1. Class 2A Covalent links at molecular level
Lecture 9 Hybrid POSS
Class 2A Covalent links at molecular level

2. Making Hybrid Materials: Class 2A (Covalent links at molecular level)
Class 2A are hybrids based on monomers with both inorganic and organic components. Silsesquioxane monomers, organotrialkoxysilanes, are by definition Class 2A. Polymerize them with water and you get a class 2A hybrid as the product. T
Chromatographic
Materials
Organic group is attached to network at molecular level
Hypercrosslinking is possible
Pendant or bridging monomers
Bridging groups can be small or macromolecule
This class also includes the organometallic polymers
Low K Dielectrics
Photoresists for Lithography

3. There are many hybrid or organometallic polymers: A quick survey
Some have been used in making hybrid materials
Many have not.
Hybrid Polymers:
Polysilanes
Polyphosphazenes
Coordination polymers
Polysiloxanes
Purely inorganic:
Poly(sulfur nitride)
Fullerenes
Carbon nanotubes
Graphene
There are a number of other mixed organic-inorganic polymers including those listed here. I have also included some that, by definition, are inorganic but everyone treats them as hybrids. Now one opportunity for new research is to incorporate some of these hybrids into sol-gels.

4. Poly(sulfur nitride) •First known conducting inorganic polymer
or Polythiazyl
•First known conducting inorganic polymer
•Superconducting below 1K
•LED’s and solar cells
Polysulfur nitride is a polymeric metal It was the first such metal and it even becomes superconducting at very low temperatures. The intermediate dimer is made by reacting the tetramer vapor with silver or palladium wool which is turned into the corresponding sulfide. The dimer is slightly explosive so be careful.The dimer ring open polymerizes at room temperature to give a golden polymer. Prep for tetramer: Dissolve 131 mL of freshly prepared sulfur dichloride in 1950 mL of benzene in a large Erlenmeyer flask. You may pour the solution through a filter to remove any impurities that may have been in the sulfur dichloride, but it is best to purify it beforehand. Bubble anhydrous ammonia gas through the solution. A brown precipitate will begin to form, continue adding ammonia until this precipitate dissolves and forms an orange-yellow solution. Now large amounts of ammonium chloride will precipitate out of solution. Carefully heat the benzene until it boils. Immediately pour this solution over a filter to remove the crystals, collect the filtrate. Wash the crystals with 200 mL of benzene and add the wash to the filtrate. Allow the benzene filtrate to evaporate until a crystalline slush remains, filter the slush, and allow to dry. Using vacuum drying or a desiccator can speed the process. The resulting product should be yellow to orange-red crystals of nitrogen sulfide. You will need a graduated cylinder for measuring liquids.
Labes, M. M.; Love, P.; Nichols, L. F. (1979). "Polysulfur Nitride - a Metallic, Superconducting Polymer". Chemical Reviews 79 (1): 1–15.

5. Just some images made by vapour deposition and polymerization of sulfur nitride dimer on a xerograph and a fingerprint.

6. Polysilanes Polysilanes exhibit σ-delocalization.
Wurtz Coupling Reaction
Dehydrocoupling
Polysilanes are the silicon analog of polyethylene. If R and R’ are H the material will catch fire, pyrophoric. If R and R’ are organic groups and not the same, the polymer will be soluble. If R = R’ they are insoluble due to crystallinity, Semiconducting polyemrs, they are generally prepared Wurtz coupling in low yield.
Polysilanes exhibit σ-delocalization.
UV absorbing/degrading
Semiconductor (4.5 eV)
Ceramic (Si-C) fiber precursor
heat resistant, almost up to 300 oC
Chem. Rev. 1989, 89,

7. Polyphosphazenes Over 600 known polymers
Glass transition temp < -60 °C
Thermal stability
Tailored solubility
Can be bioerodible
Polyphosphazenes start as all inorganic polymer and are converted to hybrids through nucleophilic substitution of the chloride groups with alkoxides, amines, or other nucleophiles. Depending on the organic group they can be elastomers or thermoplastics. They are thermally stable.
Polymer electrolytes for fuel cells
Allcock, Harry R. (2003). Chemistry and Applications of Polyphosphazenes. Wiley-Interscience.

