Pr A103 473 PENNSYLVANIA STATE UNIV UNIVERSITY PARK DEPT OF CHEMISTRY F/6 7/4CONTROLLED SYNTHIESIS OF ORGANIC-INORANIC POLYMERS THAT POSSESS-ETC(U)AUG 81 H R ALLCOCK NOOOi-75-C-O68S

UNCLASSIFIED TR-2I NL

* f l f f l f l l f l l f l f f l ND 1

qECURITY CLASSIFICATION OF THIS PAGE (When Dae Entered)REPORT DOCUMENTATION PAGE F(=S LEzD sr.cn~

1. REPORT NUMGER 1 2. GOVT ACC [,l|ot NO, ,3k"RECIP|ENT*S 0469O1NtIE

21

4. TITLE (and Subtitle) S. TYPE OF REPORT & PERIOD COVERED

CONTROLLED SYNTHESIS OF ORGANIC-INORGANICPOLYMERS THAT POSSESS A BACKBONE OF PHOSPHORUS Interim Technical Report

AND NITROGEN ATOMS s. PERFORMING ORG. REPORT NUMmER

. AUTHORfe) . CONT ACT 04 *ANT UM P E RS )

Harry R. Allcock LE N00014-75-C-0685

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. PO30:A 9M N. PROJECT. TA"K

Department of Chemistry, The Pennsylvania NR 356-577

ro-4 State University, University Park, Pa. 16802

It. CONTROLLING OFFICE NAME ANO ADDRESS 12. REPORT DATEAugust 20, 1981

Department of the Navy S. NMBER OF PAGES

Office of Naval Research, Arlington, Va. 22217 21

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Unclassified

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DitrbtinuniitdD T IC_UG :3 1 1981

AIS. SUPPLEMENTARY NOTES

Submitted to Makromol. Chem., (Proceedings of the IUPAC Conference

on Macromolecules, Florence, Italy, 1980)

IS. KEY WORDS (Continue on revere. side Itf neceeeary and Identlf. by block number)

Macromolecules, synthesis, polymers, polyphosphazenes, model systems

2O ABSTRACT (Continue on reverse side If necessary end Ident)& by block number)

- A new class of macromolecules has been synthesized based on a skeletalstructure of alternating phosphorus and nitrogen atoms. The preferred

C7 synthesis routes makes use of highly reactive polymeric intermediates.Structural diversity is achieved either by changes in the nucleophile or by

chemical modification of the side groups after they are attached to theLAi backbone. A wide ragne of different polymers have been prepared. The

role of small molecule model reactions is stressed.A.

DD A 1473 EDITION OF I NOV 6 IS OUSOLETESIN 01 02-LF .0 14-6601

SECURITY CLASSIFICATION'OF THIS PAGE (When Data Enteree)

I3 206s

Office of Naval Research

Contract No/ N00014-75-C-0685

Task No. 356-577

Technical #ep t(No. 217 ' ~ ~~ ~~ r .. >"l 7-1.5

CONTROLLED SYNTHESIS OF 0RGANIC-INORGANICJOLYMERS THAT

POSSESS A BACKBONE OF PHOSPHORUS AND NITROGEN ATOMS,

by

Harry R./ Allcock

Prepared for publication Makromol Chem.

Department of ChemistryThe Pennsylvania State University A

University Park, Pennsylvania 16802

Reproduction in whole or in part is permitted for any . .purpose of the United States Government -

This document has been approved for public releaseand sale; its distribution is unlimited

/

CONTROLLED SYNTHESIS OF ORGANIC-INORGANIC POLYMERS THATPOSSESS A BACKBONE OF PHOSPHORUS AND NITROGEN ATOMS

Harry R. Allcock

Department of Chemistry, The Pennsylvania StateUniversity, University Park, Pennsylvania 16801, USA

Abstract - A new class of macromolecules has beensynthesized based on a skeletal structure of alternatingphosphorus and nitrogen atoms (I). These are thefirst high polymers with an inorganic backbone to bedeveloped on a broad scale since the discovery of thepoly(organosiloxanes) in the 1940's.

