Polymers Containing Cyclophosphazenes as Pendant Groups

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 

Since cyclophosphazene-containing vinyl monomers are really a special case of CH2=CHG type of monomers where G is a cyclophosphazene group, in principle all the polymerization methods for polymerizing conventional vinyl monomers should also be equally effective for this system. In practice, however, only the free-radical methods have so far been found to be successful. Recently, ring-opening metathesis polymerization has also been successfully implemented in this system. The apparent limitations of the other methods of polymerization with respect to the cyclophosphazene-containing vinyl monomers arise from the following factors  

If halogenocyclophosphazenes are the pendant groups, clearly anionic initiators (such as n-butyllithium) or reactive organometallic initiators (such as Ziegler-Natta catalysts) cannot be used because they would first react with the more reactive P-X bonds before they can initiate polymerization. 

Similarly, cationic initiators such as protic or Lewis acids jne also not very effective as polymerization catalysts for these systems. The basicity of the skeletal nitrogen atoms of the cyclophosphazene rings overwhelms the reactivity of the cationic initiators towards olefin polymerization. Thus, the cyclophosphazene rings are readily protonated or metalated and thus prevent both protic and Lewis acids from initiation of polymerization. 

1 Synthesis of Cyclophosphazene Monomers Containing Vinyl Groups 

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One of the recent methods that has been applied for preparing polymers containing cyclophosphazene rings as pendant groups is by ring-opening metathesis polymerization (ROMP). The principles involved in the application of this strategy to cyclophosphazene monomers are as follows. 

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. 

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