Polymers Class 12 Notes Chemistry Chapter 15
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 on some special considerations. One of the common classifications of polymers is based on source from which polymer is derived.
There are two broad types of polymerisation reactions, i.e., the addition or chain growth polymerisation and condensation or step growth polymerisation.
In this type of polymerisation, the molecules of the same monomer or diferent monomers add together on a large scale to form a polymer. The monomers used are unsaturated compounds, e.g., alkenes, alkadienes and their derivatives.
Mechanism of Addition Polymerisation:
Free radical mechanism: A variety of alkenes or dienes and their derivatives are polymerised in the presence of a free radical generating initiator (catalyst) like benzoyl peroxide, acetyl peroxide, tert-butyl peroxide, etc.
Some Important Addition Polymers:
(a) Polythene: Polythenes are linear or slightly branched long chain molecules. 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:
(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.
This type of polymerisation generally involves a repetitive condensation reaction between two bi-functional or trifunctional mono-meric units.
Some Important Condensation Polymers:
Copolymerisation:
Copolymerisation is a polymerisation reaction in which a mixture of 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.
Rubber:
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.
Vulcanisation of rubber: Natural rubber becomes soft at high temperature (>335 K) and brittle at low temperatures (<283K) and shows high water absorption capacity.
Synthetic rubbers: Synthetic rubber is any vulcanisable rubber like polymer, which is capable of getting stretched to twice its length. However, it returns to its original shape and size as soon as the external stretching force is released.
Preparation of Synthetic Rubbers
Polymer properties are closely related to their molecular mass, size and structure. The growth of the polymer chain during their synthesis is dependent upon the availability of the monomers in the reaction mixture. Thus, the polymer sample contains chains of varying lengths and hence its molecular mass is always expressed as an average. The molecular mass of polymers can be determined by chemical and physical methods.
A large number of polymers are quite resistant to the environmental degradation processes and are thus responsible for the accumulation of polymeric solid waste materials. These solid wastes cause acute environmental problems and remain undegraded for quite a long time.
Aliphatic polyesters are one of the important classes of biodegradable polymers. Some important examples are given below:
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