X

N = n (X = Halogen or Organic, n=15,000)

L X_ n

I

The preferred synthesis route makes use of highlyreactive polymeric intermediates (I, X = Cl or F)used as substrates for halogen replacement by organicnucleophiles. Structural diversity is achieved eitherby changes in the nucleophile or by chemical modificationof the side groups after they are attached to thebackbone. Using these principles, a wide range ofdifferent polymers has been prepared ranging fromelastomers to carrier molecules for steroids, transitionmetals, metalloporphyrins, or carboranes. Biodegrad-able and biocompatible macromolecules have also beensynthesized. The critical role of small moleculemodel reactions is stressed, and examples are givenof recently discovered organometallophosphazenes thatare expected to have a profound influence on futurepolyphosphazene syntheses.

Future developments in both the science and technology ofmacromolecules may depend on the availability of polymers thatare drastically different from those that are familiar at thepresent time. Specifically, a growing interest exists in thedesign and synthesis of polymers with extremely flexiblemolecular structures on the one hand, or rigid, extended chainconformations on the other. A need exists for other materialsthat conduct electricity or can function as carrier moleculesfor biologically active or organometallic agents. Moreover,a long-ranqe interest exists in the use of the controlled

1I

sequencing of side groups along a polymer chain for informationstorage or template syntheses.

In our laboratory we are testing the idea that such alterationsin bulk properties and individual macromolecular behavior canbe accomplished by drastic changes in the macromolecularbackbone; in other words, by the incorporation of inorganicelements into the chain structure.

Inherent in this approach is the need to separate theconstruction of the polymer chain from the attachment of specificside groups to the chain. The most characteristic feature ofconventional macromolecular synthesis is that changes in polymerside group structure are accomplished by modifications to thepolymerization process, i.e. by changes in the monomer used, bycopolymerization, or by variations in the initiator or thepolymerization conditions.

The alternative approach to macromolecular diversity is basednot on changes in the polymerization process, but on thereplacement of one side group structure by another after thepolymer chain has been constructed. We will call th-sthesubstitutive approach to polymer synthesis. This approach isnot new, but it is relatively uncommon and is employed mainlyin long-established and well-known processes such as conversionof poly(vinyl acetate) to poly(vinyl alcohol), the chemicalmodification of cellulose, and the chloromethylation of poly-styrene. As will be discussed, the realities of inorganicchemistry require that this approach must usually be employedfor the preparation of new inorganic chain polymers withorganic side groups.

Because of the need for a substitutive approach to thesynthesis of inorganic-type macromolecules, two seriousproblems must be taken into account. First, substitutionreactions carried out with a macromolecular substrate may beretarded by chain coiling, adjacent charge effects, or solvent-polymer interactions. Hence, highly reactive macromolecularsubstrates may be needed before such an approach is feasible.Second, the substitution reactions must be exceedingly "clean"in the sense that skeletal cleavage or molecular coupling sidereactions can have a catastrophic effect on the macromolecularsubstrate. Hence, our approach to this problem has dependedon the extensive use of small-molecule analogues as models forthe design of macromolecular substitution reactions (Ref. 1).

MOLECULAR DESIGN (GENERAL)

We begin with the requirement that the macromolecular systemshould be based on an inherently flexible skeletal structure,since reduced flexibility or high rigidity can be ensured later,if necessary, by the choice of bulky or hydrogen bonded sidegroups. A second requirement for certain properties is theexistence of an unsaturated bonding mode in the backbone,

* preferably comprising a broadly delocalized electronicarrangement. Third, and most critical, is the need for areactive polymeric substrate that will allow the selectiveattachment of a wide range of side groups to the backbone.And fourth, reactions must be available for the facile

2

attachment of side groups that will confer hydrophobicity,hydrophilicity, water- or organic-solubility on the system,or will allow the linkage of "active" side groups such astransition metals, biologically-active units, metalloporphyrins,chromophores, etc. These requirements are met by a macro-molecular system based on a chain of alternating phosphorus andnitrogen atoms. They are called polyphosphazenes.

