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POLYPHOSPHAZENES Introduction Polyphosphazenes (1–9) belong to the class of inorganic polymers (qv). They have a heteroatomic backbone consisting of alternating phosphorus and nitrogen atoms, with two side groups attached to each phosphorus atom (Fig. 1). The side groups can be organic, inorganic, or organometallic. Perhaps the most attractive feature of polyphosphazene chemistry is the very broad range of groups that can be easily incorporated into the macromolecular chain, which opens up unlimited possibilities for derivatization. The number of different polyphosphazenes that have been synthesized will soon approach 1000; no other polymer synthesis has been studied so extensively. The ability to control the macromolecular architecture and to finetune properties of the resultant polymer has attracted the attention of many research groups throughout the world. The initial application of polyphosphazenes as high performance elastomers has expanded into solid polymer electrolytes, membranes for gas and liquid separations, optically active polymers, biomaterials, and proton-exchange membranes for fuel cells. Polyphosphazenes are strong competitors for their isoelectronic analogues, polysiloxanes, which are considered the most important of all inorganic polymers with regard to commercial applications (see SILICONES). History. The history of polyphosphazenes dates back to 1897, when Stokes (8) reported that phosphonitrilic chlorides could be converted by heating into “the rubber-like polyphosphonitrilic chloride.” He described it as a “body, or a mixture of bodies, of very high molecular weight, that is highly elastic and insoluble in all neutral solvents, but which swells enormously in benzene”. Since then there have been a number of studies devoted to polymerization of phosphonitrilic chlorides. Attempts to carry out the polymerization in organic solvents containing hydrogen Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

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R1

OC6H5

NHCH3

P N

P N

P N

R2 (a)

n

OC6H5 (b)

n

n

NHCH3 (c)

CH3 P N

n

C6H5 (d)

Fig. 1. General formula (a) and three examples of polyorganophosphazenes with phenoxy (b), methylamino (c), and methyl/phenyl (d) substituents.

were unsuccessful. Later studies performed in halogenated solvents showed some moderate success: a soluble fraction of 18–50% with a polymerization degree of 300 was reported (11). Any effort to increase the molecular weight of the polymer resulted in formation of cross-linked, insoluble “inorganic rubber”. To overcome the problem of cross-linking, Kauth (12) proposed a two-step synthesis. In the first step the monomer was heated for 12 h at 150–200◦ C, which resulted in a partially polymerized oil that could be dissolved in a suitable solvent. This solution was used to “saturate some material,” and the final polymerization was accomplished by an additional heating step. The struggle to obtain a soluble, long-chain polymer continued until the mid-1960s when Allcock and Kugel (13) reported that linear polydichlorophosphazene could be obtained. Optimization of the polymerization conditions and macromolecular substitution of chlorines with organic nucleophiles led to hydrolytically stable polyorganophosphazenes. Since then, interest by the military and polymer industries has led to an enormous growth in polyphosphazene science. Nomenclature. Of the several naming systems related to phosphorus– nitrogen compounds: phosphonitrile (the oldest one), azaphosphorine, phosphorus-µ-nitrido, nitrilo-phosphoranylidyne, and phosphazene (14), only the last one has been generally accepted and adopted by the macromolecular research community. To comply with IUPAC nomenclature, a polyphosphazene with the common name poly(diphenoxyphosphazene) should be named catenapoly[(diphenoxyphosphorus)-µ-nitrido], if rules for “inorganic and coordination polymers” (15) were applied. If on the other hand, rules for “organic polymers” (16) were used, then the preferred constitutional repeating unit should be reoriented and the polymer should be named poly[nitrilo(diphenoxyphosphoranylidyne)].

