versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.52 n.2 Concepción jun. 2007
J. Chil. Chem. Soc, 52, Nº 2 (2007) págs.: 1191-1195
POLYPHOSPHAZENES WITH P(DIOXY-BIARYL) CYCLES IN THE REPEATING UNITS. A REVIEW
GABINO A. CARRIEDO
Departamento de Química Orgánica e Inorgánica. Facultad de Química. Universidad de Oviedo. Oviedo 33071.Spain.
This review covers the chemistry of the polyphosphazene homo or random copolymers having repeating units of the type [NP(O2-biaryl)]n, were the P(dioxy-biaryl) moieties are cyclic (polyspirophosphazenes), specially those of the types [NP(O2C12H8)]n [(O2C12H8)=2,2´-dioxy-1,1´-biphenyl], and the chiral [NP(O2C20H12)]n [(O2C20H12)= 2,2-dioxy-1,1´-binaphthyl] or [NP(O2C20H10R2)]n [(O2C20H10R2)= 2,2´-dioxy-1,1´-binaphthyl functionalized in the 6,6´positions]. Special mention is made to the molecular and solid state structure, the behaviour in solution, the thermal properties, and, in the case of the chiral polymers, their chirooptical properties. The use of some of this polymers as precursors for different kind of materials, such as polymeric complexes with the transition metals and their use as supported catalysts, branched phosphazene polymers and metal containing nanoparticles is also discussed.
Keywords: polyphosphazenes / chiral / metal-containing / supported catalysts.
1. Polyspirophosphazenes homo and copolymers.
Polyphosphazenes (figure 1) are polymers having repeating units of the types [NPX2] with X=halogen or pseudohalogen, (polyhalophosphazenes) (1a), [NPR2] with R=alkyl or aryl [poly(alkyl)phosphazenes or poly(aryl)phosphazenes)] (1b), [NP(OR)2] with R=alkyl or aryl, [poly(alcoy) phosphazenes or poly(aryloxy)phosphazenes] (1c), or [NP(NHR)2] [poly(am ino)phosphazenes] (1d). A variety of copolymers combining two or more of those units is also possible.fig 2a). In fact, an alternation of shorter (P=N) and longer (P-N) bonds is usually observed along the chains4a, although in some conformations, for example in optimized helical chains, all the P-N bond distances may be almost equal4a (as observed in all the cyclic phosphazenes), and the other representation shown in fig 2b might be also appropriate. In any case, it should be taken into account that the analysis of the bonding show no electronic delocalization4b and that they are non-conducting polymers [in fact, they are isoelectronic with the polysiloxanes (silicones)].
These macromolecules are very versatile in chemical composition and properties and have a great academic and applied interest, including the very relevant fields of polymeric materials for electronics and for biomedical applications1.
A special group of phosphazenes comprises those having λ5-phosphorus cycles in the repeating units (fig 3a), that, being the linear counterparts of the spirocyclophosphazene trimers or tetramers, they can be named as polyspirophosphazenes5, although other terms, such as cardo-phosphazenes might also be appropriate.
Although there were earlier reports on the possibility of the existence of this type of polyphosphazenes with dioxy-biaryl substituents (fig 3b), the first to be prepared in high yield an purity and practically free of residual chlorine was the poly(2,2´-dioxy-biphenyl)phosphazene6 [NP(O2C12H8)]n (or polyspirophosphazene PSPP) (I) (see chart 1), that later7 was also obtained as the pure 15N isotopomer [15NP(O2C12H8)]n. Other phosphazenes with cyclic diaryloxy repeating units that could be synthetized8 were II and III, (chart 1), and the chiral binaphthoxyphosphazenes IV, either in the isotactic R or S enantiomeric forms8 or as atactic mixed R/S isomers9 , and the chemically 6-6´-functionalized derivatives V and VI (both as isotactic R isomers)10 (chart 2).
It was also possible to prepare random copolymers combining two different cyclic units, such as VII8, VIII8, and IX to XI11 (chart 3); or combining cyclic and non-cyclic units such as the polymers XII6a, XIII6a, XIV12, XV6a, XVI6a, XVII13, XVIII14, XIX15, XX16, XXI17, XXII18, XXIII-XXIV19, XXV20, and XXVI to XXVIII21 (chart 4).