CHARACTERISTICS OF POLYPHOSPHAZENES (GENERAL)

The basic structure of polyphosphazenes is illustrated bystructure I.

X XX X

1 N N IfN P- X NX N N

L x' n X" P /P X X -P -N =P -XN II

X X(n=15,000)

II III

Although the chemistry of these high polymeric species is offairly recent origin, small-molecule analogues, such as II andIII (X = Cl) have been known since the early 1800's (Ref. 2).They are prepared by the reaction between phosphorus penta-chloride and ammonium chloride in an organic solvent (Ref. 3).As long ago as 1897 Stokes (Ref. 4) recorded that the heatingof II or III brought about its conversion to an insolublerubbery elastomer. Physical characterization of this polymerwas attempted by later investigators (Refs. 5 & 6), but itsinsolubility prevented any definitive chemical exploration.

However, it was clear from the early work that the polyphos-phazene system possessed some of the key requirements listedabove for the development of a new macromolecular system - ahighly flexible backbone system (as indicated by the rubberyelasticity of the insoluble (NPCl2)n, I), a formallyunsaturated skeleton, highly reactive side group bonds (asevidenced by the hydrolytic instability of I) , and the possibleadaptability of I to the nucleophilic replacement of chlorineby hydrolytically stable organic residues.

MODEL COMPOUND STUDIES

Initially, it was found that the cyclic oligomers (I and II)could function as substrates for the nucleophilic replacementof chlorine by amino (IV), alkoxy (V), or aryloxy groupswithout detectable chain cleavage or intermolecular linkage

3

'KNH ~N H \Q CF 3CH 2 0 OCH 2CF 3

N3 N CFC N' -NH 11 NH .3 H2 1 1 OCH 2CF 3

' CF .CHCNH N NH 3 N OCH 2CF 3

IV V

(Refs. 7-10). Large numbers of such cyclic derivatives havenow been prepared (Ref. 11). These reactions have played avital part in the design of the more challenging macromolecularsyntheses (Ref. 1). Nevertheless, an extension of thesereactions to the high polymer could not be made until a methodwas found for the preparation of a soluble form of I.

BACKGROUND TO THE SUBSTITUTIVE APPROACH TO POLYPHOSPHAZENES

Insolubility in a linear type polymer can generally be attributedto high crystallinity, excessive chain branching, or cross-linking. Because poly(dichlorophosphazene) is amorphous atroom temperature in the unstretched state, this factor can beexcluded. Thus, it appeared possible that the original formsof poly(dichlorophosphazene) were excessively branched, cross-linked, or both. It was reasoned that if the chain branchingand/or crosslinking reactions were kinetically distinct fromchain propagation, it might be possible to terminate thepolymerization reaction before insolubilization took place.This idea proved to be correct (Refs. 12-14). In a typicalpolymerization reaction, the insoluble polymer is formed onlyafter =70% of the cyclic trimer has been converted to polymer.The high polymer isolated before this stage is completelysoluble in solvents such as benzene or tetrahydrofuran. Thisobservation provided the key to all the subsequent syntheticwork on polyphosphazenes.

Soluble poly(dichlorophosphazene) is highly reactive tonucleophilic substitution reactions. The initial experimentsshowed that all the chlorine atoms in I can be replaced byOCH3 , OC2H5 , OCH 2CF3 , OC6H5 , NHC6H5 , NHC2H5 , N(CH3), orNC5HI0 (piperidino) groups, without crosslinking and, in mostcases, without chain cleavage (Refs. 12-14). These reactionsare summarized in Scheme 1. Needless to say, this cannot beaccomplished with the crosslinked form of I.

