The chemical structure of the new polymers was confirmed by 1 H, 11 B, and two-dimensional (2D) NMR spectroscopy. The disappearance of the characteristic olefinic group signals and pronounced peak broadening in the 1 H NMR spectra clearly indicated successful ROMP of the strained cyclobutene rings in Dewar isomer 1 (Figure ). In the 11 B NMR spectra of both 2 and 2-Z, a significant upfield shift from ca. 53 to 45 ppm and concomitant peak broadening provided further evidence for the successful polymerization (Figure ). An upfield shift is frequently observed upon polymerization of borane monomers as a result of shielding effects of the neighboring groups along the polymer chain. To further confirm the connectivity between the four-membered BN-heterocycles and vinylene groups in the polymer main chain, heteronuclear single-quantum correlation (HSQC), heteronuclear multiple-quantum correlation (HMBC), and nuclear Overhauser effect spectroscopy (NOESY) NMR data were acquired. The HSQC and HMBC data of 2 and 2-Z showed the expected cross peaks for the mesityl and tert-butyldimethylsilyl groups (Figures S10-11, S15-16). The backbone protons of polymers 2 and 2-Z in the range from 2.7-6.5 ppm could be assigned based on a NOESY NMR analysis (Figure ) and supported by DFT methods. To gain insights into the relative distances between protons we optimized the geometry of head-totail model dimers with vinyl end groups, having either a cis-and trans-vinylene linker. The methine protons H1 and H4 were assumed to retain the syn configuration that is seen in the Dewar-azaborinine precursor 1 and only one of the possible diastereomers is illustrated (relative stereochemistry of C atoms in the B-N heterocycles). The calculated Gibbs free energy was higher for the isomer with a cis-olefin linkage by 20.8 kJ mol -1 relative to that with a trans-olefin linkage, which suggests that formation of trans-linkages is thermodynamically more favorable (Table ). The structure of the trans-isomer is displayed in Figure , illustrating some of the closest intramolecular H … H distances. The distances in this model dimer were then used to assign the NOESY spectrum of the polymer (Figure and Table ). The allylic methine protons H1 and H4 are expected more upfield than H2 and H3. The upfield signal at 3.4 ppm was assigned to H1 as it shows the expected intense NOE peak at 3.4/2.6 ppm due to its proximity to the boron-bound mesityl group (H9). The orthomesityl protons (H9) are also in close proximity to H2 resulting in another dominant cross peak 5.8/2.6 ppm. A third strong cross-peak at 3.4/5.2 ppm is attributed to the NOE between H1 and H4 which are in adjacent positions and share the same orientation. Finally, the assignment of H3 at 6.0 ppm is based on an NOE peak with H2 at δ = 6.0/5.8 ppm. The separation of H2 and H3 within a single trans-vinylene unit is large, but H2 comes in close contact to H3' in the next vinylene repeating unit and vice versa. Additional weaker NOE peaks indicated the presence of a relatively smaller number of cis-vinylene linkages, or possibly some head-to-head defects (Figure ). For polymer 2-Z which was prepared with HGM2001 as a Z-selective catalyst, the 1 H NMR signals were much sharper than those of 2 and appeared at different chemical shifts (Figure and Table ). The presence of a dominant set of sharp signals in the region from 2.5-5.5 ppm allowed for unequivocal assignment of all the protons within the B-N heterocycles and vinylene linkers. The NOE peaks were in excellent agreement with the proposed head-to-tail structure with cis-linkages. Notably, the mesityl protons were split into two sets suggesting that rotation about the B-C bond is hindered. Collectively, these data suggest the presence of predominantly but not exclusively trans-linkages for polymer 2 (the exact E/Z ratio is difficult to determine because the 1 H NMR signals are heavily broadened) and a highly regioregular structure with dominant formation of cislinkages for polymer 2-Z (>90% Z).