8. Coordination Polymers
•Many are anisotropic
•Includes metal oxide framework materials
• catalysts
• gas adsorbents
• electrical conductors & semiconductors
• Solar cells
There are tens of thousands of different coordination polymers and many mmore made each day. A great many are completely insoluble and intractable materials. Some are very conductive and becoming important for solar systems. Others like the MOF’s are of interest for hydrogen storage and carbon dioxide removal.
If bonding between metal and ligand is not reversible, then small oligomers
If bond formation is reversible, large 3-C crystals can form.
Angew. Chemie 1996, 35, 1602
& Chem. Soc. Rev., 2012,41,

9. Polysiloxanes (silicone)
Polydimethylsiloxane has been arounbd since before world war 2, but it was Rochow’s method for synthesizing the dichlorodimethylsilane monomer directly from silicon and methylene chloride, that made the amterials inexpensive enough to be used commercially today. Generally, these are elastomeric materials with some crosslinking. Without crosslinking they are oils or amorphous solids that are very weak. Silly putty is lightly crosslinked polydimethylsiloxane.
Thermally & chemically stable
Glass transition temp < -123 °C
Melts at -23 °C (liquid at room temperature)
With crosslinking – elastomer
Not flammable

10. Silsesquioxanes
Back to silsesquioxanes.

11. What about other metals with C-M bonds?
RGe(OR’)3
R-Sn(OR’)3 These are known, but not
R-B(OR’)2 commonly used
Most C-M bonds are too reactive with water with the bond polarized with the electron density on carbon.

12. Basic Polysilsesquioxane precursors
These are three different possible silsesquioxanes,
Class 2C

13. Sol-gel polymerization chemistry. A recipe
catalyst
Solvent
2 Mole/Liter
3 Moles/Liter
Catalyst:
Acid catalysts: HCl, H2SO4 (< 0.2 M/Liter)
Basic catalysts: NH3, NaOH or KOH
Nucleophilic catalyst: Bu4NF
Solvent: Alcohol. R’OH – same alcohol formed by monomer hydrolysis
EtOH for RSi(OEt)3.
Tetrahydrofuran (THF) – phase separates with base.
Acetone - not commonly used.

14. Making Polysilsesquioxane gels as Class 2A Materials: Sol-Gel Process
•Sol is a dispersion of particles in solvent
•A gel forms when those particles percolate through the solvent
•Aging is the relaxation of the network with time
•Drying removes the solvent leaving the network behind.

15. But polymerization of RSi(OR)3 does not always lead to gels.
High monomer concentration, small or reactive R groups
Low monomer concentration, bulky R groups
High monomer concentration, most R groups
POSS
Gel
Liquid or waxy solid
Insoluble
May get mixture of products. Rarely get gels

16. Why don’t most simple pendant silsesquioxanes form gels
Why don’t most simple pendant silsesquioxanes form gels? To answer we must look at formation of gels
Gel
No Gel
No Gel
• Must have solid and liquid phase
• Solid phase (usually particles) must be continuous through liquid (percolation)
• Phase separation of liquid prevents further reaction and gelation

17. What determines if phase separation occurs? How to make solid particles?
• very large polymers.
• cross-link polymers (this is easiest)
Functionality = 2, linear siloxane polymers.
Because linear (functionality = 2) siloxanes are generally liquids,
so gels don’t form
When RSi(OR)3 polymerizes and makes rings, its functionality nears 2

18. Condensation reactions during organotrialkoxysilane polymerization

19. Polymerization of RSi(OR’)3 at concentrations > 1 M.
At higher concentration, intermolecular reactions are faster
And compete better with cyclizations.
Therefore, more network and less cyclic T8.
Distill off solvent during reaction to further concentrate.
If R is too bulky, never get gels

20. Organotrialkoxysilane Monomers: Aliphatic Substituents* * *
Transparent
Opaque *
* Forms gels

21. Organotrialkoxysilane Monomers: Sterically hindered Substituents
Forms cyclic structures; no gels

22. Organotrialkoxysilane Monomers: Alkenyl and halogenated Substituents* *
Translucent
Transparent
* Forms gels

23. Organotrialkoxysilane Monomers: Aryl Substituents*
Opaque
* Forms gels

24. Organotrialkoxysilane Monomers: Electrophilic Substituents* * * * * *
*Gels with just monomer and water
Organic groups react under sol-gel conditions

25. Isocyanate Functionalized Organotrialkoxysilanes
Gels form from neat monomer at acidic, neutral and basic conds.
Gel from 1 M Monomer with tetrabutylammonium hydroxide

26. Epoxide Functionalized Organotrialkoxysilanes
Only neat Si(OMe)3 monomers gelled (with NaOH catalyst)
Epoxide Group ring opens slower than SiOR polymerization
Ring opening occurs under acidic and basic conditions

