Polyphosphazenes, polymer formation

The final class of polymers containing carboranyl units to be mentioned here is the polyphosphazenes. These polymers comprise a backbone of alternating phosphorous and nitrogen atoms with a high degree of torsional mobility that accounts for their low glass-transition temperatures (-60°C to -80°C). The introduction of phenyl-carboranyl units into a polyphosphazene polymer results in a substantial improvement in their overall thermal stability. This is believed to be due to the steric hindrance offered by the phenyl-carborane functionality that inhibits coil formation, thereby retarding the preferred thermodynamic pathway of cyclic compound formation (see scheme 12). 

Greish YE, Bender JD, Lakshmi S et al (2006) Formation of hydroxyapatite-polyphosphazene polymer composites at physiologic temperature. J Biomed Mater Res 77A 416- 25... 

Monomer 89 (Scheme 10) is a ferrocenyl-substituted cyclic phosphazene, which is ring-opened to give a polyphosphazene with ferrocenyl moieties attached as side-chains to two positions on the polymer backbone. Attempted polymerization at 250°C resulted in no polymer formation however, a catalytic amount of a perchlorinated cychc phosphazene monomer under the same conditions resulted in the isolation of polymer 90. 

Additional lack of chain flexibility is introduced in the polyphosphazene skeleton when polyspirophosphazenes are considered. These materials are obtained by reacting 2,2 -di-hydroxy biphenyl or l,r-binaphthyl derivatives with polydichlorophosphazene [305], leading to the formation of polymers having the structure shown in Formula below. 

Polyphosphazenes sulfonates XIX with the anion covalently attached to the polymer are a new class of cation conductors that have been synthesized by Shriver [625]. They were obtained by reaction of Na0C2H4S03Na with an excess of polydichlorophosphazene in the presence of 15-crown-5, followed by the reaction of the partially substituted product with the sodium salt of poly(ethylene glycol methyl ether). The conductivity at 80 °C of the polymer with x=1.8, m=7.22 is 1.7x10 S cm This low conductivity can be attributed to an extensive ion pair formation between the sodium and sulfonate ions. 

When the substituent groups in the polyphosphazenes were azobenzene [719] or spiropyran [720] derivatives, photochromic polymers were obtained, showing reversible light-induced trans-cis isomerization or merocyanine formation, respectively. Only photocrosslinking processes by [2+2] photo-addition reactions to cyclobutane rings could be observed when the substituent groups on the phosphazene backbone were 4-hydroxycinnamates [721-723] or 4-hydroxychalcones [722-724]. 

The thermal polymerization of N-trimethylsilylphosphoranimines 2215 to 2216 with elimination of CF3CH20SiMe3 2217 is the prototype for formation of inorganic polymers [5, 23, 24]. Polyphosphazene 2216 is also prepared from bromodimethyl(tri-... 

This effect has also been observed in polyphosphazenes containing alkyl- or phenyl-carborane as pendent groups.12 A typical synthetic route to poly(phenyl-carboranyl-di-trifluoroethoxy-phosphazene) having pendent phenyl-carborane groups is shown in scheme 4. A substantial improvement in the thermal stability of the polymer was observed. This is attributed to a retardation of the ring-chain de-polymerization mechanism due to steric hindrance effects of the carborane units, inhibiting helical coil formation. 

Polymers that consist of metal atoms joined together in a linear array are quite rare. This is because such systems prefer to collapse to three-dimensional clusters—literally very small chunks of metal stabilized on the surface by ligands. However, many of the intriguing electrical properties proposed for linear metallo polymers might be realized if a non-metallo polymer, such as a polyphosphazene, were used as a template and scaffold to stabilize a string of metal atoms and prevent cluster formation (structures 3.67 and 3.68). This has been accomplished only in a very preliminary and tentative way,... 

A related synthetic strategy by Allcock and Welker65 is illustrated in reaction (19). Here, borazine side groups are linked to a polyphosphazene chain. This reduces the volatility of the borazine and allows facile fabrication of the linear high polymer. Subsequent pyrolysis at temperatures up to 1,000 °C results in breakdown of the phos-phazene carrier backbone (possibly via the formation of volatile phosphorus nitride) and formation of boron nitride. 

Polyphosphazenes are a relatively new class of biodegradable polymers. Their hydrolytic stability or instability is determined not by changes in the backbone structure but by changes in the side groups attached to an unconventional macromolecular backbone. Synthetic flexibility and versatile adaptability of polyphosphazenes make them unique for drug delivery applications. For example, Veronese et al.18 prepared polyphos-phazene microspheres with phenylalanine ethyl ester as a phosphorous substituent and loaded it with succinylsulphathiazole or naproxen. The kinetics of release from these matrices were very convenient in yielding local concentrations of the two drugs that are useful per se or when mixed with hydroxyapatite for better bone formation. Polyphosphazene matrices are also considered as potential vehicles for the delivery of proteins and vaccines.19... 

Cyclophosphazenes (46) nndergo both thermal ringopening polymerization with formation of linear polymers (4 7) see Polyphosphazenes) and ring-ring eqnilibria (resnlting in ring expansion). 

Two methods have been presented for the phosphonation of poly(bro-moaryloxyphosphazenes), viz. via sodium dialkyl phosphite as reagent or via a treatment with Bu Li in combination with dialkyl chlorophosphate. The reactions are visualized by the product formation of (206) and (208) from (205) and (207), respectively. Polyphosphazenes with diphenyl phosphonate groups (209) have been synthesized by the reaction of the bromophenoxy substituted polymer (207) with Bu Li followed by a rapid addition of diphenyl chlorophosphate in thf. Only 50% of the available bromophenoxy groups are converted to diphenyl phosphonate esters groups. ... 

Block copolymers (228), consisting of a hydrophilic poly(ethylene glycol) and a hydrophobic polyphosphazene residue, have been investigated with respect to their micelle formation in aqueous solution Micelle formation in water has also been observed for polymers (229) with ethyl glycinato substituents. Hydrolytic degradation of these polymers has been studied in aqueous thf. ... 

Polyesters causing pseudoneointima formation Polyphosphazenes (inorganic polymers)... 

Thus far, the survey of phosphazene elastomers has been based on the formation and modification of poly(dlchlorophosphazene). Although a large variety of polymers can be prepared by this approach, there are limitations In the preparation of polyphosphazenes with phosphorus-carbon bonds. The reaction of poly(dlchlorophosphazene) with organometalllc agents, such as RMgX or KLl, results mainly In decomposition and not the desired polymers [NPR.]. There are three possible approaches to the preparation ox polyphosphazenes with phosphorus-carbon bonds polymerization of substituted trlmers, poly(dlfluorophosphazene), and thermolysis of small linear molecules. These three approaches will be discussed In turn. 

A very promising variant on this type of condensation polymerization involves the use of monomers that possess groups X and Y, which can be eliminated from the same molecule. This circumvents the need for careful control of reaction stoichiometry. Moreover, in certain cases, polymerization of monomers of this type can follow a chain-growth type of mechanism that leads to high molecular weights much more easily. Such processes (Scheme 1.1, Route C) have not yet been explored for the formation of metal-containing polymers, but are well-established for the synthesis of certain classes of polymers based on main-group elements such as polyphosphazenes (1.2) and polyoxothiazenes (1.4) [12, 16, 26]. 

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