We present a conceptual framework for adding molecular details of chain extension and force-coupled bond dissociation to the Lake-Thomas model of tear energy in rubbery crack propagation. Incorporating data reported from single-molecule force spectroscopy experiments provides an estimate for the stored energy per bond at fracture of ∼60 kJ mol for typical hydrocarbon polymers, well below the typical carbon-carbon bond dissociation energy in these systems. Opportunities to test and exploit the role of molecular extension and covalent bond scission in experimental systems are proposed. -1