Phosphazenes

Frederick F. Stewarta

DOI: 10.1039/9781849730839-00308

1. Introduction

Phosphazenes have been of interest for at least 100 years. Much of theinterest is due to the ability to form widely varying structures while main-taining key characteristics due to the phosphorus content. Phosphorus,coupled with nitrogen, forms a scaffold from which a multitude of chem-istries can be performed. This paper is a review of the latest developments inphosphazene chemistry. For the sake of this report, the term phosphazene isused to indicate multiple P–N bonds, which means that the scope of thiswork is limited to novel material structures and thus the monomericphosphazene bases have been specifically excluded. The theme of the reportis the pendant group chemistry that has been performed to obtain func-tional materials. In discussing potential applications, aspects of funda-mental science will be introduced with the goal of providing a descriptionof the progress that has occurred during the year 2008. Thus, the work isdivided into three general sections: (1) applications, including biomedicaland energy applications, (2) novel structures, which include both linear andcyclotrimeric phosphazenes and (3) inorganic and materials chemistry thatdiscusses both metal complexes and structured materials. Finally, a shortsection on phosphazene characterization is presented.

2. Applications

2.1 Biomedical materials

Biocompatibility of phosphazenes has spurred research for a long time andduring this year, some significant reports were published. Biocompatibilitygenerally implies that a degree of water miscibility must be achieved. Forphosphazenes, the attachment of hydrophilic pendant groups often impartsthat character onto the macromolecule. Amino acid groups with stronghydrophilic character can be attached through the nitrogen functionality,Scheme 1.1 Synthesis of these structures is performed through nucleophilicsubstitution at phosphorus. Blending of pendant groups is an additional toolthat can be employed to create macromolecules with specific characteristics.

P N

Cl

Cln

P N

HN

HNn

R

R

G

G

R = PEG 750, CH2, (CH2)6

G = COOEt, Galactopyranoside

Amino acid pendantgroup

Base

Scheme 1

aInterfacial Chemistry Department, Idaho National Laboratory, Idaho Falls, ID USA

308 | Organophosphorus Chem., 2010, 39, 308–352

�c The Royal Society of Chemistry 2010

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P N

NH

NH0.8

P N

NH

HN0.2

N

N N

NO

PEG 500NH

O

O

OH

N

O

H

NN

N

N

N

O

NH2

H

(1)

H

A demonstration of the complexity of a blending strategy is shown in (1)and Scheme 2. Scheme 2 shows the synthesis of amino acid phosphazenesusing both nitrogen and oxygen attachment points.2,3 These molecules areformed using a protection-deprotection strategy to assure that the attach-ment is regiospecific. Further work has shown that this type of phosphazenemay be formed into a porous structure suitable for tissue regeneration.4

Structure 1 shows a mixed pendant group structure in which a large andhighly functionalized pendant group has been included.5 The size of apendant group can dictate the relative backbone loading that is possible dueto steric considerations. In this structure, the large pendant group is com-plemented with dimethylaminoethylamine (DMAEA) groups, which, due totheir relative small size, can access the polymer backbone and displace the

P N

Cl

Cln

P N

NHCHC(O)OC2H5

NHCHC(O)OC2H5

nNH2CHC(O)OC2H5

R

Et3N

R

R

R =CH2OH,CH(CH3)OH

HOCHCHC(O)OCH3

R NHBOC

NaH

P N

OCHCHC(O)OCH3

OCHCHC(O)OCH3

n

R

R

R = H orCH3

NHBOC

NHBOC

Scheme 2

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remaining chlorine atoms. Variants using DMAEA with imidazole orhistidine have showed lower cytotoxicity.6,7

Amphiphilic phosphazenes are synthesized by the blending of pendantgroups having differing hydrophilicities. For example, the mixing of anamino acid and an aromatic moiety can yield a macromolecule with mod-erated characteristics.8 In this instance, the biodegradable phosphazene wasblended with poly(lactic acid-glycolic acid) to yield polymer blends withosteocompatibility, Scheme 3. Others have used phosphazenes for syntheticbone grafts.9 Polylactic acid can be combined with hydroxyl terminatedcyclotriphosphazenes to form composites with improved properties.10

Another intruguing development was the use of amino acid phosphazenes forthe formation of composites with nano-hydroxyapatite, further expandingthe physical properties that can be obtained in biocompatible formulations.11

Biocompatibility of phosphazenes have designated them as potential drugdelivery agents. Glucosyl pendant groups, combined with either ethylglyci-nato or 2-methoxyethoxy groups yield bio-compatible hydrogels, whencross-linked, that potentially have activity for drug delivery.12 Cyclotripho-sphazenes containing equal substitution of glycyl-L-lysine and methyl ter-minated poly(ethylene glycol) were found to react readily with 20-succinylpaclitaxel to form a biodegradable matrix for delivery of this important anti-cancer drug.13 The synergistic behavior of multiple adjuvants is recognizedto increase the efficacy of vaccine formulations. Polyphosphazene can act asan adjuvant in combination with CpG oligodeoxynucleotide and a specificantiviral agent, specifically Hepatitis B14 and Respiratory Syncytial Virus(RSV).15 In these works, two phosphazenes were found to be of benefit:poly[di(sodium carboxylatophenoxy)phosphazene] and poly[di(sodium car-boxylatoethylphenoxy)phosphazene]. Interactions between proteins and thehost phosphazene hydrogels must be considered. These interactions have adirect effect on the release rate of the antiviral agent. Chitosan, a naturallyderived polymer, has been observed to reduce the release rate from a phos-phazene hydrogel.16 Phosphazenes also have been demonstrated as agents forgene delivery.7 Phosphazenes substituted with DMAEA and histidene de-rivatives self-assemble with DNA to form 110nm particles.

Addition of fluorine to both cyclotriphosphazenes and linear polypho-sphazenes have been found to yield significant antimicrobial behavior.17

Linear polyphosphazenes were formed using 4-fluoroaniline using threediffering routes, Scheme 4. First, the conventional nucleophilic substitutionroute emplying poly[bis-chlorophosphazene] yielded the desired polymer in87% yield. Second, 2,2,4,4,6,6-hexa (4-fluoroanilino)cyclotriphosphazene

P N

Cl

Cln

P N

NHCHC(O)OC2H5

n

NH2CHC(O)OC2H5

CH3

CH3

2)

1) O-Na+ O

Scheme 3

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was formed and polymerized in a sealed ampoule at 250 1C. The result ofthis reaction was formation of the desired polymer in 70% yield, lower thanthe first route, but still excellent when compared to many linear phospha-zene syntheses. Third, 2-(4-fluoroanilino)-2,4,4,6,6-hexachlorocyclotripho-sphazene was polymerized at 250 1C, followed by substitution of chlorine bythe 4-fluoroaniline. Yield using this route was a low 23% suggesting that theconventional route or the polymerization of the fully substituted trimerwere more effective pathways by which this polymer can be synthesized.Antimicrobial activity was shown for a wide range of targets. Activity alsowas shown for chloroaniline cyclic trimer derivatives.18

Introduction of synthetic materials into the human body often can haveramifications on survivability and overall health. Application of vascularstents are one of the more common procedures performed using a syntheticmaterial. Use of stents is not without disadvantages, such as thrombosis.To combat thrombosis, coatings can be applied over the metal stentto discourage blood clotting. Poly[bis-(2,2,2-trifluoroethoxy)phosphazene](PTFEP) is a durable hydrophobic elastomer with excellent coating prop-erties. Applied to the renal, coronary, and iliac arteries of pigs, significantreduction in thrombosis was observed.19,20 PTFEP was used as a coatingover poly(methylmethacrylate) particles for use in recanalization of bloodvessels following embolization.21 Particles measuring 300–600 mm wereformed and were determined to induce little inflammatory effect suggestinggood biocompatibility. Coatings of PTFEP on osteopathic implants were

PN

PN

P

N

Cl Cl

Cl

ClCl

Cl

PN

PN

P

N

O O

O

OO

O

PN

PN

P

N

Cl O

Cl

ClCl

Cl

F

F

F

F

FF

F

P N

O

On

F

F

1) 250 °C

2) 4-fluorophenol, base

1) 250 °C 2) 4-fluorophenol, base

250 °C

Scheme 4

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observed to improve osseointegration of the implant into the host ascompared to an uncoated implant.22

2.2 Optical/photonic applications

Phosphazenes are known to have useful optical and photonic properties.Polymeric materials can be based upon linking together individual trimersto form a cyclomatrix polymer structure, Scheme 5. Polymerization isperformed through the linking of aromatic perfluorovinyl units to obtainperfluorocyclobutane structures. These molecules were synthesized by firstattaching either 2,2,2-trifluoroethanol or 4-fluorophenol, followed by 4-perfluorovinyl phenol. It should be noted that these reactions are notregiospecific and that formation of trimers with homogenous substitution ispossible. One fact, though, is the steric effect of each initial pendant group.4-Fluorophenol, once added to phosphorus, would be expected to some-what discourage a second attachment due to its steric bulk. However, 2,2,2-trifluorethanol is substantially less encumbering so some homogenouslysubstituted products may be possible. To assure complete substitution, thesecond nucleophile addition is made with a large excess. Polymerization ofthese structures is performed by casting the neat liquid onto a glass plateand heating at 130 1C for 1 hour followed by 180 1C for 15 hours. Obser-vation of the polymerization process by differential scanning calorimetryshows a large endotherm at 120–128 1C. Following heating, theresultant films are no longer soluble due to cross-linking.

