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The crystallization pathway of long and flexible polymer chains is debatable because of the lack of an initial melt/glass structure. To identify the crystallization pathway, we focus on two binary blends of poly(lactic acid) racemates that form stereocomplex crystals (SCCs). NMR crystallography is used to identify the stereocomplex (SC) structure and SC fraction with or without long-range order. There are significant structural analogies between glass and crystals for both high-molecular-weight (M) and low-M racemates. The observed analogies and kinetics of crystallization indicate that polymer crystallization proceeds via chain segments moving the least possible distance (“freezing in” mechanism) and that topological constraints govern nucleation barriers. 

Stereoregularity significantly influences the crystallization, mechanical, and thermal properties of polymers. In this work, we investigate crystallization behaviors and molecular dynamics of atactic (a)-, isotactic (i)-, and syndiotactic (s)-hydrogenated poly(norbornene) (hPNB)s by using small-angle X-ray scattering and solid-state (ss) NMR. a-hPNB exhibits a much higher crystallinity (Φc) (82%) and long period (L) (80 nm) than i- and s-hPNB (50–55% and 17–21 nm). Moreover, in the s-hPNB crystalline region, chain dynamics is not thermally activated up to the melting temperature (Tm), while in the crystalline regions of i- and a-hPNB, small amplitude motions occur in a slow dynamic regime of 10–2–102 s. The molecular dynamics follows Arrhenius behavior in a-hPNB up to the crystal–crystal transition temperature (Tcc), while these dynamics are surprisingly saturated in i-hPNB under these conditions. Temperature dependence of the molecular dynamics leads to different crystal–crystal transitions between i- and a-hPNBs: i-hPNB changes the trans conformation to the gauche one due to the localized bond rotations where chain dynamics is restricted, whereas a-hPNB keeps a nearly trans conformation and conducts fast chain dynamics due to the amplified C–C bond rotations in the high-temperature phase. Such fast chain dynamics leads to unique crystallization behaviors of hPNB, specifically in the atactic configuration due to configurational disorder coupled with conformational flexibility.