Polyphosphazene structure

On this basis, five classes of different polyphosphazenes are considered as outstanding examples of this type of macromolecules, in which skeletal and substituent features overlap to the highest extent. The reported materials are elastomers, flame retardants and self-extinguishing macromolecules, polymeric ionic conductors, biomaterials, and photosensitive polymeric compounds all of them based on the polyphosphazene structure. 

The development of synthetic routes to new polyphosphazene structures began in the mid 1960 s (2-4). The initial exploratory development of this field has now been followed by a rapid expansion of synthesis research, characterization, and applications-oriented work. The information shown in Figure 3 illustrates the sequence of development of synthetic pathways to polyphosphazenes. It seems clear that this field has grown into a major area of polymer chemistry and that polyphosphazenes, as well as other inorganic macromolecules, will be used increasingly in practical applications where their unique properties allow the solution of difficult engineering and biomedical problems. 

In the following sections, four different aspects of polyphosphazene structure will be reviewed briefly side-group disposition chain conformation skeletal bonding and skeletal flexibility. 

Polyphosphazenes (Structure 1.1) are a broad class of macromolecules with a backbone of alternating phosphorus and nitrogen atoms and with two side groups (R)... 

Potin P and Jaeger RD. Polyphosphazenes Synthesis, structures, properties, applications. Eur Polym J, 1991, 415, 341-348. 

Caliceti P, Veronese FM, Marsilio F, Lora S, Seraglia R, and Traldi P. Fast atom bombardment in the structural identification of intermediates in the hydrolytic degradation of polyphosphazenes. Org Mass Spectrom, 1992, 27, 1199-1202. 

To conclude this synthetic section, it appears very clear that the experimental approaches for preparation of POPs are very numerous and give accessibility to phosphazene polymers and copolymers with different structures and properties. Moreover, it has been recently estimated [10,383] that the total number of polyphosphazenes reported up to now in the literature is about 700, and that these materials can find potential practical application as flame- and fire-resistant polymers [44,283, 384-388] and additives [389, 390] thermally stable macromolecules [391] chemically inert compounds [392] low temper-... 

A major feature of the polyphosphazene skeleton is its ability to resist fire and combustion due to the inorganic elements constitutive of its structure [44,387, 388,459,460]. Moreover, the action of skeletal nitrogen and phosphorus atoms can be enhanced by inserting additional inorganic elements (F, Cl, Br, J, B, metals, etc.) in the substituent groups [459,460]. 

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. 

The general structure of polyphosphazenes substituted with fluorinated alcohols is described by the Formula below while the basic structure-property relationships for these substrates are collected in Table 9. 

When x=l and y=2,3,4,. and Z=H or F, a new class of polyphosphazene substrates is obtained, which derive from the simultaneous substitution of two different fluorinated alcohols of different lengths on the same polydichloro-phosphazene macromolecule. The general structure of the substrates is reported below. 

In this section we will describe the general principles that determined the biological applications of polyphosphazenes in different domains, putting an effort into establishing their specific utilization on the basis of structure-property relationships. This argument has been covered by several different review articles in the past [400-406,626] and has been recently highlighted by H. R. Allcock [627] and E. Schacht [407]. 

The general structure of the polyphosphazenes investigated can be described by Formula below in which only one type of substituent has been attached to the polyphosphazene chain. 

When compounds with more comphcated chemical structures were taken into consideration as possible polyphosphazene substituents, the polymers started to show spectroscopic absorptions at wavelengths longer than 240-280 nm. As a consequence, significant photochemical activity started to be observed for POPs, intimately correlated to the photochemical features of these groups. 

The purpose of this chapter is to introduce a new class of polymers for both types of biomedical uses a polymer system in which the 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. These polymers are polyphosphazenes, with the general molecular structure shown in structure 1. 

Molecular structural changes in polyphosphazenes are achieved mainly by macromolecular substitution reactions rather than by variations in monomer types or monomer ratios (1-4). The method makes use of a reactive macromolecular intermediate, poly(dichlorophosphazene) structure (3), that allows the facile replacement of chloro side groups by reactions of this macromolecule with a wide range of chemical reagents. The overall pathway is summarized in Scheme I. 

Some of the most hydrophobic synthetic polymers known are polyphosphazenes that bear fluoroalkoxy side groups (1,2,10-12). Examples are shown in structures 11 and 1. ... 

The first bioerodible polyphosphazenes synthesized possessed amino acid ester side groups (25). The structure and preparation of one example is shown in Scheme V. The ethyl glycinato derivative shown... 

At another level, water-soluble polyphosphazenes are of interest as plasma extenders. In addition, specific polymers with pendent imidazolyl units have been studied as carrier macromolecules for heme and other iron porphyrins (structures and (44,45). (In structures M and the ellipse and Fe symbol represent heme, hemin, or a synthetic heme analog.)... 

Finally, a new water-soluble polyphosphazene was recently synthesized that has the structure shown in 36 (46). This polymer has two attributes as a biomedical macromolecule. First, the pendent carboxylic acid groups are potential sites for condensation reactions with amines, alcohols, phenols, or other carboxylic acid units to generate amide, ester, or anhydride links to polypeptides or bioactive small molecules. Second, polymer forms ionic crosslinks when brought into contact with di- or trivalent cations such as Ca or Ai3+. The crosslinking process converts the water-soluble polymer to a hydrogel, a process that can be reversed when the system... 

Understanding the relationship between molecular structure and materials piroperties or biological activity is one of the most important facets of biomaterials synthesis and new-drug design. This is especially true for polyphosphazenes, where the molecular structure and properties can be varied so widely by small modifications to the substitutive method of synthesis. 

Table 1 is a summary of current knowledge of the relationship between side group structure in polyphosphazenes and biomedically important properties. Within rather broad limits two or more of these properties can be incorporated into the same polymer by a combination of different side groups attached to the same macromolecular chain. 

TABLE 1 Summary of Side Group Structure-Property Relationships in Polyphosphazenes... 

Polyphosphazenes comprise some of the most intensively studied inorganic macromolecules. They include one of the oldest known synthetic polymers and many of the newest. In molecular structural versatility, they surpass all other inorganic polymer systems (with over 300 different species now known), and their uses and developing applications are as broad as in many areas of organic polymer chemistry. 

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