Arrange the following monomers in order of decreasing ability to undergo cationic polymerisation. I. $\mathrm{CH}_2=\mathrm{CH}-\mathrm{C}_6 \mathrm{H}_4\left(\mathrm{NO}_2\right)$ II. $\mathrm{CH}_2=\mathrm{CH}-\mathrm{C}_6 \mathrm{H}_4\left(\mathrm{CH}_3\right)$ III. $\mathrm{CH}_2=\mathrm{CH}-\mathrm{C}_6 \mathrm{H}_4\left(\mathrm{OCH}_3\right)$

• A

  I $>$ II $>$ III
• B

  II $>$ I $>$ III
• Correct answer
  C

  III $>$ II $>$ I
• D

  I $>$ III $>$ II

Solution

Electron releasing groups such as $-\mathrm{CH}_3,-\mathrm{OCH}_3$ activate the monomer towards cationic polymerisation because these groups provide stability to the carbocation formed. On the other hand, $-\mathrm{NO}_2$ is a electron withdrawing group. So, it reduces the stability of carbocation formed. Thus, the correct order is

Hence, the correct answer is option: 'C'.