Question

With the aid of diagrams describe the radical polymerization of vinyl acetate using AIBN as initiator. Describe initiator formation, monomer initiation, and propagation steps. In addition, describe the various mechanisms by which chain termination (no capacity for further growth) can occur.

Describe the various ways in which charge transfer could occur (potential for further growth) for the polymerization described in part (b) above.

Answer 1

In free radical polymerization, the reactive intermediate is free radical . Here, Vinyl acetate acts as monomer. The inititator decomposes to give free radical which initiate the chain reaction. In this case, AIBN ( azidoisobutyronitrile) is used as initiator which decomposes to form isopropylnitrile radical.

Free radical polymerization occurs through 3 steps.

1) chain initiation step: In this step, initiator free radical adds to the double bond of the vinyl acetate monomer to form a new free radical of larger size.

2) chain propagation step : in this step, the addition of radical (l) to another molecule of vinyl acetate forms another radical (ll) . This process gets repeated over and over again to form a bigger size radical.

3) chain termination step : The polymer chain (lll) is terminated by consuming the radical either by combination or disproportionation.

Ways of chain transfer

1)The growth of original polymer chain may get terminated or interrupted in presence of certain compounds such as CCl₄ which compete with the monomer for the growing polymer chain. It forms a new radical which can initiate its own polymerization chain. The compounds like CCl₄ are called chain transfer agents which transfers the original chain i. e. Initiates a new polymer chain.

2) chain transfer agents can act as inhibitor by inhibiting the polymerization process. This happens if chain transfer agents form a highly unreactive radical which leads to termination of original chain. However, the new chain is not initiated. As a result, polymerization process stops. Amines, quinones can act as inhibitors.

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