It is an oxidative polymerization of aniline to form polyaniline. You can see the following links for the mechanism of PANI formation.

Article The mechanism of the oxidative polymerization of aniline and...

http://shodhganga.inflibnet.ac.in/bitstream/10603/48043/12/12_chapter%203.pdf

http://onlinelibrary.wiley.com/doi/10.1002/pola.1989.080270720/abstract

(1) Roughly said, Mr. Rahman, your hint to the link you gave is correct. I have a few more details and corrections (correcting the above authors) to share, for answering Tapas' questions:

1. The initiation is known - first (after some period after the first addition of a few drops of persulfate solution), pernigraniline oligomer (!) is formed which then oxidizes aniline. That means, persulfate does not directly oxidize the aniline monomer, but it takes some time to form pernigraniline (via an unknown intermediate) which then performs the starting oxidation.

2. It should be noted that Polyaniline does not have a long chain - only about 12 monomer units long.

3. re "exothermic": it is very important to recognize that the oxidative aniline polymerisation proceeds in qualitatively completely different ways leading to totally different products depending upon whether you just add the persulfate to the reaction mixture more or less in 1 shot or very quickly, with no or inefficient cooling,

- or if you run the polymerisation at low temperatures (like 3 - 5 °C) under very controlled colling conditions, so that the temperature change is only 1 or 2 degrees.

(see next answer part (2))

(2) If you run the reaction very quickly with no or inefficient colling, the delta H is very high (I forgot the number) and the püroduct is totally not dispersible;

if you run at low temperature and control the temperature changes within max 1 or 2 °C, the delta H is low, and the product becomes dispersible.

(3) p-toluene sulfonic acid provides the protons (H+) to charge half of the aniline units, pTs(-) is then the counterion neutralizing the chain against the outside world. That reaction (pTsH + polyaniline) is an acid/base reaction.

Bernhard Wessling sir, thank you so much for this valuable information..

Md. Abdur Rahman sir, thank you so much.