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Polymerization of dienes with acid

Description: Treatment of a diene with strong acid leads to the formation of polymers.

Notes: The acid chosen for this reaction is usually one that has a counterion that won’t add to carbocations, such as H2SO4 or TsOH



Mechanism: This is what is known as a “cationic polymerization” since it proceeds through carbocations. Protonation of the diene leads to a resonance-stabilized carbocation (Step 1, arrows A and B). One of the double bonds from a second equivalent of diene then adds to this carbocation, generating a new resonance-stabilized carbocation (Step 2, arrow C). The process repeats (Step 3, arrow C) again (Step 4, arrow C) again and again N times. Eventually the carbocation is either quenched with a nucleophile (as shown, arrow D) or elimination occurs to form an alkene.

Notes: The last step could also be elimination to form an alkene.