Chemists are generally excited by techniques that separate enantiopure molecules from racemic mixtures. However, the reversal of this process is equally intriguing for material chemists; mixing enantiopure polymers of opposite chirality creates value-added stereocomplexed polymers. For instance, while the melting point of poly(L-lactide) or poly(D-lactide) is around 170°C, the melting point of a stereocomplex of poly(L-lactide) and poly(D-lactide) increases to around 240°C. Beyond the melting point, stereocomplexation has been shown to improve various properties, including mechanical strength, storage modulus, crystallinity, solvent resistance, thermal stability, and hydrolytic stability, making these materials attractive for advanced applications.
Our research focuses on topochemical reactions, demonstrating chemical reactions in confined spaces such as crystals or gels. We developed the Topochemical Azide-Alkyne Cycloaddition (TAAC) and Topochemical Ene-Azide Cycloaddition (TEAC) reactions to create crystalline chiral polymers for various applications. Initially, we aimed to enhance the properties of our chiral polymers through stereocomplexation. This is when we noticed significant issues associated with conventional stereocomplexation, primarily due to the inefficient blending of two enantiopure polymers. The ineffective blending is mainly due to (i) mismatched lengths of polymer chains with opposite chirality, (ii) flexibility-driven looping, knotting, and coiling of individual chains, and (iii) bundling of chains with the same chirality, leading to phase-separated chiral domains. Due to these issues, stereocomplexes are generally semi-crystalline; thus, they have not yet realized their full potential!
Effective stereocomplexation requires pairing chains of equal sizes but opposite chirality, which is unlikely when two enantiopure polydisperse polymers are mixed after synthesis. Furthermore, the attractive noncovalent interactions between oppositely chiral chains must surpass the intrachain interactions as well as the interchain interactions between chains of same chirality that lead to homocrystals. This balance is crucial for maintaining hybridization along the entire length of the chains. Consequently, mixing two enantiomeric polymers typically results in imperfect and locally anisotropic stereocomplexes, making it nearly impossible to achieve a perfect and uniform stereocomplex.
To address this issue, we sought to utilize our newly discovered TEAC reaction, which has the remarkable ability to generate a new chiral center at the linkages (triazolines) between monomers. We used the TEAC reaction to create triazoline-linked chiral polymers, where the stereochemistry of the newly generated triazolines was dictated by the chirality of the chosen monomer. We hypothesized that the meticulously designed achiral or meso monomer on TEAC polymerization would yield two enantiopure polymers of opposite chirality, forming a stereocomplex. Thus, circumventing the conventional method of mixing pre-synthesized enantiopure polymers, by rational design, we made the monomers arrange in a crystal in such a way that polymerization would occur simultaneously, resulting in two enantiopure polymers of opposite chirality. As predicted, we achieved a perfect single crystalline stereocomplex with effective blending between the enantiopure polymers of opposite chirality.
How exactly did we realize this perfect stereocomplex? Check out our findings in our recent research article in Nature Communications, https://doi.org/10.1038/s41467-024-50948-z
A Perfect Marriage of Enantiopure Polymers of Opposite Chirality in Single Crystals
