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Polysilsesquioxanes and T-resins are highly crosslinked materials with the empirical formula RSiO1.5. They are named from the organic group and a one and a half (sesqui) stoichiometry of oxygen bound to silicon. T-resin, an alternate designation, indicates that there are three (tri-substituted) oxygens substituting the silicon. Both designations simplify the complex structures that have now come to be associated with these polymers. A variety of paradigms have been associated with the structure of these resins ranging from amorphous to cubes containing eight silicon atoms, sometimes designated as T8 structures. Ladder structures have been attributed to these resins, but the current understanding is that in most cases these are hypothetical rather than actual structures.


T8 cube

Hypothetical ladder

Polysilsesquioxanes are used as matrix resins for molding compounds, catalyst supports and coating resins. As dielectric, planarization and reactive ion etch resistant layers, they find application in microelectronics. As abrasion resistant coatings they protect plastic glazing and optics. As preceramic coatings they convert to silicon dioxide, silicon oxycarbide, and silicon carbide depending on the oxidizing conditions for the high temperature thermal conversion.

Polysilsesquioxane resins containing silanols (hydroxyls) can be cured at elevated temperatures. Formulation and catalysis is generally performed at room temperature or below. At temperatures above 40 °C most resins soften and become tacky, becoming viscous liquids by 120 °C. The condensation of silanols leads to cure and the resins become tough binders or films. The cure is usually accelerated by the addition of 0.1-0.5% of a catalyst such as dibutyltindiacetate, zinc acetate, or zinc 2-ethylhexanoate. The resins can also be dispersed insolvents such as methylethylketone for coating applications.

These materials are oligomeric alkoxysilane hydrolyzates and are the basis of coating resins. They can also be used as primer coatings or to modify organic resins such as polyesters. They generate small amounts of alcohols as byproducts during cure. Moisture-cure can be accelerated by adding 0.1-0.4 wt% of titanates, such as AKT855.

Polysilsesquioxanes Liquid T-Resins

Product CodeNameViscosity (cSt)Molecular Weight (approximate)Refractive IndexDensityCASComments
SLT-3SA2(80-85% octylsilsesquioxane)-(15-20% mercaptopropylsilsesquioxane) copolymer500-1,000----1.03----
SLT-3UM3(85-90% acryloxypropylsilsesquioxane)-(10-15% methylsilsesquioxane) copolymer25-50500-1,0001.451.14[1385031-14-0]ethoxy-terminated

Polysilsesquioxanes Solid T-Resins

Product CodeNameViscosity (cSt)Molecular Weight (approximate)Refractive IndexDensityCASComments
SST-3M01poly(methylsilsesquioxane)7,000-8,0004.0-6.01.42--[68554-70-1]100% methyl
SST-3M02poly(methylsilsesquioxane)--2.5-4.01.421.08[68554-70-1]100% methyl
SST-3MH1.1poly(Methyl-Hydridosilsesquioxane)------0.91--90% methyl, 10% hydride
10 wt% soln in methyltetrahydrofuran
SST-3P01poly(Phenylsilsesquioxane)1,200-1,6004.5-6.51.56--[70131-69-0]100% phenyl
SST-3PM1poly(Phenyl-Methylsilsesquioxane)----1.55--[181186-29-8]90% phenyl, 10% methyl
SST-3PM2(Phenylsilsesquioxane)-(Dimethylsiloxane) copolymer--3.0-5.0--1.08[73138-88-2]70% phenyl, 30% dimethyl
SST-3PM4(40% Phenyl- 45% Methylsilsesquioxane)-(5% Phenylmethylsiloxane) (10% Diphenylsiloxane) tetrapolymer1,400-1,6002.0-3.0--1.08[181186-36-7]85% silsesquioxane, 15% siloxane
SST-3PP1poly(Phenyl-Propylsilsesquioxane)1,500-1,8003.5-5.51.541.25[68037-90-1]70% phenyl, 30% propyl
equivalent weight: 400
SST-3PV1poly(Phenyl-Vinylsilsesquioxane)1,000-1,300--------90% phenyl, 10% vinyl