Also prepared were the related chiral binaphthoxy derivatives (always as R isomers) XXIX22, XXX22, XXXI9, XXXII-XXXIII22, XXXIV17,22, XXXV18, XXXVI22, and XXXVII-XXXVIII23(chart 5).
The weight average molecular weight of all those polymers (as measured by GPC in THF and in the presence of tetrabuthylammoniun bromide) span in the range 103-106. The individual values depended on the reaction time needed in the preparation (see later) and, therefore, on the actual groups present in the bi and monoaryls. The polydispersity indexes vary from 2 to 6, which is a normal feature of polyphosphazenes obtained by the macromolecular substitution of chlorine from [NPCl2]n1.
2a. High Mw polymers.
There are many methods suitable for the preparation of polyphosphazenes with low or high molecular weight1. For the synthesis of polyphosphazenes with λ5-phosphorus cycles in the repeating units (fig. 3a) three main strategies are possible5, namely: a) ring opening polymerization of a spirocyclic precursor; b) condensation of a cyclic phosphoranimine; and c) the macromolecular substitution of chlorines from the starting polymer [NPCl2]n using bifunctional nucleophiles. The macromolecular substitution is the method most commonly used for the preparation of aryloxyphosphazenes. However, to obtain polyspirophosphazenes of the type shown in fig 3b, it is necessary to use a dihydroxy-biarene in the presence of a base (i in scheme 1) and, therefore, unwanted collateral inter-chain substitution is rather probable, forming cross-linked insoluble and hydrolytically unstable materials with high residual unreacted chorine contents (ii in scheme 1). Non geminal substitution, which is the other possible collateral process, is rarely observed.
However, when the direct reaction of [NPCl2]n and the biphenol (HO)2C12H8 (2,2´-dihydroxy-1,1`-biphenyl) was carried out using potassium carbonate as proton abstractor in THF as solvent, a method that was very convenient for other aryloxyphosphazenes24a, no cross-linking occurred resulting in the formation of the linear polymer I(PSPP) (chart 1) in good yield and purity with only a few ppm of residual chlorine6. Furthermore, a new method at a laboratory scale7 to obtain THF solutions of [NPCl2]n or its isotopomer [15NPCl2]n starting directly from PCl5 and NH4Cl or 15NH4Cl, suggested the synthesis of the isotopically pure 15N polyspirophosphazene7 [15NP(O2C12H8)]n.
The macromolecular substitution reactions from [NPCl2]n could be extended to other biphenols in the presence of K2CO3 or Cs2CO3 (with which the substitution are much faster24b), allowing the synthesis of the polyphosphazenes shown in charts 1 and 2. It was noticed, however, that the tendency to cross-linking increased with the number of atoms between the two OH groups (i.e. those biphenols forming larger cycles). Thus, the polymers II and III were obtained in lower yields and with some larger residual chlorine contents8.
Once shown that the biphenols could react without cross-linking with polydichlorophosphazene in the presence of an alkali carbonate, it was easy to design a general method to synthesize random copolymers combining two different repeating units, based on the sequential substitution method shown in scheme 2.
In the first step, using substoichiometric amounts of the biphenol, the partially substituted intermediates are formed, which are subsequently reacted with another biphenol or other nucleophile in the presence of the alkali carbonate. This allowed the synthesis of the numerous polymers shown in charts 3 to 5 carrying different chemical functions, and with a composition that could be varied from x=0.05 to 0.9. The mild reaction conditions allowed by the use of the alkali carbonates as proton abstractors, specially in the case of the caesium salt, made it possible to use directly phenols and binaphthols carrying sensitive chemical functions (see, for example, the polymers II, III,
VI or XX). An specially interesting case was the formation of the polymers XVII carrying OC6H4-NH2 terminal groups, because the starting aminophenol HO-C6H4-NH2 could react with the P-Cl bonds either through the OH or the NH2 nucleophiles. The results showed that, although at room temperature, the NH2 is more reactive giving aminophosphazenes as the kinetically controlled products, at the higher temperatures needed to substitute the residual chlorines in the intermediate of the type shown in scheme 2, the reaction was regio-specific and only the OH was activated (thermodynamic control)13.