Organic light emitting diodes (OLEDs) are an exciting new area forcreating low energy light sources with a high degree of color control, sta-bility, and efficiency. A recent review includes a discussion of phosphazene-based OLEDs.23 Blue and green emitting OLEDs have been developedbased on phosphazene chemistry using a dendrimeric synthetic approach.24

In this pathway, the phosphorus-nitrogen ring serves as the center of thedendrimer. Sequential addition of specific reagents leads to dendrimerictwo-dimensional growth of the ring. For selection of pendant groups,aromatics are preferred due to the high electron delocalization that givesthese materials visual light absorbances. Cyclotriphosphazenes can also

PN

PN

P

N

O O

O

OO

O

R

R

R

F

FF

F

R = CF3CH2O, 4-F-C6H4O

130-180 °CP

N

PN

P

N

O O

O

OO

O

R

R

R

F

FF

FF

F

F

F

F

F

F

F

F

F

n

Scheme 5

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form block co-polymers with aromatic containing organics to form stableblue light emitting polymers.25 The phosphazene prevented thermally in-duced polymer chain aggregation that can shift the emission to green.

An example of a dendrimeric approach is shown in Scheme 6. Initialreaction of hexchlorocyclotriphosphazene with 4-bromophenol, assisted bybase, yields hexa(4-bromophenoxy)cyclotriphosphazene, which is thenreacted with an amino pyrene yielding the OLED material. Variability isshown in terms of the R- group selected for the product dendrimer. Anothervariation in this scheme was the replacement of the initially added 4-bro-mophenol with 4-bromophenoxyphenol. The changes in structure result inchanges in the wavelength of the emitted light, varying between 481 and519 nm. Physical properties of these materials include high glass transitiontemperatures (Tg) (168–191 1C) depending on the actual structure, and de-composition temperatures in excess of 400 1C. Poly(amino acid) variantsthat use a pyrene system on the periphery of the cyclotriphosphazene ringhas also been shown to act as an OLED.26

PN

PN

P

N

O O

OOO

O

N

Ir N

N

(2)

PNP

NPN

O O

OOO

O

Br

Br

Br

PNP

NPN

Cl Cl

ClClCl

Cl

Br Br

Br

BrHO

PNP

NPN

O O

OOO

O

N

N

N

N N

N

R

R

R

RAr

Ar

Ar

ArR

Ar

R

Ar

R= H and CH3

RNAr

H

Ar =

Scheme 6

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Solution processable cyclotriphosphazene OLEDs can be formed throughdirect substitution at phosphorus.27 Polyaromatic substituted phosphazene(2) has one pendant group containing nitrogen for potential complexationwith a metal. In this example, two 2-phenylpyridine ligands are complexedto the iridium metal center.

Novel photorefractive polymers were formed by the addition of carba-zole,28,29 imidazole, and azobenzene30,31 chromophores to linear phos-phazenes, Scheme 7. In this work, the imidazole pendant group is intiallyattached and the remaining chlorines are displaced with ethoxy groups.The choice of ethoxy is purposeful in that the imidazole pendant groupposes significant steric issue in substitution. Loadings of 11 and 14 % wereobtained. It is not clear how high a level of substitution theoreticallycan be achieved; however large pendant groups tend to discourage highlevels of inclusion into the polymer. To obtain a stable material, a smaller

P N

Cl

Cln

+NNH

O-Na+ P N

OEt

OEt

P N

OEt

On

NNH

EtO-Na+

0.86 0.14

Para-substituted benzenediazonium fluoroborate

P N

OEt

OEt

P N

OEt

On

NNH

0.86 0.14

NN

Y

NN

Y

Y = CN, OEt, or Cl

Scheme 7

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pendant group must be employed subsequently to displace the remainingchlorines. Further reaction chemistry produced the azobenzene derivatizedpolymers. These polymers were observed to have high Tg values, rangingbetween 122 1C and 172 1C, which the authors claim give these materialsresistance to either phase separation or crystallization at temperatures ofinterest.

N

NN

N

N

NN N

O

OP

N P

NPN

O

O O

OO

OO

PN

P

NP

N

O

O O

OO

OP

N P

NPN

O

O O

OO

OP

N P

NPN

O

O O

OO

O

O

(3)

M

M = 2 H, Ni, or Zn

The phosphazene can be a pendant group onto an organic chromophore.Structure (3) shows the first phosphazenes formed by the attachment ofcyclotriphosphazene rings to a phthalocyanine ring, with either protons,Ni, or Zn coordinated to the inner ring nitrogens.32 The result of thiscoupling is a molecule containing four phosphazene rings that has a highdegree of electron delocalization and exhibits at least three electronic ab-sorption bands, depending on the presence and identity of the coordinatedmetal and also the solvent. For example, using THF as a solvent for theprotonated phthalocyanine, Q-band absorptions at 699 nm and 669 nm anda B band absorption at 341 nm were measured.

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P

P P

O O

O

OO

O

NN

N

NN

N

(4)

N N

NO2

NN NO2

Structure (4) has served as the basis of a photorefractive glass through thediazotization of a carbazole containing phosphazene cyclic trimer.28 Thediazotization chemistry was found to only lead to one or two as shown instructure (4) azo-groups; although others have formed similar azo linkageson all six pendant groups.33 The diazo group substitution can potentiallyoccur either in a geminal manner (on carbazoles attached to the same P) oras attachments to carbazoles on adjacent phosphorus atoms. Investigationof the regiochemistry of the diazotization was performed using semi-em-pirical AM1 and PM3, and ab initio HF/STO-3G computational methods.All three methods suggested that diazotization would occur on carbazolerings trans to each other on adjacent phosphorus atoms. This result is notunreasonable due to steric considerations. Extension of this general schemeto linear polymers was performed.29 The resulting polymers were found tohave Tg values ranging from 20–65 1C. The polymer with the lowest Tg hada refraction gain coefficient of 91 cm� 1. The highest Tg polymer, with theshortest alkyl spacer between the carbazole and the phosphazene ring, gavea gain of 198 cm� 1 and a diffraction efficiency of 46 % when compoundedwith photoconductive N-ethyl-carbazole, which also acted as a plasticizer.

Related materials were formed by the reaction of hexachlorocyclo-triphosphazene with the sodium salt of 2-hydroxybenzaldehyde, followedby treatment with an amine at reflux, which forms a functionalized imine,Scheme 8.34 Photochemical absorption maxima ranged between 229 nm and245 nm, depending on the choice of R group. Fluorescence behaviors havebeen noted for cyclotriphosphazenes with naphthol (5) and napthylamino(6) pendant groups.35 Furthermore, more complicated fluorescent phos-phazene dendrimers can be formed by grafting dansyl groups onto ionicphosphazene rings. The largest dendrimer possessed 10 ammonium groupsand 5 dansyl groups to form materials that have potential biological com-patibility.36 Attachment of 1,4-phenylenediamine yields three dimensionalnetworks, also known as cyclomatrix polymers, that have both terminal andcross-linking groups.37 The product material is purple, which the authorsproposed to be due to a minor concentration of free radicals. The optical

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properties are intriguing due to the ability to form ammonium groups andthe known degree of electron delocatization in phosphazene rings. Ab-sorption maximum was noted at 575 nm and oxidation of the material in-duced changes to a more complicated UV-visible spectrum fiving peaks at330, 470, 500, and 700 nm.

PN

PN

P

N

XX

Cl

ClCl

Cl

5, X = O6, X = NH

2.3 Materials for electrolytes, membranes, and lubricants

Over the past 20 years, there has been a significant amount of research intothe use of phosphazenes as solid polymer electrolytes for use in both bat-teries and fuel cells. Specifically, materials studied for fuel cell electrodeseparators have focused on the use of sulfonated aromatic containing linearpolyphosphazenes, as shown in a review article;38 although a new strategyinvolves placement of sulfonated cyclotriphosphazenes onto an organicpolymer backbone.39 In the latter case formation of sulfonated phospha-zenes is accomplished through a ‘‘post-sulfonation’’ strategy, Scheme 9.First, the linear poly[bis-chlorophosphazene] is formed. Second, the chlor-ines are substituted with aromatic pendant groups, such as phenol,3-methylphenol, or 4-phenylphenol. Third, the isolated polymer is sulfon-ated in chlorinated solvent using SO3. Also discussed in the review is asummary of the efforts conducted into other ion carriers. A significant issue

PN

PN

P

N

O O

O

OO

O

O

O

O

O

O

O

PN

PN

P

N

O O

O

OO

O

N

N

N

N

N

N

R

R

R

R

R

R

RNH2

THFreflux

R = Ph-; Naphthyl-; 2-HOC6H4-; 2,3-Cl2C6H3-; 3,4-Cl2C6H3-; benzyl; tert-butyl; n-butyl; 3,5-di-tertbutylphenyl;

Scheme 8

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that exists in sulfonated polymers is their excessively high dependence onmaintaining humidity for conductivity. Sulfonates have waters of hydrationassociated with them that provide proton conductivity. In fact, it can besaid that sulfonated polymers are hydronium ion carriers. This dependenceon water limits the high end temperature at which these membranes canoperate. Conductivity begins to degrade at approximately 80 1C and fuelcells with sulfonated membranes require complicated water managementsystems to operate at higher temperatures. To address this problem, phos-phonate and sulfonimide ion carriers have been studied with some success.The progress in the use of phosphonates was the topic of a review.40 Aro-matic phosphonate containing trimers have also been synthesized withsuccess.41

The goal of the development efforts is to create a more effective mem-brane electrode assembly (MEA). Benchmark MEAs are typically formedusing a perfluorinated membrane, such as Nafions. Nafions is an effectiveconductive membrane; however it suffers from excessive water permeabilityand is somewhat unstable in an electrochemical environment, which leads tofree radical degradation processes liberating species such as HF. Phospha-zenes are of interest because they can be formed in the absence of fluorin-ated groups (no potential for HF formation), and they are stable in anoxidative environment. Formation of mechanically stable MEAs has beenaccomplished through blending the sulfonated phosphazene with otherpolymers such as poly(vinylidene difluoride), poly(acrylonitrile) andpoly(benzimidazole).40 The future outlook on the use of phosphazenes asfuel cell materials is directed toward the nature of the polymer itself.Randomness in the standard nucleophilic substitution process is a weak-ness. Better conductors can be envisioned if the substitution can be con-trolled and the conducting moieties can be ordered in the membrane. Also,the nature of the polymer formation process represents another problembecause reproducibility is not guaranteed in the initial ring opening poly-merization process that forms poly[bis-chlorophosphazene]. This leads tovariations in the conductive polymers from batch to batch. Furthermore,the sulfonation reaction also introduces unwanted variability.

