Mechanistic Characterization and Control of Branching in the Polymer of Intrinsic Microporosity PIM-1

Polymers of intrinsic microporosity (PIMs) have shown tremendous potential for membrane-based gas separations. PIM-1, the archetypal PIM, has demonstrated significant enhancements in gas permeability compared to conventional polymers, although significant variability in transport performance has been reported based on selected synthesis techniques and the presence and degree of branching. Herein, we re-examine the step-growth polymerization mechanism that produces PIM-1. We also employ a variety of 1D and 2D NMR spectroscopy techniques to propose a new structure of the branched polymer. The combination of reaction optimization and spectroscopy has led to the hypothesis that branching and cross-linking during the polymerization is caused by radical-mediated reactions that can be mitigated by the presence of a radical inhibitor, even when the reaction is run at high temperatures in the presence of oxygen. We use this proposed mechanism to demonstrate techniques to control the degree of branching in PIM-1, providing access to tunable branching in the polymer.