Molar Mass Dependent Polymorph Selection of Polybutene-1 Under Compressed CO₂

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 (CO₂). PB-1 with a lower M_w exhibited a larger fraction of form II than the higher M_w PB-1 at the same CO₂ 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 CO₂.