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This study investigates brittle-to-ductile transition (BDT) and re-entry phenomena in glassy polymers, against the background provided by the conventional Ludwik-Davidenkov-Wittman-Orowan (LDWO) hypothesis and available phenomenological theory for BDT (J. Chem. Phys. 141, 094905 (2014)).Through systematic experiments on bisphenol A polycarbonate (bpA-PC), polymethyl methacrylate (PMMA), polyethylene terephthalate (PET), we explore effects of polymer structure (e.g., crosslinking), temperature, strain rate, and sample geometry on mechanical characteristics of these polymers. We found adequate crosslinking to rend PMMA ductile at room temperature. We discovered a non-monotonic relationship regarding temperature and rate effects on polymer ductility: a ductile polymer can turn brittle at higher temperatures; a polymer is ductile in fast extension and becomes brittle in slow extension. Moreover, a specimen is ductile when it is shorter, but a long specimen can be brittle. These findings significantly expanded and improved our theoretical understanding of polymer ductility in glassy state.