Polyphosphazenes: Synthesis, structures, properties, applications

Eur. Polym. J. Vol. 27, No. 4/5, pp. 341-348, 1991 Printed in Great Britain.All rights reserved

0014-3057/91 $3.00+ 0.00 Copyright © 1991 PergamonPress pie

REVIEW POLYPHOSPHAZENES: SYNTHESIS, STRUCTURES, PROPERTIES, APPLICATIONS PH. POTIN ATOCHEM S.A. (Elf-Aquitaine), Groupement de Recherches de Lacq, BP 34 64170 Artix, France R. DE JAEGER LASIR Universit6 des Sciences et Techniques de Lille-Flandres-Artois,59655 Villeneuve D'Ascq, France (Received 20 June 1990)

Abstract--The first polyphosphazene synthesis dates from the work of Allcock published in 1964. Since then, new synthetic routes have been developed, each of them characterized by specific advantages, viz. simplification, better control over molecular weights and cross-linking processes, possibilityof substitution with alkyl or aryl groups. The outstanding diversity of structures accessible through the various synthetic routes is further enhanced by the possibility of chemical modifications. This article reviews the wide range of structures and potential applications of these polymers in numerous fields.

INTRODUCTION Polyphosphazenes are exceptional in macromolecular chemistry because of their extremely versatile adaptability for applications. Generally, the structure of a polymer is determined by the monomer and its resulting properties tend to be random rather than specifically engineered. Polyphosphazenes do not comply with this rule, and from the same precursor an almost unlimited number of specific structures can be selected by the worker.

SYNTHESES

Polyphosphazenes are prepared in three stages (Figs 1 and 2) viz. the synthesis of the precursor, the polymerization of this precursor leading to the formation of polydichlorophosphazene, the substitution of the chlorine atoms in this entirely inorganic polymer by organic groups, giving a polyorganophosphazene. Polydichlorophosphazene is the starting block for the whole chemical family (Fig. 2). Of course this molecule cannot be used directly. The chloride groups on the phosphorus atom are extremely reactive, and cause crosslinking on contact with atmospheric moisture. However this extreme reactivity allows substitutions with nearly all nucleophiles, with very high conversion rates. Polymerization of hexachlorocyclotriphosphazene to give polydichlorophosphazene was discovered as far back as 1897 by Stokes [1]. However, the polymer was always in the form of an insoluble, crosslinked gum, and consequently unsuitable for any applications. In 1964 Alicock filed a patent [2] claiming the preparation of a soluble polydichlorophosphazene, and the substitution for producing polyorganophosphazenes which were stable and suitable for applications.

Considerable fundamental and applied research has been conducted on the basis of this procedure. These very numerous studies resulted a few years ago in an industrial production of technical polymers. The polymerization mechanism for hexachlorocyclotriphosphazene in bulk and in solution, its catalysis and its competitive cross-linking reactions have been summarized by Hagnauer [3]. Since then, other catalytic systems have been proposed [4, 5], allowing in particular the fundamental problem of the crosslinking reactions to be minimized. More recently Hornbaker and Li have suggested a procedure [6] which allows polydichlorophosphazene to be obtained directly from basic chemicals: PCI3, CI2 and NH4CI without requiting any particular precursor to be isolated. Our research group filed a patent in 1979 [7] and published in 1982 [8] a new route to polydichlorophosphazene based upon the polycondensation of N-dichlorophosphoryl-P-trichloromonophosphazene. This monomer can be obtained with high purity and yield. The polycondensation, which leads to the elimination of phosphoryl chloride, can be conducted to total conversion with a negligible gel ratio [9]. One of the important advantages of this process is that the molecular weight of the polymer can be controlled either with a chain limiter [10] or by altering the reaction time. As in classic polycondensation, the degree of polymerization depends on the reaction time. Controlling the viscosity of the polycondensate allows the reaction to be stopped at the desired level. From the work conducted on this process, it is expected that it will be applied on an industrial scale in the near future [11-16]. Polydichlorophosphazene can be obtained from N-dichlorothiophosphoryl P-trichloromonophosphazene [17] by an analogous route. Nearly all the nucleophillic reagents which can replace the chlorine atoms of the phosphorus 341