P N

Cl

Cln

P N

O

On

O-Na+

SO3P N

O

On

SO3H

SO3H

Scheme 9

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P N P N P N

O

O

O

O

O

O

OCH3

OCH3

CH3OCH2CH2OCh2CH2

CH3OCH2CH2OCH2CH2CH3OCH2CH2OCH2CH2

n

(7)

Phosphazenes can act as membranes for gas separations. They havegarnered attention due to the unique nature of the polymers in that abackbone is formed first, followed by substitution, which allows for thesynthesis of a self consistent series of polymers only differing from eachother by their pendant groups. Such a series allows for observation ofstructure-function relationships and the direct comparison of transportbehaviors with respect to characteristics such as Tg and chemical affinity.Phosphazenes also can be formed into mixed matrix materials through theinclusion of inorganics such as SAPO-34, a silicoaluminophosphate, andmodified two dimensional aluminophosphate (ALPO).42 Inclusion of ofthese inorganics resulted in membranes with greater selectivity towards CO2

over gases such as H2, CH4, and N2. The phosphazene employed in thesestudies was substituted with three differing pendant groups (7). Hydrophilicmethoxyethoxyethanol and hydrophobic 4-methoxyphenol were usedto provide an ability to control the chemical affinity of the polymer, and2-allylphenol was included to provide a facile pathway for cross-linking.

PN

PN

P

N

O O

O

OO

O

F

CF3

F

CF3 CF3

CF3

PN

PN

P

N

O O

O

OO

O CH2CF2O(CF2CF2O)m(CF2O)nCF2CH2OH

CF3

CF3

CF3 CF3

CF3(8) (9)

(m and n range between 10 and 20)

Magnetic recording media, such as computer hard disk drives, rotate athigh frequencies (5–15 kHz) and need lubricants to maintain opertation.Phosphazenes are of interest in this application due to their ability to formfluids with little to no volatility and, due to their phosphorus content, theydo not support combustion. Two differing phosphazene fluid additives havebeen developed based on fluorine chemistry.43 Structure (8) was an early

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additive that indicated that phosphazene had value for this application.Structure (9) is an improvement on the concept that provides enhancedsurface adhesion properties. A variant of phosphazene (9) was synthesizedto have a lower molecular weight. In this variation, the trifluorophenoxygroups on the phosphazene ring were replaced with 2,2,2-trifluorethoxygroups.

3. Novel structures

3.1 Polymerization chemistry

Phosphazenes have nearly an endless variety of structures in which pendantgroup substitution and backbone selection (cyclic or linear) dictate thephysical and chemical characteristics of the materials. Formation of linearpolymers, as shown in the literature, is dominated by ring opening poly-merization (ROP), performed with or without catalyst. This method issomewhat subjective and does give a range of product molecular weights,polydispersities, and yields. Attempts have been made to use more con-trolled processes to form phosphazenes with greater reproducibility. Aperspective paper from the Manners group discusses the various methodsby which linear poly[bis-chlorophosphazene] may be synthesized usingcondensation processes with the goal of providing greater control overthe backbone formation.44 Initial discussions focused on ROP of hexa-chlorocyclotriphosphazene. This discussion included the disadvantages ofROP, namely the lack of control of the product molecular weight anddistribution, and yields that are limited to approximately 70%. Also dis-cussed are the efforts in catalysis that has reduced reaction times signifi-cantly, which includes the historical use of Lewis Acids, solution ROP usingsulfamic acid and calcium sulfate dihydrate, and a direct one-pot synthesisfrom PCl5 and NH4Cl that yielded high polymer. The nature of the ROPprocess is not completely understood; although many studies have pointedto the reaction being cationic. Attempts to employ cationic polymerizationin terms of a condensation route have yielded high polymer. A ‘‘living’’polymerization process using trichloro(trimethyl-silylphosphoranimine),Cl3PQN(Si(CH3)3), has yielded (PNCl2)n through the loss of trimethylsi-lylchloride at ambient temperature, Scheme 10. This method producespolymer with a molecular weight of approximately 104, which is consideredhigh polymer; however it is substantially lower than weights gained usingROP, which can be as high as 106 Daltons.

Catalysis of hexachlorocyclotriphosphazene ROP would be possible if thecatalyst could facilitate the initial formation of a cationic species, which is

P NCl Si

Cl

Cl

P N

Cl

Cln

CH3

CH3

CH3

PCl5

-(CH3)3SiCl

Scheme 10

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then speculated to result in rapid linear polymer formation. Cyclotripho-sphazene cations can be formed through protonation at nitrogen, providedthat there is a stabilizing anion available that has a sufficient low bindingaffinity for the proton. Carboranes are suitable weakly coordinating anions.Using carboranes, protonated (10), methylated (11), and silylated (12)complexes can be formed with hexachlorocyclotriphosphazene.45 The pro-tonated and methylated complexes were not found to significantly effecthexachlorocyclotriphosphazene ROP; however the silylated derivative wasfound to be active. The ability to catalyze ROP was found to be highlydependent on the associated anion. Complex (13), that has a brominatedcarborane anion, catalyzes the room temperature ROP of hexachloro-cyclotriphosphazene at a loading of 10mol %, Scheme 11. Due to thehydrolytic instability of phosphorus-chlorine bonds, the resulting poly[bis-chlorophosphazene] was exposed to sodium 2,2,2-trifluoroethoxide to formthe corresponding substituted phosphazene prior to analysis. The productpolymer was formed in 86% isolated yield and had a molecular weight (Mw)of 1.12� 105 and a polydispersity index (PDI) of 1.83, suggesting that theinitial polymerization process effectively provided high molecular weightpolymer without heating.

Scheme 12 shows the solid state polymerization of a tricyclic mono-azidophosphane yielding a phosphazene.46 Using 2,20dihydroxybiphenyl,the chlorophosphine is formed. Chlorine is then displaced with azide inacetonitrile as solvent in quantitative yield. Interestingly, no azide wasformed in tetrahydrofuran (THF) solvent. Removal of the solvent yields asolid that, when heated to 30 1C, polymerizes to form the correspondingphosphazene. The polymerization produces a range of molecular weightsfrom relatively small oligomers and cyclics to tractable linear polymers withan Mw of 104 and a high Tg of 161 1C. Interestingly, the high Tg is a result ofthe ring strain introduced by the tricyclic biphenyl pendant group. Gener-ally, phosphazenes are observed to have relatively low Tg values due to the

PN

PN

P

N

Cl Cl

Cl

ClCl

Cl

(10)

H

[HCB11H5Br6]-

PN

PN

P

N

Cl Cl

Cl

ClCl

Cl

(11)

CH3

[HCB11(CH3)5Br6]-

PN

PN

P

N

Cl Cl

Cl

ClCl

Cl

(12)

Si(CH3)3

[HCB11Cl11]-

PN

PN

P

N

Cl Cl

Cl

ClCl

Cl

(13)

Si(CH3)3

[HCB11H5Br6]-

PN

PN

P

N

Cl Cl

Cl

ClCl

Cl+

(10 mol %)

25 °CP N

Cl

Cln

Scheme 11

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backbone. Bonding between P and N is pp–dp with a node at each phos-phorus. This bonding results in electron delocalization between phosphorusatoms; however not through them. Thus, the rigidity seen in analogouscarbon backbones is not observed in phosphazenes. A high degree offlexibility results in low Tg values.

3.2 Chemistry of strained systems

Substituted 2-azido-1,3,2-diazaphospholenes undergo thermolysis to givespirocyclic diphosphazenes, Scheme 13.47 The azide is formed from thecorresponding chlorophosphine. The cyclodiphosphazene is formed byheating the neat azide at 110 1C for 6 hours. Characterization of thisstructure was performed using nuclear magnetic resonance (NMR) andsingle crystal X-ray diffraction. Reaction of this structure with triflic acid inmethylene chloride results in protonation of the phosphazene nitrogens.Further thermolysis in wet N,N-dimethylformamide (DMF) results incleavage of the cyclicphosphazene structure. The charge separation notedbetween phosphorus and oxygen is enhanced by a hydrogen bondinginteraction with an amine hydrogen.

In a similar scheme, a monoalkyl amino cyclotriphosphazene was di-merized upon exposure to a deprotonating agent, Scheme 14.48 Spirocyclicpropanolamine binding to a cyclic trimer results in a species, that when theamine is deprotonated with sodium hydride, results in a dimerized structurewith an eight-membered P-N ring, Scheme 15. Both novel compoundswere characterized using NMR and X-ray diffraction. Nitrogen containingorganic species such as oxamide, a two carbon di-amide, can react with PCl5

O

O

P ClNaN3

CH3CN O

O

P N330 °C

O OP N n

Scheme 12

N

P

NR

R

N3

R′

R′

110 °C

N

P

NR

R

R′

R′

N

P

N R

R

R′

R′

N

N

-N2

HOTf

CH2Cl2N

P

NR

R

R′

R′

N

P

N R

R

R′

R′

N

N

H

H

N

P

NR

R

R′

R′

N

P

N R

R

R′

R′

NH2

N

H

O

145 °Cwet DMF

R = 2,6-(CH3)2C6H3; R′ = HR = 2,4,6-(CH3)3C6H2; R′ = HR = 2,6-(CH3)2C6H3; R′ = CH3

Scheme 13

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to give phosphazene structures, Scheme 16.49 In this reaction, an inter-mediate five membered ring monophosphazene is proposed, which thendimerizes into a tricyclic diphosphazene. In an attempt to expand the utilityof this reaction, di-amides based five total carbons were found to give onlycyclic di-amides with no incorporation of phosphorus – the PCl5 acting as adehydrating agent only.

