Question

(Ionic polymerization – effect of reaction medium) Show, using structures depicting the propagating chain end, and describe with words, why the hydrobromic acid (HBr) initiated polymerization of styrene proceeds about 1,900 times faster when 1,2-dichloromethane is used as the polymerization solvent relative to benzene.

Answer 1

Chain initiation:

HBr (aq.) ----------> H+ (aq.) + Br- (aq.)

C₆H₅HC = CH₂ + H⁺ ----------> C₆H₅CH₂-CH₂⁺

Chain propagation:

C₆H₅CH₂CH₂⁺ + CH₂=CH₂ --------> C₆H₅CH₂CH₂CH₂CH₂⁺

C₆H₅CH₂CH₂CH₂CH₂⁺ + n CH₂=CH₂ -----> C₆H₅CH₂CH₂[CH₂-CH₂]_n-CH₂CH₂⁺

SInce in cahin propagation step, the cabocation is obtained as the intermediate. This positively charged particle is stabilised if the solvent is polar but less stabilised if the solvent is non polar.

Hence, the polymarization of styrene proceeds 1900 times faster when in aqueous HBr solution (polar) than in CH2Cl2 ( a non polar solvent)