**2.2 Functional siloxanes and polysiloxanes**

Siloxane oligomers (linear and cyclic) and polysiloxanes (silicones) are mainly prepared by so called a multistep "hydrolytic polycondensation" of chlorosilane monomers of different functionality. This step-growth process consists of elementary hydrolysis and condensation reactions:

$$\text{\#Si-Cl} \quad + \text{\text{H}\_2\text{O}} \quad \xrightarrow{\text{---}} \quad \text{\#Si-OH} \quad + \text{\text{HCl}} \tag{1}$$

$$\text{\tiny\text{\tiny\text{\tiny\text{Si-Cl}}}\text{ + H}\text{O-Si}\text{\rightarrow}} \longrightarrow \text{\tiny\text{\tiny\text{Si-O-Si}}\text{\rightarrow H}\text{\rightarrow HCl}}\tag{2}$$

$$\text{\#Si-OH} \text{ + \text{ HO-Si}=} \begin{array}{c} \longrightarrow \end{array} \text{\#C-Si=} + \text{\text{H}\_2\text{O}} \tag{3}$$

Linear oligo- and polysiloxanes with diferent functional (,-terminal or pendant) groups and functional cyclosiloxanes find many applications in further syntheses. These functionalities include most often silanol, chlorosilyl, hydrosilyl, vinyl, and allyl groups.
