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.

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.

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.

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.

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.