Animated Solution for Chemistry - s and p-Block Elements: Under hydrolytic conditions, the compounds used for preparation of linear polymer and for chain termination, respectively, are

Welcome to the fascinating world of silicones! These are synthetic organosilicon polymers that have a backbone of alternating silicon and oxygen atoms, with organic groups (like methyls) attached to the silicon. Because of their unique structure, they are incredibly versatile—used in everything from waterproof sealants to medical implants.

To create these polymers, we don't just mix silicon and oxygen. We start with specific precursor molecules called chlorosilanes. When these chlorosilanes are exposed to water, they undergo a process called hydrolysis, where the chlorine atoms are replaced by hydroxyl ($-OH$) groups. This sets the stage for the molecules to link together.

Linear Polymers

If our goal is to build a long, straight chain—a linear polymer—we need a building block that can connect to exactly two other molecules. Think of it like holding hands; you need two hands to form a continuous line of people.

This is where dimethyldichlorosilane, $({\text{CH}}_3{)}_2{\text{SiCl}}_2$, comes in. As the name suggests, it has two chlorine atoms. When it reacts with water, it undergoes hydrolysis to form dimethylsilanediol, $({\text{CH}}_3{)}_2{\text{Si(OH)}}_2$.

Now, these diol molecules are ready to polymerize. They undergo a condensation reaction, where the $-OH$ group of one molecule reacts with the $-OH$ group of another, releasing a molecule of water and forming a strong $\text{Si}-\text{O}-\text{Si}$ bond. Because each silicon atom has exactly two $-OH$ groups, the chain can only grow end-to-end, resulting in a beautiful linear polymer.

Chain Termination

But there is a catch. If we just let the condensation reaction run, the polymer chain could theoretically grow forever, becoming too viscous or unmanageable. To control the length of the polymer, we need a way to hit the brakes. We need a chain terminator.

A chain terminator is a molecule that can attach to the growing end of the polymer but cannot form any further connections. It needs exactly one reactive site.

Enter trimethylchlorosilane, $({\text{CH}}_3{)}_3\text{SiCl}$. This molecule has three unreactive methyl groups and only one chlorine atom. Upon hydrolysis, it forms trimethylsilanol, $({\text{CH}}_3{)}_3\text{SiOH}$.

When this molecule bumps into the active $-OH$ end of our growing linear polymer, it condenses and caps the chain. Because the newly attached end is a $-{\text{Si(CH}}_3{)}_3$ group with no more $-OH$ groups available, the chain is officially terminated. No further growth can occur at that end.

Why the Other Options Fail

Let's quickly look at why the other compounds mentioned in the options wouldn't work for this specific task.

If we used methyltrichlorosilane, ${\text{CH}}_3{\text{SiCl}}_3$, it would hydrolyze to form a triol. With three reactive sites, it would branch out in multiple directions, creating a rigid, cross-linked 3D network rather than a flexible linear chain.

Similarly, silicon tetrachloride, ${\text{SiCl}}_4$, has four reactive sites and would form an even denser 3D network (silicic acid).

Finally, tetramethylsilane, ${\text{Si(CH}}_3{)}_4$, has zero chlorine atoms. The $\text{Si}-\text{C}$ bonds are highly stable and do not undergo hydrolysis under these conditions. It is completely unreactive here.

Therefore, to get a linear polymer and successfully terminate it, we must use $({\text{CH}}_3{)}_2{\text{SiCl}}_2$ as the builder and $({\text{CH}}_3{)}_3\text{SiCl}$ as the stopper. This makes Option (B) the perfect answer!