The shock or impact initiation of high explosives (HEs) at the particle scale remains of great interest to the military and other industries, yet there lacks an exhaustive understanding of the mechanisms by which this occurs. The most prevalent hypothesis to explain this phenomenon is termed hot spot theory. Hot spot theory proposes that energy inside the HE consolidates at locally concentrated sites due to an impact and that these localized pockets create the adequate temperature required for ignition. However, there is a dearth of experimental data at the microscale where hot spots occur. This study focuses on investigating the impact dynamics of HE particle(s) inside a polymer matrix, which is similar to, however a simplified form of, a polymer-bonded explosive (PBX). Samples with inert particles were also investigated as a comparison to the energetic particles. To facilitate this investigation, a light gas gun in combination with high-speed x-ray phase contrast imaging (PCI) was employed. PCI allows for the observation of the particle's deformation, damage, and reaction, which cannot be viewed by traditional optical methods. HMX (energetic) and sucrose (inert) particles were used for this study, each captured in a Sylgard-184RTM matrix. The experiments show that at lower impact velocities (~200 m/s) the HE particle was cracked and crushed. More intense cracking of the particle and debonding from the matrix were observed at increased velocities (300--450 m/s) in both energetic and inert particles. At the highest impact velocities (~450 m/s), considerable volume expansion of the particle occurred in the energetic samples. This is thought to be the result of gas production resulting from chemical reactions inside the HE particle, which is a possible precursor to detonation.