Phosphaalkenes polymerization

Abstract Many similarities between the chemistry of carbon and phosphorus in low coordination numbers (i.e.,CN=l or 2) have been established. In particular, the parallel between the molecular chemistry of the P=C bond in phosphaalkenes and the C=C bond in olefins has attracted considerable attention. An emerging area in this field involves expanding the analogy between P=C and C=C bonds to polymer science. This review provides a background to this new area by describing the relevant synthetic methods for P=C bond formation and known phosphorus-carbon analogies in molecular chemistry. Recent advances in the addition polymerization of phosphaalkenes and the synthesis and properties of Tx-con-jugated poly(p-phenylenephosphaalkene)s will be described. 

This section will provide details of recent efforts to polymerize phosphaalkenes. It will begin with an introduction to the factors that must be considered when attempting to polymerize P=C bonds. In addition, a historical context will be provided since, perhaps ironically, it was so-called polymerization reactions that plagued early efforts to prepare compounds possessing heavier element multiple bonds. Finally, it will close with the first successful polymerization of a P=C bond to give poly(methylenephosphine)s. 

This review has shown that the analogy between P=C and C=C bonds can indeed be extended to polymer chemistry. Two of the most common uses for C=C bonds in polymer science have successfully been applied to P=C bonds. In particular, the addition polymerization of phosphaalkenes affords functional poly(methylenephosphine)s the first examples of macromolecules with alternating phosphorus and carbon atoms. The chemical functionality of the phosphine center may lead to applications in areas such as polymer-supported catalysis. In addition, the first n-conjugated phosphorus analogs of poly(p-phenylenevinylene) have been prepared. Comparison of the electronic properties of the polymers with molecular model compounds is consistent with some degree of n-conjugation in the polymer backbone. 

Soluble phosphine support 57 was prepared starting from mesityl-substituted phosphaalkene 56 (Scheme 37). This was co-polymerized with various equivalents of styrene to give a series of materials, soluble in dichloromethane, thf, and toluene, with phosphine loadings ranging from 5-39 mol%. This methodology in essence shows that P=C bonds can mimic C=C bonds in their polymerization chemistry and opens avenues for the synthesis of other phosphine-substituted supports since a wide range of phosphaalkenes are known. 

Reeently, condensation polymerization has been used for preparing poly(p-phenylene phosphaalkene) which contains P=C double bonds in the polymeric backbone. Thus, the reaction between the silylated phosphane, (Me3Si)2P-C6H4-p-P(SiMe3)2 and a diacid chloride affords an E/Z mixture of poly(p-phenylene phosphaalkene) [37]. 

Recently, an addition polymerization of the phosphaalkene, (Mes)2P=CPh2 (Mes = 2,4,6-Me3-CfiH2) has been reported [27]. Thus, after the vacuum distillation of the crude monomer (150 °C 0.1 mmHg) a residue was left behind. Analysis of this revealed it to be a high polymer. It was suspected that radical impurities might have caused the polymerization. Subsequently it was shown that the phosphaalkene can be polymerized by free-radical or anionic initiators to afford a low molecular weight M = 5,000-10,000) polymer (see Eq. 5.14) [27]. The P-NMR chemical shift of the polymer (-10 ppm) is considerably upfield shifted vis-a-vis the monomer (+233 ppm). 

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