Precise Characterization of the Sequence Distribution of Poly(vinyl butyral) (PVB) by 2D-NMR and Isotope Enrichment

The complicated chemical structure of poly(vinyl butyral) (PVB) as well as its acetalization mechanism was unveiled by multidimensional NMR with the assistance of the 13C labeling. The isotope enrichment significantly suppresses the NMR signal from the side groups, which enables us to precisely determine the sequence distribution of various monomers, i.e., vinyl butyral (VB) and vinyl alcohol (VA), along the PVB backbone. The primary, secondary, and tertiary carbons were directly identified by NMR spectral editing through 13C distortionless enhancement by polarization transfer (DEPT) NMR experiments. Various 2D-NMR experiments, including COSY, NOESY, HMQC, and INADEQUATE, were adopted to establish homo- and heteronuclear coupling through either chemical bond or space correlation. Because only the PVB backbone is 13C-enriched, all 1H and 13C NMR spectra provided unprecedented resolved information on various sequence information. In total, 13 13C and 18 1H resolved NMR peaks are assigned. Based on the accomplished 13C NMR peak assignment and quantitative 13C NMR of PVB with different acetalization degrees as well as numerical modeling, a possible acetalization reaction mechanism is proposed.