4

Cl C1

p FC Cl-

ClN . P N 250 0C FI 7X CN P N P

"P I Carefully I ProlongedCP, N controlled C1_ heatingC1n

Cl1 N polymerization n tCrosslinked,insoluble

II Soluble

NaOR RNH RNH2 2

-NaCl -HCl -HCl

OR 2 NHR1 NR2

SI I

-- N P-1 N =P N = P

ORn NHRn NR2

VI VII VIII

R = organic. n z 15,000

Scheme 1

The prototype homopolymers, VI, VII, and VIII are stable to

water, soluble in various solvents, and have properties thatdepend on the side group structure. For example, specieswith OR = OCH 3 or OC2H5 are elastomers (Tg, -70'C and -84°C,respectively); the derivative with OR = OCH 2CF 3 is a hydro-phobic, microcrystalline, film- and fiber-forming polymer(Tg, -66°C). When OR = OC6H5 the polymer is a microcrystalline

thermoplastic (Tg, -80C). Anilino side groups generateglass-like properties. The CH3NH group provides solubilityin water (Ref. 15). Most of these polymers resist burning.A substantial number of homopolymers have now been reported(Refs. 16-20), and a number of these have been subjected tostructural analysis. Certain aryloxy derivatives show .

evidence of liquid crystalline behavior (Refs. 21 & 22).

5

CONTROLLED SYNTHESIS BY SUBSTITUTION

Although the homopolymers derived from Scheme 1 are themselvesof continuing interest from both the fundamental and appliedviewpoints, special attention has been paid to t~e preparationof mixed-substituent polyphosphazenes. One of the advantagesof the polyphosphazene system is the ease with which two ormore different substituents can be introduced, often in acontrolled manner. The following examples illustrate thisfeature.

Mixed substituent elastomers

Many phosphazene homopolymers are highly microcrystalline andtherefore behave as flexible thermoplastics at room temperaturerather than elastomers (see note a). Crystallization depends,of course, on stereochemical order. Hence, a disruption ofmolecular symmetry leads to a reduction in or a loss of micro-crystallinity. This principle was first demonstrated by thecompetitive cosubstitution of (NPCl2)n by two differentnucleophiles, for example, by CF3CH2ONa and CF3CF2CH2ONa(Ref. 25). Although the two homopolymers are non-elastomeric,the mixed substituent polymer is a solvent-resistant, lowtemperature elastomer. Different aryloxy groups attached tothe same chain can generate a similar effect (Refs. 26-30),and these elastomers have stimulated a growing technologicalinterest. An alternative method exists to achieve the sameend - the metathetical replacement of one organic substituentby another (Ref. 31). The structure of the mixed substituentelastomers is quite complex, with all three of the possiblerepeating sequences shown in IX probably being present.

OR OR' OR'

N P LN=PV N=P

OR x OR y OR'/

IX

The elastomeric behavior of these derivatives is not fullyunderstood but is of continuing interest.

Note a. Exceptions are (NPF2)n and (NPC12 ) which crystallizeonly when stretched or cooled (Refs. 23 & 24, and [NP(OCH 3)2]n,or rNP(OC 2H5)2 ]n which appear to resist crystallization (Refs.13 & 19), presumably because of the high conformationalmobility of the side groups.

6

Non-geminal and geminal synthesis

Although competitive cosubstitution or metathetical exchangeleads mainly to a randomization of sterochemical structure,sequential substitution can generate structural order. Forexampl-e--2iethylamine reacts with (NPCl2)n to replace only 50%of the chlorine atoms, apparently to yield the non-geminalstructure, X (Ref. 15).

CH N N(C 2 H5 ) F N(C 2 H5 ) 2

1 (C 2H 5 2N RN H 2N -j- N- P-' j -HCl L -HCI

CCl NHnL n n in

X XI

The remaining chlorine atoms can then be replaced by a secondorganic group to yield species, such as XI. However, noevidence exists yet that isotactic or syndiotactic-type orderis present in these molecules. Some evidence has been obtainedthat the reaction of (NPCl2)n with aryloxide proceeds by anon-geminal route. For example, the treatment of (NPCI2)n withone equivalent per repeating unit of phenoxide ion, followedby an excess of p-bromophenoxide ion,yields a mainly non-geminalsubstituent array (Refs. 32 &33). By contrast, evidence hasbeen obtained that the reaction of phenyllithium with (NPF2)n(XII) proceeds initially by a geminal replacement pattern(XIII) (Ref. 34).

