Question

The rate of free-radical polymerization of styrene, induced by benzoyl peroxide in mixtures of carbon tetrachloride and benzene, is independent of the concentration of carbon tetrachloride. At high concentrations of carbon tetrachloride, the average molecular weight of the polymer is greatly reduced and chlorine is found in the polymer. Explain.

Short answer

The reduced average molecular weight of the polymer and the presence of chlorine in it, at high carbon tetrachloride (CCl4) concentrations, can be explained by the increased probability of termination reactions between the propagating polymer radicals and the chlorine radicals generated from CCl4. These termination reactions effectively end the growth of the polymer chain, resulting in a decreased average molecular weight. Additionally, chlorine radicals can react with the styrene monomers and become incorporated into the polymer structure.

Step-by-step solution

Understand free-radical polymerization

Free-radical polymerization is a process that involves the formation of covalent bonds between reactive species called free radicals. These radicals are highly reactive, short-lived species with unpaired electrons. In the case of the polymerization of styrene, the free radicals can react with the double bond present in the styrene monomers, leading to the formation of polymers.

Role of benzoyl peroxide

Benzoyl peroxide acts as an initiator in the free-radical polymerization process. It decomposes via homolytic cleavage, generating two benzoyloxy radicals that can induce polymerization by reacting with styrene monomers to form new radicals. These radicals can then react with other styrene monomers to propagate the polymer chain.

Role of carbon tetrachloride

Carbon tetrachloride (CCl4) acts as a solvent in this process but can also generate free radicals via homolytic cleavage, forming carbon radicals and chlorine radicals. However, the rate of polymerization remains unaffected, as the reactions involving the CCl4 radicals are insignificant compared to those involving the benzoyl peroxide.

Explaining the reduced molecular weight and chlorine incorporation at high CCl4 concentrations

At high concentrations of carbon tetrachloride, the probability of collisions between the propagating polymer radicals and carbon tetrachloride molecules becomes higher. This can lead to termination reactions where chlorine radicals, generated from carbon tetrachloride, react with the polymer radicals, effectively ending the growth of the polymer chain. As a result, the average molecular weight of the polymer decreases. Furthermore, the chlorine radicals can also react with the styrene monomers and get incorporated into the polymer structure, hence, the presence of chlorine in the polymer. In summary, the reduced average molecular weight and the presence of chlorine in the polymer at high carbon tetrachloride concentrations can be attributed to the increased probability of termination reactions between the polymer radicals and the chlorine radicals generated from carbon tetrachloride.

Key concepts

Benzoyl Peroxide

Benzoyl peroxide is a fascinating compound primarily used as an initiator in free-radical polymerization processes. Its molecular structure allows it to break down into radicals. The process begins with benzoyl peroxide undergoing homolytic cleavage. This cleavage splits the molecule into two benzoyloxy radicals. These newly formed radicals play a crucial role in beginning the polymerization of monomers such as styrene, a popular building block for many synthetic materials.

The activity of benzoyl peroxide is highly significant in the initiation phase. When it reacts with the styrene monomers, it forms new radicals. These new radicals propagate further by continuously reacting with additional styrene monomers, thereby elongating the polymer chain.

It's important to remember that benzoyl peroxide is not only fundamental in synthetic chemistry but also carries practical importance beyond this sphere. It's even used in acne treatments due to its antibacterial properties, showcasing its versatility. However, in the context of polymerization, its primary role remains as a powerful initiator.

Styrene

Styrene is an organic compound that serves as a building block for the creation of polystyrene through free-radical polymerization. This monomer contains a double bond, which is crucial for its reactivity in such polymerization processes. When initiated by free radicals, such as those generated by benzoyl peroxide, the double bonds in styrene monomers open up and link together to form long chains of durable plastic.

In practical terms, this reaction is the basis for a plethora of consumer products, from packaging materials to insulation. The allure of styrene lies in its ability to form polymers with desirable characteristics: strong, impact-resistant, and long-lasting.

Even as it serves indispensable purposes, it's crucial to handle styrene with care. When in its raw form, it can be hazardous, thus necessitating appropriate safety measures during handling and processing.

Carbon Tetrachloride

Carbon tetrachloride (CCl4) is often used as a solvent in chemical reactions due to its non-polar nature. However, it plays a more complex role in free-radical polymerization involving styrene and benzoyl peroxide. While it does not significantly affect the rate of polymerization, at high concentrations, carbon tetrachloride can indeed influence the final polymer properties.

This occurs because carbon tetrachloride can itself decompose to generate radical species, such as carbon and chlorine radicals. These radicals, while less reactive than those from benzoyl peroxide, can interact with the propagating radicals in the polymerization process. When chlorine radicals terminate growing polymer chains, the average molecular weight of the polymer decreases, and chlorine atoms get incorporated into the polymer matrix.

Thus, carbon tetrachloride's presence can lead to polymers with unexpected features—in particular, reduced size or altered chemical properties such as the inclusion of chlorine, which may impact stability and functionality.