Synthesis There are three general methods to synthesize polyphosphazenes. One is the ringopening polymerization of hexachlorocyclotriphosphazene, followed by a macromolecular substitution of chlorine atoms with a desired nucleophile or a combination of nucleophiles. As a result, the side groups are bonded to phosphorus through oxygen or nitrogen linkages. This is the primary method for polyphosphazene synthesis. The second method, called a “single-pot synthesis,” is based on direct reaction of ammonium and phosphorus salts. The product, poly(dichlorophosphazene), then reacts with an appropriate nucleophile to give a poly(organophosphazene). It is claimed that production costs for this second method are low. The third method is condensation polymerization of phosphoranimines. Here the substituents are

Vol. 7

POLYPHOSPHAZENES Cl Cl P N N

PCl5 + NH4Cl

Cl

P

N

Cl

250°C

P Cl Cl

Cl P N

R

R−X+

P N

n

Cl

(a)

(b)

605

n

R (c)

Fig. 2. (a) Trimer synthesis, (b) the ring-opening polymerization, and (c) the macromolecular substitution.

usually attached to phosphorus atoms prior to the polymerization. Very high molecular weights (M w = 106 ) can be reached with ring-opening polymerization but the molecular weight distribution is very broad. Only moderate molecular weights (M w = 104 –105 ) are accessible via the condensation method; however, polydipersity in this case is usually 105 ), severe gelation resulted and the copolymer was insoluble in available solvents. Gleria and co-workers (65) synthesized several copolymers by thermal or photochemical grafting of various vinyl polymers onto polyphosphazene matrices. The free-radical process was initiated by hydrogen abstraction from a primary, secondary, or tertiary carbon of the alkylphenoxy substituent. Monomers such as maleic anhidride (66), methylmethacrylate (67), N,N  dimethyl-acrylamide (68), meth(acrylic) acid (69), styrene (70), and vinyl acetate or alcohol (71,72) were used. Welker and co-workers (73,74) studied radiationinduced grafting onto allylamino-substituted polyphosphazenes and Prange and co-workers (75) prepared graft copolymers from polyphosphazene and polystyrene macromonomers. An interesting example of a phosphazene random copolymer containing thiophenoxy groups was synthesized by Carriedo and co-workers (76). Although very long reaction times were required and low yields (ca 25%) were observed, it was the first time that a thiophenoxy-substituted poly(phosphazene) was obtained. Polyphosphazene Blends. Polyphosphazene blends were investigated for thermal stabilization of organic polymers, biochemical applications, and membrane preparation. Poly[bis(carboxylatophenoxy)phosphazene]– polyurethane blends (77) were prepared through reactive mixing of the polyphosphazene with diisocyanate and diol prepolymers, and their flame-retarding potential was analyzed. Chen-Yang and co-workers (78) prepared blends of poly[bis(pchlorophenoxy)phosphazene] and polystyrene, and studied their morphology, thermal properties, and flammability. Partial compatibility was observed when the polyphosphazene content was less than 50%. Ambrosio and co-workers (79) studied degradable polyphosphazene/polyester blends with self-neutralizing potential.

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Herrero and Acosta (80) investigated the microstructure of poly(ethylene oxide)– poly[(octafluoropentoxy)(trifluoroethoxy)phosphazene] blends. Limited miscibility of both components was inferred, based on the observed shift of the components’ glass-transition temperatures. Wycisk and co-workers (81) prepared membranes from blends of sulfonated poly[bis(3-methylphenoxy)phosphazene] with polyimides, polyacrylonitrile, and Kynar FLEX PVDF. Morphology, electrochemical performance, and methanol permeabilities of the membranes were then evaluated as part of a program to investigate such blends in direct methanol fuel cells. The polymers were immiscible and a domain-type structure was observed. The best compatibility resulted when the tetrabutylammonium or sodium salt of the polyphosphazene was used (82). Cross-Linked Polyphosphazenes. Many applications require some level of cross-linking to be incorporated into the polymer to obtain an elastomer or to make it stronger, insoluble, or more thermally/chemically stable (83). Three kinds of cross-links are generally distinguished: physical (through crystallites or glassy microdomains), ionic complexes, and finally, covalent cross-links. The most stable are covalent bonds and these are of utmost importance. Two general methods, both of the radical nature, were generally used to induce covalent cross-linking in polyphosphazenes. One was based on incorporating some number of unsaturated functions like vinyl or allyl groups into the polyphosphazenes. This approach was generally adopted for vulcanization of polyphosphazene elastomers (84,85). The second method relied on labile hydrogen abstraction from secondary or tertiary carbons in the side groups (86–89). In Figure 6, an example is presented of photo-cross-linking of poly[(4-ethylphenoxy)(phenoxy)phosphazene] (90). Here, irradiation with UV light of wavelength 340 nm causes excitation of the benzophenone photoinitiator, which then abstracts hydrogen from a benzylic carbon of the polyphosphazene side groups. As a result, a macroradical is formed, where recombination of two macroradicals results in formation of a covalent crosslink. Electron beam irradiation or γ -ray exposure was also used to induce crosslinking of alkylaryloxy- or oligoethoxy-substituted polyphosphazenes (74,91–95). Interesting results were obtained with ionic cross-links. Allcock described the use of poly[bis(carboxylatophenoxy)phosphazene] for a microencapsulation of

