Question

Which of these is not a monomer for a high molecular mass silicone polymer?
(A)- $MeSiC{{l}_{3}}$
(B)- $M{{e}_{2}}SiC{{l}_{2}}$
(C)- $M{{e}_{3}}SiCl$
(D)- $PhSiC{{l}_{3}}$

Answer 1

Hint: For the formation of the silicon polymer, the methyl chlorides react with the silicon, forming the methyl chlorosilane, which on hydrolysis form the methyl silanols. These are unstable and undergo condensation with another silanol molecule to form polymers.

Complete step by step solution:
Firstly, the silicones are polymers having a continuous linkage of the Si-O-Si bonds present in it, with $({{R}_{2}}SiO-)$ forming the repeating unit. Hence, known as the organosilicon compounds.
So, the given R-substituted chlorosilane compounds where R can be an alkyl or aryl, are formed in presence of the catalyst (Cu), by the reaction between alkyl chloride and silicon at high temperature, which on hydrolysis replaces the -Cl by the -OH group, as follows:
\[2RCl+Si\xrightarrow{Cu\,\,catalyst}{{R}_{2}}SiC{{l}_{2}}\xrightarrow{2{{H}_{2}}O}{{R}_{2}}Si{{(OH)}_{2}}\]
Hence, taking a methyl group (Me), the reaction of the methyl chloride with silicon produces various methyl- substituted chlorosilanes such as $MeSiC{{l}_{3}}$, $M{{e}_{2}}SiC{{l}_{2}}$, $M{{e}_{3}}SiCl$as above.
And on further hydrolysis, $MeSiC{{l}_{3}}$ forms $MeSi{{(OH)}_{3}}$, $M{{e}_{2}}SiC{{l}_{2}}$ forms $M{{e}_{2}}Si{{(OH)}_{2}}$ and $M{{e}_{3}}SiCl$ forms $M{{e}_{3}}SiOH$.
These methyl silanols ($MeSi{{(OH)}_{3}}$ and $M{{e}_{2}}Si{{(OH)}_{2}}$) are unstable and condense eventually to form a polymer, as follows:
$OH-M{{e}_{2}}Si-OH+OH-M{{e}_{2}}Si-OH\,\,\,\,\xrightarrow[-{{H}_{2}}O]{}\,\,\,\,\,-O-{{(M{{e}_{2}}Si-O)}_{n}}-M{{e}_{2}}Si-$
where $-(M{{e}_{2}}Si-O)-$ is the repeating unit forming silicone.
But in case of $M{{e}_{3}}SiOH$, it condenses to form a dimer due to the presence of only a single -OH group. Thus, does not form the higher molecular mass silicon polymer.

Therefore, option (C)- $M{{e}_{3}}SiCl$ is not a monomer for a high molecular mass silicone polymer.

Note: $M{{e}_{3}}SiCl$ due to its inability to condense further after the dimer formation, it often acts as the chain terminating organ silane by blocking the addition of another organosilicon molecule to the polymeric chain. Thus, maintains the length of the chain.
The $MeSi{{(OH)}_{3}}$forms a cross-linked chain of polymer due to the presence of the -OH group in all three directions.