Polymers
Some Important Terms
Polymers are formed in two general ways.
a) In chain-reaction polymerizationRad. + CH2 = CH2 → RadCH2CH2→ RadCH2CH2CH2CH2× → etc.b) In step reaction polymerization,c) Free-radical vinyl polymerization:At the doubly bonded carbons — the vinyl groups — and is called vinyl polymerization. A wide variety of unsaturated monomers may be used, to yield polymers with different pendant groups (G) attached to the polymer backbone. For example.
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Some Important Polymers:
a) Natural Rubber:Addition polymer of isoprene (2-methyl-1,3-butadiene)An average chain length of 5000 monomer units of isoprene.The rubber in which the arrangement of carbon chain is trans with respect to the double bond is known as Gutta Percha and this is the natural rubber obtained from bark of various trees.'Vulcanisation of rubber involves addition of sulphur to rubber and heating the mixture to increase the strength of natural rubber. sulphur forms short chains of sulphur atoms that link two hydrocarbon (isoprene) units together.Vulcanised rubber is thus stronger and less sticky than the natural rubber.b) Synthetic rubber:(Polychloroprene) or Neoprene)It is obtained by free radical polymerisation of chloroprene inA thermoplastic and need not to be vulcanised.It is a good general purpose rubber and superior to natural rubber as it is resistant to the reaction of aire, heat, light chemicals, alkalis and acids below 50% strength.It is used for making transmission belts, printing rolls and flexible tubing employed for conveyence of oil and petrol.c) Buna rubbers:i) Buna - N or GRA: it is synthetic rubber obtained by copolymerisation of one part of acryl nitrile and two parts of butadiene.It is more rigid responds less to heat and very resistant to swelling action of petorol, oils and other organic solvents.ii) Buna -S or GRS (General purpose Styrene rubber): It is a copolymer of three moles of butadiene and one mole of styrene and is an elastomer. It is obtained as a result of free radical copolymerisation of its monomers.It is generally compounded with carbon black and vulcanised with sulphur. It is extremely resistant to wear and tear and finds use in manufacture of tyres and other mechanical rubber goods.
d) Teflon: It is polymer of tetrafluorethylene (F2C=CF2) which on polymerisation gives Telfon.It is thermoplastic polymer with a high softening point (600K).It is very tough and difficult to work. It is inert to most chemicals except fluorine and molten alkali metals.It withstands high temperatures. Its electrical properties make it an ideal insulating material for high frequency installation.e) Nylon -66:A condensation polymer formed by reaction between adipic acid and hexamethylene diamine. It is a thermoplastic polymer.h) Phenol-formaldehyde polymer: E.g., Bakelite Novolac
What is Mole? Atoms and molecules are too small to count. To solve this problem their numbers are expressed in terms of Avogadro’s number (N A = 6.023 10 23 ). Mole is the number equal to Avogadro’s number just like a dozen is equal to 12, a century means 100, a score means = 20. Mole can be defined as a unit which represents 6.023 x10 23 particles of same matter. A mole (symbol mol) is defined as the amount of substance that contains as many atoms, molecules, ions, electrons or any other elementary entities as there are carbon atoms in exactly 12 gm of . The number of atoms in 12 gm of is called Avogadro’s number . One atomic mass unit (amu) The number of moles of a substance can be calculated by various means depending on data available, as follows. Number of moles of molecules Number of moles of atoms Number of moles of gases (Standard molar volume at STP = 22.4 lit) Number of moles of particles e...