Polyphosphazenes molecular structure

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. 

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. 

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. 

Moreover, the molecular structural, synthetic, and property nuances of these polymers illustrate many of the attributes, problems, and peculiarities of other inorganic macromolecular systems. Thus, they provide a "case study" for an understanding of what may lie ahead for other systems now being probed at the exploratory level. In short, an understanding of polyphosphazene chemistry forms the basis for an appreciation of a wide variety of related, inorganic-based macromolecular systems and of the relationship between inorganic polymer chemistry and the related fields of organic polymers, ceramic science, and metals. 

With this synthetic and molecular structural diversity, polyphosphazene chemistry has developed into a field that rivals many areas of organic polymer chemistry with respect to the tailored synthesis of polymers for specific experimental or technological uses. Indeed, hybrid systems are also available in which organic polymers bear phosphazene units as side groups. This is discussed in another Chapter. 

In the rest of this chapter an attempt will be made to describe how this field developed, how polyphosphazenes are synthesized, how the system provides almost unprecedented opportunities for the design of new macromolecules, and how the molecular structure-property relationships have been developed to produce a wide range of advanced materials. 

From information presented earlier in this chapter, it will be clear that three types of molecular structures in polyphosphazenes are known to give rise to rubbery or elastomeric properties. These are summarized in Table 3.1. First, if the side groups are single atoms, such as fluorine, chlorine, or bromine, the inherent flexibility of the backbone dominates the materials flexibilty and gives rise to elastomeric character. Unfortunately, the polymers (NPF2) , (NPC12) , and (NPBr2) are slowly hydrolyzed in contact with atmospheric moisture. Hence, they are of interest as reaction intermediates but not as usable materials. 

Recently, a mixed-substituent polyphosphazene (polymer V) was synthesized and the second-order NLO properties were investigated (17). The nitrostilbene/trifluoroethoxy ratio was approximately 36 64. Due to the low glass transition temperature of V (T - 25 C), the second harmonic signal decayed to zero within a few minutes. However, polymer V is a prototype which offers many opportunities for further tailoring the molecular structure of polyphosphazenes to generate an optimum combination of NLO and physical properties (17). 

Chart 1. Molecular structures of polyphosphazenes mentioned in this article. WCA water contact angle. 

Sun, H. Molecular structures and conformations of polyphosphazenes, a study based on density functional calculations of oligomers, J. Am. Chem. Soc. 1997, 119, 3611-3618. 

JThe effect of the substituent on the properties of the polyphosphazenes is not fully understood. For instance, [NP(OCH ) ]n and [NP C CH. homopolymers are elastomers (8,29). Synthesis using lithium, in contrast to sodium, salts is claimed to produce rubber-like fluoroalkoxyphosphazene polymers (30). The presence of unreacted chlorine or low molecular weight oligomers can affect the bulk properties (31,32). Studies with phosphazene copolymers both in solution and in the bulk state (29,33-38) indicate a rather complex structure, which points out the need for additional work on the chain structure and morphology of these polymers. 

The early development of polyphosphazene science and technology focused on the preparation of polymers that are unreactive. However, the expansion of this field is being driven by the need for functional polymers that bear -OH, -COOH, SO3H, -NH2. and numerous other reactive units in the side group structure. This article reviews the methods that are being developed to accomplish this at both the molecular level and at surfaces. 

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