The crystallization of synthetic polymers, in spite of its significant industrial importance, continues to represent a primary challenge of polymer science and condensed matter physics. Although kinetic theories on polymer crystallization enjoyed widespread acceptance, the underlying dynamics of polymer crystallization is still unknown. Consequently, man's ability to control polymer structure via crystallization is limited, particularly during its early stages and at the nanoscale. In this SGER proposal, we propose to initiate research on the Terahertz (THz) dynamics of polymer crystallization. THz dynamics originates from the collective modes of inter- and intra-chain motions and is expected to be active during both stages of nucleation and growth. The spectral fingerprints of the collective modes lie in the far-IR region, from a few wavenumbers to some 300 cm-1. Energetically, they reside in the THz regime (1 THz = 1012 Hz). It is the objective of this proposal to establish the role of THz dynamics in structural formation during crystallization. Efforts will be focused on the stage of primary nucleation. The specific aim is to establish concrete microscopy evidence that THz energy input is capable of controlling nucleation behavior. Experimentally, brilliant monochromatic THz free-electron laser (FEL), whose energy is characteristic of nucleation activation, will be used. In-situ microscopy is employed to follow changes in nucleation behavior upon FEL illumination on metastable polymer melts. The goal is not only to establish the role of THz dynamics in polymer crystallization, but also to introduce the concept of THz dynamics to study structural phase transformations in polymer systems. The experimental strategy adopted in this proposal also opens the door for the development of a groundbreaking nanotechnology for molecular engineering.