Chapter 15: Q72. (page 570)

Question: As we will learn in Chapter 30, styrene derivatives such as A can be polymerized by way of cationic rather than radical intermediates. Cationic polymerization is an example of electrophilic addition to an alkene involving carbocations.
a. Draw a short segment of the polymer formed by the polymerization of A.
b. Why does A react faster than styrene (C₆H₅CH=CH₂)in a cationic polymerization?

Short Answer

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Answer

Polymerization of A

b. The methoxy group in A stabilizes an intermediate carbocation by resonance. Therefore, A has more reactivity than styrene for polymerization.

Step by step solution

Cationic polymerization

Cationic polymerization is a type of chain-growth polymerization which involves electrophilic addition to an alkene.

The polymerization is initiated by a lewis acid-base complex which adds to the monomer. The carbocation thus formed propagates the polymerization reaction.

Monomers having electron-donating groups that can stabilize the intermediate carbocation undergo cationic polymerization.

Mechanism of Cationic polymerization

Cationic polymerization involves three steps- initiation, propagation and termination.

In the initiation step, lewis acid and base react to form a lewis-acid base complex which reacts with monomer to form the carbocation.

In the propagation step, the carbocation formed reacts with the alkene monomer to form a new C-C bond. An addition reaction occurs forming another carbocation. This reaction is repeated to form long-chain polymers. The addition reaction follows Markonikov’s rule.

In the termination step, a new pi bond is formed by the loss of a proton from the carbon adjacent to the carbocation. This terminates the reaction.

Step 3: Cationic polymerization of compound A

a. Compound A undergoes cationic polymerization due to the presence of an electron-donating methoxy group.

Polymerization of A

b. Compound A has a methoxy substituent which is an electron-donating group. Therefore, the carbocation intermediately formed during the reaction can be stabilized by resonance. This stabilization is absent in styrene. Therefore, compound A can react faster than styrene in cationic polymerization.