arylphosphazenes) - American Chemical

Chapter 19

Derivatives of Poly(alkyl/arylphosphazenes) Patty Wisian-Neilson

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Department of Chemistry, Southern Methodist University, Dallas, TX 75275

The synthesis of poly(phosphazenes) from preformed poly(alkyl/arylphosphazenes) such as [Me(Ph)PN] is discussed. This approach affords polymers with a variety of side groups all of which are attached to the backbone by direct phosphorus-carbon bonds. The methods used include deprotonation-substitution reactions at methyl groups, electrophilic aromatic substitution at phenyl groups, and coordination of Lewis acids at the basic backbone nitrogen. The side groups incorporated by these approaches include silyl, alcohol, ferrocene, thiophene, carboxylic acids, carboxylate salts, fluorinated alkyl, ester, and simple transition metal moieties. Several graft copolymers are also discussed. n

The formation of the poly(phosphazene) backbone can be accomplished by either ringopening of selected cyclic phosphazenes, e.g., [Cl2P=N]3, or by condensation polymerization of certain X-P(Z2)=N-Y compounds. As discussed in greater detail in other chapters of this book, there are numerous variations of these two general methods. Generally, ring-opening polymerizations involve cyclic phosphazenes in which at least a few of the phosphorus sites have halogen substituents. The halogens on the ρ oly(pho sphazenes) formed in this manner are then readily replaced with a variety of alkoxy, aryloxy, and/or amino groups using nucleophilic substitution reactions. (7-3) Condensation polymerization, on the other hand, is a newer, less well-studied approach that has yielded fully P-C-bonded alkyl/aryl substituted poly(phosphazenes) (4, 5, 6) as well as several simple dialkoxy, diaryloxy, (7) dichloro (8), and diaryl (9) substituted polymers. A major factor in the diversity of the phosphazene polymer system, and indeed the reason polyphosphazenes) are unique among polymers, is the broad range of chemistry that can be carried out on the poly(phosphazenes) once the backbone has been formed. As mentioned above, the best examples of this are the simple, yet very

0097-6156/94/0572-0246$08.00/0 © 1994 American Chemical Society In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

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Derivatives of Poly (alkyllarylphosphazenes)

247

well-developed, nucleophilic substitution reactions of P-halogen substituted phosphazenes. Such reactions have afforded hundreds of poly(phosphazenes), which, for the most part, have P-N and P-0 bonded side-groups. Subsequent reactions on these polymers serve to further extend the diversity of substituents as well as the range of properties of these polymers. (1, 2) More recently, the derivative chemistry of poty(alkyl/arylphosphazenes) such as [Me(Ph)PN] , 1, has also been under investigation as a means of expanding the range of properties of pory(phosphazenes) with P-C bonded substituents. The synthesis, characterization, and properties of polymers prepared by this approach are discussed below. Poly(methylphenylphosphazene), 1, which is prepared by the thermal condensation polymerization of N-silylphosphoranimines, (MeXPh)P(OR)=NSiMe3 [R = OCH2CF3 (4 5) or OPh (6, 10)], offers three sites for derivatization. These are (a) the methyl group which is suitable for deprotonation-substitution reactions, (b) the phenyl group which is susceptible to electrophilic aromatic substitution, and (c) the lone pair of electrons on the backbone nitrogen atom which is an excellent site for coordination of Lewis acids.


n

y

Me

η deprotonation-substitution

1 ^ As discussed below, the reactivity of one site often interferes with desirable chemical reactions to be conducted at another site. It should also be noted that model reactions can sometimes be performed on the N-siïylphosphorariirnines precursors, (6) but, because these small molecules also contain reactive N-Si and P-O bonds, they are not always suitable prototypes. Furthermore, while derivatization of these precursors can be a suitable means of preparing new polymers with substituent diversity, some groups interfere with or inhibit the thermal polymerization. Thus, our work as described here focuses on the chemistry of the preformed polymer systems. Deprotonation-Substitution Reactions The most straightforward way to derivatize poly(methylphenylphosphazene) is through initial formation of the anionic intermediate 2 (eq 1). This reaction has generally been carried out by treatment of a 0.7 to 1.0 M THF solution of 1 with nBuLi at -78 °C, followed by stirring at low temperature for 1 to 2 h. In most cases, the -78 °C solutions of 2 have been treated with electrophiles and then allowed to warm to room temperature. This affords maximum x:y ratios of 1:1 or substitution of 50% of the methyl groups. Using less than 0.5 equivalents of ft-BuLi results in correspondingly lower degrees of substitution. (11-13)

