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Unit Objectives 15 Polymers After studying this Unit, you will be able to “Co...

Unit Objectives 15 Polymers After studying this Unit, you will be able to “Copolymerisation has been used by nature in polypeptides which explain the terms - monomer, may contain as many as 20 different amino acids. Chemists are still polymer and polymerisation and far behind”. appreciate their importance; distinguish between various Do you think that daily life would have been easier and classes of polymers and different colourful without the discovery and varied applications types of polymerisation processes; of polymers? The use of polymers in the manufacture appreciate the formation of of plastic buckets, cups and saucers, children’s toys, polymers from mono- and bi- packaging bags, synthetic clothing materials, automobile functional monomer molecules; tyres, gears and seals, electrical insulating materials and describe the preparation of some machine parts has completely revolutionised the daily important synthetic polymers and life as well as the industrial scenario. Indeed, the their properties; polymers are the backbone of four major industries viz. appreciate the importance of plastics, elastomers, fibres and paints and varnishes. polymers in daily life. The word ‘polymer’ is coined from two Greek words: poly means many and mer means unit or part. The term polymer is defined as very large molecules having high molecular mass (103-107u). These are also referred to as macromolecules, which are formed by joining of repeating structural units on a large scale. The repeating structural units are derived from some simple and reactive molecules known as monomers and are linked to each other by covalent bonds. The process of formation of polymers from respective monomers is called polymerisation. There are several ways of classification of polymers based 15.1 Classification on some special considerations. One of the common of Polymers classifications of polymers is based on source from which polymer is derived. Under this type of classification, there are three sub categories. 1. Natural polymers These polymers are found in plants and animals. Examples are proteins, cellulose, starch, some resins and rubber. 2022-23  2. Semi-synthetic polymers Cellulose derivatives as cellulose acetate (rayon) and cellulose nitrate, etc. are the usual examples of this sub category. 3. Synthetic polymers A variety of synthetic polymers as plastic (polythene), synthetic fibres (nylon 6,6) and synthetic rubbers (Buna - S) are examples of man-made polymers extensively used in daily life as well as in industry. Polymers can also be classified on the basis of their structure, molecular forces or modes of polymerisation. Intext Questions 15.1 What are polymers ? 15.2 Types of There are two broad types of polymerisation reactions, i.e., the addition or chain growth polymerisation and condensation or step growth Polymerisation polymerisation. Reactions In this type of polymerisation, the molecules of the same monomer or 15.2.1 diferent monomers add together on a large scale to form a polymer. The Addition monomers used are unsaturated compounds, e.g., alkenes, alkadienes Polymerisation and their derivatives. This mode of polymerisation leads to an increase in or Chain Growth chain length and chain growth can take place through the formation of Polymerisation either free radicals or ionic species. However, the free radical governed addition or chain growth polymerisation is the most common mode. 1. Free radical mechanism A variety of alkenes or dienes and their derivatives are polymerised in 15.2.1.1 the presence of a free radical generating initiator (catalyst) like Mechanism of benzoyl peroxide, acetyl peroxide, tert-butyl peroxide, etc. For example, Addition the polymerisation of ethene to polythene consists of heating or Polymerisation exposing to light a mixture of ethene with a small amount of benzoyl peroxide initiator. The process starts with the addition of phenyl free radical formed by the peroxide to the ethene double bond thus generating a new and larger free radical. This step is called chain initiating step. As this radical reacts with another molecule of ethene, another bigger sized radical is formed. The repetition of this sequence with new and bigger radicals carries the reaction forward and the step is termed as chain propagating step. Ultimately, at some stage the