CH3

CH3 CH2

CH3 * CH

* CH

CH3 HC

CH3 CH

O P N O

O

O

O

O

P N

P N

P N

P N

O

O

O

O

UV 340 nm

O

Fig. 6. The photo-cross-linking of poly[(4-ethylphenoxy)(phenoxy)phosphazene] with benzophenone photoinitiator.

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mammalian cells (96). Acid–base complexation was also employed for crosslinking (97). In this way a poly[bis(3-methylphenoxy)phosphazene] sodium sulfonate was blended with polybenzimidazole into a membrane. Insoluble films were obtained after conversion of the sulfonate groups into sulfonic acid moieties and heat treatment of the polymer (98). The cross-linking degree and water swelling were controlled by adjusting the polybenzimidazole content. Functional Polyphosphazenes. Functionalization can be defined as the introduction of chemical groups into the polymer chain that exert a specific function (eg chemical, physical, or biological) (99). Introduction of reactive groups into a polyphosphazene via the general macromolecular substitution route usually requires use of protected reagents, otherwise cross-linking and precipitation of the partially substituted polydichlorophosphazene may result (100,101). Derivatives containing hydroxylic (102–104), amine (105–108), and carboxylic (109) groups were prepared in this way. Carriedo and co-workers (110,111) developed an interesting method of synthesizing various poly(aryloxyphosphazenes) using K2 CO3 or Cs2 CO3 as proton abstractors to substitute for chlorine atoms in poly(dichlorophosphazene). Even if bifunctional nucleophiles were used, no cross-linking was observed when the carbonates were present and the resultant polyphosphazene was soluble. Using this method, novel poly(spirophosphazenes) (112) and the chiral poly(dioxybinaphtylphosphazenes) (45) were synthesized. A significant amount of research was devoted to post-functionalization of aryloxy- or methyl-substituted polyphosphazenes. In the first case, electrophilic aromatic substitution reactions were used to obtain sulfonated (113–116), carboxylated (117) nitrated (aminated) (118,119) and phosphonated (120,121) products (Fig. 7a). Alternatively, methyl substituents were deprotonated with n-BuLi and subsequently treated with a desired nucleophile (Fig. 7b). Using this method, polyphosphazenes bearing carboxylic (122), alcohol (123,124), ester (125), and other (63,126) groups became available. Numerous studies have been devoted

O

SO3H

O

P N

SO3

P N

n

O

n

O (a)

CH3 P N

n-BuLi n

CH2−Li+ P N

n

CO2 H+

CH2COOH P N

n

(b)

Fig. 7. (a) Sulfonation of the phenoxy substituents of the poly[bis(phenoxy)phosphazene] and (b) lithiation/carboxylation of the methyl group of poly[(methyl)(phenyl)phosphazene].

