Cyclophosphazene polymer

Very intensive research into thermal and fire resistant polymers continues. An interesting target of these investigations is the cyclophosphazene polymers. The studies aim not only at the development of new more heat-resistant and less combustible resins but also at improving the processability of the current grades. These efforts have lead to the commercialization of a large number of thermally stable resin grades, as listed in Table 5.17. 

Derivatives of the type, RjP N=P(NH2)X 2, were obtained by displacement of the chlorine atoms marked with an asterisk by ammonia. The parent halides decomposed at ca. 150 °C giving, amongst other products, cyclophosphazenes, (NPR2) , and phosphazene polymers. 

Cyclophosphazenes are a fascinating group of inorganic heterocyclic compounds whose chemistry is multi-faceted, well developed and reasonably well understood. They are closely related to the linear poly-phosphazenes this relationship is unlike any other existing between ring-polymer systems. Although cyclic siloxanes and polysiloxanes have a close interrelationship, the number and types of cyclophospha-zene derivatives that are known, together with their exact counterparts in polyphosphazenes, underscore the utility of cyclophosphazenes as models for the more complex polyphosphazenes. The literature on cyclophosphazenes has appeared earlier in the form of books (1,2), chapters of books (3-5), authoritative compilations of data (6,7), and several reviews (8-21). The current literature on this subject is reported annually in the Specialist Periodic Reports published by the Royal Society of chemistry (22). This review deals mostly with chlorocyclo-... 

Most aromatic difunctional reagents react with N3P3Cl6 to afford spirocyclic products (20,176,180,181,189,190). With catechol, the trispiro product is observed (190). This product was shown to function as a host in the formation of several inclusion adducts, including polymers (191). Ring degradation of the cyclophosphazene ring occurs in the reaction with o-amino phenol as well as in the reaction with catechol in the presence of a triethylamine (192). 

Polyphosphazenes and cyclophosphazenes are almost unique as carrier molecules for transition metals because of the wide range of binding sites that can be incorporated into the phosphazene structure. The substitutive mode of synthesis described earlier allows a structural diversity that is not found, for example, in polystyrene, polyphenylene oxide, or other organic carrier polymers. 

Various aspects of the chemistry of cyclophosphazenes have been discussed in earlier chapters, viz. synthesis (see Scheme 2.1), electronic structure (see Section 4.1.2.2 and Figure 4.2), macrocyclic ring systems (see Section 6.2.2 and Figure 6.4) and ligand behaviour (see 7.20 and 7.21 in Section 7.1.3). Phos-phazene polymers are considered in Section 11.4.3. In this section, emphasis will be placed on new developments in cyclophosphazene chemistry that are not covered elsewhere in this book. 

When butadiene and 2,3-dimethylbutadiene are included in the channels of urea and thiourea, respectively, 1,4 addition invariably results to yield polymers with chemical and stereo regularities (Scheme 39). Note that addition in the 1,2 fashion is prevented sterically by the narrow channel. Similarly, high selectivity was obtained when butadiene, vinyl chloride, and styrenes were polymerized in the channels of cyclophosphazenes. Syndiotac-tic polymer alone is obtained from vinyl chloride included in urea channels this is apparently the first example of inclusion polymerization of a vinyl polymer in which control is exerted over the steric configuration of the developing tetrahedral carbon atom (Scheme 39). Highly isotactic polymer is obtained from 1,3-pentadiene when it is included in a perhydrotriphenylene matrix (Scheme 39). Note that addition could occur at either end (i.e., Q to... 

An alternative, and more recent, approach is shown in reaction (42).9091 Here, a cyclophosphazene is synthesized with two non-geminal alkoxy chains that bear terminal olefinic groups. Treatment of these compounds with an organometallic ADMET catalyst causes loss of ethylene and formation of a cyclolinear polymer. The chain lengths achieved by this method are generally longer than those produced by the dehydrohalo-genation technique. 

If a cyclophosphazene precursor molecule has more than two reactive sites (e.g. chlorine atoms), linkage reactions with diamines of dialkoxides will generate a three-dimensional structure rather than a linear chain. Such materials are known as cyclomatrix polymers (3.18). These species form a half-way stage between linear polymers and ceramics, and occupy a critical position in materials science. The term ultrastructure covers a broad range of materials of this type. 

Modesti, M. Zanella, L. Lorenzetti, A. Bertani, R. Gleria, M. Thermally stable hybrid foams based on cyclophosphazenes and polymethanes. Polym. Degrad. Stab. 2005, 87, 287. 

Early comprehensive reviews of phosphazene chemistry by Audrieth, Steinman, and Toy (43), Gribova and Ban-Yuan (214), Paddock and Searle (337), Shaw, Fitzsimmons, and Smith (401), and Schmulbach (388) were followed by reviews on specific aspects, such as preparative methods (402), structure and bonding (336, 407), and high polymers (254). Some excellent books dealing with the chemistry of cyclophosphazenes (13, 21, 216) have also appeared. The recent reviews of Allcock (22), Sowerby (418), and Keat and Shaw (249) describe the developments up to 1970-1971. Several short articles on this topic have also appeared from time to time (264, 403, 404, 408). Phosphazene chemistry is now reviewed annually (251). 

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. 

Cyclophosphazenes (46) nndergo both thermal ringopening polymerization with formation of linear polymers (4 7) see Polyphosphazenes) and ring-ring eqnilibria (resnlting in ring expansion). 

Another approach for the preparation of organic polymers with cyclophos-phazene side groups involves the Staudinger reaction of an azido- substituted cyclophosphazene with a phosphine residue in an organic copolymer. Free-radical copolymerization of styrene with diphenyl-p-styrylphosphine yields... 

Ring opening metathesis polymerization (ROMP) of norborenes with a cyclophosphazene side group (104) in presence of the ruthenium catalyst (PCy3)2Ru(Cl2)CHPh has shown to yield organic polymers with a cyclophosphazene as pendant group (105). °... 

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