Direct functionalization of poly(spirophosphazene)s via the regioselective lithiation of the aromatic rings using a cooperative super-base

The synthesis of functional materials having tailor-made properties is a topic of growing interest due to their applications in nanotechnology, health and energy issues. In this regard, polyphosphazenes, [N=PR2]n, are among... [ view full abstract ]

The synthesis of functional materials having tailor-made properties is a topic of growing interest due to their applications in nanotechnology, health and energy issues. In this regard, polyphosphazenes, [N=PR₂]_n, are among the most versatile class of inorganic polymers allowing the creation of well-defined functional macromolecules. During the last years, our research group has developed the chemistry of polyphosphazene containing cyclic dioxy-biphenyl-phosphorus units, [N=P(O₂C₁₂H₈)], such as the homopolymer [N=P(O₂C₁₂H₈)]_n (1), and the random copolymers of general formula {[N=P(O₂C₁₂H₈)]_1-x[N=P(FG)₂]_x}_n carrying a variety of functional groups (FG). However, the direct functionalization of the cyclic dioxy-biphenyl-phosphorus units has been very little explored. Here, we report that the direct chemical functionalization of poly(spirophosphazene) [N=P(O₂C₁₂H₈)]_n (1) can be performed by the lithiation of the aromatic rings at low temperature using the Schlosser´s base (Li^tBu/KO^tBu or “super base”), and the subsequent reaction with various electrophiles such as Cl-SiMe₃, Cl-PPh₂ or MeO-B(O₂C₆H₁₂) (MeO-Bpin). The functionalized polymers, isolated in very high yields (> 90%) and without degradation of the polymeric chains, have an average degree of substitution per repeat unit ranging from 0.3 (random copolymers) to a maximum of 1.0, which corresponds to the homopolymers [N=P(O₂C₁₂H₇-FG)]_n (FG (functional group) = -SiMe₃, -PPh₂, and -Bpin). NMR studies, including bidimensional high temperature COSY, HSQC and HMBC, on silylated and deuterated polymers show that the substitution is regioselectively occurring at the C₃ carbon of the aromatic rings due to the coordination of the lithium cations to the nitrogen of the polyphosphazene chain. The introduction of functional groups on the aromatic rings leads to significant changes in the solubility (silylated polymers), T_g, and electronic properties of the material, showing how the change of substituents in the aromatic rings can lead to polyphosphazenes with properties markedly different from those of the precursor polymer.