Physics and Chemistry of Hybrid Organic-Inorganic Materials Lecture 7: Polymerizing monomers to make hybrids

Key concepts Hybrid monomers have both organic and inorganic parts in a single molecule The monomer polymerizes to form a hybrid that has both organic and inorganic components homogeneously mixed rather than phase separated Hybrid monomers (hybrid polymers): organotrialkoxysilanes (polysilsesquioxanes and POSS), Hexachlorophosphazene (polyphosphazenes), dichlorosilanes (polysilanes and polysiloxanes) Other hybrids: polymeric phthalocyanines, fullerene based polymers, and many more... Generally greater thermal stability than organic polymers Flexible and processable like organic polymers.

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  Dielectrics Photoresists for Lithography

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

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.Chemical Reviews or Polythiazyl

Poly(sulfur nitride) by Chemical Vapor Deposition

Polysilanes Wurtz Coupling Reaction Polysilanes exhibit σ-delocalization. UV absorbing/degrading Semiconductor (4.5 eV) Ceramic (Si-C) fiber precursor heat resistant, almost up to 300 o C Chem. Rev. 1989, 89, Dehydrocoupling

Polyphosphazenes Over 600 known polymers Glass transition temp < -60 °C Thermal stability Tailored solubility Can be bioerodible Allcock, Harry R. (2003). Chemistry and Applications of Polyphosphazenes. Wiley-Interscience. Monomer: Hexachlorophosphazene

Coordination Polymers Angew. Chemie 1996, 35, 1602 & Chem. Soc. Rev., 2012,41, 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 stable Glass transition temp < -123 °C Melts at -23 °C (liquid at room temperature) With crosslinking – elastomer Not flammable

Polysilsesquioxanes PolyhedralOligoSilSesquioxanes (POSS) organotrialkoxysilanes Hydrolyze alkoxide groups to silanols Condense silanols to Si-O-Si linkages Trifunctional monomer can form gels Next lectures are on polysilsesquioxanes

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

What are gels? Two phase systems: solid particles of polymer + solvent Jello: solid particles of protein + water Hybrid gels: solid particles of network polymer percolating through liquid.

Prog. Polym. Sci. 29 (2004) 1079–1141 Fullerenes covalently incorporated into polymers

First polymer known was Friedel Craft polymerization of C 60 in 1991 JACS 1991, G. Olah

Second polymerization was copolymerization with paraxylene Loy, JACS, 1992

Fullerenes will polymerize photochemically Insoluble powders Highly crosslinked.

Fullerenes will polymerize thermally by step addition, free radical polymerization

Chemical modification of fullerenes with polymerizable groups Pendant C60 groups improve thermal properties

Macromolecules 1994,27, T g went from 97 °C to 163 °C Polymers with pendant C 60 have improved thermal-mechanical properties

C 60 can be added by nucleophilic addition chemistry C 60 is electron deficient

C 60 is good acceptor dye for organic photovoltaic systems

Fullerenes as covalently attached “inorganics” Purple colored hybrid glass

Summary Hybrid monomers polymers to homogeneous hybrid solids Hybrid monomers in solvent can polymerize to form solid particles. Solid particles may form gel. Hybrid monomers include: organotrialkoxysilanes, dichlorosilanes, hexachlorotriphosphazene Sol-gel polymerizations involve hydrolysis and condensation reactions and often form gels