Question

An important use of radical-chain reactions is in the polymerization of ethylene and substituted ethylene monomers such as propene, vinyl chloride [the synthesis of which was discussed in Section \(7.6\) along with its use in the synthesis of poly(vinyl chloride), (PVC)], and styrene. The reaction for the formation of \(\mathrm{PVC}_{\text {, where }} n\) is the number of repeating units and is very large, follows. (a) Give a mechanism for this reaction (see Chapter 29). (b) Give a similar mechanism for the formation of poly(styrene) from styrene. Which end of the styrene double bond would you expect \(R\) - to attack? Why? C=CC1=CCCCC1 Styrene

Short answer

Answer: The most favorable attack site for an attacking radical on a styrene monomer during the polymerization of polystyrene is the terminal carbon (C=C) of the double bond. This is because the terminal carbon is more electron-rich, making it an attractive site for the electron-deficient radical. Moreover, the resulting new radical is stabilized by resonance with the phenyl ring, further supporting the rationale for this preferred attack site.

Step-by-step solution

(Step 1: Formation of PVC Radical Chain Mechanism)

To form PVC, we need to propose a mechanism for the polymerization of vinyl chloride. The radical chain mechanism typically involves three stages: initiation, propagation, and termination. Initiation: In this step, a radical initiator, such as a peroxide or an azo compound, decomposes to form two radicals. These radicals can then attack the vinyl chloride monomer to create a new radical. Propagation: In this step, the newly formed radical reacts with another vinyl chloride monomer to form a chain containing two vinyl chloride units and a radical at the end. This radical can further react with more vinyl chloride monomers, extending the chain. This process continues until the termination step occurs. Termination: The termination step occurs when two radicals react, forming a stable molecular product and ending the chain.

(Step 2: Formation of Polystyrene Radical Chain Mechanism)

To form polystyrene, we need to propose a similar mechanism for the polymerization of styrene. Similar to PVC formation, the radical chain mechanism consists of initiation, propagation, and termination stages. Initiation: A radical initiator decomposes to form two radicals. These radicals can then attack the styrene monomer to create a new radical. Propagation: The newly formed radical reacts with another styrene monomer to form a chain containing two styrene units and a radical at the end. This radical can further react with more styrene monomers, extending the chain. The process continues until the termination step occurs. Termination: The termination step occurs when two radicals react, forming a stable molecular product and ending the chain.

(Step 3: Attack Site on Styrene)

When the attacking radical approaches the styrene double bond, we need to consider which end of the double bond will be more favorable for the attack. In styrene, the phenyl group is electron-withdrawing due to its resonance effect, thus making the carbon atom attached to the phenyl group less electron-rich. Conversely, the terminal carbon of the double bond is more electron-rich, making it a more favorable site for the radical attack. Therefore, we expect the radical to attack the terminal carbon (C=C) of the styrene monomer.

(Step 4: Explanation for the Preferred Attack Site)

The attacking radical is electron-deficient, which means it will be attracted to electron-rich areas (sites) in the molecule. In styrene, the phenyl group's resonance effect decreases the electron density on the carbon atom attached to the phenyl group, making it less favorable for the radical attack. In contrast, the terminal carbon of the double bond is more electron-rich, meaning the radical will preferentially attack this carbon. The resulting new radical is stabilized by resonance with the phenyl ring, which further supports the rationale for the preferred attack site.