Poly organophosphazenes

Allcock HR, Nelson JM, Reeves SD, Honeyman CH, and Manners I. Ambient-temperature direct synthesis of poly(organophosphazenes) via the living cationic polymerization of organo-suhstituted phosphoranimines. Macromolecules, 1997, 30, 50-53. 

Allcock HR. Qrganometallic and bioactive phosphazenes. J Polym Sci Polym Symp, 1983, 70, 71-77. Allcock HR. Poly(organophosphazenes) Synthesis, unique properties and applications. Makromol Chem... 

Among these polymers, poly(organophosphazenes), POPs, occupy a special position [38]. 

The synthesis of poly(organophosphazenes), POPs, is a research area that has involved a lot of effort in the past by many scientists active in the phosphazene domain. There are several important reasons for this, basically related to the high cost of the starting products [44] used to prepare POPs, to difficulties in carefully controlling the reactions involved in the preparative processes [38] and to the need for accurately predicting both molecular weight and molecular weight distribution of the POPs produced [38,45]. 

In general terms, the synthesis of poly(organophosphazenes) can be obtained by using three main strategies, as reported in Fig. 1. These approaches imply ... 

Use of polycondensation processes of substituted phosphoranimines to obtain already substituted poly(organophosphazenes)... 

Polydichlorophosphazene is commonly considered as the key polymer from which almost all poly(organophosphazenes) are generated [38]. 

This ROP of hexachlorocyclophosphazene to polydichlorophosphazene is very relevant in phosphazene chemistry as it has been used in almost every laboratory in the world for the preparation of poly(organophosphazenes) starting from the middle of the 1960s up to recent times [38]. H. R. Allcock discovered in 1965 [40-42] that (NPCl2)3 can open its inorganic ring thermally, under strictly controlled experimental conditions (250 °C, vacuum of 10" torr, and reaction time of 8-12 h), to form polydichlorophosphazene in a reasonable yield, but in a rather slow and irreproducible way [38]. Moreover, the final polymer obtained shows a very variable MW and MW distribution, with a strong tendency to produce crosslinked materials [45]. 

In spite of the fact that the preparation of polydichlorophosphazene nowadays can be reached in many different ways and with great efficiency (vide supra), the substitution of the chlorine atoms of this polymer to form stable poly-(organophosphazenes) is stiU a source of problems as it can be seldom driven to completeness and a very small amount of unreacted chlorines is always present in the final phosphazene material [38]. The complete ehmination of these chlorines is mandatory if the modification of the phosphazene materials over time has to be successfully prevented. 

The problem of the thermally induced polymerization reaction of partially or completely substituted cyclophosphazenes has been considered in the past by several authors [355-357], and more recently by H. R. AUcock [358]. This is because of the ease of synthesizing these substrates, the possibihty of preparing structurally regulated poly(organophosphazenes), and the lack of any additional nucleophilic substitution processes on the poly(organophosphazenes) obtained by the ROP process of fully saturated trimers. 

Understanding the properties of poly(organophosphazenes) is a fascinating and relatively simple problem, and the analysis of the structure-property relationships of these compounds is a powerful tool for solving this problem. 

In fact, considering the basic structure of these materials (vide supra), it can be immediately realized that the basic features of poly(organophosphazenes) are the result of two main contributions. The first one is fixed and is basically related to the intrinsic properties of the -P=N- inorganic backbone, while the second is variable and mostly connected to the chemical and physical characteristics of the phosphorus substituent groups. Skeletal properties in phos-phazene macromolecules intrinsically due to the polymer chain are briefly summarized below. 

Considering first Table 5, it can be seen that Tg values for the reported poly(organophosphazenes) spanned from very low (-105 °C in the case of poly[bis(n-butoxy)phosphazene]) up to very high (-1-220 °C for poly[tris(2,2 -dioxy-l,T-binaphthyl)phosphazene]), covering almost all the intermediate temperatures between these two limits. Low TgS are indicative of very high torsional freedom of the polyphosphazene chain, which is manifested clearly when flexible substituents of reduced bulkiness are used in the substitution... 

As a conclusion of this section we would like to stress once again the strategic importance of POPs as biological substrates that appear to be one of the most probable break-throughs for a wide-range industrial and commercial utilization of poly(organophosphazenes). 

Similarly, energy-transfer processes, together with electron transfer and hydrogen abstraction reactions could be induced in poly(organophosphazenes) in an intramolecular way by preparing POPs geminally substituted at the same phosphorus with two different substituent groups. 

Allcock, H. R., Schmutz, J. L., and Kosydar, K. M., A new route for poly(organophosphazene) synthesis. Polymerization, copolymerization, and ring-ring equiUbration of trifluoroethoxy-and chloro-substituted cyclotriphosphazenes, Macromolecules. 

Allcock, H. R., and Kwon, S., Covalent linkage of proteins to surface-modified poly(organophosphazenes) Immobilization of glucose-6-phosphate dehydrogenase and trypsin, Macromolecules. 19, 1502, 1986. 

Chun C, Lim HJ, Hong KY et al (2009) The use of injectable, thermosensitive poly (organophosphazene)-RGD conjugates for the enhancement of mesenchymal stem cell osteogenic differentiation. Biomaterials 30 6295-6308... 

It should be noted that most of the substitution-based synthesis work with poly(organophosphazenes) has been preceded by exploratory studies at the small molecule, model compound level, often with the use of cyclic trimer I as a model for polymer II (6). 

Figure 3. The approximate sequence of developments in the synthesis of poly(organophosphazenes) from the early 1950 s to the present.

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. 

Figure 1. Synthesis of poly(dichlorophosphazene) and poly(organophosphazenes).

This paper will provide an overview of the polymerization processes and the properties of poly(dichlorophosphazene). This paper will also discuss the various factors which influence the properties of the poly(organophosphazenes) and show how these factors have resulted in a class of polymers with a wide range of properties, including several examples of current commercial importance. 

The synthesis of poly(organophosphazenes) represents probably the best example of a central theme of Inorganic macromolecules Preparation of a reactive polymeric intermediate, poly(dichlorophos-phazene), and subsequent use in a wide variety of side group replacement reactions (Figure 1). This concept has been demonstrated in a number of laboratories (3) and has provided a wide variety of polymers with different properties. 

Table I serves to illustrate how the nature and size of the substituent attached to the P-N backbone can influence the properties of the poly(organophosphazenes). The glass transition temperatures range from -84 °C for (NP CH-CH ) to around 100 °C for the poly(anilinophosphazenes). Polymers range from elastomers to flexible film forming thermoplastics or glasses at room temperature.

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