Spermine reacts with a geminal tetrachlorocyclotriphosphazene to yield atetracyclic structure, Scheme 17.50 Versatility of this reaction was shown bythe differing substitutents that can be added to the ring, which includedphenoxy, methoxy, ethoxy, and 2,2,2-trifluoroethoxy. Most interestingly,the order of addition did not appear to effect the overall outcome of thereaction. Spermine can be added first, followed by the oxy-nucleophiles todisplace the final two chlorines. Likewise, two chlorines can be displacedgeminally and the resulting complex can then react with spermine.

Bifunctional reagents can give either spirocyclic pendant groupattachment, bridging to adjacent phosphorus atoms on the same

PN

PN

P

N

Cl NH

Cl

ClCl

Cl PN

PN

P

N

Cl

ClCl

Cl

PN

PN

P

N

Cl

Cl Cl

Cl

N NNaH, THF

-2 HCl

Scheme 14

PN

PN

P

N

Cl

Cl

Cl

Cl

NHO

NaH, THF

-2HCl

P

N

P N

P

N

N

O

Cl

Cl

Cl

N PN

P

N P

N

OCl

Cl

Cl

Scheme 15

O

O NH2

NH2 NP

N

Cl

Cl

Cl

Cl

Cl

N

N

PN

PNCl

Cl ClCl Cl

Cl Cl

ClCl Cl

PCl5

-HCl-POCl3

Scheme 16

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cyclotriphosphazene ring, or cross-linking between rings.51 By maintaininga relatively short carbon chain, substitution onto more than one phos-phorus can be discouraged, Scheme 18. Product distribution for this re-action was measured at 53.2% (14), 21.9% (15), and 2.2% (16), asdetermined by 31P NMR data. No explanation is given for the productdistribution; although one could infer steric influences.

PN

PN

P

N

O O

N

O

O

O

CH3P

N

PN

P

N

O O

N

O

O

O

CH3

(17) (18)

PN

PN

P

N

X X

ClCl

Cl Cl

PN

PN

P

N

X X

N

N N

N

H H

PN

PN

P

N

Cl Cl

N

N N

N

H HNaX

Spermine

X = OPh, OCH3, OEt, OCH2CF3

Scheme 17

PN

PN

P

N

Cl Cl

ClClCl

ClOH OH

BrBr

+THF

PyridineP

N

PN

P

N

ClClCl

Cl

OO

BrBr

PN

PN

P

N

ClCl

OO

BrBr

O

O

Br

Br

PN

PN

P

N

OO

BrBr

O

O

Br

Br

O

OBr

Br

+ +

(14) (15) (16)

Scheme 18

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Other spirocyclic phosphazenes have been developed with diastereos-electivity through the inclusion of chiral pendant groups. 1,10-Bi-naphthol(BINOL) has a rotational barrier between the two napthyl rings preventingfree rotation and imparting chirality.52 Thus, use of this pendant group in aphosphazene synthesis also has the potential of imparting chirality into thephosphazene. Structures (17) and (18) are the – andþ diasteromers, re-spectively. Another unique feature of these compounds are the use of threediffering bidendate pendant groups attached to the ring. Prior to this study,only one report showed this type of substitution.

Tricyclic phosphazenes can be formed using a linear polymer backbone.Earlier in this report, the synthesis of a polyphosphazene with 2,20-dioxy-1,10-biphenyl pendant groups was discussed as the product of the ringopening polymerization of the corresponding cyclotriphosphazene. Nitra-tion was used as a tool to study the regiochemistry of functionalizationprocesses for this polymer, Scheme 19.53 It was found that the polymer iscompletely stable in 98% H2SO4 and the nitration was performed usingeither HNO3 or LiNO3. Treatment of the polymer with excess nitrate resultsin a mixture of 5,50 and 5,30 substitution which is consistent with the resultsof nitration of the free pendant group. Less than stoichiometric addition ofnitrate results in incremental attachment. Thermal analysis of the products,that varied from 0.2 to 1.9 nitrates per mer, produced a proportional in-crease in Tg from approximately 170 1C to 230 1C.

Reaction of linear poly[bis-chlorophosphazene] with substiochiometricamounts of 2,20-dioxy-1,10-biphenyl results in partially chlorinated poly-mers, Scheme 20.54 Three differing loadings of biphenyl were reported:12%, 28%, and 43%. Hydrolysis of poly[bis-chlorophosphazene] is fast.The polymers containing varying loadings of 2,20-dioxy-1,10-biphenylhydrolyze far slower, suggesting that the hydrophobic pendant groupshields the backbone to some degree from hydrolysis. Hydrolysis

PN

O O

PN

O O

PN

O O

PN

O O

NO2

NO2

NO2NO2

NO2

n

n

n

x

x HNO3

excessHNO3

O2N y x + y = 0.2 - 1.9

x and y < 1

Scheme 19

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experiments were performed in both room temperature and hot water andthe data suggested that the rate and extent of hydrolysis was more of afunction of the temperature than the water. Furthermore, the rate of hy-drolysis was a function of the chlorine content. The polymers with higheramounts of chlorine hydrolyzed faster. More than 50% chlorine contentresulted in polymers that rapidly hydrolyzed and cross-linked forminginsoluble materials.

The stability of poly[2,20-dioxy-1,10-biphenylphosphazene] was probed byimmersion of the polymer in aqueous acidic solutions.55 The polymer can bereversibly protonated using HBF4 and restored using a base such at trie-thylamine; however there is some loss in Mw. Reflux of the polymer inaqueous HCl revealed a a more significant loss of molecular weight with time.Initial polymer Mw was measured at 800,000 Da. Degradation was found tooccur asymptotically with the most significant mass losses occuring in the firsthour with Mw falling to 35,000. As a control, similar reflux experimentsperformed in pure water and at low concentrations of HCl did not givesignificant Mw loss suggesting that the HCl is not catalytic in its action.Poly[2,20-dioxy-1,10-biphenylphosphazene] is stable in concentrated H2SO4,which is surprising because poly[bis-phenoxyphosphazene] decomposes.Samples dissolved in concentrated H2SO4 can be recovered and give nearlyidentical spectroscopic data and Tg values. Experiments were performed inconcentrated HNO3 and the Mw for the 800,000 Da polymer only decreasedto approximately 400,000 Da at room temperature. Increasing the tem-perature to reflux resulted in the complete degradation of the polymer.

O R1 O

N N

PN

PN

P

N

R2

ClCl

Cl Cl

(19)

PN

O O

1-n

PN

Cl Cl

nP

N

Cl Cl

n

O-Na+

O-Na+

n = 0.12, 0.28, 0.43

Scheme 20

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Spiro-crypta phosphazenes with multiple ring systems can be synthesizedusing cryptands and they exhibit geminal attachment to the phosphazene.56

In phosphazene (19), three ring systems were synthesized: 1) R1¼R2¼(CH2)3; 2) R

1¼ (CH2)3 and R2¼ (CH2)4; and 3) R1¼R2¼ (CH2)4. Struc-tures were proposed from crystallographic and multidimensional NMR data.Interestingly, the reaction of phosphazene (19) with excess pyrrolidine yieldsonly geminal di-substitution on one of the remaining phosphorus centers,while the other maintains chlorine substitution.

PN

PN

P

N

Cl

ClCl

Cl

NN

PN

PN

P

N

Cl

ClCl

Cl

NN

(20) (21)

NO2O2NHHN

P

N P

N

P

NPN3

N3

N3

N3

N3

N3

N3

N3

(22)

A study of phosphazene as potential high energy compounds resulted inthe characterization of a cyclotriphosphazene ring with an ethylenediaminegroup (20).57 Further, N,N0-dinitroethylene diamine yielded a similarcomplex (21). An additional report from the same group discussed thetheoretical study of octaazidocyclotetraphosphazene (22).58 In this report,the molecular structure, vibrational fequencies and infrared intensities werecalculated using Hartree-Fock, B3LYP, and B3PW91 calculational toolswith a 6-31G basis set. The results of the study suggested that the compoundwould have a high heat of formation, and thus be useful as a high energymaterial. Oxygen combustion of related nitro-containing phosphazenes hasalso been reported.59

3.3 Novel pendant group attachments

Substitution with the bulky aminoadamantane results in incomplete re-action with only four of the six positions substituted, Scheme 21.60

Most interestingly is the fact that geminal substitution was observed inrelatively high yield (63%). This conclusion was supported by NMR andX-ray crystallography. Memantine (1-amino-3,5-dimethyladamantane)gave similar results with a yield of 69%, suggesting that the steric bulk

Cl

PN

PN

P

N

Cl

NH

NH NH

NH

R

R

R

R

R

R

R

R

PN

PN

P

N

Cl Cl

Cl

ClCl

Cl

NH2

R

R+

Et3N

Toluene-HCl

R = H, CH3

Scheme 21

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added by the methyl groups was negligible. Additional crystallographic datawas collected from cyclotriphosphazenes with aromatic pendant groups.61

N

NNH

N

NH2

HN N

NH

N

O

NH2

N

NH

NH2

O

(23) (24) (25)

4-Hydroxy-3-methoxybenzaldehyde gave complete substitution in 74%yield, Scheme 22.