Silsesquioxanes containing β-electron withdrawing groups can be converted to silicon dioxide via elimination and hydrolysis at low temperatures or under UV exposure.1 The thermal reaction cascade for β-substituted silsesquioxanes leading to SiO2-rich structures with a low level of carbon occurs at temperatures above 180 °C.2 UV exposure results in pure SiO2 films and suggests that patterning β-substituted silsesquioxane films can lead to direct fabrication of dielectric architectures.

1. Arkles, B.; Berry, D.; Figge, L.; J. Sol-Gel Sci. & Technol. 1997, 8, 465.
2. Ezbiansky, K. et al, Mater. Res. Soc. Proc., 2001, 606, 251.

Thermally- and UV-Labile Polysilsesquioxanes

Product CodeNameMolecular Weight (approximate)% OHConversion to SiO2% in MethoxypropanolCAS
SST-BAE1.2poly(2-acetoxyethylsilsesquioxane)----> 350 °C8-20[349656-50-4]
SST-BCE1.2poly(2-chloroethylsilsesquioxane)800-1,4003.0-5.55> 300 °C14-16[188969-12-2]

Specialty polysilsesquioxanes can be utilized as models and precursors for silica surfaces and zeolites. If a silicon is removed from a T8 cube, the open position of the remaining T7 cube can be substituted with catalytically active metals.1 T7 cubes can be converted to functionalized T8 cubes. Functionalized T8 cubes are sometimes designated POSS (polyhedral oligomeric silsesquioxane) monomers. Methacrylate T8 cubes can be copolymerized with a variety of monomers to form homopolymers and copolymers. The polymers may be viewed structurally as nanocomposites or hybrid inorganic-organic polymers. The cube structures impart excellent mechanical properties and high oxygen permeability.2 Hydride-substituted T8 cubes can be introduced into vinyl-addition silicone rubbers.3 T8 cubes in which all silicon atoms are substituted with hydrogen have demonstrated utility as flowable oxide precursors in microelectronics. Fluorinated polysilsesquioxanes demonstrate tunable oleophobicity.4

1. Feher, F.; et al, J. Am. Chem. Soc., 1989, 111, 1741.
2. Lichtenhan, J.; et al, Macromolecules, 1995, 28, 8435.
3. Lichtenhan, J.; Comments Inorg. Chem. 1995, 17, 115.
4. Choi,W.; et al, Adv. Mater., 2009, 21(21) 2190.

Polyhedral Oligomeric Silsesquioxanes (POSS Materials)

Product CodeNameMolecular Weight (approximate)SolubilityDensityDescriptionCAS
SST-A8C42allyl-substituted poly(isobutylsilsesquioxane)851.55THF, hexane1.44T8 cube with single substitution, employed in epoxy nanocomposites--
SST-F8F41poly(tridecafluorooctylsilsesquioxane)1,000-2,000----T8 cube[1610607-30-1]
SST-H8H01poly(hydridosilsesquioxane) - polymeric T8 with all silicons hydride-substituted3,000-5,000--0.88T8 cube
17-20% hazy solution in methylisobutylketone
SST-H8HS8poly(hydridosilsesquioxane) - T8 with all silicons dimethylsiloxy (HSiMe2O)-substituted1,017.98--1.23T8 cube
see also HQM-107
SST-R8C42methacryloxypropyl-substituted poly(isobutylsilsesquioxane)943.64THF, hexane1.13T8 cube with single substitution with polymerizable functionality[307531-94-8]
SST-S7C41silanol-functional poly(isobutylsilsesquioxane)791.42----T7 cube[307531-92-6]
SST-V8V01poly(vinylsilsesquioxane) - T8 with all silicons vinyl-substituted633.04THF, chloroform, hexane--T8 cube[69655-76-1]