product radical thus formed reacts with another radical to form the polymerised product. This step is called the chain terminating step. The sequence of steps involved in the formation of polythene are depicted as follows: Chain initiation steps Chemistry 434 2022-23  Chain propagating step Chain terminating step For termination of the long chain, these free radicals can combine in different ways to form polythene. One mode of termination of chain is shown as under: The addition polymers formed by the polymerisation of a single monomeric species are known as homopolymers, for example polythene discussed above is a homopolymer. The polymers made by addition polymerisation from two different monomers are termed as copolymers. Buna-S, which is formed by polymerisation of buta–1, 3–diene and styrene is an example of copolymer formed by addition polymerisation. (a) Polythene 15.2.1.2 Some Important Polythenes are linear or slightly branched long chain molecules. Addition Polymers These are capable of repeatedly softening on heating and hardening on cooling and are thus thermoplastic polymers. There are two types of polythene as given below: (i) Low density polythene: It is obtained by the polymerisation of ethene under high pressure of 1000 to 2000 atmospheres at a temperature of 350 K to 570 K in the presence of traces of dioxygen or a peroxide initiator (catalyst). The low density polythene (LDP) is obtained through the free radical addition and H-atom abstraction. It has highly branched structure. These polymers have straight chain structure with some branches as shown below. Low density polythene is chemically inert and tough but flexible and a poor conductor of electricity. Hence, it is used 435 Polymers 2022-23  in the insulation of electricity carrying wires and manufacture of squeeze bottles, toys and flexible pipes. (ii) High density polythene: It is formed when addition polymerisation of ethene takes place in a hydrocarbon solvent in the presence of a catalyst such as triethylaluminium and titanium tetrachloride (Ziegler-Natta catalyst) at a temperature of 333 K to 343 K and under a pressure of 6-7 atmospheres. High density polythene (HDP) thus produced, consists of linear molecules as shown below and has a high density due to close packing. Such polymers are also called linear polymers. High density polymers are also chemically inert and more tough and hard. It is used for manufacturing buckets, dustbins, bottles, pipes, etc. ( b )Polytetrafluoroethene (Teflon) Teflon is manufactured by heating tetrafluoroethene with a free radical or persulphate catalyst at high pressures. It is chemically inert and resistant to attack by corrosive reagents. It is used in making oil seals and gaskets and also used for non – stick surface coated utensils. ( c ) Polyacrylonitrile The addition polymerisation of acrylonitrile in presence of a peroxide catalyst leads to the formation of polyacrylonitrile. Polyacrylonitrile is used as a substitute for wool in making commercial fibres as orlon or acrilan. Example 15.1 Is a homopolymer or a copolymer? It is a homopolymer and the monomer from which it is obtained Solution is styrene C6H5CH = CH2. Chemistry 436 2022-23 15.2.2 This type of polymerisation generally involves a repetitive Condensation condensation reaction between two bi-functional or trifunctional Polymerisation mono-meric units. These polycondensation reactions may result in or Step Growth the loss of some simple molecules as water, alcohol, hydrogen Polymerisation chloride, etc., and lead to the formation of high molecular mass condensation polymers. In these reactions, the product of each step is again a bi-functional species and the sequence of condensation goes on. Since, each step produces a distinct functionalised species and is independent of each other, this process is also called as step growth polymerisation. The formation of terylene or dacron by the interaction of ethylene glycol and terephthalic acid is an example of this type of polymerisation. O O n HOH2C – CH2OH + n HOOC COOH OCH2–CH2–O– C C n Ethylene glycol Terephthalic acid Terylene or dacron (Ethane-1, 2 - diol) (Benzene-1,4 - di carboxylic acid) 15.2.2.1 (a) Polyamides Some Important These polymers possessing amide linkages are important Condensation examples of synthetic fibres and are termed as nylons. The Polymers general method of preparation consists of the condensation polymerisation of diamines with dicarboxylic acids or condensation of amino acids or their lactams. Nylons (i) Nylon 6,6: It is prepared by the condensation polymerisation of hexamethylenediamine with adipic acid under high pressure and at high temperature. Nylon 6, 6 is fibre forming solid. It possess high tensile strength. This characteristic can be attributed to the strong intermolecular forces like hydrogen bonding. These strong forces also lead to close packing of chains and thus impart crystalline nature. Nylon 6, 6 is used in making sheets, bristles for brushes and in textile industry. (ii) Nylon 6: It is obtained by heating caprolactum with water at a high temperature. 