Synthesis of Poly(alkyl/arylphosphazenes) and Their Precursors

Abstract

Poly(alkyl/arylphosphazenes), [R 2 P=N] n , are a class of phosphazene polymers in which all of the side groups are attached to the P-N backbone via direct phosphorus-carbon bonds. While other types of poly(phosphazenes) are usually prepared by the ring-opening polymerization of the cyclic trimer, [Cl 2 P=N] 3 , followed by nucleophilic substitution of the P-Cl groups along the polymer backbone, the poly(alkyl/arylphosphazenes) are directly obtained by the thermal condensation polymerization of N-silylphosphoranimines, Me 3 SiN=PR 2 X. This paper reviews (1) the overall scope and limitations of this condensation polymerization, (2) some general properties of the title polymers, (3) recent improvements in the synthetic methodology including catalysis of the polymerization process as well as simplification of the precursor synthesis, (4) observations relevant to the polymerization mechanism such as leaving group effects, and (5) the synthesis, structure, and reactivity of some related small molecule species including N-silylphosphoranimines with bulky silyl groups, acetylene-substituted Si-N-P systems, and new Nphosphoryl-phosphoranimines, e.g., Cl 2 P(O)-N=PR 2 X.Since the mid-1960's, a large number of polyphosphazenes, [R2PN] n , with a great variety of substituents at phosphorus have been prepared and characterized.(1) Because of the diversity of possible side groups, phosphazene polymers exhibit a very broad spectrum of useful chemical, physical, electrical, optical, and biological properties. (2-4). This diversity of side groups is a direct result of the fact that there are varied and complimentary methods for the synthesis of the phosphazene backbone and, more importantly, for the attachment of the desired side groups to the phosphorus centers along the P-N chain. The first, and most extensively studied, approach is the ring-0097-6156/94/0572-0232$08.00/0

Deprotonation−Substitution Reactions of Poly(Methylalkylphosphazenes) and TheirN-Silylphosphoranimine Precursors

“…Preparation of Phosphoranimines 4 and 5, Me 3 SiN  P(OPh)(CH 3 )(R). The following preparation is an adaptation of two published methods. , A 5 L, three-neck flask, equipped with an N 2 inlet, a mechanical stirrer, and a pressure equalizing addition funnel with a rubber septum, was charged with hexamethyldisilazane (typically, 0.8 mol) and diethyl ether (500 mL). The solution was cooled to 0 °C, and an equal molar portion of n -BuLi was added.…”

“…With the exception of poly(dimethylphosphazene), [Me 2 PN] n , the semicrystalline dialkyl-substituted homopolymers (e.g., [Et 2 PN] n , [Pr 2 PN] n ) are insoluble in common organic solvents, and all are insoluble in coordinating solvents such as THF or ether, , thus limiting the scope of their derivative chemistry to surface reactions. The methylalkyl -substituted phosphazene homopolymers, [Me(R)PN] n where R = Et, Pr, Bu, or Hex, on the other hand, are amorphous and are soluble in solvents such as THF, CH 2 Cl 2 , hexane, ether, benzene, ethyl acetate, and methanol. , In addition to enhanced solubility, the poly(methylalkylphosphazenes) generally have glass transition temperatures well below 0 °C. , Thus, deprotonation−substitution reactions of these systems offer ample opportunity to further vary and control the properties of poly(phosphazenes). In the work reported here, we examined the deprotonation−substitution reactions of both the methylbutyl- and methylhexyl-substituted poly(phosphazenes) and their phosphoranimine precursors.…”

Synthesis of Trifluoromethyl- and Methylphosphazene Polymers: Differences between Polymerization and Initiator/Terminator Properties

“…The development of new synthetic methods for the production of polyphosphazenes, (NPR 2 ) n with unique physical and chemical properties, is an important objective. − The various properties of these polymers are normally controlled by the side groups along the polymer backbone. Traditionally, most polyphosphazenes have been prepared by the thermal ring-opening and macromolecular substitution route developed in our program .…”

“…Traditionally, most polyphosphazenes have been prepared by the thermal ring-opening and macromolecular substitution route developed in our program . The initial step in this approach is the thermal ring-opening polymerization of hexachlorocyclotriphosphazene, (NPCl 2 ) 3 , which yields the reactive macromolecular intermediate, poly(dichlorophosphazene), (NPCl 2 ) n . − Subsequent replacement of the chlorine atoms in (NPCl 2 ) n with alkoxy, aryloxy, and/or amine nucleophiles has yielded several hundred different poly(organophosphazenes). However, this route allows for only minimal control of the molecular weight and generates polymers with broad polydispersities …”

“…This synthetic method involves the living cationic-induced polymerization of Me 3 SiNPCl 3 ( 1 ) with trace amounts of PCl 5 (Scheme ) to yield polymers with well-defined molecular weights and narrow polydispersities . To date, the versatility of this living route has been extended to the synthesis of block copolymers, − star-branched polymers, and end-functionalized polyphosphazenes. ,

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