Structural Confirmation of Exo‐Olefin‐Coupled Polyisobutylene via Model Compound Synthesis and Characterization

Abstract

We report the synthesis and characterization of a model compound representing exo‐olefin coupled polyisobutylene (PIB). Under certain conditions during the in situ quenching of quasiliving PIB with hindered bases to yield exo‐olefin‐terminated PIB, remaining PIB carbocations can undergo addition to exo‐olefin PIB to yield coupled PIB with a molecular weight approximately double that of the primary chains. Characterization of the signals that arise in 1H‐NMR from the presence of coupled product has not yet been properly performed; the accurate chemical shifts of these products, specifically the exo‐olefin product, have been in debate recently. Therefore, we carried out the synthesis of a model compound which mimics the exo‐olefin coupled product, and through variations in the synthetic method were able to produce a range of exo‐ and endo‐olefin coupled product mixtures. The model compounds have been fully characterized using NMR techniques, and we herein conclusively report the proton shift (for 500 MHz 1H‐NMR in CDCl3) of 4.82 ppm for the exo‐coupled product and that of 5.11 ppm for the endo‐coupled product.