The pursuit of synthetic routes for design and preparation of nanoporous polymeric networks with inherent permanent microporosity and functionality through bottom-up methodologies remains a driving force in developing CO2-philic materials. We report nanoporous, processable, benzoxazole-linked covalent organic polymers (Box-COPs) that show exceptional thermal stability up to 576 °C. Box-COPs can be formed into films thanks to the silylation that is used to guide polymeric network formation. Surface areas of up to 606 m2 g–1 and narrow pore sizes of 4.36 Å were observed with a CO2 uptake capacity of 139.6 mg g–1 at 273 K and 1 bar. Box-COPs were stable in boiling water for a week without deteriorating CO2 capture capacity.