Precise Characterization Of The Sequence Distribution Of Poly(Vinyl Butyral)(PVB) By NMR And Isotope Enrichment

Vinyl polymer obtained through chain polymerization of vinyl monomers is a major class of functional polymer materials.It has superior physical properties and is widely used in coating,packaging,construction materials,adhesives,etc.The side-chain substitution of these polymers usually introduces chiral centers at the methine sites,leading to obvious stereoisomeric sequence differences.Whether it be natural or artificial large molecules,sequence distributions are very important.Copolymers with similar components often show different physical properties due to differences in composition sequence(relative arrangement of structural units)and tactictity(stereoisomeric sequence).Therefore,the characterization and controllability of sequence distribution has always been a major research topic and challenge in the field of polymer science.Among the commonly used sequence distribution characterization means,NMR method has the advantages of high sensitivity to sequence structure and quantification.This work precisely characterizes the complex chemical structure and sequence distribution of polyvinyl butyral(PVB)by isotope enrichment and multidimensional NMR methods.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 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,INADEQUATE and H2BC were adopted to establish homo and heteronuclear coupling through either chemical bond or space correlation.Since only the PVB backbone is 13C enriched,all 1H and 13C NMR spectra provided unprecedented resolved information of various sequence information.Totally 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.