The chain length of silicones can be controlled by adding

[CH₃]₃SiCl blocks the end of chain, hence chain length is controlled.
(1) SiCl₄-used to produce high purity silicon and silica for commercial application
(2) [CH₃]₃ SiCl₂ on hydrolysis produces chain silicone
(3) [CH₃] SiCl₃ on hydrolysis from complex cross linked polymer
The chain length of the silicone polymer can be controlled by adding Me₃SiCl[CH₃]₃ SiCl. The reaction of the sequence can be represented as :
R+Me₃ SiCl→Me₃ Si-R+Cl
This blocks the end of the silicon polymer and thus terminates the chain propagation.
2Cl→Cl₂