Alkyne-Palladium(II)-Catalyzed Living Polymerization of Isocyanides: An Exploration of Diverse Structures and Functions

• Published in: Accounts of chemical research 2021-10, Vol.54 (20), p.3953-3967
• Main Authors: Liu, Na, Zhou, Li, Wu, Zong-Quan
• Format: Article
• Language: English
• ISSN: 0001-4842; 1520-4898
• DOI: 10.1021/acs.accounts.1c00489

Conspectus Inspired by the perfect helical structures and the resulting exquisite functions of biomacromolecules, helical polymers have attracted increasing attention in recent years. Polyisocyanide is well known for its distinctive rodlike helical structure and various applications in chiral recognition, enantiomer separation, circularly polarized luminescence, liquid crystallization, and other fields. Although various methods and catalysts for isocyanide polymerization have been reported, the precise synthesis of helical polyisocyanides with controlled molecular weight, low dispersity, and high tacticity remains a formidable challenge. Owing to a limited synthesis strategy, the controlled synthesis of topological polyisocyanides has barely been realized. This Accounts highlights our recent endeavors to explore novel catalysts for the living polymerization of isocyanides. Fortunately, we discovered that alkyne-Pd­(II) catalysts could initiate the living polymerization of isocyanides, resulting in helical polyisocyanides with controlled structures, high tacticity, and tunable compositions. These catalysts are applicable to various isocyanide monomers, including alkyl isocyanides, aryl isocyanides, and diisocyanobenzene derivatives. Incorporating chiral bidentate phosphine ligands onto alkyne-Pd­(II) complexes formed chiral Pd­(II) catalysts, which promoted the asymmetric living polymerization of achiral isocyanide, yielding single left- and right-handed helices with highly optical activities. Using alkyne-Pd­(II) catalysts, various topological polyisocyanides have been facilely prepared, including hybrid block copolymers, bottlebrush polymers, core cross-linked star polymers, and organic/inorganic nanoparticles. For instance, various hybrid block polyisocyanides were easily produced by coupling alkyne-Pd­(II)-catalyzed living isocyanide polymerization with controlled radical polymerization and ring-opening polymerization (ROP). Combining the ring-opening metathesis polymerization (ROMP) of norbornene with Pd­(II)-catalyzed isocyanide polymerization, bottlebrush polyisocyanides and core cross-linked star polymers were easily prepared. Pd­(II)-catalyzed living polymerization of poly­(lactic acid)­s with isocyanide termini resulted in densely grafted bottlebrush polyisocyanides with closely packed side chains. Moreover, the surface-initiated living polymerization of isocyanides produced a family of polyisocyanide-grafted organic/inorganic hybrid nanoparticles