Polyphosphazenes condensation polymerization

The conventional route to prepare I generally involves a high temperature melt polymerization of hexachlorocyclotriphosphazene, or trimer (IV). Recent studies have demonstrated the effectiveness of various acids and organometalllcs as catalysts for the polymerization of IV (8). Alternate routes for the preparation of chloro-polymer which do not involve the ring opening polymerization of trimer have been reported in the patent literature (9. 10). These routes involve a condensation polymerization process and may prove to be of technological importance for the preparation of low to moderate molecular weight polyphosphazenes. 

In many respects, the polyphosphazenes are the prototype inorganic backbone polymers, that exemplify the principles of ring-opening and condensation polymerization, macromolecular substitution reactions and their potential for molecular design, and an enormous range of derivatives with the same backbone but different organic side groups. 

The polymeric phosphazenes are treated in chapter (see Polyphosphazenes) A recent monograph covers the chemistry of polyphosphazenes (nomenclature, synthesis of cyclic monomers , ring opening polymerization, condensation polymerization, substitution, polymer properties, and applications more than 1000 literature citations). Other reviews have also been published recently. Sulfur-containing polyphosphazenes have also been described. ... 

Cationic condensation polymerizations of Cl3P=NSiMe3 and PhCl2P=NSiMe3 in the solvents benzene, toluene and dioxane, and initiated by PCI5, appear to be reproducible and result in polymers with a low polydispers-ity. Diblock and triblock polyphosphazene-polystyrene copolymers have been synthesized by quenching the living polymer (135) by a polystyrene phos-... 

This synthetic gap has been bridged by the development of another process that involves the condensation polymerization of suitably constructed sllicon-nitrogen-phosphorus compounds. The most significant feature of this method is that it leads directly to fully P-C substituted polyphosphazenes, i.e., poly(alkyl/arylphosphazenes), [RR PN]p, where R and R are alkyl and/or... 

A very promising variant on this type of condensation polymerization involves the use of monomers that possess groups X and Y, which can be eliminated from the same molecule. This circumvents the need for careful control of reaction stoichiometry. Moreover, in certain cases, polymerization of monomers of this type can follow a chain-growth type of mechanism that leads to high molecular weights much more easily. Such processes (Scheme 1.1, Route C) have not yet been explored for the formation of metal-containing polymers, but are well-established for the synthesis of certain classes of polymers based on main-group elements such as polyphosphazenes (1.2) and polyoxothiazenes (1.4) [12, 16, 26]. 

An alternative approach to the synthesis of polyphosphazenes has also been reported [29, 30). In this process, the condensation reactions of N-silylphosphor-animines yield medium molecular weight poly(alkyl- and atyl-phosphazenes). The advantage of this route is that it yields polymers in which all the side groups are alkyl-or aryl- units bonded to phosphorus through carbon-phosphorus bonds - precisely those structures that are the most difficult to prepare by the classical route. Catalysis of these condensation polymerizations by BU4NF has also been described [31]. It has been demonstrated [32] that alkyl side groups can be lithiated, and these sites can form the basis of a lithium-replacement macromolecular substitution chemistry. 

Polyphosphazenes with simple alkyl and aryl substituents directly attached to the backbone by P-C linkages can be prepared by the condensation polymerization of N-silylphosphoranimine precursors. These simple polymers can then be converted to a variety of functionalized polyphosphazenes by derivatization reactions. In this paper, the synthesis and characterization of some derivatives of poly(methylphenyl-phosphazene), [Me(Ph)P=N]and the copolymer, [Me(Ph)P=N]j [Me2P=N)y, are discussed. These polymers include grafted copolymers, water soluble carboxylated polymers, and polymers with silyl, vinyl, alcohol, ester, ferrocene, phosphine, thiophene, and/or fluoroalkyl groups. 

The poly(alky1/arylphosphazenes) are a special type of phosphazene polymer in which all substituents are attached to the backbone phosphorus by direct P-C linkages. Unlike the majority of polyphosphazenes, which are usually prepared by ring opening of fully (3 - 5) or partially (6 -8) halogenated cyclic phosphazenes followed by nucleophilic substitution of the halogens, the poly(alkyl/arylphosphazene) homopolymers and simple copolymers are made by the condensation polymerization (9, 10) of Si-N-P compounds known as N-silylphosphoranimines (eg 1 and 2). 

Other methods of polyphosphazene synthesis have also been reported [17]. The most interesting is the living cationic condensation polymerization of phosphoranimines [21,22] as it offers access to polyphosphazenes with controlled molecular weight, low polydispersity, and more complex architectures, e.g. block or graft copolymers. At the present time, only moderate molecular weights (Mw = 10 -10 ) are available via the condensation route. 

Later on, such ambient temperature synthesis approach was extended to a variety of organo-substituted phosphoranimines, to directly synthesize polyphosphazenes with controlled molecular weight and low polydispersities, so that PDCP preparation and following chlorine substitution steps were eliminated [26]. Such living cationic condensation polymerization method also allows the preparation of polyphosphazenes with complex structures such as comb, star, and dendritic architectures, as well as block and graft copolymers with organic macromolecules [18]. 

Modifications in the preparation of polyphosphazenes by the condensation polymerization of N-silylphosphoranimines are also reported. The preparation of a series of new poly(alkyl/arylphosphazenes), relevant chemistry of the Si-N-P precursors, and catalysis by phenoxide anions are presented (NeUson, Chapter 18). Matyjaszewski (Chapter 24) discusses the use of fluoride catalysts to prepare polyphosphazene random and block copolymers with alkoxy and fluoroethoxy substituents and proposes mechanisms for their formation. 

The most striking difference between conventional polymers and poly(organophosphazenes) is in their method of synthesis. The normal techniques for the synthesis of macromolecules - i.e. the polymerization of unsaturated monomers or the condensation reactions of difunctional monomeric reagents - are not applicable to polyphosphazene synthesis. Monomers of structure, N=P(0R)2, N = P(NHR)2, N = P(NR2)2> N=PR2, have not yet been isolated. 

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