342

Review

[,~d..~ &KUG~ FKx.rrE}

[HORNBAKER & LIROUTEJ

PCI5 + NH4CI

PCI5 + NH4CI

'

/

Cl2p1 N%pcI2 II I N,~p//N Cl2

o~ 5 Special rubber ~ ~ ~lnaulatlon Membrane= (Fluorlneted"~ Cable covering Cushonlng Floor covering Paints Adhesives CI Membranes

/

I

"*-N=~-~n

CI O

~

(AmlAn:l:~a ~')(~MIcelleneou~ \

POCI3

II CI2P- N - PCI3

X

HNR

I~

I

PCl5 + (NH4)2SO 4

Biomedical

Conducting polymers Ctlalytlt Blomedlct Liquid crlttalt

Fig. 1. Access routes to polydichlorophosphazene.

Fig. 2.

compounds can be substituted onto the polyphosphazene chloride groups, except for organometallic compounds such as organo-magnesium or organolithium which, simultaneously with the chloride substitution, cause the degree of polymerization to collapse because of the breaking of the N - P bonds [18]. This exception is regretable since this route would have led to a simple synthesis of polyalkyl or polyaryiphosphazenes with P-C bonds. These exhibit a higher thermal stability than polyphosphazenes carrying only P-O, P-S or P - N bonds. However these polymers can be obtained by two routes: unlike the polydichloro, polydifluorophosphazene can be substituted by organo-lithium up to 70% without any appreciable degradation. Polydifluorophosphazene, which was used by Evans and Allcock [19], is obtained from hexachlorocyclotriphosphazene.

Another route gives access to a polymer carrying one or two alkyl or aryl substituents, by the polymerization of incompletely substituted cyclic triphosphazenes [22, 24].

Cl2pt N~PCI2 +NaF II I -NaC/ N%p~N CI2

IN~... /R II I CI N~.p~N CI2

The process for the synthesis of these polymers thus allows an almost unlimited number of new structures to be created. To date about 300 have been described [18]. In addition to the polymers obtained

FI

3SO°C

I

"LiFI+LiR

~ N

R I

n

Me3Si'N'~OCH2CF~

k + (n-l) MesSiOCH2CF3

R CI ~)T('N-' CI CI

CHEMICAL MODIFICATIONS

F2p l N~PF2 II I N..p~N F2

A direct synthetic route to the substituted polymer was found by Neilson and Wisian-Neilson [20, 21]. It is based on the polycondensation of an already substituted monomer carrying the two leaving groups trifluoroethoxy and trimethylsilyl.

~-

R I =PW I R

by direct synthesis, those resulting from their chemical modification must also be considered. Most of them are functionalized polyphosphazenes, often aimed to be bound to other molecules such as biological active constituents, or to other polymers, thus creating novel materials. Several examples are described below (Fig. 3). Starting from the p-bromophenoxy substituted polymer, and by substitution of the p-bromine atom with lithium, Allcock [25] obtained a series of

Review CCCH

Br (25)

Li

/

0-0 I

P.

,

I

I -(- P - N'),n I

Ph- p-Ph

""

I

I

-(- P = N)n I

o -R I

(28)

NaO --X -Y

-{- P=N ~

~"

