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Question:

Arrange the following monomers in order of decreasing ability to undergo cationic polymerization:
I. CH2=CH−C6H4(NO2)
II. CH2=CH−C6H4(CH3)
III. CH2=CH−C6H4(OCH3)

II>I>III

I>II>III

III>II>I

I>III>II

Solution:

Electron releasing groups such as −CH3, −OCH3 activate the monomer towards cationic polymer as it stabilizes the carbocation formed. On the other hand, −NO2 is an electron-withdrawing group. So, it reduces the stability of carbocation formed. Thus, the correct order is:
CH2=CH−C6H4(OCH3) > CH2=CH−C6H4(CH3) > CH2=CH−C6H4(NO2).