Preparation of Important Addition Reaction
Polythene: - There are two types of 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.
- 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.
- Polytetrafluroethene (Teflon)
- Teflon is manufactured by heating tetrafluoroethene with a free radical or per sulphate 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.
- 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.
Condensation Polymerization or step growth polymerization
- This type of polymerisation involves a repetitive condensation reaction between two bi-functional monomers.
- These poly condensation reactions results in the loss of some simple molecules as water, alcohol, etc., and lead to the formation of high molecular mass condensation polymers.
- In these reactions, the product of each step is again a bi-functional species and the sequence of condensation goes on.
- Since, each step produces a distinct functionalised species and is independent of each other; this process is also called as step growth polymerisation.
- The formation of Terylene or Dacron by the interaction of ethylene glycol and terephthalic acid is an example of this type of polymerisation.