isomers may provide an avenue to new classes of highly functionalized polyolefins via ring-opening metathesis polymerization (ROMP). Although relatively less studied, ROMP of strained cyclobutenes is well established and typically accomplished using Grubbs 2 nd (G2), Grubbs 3 rd (G3), or Hoveyda-Grubbs 2 nd (HG2) generation catalysts. 5b, 9 For instance, Bielawski and coworkers reported the ROMP of a dibromo derivative of Dewar benzene, which upon MeLitriggered elimination rapidly converted into transpoly(acetylene) (Figure ). More recently, Xia and coworkers demonstrated the ROMP synthesis of poly(ladderane)s and poly(benzoladderene)s that could be mechanochemically transformed into polyacetylene derivatives. Different from these polymeric materials, in which rearrangements are triggered by ring-opening of cyclobutene or unzipping of multiple fused rings, the ROMP of pyridinones has recently been shown to result in functional polymers that incorporate 4-membered lactam heterocycles (Figure ). 5b Similarly, we anticipated that ROMP of BN-Dewar isomers could offer access to a novel class of polyolefins that retain the four-membered heterocycle containing an amine and a borane moiety. Here we report the first synthesis of poly(BN-Dewar benzene)s via Ru-catalyzed ROMP of a bicyclic azaboretidine and their structural corroboration by multinuclear and two-dimensional NMR, GPC, FTIR and Raman experiments. The new approach to highly functionalized polyolefins that is presented here is very attractive as the B-N heterocycles can potentially be further elaborated into amine, hydroxyl and other polar functional groups through postmodification approaches that involve B-C and/or B-N bond cleavage. [a] Analyzed by gel permeation chromatography with refractive index (GPC-RI) detection relative to narrow polystyrene standards; Đ = Mw / Mn. [b] Conversion estimated for the crude product based on 1 H NMR integration of the t-butyl H NMR signal of the residual monomer relative to the t-butyl H signal of the polymer; GPC analysis of crude product in THF. [c] Conversion estimated for the crude product based on 1 H NMR integration of the olefinic signals of the residual monomer relative to anisole as a reference; GPC analysis of isolated product in THF.