Abstract::Computational investigation of interfacial failure in composite materials is challenging because it is inherently multi-scale: the bond-breaking processes that occur at the covalently bonded interface and initiate failure involve quantum mechanical phenomena, yet the mechanisms by which external stresses are transferred through the matrix occur on length and time-scales far in excess of anything that can be simulated quantum mechanically. In this work, we demonstrate and validate an adaptive quantum mechanics / molecular mechanics (QM/MM) simulation method that can be used to address these issues and apply it to study critical failure at covalently bonded carbon nanotube (CNT)-polymer interface. In this hybrid approach, the majority of the system is simulated with a classical forcefield, while areas of particular interest are identified on-the-fly and atomic forces in those regions are updated based on QM calculations. We demonstrate that the hybrid method results in excellent agreement with fully-QM benchmark simulations and offers qualitative insights missing from classical simulations. We use the hybrid approach to show how chemical structure at the CNT-polymer interface determines its strength and we propose candidate chemistries to guide further experimental work in this area.