−100

−50

0

611

OC6H4C6H5-p

NHCH3

NHC2H5

OC6H4CH3-p

OC6H4CH3-m

CH3

Cl

OCH3

POLYPHOSPHAZENES

OC4H9

Vol. 7

50

100

Glass-transition temperature, °C

Fig. 8. The dependence of glass-transition temperature on the type of side group R in symmetrically substituted polyphosphazenes, [R2 PN]n .

to a surface modification of polyphosphazenes (98,100). The most important objectives were balancing hydrophobicity or hydrophilicity, and immobilization of biologically active compounds (113,127–130).

Properties Polyphosphazenes, owing to their inorganic backbone, are characterized by properties that are not common to organic polymers, namely low temperature flexibility, nonflammability, good thermal stability, and biocompatibility. Backbone Flexibility. A characteristic feature of all the polyphosphazenes is their high skeletal flexibility, which is reflected in their very low glasstransition temperatures (Fig. 8). The high segmental mobility of the chain was explained by Allcock (4) as originating from cylindrical symmetry of the P N bond because of broad overlapping of the nitrogen p-orbital with any of the five d-orbitals of the phosphorus (131). For example, the backbone torsional barrier in poly(difluorophosphazene) was found to be as low as 0.1 kcal per repeat unit (1). Although the formal structure of the polymers comprised a system of alternating single and double bonds, no electron delocalization was evidenced and an “island” pibond structure was postulated. Structural studies on poly(dichlorophosphazene) suggested a slight alternation in P N bond lengths in the polymer backbone with values of 0.144 and 0.168 nm (132). Crystallinity. Polyphosphazenes with a single substituent that is either small or rigid are semicrystalline. Their melting behavior consists of two firstorder transitions, T(1) and Tm , separated by a 150–200◦ C gap. Optical microscopy showed that the crystalline structure was not lost at T(1), but rather existed as a mesophase until Tm was reached (2). Most fluoroalkoxy and aryloxy derivatives exhibited very high thermal stability with decomposition temperatures of 300– 400◦ C.

Applications Elastomers. An elastomer is a material that can be stretched repeatedly to a high extent (elongation greater than 200%) and always retracts to its original dimensions when the stress is released. Chain flexibility resulting from a freedom of

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POLYPHOSPHAZENES

Vol. 7 OCH2CF3 P N

n

OCH2C3F6CHF2

Fig. 9. An example of a typical polyphosphazene elastomer (EYPEL-F).

bond rotation within the polymer backbone is the main prerequisite for achieving elastomeric properties. Polyphosphazenes with low glass-transition temperatures proved to be best suited as high performance elastomers (61,133,134), especially for military applications. Substitution of two different types of fluoroalkoxy side groups on the same chain gave phosphazene polymers that were elastic at temperatures as low as −60◦ C, nonflammable, and resistant to hydrocarbon solvents, oils, and hydraulic fluids. A representative structure is shown in Figure 9 (135). Several polyphosphazene fluoroelastomers were commercialized under the trade name PNF or EYPEL-F. Polyphosphazenes were also used for toughening of thermosetting resins (136). Polydialkoxyphosphazenes were tested as low temperature greases and sealants (137). These materials proved to have high plasticity over a wide temperature range (−100◦ C to +100◦ C) with a decrease in the compressive/tensile modulus as the temperature was lowered. Solid Polymer Electrolytes. Poly(ethylene oxide) (PEO) doped with a lithium salt has been used as the solid polymer electrolyte in rechargeable lithium batteries. The oxygen atoms in the PEO backbone coordinate metal cations, thus facilitating dissociation of the dissolved salts. However, because of its crystallinity, high conductivity could only be achieved when heated above 65◦ C. In 1984 Allcock and co-workers (138) found that poly[bis(methoxyethoxyethoxy)phosphazene] (MEEP, Fig. 10) doped with certain lithium and silver salts exhibited a room temperature ionic conductivity that was 2–3 orders higher than that of doped PEO. The repeat unit of MEEP has six oxygen atoms for cation coordination. The side groups are very flexible, which together with the inherent backbone flexibility, contributed to a glass-transition temperature being as low as −84◦ C. The polymer, however, was totally amorphous, which resulted in problems with its dimensional stability (139). A significant amount of research was directed to overcome this disadvantage. There were reports on cross-linking (140–142), blending with PEO (143,144), composites with silicates (145,146), and synthesis of poly(phosphazene-ethylene oxide) block copolymers (59). A new mechanical stabilization strategy, based on ceramic composites, was invented and developed at the Idaho National Engineering and Environmental Laboratory (147). Polyphosphazenes bearing crown ethers (148), sulfone or sulfoxide (149), or sulfonate (150) groups were also tested as potential Li+ ion conducting solid polymer electrolytes. OC2H5OC2H5OCH3 P N

