Polymethylsilsesquioxane or PMSSQ is a chemical compound, silicone resin, a spherically cross-linked siloxane particle (microsphere).

The name describes the structure of the molecule:

• Poly indicates that the compound is a polymer, a large molecule composed of repeated subunits.
• Methyl refers to a methyl group, a component of organic molecules and consists of a carbon atom bound to three hydrogen atoms (CH3).
• Silsesquioxane is a type of silicone compound. The "Sil" part refers to silicon, a chemical element often used in the creation of polymers. The "sesquioxane" part refers to a specific type of structure in the molecule where each silicon atom is connected to one and a half (Sesqui-) oxygen atoms on average.

The synthesis process takes place in different steps:

• Preparation of starting materials. PMSSQ synthesis begins with a specific type of organosilicone compound known as methyltrimethoxysilane (MTMS). This compound contains a silicon atom bound to a methyl group and three methoxy groups.
• Hydrolysis and condensation. MTMS undergoes a process of hydrolysis and condensation. In the hydrolysis phase, methoxy groups on MTMS are replaced with hydroxyl groups. In the condensation phase, these hydroxyl groups react with each other to form siloxane bonds (Si-O-Si), creating a three-dimensional network structure of PMSSQ.
• Purification and processing. The resulting PMSSQ is then purified and transformed into the desired shape for use in various products. This may include steps such as drying, grinding or incorporation into a cosmetic formulation.

It comes in the form of a white silicone elastomer powder with a particle size of 2.5 μm with high thermal stability and excellent heat resistance, superior to conventional organic polymer. Good solvent resistance, insoluble and non-swelling in common organic solvents.

What it is used for and where

Lighting

As an organic light-diffusing agent, it has a polymer network structure and high transparency. ) Compared to classic acrylic light-diffusing agents, the heat resistance and high temperature resistance are better and the lamp tube has a longer life. The spherical powder has an average particle diameter of 1-2 micron and a refractive index of 1.43. It is used in LED lampshades, LED tubes, LED light boxes, LED packages (high power), LCD light diffusers (1).

Cosmetics

Properties. Provides a silky, smooth feel to cosmetic products, enhancing application and longevity.

Used as a film-forming agent, it can spread a microscopic film on the skin or hair to protect them. As a wetting agent it produces excellent lubrication of skin creams and lipsticks with a good spreading effect. It also reduces the agglomeration of pressed powders. Does not clog pores.

Opacifying agent. It is useful into formulations that may be translucent or transparent to make them opaque and less permeable to light.

Commercial uses

Cosmetic Industry. Polymethylsilsesquioxane is widely used in cosmetic products such as powders, foundations, and sunscreens for its light-diffusing properties, creating a mattifying and smoothing effect on the skin.

Skin Care Products. Used in skincare formulas to provide a soft-focus effect, visually minimizing the appearance of fine lines and imperfections.

Other Uses

• Coating ink
• Anti-adhesion film
• Matting powder 

Dosage

• Color Cosmetic: 2-50% 
• Hair Care: 1-5%
• Skin Care: 2-30% 

CAS    68554-70-1

The most relevant studies on this ingredient have been selected with a summary of their contents:

Polymethylsilsesquioxane studies

Synonyms:

• Poly(methylsilsesquioxane)
• Methyl silsesquioxanes
• Poly methylsilsesquioxane

References__________________________________________________________________

(1) Maleki H, Whitmore L, Hüsing N. Novel multifunctional polymethylsilsesquioxane-silk fibroin aerogel hybrids for environmental and thermal insulation applications. J Mater Chem A Mater. 2018 Jul 14;6(26):12598-12612. doi: 10.1039/c8ta02821d. Epub 2018 Jun 12. PMID: 30713688; PMCID: PMC6333272.

Abstract. The development of aerogels with improved mechanical properties, to expand their utility in high-performance applications, is still a big challenge. Besides fossil-fuel based polymers that have been extensively utilized as platforms to enhance the mechanical strength of silsesquioxane and silica-based aerogels, using green biopolymers from various sustainable renewable resources are currently drawing significant attention. In this work, we process silk fibroin (SF) proteins, extracted from silkworm cocoons, with organically substituted alkoxysilanes in an entirely aqueous based solution via a successive sol-gel approach, and show for the first time that it is possible to produce homogeneous interpenetrated (IPN) polymethylsilsesquioxane (PMSQ)-SF hybrid aerogel monoliths with significantly improved mechanical properties. Emphasis is given to an improvement of the molecular interaction of the two components (SF biopolymer and PMSQ) using a silane coupling agent and to the design of pore structure. We succeeded in developing a novel class of compressible, light-weight, and hierarchically organized meso-macroporous PMSQ-SF IPN hybrid aerogels by carefully controlling the sol-gel parameters at a molecular level. Typically, these aerogels have a compressive strength (δ max) of up to 14 MPa, together with high flexibility in both compression and bending, compressibility up to 80% strain with very low bulk density (ρ b) of 0.08-0.23 g cm-3. By considering these promising properties, the superhydrophobic/oleophilic PMSQ-SF aerogel hybrids exhibited a high competency for selective absorption of a variety of organic pollutants (absorption capacities ∼500-2600 g g-1 %) from water and acted as a high-performance filter for continuous water/oil separation. Moreover, they have demonstrated impressive thermal insulation performance (λ = 0.032-0.044 W m-1 K-1) with excellent fire retardancy and self-extinguishing capabilities. Therefore, the PMSQ-SF aerogel hybrids would be a new class of open porous material and are expected to further extend the practical applications of this class of porous compounds.