O-X- Y I +P=N~B

I

I

X=Organicgroup

Y=OH,NH2 R I

OOH

(CH2CH"~n

(26) O I -6-P=N ~ I

=

O I +P=N~ I

~

O I + P=N'M I

Fig. 3. Examples of chemical modifications. functional polymers including functional groups such as carboxylic acid and diphenylphosphine. Neilson [21] has also obtained a number of new structures by lithiation of polymethylphosphazene, such as silyl derivatives, metallocenes or grafted polystyrene. Gleria [26] grafted various vinyl polymers, in particular polystyrene, by initiating hydroperoxides on the ternary carbon of the p-isopropylphenoxy substituents. Alkyloxy or aryloxy substituents are transesterified by alkoxylates [27]. This technique was used by Matsuki in the functionalization of the polymers with amino-alcohols, diols or polyglycols [28]. This process is particularly useful to modify the surface properties. The catalytic hydrogenation of p-nitrophenoxy gives an amine-functionalized polymer [29, 30] which can cure epoxy resins, or can be converted into a polymeric diazoic dye. The treatment of polydiphenoxyphosphazene with oleum, which does not affect the polymer backbone, allows grafting of sulphonic acid groups [31]. A carboxylic acid group can be obtained by hydrolysis of an ethyl carboxylate function in the para position on a phenoxy substituent [32]. GENERAL PROPERTIES This great variety of structures obviously gives rise to very diverse physical and chemical properties. The polymers can be hydrophobic or hydrosoluble, electrical conductors or insulators, photodegradable or photoresistant, resistant or not to solvents and chemical attack, biologically active or not, and so on.

343

There is, however, one property that the whole family has in common viz. the flexibility of the chain. In fact this backbone consists of alternating P - N single and double bonds without any resonance between them, since the phosphorous atom does not transfer n-electrons [33-35]. The strong polarization of these bonds gives rise to a high dipole moment [36]. The rotation energy around the N - P bond is weak (3.38 and 21.8 kJ.mol -l respectively for polyditrifluoroethoxy and polydiphenoxyphosphazene [37]) so giving a structure with a high degree of freedom and a low glass transition temperature. Small and unhindered substituents such as alkoxys give very low Tg, below - 6 0 ° ( - 105 ° for n-butyl) [38]. Aromatic rings give more stiffness, leading to a transition temperature of between - 3 4 and 0 ° with one ring and between 0 and 100° with two rings [39]. Interatomic interactions can also limit the freedom of movement of the chain. This is particularly true of primary amine substituted polymers which exhibit a Tg about 100° higher than for polymers substituted with the corresponding alcohols. This effect is explained by segment immobilization due to hydrogen bonding [40]. Another consequence of the P - N backbone flexibility is the possibility for these polymers to undergo structural changes in the solid state. Crystalline polyphosphazenes generally exhibit polymorphism with a mesomorphic state between the crystalline and the melted state [41, 42]. It will be seen later that the flexibility of the chain allows some of the polymers to be used as speciality elastomers. Various applications such as liquid crystals, catalysis and conductive polymers can also be expected. Another characteristic common to nearly all the polyphosphazenes is their combustion behaviour. They exhibit a high oxygen index, low smoke emissions, no corrosiveness and a low toxicity of the combustion gases [43-46]. This property has proven to be essential for the commercial development of polyaryloxyphosphazenes. The physical properties of these polymers depend on the nature and the number of substituents. Polyalkoxy- or polyaryloxy-phosphazenes generally exhibit a semi-crystalline structure when there is only one type of substituent, whereas two or more substituents turn them into an amorphous gum. Amine substituted polymers are amorphous. Those with a high Tg (aromatic amines) are glassy polymers. STRUCTURES AND APPLICATIONS Such a large diversity of structures obviously gives an equally large variety of potential applications. To date, industrial developments have involved applications with large markets, mostly flame resistant materials. More recently research efforts have turned to areas where the great versatility of polyphosphazenes will open markets of low volume and high added value.