The substitution mechanism in all those reactions was found to be strictly random along all the N=PCl2- groups of the chain, as shown by the nature of the products that were strictly random copolymers17 (see later).
The average Mw (measured by GPC) of the polymers obtained in those reactions were slightly lower in the cases requiring more refluxing time to complete the substitution, i.e., with the less acidic phenols. But, also, it was noticed that the biphenol (HO)2C12H8 tends to induce some, not very extended, degradation in the chains. Thus, depending on the reaction conditions, the homopolymer I (PSPP) may result with Mw between 200.000 to 800.000, while the copolymers having less proportion of the biphenoxy units have higher Mw, except in the cases of the amino derivatives XXVI to XXIX. The binaphthoxy derivatives tend to have higher Mw (closer to the 1.000.000). However, the measurement of Mw by GPC in phosphazenes should be taken with caution. Because of their tendency to form aggregates in solution causing some uncertainty in the high Mw range of the GPC chromatograms25. In the case of the polyspirophosphazenes, specially those with naphthoxy groups, the broadness of the signals in the NMR spectra, which are sharpened only at high temperatures8, suggests that this may be the case.
The synthesis of several of the polymers in charts 3-5 required a chemical derivatization step starting from a convenient precursor, a field that, in the case of polyphosphazenes has and special interest on its own26. Examples are the preparation of the COOH derivatives XXII and XXXV, that were made by basic hydrolysis of the CO2Pr precursors XXI and XXXIV respectively18. Other interesting example of chemical derivatization was the preparation of the silicon containing phosphazenes XXXVIII. The SiR1R2R3 groups were introduced23 by the lithiation and substitution sequence shown in scheme 3.
In those reactions only a slight decrease in the Mw with respect to the starting polymers was observed. During the synthesis a fraction of the PPh2 sites resulted oxidized by traces of oxygen to OPPh2- that could be re converted quantitatively into oxygen the free PPh2 groups by a treatment with SiHCl3/ PPh3 in THF/toluene at 100ºC.
2b. Lower Mw distributions.
The controlled heating of the polymer I for a pre-determined time at temperatures between 100ºC to 250ºC allowed the formation in quantitative yield of different distributions of the same polymer with average Mw in the range decreasing from 800.000 to 20.000 without affecting the chemical composition27. Similar effects were observed with the binaphthoxy polymers IV. The same range of Mw distributions of I could be obtained by refluxing the high Mw polymer in aqueous hydrochloric acid for times varying from 1 to 4 hours (more prolonged times resulted in the complete hydrolysis to NH4H2PO4 and biphenol) 28. A very low Mw I (ca. 15.000) was also obtained by dissolving I (with Mw of 700.000) in concentrated H2SO4 for a while and re precipitating into water 28.
Those results are important not only because they afforded more soluble and more procesable materials, but also because the availability of different Mw distributions of the same polymer allows the study of the effects of the Mw on the properties.
An study of the variation of the glass transition temperatures with the composition (ratio of the two different phosphazene, units given by x) in the series XXI and XXXIV, in the range x= 0 to 1, clearly demonstrated that they can be considered as strictly random17. As a consequence, all the copolymers in charts 4 and 5, that were made by the same method, are very close to strictly alternating copolymers17. Therefore, the distribution of the functional R groups on the monophenoxy units is very regular along the chains and, when x=0.5 (i.e., those having 50% of each phosphazene unit), the structure is almost that of a mixed diphosphazene homopolymer (Fig. 4):
An important consequence of this fact is that, in the binaphthoxy random copolymers shown in chart 5, when the value x is lower than 0.8, all the non cyclic units are in the centre of the space defined by two binaphthoxy units (figure 5). This space with C2 symmetry is a chiral pocket, the stereochemical shape and size of which can be easily controlled by changing the substituents R in the 6-6´ possitions of the binaphthyl rings23. For this reason, the support of transition metal complexes in the non cyclic units was considered as very interesting for enantiomeric catalysts (see later)29.
