Structurally, polyorganosiloxanes have the following characteristics:
(1) The main chain Si-O-Si bonds are inorganic, while the side chains are organic groups, making them a typical semi-inorganic, semi-organic polymer.
(2) The bond angle (∠SiOSi) of the Si-O-Si bonds in the main chain is 130°–145°, the bond length of the Si-O bonds is 0.163 nm–0.164 nm, and the bond energy is 367.8 kJ/mol. Compared with carbon oxides in the same group (bond angle ∠COC 108°, C-O bond length 0.142 nm, bond energy 357.4 kJ/mol), polysiloxanes have a large bond angle, long bond length, and high bond energy in their main chain.
(3) The C-Si-O bond angle in the side chain is 160°, and the C-Si bond length is 0.188 nm. Therefore, the activation energy required for the methyl group in the side chain to rotate around the Si-O bond is low, only 8 kJ/mol.
(4) The dipole-dipole interactions between the Si-O bonds in the polysiloxane backbone and the interactions between the Si-CH3 groups in the side chains cause the polysiloxane molecule to exist in a certain α-helical structure with the methyl group facing outward and the silicon-oxygen bond facing inward. This unique structure results in weak inter-chain interactions, a large molar volume, and low surface tension in polysiloxanes.
(5) Polysiloxanes with Si-H, Si-OR, or Si-OH groups as side groups are all reactive.
