Side-chain polyphosphazenes

In conclusion, polymer electrolytes based on phosphazene backbone and containing ether side chains are, after complexation with alkali metal salts, among the highest ionically solvent-free polymer salt complexes, with conductivities in the order of 10" -10" S cm However, these conductivities are still below the value of 10 S cm" which is considered to be the minimum for practical applications. Therefore the design of new polyphosphazenes electrolytes with a higher conductivity and also a higher dimensional stability still remains a challenge for future researchers. 

Table 19 also describes polyphosphazenes bearing oligopeptide side chains (Gly-Pro-Gly and Gly-Val-Ala tripeptides) [636], potentially useful for tissue engineering, and polymers bearing AT-acetylglycine substituents [637]. The... 

The side chain substituents can affect the properties of the polyphosphazenes in yet another way. Whereas (NP CH.CH ), is amorphous, increasing the side chain length by using long chain alcohols can result in polymers which are semicrystalline (13). Presumably these polymers assume more of the character of poly(ethylene oxide), as the side chain length increases. 

Polyphosphazene-based PEMs are potentially attractive materials for both hydrogen/air and direct methanol fuel cells because of their reported chemical and thermal stability and due to the ease of chemically attaching various side chains for ion exchange sites and polymer cross-linking onto the — P=N— polymer backbone. Polyphosphazenes were explored originally for use as elastomers and later as solvent-free solid polymer electrolytes in lithium batteries, and subsequently for proton exchange membranes. 

Metals ions such as silver or mercury have been coordinated to thioether side chains linked to a polyphosphazene skeleton.109... 

The principal polyphosphazenes that have been used in hydrogels are those with linear or branched ethyleneoxy side chains, aryloxy groups with carboxylic acid substituents, or mixed-substituent polymers that bear hydrophilic methylamino side groups plus a hydrophobic cosubstituent such as phenoxy or trifluoroethoxy. Cross-linking is usually accomplished by gamma-ray irradiation or, in the case of the carboxylic acid functional species, by treatment with a di- or tri-valent cation. Here, we will consider another example based on MEEP (3.79), a polymer that is well suited to the clean method of radiation cross-linking. 

First, we will consider the design of polyphosphazenes as side-chain liquid-crystalline materials.241 248 Side-chain liquid-crystalline polymers are a subclass of species described earlier as structure 3.72. Liquid crystallinity occurs when the rigid side groups become organized, usually in the semi-liquid state. The organization may be nematic (oriented but unlayered) or smectic (layered) as illustrated in Figure 3.25. 

Polyphosphazene block copolymers were synthesized by these chain-growth polymerization methods. The successive anionic polymerization of N-silylphosphoranimines 19d and 19a at 133 °C yielded the block copolymer with Mw/Mn of 1.4-2.3 (Scheme 80) [278,279]. However, due to the presence of two possible leaving groups in 19d, this approach yielded block copolymers where one of the block segments contained a mixture of side groups. On the other hand, the cationic polymerization of 19b with PCI5 was carried out at ambient temperature, followed by addition of a second phosphoranimine to yield a block copolymer with Mw/Mn of 1.1-1.4, where each block segment had one kind of side chain (Scheme 81) [280]. 

Polyphosphazene has good chemical and thermal stability. Its polyphosphazene backbone is highly flexible. Various side chains can be introduced to this backbone readily. Cross-linking is needed in order to reduce the dimensional changes in the presence of methanol or water. The membranes have shown reasonable proton conductivity and low methanol crossover. However, an improvement in mechanical strength is needed for practical fuel cell applications. 

Polyphosphazenes have also been prepared using modified amino acid ester side-chain substituents (Table 7). Studies show that the polymer containing the smallest hydrophobic side-chain constituent, glycine ethyl ester, exhibited the most rapid degradation. This was attributed to the lower steric hindrance that the small side-chain group could provide in... 

Polyphosphazenes represent a new approach to the design and synthesis of side-chain liquid crystal polymers. Polyphosphazenes are inorganic main-chain polymers consisting of alternating phosphorus-nitrogen atoms in the main chain with two substituents attached to each phosphorus atom. The top of Figure 1 shows the general structure for a side chain liquid crystal polymer a polymer... 

The two-step synthesis process shown in Figure 2 affords several possibilities for preparing new side-chain liquid crystal polymers. The polymerization process allows one to vary the molecular weight and molecular weight distribution (MWD), and potentially change the properties of the liquid crystalline state. Most of the polyphosphazenes reported in the literature, including the examples in this paper, are derived from the bulk uncatalyzed process this... 

SINGLERETAL. Side-Chain Liquid-Crystalline Polyphosphazenes... 

For the synthesis of side chain liquid crystal polyphosphazenes, the most important examples in Figure 2 are the alkoxy- and aryloxy-polymers. Mesophase behavior has been noted with simple side chains, such as trifluoroethoxy and aryloxy side chains (9). This mesophase behavior is not conventional liquid crystal order, but polymers which exist in a conformationally disordered state (10). 

Figure 3. Side-chain liquid crystal polyphosphazenes.

The study by Percec, Tomazos and Willingham (15) looked at the influence of polymer backbone flexibility on the phase transition temperatures of side chain liquid crystalline polymethacrylate, polyacrylate, polymethylsiloxane and polyphosphazene containing a stilbene side chain. Upon cooling from the isotropic state, golymer IV displays a monotropic nematic mesophase between 106 and 64 C. In this study, the polymers with the more rigid backbones displayed enantiotropic liquid crystalline behavior, whereas the polymers with the flexible backbones, including the siloxane and the polyphosphazene, displayed monotropic nematic mesophases. The examples in this study demonstrated how kinetically controlled side chain crystallization influences the thermodynamically controlled mesomorphic phase through the flexibility of the polymer backbone. 

Part of the rationale for investigating liquid crystal polyphosphazenes is for nonlinear optical (NLO) applications. In general, for NLO activity, polymers must either contain noncentrosymmetric side chains (eg., side-chain liquid crystal polymers) or highly delocalized (conjugated) backbones (16). Alternatively, polymers can be doped with low molar mass compounds to obtain NLO activity. 

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