C H Li5

-N = P N=P - etc.i I -LiF I I1L F J n C 6 H5 x F V

XII XIII

With diethylaminolysis and aryloxide attack, the non-geminalpattern is probably caused by steric effects. However,electronic effects must be invoked to explain the geminalpathway with phenyllithium.

It should be noted that poly(difluorophosphazene)(XII) is apreferred substrate for organometallic substitution reactionsbecause it is less prone to skeletal cleavage reactions withstrong nucleophiles than is poly(dichlorophosphazene) (Ref. 34).

.7

Attachment of "active" substituent cr~The use of macromolecules as "carrier molecules" for catalytic-ally active or biologically active small molecule species iswell known. Polyphosphazenes possess definite advantaqes ascarrier molecules because of the ease with which activesubstituent groups can be attached to the skeleton and (as willbe illustrated later) because of the possibility that bio-degradability can be ensured by the choice of a suitablecosubstituent unit. Five examples will be given here toillustrate the scope of this approach.

First, a successful attempt has been made recently to attachsteroid molecules to a polyphosphazene chain. This can beaccomplished via the sodium salts of steroids that possess ahydroxyl group at the 3-position (Ref. 35), as shown in theformation of XIV.

CH3

CH3 r OHF3Cl 0-oCl

kN=Pj NaO g] N P N= N P

-NaCl p ICl n C1, C /

x y n

XIV

The steroidoxide ion behaves as a bulky aryloxide analogue,with the expected steric limits to complete chlorine replace-ment. The remaining chlorine atoms can be replaced by watersolubilizing units (NHCH3 ) or biologically compatible anddegradable side groups, such as amino acid ester residues (seelater). Interestingly, the steroid coupling reaction to(NPCl2). does not occur if the A-ring is saturated. In suchcases the (NPCI2)n acts as a dehydrating agent for the steroid.

Second, active groups can be attached to the polymer viareactions that are carried out on the perimeter of side groupsalready present. As mentioned earlier, polyphosphazenes can beprepared by the controlled attachment of phenoxy and 2-bromo-phenoxy groups to the skeleton (XV). A metal-halogen exchangereaction can then be carried out to yield XVI, which functionsas a polymeric organometallic intermediate for the preparationof species XVII-XX (Refs. 32 &33). Species XVII is a carriermolecule for transition metal catalyst systems.

8

- I

0- B 0 j, 7-N Li1 -- 2 n-C 4H9Li ) I

-N =P- " -N =P-

XV -" XVI

(C6H5 ) 2PCI,// /

6 5o r-/ 'P'"CH

(C6H5 ) 3SnC /

XVI I

O _ / CO2/H+

-Sn (C6 H5 ) 3 2

-N -

I -

xvixx

0-.Q AuP (C 6 H5 ) 3

1 3

-N P-

0

XIX 0 -o COOH

-N P-

XI

L

Third, the coordination ability of polyphosphazenes can beexploited in another way. The backbone nitrogen atoms aresufficiently basic that they can function as coordinativedonor sites for transition metals. Perhaps the best exampleof this, to date, is the ability of carrier polymer XXI to bind

HCH3 /1N N N

NHCH 3 Ptn "Cl Cl

XXI XXII

square planar platinum species (XXII) (Ref. 36). In this waythe polymer may alleviate some of the physiological sidereactions encountered when platinum compounds are employed inantitumor therapy.

A fourth approach to the attachment of active side groupstructures is of special interest for the binding of metallo-porphyrins. Units of type XXIII can be incorporated into apolyphosphazene structure by the sequential reaction of(NPCI2)n with NH2-(CH 2 )3-C3N2H3 and methylamine (Ref. 37).

