NEET Chemistry – Polymers- Study Notes

POLYMERS

CLASSIFICATION OF POLYMERS

• Natural polymers – They are obtained from natural sources eg poly saccharides (starch, cellulose), Proteins (polymers of amino acids), gums, resins (cross linked polymers formed by compounds containing double or triple bonds slowly oxidised by atmospheric oxygen). natural rubber (polymer of isoprene), Nucleic acids (polymers of nucleotides) silk and wool (polymers of amino acids).
• Synthetic polymers – Polymers prepared by synthesis (man made), are known as synthetic polymers eg. polystyrene, nylon, PVC etc.

CLASSIFICATION BASED ON STRUCTURE

• Linear polymers – Polymers containing the monomeric units linked together to form long straight chains stacked over one another to give packed structure.

• Branched polymers – Long chain of monomer units containing side chains of different length forms branched polymers. The chains are loosely packed, hence polymers have low density, low m.p. and low tensile strength. Examples – Amylopectin and glycogen.
• Cross-linked or three dimensional polymers – Such polymers have three dimensional network and are hard, brittle and rigid. Examples – Bakelite, melamine.

STEREOCHEMICAL CLASSIFICATION

• Isotactic – with all methyl groups on one side of extended chain

• Syndiotactic – with methyl groups alternating regularity side to side

• Atactic – with random distribution of methyl groups

CLASSIFICATION BASED UPON MODE OF SYNTHESIS

• Addition polymers – Polymers formed without the elimination of any elements and the molecular weight of the polymer is the exact multiple of the monomer unit are known as addition polymers

• Condensation polymers – They are formed with elimination of certain elements in the form of H2O, C2H5OH, NH3 etc.  and molecular weight of polymer is not exact multiple of monomer unit eg. terephthalic acid condenses with ethylene glycol to give polyester

CHAIN GROWTH AND STEP GROWTH POLYMERS

• Chain growth polymers – In chain growth polymerisation each step consumes a reactive particle and produces another similar particle.

• Step growth polymers – Polymerisation proceeds through various steps independent of each other and it happens when monomer units contain more than one functional group. For example formation of nylon 66, bakelite etc proceed by step growth polymerisation.

CLASSIFICATION ON THE BASIS OF PHYSICAL PROPERTIES AND INTERPARTICLE FORCES

• Elastomers – They can be easily stretched and can be made rigid to some extent by adding cross linking agents eg vulcanised rubber. They possess weakest intermolecular forces.
• Thermoplastics – They become soft on heating and harden on cooling. eg polythene, polystyrene. They can be worked up again and again. Forces between molecules are intermediate of elastomers and fibers.
• Thermosetting – Such polymers undergo chemical changes when heated and set to hard mass when cooled eg Bakelite. Such polymers can not be reworked. There is excessive cross linking on heating and three dimensional network of bonds.
• Fibres – Linear polymers containing polymeric units joined by hydrogen bonding. They can be woven into fabrics. Rayons, nylons are the examples.

FORMATION OF ADDITION POLYMERS

• Free radical polymerisation
• Cationic polymerisation
• Anionic polymerisation
• Ziegler-Natta polymerisation or coordination polymerisation

VULCANIZATION OF RUBBER

• Heating of rubber with sulphur which causes formation of sulphur bridges between molecules which are then cross linked is known as vulcanization of rubber.
• The vulcanised rubber is more elastic than natural rubber.
• Vulcanization may be brought about by free radical generators (peroxides, azo compounds) and metal oxides (ZnO or MgO) also.

ANTIOXIDANTS

Natural rubber is very sensitive to oxidation by air and ozone which can be inhibited by adding antioxidants. They undergo easy oxidation and prevent the oxidative degradation of rubber. eg.
N-phenyl-naphthylamine and Di--naphthyl-p-phenylenediamine.

MOLECULAR MASS OF POLYMERS

• Number average molecular mass (Mn–) is given by 

• Mass-average molecular mass is given by

POLYDISPERSITY INDEX

PDI =  

IMPORTANT ADDITION POLYMERS

POLYETHYLENE

POLYPROPYLENE

POLYSTYRENE

POLYMETHYLMETHACRYLATE, LUCITE OR PLEXIGLASS

POLYVINYL CHLORIDE (PVC)

TEFLON, POLYTETRAFLUOROETHYLENE (PTFE) OF FLUON

POLYACRYLONITRILE (PAN), ACRILON OR ORLON

RUBBERS

BUNA – S OR SBR OR GSR (GOVERNMENT STYRENE RUBBER)

NITRILE RUBBER (GR-A OR BUNA – N)

NEOPRENE

HEAVY RUBBER

GUTTA PERCHA

IMPORTANT CONDENSATION POLYMERS

NYLONS

       

POLYESTERS

THERMOSETTING RESINS

 

SILICON RESINS

• liquid silicones or silicone oils
• silicon greases
• silicon rubbers
• solid silicones

RESIN

RAYON

• Acetate process
• Viscose process

PLASTICIZERS