Q.If the starting material for the manufacture of silicones is RSiCl3, write the structure of the product formed.

$\text{RSiCl}_3 + 3\text{H}_2\text{O} \rightarrow \text{RSi(OH)}_3 + 3\text{HCl}$ …

1. Hydrolysis of RSiCl$_3$. In moist air/water, each Si–Cl bond is hydrolysed to Si–OH:

This gives a silanol, RSi(OH)$_3$, with three –OH groups on each silicon (i.e. RSiCl$_3$ is "trifunctional").

2. Condensation polymerisation. These silanol units then undergo intermolecular condensation: an –OH on one silicon reacts with an –OH on a neighbouring silicon, eliminating a water molecule and forming an Si–O–Si linkage:

3. Why the product is cross-linked (3-D), not linear. Because each silicon atom retains three –OH groups (after losing only one Cl-derived bond to its own R group), it can condense with up to three different neighbouring silicon atoms — one Si can therefore branch out to link with three others rather than just two, as would be the case for the difunctional R$_2$SiCl$_2$ (which gives simple linear chains). This trifunctional linking builds a cross-linked, three-dimensional (sheet-like/network) silicone structure.

4. Resulting structure. Each silicon atom sits at a branch point, bonded to its own R group and to three oxygen bridges connecting it to neighbouring silicon atoms, each of which is in turn bonded to its own R group and further oxygen bridges — extending indefinitely into a cross-linked polymer: …

Hydrolyse RSiCl$_3$ to the trifunctional silanol RSi(OH)$_3$, then apply condensation polymerisation (loss of water between –OH groups) to build the Si–O–Si backbone; since each Si retains three condensable –OH groups (vs. two for R$_2$SiCl$_2$), the product must be cross-linked/three-dimensional rather than a simple linear chain. C …

• Drawing a simple straight-chain silicone (as would form from the difunctional R$_2$SiCl$_2$) instead of recognising RSiCl$_3$'s trifunctionality forces a cross-linked, 3-D structure.
• Omitting the hydrolysis step (RSiCl$_3\to$RSi(OH)$_3$) and jumping straight to a guessed polymer structure without justifying it. …