Another interesting structural feature of polyphosphazenes at the molecular level is the helicity of the -N=P-N=P- chains30. In fact, theoretical calculations have shown that this is also the case of the spirophosphazene homopolymer (I)27, and of the chiral binapthoxy analogues31. Moreover, the formation of helical chains has been proven experimentally in the case of the isotactic polymer IV(S) in the solid state by a combination of X-ray energy dispersion and molecular mechanics. The best model indicated that the chains packed parallel to each other in a columnar hexagonal way32.
In solution, in good solvents, the homopolymer (I)27, the random biphenoxy copolymers33, and the bipnaphthyl derivatives9, behave as random coils, although, in the later the helical segments are maintained thought the chain conformational dispositions9,31. The helicity of the chain has important implications in the case of the chiral poly(binaphthoxy)phosphazenes, affecting all their chirooptical properties (see later).
Finally, in the solid state, as many other polyphosphazenes34, the spirophosphazenes, both homo o random copolymers, are mainly amorphous, but the X-ray diffractograms always show the presence of a sharp low angle reflection that indicated the formation of a mesophase having bidimensional order 6b,17,18. A very interesting fact is that the plane separation varies accordingly with the shape and size of the substituents in the pendant groups and, in the case of the random copolymers, almost linearly as a function of the chemical composition17.
The spirophosphazenes are white solids (V and XIX are yellow) very soluble in organic solvents (specially THF, dimethyformamide, N-methyl pyrrolidone, CHCl3 and CH2Cl2), and give easily transparent hydrophobic films, that are rather fragile. The micrographs show that the solids directly precipitated pouring their concentrated THF solutions into water are very porous and, in occasions, they form hexagonal channels17.
In solution they behave normally as random coils 9,27,33 with square radius of gyration that depend on the susbtituents in an expected manner.
The isotatic chiral binaphthyl polymers are optically active having large specific rotation [α]D30 always opposite in sign to that of the binaphthols used in their synthesis8,10. Moreover, because of the helical nature, the specific rotation is very dependent on the temperature and on the wavelenght35. By contrast, the optical rotation of the non-isotactic polymers (with an excess of one of the two R or S configurations) is temperature independent, indicating the absence of a preferential screw sense along the chains35 (i.e. the majority rules principle was not observed (for a discussion see referencences in ref. 3). The binaphthoxy derivatives form excimers by energy transfer, but too short lived to evidence the effects of the helicity in the fluorescence spectra35.
In the solid state they have a high thermal resistance6,27 (II and III are exceptions8). Most of them begin to undergo fast decomposition (sharp weight losss) in the TGA chromatograms (at 10ºC min-1) only above 300ºC (near 400ºC for I6b and 500ºC for IV8) and leave rather high residues even at 800ºC, also depending on the chemical compostion17,18. At temperatures lower that 250ºC only a smooth decrease on the average Mw is observed27, and, in the case of the binaphthoxy derivatives only at 300ºC the atropisomerisation process (absolute configuration of the R/S rings interconversion by rotation of the 1-1´-C-C bonds) with loss of the optical activity begins to be observed35.
An important characteristic feature of the polyspirophosphazenes is their high glass transition temperatures (all measured in the second heating run of the DSC experiments), first observed in the case of the homopolymer (I) 6, that has a Tg=160ºC. This show the significant effects of the cyclic units on the chain rigidity, that are probably due to strong interactions between the adjacent dioxy-biaryl moieties. The higher values correspond to the binaphthoxy homopolymers (300-329ºC) 9,35. Interestingly, it is precisely in this range of temperatures were the atropisomerization of the dioxybinaphthyl units is faster, a fact that is in agreement with the predictions for the behaviour of binaphthyl groups immersed into solid polymeric matrices (see references in reference 35).
In the case of the copolymers of charts 3-5, the Tg varies as expected with the polarity and bulkiness of the substituents, but also, with the composition (x) (the two extremes are the values corresponding to the two homopolymers. i.e, x=0 or 1). In fact, as commented above, the variations allowed a confirmation of the strictly randomness of the substitution reaction in its formation from polydichlorophosphazene17.