Special rubbers Polymers with very low Tg are produced by substitution with alkoxy groups [38]. However the low thermal stability of these compounds [47-49] makes them totally unsuitable for any application in the field

344

Review

of materials. In 1965, Allcock [2] obtained a polymer with a very low Tg and with a satisfactory thermal stability using sodium trifluoroethoxylate as a substitution reagent. In 1968, Rose [50] obtained the first elastomeric polyphosphazene by co-substitution of two different fluorinated alcohols. Since then this type of structure has been optimized [51, 52]. A polymer with the following structure is now commercially available:

corrosiveness and low toxicity of combustion gases [43--46]. Type I polymers have a greater elastomeric character with Tg between - 1 5 to - 2 0 ° compared with about 0 ° for type II polymers; the latter however emit very little smoke on burning. These two types of polymers are aimed at very similar applications, mainly insulating foams [57, 58], flexible foams [59], electric cable covering [60, 61], floor covering [62], paint binders [63]. Membranes

I

4P-N~

& I

CH24CF262.sCFaH Approximately 1% of the available sites of this polymer carry an o-aUylphenoxy group. The presence of this reactive double bond allows this rubber to be crosslinked either with a peroxide or with a sulphur curing agent [53]. The polymer exhibits a very low Ts ( - 6 5 °) and a good thermal stability and has a service temperature ranging from - 6 0 to +175 °. This rubber also shows good resistance to solvents, particularly hydrocarbons, as well as to a large variety of other chemicals [54]. Obviously, it is also extremely resistant to fire. Flame resistant materials

Transport growth, the increasingly frequent occurrence of large crowds in confined spaces and the growing awareness of public opinion to catastrophic events have tremendously increased the demand for flame resistant materials. Polyaryloxyphosphazenes fully satisfy this requirement. Two main types of structures are currently being commercially developed [45-55]:

Membrane separation processes are steadily gaining importance in all areas where molecules must be selected according to size or polarity. The use of these methods in chemical engineering is closely dependent on the development of membranes exhibiting good resistance to solvents and chemicals. Polyfluoroalkoxy and polyaryloxyphosphazenes, which are particularly suited to these applications, are the two structures most often used in this field [64, 65]. The main applications mentioned are enrichment of air with oxygen [61, 66-68] and the purification of sour gas [64-69]. Separation of mixtures of alcohols [70], of various organic compounds [71, 72] and ions [70] are also mentioned. Polymer conductors

It was mentioned earlier that the polyphosphazene chain cannot transfer electrons. Any electronic or ionic conduction must therefore occur within the substituents. Ionic conduction has been the object of a number of studies, in particular by Allcock, Blonsky, Shriver et al. [73-78]. The principle of their study consisted of grafting polyethylene glycol oligomers onto the polyphosphazene backbone. The addition of a lithium or silver salt gives a much higher conductibility than polyglycols doped with the same salts [85].

o-<3-oc. -(,. P = N~

Type I

CH3.

-(. P= N~

",.:_f"%_o I c c./ x = / -

Type II

Like their fluorinated homologues, these two structures generally contain a few percent of o-allylphenol or eugenol substituents [53, 56] which allow a peroxide or sulphur system cure. However, they can also be cross-linked with free-radicals even when there are no reactive double bonds. Although these polymers do not exhibit the same mechanical properties as true elastomers, they can be transformed and formulated according to the same rules. The properties and applications of these polymers are very varied and depend to a large extent on the nature and ratio of the substituents, on the curing process, and on the formulation and transformation processes. However they all exhibit remarkable fire resistance, characterized by a high oxygen index (27-33 for halogen-free polymers and over 40 for the final material), low smoke emission, no

-

CH2

A-

I

CH2

CH~ A- CH2

M* I

I "

I

.CH2,`_ CH2 A - CH2

M" ~ N = P AM*

d) " 6 "t'x "t"x N=P-

' ~I A CH2 I

CH2

CH;

(~ M" 6

CH2

A = CF3SO3,Br,SCN

~'.,'x

N =~-

"~M÷ ~H 2 M+ /

CH2 M*

N=P~%I%I~

'~M÷ ~H 2 A-I

A-

CH2

" ~I CH2 A-

,A-

I

CH2 ""

M = Li,Ag

Despite their very high molecular weight, these polymers do not have the necessary mechanical properties to be transformed into thin films needed for the production of compact batteries. Various crosslinking methods have been suggested [76, 78, 79, 128]. Combined substitution with polyethylene and with substituents carrying ionic groups [80, 81] has been tried in order to prevent the doping salts from migrating out.

