Polymers Flashcards
(23 cards)
Classification of Polymers
- Natural polymers: proteins, cellulose, starch, some resins and rubber.
- Semi-Synthetic polymers: cellulose acetate (rayon) and cellulose
nitrate, etc. - Synthetic polymers: plastic (polythene), synthetic fibres (nylon 6,6) and synthetic rubbers (Buna - S)
Addition of Chain Growth Polymerisation
Chain growth can take place through the formation of either free radicals or ionic species. Free radical mechanism is more common.
Chain initiation: A certain catalyst (usually a peroxide) is used to form a free radical initiator. e.g. benzoyl peroxide, acetyl peroxide, tert-butyl peroxide. The free radical formed combines with the monomer to form a larger free radical.
Chain propagation: These free radicals of monomers react with one another to form large chains of repeating units.
Chain Termination: Ultimately, at some stage the product radical thus formed reacts with another radical to form the polymerised product.
Low density polythene
It is obtained by the polymerisation of ethene under high pressure at a temperature of 350 K to 570 K in the presence of traces of dioxygen or a peroxide initiator (catalyst like benzoyl peroxide ). The low density polythene (LDP) is obtained through the free radical addition. It has highly branched structure.
It is chemically inert and tough but flexible and a poor conductor of electricity. Hence, it is used in the insulation of electricity carrying wires and manufacture of squeeze bottles, toys and flexible pipes.
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). High density polythene (HDP) thus produced, consists of linear molecules 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.
Polytetrafluoroethene (Teflon)
Teflon is manufactured by heating tetrafluoroethene (CF2=CF2) 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.
Polyacrylonitrile
The addition polymerisation of acrylonitrile (CH2-CH-CN) 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.
Buna-S
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. 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.
Condensation Polymerisation or Step Growth Polymerisation
This type of polymerisation generally involves a repetitive condensation reaction between two bi-functional or trifunctional mono-meric units. It involves the loss of some simple molecules as water, alcohol, hydrogen chloride, etc.,
Polyamides
polymerisation of diamines with dicarboxylic acids or condensation of amino acids or their lactams.
Nylon 6,6
It is prepared by the condensation polymerisation of hexamethylenediamine (H2N-(CH2)6-NH2) with adipic acid
(HOOC-(CH2)4-COOH) under high pressure and at high temperature.
Nylon 6, 6 is fibre forming solid. It possess high tensile strength due to the strong intermolecular forces like hydrogen bonding. These strong forces 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.
Nylon 6
Nylon 6: It is obtained by heating caprolactum with water at a high temperature.It is used for the manufacture of tyre cords, fabrics
and ropes.
Caprolactum is a 7-membered ring with 6 C atoms and 1 N atom. During polymerisation the rring breaks to form [-CO-(CH2)4-NH-] units.
Polyesters
products of dicarboxylic acids and diols. Dacron or terylene is the best known example.
It is manufactured by heating a mixture of ethylene glycol
(HO-CH2-CH2-OH) and terephthalic acid (HOOC-Ph-COOH) in the presence of zinc acetate-antimony trioxide catalyst.
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.
Phenol – formaldehyde polymer
These are obtained by the condensation reaction of phenol with formaldehyde in the presence of either an acid or a base catalyst.
Initially 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. This linear product is called Novolac. It is used in paints.
Novolac on heating with formaldehyde undergoes cross linking to form an infusible solid mass called bakelite. It is thermosetting polymer which cannot be reused or remoulded.
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.
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.
Natural rubber may be considered as a linear polymer of isoprene (2-methyl-1, 3-butadiene) [(CH2)2-CH=CH2-CH3] 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.
Vulcanisation of rubber
To improve upon these physical properties of rubber, a process of
vulcanisation is carried out.
This process consists of heating a mixture of raw rubber with sulphur and an appropriate additive at a temperature range between 373 K to 415 K. On vulcanisation, sulphur forms cross links at the reactive sites of double bonds and thus the rubber gets stiffened.
Synthetic Rubber
Synthetic rubber is any vulcanisable rubber like polymer, which
is capable of getting stretched to twice its length.
- Neoprene
Neoprene or polychloroprene is formed by the free radical polymerisation of chloroprene (CH2=CCl-CH=CH2) - Buna – N
Buna–N is obtained by the copolymerisation of 1, 3 – buta–1, 3–diene
and acrylonitrile (CH2=CH-CN) in the presence of a peroxide catalyst.
Biodegradeable polymers
-
Poly b-hydroxybutyrate or b-hydroxy valerate (PHBV): It is obtained by the copolymerisation of 3-hydroxybutanoic acid and 3 -hydroxypentanoic acid.
PHBV is used in speciality packaging, orthopaedic devices and in controlled release of drugs. PHBV undergoes bacterial degradation in the environment. -
Nylon 2–nylon 6
It is an alternating polyamide copolymer of glycine (H2N–CH2–
COOH) and amino caproic acid [H2N (CH2)5 COOH] and is
biodegradable.
Polypropene
Monomer: Propene
Structure: [-CH2-CH(CH3)-]
Uses: Manufacture ofropes, toys, pipes, fibres, etc.
Polystyrene
Monomer: Styrene
Structure: [-CH2-CH(C6H5)-]
Uses: As insulator, wrappingmaterial, manufacture of toys, radio and television cabinets.
Poly vinyl chloride
Monomer: Vinyl CHloride
Structure: [-CH2-CHCl-]
Uses: Manufacture of rain coats, hand bags, vinyl flooring, water pipes.
Urea formaldehyde resin
Monomer: urea and formaldehyde
Structure: [-NH-CO-NH-CH2-]
Uses:For making unbreakable cups and laminated sheets.
Gyptal
Monomer: Phthalic acid and ethelene glycol
Structure: [-OCH2-CH2O-OOC-Ph-CO-]
Uses:Manufacture of paints and lacquers.
