Explain the polymerization of alkenes with examples.

(N/A) Definition: Polymerization is a process in which a large number of small monomer molecules combine to form a large macromolecule called a polymer under high temperature,high pressure,and in the presence of a catalyst.
$(b)$ Examples:
$(i)$ Polymerization of ethene: $nCH_2=CH_2 \xrightarrow[\text{High pressure, catalyst}]{473-673 \ K, O_2} (-CH_2-CH_2-)_n$ (Polythene)
$(ii)$ Polymerization of propene: $nCH_3-CH=CH_2 \xrightarrow[\text{High pressure, catalyst}]{\text{High temperature}} (-CH(CH_3)-CH_2-)_n$ (Polypropene)
$(c)$ Uses: Polymers are used for the manufacture of plastic bags,squeeze bottles,refrigerator dishes,toys,pipes,radio and $T$.$V$. cabinets,etc. Polypropene is used for the manufacture of milk crates,plastic buckets,and other moulded articles.
Limitations: Though these materials have now become common,excessive use of polythene and polypropylene is a matter of great concern for the environment.