Molar Mass Dependent Polymorph Selection of Polybutene‐1 Under Compressed CO ₂

Isotactic polybutene‐1 (PB‐1) shows peculiar crystallization behavior during crystallization from the melt, the metastable form II is always obtained. During aging at room temperature, the metastable crystals further undergo a solid‐solid transition into the stable form I with a corresponding change in volume. Therefore, direct formation of the stable crystalline form I’ in PB‐1 has attracted significant attention in research. In this work, we report a molar mass ( M w ) dependency of polymorph selection in PB‐1 under compressed carbon dioxide (CO 2 ). PB‐1 with a lower M w exhibited a larger fraction of form II than the higher M w PB‐1 at the same CO 2 pressure, and the former required a higher pressure to completely suppress the formation of form II. It was shown that differences in chain morphologies in the formed crystals played a decisive role in polymorph selection. PB‐1 with a lower M w crystallized into form II with chain‐extended crystals, whereas PB‐1 with a higher M w crystallized into form II with folded‐chain crystals. A lower nucleation free energy barrier was expected in the former than in the latter, weakening the suppression of form II in a lower M w PB‐1 under compressed CO 2 .