chemistry

Silsesquioxane

Silsesquioxane is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Silsesquioxane rather than just read about it. In short: A silsesquioxane is an organosilicon compound with the chemical formula [RSiO3/2]n (R = H, alkyl, aryl, alkenyl or alkoxyl.). Silsesquioxanes are colorless solids that adopt cage-like or polymeric structures with Si-O-Si linkages and tetrahedral Si vertices.

Key takeaways

Silsesquioxane belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Silsesquioxane to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Silsesquioxane from memory before moving on to harder problems.

Reference excerpt

A silsesquioxane is an organosilicon compound with the chemical formula [RSiO3/2]n (R = H, alkyl, aryl, alkenyl or alkoxyl.). Silsesquioxanes are colorless solids that adopt cage-like or polymeric structures with Si-O-Si linkages and tetrahedral Si vertices. Silsesquioxanes are members of polyoctahedral silsesquioxanes ("POSS"), which have attracted attention as preceramic polymer precursors to ceramic materials and nanocomposites. Diverse substituents (R) can be attached to the Si centers. The molecules are unusual because they feature an inorganic silicate core and an organic exterior. The silica core confers rigidity and thermal stability.

Structure

Silsesquioxanes are known in molecular form as well as polymers. The 6-, 8-, 10-, and 12-Si cages are sometimes labeled T6 T8, T10, and T12, respectively (T = tetrahedral vertex). The T8 cages, the most widely studied members, have the formula [RSiO3/2]8, or equivalently R8Si8O12. In all cases each Si center is bonded to three doubly bridging oxo groups, which in turn connect to other Si centers. The fourth group on Si is usually an alkyl, halide, hydride, alkoxide, etc. In the cubic clusters with Oh symmetry the Si-O-Si angles are in the range 145–152°, being bowed out, allowing the Si centers to better adopt tetrahedral geometry. The O-Si-O angle are in the range: 107–112°, Si-O bond: 1.55–1.65 Å.

Synthesis Silsesquioxanes are usually synthesized by hydrolysis of organotrichlorosilanes. An idealized synthesis is:

8 RSiCl3 + 12 H2O → [RSiO3/2]8 + 24 HCl The formation of HCl negatively impacts the relative rates of hydrolysis and condensation of intermediate silanols. Consequently, silsesquioxanes can be obtained directly by condensation of the corresponding silanetriols which occurs at neutral pH and works even for sterically very bulky substituents.

8 RSi(OH)3 → [RSiO3/2]8 + 12 H2

Reactivity

Substituent modifications Depending on the R substituent, the exterior of cage can be further modified. When R = H, the Si-H group can undergo hydrosilylation or oxidation to the silanol. Bridged polysilsesquioxanes are most readily prepared from clusters that contain two or more trifunctional silyl groups attached to non-hydrolysable silicon-carbon bonds, with typical sol-gel processing. Vinyl-substituted silsesquioxanes can be linked by the alkene metathesis.

Cage-Rearrangement Reorganization of the siloxane cage-like core (T8 → T10) can be induced by trifluoromethanesulfonic acid (CF3SO3H). In this case, reaction of hexahedral silsesquioxane and CF3SO3H in DMSO conducted in 1 : 12 molar ratio gives heptahedral silsesquioxane. In the first step CF3SO3H acid attacks siloxane Si-O-Si bonds and the formation of Si-O-SO2CF3 bond, opening one edge of the cage (Figure below). Such an inversion is observed at silicon atom during nucleophilic displacement reaction that is usually noticed when leaving groups are replaced by soft nucleophiles. Upon further acid attack, both T6(OH)4 C and siloxane dimer D are formed. Because this reaction takes place in an aqueous conditions, compound E of general formula T8(OH)4 as a consequence of hydrolysis reaction was obtained. E is prone to reaction with D and due to this, the abstraction of CF3SO−3 anion occurs and the closure frame with the spontaneous cage-rearrangement to heptahedral T10 structure F is observed. Although, heptahedral F is less favorable energetically (MM2 data), in this case its creation is forces by the formation of a new Si4O4 moiety from much more less stable substrates D and E.

Polymeric silsesquioxanes Polymeric silsesquioxanes have been prepared starting with the hydrolysis of phenyltrichlorosilane. Hierarchical organic-inorganic (hybrid) polysilsesquioxane (PSQ) materials using polyhedral oligomeric silsesquioxanes (POSS) cages as singular building blocks of the inorganic framework exhibit high specific surface area and hydrothermal stability and micro and/or mesoporosity. In addition, Marchesi et al. developed a series of amorphous POSS-based polysilsesquioxanes, in which the POSS cages (of partially-condensed T7-POSS or anionic completely-condensed T8-POSS molecules) act as structural units of the polymer-like inorganic network, both alone or in conjunction with metal ions of the lanthanide series (europium and terbium, especially).

Hydridosilsesquioxanes A well-known hydrogen silsesquioxane is [HSiO3/2]8. Early syntheses involved treatment (protonation) of trichlorosilane with concentrated sulfuric acid, and fuming sulfuric acid, affording T10-T16 oligomers. The T8 cluster was also synthesized by the reaction of trimethylsilane. The Si-H groups are amenable to hydrosilylation.

Applications Some high molecular weight soluble polymethylsilsesquioxanes have found applications in cosmetics. resins, and lithography.

Potential applications

Electronic materials Films of organosilsesquioxane, e.g., poly(methylsilsesquioxane), have been examined for semiconducting devices. Poly(hydridosilsesquioxane), which has a linked-cage structure, was sold under the name Fox Flowable Oxide. Methylsilsesquioxanes have been examined for spin-on-glass (SOG) dielectrics. Bridged silsesquioxanes have been used for quantum confined nano-size semiconductors. Silsesquioxane resins have also been used for these applications because they have high dielectric strengths, low dielectric constants, high volume resistivities, and low dissipation factors, making them very suitable for electronics applications. These resins have heat and fire resistant properties, which can be used to make fiber-reinforced composites for electrical laminates. Polyhedral oligomeric silsesquioxanes have been examined as a means to give improved mechanical properties and stability, with an organic matrix for good optical and electrical properties. The mechanisms of degradation in these devices is not well understood, but it is believed that material defect understanding is important for understanding the optical and electronic properties. Hydridosilsesquioxanes can be converted to silica coatings for potential application in integrated circuits.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Silsesquioxane

Start with the simplest possible case. Write down what Silsesquioxane claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Silsesquioxane before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Silsesquioxane ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Silsesquioxane

In research: Silsesquioxane appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Silsesquioxane in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Silsesquioxane is common in secondary-school and first-year university syllabi. It links to neighbouring topics Organosilicon compounds, Oxygen compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Silsesquioxane outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Silsesquioxane in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Silsesquioxane means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Silsesquioxane out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Silsesquioxane in simple terms?

Why does Silsesquioxane matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Silsesquioxane?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Silsesquioxane.