In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

248

INORGANIC AND ORGANOMETALLIC POLYMERS II

Ph

Ph

Me

Me

Ph

6Η ~Ι-Ι


+

2

1

2

Silyl Substituted Polymers from Chlorosilanes. One of the simplest deprotonationsubstitution reactions studied in our labs was the reaction of 2 with Me3SiCl (eq 2). Typically substitution of 10 and 50 % of the methyl groups, which was confirmed by NMR spectroscopy and by elemental analysis, was easily obtained by controlling stoichiometry. More importantly, gel permeation chromatography (GPC) and intrinsic viscosity measurements clearly indicated that no chain degradation occurred in the deprotonation-substitution process. (77) NMR spectroscopy also showed the presence of small quantities of disubstituted methyl groups, CH(SiMe3)2- Sequential treatment with w-BuLi and Me3SiCl or using more dilute THF solutions of 1 increased the degree of disubstitution. Differential scanning calorimetry (DSC) measurements of the glass transition temperature (7^) showed only one transition (ca. 73 °C, when χ = y) indicating that the substitution was random. Ph Ph Ph Ph r I -i r -. +Me SiCl _| _ _ I -Ep=N^E-p=N3y £ * _ 9 Ρ = ^ Ρ _ Me «WLÎ*" CH^SiMe3 2 3 3

Τ

ΐ

Ν

Γ

M

=

Ν

3

(

2)

e

Reactive Functional Groups. Other simple chlorosilanes provided access to poly(phosphazenes), 4, [R = H, CH=CH2, and (CH2)3CN] with potentially reactive SiH, vinyl, and cyanopropyl groups (eq 3). (77) Currently, we are using hydrosilylation reactions of the vinyl substituted polymers for the attachment of Si(Me2)[CH2CH20] C6H4C6H4R groups. Several attempts to use the Si-H substituents in hydrosilylation reactions have had limited success. A number of simple ether groups have also been attached through the Me2Si spacer group using ClSiMe R where R = Si(Me) C H OC4H or Si(Me)2C3H6(OC2H4)20CH . (14) n

2

2

3

6

9

3

Ph Ph Ph Ph I I _ +RMe SiCl A _ _ -Ef=*%tf*% - E p N ^ t - ^ 2

Me

^

CU^L^

M

e

(3)

CH2SMe R 2

Alkyl and Fluoroalkyl Groups.. A series of silylated poly(phosphazenes) (eq 3) with long chain alkyl [R = ( C ^ ^ C ^ , m = 2, 3, 7, 9, 17] and fluoroalkyl [R = CH2CH2(CF2) CF3, m = 0, 5, 7] groups were prepared recently in our laboratory. (75) These polymers, where the degree of silyl substitution was between 40 and 70 %, were characterized by NMR spectroscopy, GPC, and elemental analysis. The higher degrees of substitution were achieved by using elevated reaction temperatures and higher proportions of «-BuLi to form the anion as discussed later in this paper. The Tg values in both series decreased with increasing chain lengths to a minimum of m


19.

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249


-20 °C for the polymers with decyl and fluorodecyl substituents. With longer side chains, the Tg values were higher as has been observed with other systems. (16, 17) Contact angle measurements showed that the polymers became rriore hydrophobic as the length of the alkyl side group increased (ca. 90 to 96 °) . The polymers with fluoro alkyl groups were slightly less wettable (96 to 100 °). Both of these are significantly less wettable than the parent polymer 1 where the contact angle is 73 °. Alcohol Substituted Polymers from Aldehydes and Ketones. As demonstrated by the chemistry in the previous section, the polymer anion 2 can be viewed as a polymeric version of an organolithium reagent. This is further illustrated by the successful reactions of 2 with aldehydes and ketones (eq 4). (12, 13) The alcohol derivatives 5 (R, R' = H, Me, Ph, thiophene, ferrocene) were prepared by initial reaction of 2 with the electrophiles at either 0 or -78 °C followed by quenching of the alkoxy anion with aqueous NH4CI. Typically the degree of substitution in these reactions was between 30 and 45% with best results with the aldehydes. In addition to attaching potentially electroactive groups such as ferrocene and thiophene, this approach has also been used to attach organofluoro [R= R' = CF3; R = H, R - C6F5] groups that may serve to modify the polymer surface. (18) The Tg values of the alcohol derivatives rangedfrom49 to 102 °C which is significantly higher than the 37 °C of the parent polymer 1. This can be attributed to both increased steric size of the substituents and to hydrogen bonding of the hydroxy groups. The Tg values increased with higher degrees of substitution and larger R and R groups. (75) The fluorinated alcohols have Tg values near those of the non-fluorinated analogs and exhibit slightly lower onsets of decomposition in TGA analysis. (18) 1

