Methylphenyl silicone oil, also known as phenyl silicone oil, is synthesized by replacing some of the methyl groups in the silicone oil molecule with phenyl groups. Its chemical structure mainly consists of the following two types: where R1 and R2 are methyl or phenyl.。

Synthesis of methyl phenyl silicone oil 
There are mainly two synthetic routes in industry: 
Method I: The synthesis can be achieved by using the hydrolysis products of methylphenyl cyclosiloxane or methylphenyl dialkoxysilane, through catalytic equilibrium polymerization with methyl D4 and MDnM. 
Method II: First, conduct a co-hydrolysis of diphenyl dichlorosilane and dimethyl dialkyl silane. Then, add the resulting hydrolysis product to the catalytically balanced polymerization mixture containing methyl D4 and MDnM. 
Properties of Phenyl Silicone Oil 
Viscosity-temperature coefficient: A higher value indicates that the viscosity of the substance changes less sensitively with temperature. 
Freezing point and flash point: Lower, enhancing its performance in low-temperature environments. 
Compression resistance performance: Lower than that of methyl silicone oil, but the viscosity changes more rapidly under pressure. 
Oxidation stability and radiation resistance performance: Higher than methyl silicone oil, suitable for use in harsh environments. 
The influence of phenyl content: 
As the content of phenyl groups increases, the specific gravity and refractive index of the silicone oil also increase. 
Silicone oils with low phenyl content (such as those containing 5% molar fraction of phenyl) have lower crystallization properties and solidification points, reaching approximately -70℃, making them suitable for ultra-low temperature applications. 
Thermal stability and radiation resistance: 
The physical properties of the silicon oil with medium and high phenyl content remained basically unchanged after being heated at 250℃ for several thousand hours, demonstrating excellent thermal stability. 
These silicone oils can still be used after receiving a radiation dose of up to 2×108 R at room temperature, demonstrating excellent radiation resistance. 
Lubrication performance: 
The lubricating performance of methyl phenyl silicone oil is superior to that of methyl silicone oil. It is suitable as a lubricant at high temperatures (such as 200℃), but the demolding effect is relatively poor. 
Organic compatibility: 
The organic compatibility of methyl phenyl silicone oil is better than that of methyl silicone oil. It can be used in combination with vegetable oil, animal oil and mineral oil. 
Ultra-high vacuum diffusion pump oil: 
Low-viscosity methylphenyl silicone oil is suitable for ultra-high vacuum diffusion pump oil. It is required to have high thermal stability, oxidation stability, and low vapor pressure at cooling temperatures. 
Compared with other types of high vacuum diffusion pump oils such as silicone oil, polyphenylene ether, and perfluoro polyether, methyl phenyl silicone oil has a steeper vapor pressure/temperature curve, lower vapor pressure and freezing point, as well as excellent heat resistance and oxidation resistance. 
Even at low temperatures, the viscosity of methyl phenyl silicone oil changes very little, and it has the characteristics of being non-toxic, non-corrosive and chemically stable. It is an ideal high-vacuum diffusion pump oil. 
Methyl phenyl silicone oil has a long service life. It can be used for a long time (more than 103 hours) at 250℃, and can also be used at higher temperatures in a vacuum environment. 
The methylphenyl silicone oils in circulation on the market come in various grades, such as 250, 255, 275, etc.