Poly(iminoborane)s: An Elusive Class of Main Group Polymers

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Polymers with a backbone comprised of inorganic elements often show useful properties and functions that complement those of state-of-the-art organic macromolecular materials. Prominent examples are the well-known silicones, which are nowadays regarded as commodity products. Poly(iminoborane)s (PIBs) constitute a class of inorganic main group polymers with a linear chain of alternating boron and nitrogen atoms. Considerable theoretical interest has been devoted to these species, as they are formally BN analogs of the important semiconducting organic polymer polyacetylene. However, despite extensive experimental effort, poly(iminoborane)s have never been prepared and convincingly characterized thus far.The aim of this research project is to develop novel synthetic routes that enable the synthesis of poly(iminoborane)s with well-defined microstructure for the first time. The failure of previous attempts to obtain linear PIBs was generally due to the susceptibility of potential BN monomers to form cyclic iminoborane trimers (i.e., borazines, BN analogs of benzene) instead. This project focuses on two strategies to prevent ring formation during the polymerization process. The idea of the first approach is to link two boron or two nitrogen atoms of the backbone via a hydrocarbon bridge. This provides a geometrical constraint that makes the unwanted cyclization with formation of borazines impossible. Poly(iminoborane)s composed of various types of heterocyclic systems are targeted that may be aromatic, non-aromatic, or anti-aromatic.The idea of the second approach is to develop novel chain growth polycondensation processes to access poly(iminoborane)s. The growing polymer chain generated should have functional groups of the same kind on both chain ends. Consequently, the competing formation of cyclic species, which would require the presence of two complementary end groups, should be prevented at any stage of the polymerization process. Novel, possibly catalytic B-N bond formation methods will be developed and their applicability in polymerization procedures will be explored.Besides interesting electronic features anticipated to be associated with this novel class of macromolecules, solution-processable poly(iminoborane)s may be valuable polymeric precursors for the fabrication of shaped ceramics such as hexagonal boron nitride, a desirable insulator and thermal conductor that is isoelectronic with graphite/graphene. Boron-rich materials are also of interest for use in boron neutron capture therapy (BNCT) for cancer. Moreover, the synthesis of PIB block copolymers will be pursued with this project. Self-assembly of the resulting materials will be investigated for applications in nanoscience.