Ph •I Me

χ τ Ί

r

II (l)R-C^R'

Ph I

Ph ι

/ ι χ

χ τ Ί

Λ Hχ τ2-Lτ ίÎ + C

W

n

r

Me

4

|

Ph

R - C1 - O H

5

R'

Ferrocene Substituents. Some of the most interesting poly(phosphazenes) that have been prepared from the reactions of the polymer anion 2 with carbonyl compounds are those with ferrocene substituents. Ph Me

Ph ÇH R—C-OH 2

R = H,Me



250


First, thermal gravimetric analysis showed up to 50 % retention of weight even on heating to 900 °C. (73) Second, the revalues of a series of these polymers [x = 0.94, y = 0.06, R = Η; χ = 0.80, y = 0.20, R = Η; χ = 0.64, y = 0.36, R = Me; and x = 0.56, y = 0.44, R = H) increased systematically from 40 to 89 °C. Third, the electrochemistry of this series was examined both in solution and asfilmson electrode surfaces. Reversible electrochemistry was observed for each polymer and the chargetransfer efficiency increased as the degree of ferrocene substitution increased. (79) Ester Derivatives. In addition to simply altering the properties of the poly(phosphazenes), attaching reactive functional groups extends the types of derivative chemistry that may be used to modify the polymers. An example of this is the preparation of a series of ester derivatives 6 from the alcohol obtained from acetone (eq 5). The Tg values of the esters were lower than that of the alcohol precursor (55 °C) presumably because hydrogen bonding was eliminated. These values also decreased systematically as the length of the alkyl ester group increased until a minimum value of 10 °C was reached for m = 8. Then, like poly(dialkoxyphosphazenes) (77), as the chain length became longer (m = 10 to 14), the Tg values became higher reaching 27 °C for m = 14. (16)

P

h

Me Me—Ç-Me

Ν _

E t 3 N H

Ph + f

Me Me—Ç-Me

c

R = (CH ) CH3 m = 2, 4, 6, 8,10, 12,14 2

Ph

U

m

6

-C* R

Carboxyl Substituted Polymers from Carbon Dioxide. Reactions of 2 with carbon dioxide afforded a series of carboxylate salt, 7, carboxylic acid, 8, and ester, 9, substituted polymers [where χ = y; χ =3, y = 1; and χ = 9, y = 1] (Scheme 1). (20) The 50% substituted salt (where χ = ν, R = L i ) is thefirstwater soluble derivative of a poly(alkyl/arylphosphazene). The salts are readily converted to the carboxylic acids upon protonation or mto esters when treated with an activated species such as 4NC^CGfi^CI^Br. Fluorescence studies of these simple acids indicate that they form a moderately hydrophobic environment in aqueous media. (27) The salts and acids are easily cross-linked by addition of divalent metal cations which suggests applications as hydrogels. (20) It should be noted that spectral data suggest that a better representation of the acid 8 is the zwitteriomc form 8b shown in Scheme 1. This is substantiated by studies of coordination of the backbone nitrogen as discussed later. +


19. WISIAN-NEILSON

Derivatives ofPoly (alkylIarylphosphazenes)

251

Scheme 1 Ph

Ph

Ph

œ

^Vf*% M

e

CH9~U ^

2

Ph

^ - E p ^ H V +

M

e

+

CH2COO~Li

7


ET*"

