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

Abstract

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 using nanoparticles with alkyne-Pd­(II) catalysts anchored on the surfaces. Surprisingly, the nanoparticles and star polymers with helical polyisocyanide arms performed exceptionally well in terms of chiral recognition and resolution. Incorporated organocatalysts such as proline and prolinol units onto the pendants of optically active helical polyisocyanides, a family of polymer-based chiral organocatalysts, were generated, which showed significantly improved stereoselectivity for the asymmetric Aldol reaction and Michael addition and can be easily recycled. Us...

Controlled Synthesis of Cyclic‐Helical Polymers with Circularly Polarized Luminescence

“…The heights of the cyclic morphologies are ca. 2.5 nm for all cyclic bottlebrush polymers, supporting the monolayer of the macromolecules [30] . These studies confirm the cyclic topology of the synthesized polymers and the living nature of the ring‐expansion polymerization.…”

“…[31,32] The molecular CD intensities at 364 nm (Δɛ 364 ) are up to + 20 when the DP of the backbone is larger than 100, indicating the excess of the right-handed helix was more than 99 % (Figure 2a). [30,43] The sc-poly-1 m s show UV/ Vis absorption profiles similar to those of rc-poly-1 m s, while the CD profiles were in mirror images, suggesting sc-poly-1 m s had left-handed helix and the helicity depends on the catalyst chirality.…”

“…Exquisite biological helices, such as double helix of DNA and right‐handed α ‐helix in proteins and peptides, have stimulated intense research enthusiasm in artificial helical polymers and supramolecular helical architectures [22–29] . Helix is chiral because left‐ and right‐handed helices are enantiomers [22, 30] . Apart from the intriguing structure, helical polymers have numerous applications, including chiral self‐assembly, asymmetric catalysis, chiral drug delivery, chirality recognition, enantiomer separation, and circularly‐polarized light (CPL) generation [30–32] .…”

“…The densely grafting side‐chain prohibits chain rotation and vibration. The absorption optical energy could not be consumed and thus emit light to release the absorbed energy [30] . Moreover, the emission intensity of the cyclic polymer brushes increased after the addition of methanol to the THF solution, indicating the aggregation‐induced emission effect (Figure S33, Supporting Information).…”

“…[22][23][24][25][26][27][28][29] Helix is chiral because left-and righthanded helices are enantiomers. [22,30] Apart from the intriguing structure, helical polymers have numerous applications, including chiral self-assembly, asymmetric catalysis, chiral drug delivery, chirality recognition, enantiomer separation, and circularly-polarized light (CPL) generation. [30][31][32] CPL materials can emit circularly-polarized light directly and have aroused attention for applications in data storage, chiral sensors, biological probes, liquid-crystal, three-dimensional displays, etc.…”