NHCH 3 Such polymers are water-soluble and bindI strongly to metalloporphyrins, such as

-N = P- heme or hemin. In a sense, they functionI as model "polypeptide analogues", andNH allow the protective role of the polymer

in solution to be explored. SomeCH 2 evidence also exists that the poly-I phosphazene has a capacity to reduceCH 2 Fe(III) to Fe(II). Carrier polymer XXI

2 does not bind strongly to heme or hemin,CH2 even though it possesses skeletal donor

sites.N -- 7

N 2 And fifth, although not an "active" unitSN in a conventional sense, stabilizinq

units of the carborane type can beattached to the skeleton by organo-metallic reactions. For example, the

Fe - lithio-derivative of methylcarboranereacts with (NPC1 )n to yield polymers ofstructure XXIV (Rif. 38). These are

XXIII considered to be exploratory precursorsfor the attachment o: catalyticallyactive metallo-carborane units.

10

S I CH2CF3

-N=P N=P-I IOCH2 CF3 OCH 2CF3

XIV

CONTROLLED SYNTHESIS BY POLYMERIZATION

The substitutive approach to polyphosphazene preparation remainsthe most important method for controlled synthesis. However,recently a few new types of polyphosphazenes have been preparedby changes in the polymerization process, followed bysubstitutive modification. This provides a new dimension tothe preparation of phosphazene polymers.

An early problem with the preparation of polyphosphazenes wasthe observation that, although halogenocyclophosphazenes, suchas (NPF2 )3 , (NPCl2 )3 , or (NPBr2 )3 polymerize thermally, cyclicphosphazenes that bear organic side groups do not. Trimers,such as [NP(CH 3)213 , [NP(OCH 2CF3)2J 3 , [NP(OC6H5 )2 13 , or[NP(C6H) 2 13 may undergo ring-ring equilibration, but they donot yield high polymers (Refs. 39-41). Thermodynamic andmechanistic arguments have been put forward to explain this(Ref. 42). However, if the trimeric ring bears both halogenoand organic substituents, polymerization may be possible.

For example, monoalkylpentachlorocyclotriphosphazenes (XXV,where R = CH3 , C2H5 etc.)polymerize thermally to yield macro-

R Cl

SR Cl ClN N I= I INP P

Cl 1 1 C1 - N P N P - N P

CN cCll Cl Cl nN

XXV XXVI

NaOCH 2CF3 -NaCl

R 0CH2CF 3 OCH2CF]

-- N =P - N =P- N P

L OCH2 CF3 OCH 2CF3 OCH2 CF3n

XXVII

II

molecules XXVI, which can be modified by substitutive methods(Ref. 43). The disruption of molecular symmetry in speciesXXVII allows them to behave as elastomers, for similar reasonsto those discussed earlier. A related polymerization hasrecently been carried out for carboranyl-substituted phosphazenetrimers, XXVIII (Ref. 38).

, C l

cl/ N "/ Cl Cl Cl Cl n

XXVIII

Trimers such as N3P3CH3Cl5 , N3P3 (C6H5)2C1 4, or N3P3(OCH2CF 3)6undergo thermal copolymerization with (NPC 2 )3 (Refs.40, 41, 43). These observations suggest that the presence ofP-Cl bonds in the system is a prerequisite for chain propagation,perhaps because of the capacity of P-Cl bonds to ionizethermally (Ref. 44) or the facility with which traces of watercan induce hydrolysis to P-OH units (Refs. 41 &45).

ADVANCED DEVELOPMENTS

As illustrated, the pattern of synthetic advances in thisfield depends heavily on new discoveries at the small moleculelevel. Hence, pointers for future developments at the macro-molecular level may be found in recent cyclophosphazeneresearch.

First, recent work has shown that two classes of aminocyclo-phosphazenes hydrolyze readily to phosphate, ammonia, and thefree side group at biological pH conditions in a manner thatsuggests promise for the preparation of biodegradable polymers.The hydrolytically sensitizing side groups are the imidazolegroup and various amino acid ester units, both linked throughnitrogen to phosphorus (XXIX and XXX) (Refs. 46 & 47).This has wide-ranging implications for the design of carrierpolymers for the slow release of drug molecules.