Purine and pyrimidine substituted cyclotriphosphazenes can be syn-thesized by attachment of the pendant group through nitrogen.62 Thepurines employed were guanine (23) and adenine (24), while the pyrimidinewas cytosine (25). Formation of 100% substituted cyclic trimers from eachof these three groups resulted in insoluble materials, thus making productcharacterization difficult. Soluble materials can be formed through thecontrolled incomplete substitution and addition of another pendant group.To demonstrate this strategy, 2,2,2-trifluoroethoxide was added as a co-substitutent, which yielded soluble materials suitable for characterization.Mass spectral data confirmed that, in many cases, more than one nitrogenper pendant group was attached to phosphorus. Extension of this chemistryto linear polymers through conventional macromolecular substitution re-vealed that the highest loading possible with any of the purines/pyrimidinewas approximately 70%, presumably due to steric effects. Additionally, theproducts were insoluble in THF and most other organic solvents, necessi-tating a change in focus to mixed substituent polymers. Chosen co-pendantgroups included glycine ethyl ester, alanine ethyl ester, and 2-methoxy-ethoxyethanol. These sterically smaller pendant groups serve to assurecomplete removal of all chlorines resulting in stable polymers.

PN

PN

P

N

PN

PN

P

N

Cl Cl

Cl

ClCl

Cl+

Et3N

Toluene-HCl

R = H, CH3

CHO

OCH3

OH

CHO

CH3O

O

CHOOCH3

O

CHO

OCH3

O

CHO

OCH3

O

CHO

OCH3

O

CHO

OCH3

O

Scheme 22

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PNP

NPN

O O

OO O

O

N

O

O

NR R

PNP

NPN

O O

OO O

O

N

OR

PNP

NPN

O O

OO O

O

N

OR

N

O

O

NO

N

O

N

O

N

R

R

R

R

R

(26) (27) (28)

R = H or CH3

Oxazolines are versatile structures that can undergo chemical transfor-mation into dendrimeric and polymeric systems. Structures (26–28)show cyclotriphosphazenes with phenoxy-oxazoline pendant groups.63

Both oxazoline and methyl oxazoline groups can be attached to thecyclotriphosphazene ring. Methyl group attachment results in a chiraloxazoline. In a brief report, 1,3,4-oxadiazole substituted cyclotripho-sphazene and linear polyphosphosphazene also were synthesized andcharacterized.64

‘‘Click’’ chemistry is an alternate method for forming dendrimericstructures.65 In this report, an 4-iodophenol is added to a cyclotripho-sphazene followed by 1) the paladium catalyzed coupling between the theiodinated phenol and a functionalized a-D-mannopyroside, protected withacetyl groups, and 2) deprotection to yield the hydroxyl functionalities,Scheme 23.

Trimethylsilylacetylene readily reacts with phosphazene (29) yield-ing hexa-4-trimethylsilylethynylphenoxyphosphazene using coupling con-ditions previously shown in Scheme 23. Removal of the trimethylsilyl groupoccurs in nearly quantitative yield to afford hexa-4-ethynylphenoxypho-sphazene (30). Compound (30) reacts with aromatic iodides, such asshown in Scheme 24, to yield a diphenyl acetylene bridge between themannopyroside and the phosphazene ring. Another pathway to effectthe connection between the sugar and the ring was demonstrated using2-azidoethyl-2,3,4,6-tetra-O-acetyl-a-D-mannopyroside.

Phosphazene rings substituted with oxime functionality can be syn-thesized by the attachment of hydroxyacetophenone followed by reactionwith hydroxylamine hydrochloride.66 This chemistry was demonstratedusing phosphazene (31) that was functionalized with two biphenyl ringsprior to attachment of the hydroxyacetophenone. An exploration of thechemistry of the oxime functionality revealed high yielding reactions be-tween organohalides and acid chloride, Scheme 25. Eugenol derivatives ofhexachlorocyclotriphosphazene offer a pathway to incorporate a reactivealkene functionality onto the ring.67 This versatile addition allows for in-clusion of epoxide groups through the reaction with m-chloroperbenzoicacid (MCPBA), Scheme 26. An additional interesting point is that thecyclotriphosphazene was stable in this oxidizing environment. Epoxidesprovide a facile route for the formation of block co-polymers.

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PN P

NPN

OO

O OOO

I

PN P

NPN

Cl

Cl

Cl Cl

ClCl

I

I

II

I

K2C

O3

Ace

tone

OH

I

PN P

NPN

OO

O OOO

O

HH

O

H

OH

OH

OHH

H

OH

O

HO

H

H

HO

HO

OH H

H

HO

OH

OH

H

OH

OH

OH

HH

HOO

H

OH

HO

H OH

OH

H

H

OH

O

H

OH

HO

H OH

OH

H

H

OH

O

HH

O

H

OH

OH

OHH

H

OH

1)O

H

RO

HR

O

RO

OH

H

H

OR

Cl 2

Pd(

PP

h)3,

CuI

, Et 3

N, D

MF

2) C

H3O

Na,

CH

3OH

(29)

R =

OA

c

Schem

e23

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PN P

NPN

OO

O OOO

(30)

PN P

NPN

OO

O OOO

OH

RO

HR

O

RO

OH

H

HR

OI

PN P

NPN

OO

O OOO

O

HH

O

HHO

OH

OHH H

OH

O

H

OH

HHO

OH

OH

H

H

OH

O

HOH

HO

H

OH

OH

H

HO

H

O

HO

H

H OH

HO

O HH

H

HO

O

H HO

HO

H

HO

OH HH

HO

OH H

OH

HO

HO

OH

H

HH

O

-OA

c

NN

N

NNN

NN

NN

NN

NN

N

NN

N

O

H

HO

H

HO

HO

OH

HH

OH

O

H

OH

HO

H

OH

OH

H

H

OH

O

HOH

HO

H OH

OH

H

H

OH

O

HOH

HO

H OH

OH

H

H

OH

O

H

HO

HH

O

HO

OH

HH

OHO

H

HO

H

HO

HO

OH

HH

OH

N3

O

H

RO

HR

O

RO

OH

HH

OR

-OA

c

Schem

e24

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Synthesis of block co-polymers with phosphazenes requires the control ofend groups such that attachments to other polymers can be formed. Poly-caprolactone, terminated with an amino functionality allows for attachmentto a phosphoranimine, to produce structure (32), Scheme 27.68 Reactionof structure (32) with trifluoroethoxy-trimethylsilylphosphoranimineand trichlorotrimethylsilyl-phosphoranimine yields the block poly(capro-lactone)- poly(2,2,2-trifluoroethoxyphosphazene) copolymer, (33). In an-other report, similar structures were formed by formation of phosphazeneswith hydroxy-terminated pendant groups, which were used in the ROP ofthe caprolactone resulting in the incorporation of phosphazene into thepoly(caprolactone).69

Phosphazenes can exhibit Lower Critical Solution Temperature (LCST)behavior. LSCT behavior is exhibited by a phosphazene that becomes less

PNP

NPN

O O

OO O

O

(31)

NOH

NHO

PNP

NPN

O O

OO O

O

NO

NO

PNP

NPN

O O

OO O

O

NOR2

NR2ORX

R1

O

R1

O

R1 X

O

R1 = CH3, C6H5, 4-CH3OC6H4

R2 = CH3, C6H4CH2

X = a halide

Scheme 25

PNP

NPN

O O

O

O OO

CH3O

CH3OOCH3

OCH3

OCH3CH3O

PNP

NPN

O O

O

O OO

O

O

CH3O

CH3OOCH3

OCH3

OCH3CH3O

OO

O

O

MCPBA

Scheme 26

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soluble as the solution temperature is increased. Copolymers consisting ofphosphazenes substituted with either L-isoleucine ethyl ester or L-valineethyl ester, and a-amino-o-methoxypoly(ethylene glycol) (550 and 750Daltons) have been reported to undergo LCST behavior in which the so-lution thickens, as opposed to other systems that result in precipitation ofthe polymer.70,71 This behavior is reported to be of potential benefit forbiomedical applications.

Chlorinated phosphazenes, both cyclic and linear, can function asgrafting agents onto a polymer substrate. Polyamide 6 can be treated with aplasma and air to yield a surface functionalized with ethers, alcohols, andacids, which will react with both poly[bis-chlorophosphazene] and hexa-chlorocyclotriphosphazene.72,73 Once grafted, the remaining chloro groupson the phosphazene are active to further reaction with an appropriatenucleophile. The grafted surfaces can be simply dipped into an alkoxidecontaining solution to obtain a functionalized and stable surface. In thisreport, the validity of the strategy was shown through the use of 2,2,2-tri-fluoroethanol, heptadecafluorononanol, and 4-hydroxyazobenzene. Thisapproach was also found to be valid for the surface treatment of poly-ethylene-co-polyvinylalcohol.74

Cross-linking in phosphazenes is a critical ability that can be used toadjust the physical characteristics of the polymer to meet a need. There aretwo general methods for forming cross-linked phosphazenes. First, phos-phazenes can be directly irradiated. UV irradiation can be performed onpolymers that have pendant groups that can absorb UV light. Typically,these are polymers containing aromatic groups. Electron beam and 60Cogamma irradiation can cross-link phosphazenes by activation of carbon-hydrogen bonds, followed by formation of carbon-carbon bonds. Second,phosphazenes can be cross-linked using relatively mild thermal conditionsby incorporating pendant groups into the polymer that are specifically in-cluded because of their ability to create a cross-link, such as an allyl group.For example, allylamine can be attached to the phosphazene backbone toyield a cross-linkable group that was shown to give cross-links at 100 1Cinitiated by benzoyl peroxide.75 From integration of the 1H NMR spectrum,a loading of allylamine was estimated to be 50%, with the balance con-sisting of phenol in this particular example.

