Strong memory effect in higher α-olefin homopolymers with crystalline side chains

Abstract

The homopolymers of higher α-olefins with 12–20 carbon atoms were successfully synthesized using the dimethylpyridylamidohafnium/organoboron catalyst. With these model samples, the relationship between crystallization and chain topology were investigated employing the differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The DSC results show that for these homopolymers of higher α-olefins, the crystallization temperature is significantly elevated with increasing the length of side chains. The WAXD evidences demonstrate that cooling crystallization of these homopolymers at 10 °C/min can form the Type ІІ crystallites with zigzag arrangement of side chains. Interestingly, these higher α-olefin homopolymers exhibit the strong memory effect of crystallization, where the accelerated crystallization of poly(1-hexadecene) was observed after annealing at the elevated temperature up to 70 °C higher than melting range. Quantitatively, the temperature width of memory Domain II changes in a bell type with increasing side-chain length, where poly(1-hexadecene) shows the largest Domain II width of 74 °C. However, the strong memory effect beyond the melting range that is observed for the Type ІІ crystallites, is absent in Type І crystallites packed by the main chains, as demonstrated by the results of poly(1-tetradecene). Moreover, it was found that this memory effect of Type II crystallites is stable and independent on the annealing time.