Phosphazene architectures

The parent polymer by itself is not a useful material owing to the extreme hydrolytic sensitivity of the P-Cl bond. However, this feature has been turned around and used as an advantage. Nucleophilic substitution of the chlorines in the polymer results in substituted polyphosphazenes which are hydrolytically stable. Also, using this method the polymer architecture and properties are readily fine-tuned by a subtle variation of the substituent. Over three hundred types of polyphosphazenes have been synthesised by this method. Assembly of organic polymers containing cyclo-phosphazenes as pendant groups is another approach that is gaining importance [6]. 

In addition to linear polyphosphazenes with one type of side group, as shown in 3.1, other molecular architectures have also been assembled. These include polyphosphazenes in which two or more different side groups, R1 and R2, are arrayed along the chain in random, regular, or block distributions (3.2-3.4). Other species exist with short phosphazene branches linked to phosphorus atoms in the main chain (3.5,3.6). Also available are macromolecules in which carbon or sulfur replace some of the phosphorus atoms in... 

One group of polyphosphazenes has received considerable attention because of their advantages for lithium ion conduction. These are polyphosphazenes with oligo-ethyleneoxy side groups. They are characterized by their high molecular flexibility and their ability to coordinate weakly to lithium and other monovalent ions. A well-known example is poly[to(methoxyethoxyethoxy)phosphazene) (3.79), also known by the acronym MEEP (MethoxyEthoxyEthoxyPhosphazene). Numerous other polyphosphazenes with different alkyl ether side groups and different architectures have also been studied as ionic conductors. 

Step-by-step growth of phosphazene chains can be achieved and the five-phosphoms chain can be converted into a dendrimeric stmcture. The fascinating chemistry of dendrimeric phosphors nitrogen molecnlar architecture has been reviewed. ... 

Phosphazene polymers comprise a class of several hundred different macromolecules with the general formula shown in Structure 1, where R represents organic, organometallic or inorganic side groups. Two structural factors can be varied for these polymers. First, the basic skeletal architecture varies from linear polymers or block copolymers to stars, dendrimers, combs, cyclolinear or cyclomatrix structures. Second, more than 250 different side groups have been linked to the various skeletons. Most of the polymers that have been studied in detail are linear macromolecules of type 1 or block copolymers formed between 1 and classical organic polymers. 

Furthermore, despite still being the route able to prepare the highest Mjy, ROP inherently produces polymers with broad polydispersities (Mjy/M >2) due to its initiation mechanism, in which the formation of new chains can occur throughout. Although such polydispersity is perfectly tolerable for many medical applications, for example, as inert biomaterials, the method is less suitable for some biomedical applications, in which precise molecular size is often an essential property. Furthermore, advanced polymer architectures and macromolecular constructs cannot be readily attained via this method, due to the absence of end-group control, and hence the development of poly(dichloro)phosphazene with controlled... 

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