Phosphazenes containing hydroxybenzaldehyde pendant groups can becross-linked using a Claisen-Schmidt condensation.76 p-Phthaldialdehydereacts with the acetyl moieties to form b-hydroxyketone linkages, that

O

O

NH2

O

+ Br P N Si(CH3)3

OCH2CF3

OCH2CF3

O

O

N

O

P N Si(CH3)3

OCH2CF3

OCH2CF3H(32)

CF3CH2O P N Si(CH3)3

OCH2CF3

OCH2CF3

Cl P N Si(CH3)3

Cl

Cl

+ + (32)O

O

N

O

P N P

OCH2CF3

H

(33)

N

OCH2CF3

OCH2CF3 OCH2CF3n

Scheme 27

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subsequently undergo dehydration to form an a-b unsaturated ketonecross-links between adjacent polymer chains, Scheme 28. Hydroxyl termi-nated pendant groups can undergo chemistries that yield cross-links.77 Anunconventional route was employed to gain the hydroxyl functionality.Initial substitution of the polymer backbone was performed using a aro-matic compound, such as m-cresol, phenol, or 4-phenylphenol. Subsequentattachment of 4-hydroxybenzaldehyde was performed by initially depro-tonating the phenolic hydroxyl with sodium hydride. The authors indicatethat the deprotonation was directed away from the benzylic hydroxyl groupdue to pKa differences (Scheme 29). Further, the attachment occurredregiospecifically without any unwanted side reactions. Cross-linking of theresulting polymers was performed thermally, presumable obtaining cross-links as shown in Scheme 30.

P NO

O

CH3

O

CH3

O

n

H

O

H

OP NO

O

CH3

O

n

H

O

O

P NO

O

CH3

O

CH3

O

n+

OH

P NO

O

CH3

O

n

H

O

O

OHP NO

O

CH3

O

nP NO

O

CH3

O

n

O O

1) KOH

2) -H2O

Scheme 28

P N P

O

O

N

O

O

OH

n

R

R R

P N P

O

O

N

O

O

OH

n

R

R R

H2SO4 - SO3

R = CH3

SO3H SO3H

Scheme 29

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4. Inorganic complexation and materials chemistry

4.1 Inorganic complexes

Cyclotriphosphazene containing alkynes react with a cobalt cyclopenta-dienyl complex.78 The cobalt facilitates both 2þ 2 and 2þ 2þ 2 cyclo-additions, Scheme 31. When indene is added, it takes part in the 2þ 2þ 2variant in creating a substituted tricyclic phosphazene. The 2þ 2 cyclobu-tane product is formed when indene does not take part in the reaction. Thisreaction favors the tricycle over the cyclobutane by an approximate 2 to1 ratio.

Spirocyclic cyclotriphosphazenes can be functionlized with donor groupsto effect coordination with divalent metals such as Co, Cu, and Zn, Scheme32.79 Cobalt binds to the pyridinyl rings on adjacent phosphorus atoms andto a ring nitrogen. Two nitro groups remain attached to the metal center.Copper binds similarly; although coordination to an additional pyridinylring is evident. Zinc binds similarly to cobalt. Linear phosphazenessubstituted with 2-hydroxypyridine, giving a similar bidentate bindingcapability, will bind copper (II) acetate and cobalt (II) acetate.80 Thesemetal-polymer complexes can act as catalysts for the molecular oxygenoxidation of alkenes. High yields of epoxidized products are noted for thereaction with indene and limonene (W67%); although lower yields weremeasured for aliphatic 1-dodecene.

A more intricate copper containing complex is shown in (34).81 Phos-phazene hydrazides bind in a bidentate manner with copper (II) salts givingone half of (34). The unanticipated aspect of the chemistry was the

O

OH

O

+

O

O

O

O

+

Scheme 30

Co

CO2CH3 Ph P3N3F5

+

RefluxingXylene

-COD

P3N3F5

F5N3P3

Ph

Ph

Co

CO2CH3

P3N3F5

PhF5N3P3

PhCo

CO2CH3

+

Scheme 31

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dimerization forming a structure with four metal centers and 15 inorganicrings. The structure was determined from X-ray crystallographic analysis.Copper (II) complexes can be formed from cyclotetraphosphazenesfunctionalized with 2-hydroxypyridine, as shown in phosphazene (35),Scheme 33.82 Attempts to form complexes between phosphazene (35) and 1equivalent of CuCl2 only gives products with a 2:1 stoichiometry betweenCu and phosphazene. Greater yields can be obtained by treatment ofphosphazene (35, R¼CH3) with two equivalents of CuCl2, which results inbidentate coordination with adjacent nitrogens. Abstraction of chloridefrom the complex using Ag(MeCN)4(PF6) results in the formation of a newcomplex with extensive coordination to other pyridinyloxy groups, asproposed from spectroscopic and X-ray crystallographic data. Phospha-zene (35, R¼H) gave polymeric material (36). Studies of these complexesrevealed weak antiferromagnetic coupling suggesting a low level electroniccommunication between metal centers. Since coupling must be transmitted

N

P

N PN

P

NPO O

NN

O

O

N

N

OON

N

O

O

N

N

CuCl

ClCu

ClCl

N

P

N PN

P

NPO O

NN

O

O

N

N

OON

N

O

O

N

N

Cu

Cl

ClCu

ClCl

(36)n

PN

PN

P

N

O O

O

OO

ON

NN

N

PN

PN

P

N

O O

O

OO

ON

NN

N

PN

PN

P

N

O O

O

OO

O

N

NN

N

PN

PN

P

N

O O

O

OO

ON

NN

N

CoO

OOO

N NOO

Cu

OO NO2

NO

Cu(NO3)2

Co(NO3)2ZnCl2

ZnCl Cl

Scheme 32

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N PN

PNP

NP

OO

NN

R

R

O O

N

N

R

R

OO

N

NR

R

O

ON

N

R R

N PN

PNP

NP

OO

NN

O

O

N

N

OO

N

N

O

ON

NC

uC

lC

l

Cu

Cl

Cl

2 C

uCl 2

N PN

PNP

NP

OO

N

NO O

N N

OO

N

N

O

ON

N

Cu

Cl

Cu

Cl

2 [A

g(M

eCN

) 4]+

(35)

2+

PN P

NPN

Ph

Ph

NN C

u

NN

O O

CH

3

CH

3

NN

Cu

NN

O O

CH

3

CH

3

PN P

NPN

Ph

Ph

NN

Cu N

NO O

CH

3 CH

3

NN

Cu

NN

O O

CH

3

CH

3(34)

Schem

e33

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PN

P

NP

N

N N

N NPd Cl

Cl

N

NN

N

Pd

ClCl

N

N N

N

Pd

Cl Cl

(37)

N

PN

P

NP

N

N

N

N

N

N

Cd

OO

OO

N

N

O

ON

N

N

NCd

O

OO O

N

N

O

O

NN

(38)

through the phosphazene ring, this data also suggests a predominanceof the ionic component in the P–N bond; however there is a lack of electrondelocalization in contrast to analogous carbon p–p bonding. In a pair ofbrief X-ray crystallographic structure reports, the coordination of pal-ladium (II)83 and cadmium (II)84 with a pyrazole substituted cyclotripho-sphazene was described, as shown in complexes (37) and (38). Pd (II)corrdinates with geminal pyrazole groups at each phosphorus while cad-mium binds on adjacent groups with only two metal centers per phospha-zene ring.

PN

PN

P

N

N N

N

N

N

N

Ag

Ag

Ag

Ag

Ag

(39)

Cyclotriphosphazenes substituted with allylamino groups form com-plexes (39) with silver (I).85 Coordination can be described as occuringthrough one or more p-bond interactions in which cross-linking by themetal center creates a macromolecular structure. Another exampleof a metal coordinated polymerization is shown through the use of tet-rapyridyloxy 1,10-dioxy-2,20-binaphthol substituted cyclotriphosphazene.86

The significance of this work is not just the polymer formation, but theuse of the binaphthol group that imparts chirality into the structure, (40).

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PN

PN

P

N

O O

OOO

ON

N

N

NP

N

PN

P

N

OO

O

O

O

O N

NN

N

PN

PN

P

N

OO

O

O OO

N

N

N

N

Ag Ag

(40)

Phosphazene coordination complexes can act as facilitators for probes ofthe electronic structure of metal centers. Gadolinium (III) complexes withhexa(diphenylphosphine oxide)cyclotriphosphazene, (41), can be stabilizedand immobilized into a silicate xerogel matrix and studied using ElectronParamagnetic Resonance (EPR) at X (9.4 GHz) and W (94 GHz) bandfrequencies, which revealed the first reported information concerning thephase change that occurs at approximately 5.4 K.87

P

N

P

N

P

N

P

PP

P

P P

OOPh

OPh

O

PhO

PhO

O

OPh

OPh

O

OPh

OPhO

PhO

PhO

O

PhO

PhO

(41)

4.2 Structured materials

Phosphazenes, by their very nature, are three dimensional structures thatcan be manipulated to form larger functional materials. Calculationalmethods have been applied to understand fundamentally simple questionssuch as where do the pendant groups lie with respect to the P–N ring.88,89

Crystallographic analyses, such as shown by Tumer61, have provided someinsight; however this data can be viewed as a ‘‘snapshot’’ of atomic lo-cations in a crystalline matrix. Phenoxy substituents have been calculated tooccupy the space above and below the plane of the ring and do not appearto have any additional interactions with the ring that would tend to bringthe substituent more into the P–N plane. It should be noted that thephosphazene ring is not completely planar and does have a degree of out ofplane pucker, which may be due to the lack of complete electron delocati-zation in the ring. Moreover, there is some degree of chair-boat isomerismin phosphazene rings as shown in both experimental and computational

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studies.90,91 Benzene, on the other hand, as the carbon analog, stronglyresists distortion from planarity due to pp-pp orbital overlap, which thephosphazene ring does not enjoy. Functionalization of phenoxy substitu-ents with oxygen and nitrogen containing groups have been found to alterthe spatial relationships through introduction of new intereactions, specif-ically hydrogen bonding and oxygen-oxygen interactions. Phosphazenes canbe the individual building blocks for structured materials and since thespatial relationships of ring (or linear) systems dictate the three dimensionalstructure of condensed structures, this is a significant parameter that mustbe considered.