Review The optimization of this type of structure now seems sufficiently advanced to allow its application for rechargeable batteries [82-86]. A satisfactory lifespan has been obtained, with up to 1000 charge/discharge cycles [87]. Other structures designed for ionic conduction have also been described. Some of them rely on ether/metal complexes with crown-ether [88] or phenoxyethanol [89] substituents. Others are based on metal complexes of various types, such as phthalocyanine/copper [90], tetracyanoquinodimethane/ lithium [91] or ferrocene complexes. Iodine-doping of polybis(N- dimethylamino - 4 - phenoxy)phosphazene [92] has given an ionic conductivity of 10-4S •cm -~ . Semi-conductor type electronic conductivity has been obtained by Bowmer [93, 94], without any doping, by electrochemical oxidation of polybis(pyrrolyl)phosphazene.

O

I +P=N~ I

electrochemical

~ oxydation

I I t %1~I~'P = N -- P = N - - P= N ~ I I I

Gleria et al. [95, 96] obtained one of the highest levels of photoconductivity ever recorded for a polymer by doping polydinaphthoxyphosphazene with trinitrofluorenone. This phenomenon is explained by the favourable orientation of the naphthoxy groups under irradiation. Other similar structures also exhibit photoconductivity [97]. Liquid crystal polymers The flexibility of the chain gives a high freedom of motion to the substituents, which is a favourable feature for the synthesis of liquid crystal polymers. Simultaneous studies by Allcock et al. [98] and by Singler et al. [99, 100] achieved similar structures i.e. phenylazophenoxy mesogenic substituents linked to the polymer backbone by a polyether spacer. O-(-CH2CH20-~ i -(- P - N.~

N - N'~

OI I . ( . C H 2 C H 2 0 - ) . ~

N- N-~-

R

R

[ o r -CH2CF 3 x=l

to3

R = OCH3,nC4H9

Biomedical applications The various types of potential applications in this field can be classified according to the polymer's affinity for water. They range from a flexible prosthesis to an injectable solution, including surface-treated prostheses and implants which are more or less bydrolyzable. Polyfluoroalkoxy and polyaryloxyphosphazenes are well suited as prosthesis materials [102] since they are inert in biological media, and are well tolerated as sub-cutaneous implants [101). Heparin [103] and dopamine [104] can be fixed onto these polymers.

345

The following diagram summarizes the structural features necessary for applications requiring hydrophilic polyphosphazenes:

® ®00 I

O

I

I

~ P - N - -

P=N-- p . Ikl~,~

O=

O=~-OEI

Et

= Hydrophilic group NHCH 3 Polyether Glucosyl - Imidazol

~

O

= Bonding group - Ester - Peptidic - Schiff base - Ionic Complexation

= Active ingredient

These polymers generally comprise a hydrophilic substituent in a ratio corresponding to the required hydrophilicity, and an active principle either grafted or merely trapped in the polymeric network. Optionally a substituent capable of accelerating the hydrolysis of the chain by an assistance mechanism can also be added. The most frequently mentioned hydrophilic substituent is monomethylamine [105-111]. Polyethers [130], glucosyl [112] and imidazole [113] are also cited. The presence of amino-acid esters among the substituents, in particular ethyl glycinate, helps the hydrolysis of the polyphosphazene backbone. These esters facilitate chain breakages by an assistance mechanism [114]. The rate of hydrolysis of the system, and hence the rate of release of the active principle can be programmed as a function of the polymer hydrophilic character and the ratio of glycinate grafted. Various ways of chemically linking the active principle have been used. Some can be directly fixed onto the chain with a P--O bond [108] (steroids) or with a P - N bond [110] (procaine, benzocaine, chloroprocaine). Other types of attachments use a functionalized substituent, for instance Schiff base [109] (sulphodiazine), peptide link [115] (nicotinic acid, N-acetylpenicillamine) and ionic bond on a quaternized site [116] (heparin). Finally, attachment of platinum dichloride by complexation on the phosphazene's nitrogen doublets [107] must also be mentioned. Until recently this work has been confined to the domain of pure research but studies on practical applications have begun to appear. In particular the work of Van der Groot et al. [117-119] must be mentioned. Catalysis The flexibility of their structures makes these polymers particularly suitable as supported catalysts. In general the insertion of the cataly, tic sites within this polymeric system only results in a small loss in their activity. Some of them retain their catalytic efficiency

