The Microstructure of Poly(Isobutylene-co-p-Methylstyrene) by NMR Spectroscopy

The microstructure of isobutylene-para-methylstyrene (IB-pMeSt) copolymers was studied by NMR spectroscopy. 1H- and 13C-NMR spectra were used to obtain overall copolymer compositions. 13C-NMR signals were assigned in terms of triad monomer sequences, and triad distributions were obtained over a wide copolymer composition range. According to statistical tests, the IB-pMeSt copolymerization cannot be described by zero- (Bernoullian) or first-order Markov models because reactivity ratios r IB and r pMeSt were found to change with the monomer feed composition. Additional insight into the microstructure of IB-pMeSt copolymers was gained by calculating sequence numbers, run numbers, and sequence lengths from triad distributions. Further, the Kelen-Tüdös plot showed a distinct curvature indicating that the Kelen-Tüdös method, applied over the entire monomer feed composition range, cannot give meaningful reactivity ratios for this monomer pair. Evidently the simple two-parameter Mayo-Lewis model is inadequate to describe the IB-pMeSt copolymerization system.