Chapter 3: Structural Diversity in Fluorinated Polyphosphazenes: Exploring the Change from Crystalline Thermoplastics to High-performance Elastomers and Other New Materials

Fluorine-containing polyorganophosphazenes, (NPR^F₂)_n, constitute one of the largest and best developed areas of inorganic-organic polymer chemistry. Linear high polymers in this class are fire resistant, hydrophobic, fiber-, film- and elastomer-forming materials that, in many cases, have property combinations that cannot be matched by conventional fluorocarbon or non-fluorinated polymers. Most macromolecules of this type are synthesized by using the alkali metal salts of fluoroalcohols or fluorophenols for chlorine-replacement reactions with poly(dichlorophosphazene). This approach allows the synthesis of polymers with only one type of fluorinated side-group, mixed-substituent polymers or block copolymers. Single-substituent polymers in this series generate either semicrystalline or amorphous materials, depending on the length and flexibility of the side-chains. Mixed-substituent polymers are usually amorphous materials that can be covalently crosslinked to generate elastomers. In addition, elastomers have been produced by the incorporation of small percentages of rigid bulky side-groups that favor side-chain interdigitation, a process that leads to physical crosslinking. Finally, fluorophosphazene elastomers are accessible by the incorporation of organosilicon side-groups and by the formation of polyphosphazene-polyorganosilicon block copolymers.