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Polymer-urea inclusion compounds (P-U-ICs) were formed using a series of linear aliphatic polyesters with varying crystallizabilities: from highly crystalline to wholly amorphous. The traditional hexagonal P-U-ICs were obtained irrespective of the crystallinities of the neat guest polyesters. Two distinct co-crystallization mechanisms were evident based on the observation of the change in thermal stabilities of the ICs using DSC and the crystal morphologies by SEM; one involves polymer chain folding back and forth in a lamella-like crystal structure and the other grows much like short chain molecule U-ICs absent of chain reentering different channels. For polymers with sufficient chain length, their inherent flexibility is the key factor determining the co-crystallization mechanism while their crystallizability affects the kinetics, the consequences of which are more pronounced during recrystallization from melt. The amorphicity induced by random ester group placement is an interchain property, which does...