n

OC2H5OC2H5OCH3

Fig. 10. Poly[bis(methoxyethoxyethoxy)phosphazene] (MEEP), a solid polymer electrolyte that was studied for use in lithium batteries.

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Membranes for Gas Separation. Many linear polyphosphazenes have been tested as membranes for gas separation (151). Derivatives with phenoxy, trifluoroethoxy, or n-alkoxy were of most interest. Some of the work focused on O2 and N2 permeability (152–155) but other gases were also investigated (156– 160). The literature data show a wide scatter of measured permeabilities for the same polymers. Most of the confusion can be attributed to the quality of the polymer samples, their thermal history, and experimental factors. Nevertheless, some trends are apparent. In general, low-T g polyphosphazenes with alkoxy substituents have very high oxygen permeabilities, comparable to those of siloxane polymers, but with better selectivities (151). Also, polyphosphazenes showed a very high permeability for CO2 . Polyphosphazene membranes were applied to the recovery of helium from natural gas (158). Poly[bis(phenoxy)phosphazenes] have been tested in mixed gas separations where SO2 /N2 separation factors as high as 17.2 were observed at 190◦ C (161). The effect of trimethylsilyl substituents on the gas transport properties of a series of polyphosphazenes has also been reported (162). When substituted at the para position of the phenoxy side groups, the polymers promoted an increase in selectivity accompanied by a decrease in gas permeability. Alternatively, incorporation of the trimethylsilyl group on the methyl group of poly[(methyl)(phenyl)phosphazene] resulted in an increase of both permeability and selectivity. While searching for chemoselective membranes, Orme and co-workers (163) incorporated three pendant groups into a polyphosphazene chain: 2-(2-methoxyethoxy)ethanol, 4-methoxyphenol, and 2-allylphenol. The first group gave the polymer proper hydrophilic character, the second group provided hydrophobic counterbalance and film-forming abilities, and finally, the third group enabled cross-linking (Fig. 11). The membranes showed some promise for CO2 separation. The gas permeability of poly(organophosphazenes) was also the subject of theoretical studies and atomistic simulations (164). Pervaporation. Separation of water–organic and organic–organic solutions by pervaporation is another area for potential applications of polyphosphazene membranes (151). Selectivities as high as 104 have been reported for the separation of dichloromethane from water by pervaporation, using a membrane prepared from poly[bis(trifluoroethoxy)phosphazene] (161,165). High selectivities for the separation of toluene and heptane were observed with a similar membrane (166). Pervaporation of water and alcohols was also investigated (167–169).

O

OCH3

O

H2C CH CH2 P N P N

H3CO

O

n

O(C2H4O)2CH3

Fig. 11. Polyphosphazene with methoxyethoxyethoxy, 4-methoxyphenoxy and 2allylphenoxy substituents used by Orme and co-workers (144) for gas separation studies.