437 Polymers 2022-23  Nylon 6 is used for the manufacture of tyre cords, fabrics and ropes. (b) Polyesters These are the polycondensation products of dicarboxylic acids and diols. Dacron or terylene is the best known example of polyesters. It is manufactured by heating a mixture of ethylene glycol and terephthalic acid at 420 to 460 K in the presence of zinc acetate-antimony trioxide catalyst as per the reaction given earlier. Dacron fibre (terylene) is crease resistant and is used in blending with cotton and wool fibres and also as glass reinforcing materials in safety helmets, etc. (c) Phenol – formaldehyde polymer (Bakelite and related polymers) Phenol – formaldehyde polymers are the oldest synthetic polymers. These are obtained by the condensation reaction of phenol with formaldehyde in the presence of either an acid or a base catalyst. The reaction starts with the initial formation of o-and/or p-hydroxymethylphenol derivatives, which further react with phenol to form compounds having rings joined to each other through–CH2 groups. The initial product could be a linear product – Novolac used in paints. Novolac on heating with formaldehyde undergoes cross linking to for m an infusible solid mass called bakelite. It is thermosetting polymer which cannot be reused or remoulded. Thus, bakelite is formed by cross linking of linear chains of the polymer novolac. Bakelite is used for making combs, phonograph records, electrical switches and handles of various utensils. Chemistry 438 2022-23  OH OH OH ~~ ~~ H2C CH2 CH2 CH2 ~~ ~~ CH2 CH2 CH2 ~~ ~~ H2C CH2 CH2 CH2 ~~ ~~ OH OH OH Bakelite (d) Melamine — formaldehyde polymer Melamine formaldehyde polymer is formed by the condensation polymerisation of melamine and formaldehyde. It is used in the manufacture of unbreakable crockery. Intext Questions 15.2 Write the names of monomers of the following polymers: 15.3 Classify the following as addition and condensation polymers: Terylene, Bakelite, Polythene, Teflon. 15.2.3 Copolymerisation is a polymerisation reaction in which a mixture of Copolymerisation more than one monomeric species is allowed to polymerise and form a copolymer. The copolymer can be made not only by chain growth polymerisation but by step growth polymerisation also. It contains multiple units of each monomer used in the same polymeric chain. 439 Polymers 2022-23  For example, a mixture of buta–1, 3–diene and styrene can form a copolymer. Copolymers have properties quite different from homopolymers. For example, butadiene - styrene copolymer is quite tough and is a good substitute for natural rubber. It is used for the manufacture of autotyres, floortiles, footwear components, cable insulation, 15.2.4 Rubber etc. 1. Natural rubber Rubber is a natural polymer and possesses elastic properties. It is also termed as elastomeric polymer. In elastomeric polymers, the polymer chains are held together by the weak intermolecular forces. These weak binding forces permit the polymer to be stretched. A few ‘crosslinks’ are introduced in between the chains, which help the polymer to retract to its original position after the force is released. Rubber has a variety of uses. It is manufactured from rubber latex which is a colloidal dispersion of rubber in water. This latex is obtained from the rubber tree which is found in India, Srilanka, Indonesia, Malaysia and South America. Natural rubber may be considered as a linear polymer of isoprene (2-methyl-1, 3-butadiene) and is also called as cis - 1, 4 - polyisoprene. The cis-polyisoprene molecule consists of various chains held together by weak van der Waals interactions and has a coiled structure. Thus, it can be stretched like a spring and exhibits elastic properties. Vulcanisation of rubber: Natural rubber becomes soft at high temperature (>335 K) and brittle at low temperatures (