Ph

Ph H+

Ph

Ph

Ph

Me

CH COO~

Me

CH2ÇOOH

Me

2

8b

8a

CH

2

C=0 P-NO2-C5H4CH2—Ο

9

Graft Copolymers from Anionic Polymerization. The similar reactivity of the polymer anion 2 and organolithium reagents suggested that the anion sites could be used to initiate anionic polymerization reactions. In this manner, both organic and inorganic graft copolymers of poly(phosphazenes) have been prepared. Polystyrene Graft Copolymers. The anionic polymerization of styrene was initiated with 2 giving poly(phosphazene)-grq/?-polystyrene copolymers 10 (eq 6). Both the number of grafts (y = 1, χ = 4 - 9) and the length (z = 20 - 150) of the graft were varied such that copolymers with 65 to 90 % styrene were prepared. Grafting was demonstrated by marked changes in GPC molecular weight data in terms of higher molecular weights and larger polydispersity values, and by increased absolute molecular weight as determined by membrane osmometry. Two distinct glass transition temperatures were observed for each graft at ca. 36 to 43 °C and 96 to 108 °C. These correspond to the 7Vs of each of the homopolymer components and indicate that two well-separated pnases form in the graft systems. (22) This is also evident in SEMs of these copolymers. (23) J* M

p

e 2

f AHftf

(l)CH =C(H)Ph J

h

h

?

2

(

2

)

H

M

^

Ç 2Η

e

H

10

[CH -C}-H I z Ph 2

Poly(methyl Methacrylate) Graft Copolymers. A second type of inorganic-organic graft system was prepared using anionic polymerization of methyl methacrylate (MMA). The reaction of 2 with M M A resulted in insoluble materials due to reactions at both vinylic and carbonyl carbons. Thus the reactivity of the anion sites was modified by intial reaction with diphenylethylene (eq 7). The more sterically hindered anion reacted cleanly to give the poly(metiiyl methacrylate) (PMMA) graft


252


copolymers. These were characterized as above. Unlike the polystyrene grafts, the PMMA graft copolymers exhibited some phase miscibility. Although two distinct glass transition temperatures were observed, the Tg of the PMMA segment (ca. 100 °C) was somewhat lower than that of PMMA homopolymers and the second T g at 60 to 65 °C was significantly higher than that of the phosphazene segment. Similar merging of the TgS of a blend of PMMA and [Me(Ph)PN] were also observed. (24) The increased homogeneity of the components was also demonstrated by SEM. (27) n


—rvu

P

h

( 1 )

Ph

CH =(Me

Ph

2

(2)H

Me ÇH CPh -Li

Ph

Me

+

ÇH

i

;

2

H2

P=N}y .„ » ^ P = N ^ P = N } I y (2)Me^SiCl I x I y 2

w

x

w

3

Me

M

e

C H ^

2

8

12

m

c

H

Ç 2 [Me Si—O^SMe 2

3

Polymers with Sulfur-Containing Substituents. Sulfide groups have also been attached to the polyphosphazenes) using either elemental sulfur (eq 9) or disulfides (eq 10). The reaction with sulfur proceeded through an intermediate, [Me(Ph)PlSG [(CH2S-Li )(Ph)PN]y which was characterized by *H and P NMR spectroscopy. In addition to simple reactions with electrophiles such as Mel (eq 9), this intermediate formed S-H terminated groups and CH2S-SCH2 crosslinks upon addition of acids. Addition of strong base removed crosslinking, but the reaction +

3 1

x


19. WISIAN-NEILSON

Derivatives of Poly (alkylIarylphosphazenes)

253

could successfully be repeated over only a few cycles. The sulfide groups are also of interest as potential sites for coordination of transition metals. (27) (i)

Ph M

Ph

e

CH "Li+ \

2

8

Ph M

RSSR

2

2


s

/ (2) RI *

N

^

9

Ph

e

()

H

9 2 SR

13

(10)

Other Polymer Derivatives with Reactive Functional Groups. The polymer anion 2 reacts with a number of other electrophiles such as Ph PCl, CH2=CHCH Br, and B r all of which contain reactive element-halogen bonds (eq 11). While both phosphine and allyl groups provide sites for coordination of transition metals, the unsaturated allyl group may also be a useful site for hydrosilylation or crosslinking reactions. The bromomethyl group has, however, proven to be quite unreactive. (24) 2

2

2

Ph

Ph

I

I

Ph i?Y

-&P=N^P=N3y CH ~Li+ M

I

^P=N3^P=N3CH R

e

2

Ph

I M

2

(11)