27. Acrylate Functionalized Organotrialkoxysilanes
Most cases-sol-gel polym. with retention of vinyl.
No vinyl polymerization detected by NMR
Trimethoxysilane monomer-also exhibited ester hydrolysis
Methacrylic acid detected by NMR, odor
neat monomer conc 1.5 equiv H2O/basic-only gel obtained

28. Amine & Thiol Functionalized trialkoxysilanes
*Gels will revert to solutions with heating, solvent or with time

29. Amine Functionalized trialkoxysilanes
No point in adding acid it will just protonate amine group
Just add water. No catalyst is needed

30. Summation of Gelation for Organotrialkoxysilanes
Most sol-gel reactions with shown organotrialkoxysilanes do not give gels.
Gelation generally does occur when:
-the electrophilic functional group reacts under sol-gel conditions.
-neat monomer is used.
None of the nucleophilic functionalized monomers formed irreversible gels.
Insoluble Gels-Usually neat monomer
Soluble Thermally Reversible Gels
-Usually neat monomer
No Gels-Under any circumstances

31. But polymerization of RSi(OR)3 does not always lead to gels.
High monomer concentration, small or reactive R groups
Low monomer concentration, bulky R groups
High monomer concentration, most R groups
POSS
Gel
Liquid or waxy solid
Insoluble
May get mixture of products. Rarely get gels

32. Ladder polymers: A hypothesis proposed to explain solubility of polysilsesquioxanes
Rigid rod polymer
Researchers have clung to the ladder polymer hypothesis even after a number of viscosity studies, & NMR experiments have shown it is false

33. If Ladder polymers existed: soluble polysilsesquioxanes would be thermoplastics with higher Tg’s and some crystallinity
In reality:
•Most tg < 50 °C
•Soluble polysilsesquioxanes are weak
Ladder polymers should be stronger
Pack better and have greater non-bonding interactions
Do not expect liquids or low tg solids as with soluble polysilsesquioxanes

34. Ladder polymers: How to test hypothesis
Ladder polymers: How to test hypothesis? Dilute solution viscosity studies
Mark Houwink Sakurada equation
Inherent viscosity
M = molecular weight of polymer
K and a are Mark Houwink Sakurada parameters
For theta solvent and random coil polymer, a = 0.5
For flexible polymers 0.5 < a < 0.8
For semiflexible polymers 0.8 <a < 1.0
For rigid polymers a > 1.0
And for rigid rod polymers, like a ladder polymer, a = 2.0

35. Ladder polymers(No!!): Dilute solution viscosity studies
In Chinese Journal of Polymer Science 1987, 5, 335, Fang showed that a for polyphenylsilsequioxanes was between (These are not ladder polymers!!!!!)
For theta solvent and random coil polymer, a = 0.5
For flexible polymers 0.5 < a < 0.8
For semiflexible polymers 0.8 <a < 1.0
For rigid polymers a > 1.0
And for rigid rod polymers, like a ladder polymer, a = 2.0

36. There no ladder polymers, but still researchers claim to have made them without proof!!! And with impossible stereochemistry
PolyhedralOligoSilSesquioxane
Syn-isotactic
POSS
Zhang, R. et al. Angew. Chemie. 2006, 45, 3112
•Impossible to make high molecular weight polymer!!!
with cis isotactic stereochemistry.
•Need cis syndiotactic for it to work

37. Ladder polysilsesquioxanes do not form through polymerizations, however, they can be made step-by step

38. Back to the real world
Gels form with small R
R = H, CH3, Vinyl, ClCH2-, ClCH2Ph-
No ladder polymers from sol-gel polymerizations!!

39. Polysilsesquioxane Gels: Class 2A Hybrid
• Don’t form when R is big or bulky pendant group
• Gels with R = H, Me, Vinyl, ClCH2-, small or reactive R
• Mild Conditions
• Concentrations usually > 1M
nanoporous
• After drying, often get high surface area, porous “xerogel” with nanoscale pores
• Gels are insoluble and intractable.
• Stable to > 300 °C
• Glassy, brittle, hard gels.
• Stronger & more hydrophobic than silica

40. So what can you do with polysilsesquioxane xerogels and aerogels
Most applications are for thin films, rather than bulk:
Optical coatings
Corrosion protection coatings
Water repellant coatings
Waveguide materials for optoelectronics
Encapsulant material for enzymes and cells
Sensor coatings
Particles for chromatographic supports
Bulk adsorbents for volatile organic contaminants