Deuterated Isotactic Polybutene-1 Exhibits Fast Form II to I Transition at Isothermal Crystallization Temperatures

Deuterated isotactic polybutene-1 (dPB-1) (Mw: 349 kg/mol) and its hydrogeneous counterpart (hPB-1) (Mw: 346 kg/mol) have been synthesized using deuterated 1-butene and hydrogeneous 1-butene monomers, respectively. The dPB-1 shows similar crystallization habit as hPB-1, i.e., crystallizing into metastable form II, which then transforms into stable form I. This peculiar polymorphic transition in hPB-1 normally occurs after cooling to lower temperatures after the crystallization of form II crystals because an internal stress along the lamellar normal built-up during cooling helps to overcome the nucleation barrier of the transition. Surprisingly, a spontaneous form II to I transition at the isothermal crystallization temperature is observed in dPB-1. The crystallization kinetics of form II in dPB-1 is essentially similar to that of hPB-1. The kinetics of the II to I transition in dPB-1 at the isothermal crystallization temperature exhibits a two-step process with the first one with kinetics nearly the same as the crystallization and the second one much slower. The results indicate a very low overall free energy barrier for the II to I transition in dPB-1 suggesting a critical role of the amorphous phase. Indeed, unlike in hPB-1 the conformational energy of 3/1helix in form I is even slightly higher than the 11/3 helix in form II in dPB-1 meaning that the driving force for the II to I transition from inside the crystalline phase is at most similar in both hPB-1 and dPB-1. It turns out that the entropy in the amorphous phase of the dPB-1 is much lower than that of hPB-1 because the vibrational entropy of bonds in both systems is largely different. As such, a much smaller free energy barrier is obtained for the II to I transition in dPB-1 so that a spontaneous transition is observed.