Problem 21 - Basic Principles of Organic Chemistry

Question

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

Step-by-Step Solution

Answer

At high CCl₄ concentrations, a chain transfer reaction adds chlorine to the polymer, reducing its molecular weight.

Step 1: Understand Polymerization Reaction

In the polymerization of ethenylbenzene (styrene) initiated by benzoyl peroxide, radicals are generated which can initiate the polymerization chain reaction. This typically involves a radical coupling with the ethenylbenzene monomer.

Step 2: Analyze Solvent Effects

The solvent system in this reaction is a mixture of tetrachloromethane (CCl₄) and benzene. The rate of the polymerization is independent of tetrachloromethane concentration, implying that CCl₄ does not participate in the initiation or propagation steps directly affecting the rate.

Step 3: Consider High CCl₄ Concentrations

At high concentrations of tetrachloromethane, the molecular weight of the polymer decreases and chlorine incorporation occurs. This suggests a chain transfer reaction mechanism involving CCl₄ that limits the chain length of the growing polymer.

Step 4: Identify Chain Transfer Reaction

A chain transfer reaction with tetrachloromethane involves the abstraction of a chlorine atom by the polymer chain radical, forming a chloroalkane. This ends the growing chain and produces a new radical that can initiate the growth of another polymer chain.

Key Concepts

Ethenylbenzene

Ethenylbenzene, commonly known as styrene, is an organic compound and an important monomer in polymer chemistry. It serves as the building block for polystyrene plastics, which are used in a wide range of products from packaging materials to insulation.
Styrene is composed of a benzene ring with a vinyl group attached, making it an aromatic monomer. This structure allows it to undergo radical polymerization effectively, which involves adding many styrene molecules together to form a long chain polymer.
The process begins with an initiator that forms free radicals. These radicals react with the double bonds of styrene, resulting in polymer growth. Ethenylbenzene’s simplicity and availability make it an ideal model for studying polymerization kinetics and mechanisms.

Benzoyl Peroxide

Benzoyl peroxide is a well-known initiator in radical polymerization processes. It serves a vital role by decomposing to form free radicals under specific conditions, particularly when heated.
These free radicals are highly reactive and initiate the chain reaction needed to form polymers. When used with ethenylbenzene, benzoyl peroxide decomposes to produce phenyl radicals, which then start polymerizing the vinyl groups in ethenylbenzene. This initiation step is crucial for starting the polymerization reaction and ensuring the propagation of polymer chains continues efficiently.
In industry, benzoyl peroxide is favored for its ability to initiate polymerization at moderate temperatures, which makes it both effective and energy efficient. Furthermore, it allows for controlled polymerization, which is essential for tailoring the physical properties of polymers.

Chain Transfer Reaction

The concept of a chain transfer reaction is important in understanding why the molecular weight of polymers changes. In radical polymerization, the reaction does not simply stop when all the available monomers react; instead, other reactions like chain transfer can occur.
A chain transfer reaction involves transferring the radical site from a growing polymer chain to another molecule. In this case, tetrachloromethane (CCl₄) plays a key role. During the polymerization of ethenylbenzene, high concentrations of CCl₄ can lead to a chain transfer where the radical on the growing polymer abstracts a chlorine atom from CCl₄.
This process results in the formation of a chloroalkane and ends the chain growth, while simultaneously creating a new radical that can initiate another polymer chain. Thus, the overall effect is shorter polymer chains and incorporation of chlorine atoms into the polymer, decreasing the molecular weight and altering its properties.

Tetrachloromethane Effects

Tetrachloromethane, also known as carbon tetrachloride, impacts the polymerization process in notable ways despite not affecting the polymerization rate directly.
When mixed with benzene in polymerizations, CCl₄ acts as a solvent but can also participate in secondary reactions like chain transfer. The chain transfer reaction with CCl₄ causes the inclusion of chlorine atoms in the resulting polymer chain, thus leading to a reduction in the average molecular weight of the polymer.
The presence of chlorine in the polymer modifies its chemical structure and properties, potentially making it less stable or changing its thermal and mechanical characteristics. This is why controlling the concentration of CCl₄ in the reaction mixture is crucial to achieving the desired polymer specification. Understanding these effects helps in tailoring the final characteristics of the polymer product.