12

N

N NHCH 2 COOC 2 H5

-N =P- -N P P-I I

N NHCH 2 COOC 2 H 511 - N______ -NXXIX XXX

Second, it is now possible at the cyclic oligomer level tointroduce hydrogen as a substituent group. This has beenaccomplished via organometallic reactions of the type shownfor the conversion of II to XXXII (Ref. 48).

C1 Cl R

p RMgX p

ClN C [Bu 3 PCuI 4 jl N N-N' 17 I I !I --,PP j P Pcl / / cl cl k c

N N

II / XXXI

ROH/

/ R HRIC1 P

RC N' N

", I Is Izp p

C N "Cl

~XXX II

12

R .R' R IP P

z NN

Cl Cl cN N Cl

Cl N '.

XXXIII XXXIV

13

" .. .. .. .. il.... .. .... . .. IIIII ..... ... . ..... I| ... il[ ..... . ... .. 'l MIAI

Because of the high reactivity of XXXI and 4XII, these speciescan be used an intermediates for the introd ction of, forexample, unsaturated alkyl groups, R (XXXI I) (Ref. 49) oriodine as a side group (XXXIV). Unsaturat d side groupsprovide a n-ligand site for the binding of ransition metals(Ref. 50).

Third, it has recently been shown that phosjihazenes with directphosphorus-metal side group units can be prdipared by theinteraction of (NPCl2 )3 with organometallicireagents, as inthe formation of XXXV and XXXVI (Ref. 51).

CO I CO

F F CO -Fe Fe - CO

P Cp P CpF N N FNN

1 11 / NaFe(CO) 2 CP 1

P P F F P P FF'> 7' F FZ N F

N N

XXXV

hv t-CO

CO

CO CO COFe Fe

Cp / P Cp

FF

PF N

N

XXXVI

The implications of structure XXXVI in a polymeric system arequite far-reaching.

SUMMARY OF PRESENT IDEAS AND FUTURE PROSPECTS

The developments seen so far in polyphosphazene chemistry rein-force the view that drastic changes in the backbone structureof macromolecules bring forth a whole range of new and oftenunexpected phenomena. It seems clear that the construction ofnew polymer systems based on other combinations of the inorganicelements is a worthwhile challenge for the future. Meanwhile,the synthetic and structural ramifications of the polyphos-phazene system have hardly been touched by the present work,and it seems clear that this field may develop into a major areaof polymer chemistry. Well over 100 different poly(organo)-

14

phosphazenes) have been synthesized and characterized, and thisrepresents only a small fraction of those that are accessibleeven with known techniques. Certain lessons can be learnedfrom the development of the poly(organophosphazene) field.First, the lonq-standing preoccupation of polymer chemists withpetroleum-based monomers and polymers is the result oftraditional rather than practical restrictions. Inorganicchains can display all the structural and physicochemicalattributes of carbon-based chains and can, at the same time,offer intriguing new phenomena. This was recognized many yearsago by workers in polysiloxane chemistry, but the generality ofthis idea is only slowly gaining acceptance.

Second, the discovery and development of new inorganic-basedmacromolecular systems need not be inhibited by the failureof most potential monomers to polymerize. Provided one ortwo monomers or cyclic oligomers with the required skeletalstructure can be converted to a linear high polymer, other

species can be generated by substitutive (or additive) methods.This is a key concept for the future development of this field.

And finally, because inorganic small-molecule ring systems aremuch more readily available at present than are the analogoushigh polymers, it is essential that the reactivities of thesemodels should be explored in detail. This seemingly indirectapproach is an essential prerequisite for the synthesis ofhigh polymeric analogues.

Acknowledgements - Our work has been supported by grantsfrom the National Institutes of Health, the Army ResearchOffice, the Office of Naval Research, N.A.S.A., and theFirestone Tire and Rubber Company.

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17

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