Animated Solution for Chemistry - Organic Chemistry: Buna-N synthetic rubber is a copolymer of

Have you ever wondered how chemists name complex polymers? It might seem like a random jumble of letters and numbers, but in reality, it's a highly logical code. Take Buna-N, for example. This isn't just a catchy trade name; it's a complete recipe for making one of the most important synthetic rubbers in the modern world. Let's break down this code and uncover the chemistry behind it.

When we look at the word Buna-N, we can split it into three distinct parts: Bu, na, and N. Each of these parts tells us something crucial about how this polymer is synthesized.

The first part, Bu, stands for butadiene. Specifically, it refers to $1,3\text{-butadiene}$, which is a simple hydrocarbon chain containing four carbon atoms and two double bonds. This is the primary building block of our polymer.

The second part, na, might seem a bit out of place. It doesn't stand for a monomer, but rather for the elemental symbol of Sodium ($\text{Na}$). In this reaction, sodium acts as a catalyst, initiating the polymerization process.

Finally, the N stands for nitrile. This refers to the second monomer used in the process, which is acrylonitrile (also known as vinyl cyanide).

To truly understand Buna-N, we need to look closely at its two monomers.

The first monomer is $1,3\text{-butadiene}$. Its chemical structure is ${\text{H}}_2\text{C}=\text{CH}-\text{CH}={\text{CH}}_2$. Notice the two double bonds separated by a single bond? This is called a conjugated system. It's highly reactive and perfect for forming long polymer chains.

The second monomer is acrylonitrile. Its structure is ${\text{H}}_2\text{C}=\text{CH}-\text{CN}$. It consists of a vinyl group (${\text{H}}_2\text{C}=\text{CH}-$) attached to a highly electronegative cyanide group ($-\text{CN}$). This cyanide group is what gives Buna-N its unique properties, which we'll discuss later.

Now, what happens when we mix these two monomers together in the presence of sodium? A fascinating chemical dance begins, known as addition copolymerisation.

Unlike homopolymers, which are made from a single type of monomer (like polyethene from ethene), copolymers are made from two or more different types of monomers. In this case, $1,3\text{-butadiene}$ and acrylonitrile join forces.

During the reaction, the double bonds in both monomers break open. The electrons from these double bonds are used to form new single bonds between the adjacent molecules.

The reaction can be represented as:

Notice how the double bonds in butadiene shift to the center, and new bonds extend from the ends to connect with the acrylonitrile molecules. This creates a massive, continuous chain.

You might be wondering, why do we need sodium? Sodium is an alkali metal that readily gives up its valence electron. In this reaction, it acts as an initiator for anionic polymerization. It transfers an electron to the monomer, creating a reactive intermediate (a radical anion) that kicks off the chain reaction. Without sodium, the monomers would just sit there, staring at each other.

Nitrile Rubber

The resulting polymer, Buna-N, is also commonly known as nitrile rubber.

Because of the polar nitrile groups scattered throughout the polymer chain, Buna-N is incredibly resistant to non-polar substances like oils, greases, and gasoline. Natural rubber, on the other hand, would swell and degrade if exposed to these chemicals.

This makes Buna-N the material of choice for manufacturing automotive oil seals, fuel hoses, and protective gloves used in chemical laboratories.

Returning to our original question, we were asked to identify the monomers of Buna-N. By decoding the name, we found that it is a copolymer of $1,3\text{-butadiene}$ (${\text{H}}_2\text{C}=\text{CH}-\text{CH}={\text{CH}}_2$) and acrylonitrile (${\text{H}}_2\text{C}=\text{CH}-\text{CN}$).

Looking at the given options, option (c) perfectly matches our findings. It's a beautiful example of how understanding chemical nomenclature can lead you straight to the answer!