Some Important Polymers

Non – Biodegradable Polymers

1. Polythene

There are two types of polythene as given below:

i. Low density polythene:

· It is obtained by the polymerisation of ethene under high pressure of 1000 to 2000 atmospheres at a temperature of 350 K to 570 K in the presence of traces of dioxygen or a peroxide initiator (catalyst).

· The low density polythene (LDP) obtained through the free radical addition and H-atom abstraction has highly branched structure.

· Low density polythene 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.

Low Density Polyethene (LDPE) structures

ii. 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) at a temperature of 333 K to 343 K and under a pressure of 6-7 atmospheres.

· High density polythene (HDP) thus produced, consists of linear molecules and has a high density due to close packing.

· It is also chemically inert and more tough and hard. It is used for manufacturing buckets, dustbins, bottles, pipes, etc.

High Density Polyethene (HDPE) structures

2. 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.

3. Polyacrylonitrile

· The addition polymerisation of acrylonitrile 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.

· Acrylic fibres have good resistance to stains, chemicals, insects and fungi.

4. Polyamides

· These polymers possessing amide linkages are important examples of synthetic fibres and are termed as nylons.

· The general method of preparation consists of the condensation polymerisation of diamines with dicarboxylic acids and also of amino acids and their lactams.

Preparation of nylons

i. Nylon 16,6:

· It is prepared by the condensation polymerisation of hexamethylenediamine with adipic acid under high pressure and at high temperature.

· Nylon 6, 6 is used in making sheets, bristles for brushes and in textile industry.

ii. Nylon 6:

· It is obtained by heating caprolactum with water at a high temperature.

· Nylon 6 is used for the manufacture of tyre cords, fabrics and ropes.

5. Polyesters

· These are the polycondensation products of dicarboxylic acids and diols.

· Dacron or terylene is the best known example of polyesters.

· It is manufactured by heating a mixture of ethylene glycol and terephthalic acid at 420 to 460 K in the presence of zinc acetateantimony trioxide catalyst as per the reaction given earlier.

· 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.

6. Phenol - formaldehyde polymer (Bakelite and related polymers)

· Phenol - formaldehyde polymers are the oldest synthetic polymers.

· These are obtained by the condensation reaction of phenol with formaldehyde in the presence of either an acid or a base catalyst.

· The reaction starts with the 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 groups.

· The initial product could be a linear product – Novolac used in paints.

· Novolac on heating with formaldehyde undergoes cross linking to form infusible solid mass called bakelite.

· It is used for making combs, phonograph records, electrical switches and handles of various utensils.

Bakelite structure

7. Rubber:

Natural rubber:

· Rubber is a natural polymer and possesses elastic properties. It is also termed as elastomer and has a variety of uses.

· It is manufactured from rubber latex which is a colloidal dispersion of rubber in water.

· This latex is obtained from the bark of rubber tree and is found in India, Srilanka, Indonesia, Malaysia and South America.

· Natural rubber may be considered as a linear polymer of isoprene (2-methyl-1, 3-butadiene) 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. Thus, it can be stretched like a spring and exhibits elastic properties.

Vulcanisation of rubber:

· Natural rubber becomes soft at high temperature (>335 K) and brittle at low temperatures (<283 K) and shows high water absorption capacity.

· It is soluble in non-polar solvents and is non-resistant to attack by oxidising agents.

· To improve upon these physical properties, 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.

· In the manufacture of tyre rubber, 5% of sulphur is used as acrosslinking agent.

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.

· Thus, synthetic rubbers are either homopolymers of 1, 3 - butadiene derivatives or copolymers of 1, 3 - butadiene or its derivatives with another unsaturated monomer.

Preparation of Synthetic Rubbers

1. Neoprene

· Neoprene or polychloroprene is formed by the free radical polymerisation of chloroprene.

· It has superior resistance to vegetable and mineral oils. It is used for manufacturing conveyor belts, gaskets and hoses.

2. Buna – N

· Buna –N is obtained by the copolymerisation of 1, 3 – butadiene and acrylonitrile in the presence of a peroxide catalyst.

· It is resistant to the action of petrol, lubricating oil and organic solvents. It is used in making oil seals, tank lining, etc.

Biodegradable Polymers

· 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.

· In view of the general awareness and concern for the problems created by the polymeric solid wastes, certain new biodegradable synthetic polymers have been designed and developed.

· These polymers contain functional groups similar to the functional groups present in biopolymers.

· Aliphatic polyesters are one of the important classes of biodegradable polymers.

Some important examples are:

Poly ꞵ-hydroxybutyrate – co-ꞵ-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)₅COOH]

· It is biodegradable.

Some Other Commercially Important Polymers