Animated Solution for Chemistry - s and p-Block Elements: Among the following substituted silanes the one which will give rise to cross linked silicone polymer on hydrolysis is

Silicones are a fascinating class of synthetic polymers. Unlike organic polymers that rely on a carbon-carbon backbone, silicones are built upon a robust, alternating silicon-oxygen-silicon ($-\text{Si-O-Si}-$) framework. This unique inorganic backbone gives them incredible thermal stability, water resistance, and flexibility. But how do chemists actually construct these versatile materials? The secret lies in the starting materials: substituted chlorosilanes.

Hydrolysis

The journey from a small molecule to a massive polymer begins with a simple reaction: hydrolysis. When alkyl or aryl substituted chlorosilanes are exposed to water, the highly reactive silicon-chlorine ($\text{Si-Cl}$) bonds are broken. The chlorine atoms are replaced by hydroxyl ($-\text{OH}$) groups, transforming the chlorosilanes into silanols.

Once these silanols are formed, they don't stay isolated for long. They undergo a condensation reaction, where two $-\text{OH}$ groups react, releasing a molecule of water and forming the crucial $\text{Si-O-Si}$ linkage. The structure of the final polymer depends entirely on how many $-\text{OH}$ groups are available on each monomer.

The Power of Chlorine

Let's evaluate the options provided in the question to see how the number of chlorine atoms dictates the polymer's architecture.

1. The Inert Molecule: $R_4\text{Si}$ This molecule has no chlorine atoms. It consists entirely of strong, non-polar $\text{Si-C}$ bonds. Because it lacks hydrolyzable groups, it cannot form silanols and therefore cannot participate in polymerization.

2. The Chain Terminator: $R_3\text{SiCl}$ With only one chlorine atom, hydrolysis yields a silanol with a single reactive site: $R_3\text{SiOH}$. When this molecule condenses, it can only attach to one other molecule. It effectively "caps" the end of a growing chain, preventing further extension. Hence, it forms simple dimers ($R_3\text{Si-O-Si}{R}_3$) and acts as a chain terminator.

3. The Chain Builder: $R_2{\text{SiCl}}_2$ This molecule possesses two chlorine atoms, leading to the formation of a diol: $R_2{\text{Si(OH)}}_2$. Having two reactive sites allows these molecules to link end-to-end continuously. This results in the formation of long, linear silicone chains or, occasionally, cyclic rings. However, they cannot branch out to form a network.

Finally, we arrive at $R{\text{SiCl}}_3$. This molecule is the key to our problem. It contains three hydrolyzable chlorine atoms. Upon hydrolysis, it forms a triol: $R{\text{Si(OH)}}_3$.

This triol is a game-changer. Because it has three reactive $-\text{OH}$ groups, it can undergo condensation in three different directions simultaneously. It doesn't just form a straight line; it branches out, connecting multiple linear chains together. This extensive branching creates a rigid, interconnected, three-dimensional cross-linked network.

To synthesize a cross-linked silicone polymer, the monomer must be capable of forming bonds in more than two directions. Among the given choices, only $R{\text{SiCl}}_3$ provides the three necessary reactive sites after hydrolysis to act as the architect of a 3D network. Therefore, the correct answer is (b).