Lecture 9 Hybrid POSS

Class 2A Covalent links at molecular level

Making Hybrid Materials: Class 2A(Covalent links at molecular level)

• 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

Chromatographic Materials

Low DielectricsPhotoresists for Lithography

There are many hybrid or organometallic polymers: A quick

survey

Some have been used in making hybrid materialsMany have not.

Hybrid Polymers:PolysilanesPolyphosphazenesCoordination polymersPolysiloxanes

Purely inorganic:Poly(sulfur nitride)FullerenesCarbon nanotubesGraphene

Poly(sulfur nitride)

•First known conducting inorganic polymer•Superconducting below 1K•LED’s and solar cells

Labes, M. M.; Love, P.; Nichols, L. F. (1979). "Polysulfur Nitride - a Metallic, Superconducting Polymer". Chemical Reviews 79 (1): 1–15.

or Polythiazyl

Poly(sulfur nitride) by CVD

Polysilanes

Wurtz Coupling Reaction

Polysilanes exhibit σ-delocalization.UV absorbing/degradingSemiconductor (4.5 eV)Ceramic (Si-C) fiber precursorheat resistant, almost up to 300 oC

Chem. Rev. 1989, 89, 1359-1410

Dehydrocoupling

Polyphosphazenes

Over 600 known polymersGlass transition temp < -60 °CThermal stabilityTailored solubilityCan be bioerodible

Allcock, Harry R. (2003). Chemistry and Applications of Polyphosphazenes. Wiley-Interscience.

Polymer electrolytes for fuel cells

Coordination Polymers

Angew. Chemie 1996, 35, 1602& Chem. Soc. Rev., 2012,41, 115-147

•Many are anisotropic•Includes metal oxide framework materials• catalysts• gas adsorbents• electrical conductors & semiconductors• Solar cells

If bonding between metal and ligand is not reversible, then small oligomers If bond formation is reversible, large 3-C crystals can form.

Polysiloxanes (silicone)

Thermally & chemically stableGlass transition temp < -123 °CMelts at -23 °C (liquid at room temperature)With crosslinking – elastomerNot flammable

Silsesquioxanes

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.

Basic Polysilsesquioxane precursors

Class 2C

Sol-gel polymerization chemistry. A recipe

Acid catalysts: HCl, H2SO4 (< 0.2 M/Liter)Basic catalysts: NH3, NaOH or KOH Nucleophilic catalyst: Bu4NF

2 Mole/Liter 3 Moles/Liter

catalyst

Solvent

Solvent: Alcohol. R’OH – same alcohol formed by monomer hydrolysisEtOH for RSi(OEt)3.

Tetrahydrofuran (THF) – phase separates with base.Acetone - not commonly used.

Catalyst:

•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.

GelMaking Polysilsesquioxane gels as Class 2A Materials: Sol-Gel Process

But polymerization of RSi(OR)3 does not always lead to gels.

Low monomer concentration, bulky R groups High monomer

concentration, most R groups

High monomer concentration, small or reactive R groups

May get mixture of products. Rarely get gels

Insoluble

POSS

Liquid or waxy solid

Gel

Why don’t most simple pendant silsesquioxanes form gels? To answer we must look at formation of gels

• 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

No Gel No Gel

Gel

Functionality = 2, linear siloxane polymers.

What determines if phase separation occurs? How to make solid particles?

• very large polymers.• cross-link polymers (this is easiest)

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

Condensation reactions during organotrialkoxysilane polymerization

Polymerization of RSi(OR’)3 at concentrations > 1 M.

At higher concentration, intermolecular reactions are fasterAnd 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

Organotrialkoxysilane Monomers: Aliphatic Substituents

* Forms gels

* *

*

*

Organotrialkoxysilane Monomers: Sterically hindered Substituents

Forms cyclic structures; no gels

Organotrialkoxysilane Monomers: Alkenyl and halogenated Substituents

* Forms gels

*

*

Organotrialkoxysilane Monomers: Aryl Substituents

* Forms gels

*

Organotrialkoxysilane Monomers: Electrophilic Substituents

*Gels with just monomer and waterOrganic groups react under sol-gel conditions

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

Isocyanate Functionalized Organotrialkoxysilanes

Only neat Si(OMe)3 monomers gelled (with NaOH catalyst) Epoxide Group ring opens slower than SiOR polymerizationRing opening occurs under acidic and basic conditions

Epoxide 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

Acrylate Functionalized Organotrialkoxysilanes

*Gels will revert to solutions with heating, solvent or with time

Amine & Thiol Functionalized trialkoxysilanes

No point in adding acid it will just protonate amine group

Just add water. No catalyst is needed

Amine Functionalized trialkoxysilanes

Summation of Gelation for Organotrialkoxysilanes

Insoluble Gels-Usually neat monomer

Soluble Thermally Reversible Gels-Usually neat monomer No Gels-Under any circumstances

•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.

But polymerization of RSi(OR)3 does not always lead to gels.

Low monomer concentration, bulky R groups High monomer

concentration, most R groups

High monomer concentration, small or reactive R groups

May get mixture of products. Rarely get gels

Insoluble

POSS

Liquid or waxy solid

Gel

Ladder polymers: A hypothesis proposed to explain solubility of polysilsesquioxanes

Researchers have clung to the ladder polymer hypothesis even after a number of viscosity studies, & NMR experiments have shown it is false

Rigid rod polymer

If Ladder polymers existed: soluble polysilsesquioxanes would be thermoplastics

with higher Tg’s and some crystallinity

Ladder polymers should be strongerPack better and have greater non-bonding interactionsDo not expect liquids or low tg solids as with soluble polysilsesquioxanes

In reality:•Most tg < 50 °C•Soluble polysilsesquioxanes are weak

Ladder polymers: How to test hypothesis? Dilute solution viscosity studies

Mark Houwink Sakurada equation

Inherent viscosity

M = molecular weight of polymerK and a are Mark Houwink Sakurada parameters

For theta solvent and random coil polymer, a = 0.5For flexible polymers 0.5 < a < 0.8For semiflexible polymers 0.8 <a < 1.0For rigid polymers a > 1.0And for rigid rod polymers, like a ladder polymer, a = 2.0

Ladder polymers(No!!): Dilute solution viscosity studies

For theta solvent and random coil polymer, a = 0.5For flexible polymers 0.5 < a < 0.8For semiflexible polymers 0.8 <a < 1.0For rigid polymers a > 1.0And for rigid rod polymers, like a ladder polymer, a = 2.0

In Chinese Journal of Polymer Science 1987, 5, 335, Fang showed that a for polyphenylsilsequioxanes was between 0.6-0.86 (These are not ladder polymers!!!!!)

There no ladder polymers, but still researchers claim to have made them without proof!!! And

with impossible stereochemistry

Syn-isotactic

•Impossible to make high molecular weight polymer!!!with cis isotactic stereochemistry.•Need cis syndiotactic for it to work

PolyhedralOligoSilSesquioxane

POSSZhang, R. et al. Angew. Chemie. 2006, 45, 3112

Ladder polysilsesquioxanes do not form through polymerizations, however, they

can be made step-by step

Back to the real world

No ladder polymers from sol-gel polymerizations!!

Gels form with small RR = H, CH3, Vinyl, ClCH2-, ClCH2Ph-

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

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