Phosphazenes polymerization

Allcock in his work systematized and elucidated the role of the substituents and, more importantly, substituents inducing ring strain in the ROP process. Fundamentals of ring-ring and ring-chain equilibria in phosphazene polymerization were established. ... 

Functionalized linear polyphosphazenes can be included into copolymers as demonstrated through the formation of a polyurethane material with fiuorinated linear phosphazene polymeric content.A precursor material is formed from 2-methyl-1,4-phenylene diisocyanate and poly(tetramethylene glycol). This material is then reacted with a fiuorinated phosphazene (42) in the presence of 2,2-bishydroxymethyl-l-butanol to yield a cross-linked polyurethane-polyphosphazene material. The methacrylate moiety also has been demonstrated to create cross-linked phosphazene-organic hybrid materials. Poly(bis-methacrylate)phosphazene (43) was formed under mild (ambient) conditions to prevent premature cross-linking. 

Drug delivery is an application that has been investigated using amphiphilic phosphazene polymeric materials. An example of this was provided using polyphosphazenes substituted with N,N-diisopropylethylenediamine, an amine functionalized MW2000 poly[ethylene glycol] (AMPEG), and 4-ethyl-aminobenzoate These polymers were found to self-assemble into nanoparticles and were seen to be effective in the delivery of doxorubicin. 

The first phosphazene polymers containing carbon (79), sulfur (80,81), and even metal atoms (82) in the backbone have been reported. These were all prepared by the ring-opening polymerization of partially or fully chloro-substituted (or fluoro-substituted) trimers containing one hetero atom substituting for a ring-phosphoms atom in a cyclotriphosphazene-type ring. 

Phosphazene polymers are normally made in a two-step process. First, hexachlorocyclotriphosphazene [940-71 -6J, trimer (1), is polymerized in bulk to poly(dichlorophosphazene) [26085-02-9], chloropolymer (2). The chloropolymer is then dissolved and reprecipitated to remove unreacted trimer. After redissolving, nucleophilic substitution on (2) with alkyl or aryloxides provides the desired product (3). 

Allcock HR. Small-molecule phosphazene rings as models for high polymeric chains. Acc Chem Res, 1979, 12, 351-358. 

Abstract In this paper the synthesis, properties and applications of poly(organophos-phazenes) have been highlighted. Five different classes of macromolecules have been described, i.e. phosphazene fluoroelastomers, aryloxy-substituted polymeric flame-retardants, alkoxy-substituted phosphazene electric conductors, biomaterials and photo-inert and/or photo-active phosphazene derivatives. Perspectives of future developments in this field are briefly discussed. 

At the same time, ring-opening-polymerization (ROP) processes, which dominated the phosphazene field for decades [38], tend now to be substituted by polycondensation reactions. These seem to be more feasible and reproducible, easier to carry out, and able to guarantee predictable MWs and MW distributions for these materials [10]. 

Finally, substituted phosphoranimines seem to be able to avoid substitutional processes carried out on highly reactive polymeric phosphazene inter-... 

In the same scheme, moreover, it is evident that, besides phosphazene homopolymers, the substitution of the chlorines with two (or more) different substituents leads to the preparation of substituent phosphazene copolymers [263] containing different homosubstituted and heterosubstituted monomeric units. Moreover, the cationic polymerization of phosphoranimines [215-217] produces polymers with hving reactive ends (vide supra) from which the preparation of chain phosphazene copolymers (block copolymers) [220,223,225, 229,232-235,239, 240] formed by different polymeric backbones linked together in a unique macromolecule could be obtained. 

In this context, phosphoranimine compoimds (both homosubstituted with an unique group or bearing two different groups at the phosphorus) play a fundamental role because their polymerization under different experimental conditions eventually leads to fully substituted polyphosphazenes with no residual chlorines on the phosphazene skeleton. The general scheme of the phosphoranimine polymerization processes is reported in Fig. 10. 