e

1 4

2

Variations in the Deprotonation-Substitution Reactions. As discussed in the previous paper in this volume, the preparation of copolymers with combinations of alkyl and aryl groups attached to the backbone by P-C bonds is readily achieved by the condensation polymerization of appropriate mixtures ofN-siïylphosphoranirnines. The highly methylated copolymers 15 are of particuluar interest in terms of deprotonationsubstitution reactions for several reasons. First, with even a low proportion of phenyl groups, these copolymers remain soluble in THF, a solvent suitable for deprotonationsubstitution reactions. Second, the TgS of the copolymers are significantly lower than in the homopolymer, [Me(Ph)PN] . Third, the copolymers show a strong affinity for water, and like the dimethyl homopolymer, [Me PN] , are often solubilized by only slightly acidified solutions. These differences in properties relative to poly(methylphenylphosphazene) are reflected in the derivatives of the copolymers. Furthermore, additional control of properties is enhanced by the ease of variation of the proportion of methyl to phenyl groups. Using copolymers with nearly equal portions of [Me(Ph)P=N] and [Me PN] units, 15 (m = n\ we have found that the conditions needed to deprotonate the methyl groups differ substantially from those needed for the homopolymer, [Me(Ph)PN] Not surprisingly, since the copolymers are less soluble in THF, generation of the anion at -78 C requires at least 3 hours to get the highest degrees of substitution. When this intermediate anion is treated with Me3SiCl (eq 12), the resulting silylated polymers, 16, have Me3SiCH groups on up to 60% of the backbone phosphorus atoms. Unlike the derivatives of [Me(Ph)PN] , the silylated copolymers are soluble in hexane and acetone as well as in THF and chlorinated hydrocarbons, and they are partially soluble in water. The glass transition temperatures of the silylated derivatives n

2

n

2

n

2

n


254


are also higher than the parent copolymer (e.g., 37 °C for the 40% substitution product versus 0 °C for 15). (28) e

e

e

p T (D*-BuLi f T ρ ¥ -fP=N^r>P=N]> ^P=N^P=N^P=N^-fP=N^(12) I m Me SiCl I I I I Me ^ Me ÇH ÇH Me SiMe3 SMe3 16 y

( 2 )

Z

33

2

e

2


1 5

The carboxylic acid and salt derivatives, 17, of the copolymers also exhibit altered solubility (eq 13). Both the 50% substituted acid and the lithium salt of the 25% substituted acid are water soluble whereas the only the 50 % salt derivative (Le., 7) of [Me(Ph)PN] was soluble in water (Scheme 1). (28) n

Ρ

Me

( 1 ) n

.

B u L i

Ph

Me

Ph

Me ( i 3 )

Me

4,

( 2 ) C 0 2

Me

15

tn

CK

2

Me

2

COfLi^ C0 Li+ 17 2

A series of ferrocenyl derivatives, 18, of the copolymers were also prepared and fully characterized (eq 14). Glass transition temperatures of these polymers range from 62 to 73 °C for polymers with 25 to 40 % substitution, which reflects the high degree of hydrogen bonding between hydroxy groups. Because these derivatives have significantly lower 7gS than observed for the homopolymer derivatives discussed above, their electrochemistry is of interest. Ph Me H Me(Ph) Me(Ph) r-1 _ _I (1) H—C—R I —ι ρ I -, -&Ρ=Ν9^Ε-Ρ=Ν^ - t p N ^ ^ N ^ r

L

M

°

e

CH

(14)

2

ûrr* Fe 18

In most of the deprotonation-substitution reactions discussed thus far, the degree of substitution has usually not exceeded 50 %. Although this could be due to either electronic factors associated with the formation of charged sites along die chain or to simple steric effects, a recent *P NMR spectroscopic study of the anion indicates that steric size of the electrophile is the limiting factor in substitution. When one-half equivalent of H-BuLi was added to [Me(Ph)PN] and the mixture was stirred at room 3

n


19. WISIAN-NEILSON

Derivatives of Poly (alkylI arylphosphazenes)

255

temperature, two signals at ca. δ 4 and δ 33 were observed in the 3 *P NMR spectrum. These correspond to simple P-Me and to PCH2" groups, respectively. However, when one equivalent of w-BuIi was added, only the signal at δ 33 was observed. When this solution was treated with the relatively small electrophile Mel, essentially all of the methyl groups were converted to ethyl groups giving [Et(Ph)PN] (eq 15). (27) n

