Coexistence of Isodimorphism, Isomorphism, and Comonomer Exclusion within a Single Family of Poly(butylene- <i>ran</i> -octamethylene carbonate) Random Copolymers

In this study, a series of new random poly(butylene-ran-octamethylene carbonate) (PBxOyC) copolymers was synthesized via melt polycondensation, spanning the entire 0–100% range of butylene carbonate (BC) fraction (fBC), in 10% increments. Their crystallization behavior was examined in detail. Differential scanning calorimetry (DSC) characterization showed a pseudoeutectic-like behavior, where both melting and crystallization temperatures decreased to a minimum at fBC = 53 mol %. As fBC was further increased, copolymer crystallization was completely suppressed for fBC = 66–75 mol % and then reappeared at fBC = 87 mol %, suggesting a transition from isodimorphism to complete comonomer exclusion in the composition range where only poly(butylene carbonate) (PBC) could crystallize. Additionally, Wide Angle X-ray Diffraction (WAXD) and Fourier Transform Infrared Spectroscopy (FTIR) were used to analyze the composition-dependent crystalline structures. It was found that copolymers with low BC content, fBC ≤ 19 mol %, crystallized into poly(octamethylene carbonate) (POC) crystals, while those with high BC content, fBC ≥ 87 mol %, produced PBC crystals. However, within the intermediate composition range (fBC = 25–53 mol %), a γ phase was formed, with a crystalline structure distinct from those of the parent components and their polymorphic forms. WAXD results suggest that this γ phase results from the cocrystallization of both BC and OC units (i.e., isomorphic crystallization). Combining DSC, WAXD, and FTIR findings, we identified for the first time the coexistence of three crystallization modes dictated by composition in these copolymers: (1) isodimorphism at fBC ≤ 19 mol %; (2) isomorphism at 25 mol % ≤ fBC ≤ 53 mol %; and (3) total exclusion at fBC ≥ 66 mol %. Furthermore, isothermal crystallization experiments indicated that the crystal structures are governed by a kinetically controlled balance between comonomer exclusion and inclusion. During cooling, BC units may become trapped within the crystal lattice, helping to facilitate the formation of the γ phase. Conversely, during isothermal crystallization, BC units seem to be preferentially excluded to the amorphous regions, leaving sufficient OC units to form a POC-type crystal lattice, which limits the compositions where isomorphic crystallization can occur. These findings highlight the crucial influence of molecular composition and crystallization conditions on the crystallization behavior of these random copolymers.