Stereocomplexed Materials of Chiral Polymers Tuned by Crystallization: A Case Study on Poly(lactic acid)

Abstract

ConspectusWidespread in chemistry and biochemistry, chirality strongly affects the biological, chemical, material, and physical properties. As a downstream derivative, the enantiopure polymers of opposite chirality can cocrystallize in the stereocomplex (SC) in their enantiomeric blends or block copolymers, which endows the materials with improved physical properties and specific functions such as enhanced melting temperature, thermal stability, crystallizability, and hydrolytic resistance. A well-studied stereocomplexable polymer is poly(lactic acid) (PLA); both of its two enantiomers, poly(l-lactic acid) and poly(d-lactic acid), are biobased and biodegradable. SC crystallization is a simple and practical approach to improve the essential properties and functions of chiral polymers, which advances existing polymer applications and paves a new way to develop a variety of novel materials. However, it is noteworthy that not all of the enantiomeric blends of l- and d-polymers can exclusively crystallize into SC crystals in a typical crystallization process, which is usually accompanied by the homocrystallization of each individual enantiomer. Developing stereocomplexed materials with controlled crystalline structure, physical properties, and functions is still challenging. Consequently, studying and discussing the stereocomplexed materials developed by harnessing SC crystallization are of fundamental importance not only to obtain novel functional materials but also to promote the advancement of material researches.In this Account, we concisely summarize and analyze our recent progresses in SC crystallization and stereocomplexed materials of PLA, which initializes a conceptual methodology to study and develop the stereocomplexed materials from chiral polymers. We first introduce various approaches for controlling the polymorphic crystallization structure, in order to promote the SC crystallization of high-molecular-weight PLA and to prepare the stereocomplexed materials. Then, we present our efforts on the preparation of novel stereocomplexed materials such as micelles, physical hydrogels, and elastomers by harnessing SC crystallization. These stereocomplexed materials show good physical properties (e.g., thermomechanical property and thermal resistance) and can further be functionalized toward shape memory, drug release, and thermoresponsive materials. These works have established a practical platform to unveil the relationships between crystalline structure, physical properties, and functions of stereocomplexed materials. At the end of the Account, we offer a brief summary and outlooks in this field. Even though a case study on PLA is focused in this Account, a similar methodology can be generalizable to other chiral polymer systems. We hope that this Account will evoke new inspirations and innovative work in the field of stereocomplexed materials of chiral polymers in the near future.