Cyclophosphazene-Containing Polymers

In the previous chapter we have seen inorganic polymers that contained a [P=N] backbone. These polymers can be prepared by two principal methods. 

Ring-opening polymerization of monomers such as N3P3CI6 or N3P3CI5R to the corresponding linear polymers followed by replacement of chlorines on the macromolecules by a variety of nucleophiles to afford polymers that contain P-0 or P-N linked side-chains. 

Catalytic or uncatalyzed polymerization of various kinds of N-silylphosphoranimines such as McsSiNPC, MesSiNPRRX (X=0CH2CF3, OPh, Cl). This method is the most important route for the preparation of poly(aUcyl/arylphosphazene)s. 

In this chapter we will examine polymers that contain intact cyclophos-phazene units [1-4]. Examples of these kinds of polymers are shown in Fig. 4.1. 

Polymer 2 (Fig. 4.1) is an example of a condensation polymer where a difionctional cyclophosphazene can react with an organic or an inorganic difiinctional reagent to afford a linear polymer. These polymers contain the cyclophosphazene ring as a repeat motif in the polymer backbone. These polymers have also been termed cyclolinear polymers. In this type of polymers also, the scope for variation is considerable, although in practice this has not been realized. Polymers of the type 3 (Fig. 4.1) are examples of in-termolecularly crosslinked cyclophosphazenes. These are reminiscent of thermoset polymers such as phenol-formaldehyde or melamine-formaldehyde resins. The presence of the cyclophosphazene units in the crosslinked matrix is expected to impart special properties. However, this family of polymers also has not yet fulfilled the promise that they seem to hold. 

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Chandrasekhar, V. and Athimoolam, A. (2002) New hybrid inorganic-organic polymers as supports for heterogeneous catalysis a novel Pd(0) metalated cyclophosphazene-containing polymer as an efficient heterogeneous catalyst for the Heck reaction. Org. Lett., 4, 2113-6. 

Most of the polymers that contain cyclophosphazenes as pendant groups are prepared by polymerizing cyclophosphazene containing vinyl monomers. The vinyl group is attached to the cyclophosphazene ring either directly or through a spacer group. Recently, alternative procedures are also... 

Hexakis derivatives of cyclophosphazenes containing terminal phenol units (79) have been prepared (Scheme 13). Such compounds (79) are believed to have potential uses in polymer hybrid composites. ... 

During the past 5 years considerable progress has occurred in the structural chemistry of phosphazenes, substitution reactions, reaction mechanisms, synthetic procedures, and phosphazene high polymers. In this review, a broad outline of cyclophosphazene chemistry will be presented with an emphasis on the most recent work. The chemistry of phosphazene high polymers has been reviewed comprehensively in recent years (21, 22, 24, 412), and this topic will be mentioned only briefly. Cyclophosphazanes (21, 216) and phosphorines containing skeletal heteroatoms other than nitrogen and phosphorus (21, 249) are outside the scope of this review. 

There have been few important advances in this area during the past year. The first crystal structure of a phosph(m)azene, Bu (Me3Si)NP=NBu, has been reported, and the claim made last year, of the formation of a compound containing a phos-phazene linkage which forms part of a four-membered ring, has been shown to be erroneous. The formation and properties of cyclophosphazenes (mainly amino-derivatives) and the phosphazene polymers have been reviewed. The patent literature is the most important growth area, particularly where flameproofing applications are concerned. 

For a review on hybrid polymeric ligands and polymer-supported catalysts containing cyclophosphazenes see Chandrasekhar, V. Krishnan, V. Thilagar, P. C. R. Chim. 2004, 7, 915-925. 

An organic polymer containing a cyclophosphazene ring as a pendant group... 

There are many other polymers that are related to the polyphosphazene family. Thus, there are polymers which are actually made up of an organic backbone but contain a cyclophosphazene ring as a pendant group appended in their side chain. Other polymers that are related to polyphosphazenes are those that contain a third hetero atom in the backbone. Examples of such polymers are shown in Fig. 3.2. 

Polyphosphazenes, [N=PR2]n, are the largest family of inorganic polymers. Over 800 different types of polyphosphazenes are known [16, 21-23]. The common feature of all of these polymers is the backbone which is made up of alternate nitrogen and phosphorus atoms. While the nitrogen is trivalent and dicoordinate, phosphorus is pentavalent and tetracoordinate. Thus, nitrogen does not contain any substituents. Phosphorus, on the other hand, is attached to two substituents. Thus, the basic sfructural feature of the repeat unit of polyphosphazenes is exactly similar to what is found in cyclophosphazenes. 

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