Question

Polymerization of butadiene \(\left(\mathrm{CH}_{2}=\mathrm{CHCH}=\mathrm{CH}_{2}\right)\) does not yield the same polymer as polymerization of ethylene \(\left(\mathrm{CH}_{2}=\mathrm{CH}_{2}\right)\) a. How could we convert poly(butadiene) into poly(ethylene)? b. Predict the reactivity of poly(butadiene) with HBr.

Short answer

Answer: The main structural difference between poly(butadiene) and poly(ethylene) is the presence of double bonds in the backbone chain of poly(butadiene). Poly(butadiene) would be expected to react with HBr due to these double bonds, undergoing an addition reaction that incorporates bromine atoms into its structure.

Step-by-step solution

Understand the structures of poly(butadiene) and poly(ethylene)

Poly(butadiene) is formed from the polymerization of butadiene \((\mathrm{CH}_{2}=\mathrm{CHCH}=\mathrm{CH}_{2})\). Its structure contains double bonds along the backbone chain, and these double bonds provide unique reactions and properties that distinguish it from poly(ethylene), which has a structure of \((\mathrm{CH}_{2})(\mathrm{CH}_{2})_{n}\).

Conversion of poly(butadiene) to poly(ethylene)

To convert poly(butadiene) into poly(ethylene), we need to remove the double bonds from the backbone chain. This can be achieved by performing a hydrogenation reaction, where hydrogen gas is added across the double bonds in the presence of a metal catalyst (commonly palladium or platinum), turning the double bonds in poly(butadiene) into single bonds, resulting in poly(ethylene).

Predict the reactivity of poly(butadiene) with HBr

The reactivity of poly(butadiene) with HBr depends on the presence of double bonds in its structure. HBr can undergo an addition reaction with these double bonds, adding bromine to one carbon and hydrogen to the other carbon involved in the double bond. This reaction will lead to the formation of a new compound with bromine atoms incorporated in the backbone chain. Poly(butadiene) is expected to react with HBr because of the presence of the double bonds, whereas poly(ethylene), which has no double bonds, would not react with HBr under normal conditions.

Key concepts

Polyethylene

Polyethylene is a commonly used polymer derived from the polymerization of ethylene, which is a simple hydrocarbon with the formula \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\).
Polyethylene is characterized by its repeating \(\mathrm{-(CH_2-CH_2)-}_n\) structure, which means it consists of long chains of carbon atoms with hydrogen atoms attached.
The absence of double bonds in polyethylene makes it chemically stable and less reactive than other polymers with double bonds, such as polybutadiene.There are several key features of polyethylene:

• It is chemically inert – meaning it does not easily react with other substances.
• Polyethylene can be easily processed and molded into various shapes, making it versatile for many applications such as plastic bags, bottles, and containers.
• Despite its simplicity, it can have different densities and branch structures, resulting in varying mechanical properties and uses.

Polybutadiene

Polybutadiene is a type of synthetic rubber obtained from the polymerization of butadiene \(\left(\mathrm{CH}_{2}=\mathrm{CHCH}=\mathrm{CH}_{2}\right)\).
This polymer contains double bonds along its backbone, which provides unique mechanical properties like elasticity and resilience.
The presence of these double bonds also makes polybutadiene more chemically active compared to polyethylene.Polybutadiene has several notable properties and applications:

• It is highly resilient and capable of being stretched, making it essential in the manufacturing of tires, where it helps provide grip and performance.
• The double bonds allow for modifications and reactions that can enhance its chemical properties, such as cross-linking to improve durability.
• Polybutadiene can be used in combination with other polymers for creating diverse materials with specific desired properties.

Hydrogenation

Hydrogenation is a chemical reaction that involves the addition of hydrogen to a compound, typically in the presence of a catalyst.
This process can be applied to unsaturated polymers like polybutadiene, which contains double bonds, converting them into single bonds by adding hydrogen atoms.
When polybutadiene undergoes hydrogenation, its structure changes, potentially leading to a material more similar to polyethylene.The hydrogenation process:

• Involves hydrogen gas ( \(\mathrm{H}_2\)) and a metal catalyst – commonly palladium or platinum.
• By breaking the double bonds and adding hydrogen, the unsaturated polymer becomes saturated, increasing stability but reducing reactivity.
• This conversion can result in materials with varying properties such as enhanced resistance to oxidation and degradation, making the end product more durable.

Addition Reaction with HBr

An addition reaction occurs when atoms are added to a double bond in an unsaturated molecule.
For polybutadiene, which has multiple double bonds, reactions with hydrogen bromide (HBr) are a common example of this process.In the addition reaction with HBr:

• HBr splits into \(\mathrm{H}^+\) and \(\mathrm{Br}^-\) ions, allowing each to attach to the carbons involved in the double bond.
• Typically, the hydrogen ion adds to the less substituted carbon (Markovnikov's rule), while bromine attaches to the more substituted one.
• This reaction can lead to a variety of brominated products, depending on the structure and the number of double bonds present in the polymer.

Reactions like these highlight the reactivity of polybutadiene, as similar reactions would not naturally occur in polyethylene due to its lack of double bonds.