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Separation of Tritiated Water. Aromatic polyphosphazene membranes were investigated in the separation of tritiated water from normal water, because of their excellent radiological, thermal, and chemical stability (170,171). For these experiments carboxylated poly(diaryloxy)phospohazenes were used. A tritium depletion as high as 33% was reported. Proton-Exchange Membranes for Fuel Cells. Fuel cells are a promising power source of the future. In current hydrogen/air PEM fuel cells, Nafion perfluorinated membranes are used as the proton-conducting separator of the anode and cathode half-cells. In addition to its high price, Nafion has two serious drawbacks that limit its use: high methanol crossover in direct methanol fuel cells (DMFC) and loss of conductivity (dehydration) at temperatures above 100◦ C in hydrogen/air fuel cells. Polyphosphazenes are a serious alternative to Nafion. Preliminary studies showed that films from sulfonated and UV-cross-linked poly[bis(3methylphenoxy)phosphazene] possessed high proton conductivity, low methanol diffusivity, and good thermo-oxidative stability (172–174). Blends of sulfonated polyphosphazene and an inert organic polymer like a polyimide, poly(vinylidene fluoride), or polyacrylonitrile (PAN) were also investigated (175). The polyphosphazene component of the blend was stabilized by UV or e-beam cross-linking. The resultant membranes had conductivities from 0.01 to 0.06 S/cm (in water, 25◦ C) and an equilibrium water swelling from 20 to 60% (25◦ C). Direct methanol fuel cell (DMFC) tests were performed using membrane-electrode assemblies (MEA) prepared from SPOP/PAN and SPOP/Kynar FLEX (copolymer of vinylidene fluoride and hexafluoropropylene) blends. With a three-layer composite MEA (a methanol blocking film sandwiched between two high-conductivity membranes), a significant reduction in methanol crossover was observed with a modest decrease in current–voltage behavior, as compared to Nafion 117 (176). Phosphonated polyaryloxyphosphazenes were synthesized by Alcock and co-workers (120,121) and investigated as potential membrane materials for use in direct methanol fuel cells. The membranes cast from N,N-dimethylformamide were found to have conductivities between 0.01 and 0.1 S/cm and methanol diffusion coefficients (in 3 M MeOH) between 0.14 × 10 − 6 and 0.27 × 10 − 6 cm2 /s. Phenyl phosphonic acid functionalized poly[aryloxyphosphazenes] were prepared and evaluated as proton-conducting membranes in DMFC tests (177). Sulfonated derivatives of polyphosphazenes were also tested but their methanol barrier properties were found to be less attractive (178). Polyphosphazenes bearing sulfonimide groups were examined in a H2 /O2 fuel cell at temperatures above 80◦ C. Less deactivation of the membrane due to dehydration (as compared to Nafion) was observed (179,180), which is a significant finding if these polymers are to be used in high temperature PEM fuel cells. Biomedical Applications. Biomedical applications were investigated by a number of polyphosphazene research groups (181). The most important studies concerned biocompatibility, biodegradation, enzyme immobilization, and drug delivery (182,183). Polyphosphazene implants and prostheses were also examined. Biocompatibility through appropriate manipulation of surface or bulk chemistry was studied extensively (184). Works on the synthesis and cross-linking of amphiphilic polyphosphazenes (94), heparin immobilization (185–187), and other surface functionalizations (4,101) were reported.

