Tris(2-methoxyethoxy)vinylsilane

Endpoint summary

Description of key information

The substance, tris(2-methoxyethoxy)vinylsilane is not stable in water, which affects the approach to the determination of physicochemical properties.

 

Tris(2-methoxyethoxy)vinylsilane is a liquid at standard temperature and pressure, with a melting point of -130°C and a predicted boiling point of 280°C at 1013 hPa. It has a predicted density of 1.02 g/cm³ at 20°C and a viscosity value of 2.8 mPa.s at 20°C. It is not expected to be surface active.

 

The substance is not classified for flammability according to Regulation (EC) No. 1272/2008 on the basis of a measured flash point of 133°C at 1013 hPa and predicted boiling point of 280°C at 1013 hPa. It has an auto ignition temperature of 213°C at 1013 hPa and is not explosive and is not oxidising on the basis of structural examination.

 

The reliability of the reported data for viscosity cannot be assigned. Therefore, further testing is planned for this endpoint.

 

In contact with water, tris(2-methoxyethoxy)vinylsilane reacts rapidly (half-lives of 1.6 minutes at pH 4, 61.5 minutes at pH 7 and 0.94 minutes at pH 9 and 25°C) to form vinylsilanetriol and 2-methoxyethanol according to the following equation:

 

(CH₃OCH₂CH₂O)₃SiCH=CH₂ + 3H₂O --> (HO)₃SiCH=CH₂ + 3CH₃OCH₂CH₂OH

 

Therefore, the requirements for testing of water-based physicochemical properties for the substance are waived on the basis of instability in water. The properties of the hydrolysis products, vinylsilanetriol and 2-methoxyethanol are assessed instead.

 

However, the key physicochemical properties of tris(2-methoxyethoxy)vinylsilane have been predicted using validated QSAR estimation methods. The submission substance has a predicted log K_ow of 0.3 at 20°C, predicted water solubility of 2.2E+05 mg/L at 20°C and a vapour pressure value of 0.43 Pa at 25°C.

 

2-Methoxyethanol has a vapour pressure of approximately 1300 Pa at 25°C, it is miscible with water and has a log K_ow of -0.77.

 

The silanol hydrolysis product, vinylsilanetriol may undergo condensation reactions in solution to give siloxane dimers, linear and cyclic oligomers and highly cross-linked polymeric particles (a colloidal suspension of small solid particles known as a sol) that may over time form an insoluble gel and a dynamic equilibrium is established. The overall rate and extent of condensation is dependent on nominal loading, temperature, and pH of the system, as well as what else is present in the solution.

 

The condensation reactions of silanetriols may be modelled as an equilibrium between monomer, dimer, trimer and tetramer, with the linear tetramer cyclising to the thermodynamically stable cyclic tetramer. At higher loadings, cross-linking reactions between the cyclic tetramers may occur. The reactions are reversible unless the cyclic tetramer concentration exceeds its solubility; in this case, the cyclic tetramer forms a separate phase, driving the equilibrium towards the tetramer. At loadings below 500 mg/L of vinylsilanetriol, the soluble monomer is expected to predominate in solution (>99%), with small amounts of dimer and oligomers. Condensation reactions are expected to become important at loadings above about 1000 mg/L causing the formation of insoluble polymeric particles and gels over time. Further information is given in a supporting report (PFA 2016am) attached in Section 13.

 

The saturation concentration in water of the silanol hydrolysis product, vinylsilanetriol is therefore limited by condensation reactions to approximately 1000 mg/L. However, it is very hydrophilic (calculated solubility is 1E+06 mg/L at 20°C using a QSAR method) with a predicted low log K_ow of -2.0. It is not surface active. The first dissociation constant of vinylsilanetriol is predicted to be around pKa = 10. The silanol hydrolysis product is much less volatile than the parent substance with a predicted vapour pressure of 0.017 Pa at 25°C.

 

 

Reference:

PFA (2016am). Peter Fisk Associates, Silanols and aquatic systems, 404.105.003