A polymer is a large molecule or a macromolecule formed by joining many repeated subunits. They may be naturally found in plants and animals (known as the natural polymer) or may be man-made (called the synthetic polymer). Different polymers have a number of unique physical and chemical properties due to which they have become part and parcel of our life. In this article, we will discuss the preparation of some important addition polymers like polyethylene, Teflon, and polyacrylonitrile.
Polythene is the most common plastic used broadly in the packaging industry. Based on the density, polythene can be classified into two types:
- Low-density polythene: It has a density range of 0.910–0.940 g/cm3 and is prepared by the free-radical polymerization of ethane. The reaction is carried out at a temperature of 350 K to 570 K under the pressure of 1000 to 2000 atmospheres in the presence of a catalyst, dioxygen (in traces) or a peroxide initiator. The highly branched structure of LDP gives it a unique flow property in the molten state. It is a poor conductor of electricity and is chemically inert. The LDPs are used for making plastic bags and film wrap.
- High-density polythene: It has a density greater than or equal to 0.941 g/cm3 and has a low degree of branching. It is obtained when addition polymerization of ethene takes place in a hydrocarbon solvent. The reaction is carried out under a pressure of 6 to 7 atmospheres and at a temperature of 333 K to 343 K in the presence of Ziegler-Natta catalysts or metallocene catalysts. HDPs are chemically inert as well and are used in making bottles, butter tubs, milk jugs, water pipes and garbage containers.
Teflon is manufactured by free-radical polymerization of tetrafluoroethylene. The catalyst used is persulphate at high pressures. The reaction is given as:
n F2C=CF2 → −(F2C−CF2)n−
Teflon is hydrophobic and is inert in nature. It is used in making a non-stick coating for cookware and also as a lubricant in machinery to reduce friction.
It is manufactured by free radical polymerization of acrylonitrile. Peroxide is used as a catalyst in the reaction given by:
n H2C=CHCN → −(H2C−CHCN)n−
Polyacrylonitrile finds application in making tennis rackets, bicycles, missiles, fishing rods, rocket motors, etc.
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