The Dewar isomer 1 was prepared as previously reported by photoisomerization of B-mesityl-N-(t-butyldimethylsilyl)azaborinine (A) as described in Figure . To preclude any thermal reversion to the azaborinine, the monomer was stored in a freezer at -28 ºC prior to use. We explored different generations of Grubbs and Hoveyda-Grubbs catalysts for the ROMP of monomer 1 (Figure ). The monomer conversion was determined by 1 H NMR integration and the results are summarized in Table . Grubbs 3 rd generation catalyst (G3) is one of the most widely used ruthenium catalysts for ROMP. In comparison to 2 nd generation catalysts (G2, HG2), G3 exhibits very fast initiation rates, which typically enables the formation of polymers with very narrow dispersities and excellent control over their molecular weights. However, G3 was found to be not very effective at converting Dewar isomer 1 to polymer 2. At 0.1 M monomer concentration in benzene or dichloromethane, G3 gave only low monomer conversion even after very long reaction times at either room temperature or 0 ºC (Table , entry 1, 2), and conversions were even lower when using THF as the solvent. This contrasts the successful controlled polymerization of other cyclobutene derivatives reported in the literature. A possible reason could be that the pyridine base that is liberated during the initiation step interferes by binding to the centers. However, no change in chemical shift was observed in the 1 H or B NMR spectra when monomer 1 was treated with an equimolar amount of pyridine. Another possibility is that the initiation step is very fast, as expected, but the stability of the initial ring-opening product and the propagating species in its resting state is insufficient. An indication that this is the case comes from an 1 H NMR experiment in which the monomer was mixed with G3 in a 10:1 molar ratio in C6D6. Upon mixing the color of the solution immediately changed from yellow-green to orange-brown. An NMR spectrum taken within <10 mins showed that the carbene proton of G3 disappeared completely and multiple new peaks arose in the region from -16 to -20 ppm (Figure ). These peaks gradually disappeared with longer reaction times. This is indicative of very rapid but unselective initiation and insufficient selectivity/stability of the propagating species. G2 and HG2, which do not contain unhindered basic pyridine ligands, are known to exhibit improved thermal stability, as well as oxygen-and moisture-tolerance. Although G2 and HG2 are usually not suitable for living polymerization due to the slow initiation and small initiation/propagation rate ratio (ki/kp), they have been successfully applied to the polymerization of cyclobutenes when other catalysts gave poor results. An 1 H NMR analysis after 10 mins of mixing 1 and G2/HG2 in a 10:1 molar ratio in C6D6 again revealed complete disappearance of the carbene proton of the catalyst with formation of several new peaks. The new carbene peaks were present almost exclusively in the region of 17.5 to 18.5 ppm, indicating more selective formation of isomeric propagating species with G2 and HG2 (Figure ). Indeed, performing the ROMP of 1 at room temperature with either G2 or HG2 in a 100:1 molar ratio proved to be more effective, resulting in higher monomer conversion (>90%) over a shorter time period of 7-9 hours (Table , entry 3-4). As seen in Table , entries 5-6, higher monomer concentration (0.3 M) promoted propagation, further shortening the reaction time to 4.5 h. HG2 consistently gave slightly higher conversions, indicating a faster rate of polymerization. The use of HGM2001, which is a Z-selective catalyst, yielded polymers with an increased number of cis-linkages than G2 and HG2 (vide infra). Remarkably, an 1 H NMR analysis after 10 mins of mixing 1 and HGM2001 in a 10:1 molar ratio showed the formation of only two broad overlapping peaks at ca. 15.5 ppm, suggesting far superior regio-and stereoselectivity in the initiation and propagation steps (Figure ). However, only 31% conversion was achieved over 24 hours at room temperature when using a 100:1 molar ratio of 1 to HGM2001 (Table , entry 7). An increase of the reaction temperature to 50 °C did not significantly increase the monomer conversion but led to competing cycloreversion to the azaborinine A (not observed at RT, see Table ).