The Protein is a group of complex polymers that takes place broadly in all plants and animals. Typical illustrations of proteins are collagen (that is, found in tissue and skin); keratin (that is, found in hair and nails) and haemoglobin (that is, oxygen carrier in blood). Proteins are formed of amino acids joined by what are known as the peptide links. These are made by the elimination of water molecule between the - COOH group of one acid and the -NH2 group of an adjacent acid that is,

Fig: Protein

Proteins are thus polymers whose monomer is amino acids joined altogether via the peptide link.

a) Hydrolysis of proteins:

However different proteins differ broadly in the physical properties aid functions they can all be hydrolyzed to a mixture of amino acids. The hydrolysis can be brought concerning by acids, alkalis or enzymes. Around twenty different amino-acids have so far been isolated as the product of protein hydrolysis. The protein, insulin, for illustration, yields 16 various amino-acids on the hydrolysis.

b) Uses of proteins in the living systems:

Simple mains such as collagen, is the structural material in connective tissue, skin and cartilage; keratin as the structural material in skin, hair and nails; insulin as a hormone regulating sugar metabolism; and haemoglobin as the oxygen carrier in the blood. A few other proteins act as the enzymes and as plant viruses.

Synthetic Polymers:

The Synthetic polymers are the man-made polymers. All the addition polymers illustrated in the table above are the synthetic polymers. Some of the condensation polymers are as well man-made, among which comprise polyesters (terylene) and polyamides (Nylon -6,6).

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