As can be seen, the first example of phosphoranimine polymerization process was proposed by E. P. Flindt and H. Rose [314] in 1977 for tris(trifluoroethoxy)-AT-(trimethylsilyl)phosphoranimine, which could be polymerized to poly[bis-(trifiuoroethoxy)phosphazene] (MW 4000-10,000) by simple heating at 200 °C. 

A few year later (1980) R. H. Neilson and P. Wisian-Neilson started their long term research project on the preparation of alky, aryl, and alkyl/aryl phosphazene polymers and copolymers [55,56,315-334] prepared in the same way by thermal polymerization of a variety of phosphoranimine derivatives [55, 329, 335, 336] to the corresponding phosphazene macromolecules. The polymers obtained by long heating (several days) at high temperatures (160-220 °C) showed relatively low (about 50,000) molecular weight. 

The possibility, through living cationic polymerization processes, to produce linear chain phosphazene copolymers [486]... 

The presence in these copolymers of hetero-substituted monomeric units randomly dispersed along the phosphazene skeleton brings about the extreme difficulty of the polymeric chains to be packed in regular structures. They lose, therefore, the original stereo-regularity of the parent phosphazene homopolymers (microcrystalline materials), and show only amorphous structures, with sharp decrease in the values of the Tg (collapsed up to about -90 °C) and with the onset of remarkable elastomeric properties [399,409,457]. 

Based on the synthesis of polyphosphazenes and of diblock copolyphosp-hazenes by the living cationic polymerization of phosphoranimines [237,241], the triblock poly(phosphazene-ethylene oxide) copolymer XVIII was synthesized by Allcock [223]. 

Thus, the photo-activity of poly[bis(4-benzoylphenoxy)phosphazene] under illumination could be finely tuned by irradiating the polymer in the presence of variable amount of naphthalene, a typical triplet state energy quencher [474]. The same polymer could be used as polymeric photosensitizer to induce the... 

Allcock, H. R., and Brennan, D. J., Organosilicon derivatives of cyclic and high polymeric phosphazenes, J. Organomet. 

Allcock, H, R., and Scopelianos, A. G., Sjmthesis of sugar-substituted cyclic and polymeric phosphazenes, and their oxidation, reduction, and acetylation reactions. Macromolecules. 36. 715, 1983. 

D. Pseudohalogeno-derivatives.—Little work has been carried out in this area. Isocyanates of cyclic phosphazenes, previously unknown, are thought to be formed in the reaction of NgPaBrg with AgOCN in nitro-methane. They were detected by i.r. spectroscopy, and underwent ready polymerization, which precluded their isolation. On the other hand, isothiocyanates, [NP(NCS)2] (n = 3 or 4), are well known and a detailed study of their spectra has been reported. The azide, N3Pa(N3)8, has been the subject of an i.r. study which suggests that the molecule has Z)3A symmetry. 

Work on phosphazene high polymers continues to attract increased interest. Advances in the study of the ring-opening polymerization, and physical characterization in the solid state, of the materials produced by these reactions have been reported. 

Third, metallocene units, such as ferrocene or ruthenocene, have been linked to phosphazene cyclic trimers or tetramers and these were polymerized and substituted to give polymers of the type mentioned previously (41). Polyphosphazenes with ferrocenyl groups can be doped with iodine to form weak semiconductors. Polymer chains that bear both ruthenocenyl and ferrocenyl side groups are prospective electrode mediator systems. 

An overview of the synthesis and characterization of a unique class of polymers with a phosphorus-nitrogen backbone Is presented, with a focus on poly(dichloro-phosphazene) as a common Intermediate for a wide variety of poly(organophosphazenes). Melt and solution polymerization techniques are Illustrated, Including the role of catalysts. The elucidation of chain structure and molecular weight by various dilute solution techniques Is considered. Factors which determine the properties of polymers derived from poly(dichlorophos-phazene) are discussed, with an emphasis on the role that the organic substituent can play In determining the final properties. 

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