Alkyl/arylphosphazenes: new polymers, cyclic analogs, and precursors

Abstract

The alkyl/arylphosphazenes are a new class of linear polymers and cyclic analogs which are fully substituted with alkyl or aryl groups that are directly bonded to the PN backbone by P-C bonds. Depending on the substituents, the properties of poly(phosphazenes) may vary considerably and in this study the effects of varying alkyl substituents were investigated. The polymeric compounds of this type can be synthesized from the thermal condensation polymerization of P-trifluoroethoxy-N-silylphosphoranimines Me$\sb3$SiN=P(R)(R$\sp\prime$)OCH$\sb2$CF$\sb3$ or P-phenoxy-N-silylphosphoranimines Me$\sb3$SiN=P(R)(R$\sp\prime$)OPh. These compounds polymerize with the elimination of the corresponding trimethylsilyl ether Me$\sb3$SiOCH$\sb2$CF$\sb3$ or Me$\sb3$SiOPh. In our investigation, two series of phosphazene homopolymers $\rm \lbrack N{=}P(R\sp1)(R\sp2)\rbrack \sb{n}$ were prepared which were based on poly(dimethylphosphazene) (R$\sp1$ = R$\sp2$ = Me). The two series consisted of the dialkyl homopolymers (R$\sp1$ = R$\sp2$ = Et, n-Pr, n-Bu, n-Hex) and the methylalkyl homopolymers (R$\sp1$ = Me; R$\sp2$ = Et, n-Pr, n-Bu, n-Hex). These two types of polymers differed substantially in their physical properties. The dialkyl homopolymers were microcrystalline solids which exhibited high melting transitions (ca. $>$ 150$\sp\circ$C) and insolubility in common organic solvents. However, they could be dissolved in acidified solutions of CH$\sb2$Cl$\sb2$ or CHCl$\sb3$. The methylalkyl homopolymers were soluble, amorphous materials (gums, waxes) which typically exhibited low glass transition temperatures (ca. $-50\sp\circ$C). Two copolymer series based on the well characterized poly(dimethylphosphazene) $\rm (\lbrack Me\sb{2}PN\rbrack \sb{x}\lbrack R\sp{1}(R\sp{2}) PN\rbrack \sb{y})\sb{n}$ and poly(methylphenylphosphazene) $\rm (\lbrack Ph(Me)PN\rbrack \sb{x}\lbrack R\sp{1}(R\sp{2}) PN\rbrack \sb{y})\sb{n}$ were also synthesized by the copolymerization of the two appropriate P-trifluoroethoxy-N-silylphosphoranimine precursors in the ratio of x to y. In the comparison between the dimethyl dialkyl copolymers (R$\sp1$ = R$\sp2$ = Et, n-Pr, n-Bu, n-Hex) and the dimethyl methylalkyl copolymers (R$\sp{1}$ = Me; R$\sp{2}$ = Et, n-Pr, n-Bu, n-Hex), it was apparent that the symmetrical distribution of substituents in the dimethyl dialkyl copolymers tends to yield microcrystalline solids with increasing solubility as the x:y ratio increases. However, the unsymmetrical distribution of substituents seen in the dimethyl methylalkyl copolymers and the phenylmethyl copolymers leads to amorphous structures (gums, waxes) with no observable melting point. The dialkyl cyclic phosphazenes $\rm \lbrack R\sb2 PN\rbrack \sb{3,4,5}$ (R = Et, n-Pr, n-Bu, n-Hex) were synthesized by the thermal decomposition reactions of P-halo-N-silylphosphoranimines Me$\sb3$SiN=PR$\sb2$X (X = Br, Cl) with the elimination of the corresponding trimethylsilyl halide Me$\sb3$SiX. In their solubility and spectral properties, these compounds were reasonably good models for the dialkyl homopolymers, since the cyclic analogs were insoluble in common organic solvents, but could be dissolved in solutions of CH$\sb2$Cl$\sb2$ or CHCl$\sb3$ acidified with weak organic acids.