PN

PN

P

N

PN

PN

P

N

PN

PN

P

N

PN

PN

P

N

PN

PN

P

N

S

O

O

S

O

OCl Cl

SO

O

Cl

RR R R R R

RR

RCl

S

O

O

SO

O

RR

RR R R

Cl

R = S

O

O

O O

(42)

OOO

O O

OO

O O

O

Condensation processes involving cyclotriphosphazenes can yield nano-tube materials, which result from the cross-linking of phosphazene ringsusing bifunctional pendant groups. Phosphazene (42) is the result of cross-linking hexachlorocyclotriphosphazene with 3 equivalents of 4,40-sulfo-nyldiphenol.92 Synthesis of phosphazene (42) was straightforward in whichthe phosphazene was allowed to react with the 4,40-sulfonyldiphenol in thepresence of triethylamine (TEA) by first treating the mixture ultrasonic bathfor 2 hours followed by an addition of a small amount of TEA and con-tinued sonication for another 10 hours. The reaction product was separatedfrom the reaction solvent by centrifugation followed by washing with THF.Characterization of the product by Scanning Electron Microscopy(SEM), Tunneling Electron Microscopy (TEM), FT-IR, and Energy Dis-persive X-ray Spectroscopy (EDS) revealed, as the authors described, an‘‘octopus’’-like nanotubular structure with main body diameters of ap-proximately 150 nm and tentacle diameters between 50–100 nm.93 Alteringthe synthetic method by adding the phosphazene as a toluene solutiondropwise to a solution of the 4,40-sulfonyldiphenol in acetone with TEA andin the absence of sonication yields nanotubes with closed ends.94 Producttubes were measured to range from 2–6 mm in length with diameters from100–500 nm. Many of the tubes had a more conical geometry than cylin-drical. Using the same stoichiometry, all components were added togetherat once and the resulting mixture was sonicated at 30 1C for 5 hours to yieldmicrospheres, which could be made hollow by subsequent immersion in

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water.95 TEM revealed sphere diameters of approximately 500 nm. Add-itional characterization of the microspheres suggested that potentially activehydroxyl groups could be present.96 Altering the phosphazene/4,40-sulfo-nyldiphenol ratio between 3:1 and 6:1 yields changes in the relative amountsof potential free hydroxyl groups. To verify the presence of hydroxylgroups, microspheres were reacted with benzoyl chloride to yield estermoieties, which were detected using FT-IR. It was found that using agreater ratio of pendant group gave a higher loading of hydroxyl groupssuggesting less cross-linking between phosphazene rings. The nanotubesalso can be formed around a metal core.97 Nanowires consisting of silver arefirst formed followed by condensation of the phosphazene-4,40-sulfonyldi-phenol yielding an encapsulated nanocable.

The conversion of the phosphazene nanotubes into carbon nanotubeshas been demonstrated by thermolysis.98 Specific conditions employed atemperature ramp of 3 1C/min. up to 800 1C in a nitrogen atmosphere. The

PN

PN

PN

O O O

CH2CN{Ru}

O

CH2CN{Ru}

O O

CH2CN{Ru}

0.50.150.85(43)

Ru =Ru

PPh3Ph3P

PN

PN

P

N

NH NH

NHNH

NHNH

Si(OEt)3(OEt)3Si

Si(OEt)3

Si(OEt)3(OEt)3Si

(OEt)3Si

PN

PN

P

N

O O

O

O

O

O

CN{Ti}{Ti}NC

CN{Ti}

CN{Ti}{Ti}NC

{Ti}NC

Ti

(44) (45)

PF6

Ti = Cl

materials were held at 800 1C for two hours. The product tubes weremeasured to have inner diameters of 25 nm with wall thicknesses of 60 nm.Carbon content in the tubes was found to increase from 45.5% for the 4,40-sulfonyldiphenol phosphazene to 93.3% for the product carbon nanotubes.

Pyrolysis products from organometallic phosphazenes can give nano-structured materials.99 Mixtures of AuCl(PPh3) and poly[2,2 0-dioxy-1,10-biphenylphosphazene] were pyrolyzed by heating the sample at 10 1C/min.to 800 1C, which was held for 2 hours under a flow of air. Characterizationof the products by X-ray diffraction revealed a nanostructured Au material.Additional experiments were performed with metal centers directed boundto the phosphazene substrate. Structure (43) shows a phosphazene linearpolymer substituted with 2,20-biphenol and 4-hydroxyphenylacetonitrilependant groups, with ruthenium coordinated to the terminal cyano groups.

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This polymer was mixed with tert-butyldimethylchlorosilane, which servedas a method for incorporation of silicon into the material. Pyrolysis ofthe mixture was performed under similar conditions as the Au exampleaffording a white material consisting of RuO2 and P4O7. No Si was observedin the product, suggesting that the silane evaporated during pyroly-sis without being incorporated into the structure. SEM revealed a three-dimensional porous structure. Cyclotriphosphazene precursors wereinvestigated because it was proposed that higher yields of nanostructured

PNP

NPN

N NH

NHN

NNH

Si(OEt)3NC

Si(OEt)3

CN(OEt)3Si

NC

PNP

NPN

O N

O

N

N

O

NC

CN

NC

(46) (47)

Si(CH3)3

Si(CH3)3

(CH3)3Si

materials could be obtained. Further, it was proposed that the incorpor-ation of the silicon into the phosphazene precursor could yield siliconcontaining materials. Structures (44) and (45) are cyclophosphazenesformed for this study where structure (44) serves to provide the siliconcontaining component and structure (45) is the organometallic portion.Diffraction studies of the pyrolysis products revealed the formation of SiO2,SiP2O7, and Ti(PO3)3.

Combining together the organometallic and silicon containing com-ponents has led to the synthesis and characterization novel cyclotripho-sphazenes, (46) and (47).100 Pyrolysis of the two phosphazenes wasconducted at 800 1C to yield gray solids in yields of 20–30%. Microscopicanalysis revealed that the pyrolysis products of compound (46) gave ir-regularly shaped solids and compound (47) gave porous materials. Pyrolysisof compound (46) at temperatures of 600 1C, 800 1C, and 1000 1C gavediffering morphologies. At the highest temperature, the product resembled adense ceramic consisting of silicon, phosphorus, and trace oxygen.

PN

O O

0.5

PN

O O

0.5-x

Si(CH3)3Si(CH3)3

PN

O O

x

Si(CH3)3

PN

O O

0.5

PN

O O

0.2

Si(CH3)2PhSi(CH3)2Ph

PN

O O

0.3

Si(CH3)2Ph

nn

(49) (50)

Silicon containing phosphazenes also can be formed by lithiation of anaryl bromide followed by exposure to trimethylsilyl chloride (TMSCl),Scheme 34.101 Pyrolysis of this silylated phosphazene in air was performedat 600 1C and 800 1C and the residue yields were between 15% and 20%.

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Diffraction and SEM studies of the residue revealed agglomerated particlesof SiP2O7 in one or more of its possible crystalline phases. At 600 1C, fusedparticles with an approximate diameter of 300 nm were formed. At 800 1C,structures compared to spinal-like columns were observed. Extension of thischemistry to linear polymers can be performed under similar conditions.102

Using a phosphazene substituted to 50% with 2,20-dioxy-1,1 0-biphenol andthe remainder 4-bromophenol, attachment of silicon readily proceeds byfirst lithiation with n-butyllithium, followed by treatment with TMSCl ordimethylphenylsilyl chloride. A tin containing variant also was demon-strated by replacement of the silyl chloride with trimethyltin chloride,Scheme 35. Pyrolysis of the tin containing structure, (48), and the siliconcontaining compounds (49) and (50) was performed in air at temperaturesas high as 800 1C. Yields after pyrolysis were 33% for (48), 15–20% for (49),and only 5% for (50). For the silicon containing compounds (49) and (50),phases of SiO2, P2O5, P2O7.9, and SiP2O7 were noted in the residue. For thetin containing compound (48), SnP2O7 was detected. Additionally, for thesilicon containing compounds, much of the Si was lost during pyrolysis, asopposed to the tin compound where the majority was incorporated into thepyrolyzed structure.

P N

O

O

OO

OO

n(51)

An alternate route to tin containing nanomaterials was shown throughthe development of SnS2-phosphazene intercalates.103 The phosphazeneused in this work was poly[bis(methoxyethoxyethoxy)phosphazene](MEEP), (51). Intercalates were prepared by lithiation of SnS2, givingLiSnS2. This lithium salt was placed into a 3:1 mixture of DMF and waterand sonicated for three days, followed by the addition of the MEEP.The resulting mixture was stirred for three days and then freeze-dried toremove solvent. Final drying was performed in a vacuum oven at 120 1C for24 hours. A relatively wide interchain spacing of 18.01 A, as measured byX-ray diffraction, was attributed to the large size of the MEEP pendantgroup, as compared to the other polymer systems used in this study,which includes poly(ethylene oxide) and two of its derivatives, pluspoly(vinylpyrrolidone).

