The structural integrity of the four-membered BNheterocycles was further verified by comparison of the FT-IR spectra of the monomer and the polymers (Figure , for full spectra see Figure ). As expected, poly(1,2-azaborinine)s 2 and 2-Z displayed very similar spectral features. The individual IR bands were assigned based on comparisons with results from theoretical calculations (B3LYP/6-31g(d)) on monomer 1 and the trans-and cis-vinylenelinked head-to-tail model dimers (Table ), as well as previously reported experimental data for 1. Strong B-N stretching modes were observed for both the monomer and polymers (ca. 1354 / 1360-1361 cm -1 ). The characteristic C-H bending modes in the BN heterocycle of 1 (ca. 1179, 1136 and 979 cm -1 ), as well as the C-B (ca. 1038 cm -1 ) and C-N stretching modes (ca. 1252 cm -1 ) are also seen in the poly(1,2-azaborinine)s 2 (1250, 1041 cm -1 ) and 2-Z (1250, 1034 cm -1 ) with some peak broadening, further confirming the integrity of the BN-heterocycle. The C-B stretch for 2-Z is shifted slightly to lower wavenumbers as also predicted computationally. For monomer 1, an additional set of strong bands was found at 1275, 1228, 1158, 1120-1106 and 941-883 cm -1 and assigned to C-H bending modes in the cyclobutene ring. The disappearance of these bands in poly(1,2-azaborinines) 2 and 2-Z confirms the ring opening of the cyclobutene ring during polymerization.