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615

Trypsin and glucose-6-phosphate dehydrogenase were immobilized on surface-aminated poly[bis(phenoxy)phosphazene] and successfully tested in continuous-flow reactors (130). Biodegradable polyphosphazene/poly(lactite-coglycolide) blends were tested for controlled drug delivery using p-nitroaniline as a model release agent (188). Other polyphosphazenes that were susceptible to erosion under physiological conditions were also investigated; they contained amino acid ester (189–193), imidazolyl (194,195), glyceryl (104), or glucosyl (103) groups. In vivo performance under clinically relevant conditions were planned for poly[(p-methylphenoxy)-co-(ethylglicinato)phosphazene] matrices (196). Water-soluble methoxyethoxy–aminoaryloxy co-substituted polyphosphazenes were synthesized and tested as carriers for N-acetylglycine (106). Synthetic vectors for gene delivery based on polyphosphazenes were also reported (197). Song and co-workers (198,199) investigated the synthesis and antitumor activity of poly(organophosphazene)/diamineplatinum and poly(organophosphazene)/deoxorubicin conjugates. Only the first conjugate exhibited high antitumor activity due to controlled release of diamineplatinum(II). Polyphosphazene membranes and microspheres were investigated for the treatment of periodontal diseases (200). Antibacterial drugs, useful in periodontal tissue regeneration, could be entrapped in the membranes and released at appropriate rates to ensure therapeutic concentrations in the tissue. A hydroxyapatite– polyphosphazene composite was tested as a possible artificial bone-replacement material (201). Peripherial nerve regeneration with a poly[(organo)phosphazene] tubular prosthesis was presented as an alternative to silicone tubes usage (30). In other studies mammalian liver cells were encapsulated using calcium-crosslinked polyphosphazene (96), and polyphosphazene microspheres for insulin delivery were prepared (203) and their release behavior were evaluated. The use of polyphosphazenes as resilient denture liners and maxillofacial prostheses was also reported (204). Water-soluble phosphazene polymers were used to enhance the immunogenicity of HIV-1 secreted viral antigens (205). The system was found to have potential as a new HIV-1 vaccine candidate for human trials. Flame Retardants. The high phosphorus and nitrogen content of polyphosphazenes make them nonflammable (206). Their properties are characterized by a high oxygen index (2), low smoke emission, noncorrosiveness, and low toxicity of combustion gases. Several polyphosphazenes were tested as coatings, foams, and coverings for electric cables. In flaming combustion, a commercially viable polyphosphazene exhibited a 75% reduction in heat release rate compared to the polyurethane rubber currently used in fire-blocked aircraft seat cushions (207). Cyclic, oligomeric, and polymeric phosphazenes have been investigated as flame-retarding additives and blends for several commodity polymers. For example, flame retardation improvement of polyurethane has been achieved by the use of aziridinyl substituted cyclotriphosphazene (208). The trimer served a dual function as cross-linker and as flame-retardant. Improvement of flame resistancy by incorporation of cyclic phosphazenes into organic polymers was the subject of many studies (209–213). Chen-Yang and co-workers (78) showed that blends of poly[bis(p-chlorophenoxy)phosphazene] and polystyrene had thermal stability and flame resistance significantly better than those of a pure polystyrene. Reed and co-workers (214) blended poly[bis(carboxylatophenoxy)-phosphazene] with structural polyurethane using reactive mixing and analyzed the thermal

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stabilities of the resultant foams. An increase in flame resistance at loadings equal to or greater than 20 wt% was observed. Chen-Yang described the syntheses of an epoxy resin (215) and polyimides (216) containing flame-retarding cyclophosphazene. Optical Devices. Of most interest in this application are those studies related to the nonlinear optical effects in polyphosphazene derivatives bearing aromatic azo units (217–219) and the photorefractive effect with polyphosphazenes bearing carbazolyl substituents (220). Possible applications include optical switches, frequency doublers for lasers, holographic data storage, and real-time image processing.

Hybrid Polyphosphazenes In the last decade a number of hybrid polyphosphazenes containing backbone elements such as carbon, sulfur, or metal, in addition to phosphorus and nitrogen, have been synthesized and studied. Ring-opening polymerization of appropriate cyclic heterophosphazene has led to polymers like polycarbophosphazenes, polythiophosphazenes and polythionylphosphazenes (Fig. 12). Several polycarbophosphazenes have been synthesized (221,222). The general observation is that the polymers have a higher glass-transition temperature than the corresponding polyphosphazenes, which attributed to the lower rotational freedom of the C N bond as compared to P N. The first well-characterized examples of polythiophosphazenes, were reported by Dodge and co-workers (223). The polymers were moisture-sensitive and degradation was retarded only when bulky substituents such as o-phenylphenoxy were present. It was found that the S Cl bond was more reactive than the P Cl bond, which allowed for a regioselective substitution to yield polythiophosphazenes with different aryloxy substituents at the sulfur and phosphorus atoms. Synthesis of polythionylphosphazenes was reported by Liang and Manners (224). The perchlorinated polymers were moisture-sensitive, but reactions with aryloxy nucleophiles or primary amines converted them into stable poly(aryloxythionylphosphazenes) and poly(aminothionylphosphazenes), respectively (225,226). The substitution with aryloxides was regioselective but, contrary to the situation observed with polythiophosphazenes, only substitution of the P Cl bonds was observed and the S Cl bonds were intact. On the other hand, amines Cl