346

Review

only when they are linked to the polymeric chain by means of a spacer. Others exhibit high activity even when grafted directly. The studies undertaken in this area are aiming to design supported catalysts for use in coordination catalysis, photochemistry and enzyme catalysis. Allcock and co-workers have described many grafted complexes in a detailed review on coordination catalysis. The work of Allcock has concentrated on grafting triphenylphosphine, metallocenyls, cyclopentadienyl-metal carbonyls and carboranyls [18]. Allcock [111] and Toshitsugu [131] have also achieved enzyme binding. Complete substitution with 4-hydroxybenzophenone has allowed Gleria to obtain a polymer insoluble in all solvents, which exhibits a high photoinitiating activity when used as a powder in suspension [120, 121]. Paints

Polyphosphazenes used as binders in paint formulation provide coatings which exhibit excellent fire resistance [63]. In the specific field of marine antifouling paints Nanishi and Nakayama [132] designed a self-degradable system with alkyl glycinate substituents, which avoids the use of toxic additives. Adhesives

This area has only been tackled in an industrial research context. Most of relevant patents are on the use of polyphosphazenes as additives for various adhesives or as hardeners for epoxy resins. Some patents claim polyphosphazenes carrying an ethyl methacrylate group which allows room temperature, and probably anaerobic curing [122, 123]. Photocuring polymers

Grafting two co-reactive functional groups on the same polymer backbone allows their reactivity to be substantially increased. Gleria [124] synthesized a polymer exhibiting faster photopolymerization than classical systems. This was achieved by fixing a photoinitiator and a p-isopropylphenoxy on the same chain, thus allowing crosslinking by formation of a free radical on the tertiary carbon. Self-stabilized polymers

An article [125] describes a u.v.-stabilized polyphosphazene by dihydroxybenzophenone grafting. Antioxidant protection has been claimed in a patent [126] through 4-hydroxy 2,6-ditertioobutylphenol grafting. Polyphosphazene as f l a m e retardant polymer additives

Materials with oustanding flame resistance can be obtained by grafting polymers onto a polyphosphazene structure. Brossas et al. [127] achieved it by the reaction of a living anionic polymer on polydichlorophosphazene. Gleria et al. [26] grafted various vinylic polymers, in particular polystyrene, by initiating the polymerization of their monomers with hydroperoxides generated on the p-isopropylphenoxy substituents of a polyphosphazene. In both cases, very high oxygen index values (about 35 and even up to 57) were reported.

CONCLUSION

Problems related to the basic chemistry of these systems, in particular the difficulty of access to polydichlorophosphazene, prompted many years of research both for the industrial process and for the formulation'of the speciality polymers under development. Recently the research effort has switched to new areas where the adaptability of polyphosphazene structures will allow their use in totally new fields of applications. It could already be expected that the present efforts will result in a technological breakthrough in the fields of membranes and polymer electrolyte batteries. It is also likely to assure polyphosphazenes a place in the future of galenic. Finally, rubbers and flame resistant materials, which until now have been the only area of commercial development for these polymers, will doubtless continue to improve, to be better for their present uses and to gain access to new applications. The creativity of researchers in this area of chemistry is far from coming to an end.

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