PNP

NPN

O O

O

OBr

Br

O

O

PNP

NPN

O O

O

OLi

Br

O

OP

NP

NPN

O O

O

OSi(CH3)3

Br

O

O

n-BuLi

0 °CTHF

TMSCl

R.T.THF

Scheme 34

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Other silicon-phosphazene attachments can be made through grafting ofcyclotriphosphazenes onto silica nanoparticles.104 Functionalization of thesilica nanoparticles was required to provide the necessary nucleophilicmoiety for reaction with the phosphazene. Treatment of the nanoparticleswith g-aminopropyltriethoxysilane provided an amine tether to whichhexachlorocyclotriphosphazene was added. Substitution of the remainingchlorines from the tethered phosphazene ring was accomplished by additionof hexmethylene diamine, which provided additional amine groups forattachment of more phosphazene rings. Repeated treatments withhexachlorocyclotriphosphazene and hexamethylene diamine yielded adendrimeric structure tethered to the silica nanoparticle, Scheme 36. Add-ition of sulfonic acid functionality through the reaction of the hexa-chlorocyclotriphosphazenes with sulfanilic acid yielded an electronicallyconductive material. A similar pathway was followed to yield carbon blackgrafted phosphazenes, which provides both ionic and electrical conduct-ivity. Grafting was assessed with respect to theoretical loading. Upon re-peated treatments, the grafting percentage was both measured andcalculated gravimetrically at each step. For example, for the silica particles,the grafting percentage was measured after the addition of phosphazenerings. After the first treatment, the grafting percentage was 4.7%, comparedto a theoretical maximum of 19.3%. This disparity was carried throughout

PN

O O

0.5

PN

O O

0.5

BrBr

n

PN

O O

0.5

PN

O O

0.5

Sn(CH3)3Sn(CH3)3

n

1) n-BuLi2) (CH3)3SnCl

(48)

Scheme 35

NH2

PN

PN

P

N

ClCl

Cl

ClCl

Cl

+

NH

PN

PN

P

N

ClCl

Cl

Cl

Cl

NH2NH

PN

PN

P

N

NH2

NHNH

PN

PN

P

N

NH

PN

PN

P

N

ClCl

Cl

Cl

Cl

P

N P

N

PNCl ClCl

ClCl

H2N(CH2)6NH2

PN

PN

P

N

ClCl

Cl

ClCl

Cl

Scheme 36

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each addition. For example, after two and three courses, the percentageswere measured at 17.3% and 46.4%, respectively. This compares to theo-retical maxima of 96.5% for the second addition and 405.2% for the third.The inability to achieve higher grafting loads was attributed to steric effects.

Methods to graft phosphazenes onto lanthanide phosphate nanoparticlescan employ quaternary ammonium salts as the linkage.105 Quaternaryammonium ion containing cyclotriphosphazenes are synthesized in twosteps. First, N,N-dimethylaniline is attached to the phosphazene ring,Scheme 37. Second, the cationic centers are formed by treatment of thetertiary amines with methyl iodide, compound (52). Interestingly, cyclo-triphosphazene substituted with dimethylamine was found to be non-reactive to methyl iodide. Additional structures can be formed usingaliphatic amines, Scheme 38. 3-Dimethylamino-1-propylamino cyclotripho-sphazene readily reacts with methyl iodide stoichiometrically allowing for thefacile preparation of a predetermined number of quaternary ammoniumcenters per ring. In fact, as shown in Scheme 38, cyclotriphosphazenes witheither three of six cations per ring were characterized. The more highly cat-ionic structures, (52) and (54), were found to more strongly bind to thelanthanide phosphate surface. Also noted was the fact that the aromaticcontaining cyclotriphosphazene, (52), interfered with UV characterization ofthe particles, a disadvantage that was eliminated by the use of either com-pound (53) or compound (54) as the surface modification agent.

4.3 Phosphazene characterization

This final section of this review addresses advances in analytical toolsfor phosphazene characterization. Most examples do not address novel

PNP

NPN

PNP

NPN

ClCl

ClCl

ClCl NHN

NH

N

NH

N

NH NNH

N

NH

N

NH2NP

NP

NPN

NHN

NH

N

NH

N

NH NNH

N

NH

N

CH3I

II

I

II

I

(52)

Scheme 37

PNP N P

NNH

NH NH

NHNH NH

NN

N

NN

N

PNP N P

NNH

NH NH

NHNH NH

NN

N

NN

N

PNP N P

NNH

NHNH

NHNH NH

NN

N

NN

N

I

I

I

I

I

I

II

I

(53) (54)

6 CH3I3 CH3I

Scheme 38

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structures, only new ways to gain structural information. As mentionedearlier in several examples, NMR spectroscopy is a tool that has been widelyapplied to phosphazenes. In fact, phosphazenes are an ideal system tostudy with NMR due to the ease of observation of phosphorus in thebackbone, and carbon, hydrogen, among others typically found in pendantgroups. Most phosphazenes are characterized using liquid state techniques.However, solid state techniques also can be applied. A recent example showsthe benefits of studying poly[bis(2,2,2-trifluoroethoxy)phosphazene](PTFEP) using 1H, 13C, 19F, and 31P solid state NMR spectroscopy.106 Fromthe NMR experiments performed under fast Magic Angle Spinning con-ditions, very high resolution spectra were obtained. The authors have at-tributed this high quality of data to the high flexibility and mobility of thepolymer chains, which can be reflected in this polymer’s low Tg ranging from� 62 1C to � 82 1C, depending on preparation details and thermal history.This may suggest that higher Tg phosphazenes may not yield the same qualityof data. PTFEP is a semicrystalline polymer that NMR experiments showedincreased from approximately 70% to 80% in crystallinity upon thermo-cycling by assignment of signals for both the crystalline and amorphousphases.

P N

N

N

OO

n(55)

NMR also can serve as a probe for dynamic systems. 7Li NMR spec-troscopy can be used to probe potential locations of lithium ion coordin-ation within phosphazene-alumina composites, which are of interest for useas an solid electrolyte.107 Composites were studied using several NMR ex-periments including proton decoupled 7Li CPMAS (cross-polarizationmagic angle spinning), REDOR (rotational echo double resonance), andproton decoupled 7Li CPMAS – 27Al REAPDOR (rotational echo adi-abatic passage double resonance). The phosphazene employed in this workwas poly[bis(2-methoxyethylamino)1.6(propylamino)0.4phosphazene], (55).Composites were formed by dissolution of the polymer in THF followed byaddition of lithium ion, in the form of the triflate. To this was added Al2O3

and the resulting mixture was poured into a Teflon dish and the solvent wasallowed to slowly evaporate to form a membrane, which was then dried at60 1C prior to use. NMR analysis of these composite membranes revealedthat approximately 25% of the lithium ions are coordinated to the phos-phazene in localized mobile enviroments, which in combination withpolymer chain motions, respond to temperature. Cooling causes the motionto be ‘‘frozen’’, while heating increases fluxionality. Approximately 10 to15% of the lithium cations are quite mobile and are proposed to be looselybound to the polymer nitrogens found both in the pendant groups and thebackbone. Nearly a third of the lithium ions are found in an unbound phasethat is immobile at ambient temperature, thus does not contribute to ionic

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conductivity, and is non-responsive to temperature suggesting precipitation;however no crystallinity was found. Thus, it is proposed that these ions existin an amorphous lithium triflate agglomerate. A fourth lithium speciescoordinates to the alumina within the composite.

PN

PN

P

N

OO

O

OO

O

N

O

(56) (57)

Electron Spin Resonance (ESR) can be applied to studies of free radicalprocesses. Cyclotriphosphazene (56), that is substituted with catechol,can be used as an inclusion compound host for 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), (57), using a small amount of 2,2,6,6-tetra-methylpiperidine (TEMP) to stabilize the structure.108 Mesitylene was usedas the solvent for inclusion compound formation. Temperature dependentESR revealed that the TEMPO resides in channels formed by the phos-phazene and the rotational barrier for this species within the channel wasapproximately 4.5 kJ/mol. Also, the ESR data suggests that hydrogenbonding between TEMPO and TEMP does not contribute significantly tothe molecular rotation of TEMPO, and likewise, does not have an effect onthe guest-host relationship between the phosphazene and the TEMPO.A further probe of molecular motion in phosphazenes was performedusing Small-Angle Neutron Scattering (SANS).109 The phosphazene used inthis work was poly[bis(methoxy)phosphazene], a linear polymer with a Tg of� 70.9 1C. Solutions of the polymer in N,N-dimethylformamide with andwithout lithium triflate were analyzed. Inclusion of the triflate raised the Tg

to � 55.8 1C at 20% loading suggesting some interaction between the saltand the polymer. However, SANS experiments determined that there waslittle difference of the coil-like structure in the polymer melt conformationboth with and without salt, suggesting a limited polymer-salt interaction.

Computational methods can play a role in leading to the understandingof phosphazene structure.110–112 An example is cyclotriphosphazene (58)that has been characterized using common chemical methods. DensityFunctional Theory (DFT) was used to further elucidate the structure of themolecule as a precursor to more complex dendrimeric structures. DFTsuggested a concave structure for cyclotriphosphazene (58) with planarpendant group arms and a non-planar phosphazene core. These obser-vations suggest that the terminal groups are spatially available for furtherchemistry to create a more extensive dendrimeric system.

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PN

PN

P

N

OO

O

OO

O

CH

N

N

PS

ClCl

CH3

CHN

NP

S

ClCl

CH3

CHN

NP

SCl

Cl

CH3

CHN

NP

S

ClCl

CH3

CHN

NP

SCl

Cl

CH3

CH

N

N

PS

ClCl

CH3

(58)

In a final note, during 2008, articles addressing some of the simplestorganophosphazenes were published. An improved synthesis of hex-aphenylphosphazene, with direct C-P linkages, from the correspondingGrignard was reported.113 Yields have been increased to 33.4% yield by thesimple treatment of hexachlorocyclotriphosphazene with a 72 fold excess ofthe Grignard reagent phenylmagnesium bromide. Additionally, a paperwas published discussing the substitution mechanism of phenoxide inthe formation of hexaphenoxycyclotriphosphazene. Several conclusionswere presented. First, sodium phenoxide preferentially reacts with hexa-fluorocyclotriphosphazene as compared to hexachlorocyclotriphosphazene.Second, in the substitution of difluorotetrachlorocyclotriphosphazene, thephenoxide will substitute fluorine first. Third, DFT calculations suggest thatan associative mechanism is favored kinetically. Fourth, an increase in re-action rate is seen for both the first and second substitution steps as thesolvent is changed from diethyl ether to more polar THF.114

Acknowledgement

Work supported by the U.S. Department of Energy, Office of NuclearEnergy, under DOE Idaho Operations Office Contract DE-AC07-05ID14517.

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