Ph

Ph

-£t= j-

excess

N

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M

e

η

*-

BULL

Ph

» -EP=NJ-

25 °C

-T>P=N3-

I CH "Li

1

15

(>

11

I CH CH 19

2

2

3

Electrophilic Aromatic Substitution Reactions The phenyl ring in [Me(Ph)PN] appears to be well-suited to electrophilic aromatic substitution reactions. However, numerous attempts to acylate the ring were unsuccessful, presumably due to the coordination of catalysts such as AICI3 with the backbone nitrogen atom. Nonetheless, the phenyl ring can be nitrated at the meta position using a mixture of nitric and sulfuric acids (Scheme 2). (29) Between 15-55 % of the phenyl rings were derivatized by varying the reaction times from 30-60 min. Characterization of the nitrated polymers 20 was complicated by coordination of the backbone nitrogen atoms to protons. Even upon workup with Et3N, inconsistent elemental analyses were common. This problem, which can be attributed to the presence of counter anions such as SO4?", was later circumvented by workups involving NaOH. The nitrated polymers 20 were subsequently reduced to the amino substituted polymers 21 with Lalancette's reagent, NaBH2S3- Conversion to the amides 22 was accomplished with acid chlorides, which allowed for better characterization by *H NMR spectroscopy. The TgS of the new polymers were as high as 80 °C (parent polymer 1, 37 °C) as expected for the incorporation of more polar substituents. Scheme 2 n

_

,

_

HNO3/H2SO4

ΐ«ϊ3-

^

Me 1

Γ L

, -, I n Me 22 J

RC(=0)C1 Et N " " 3

E t 3 N H + C 1

Γ L

, -, I ri Me 1 J

2


256


Coordination of Backbone Nitrogen Atoms Throughout the studies of deprotonation-substitution and electrophilic aromatic substitution reactions, a recurring problem with coordination of the backbone nitrogen atoms to various reagents has been encountered. We have also made several attempts to attach transition metal substituents to the poly(alkyl/arylphosphazenes), but these were also complicated by coodination. Thus we recently conducted a more thorough investigation of this phenomenon. (30) A portion of this study involved the preparation of lithium and silver complexes of [Me(Ph)PN] , 1, and [Me2PN] , 23 (eq 16). Each of these complexes (24 - 28) was soluble in CH2CI2 and had a single resonance in the room temperature * lp NMR spectra. Since this was true even when small amounts of metal were incorporated, it appeared that the single resonance involved a fluxional process. Indeed, this signal was resolved into two resonances in the - 90 °C spectrum of 28. Hence, it appears that the metal ions are mobile at room temperature. When 1 was treated with RCI2, an insoluble material and trace amounts of soluble polymer were obtained. Presumably the multiple coordination sites in platinum allow for crosslinking of the polymer. Polymers 1 and 23 were also protonated with anhydrous HC1 and were recovered unchanged by washing with K2CO3. As expected for the incorporation of the ionic groups, the TgS of these complexes were higher than that of 1 (37 °C) and 23 (-42 °C) (4) and rangedfrom-20 °C for 28 to 121 °C for 25. (30)


n

n

R L

j Me

-"n

l , R = Ph 23,R = Me

CH2CI2

-ΕΡ=Ν9-ΪΡ=Ν9Τ

yBF I x j -y Me Me 24, R = Ph, χ = 0.83,7 = 0.17, M = Ag 25, R = Ph, χ = 0.70, y = 0.30, M = Ag 26, R = Me,x = 0.85, v = 0.15,M = Ag 27, R = Ph, χ = 0.84, y - 0.16, M = L i 28, R = Me, χ = 0.80, y = 0.20, M = L i L

J

L

7

4

(16)

1

Conclusion The reactivity of poly(alkyl/arylphosphazenes) is proving to be surprisingly diverse and provides access to a large number of new poly(phosphazenes). Substitution at the simple methyl and aryl groups allows for the attachment of many functional groups most of which can be used for further derivatization chemistry. Though less well developed than the substitution chemistry of poly(phosphazenes) prepared by ring opening polymerization, the chemistry of these condensation polymers is significantly broad and yields polymers with a vast range of properties. Moreover, the coordinating ability of die backbone nitrogen is enhanced by the electron releasing effects of the directly bonded alkyl and aryl groups and thus offers a relatively new dimension to poly(phosphazene) chemistry.


19.

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Derivatives of Poly (alkylIarylphosphazenes)

257

Acknowledgments. This work has been supported primarily by the the U. S. Army Research Office and the Robert A. Welch Foundation. Acknowledgement is also made to the donors of the Petroleum Research Fund administered by the American Chemical Society, The Texas Advanced Technology Program administered by the Texas Higher Education Coordinating Board, and Southern Methodist University (e.g., the SMU Research Council) for additional support of this research. Literature Cited


1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.

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RECEIVED June 24, 1994 In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

arylphosphazenes) - American Chemical

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