Introduction

Polymers of Class 12

Polymers are compounds of very high molecular mass formed by the combination of a large number of simple molecules. The simple molecules which combine to give polymer are called monomers. The process by which the simple molecules such as monomers are converted into polymers is called polymerization.

A polymer formed from one type of monomer is called homopolymer. For example, polyethylene is a homopolymer of monomer ethylene. A polymer formed from two or more different monomers is called copolymer or mixed polymer. Such as, terylene is a polymer of two types of monomers, ethylene glycol and terephthalic acid.

The polymers are also called as macromolecules because of the large size of their molecules. They are used frequently without any distinction. But a polymer will always consist of thousands of repeating monomer units. Such as chlorophyll is macromolecule but not a polymer because it does not contain monomers. Polyethene can be called as polymer as well as macromolecule. Therefore, all polymers are macromolecules but all macromolecules are not polymers.

Classification of polymers on the basis of origin

(a) Natural polymers: They are available in nature (animals or plants). Examples of such polymers are: natural rubber (1,4−cis−polyisoprene), natural silk, cellulose, starch, proteins, etc. Polymers such as polysacharides (starch, cellulose), proteins and nucleic acids etc., which control various life processes in plants and animals are called biopolymers.

(b) Semisynthetic polymers: They are chemically modified natural polymers such as hydrogenated, halogenated or hydro−halogenated natural rubber, cellulosics, i.e., esters and ethers of cellulose such as cellulose nitrate, methyl cellulose, etc.

(c) Synthetic polymers: They are man made polymers prepared synthetically such as polyethylene, polystyrene, polyvinyl chloride, polyesters, Bakelite, Buna−S, Nylon, Dacron etc.

Classification on the basis of thermal response

(a) Thermoplastic polymers: Polymers which can be easily softened when heated and hardened with little change in their properties. They can be softened or plasticizied repeatedly on application of thermal energy, without much change in properties if treated with certain precautions, e.g. polyolefins, polystyrene, nylons, linear polyesters and polyethers, polyvinyl chloride, Teflon etc. They normally remain soluble and fusible after many cycles of heating and cooling.

(b) Thermosetting polymers: Polymers which undergo permanent change on heating. They can be obtained in soluble and fusible forms in early or intermediate stages of their synthesis, but they get packed or cured and become insoluble and infusible when further heated or thermally treated. The curing or packing process involves chemical reactions leading to further growth and cross linking of the polymer chain molecule and producing giant molecules, e.g. Bakelite, melamine formaldehyde, diene rubbers, unsaturated polyesters etc.

Certain plastics do not soften very much on heating. These can be easily softened by the addition of some organic compound which are called plasticizers. For example, Polyvinyl chloride (PVC) is very stiff and hard but it is made soft by adding plasticizer e.g. Dioctyl phthalate (DOP).

(c) Fibres: Polymers which have strong intramolecular forces between chains. These forces are either H−bonds or dipole−dipole interaction. These are closely packed with a high tensile strength and less elasticity. Therefore, they have sharp melting points. These polymers are long, thin and thread like and can be woven in fabrics. Some of the example of these polymers are Nylon−66, Dacron, etc.

(d) Elastomers: Polymer with elastic character like rubber. In elastomers the polymer chains are bound together by weakest intermolecular forces. These are easily stretched by applying small stress and regains its original shape when stress is removed. For example, natural rubber.

The natural rubber is a gummy material which has poor elasticity. However, when natural rubber is heated with 3−5% sulphur, it becomes non−sticky and more elastic. This process is called vulcanization and product formed is vulcanized rubber which has better tensile strength and resistance to abrasion than natural rubber.

Molecular masses of Polymers

The molecular mass of a polymer can be expressed in two ways

(a) Number average molecular mass: If N1, N2, N3 … are the number of molecules with molecular masses M1, M2, M3 ….respectively.

Polymers

This may be expressed as

Polymers

where Ni is the number of molecules of the ith type with molecular mass Mi.

(b) Weight average molecular mass:

If m1, m2, m3….. are the masses of species with molecular masses M1, M2, M3…. respectively, then the weight average molecular mass is

Polymers

or Polymers

But mi = NiMi,

so that polymers

where Ni is the number of molecules of mass Mi.

Poly dispersity index: The ratio of weight average molecular mass to the number average molecular mass is called poly dispersity index, PDI.

PDI = Polymers

This gives an idea about the homogeneity of a polymer.

For natural polymers, PDI is usually unity and therefore, natural polymers are monodisperse. For synthetic polymers, the PDI is greater than one and therefore polymers is greater than polymers.

The number average molecular mass, polymers is measured on the basis of colligative properties like osmotic pressure. On the other hand, the weight average molecular mass, polymers is determined with the help of methods like ultra centrifugation, sedimentation etc.

Classification on the basis of mode of formation

(a) Addition polymers: They are formed from olefinic, diolefinic, vinylic and related monomers. They all have −C−C− linkages along the main chains of the polymer molecules and usually no other atom appears in the main chain. Those polymers are formed by simple additions of monomer molecules to each other in quick succession by a chain mechanism. This is known as addition polymerization or chain−growth polymerization. The examples of such polymers are: polyethylene, polypropylene, polystyrene, polybutadiene, polyvinyl chloride, etc.

(b) Condensation polymers: A polymer formed by the condensation of two or more than two different monomers with the elimination of the species like water, ammonia, hydrogen chloride or alcohol etc., is called condensation polymer. In this type of polymerization generally each monomer contains two functional groups.

Besides −C−C− linkages, they contain atoms such as O, N, S, etc., at regular intervals in the main chain. The process of their formation is called condensation polymerization or step−growth polymerization. Polyamides, polyesters, polyethers, polyurethanes, terylene, bakelite, epoxy resins and alkyd resins, etc., are examples of condensation polymers.

Classification on the basis of structure

(a) Linear polymers: These can schematically be represented by lines of finite lengths with well packed structure, having high densities, high tensile (pulling) strength and high melting points. They are formed from olefinic, vinylic or related polymerization under suitable conditions or by condensation polymerization of bifunctional monomers. Linear polymers such as high density polyethylene, polyvinyl chloride, polystyrene, Nylon−6, etc. are soluble and fusible.

(b) Branched polymers: They can be schematically represented by lines of finite lengths with the short or long branch structures of repeated units. The branches appear as a consequence of uncontrolled side reactions during polymerization or by design of polymerization. Branched polymers are usually more readily soluble and fusible than linear polymers of comparable chain length or molecular weight. For example, low density polythene, glycogen, starch etc.

(c) Cross−linked or Network polymers: They can be represented by a network structure, planar−network as in graphite or space−network as in diamond. Cross−linked polymers are insoluble and infusible as the molecules in them are giant molecules, often of unusually high or infinite molecular weight. Depending on the nature and frequency of cross−links, such polymers may show different orders of swelling in solvents. These polymers are hard, rigid, brittle because of network structure. Examples are: Phenol−formaldehyde resins, epoxy resins, vulcanized rubber etc.

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