Cl

C N P N P N Cl

Cl

Cl n

Cl

Cl

N S N P N P

Cl

Cl

(a)

n

Cl

(b) Cl

Cl

Cl

S N P N P N O

Cl (c)

n

Cl

Fig. 12. (a) Polycarbophosphazene, (b) polythionylphosphazene, and (c) polythionylphosphazene, as examples of hybrid polyphosphazenes.

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POLYPHOSPHAZENES

CH CH2

N P

N n

O

N

P

P

N

(a)

N

P N

P

n

N P (b)

N

R P P R N R R

N P R

P

617

P N

N

N

P R P

P N

N P R

(c)

Fig. 13. Polymers containing cyclic phosphazene trimers: (a) poly(alkenylcyclophosphazene), (b) cyclolinear polyphosphazene, and (c) cyclomatrix material.

substituted easily at both phosphorus and sulfur atoms. Depending on the type of substituent, the polythionylphosphazenes showed properties typical of elastomers or glasses. Condensation routes to sulfur–nitrogen–phosphorus polymers were also reported (227,228). Polymers Containing Cyclophosphazenes. Besides linear polyphosphazenes, three other macromolecular structures can be obtained based on the cyclic trimer: organic polymers with cyclophosphazene side groups, cyclolinear polymers, and cyclomatrix materials (Fig. 13). Poly(alkenylcyclophosphazenes) were studied in Allen’s group (210). His approach involved the synthesis of cyclophosphazenes with an unsaturated group and subsequent radical copolymerization with various organic monomers, eg styrene or methyl methacrylate (229–232). The polymers consisted of an organic backbone, with the phosphazene rings as side groups. Other approaches involved either incorporation of phosphazene trimers into condensation polymers (233,234) or linkage to preformed organic polymers (213,235). Lately, Allcock presented the synthesis of cyclolinear-phosphazene-containing polymers via ADMET polymerization (236,237). Phosphazene–triazine cyclomatrix polymers were prepared by Mathew and co-workers (238). The group of Kumar and Gupta (239–241) developed several cyclolinear and cyclomatrix derivatives of polyimides. These materials showed good thermo-oxidative stability and a high char yield of 50–60% in air at 800◦ C.

BIBLIOGRAPHY “Polyphosphonitrilic Polymers” under “Phosphorus-Containing Polymers” in EPST 1st ed., Vol. 10. pp. 139–144, by H. R. Allcock, The Pennsylvania State University; “Polyphosphazenes” in EPSE 2nd ed., Vol. pp. 31–41, by H. R. Allcock, Pennsylvania State University. 1. H. R. Allcock, Phosphorus–Nitrogen Compounds, Academic Press, Inc., New York, 1972, p. 337. 2. R. E. Singler, N. S. Schneider, and G. L. Hagnauer, Polym. Eng. and Sci. 15, 321 (1975). 3. Ph. Potin and R. De, Jaeger, Eur. Polym. J. 27, 341 (1991). 4. H. R. Allcock, in J. E. Mark, H. R. Allcock, and R. West, eds., Inorganic Polymers, Prentice Hall, Englewood Cliffs, 1992, p. 61. 5. H. R. Allcock, in P. Wisian-Neilson, H. R. Allcock and K. J. Wynne, eds., Inorganic and Organometallic Polymers II (ACS Symposium Series 572), American Chemical

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RYSZARD WYCISK PETER P. PINTAURO Case Western Reserve University

POLYPROPYLENE.

See PROPYLENE POLYMERS.

POLYSACCHARIDES.

See CARBOHYDRATE POLYMERS.