Mixed-substituent polyphosphazenes

The mixed substituent polyphosphazene shown in Formula below is an example [391]. In the typical formulation ... 

The overall synthetic pathway to mixed-substituent polyphosphazenes... 

Mixed-substituent polyphosphazenes are also accessible by utilizing the different reactivities of halogens according to their location in the polymer. For example, the chlorine atoms in the side groups of phosphazo polymers (3.5 and 3.6) are more reactive than those linked directly to the main chain hence, the side-group halogens react first,... 

Recently, a mixed-substituent polyphosphazene (polymer V) was synthesized and the second-order NLO properties were investigated (17). The nitrostilbene/trifluoroethoxy ratio was approximately 36 64. Due to the low glass transition temperature of V (T - 25 C), the second harmonic signal decayed to zero within a few minutes. However, polymer V is a prototype which offers many opportunities for further tailoring the molecular structure of polyphosphazenes to generate an optimum combination of NLO and physical properties (17). 

FIGURE 7.5 Mixed-substituent polyphosphazenes with both alkoxy and alkoxy ether side groups. (Reprinted with permission from Allcock, H.R., Napierala, M.E., Cameron, C.G., and O Connor, Synthesis and characterization of ionically conducting alkoxy ether/... 

Properties. One of the characteristic properties of the polyphosphazene backbone is high chain dexibility which allows mobility of the chains even at quite low temperatures. Glass-transition temperatures down to —105° C are known with some alkoxy substituents. Symmetrically substituted alkoxy and aryloxy polymers often exhibit melting transitions if the substituents allow packing of the chains, but mixed-substituent polymers are amorphous. Thus the mixed substitution pattern is deUberately used for the synthesis of various phosphazene elastomers. On the other hand, as with many other flexible-chain polymers, glass-transition temperatures above 100°C can be obtained with bulky substituents on the phosphazene backbone. 

Polyphosphazenes bearing crown ethers (12-crown-4,15-crown-5 and 18-crown-6) as single or as mixed substituents with trifluoroethoxy or methoxy-ethoxyethoxy groups were synthesized by Cowie [601,602] and Allcock [484] and their conductivity studied because it was shown that the incorporation of crown ether molecules into a polymer electrolyte could increase their ionic conductivity. In these macromolecules, the crown ether units were linked to the backbone through oxymethylene spacer groups. 

A series of cobalt carbonyl complexes of polyphosphazenes have been prepared via arene coordination sites. Examples are shown as 3.65 and 3.66.112 These are synthesized via the reactions of (NPC12) with the sodium salt of the appropriate metal-arene terminated alcohol. Mixed-substituent polymers with trifluoroethoxy or phenoxy cosubstituents have also been prepared. 

Many polyphosphazenes form strong films and membranes when solutions are spread on a flat surface and the solvent is allowed to evaporate. Some of these films have special properties such as resistance to UV radiation, and are thus of interest as coatings for solar cells, aircraft, or space craft. The polymer [NP(OCH2CF3)2] and related mixed-substituent derivatives have been studied from this viewpoint.1... 

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. 

Schacht and coworkers in Belgium201-212 used pellets of a polyphosphazene with both ethyl glycinate and ethyl phenylalanate side groups for the controlled release of the antitumor agent, mitomycin-C. A 100% ethyl glycinato polymer released the drug too rapidly, but mixed-substituent polymers with 50-65% phenylalanine ester released the drug at an optimum rate. 

This combination of macromolecular substitution and access to mixed-substituent polymers underlies the extraordinary versatility of the polyphosphazene platform. By mid-1997 more than 3000 publications and patents had appeared on this subject, and... 

The phosphorus NMR chemical shifts of a few selected polyphosphazenes are given in Table 3.7. Usually slightly broad signals are seen in the NMR spectrum of the polymers because of the inerease in viscosity when polymers are dissolved in solution. The P-NMR chemical shifts of polyphosphazenes indicate that these are about 20-30 ppm upfield shifted with respect to the cyclic trimers. The cyclic tetramers have intermediate chemical shifts (entries 1, 2, 5, 8, 11 in Table 3.7). In polyphosphazenes containing mixed substituents the P-NMR chemical shifts are diagnostic... 

FIGURE 7.7 Structure of mixed-substituent phenoxy/oligoethyleneoxy polyphosphazenes and single-substituent aryloxy polyphosphazenes with oligoethyleneoxy units attached to the aromatic rings in the para position. (Reprinted from Solid State Ionics, 156, Allcock, H.R. and Kellam, E.C., The synthesis and applications of novel aryloxy/oligoethyleneoxy substituted polyphosphazenes as solid polymer electrolytes, 401-414, Copyright 2003, with permission from Elsevier.)... 

Polyphosphazenes are a relatively new class of biodegradable polymers. Their hydrolytic stability or instability is determined not by changes in the backbone structure but by changes in the side groups attached to an unconventional macromolecular backbone. Synthetic flexibility and versatile adaptability of polyphosphazenes make them unique for drug delivery applications. For example, Veronese et al.18 prepared polyphos-phazene microspheres with phenylalanine ethyl ester as a phosphorous substituent and loaded it with succinylsulphathiazole or naproxen. The kinetics of release from these matrices were very convenient in yielding local concentrations of the two drugs that are useful per se or when mixed with hydroxyapatite for better bone formation. Polyphosphazene matrices are also considered as potential vehicles for the delivery of proteins and vaccines.19... 

In general, the synthesis of polyphosphazene polymers is unique in that, in theory, an infinite number of polymers with a variety of properties can be derived from the common polymeric intermediate, poly(dichlorophosphazene) (PNCI2), by replacing the chlorines with different nucleophiles. If the polydichlorophosphazene precursor is reacted with the sodium salts of trifluoroethanol and a mixed fluorotelomer alcohol, a poly(fluoroaIkoxyphosphazene) elastomer (FZ elastomer) is obtained. It contains a small amount of an unsaturated substituent as a curing site. The polymer is a soft gum, which can be compounded with carbon blacks and fillers and cured with sulfur or peroxides or by radiation. 

The synthesis of N-silyl-phosphoranimines bearing mixed alkoxyalkoxy and trifluoroethoxy substituents is reported. The polymerization kinetics and the proposed anionic mechanism are also discussed. These monomers are utilized for the preparation of polyphosphazene random copolymers by the simultaneous polymerization of two phosphoranimines. Block copolymers have also been synthesized by addition of a second phosphoranimine after conversion of the first. Evidence for copolymer formation includes and 3lp NMR, SEC, solubility and DSC data. The differences between analogous random and block copolymers are discussed. 

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