While single-molecule (SM) experiments using fluorescent probes have provided critical insight for molecular and polymeric glass formers, the microscopic mechanism of probe transport remains largely unexplored in such systems at the molecular length scale. Here, we report SM translational dynamics of molecular probes in high molecular weight polystyrene at three temperatures near the glass transition temperature ($T_{\mathrm{g}}$). The probes exhibit quasistationary position fluctuations, non-Gaussian displacement distributions, subdiffusive transport with anticorrelated displacements, and a characteristic translational relaxation time. The observations are quantitatively described using a microscopic framework based on the generalized Langevin equation and supported by numerical modeling for heterogeneous transport. The probe dynamics reveal a progressive softening of effective viscoelastic confinement with increasing temperature and direct microscopic evidence of dynamic heterogeneity and suggest a pathway to more fully understand glassy dynamics in glass formers near $T_{\mathrm{g}}$.

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• Polymer behavior
• Polymer glasses
• Polymers
• Langevin equation
• Non-Markovian processes
• Optical microscopy
• Single molecule techniques