The polymers with pendant chromophores (XIX and XX in chart 4) had interesting non-linear optical properties15,16, and, those having terminal NH2 groups (XVII in chart 4) were found to be very efficient to form modified electrodes capable of detecting antibiotics (rifamycin) very selectively 36.
The polymers having pendant COOH were soluble in basic water forming the corresponding COO- salts that providing aqueous polymeric materials17,18.
So far only the rheological behaviour of I has been studied 37.
5. Polyspirophosphazene ligands and complexes.
The possibility of introducing almost any chemical function to the lateral groups in the phosphazene chains, allowed the incorporation of ligands (L) with the capability of coordinating transition metal fragments (MLn) (a topic of great interest in the phosphazene field1,38a-b), forming polymeric complexes of the general type shown in figure 6. Related complexes without the cyclic units were also prepared 38c.
Thus a variety of very soluble (in many cases highly coloured) carbonyl and organometallic complexes with different MLn fragments based on W, Mn, Ru and Au, such as XXXIX20, XL39, XLI12, XLII40, XLIII14, XLIV29, XLV41 and XLVI to XLVIII42, were obtained (Chart 6).
Note that XLVI to XLVIII (with x of the order of 0.4-0.5), combine the carbonyl complex with the esther chemical function.
The synthesis of the complexes (scheme 4) could be accomplished by replacement of chlorine atoms using directly a phenolic-complex14,40 , or by the classical substitution reaction method of a labile ligand (S), such as THF, MeCN, tetrahydrothiophene (THT), etc., from a precursor complex MLn(S) and the polyphosphazene carrying the coordinating L groups 11,12,20,29,39,41,42.
By the same methods, a variety of chiral complexes such as XLIX39, L29, LI29, and LII11 were also obtained (Chart 7).
Several of these, namely XL and XLIV, were used as catalysts in the hydrogen transfer from isopropyl alcohol to ketones29. The results showed a high efficiency, and very good capability of recovery and reutilization, specially in the case of the Ru complex XLIV. However, the chiral analogues, having the MLn complexes inside the chiral pocket showed no enantiomeric induction29.
In the ligand substitution method (scheme 4), using substoichiometric amounts of the MLn(S) precursors gave crosslinked insoluble materials, that had the coordinated MLn fragments inserted into a solid chiral matrix29,39.
It should be mentioned that the supported catalysts is a very important topic43, aiming not only to enantioselective synthesis, but also to the recovery and reutilization of the catalyst and the no-contamination of the reaction products with metals, and, in the case of the phosphazenes, this aspect has been scarcely attended 44.
6. Other materials (grafted copolymers and metal-nanoparticles).
The possibility of obtaining hybrid materials by grafting carbon based polymeric chains on linear phosphazenes or the surfaces of phosphazene films or, conversely, grafting phosphazene chains on carbon polymers or surfaces, is a very interesting and challenging area of research45.
Because of the presence of the terminal COOH groups, the complexes XXII and XXXV promoted the thermal ring-opening polymerisation of ε-caprolactone producing polyamide segments attached to the O-C6H4-COO-side groups of the main phosphazene chain, yielding the branched materials18 shown in fig 7.
This opened a new route to other polyphosphazenes grafted with different carbon-based polymeric chains with combined properties, now under progress.
In various works it has been shown that the metal containing polymers of the types shown in chart 6 can be pyrolized to formed size controlled nanoparticles of different chemical compositions46. Recent results have proved that the gold (I) complex XLV can generate stabilized sized-controlled gold nanoparticles41which is interesting considering recent advances in gold-catalysis that suggest the use of gold nanoparticles advantageously47.
The works mentioned in this review have been possible thanks to the fruitful and efficient contributions of all the scientists mentioned as coauthors in the given references. The financial support of the spanish DCYCYT and FYCYT (and the other Institutions also mentioned in the references provided) is also acknowledged (the last project to be mentioned being the CTQ2004-01484).
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