Synthesis of core cross-linked star polymers carrying helical poly(phenyl isocyanide) arms via “core-first” strategy and their surface chiral recognition ability

Chiral Nanostructures from Artificial Helical Polymers: Recent Advances in Synthesis, Regulation, and Functions

Helical structures such as right-handed double helix for DNA and left-handed α-helix for proteins in biological systems are inherently chiral. Importantly, chirality at the nanoscopic level plays a vital role in their macroscopic chiral functionalities. In order to mimic the structures and functions of natural chiral nanoarchitectures, a variety of chiral nanostructures obtained from artificial helical polymers are prepared, which can be directly observed by atomic force microscopy (AFM), scanning tunneling microscopy (STM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). This review mainly focuses on the formation of chiral nanostructures and the morphology regulation triggered by polymer chain length, concentration, solvent, temperature, photoirradiation, and chemical additives. In addition, the distinct chiral functions including chiral recognition, circularly polarized luminescence, drug release, cell imaging, and antibiosis are also discussed.

One-Pot Synthesis, Circularly Polarized Luminescence, and Controlled Self-Assembly of Janus-Type Miktoarm Star Copolymers

With the aim of broadening the scope of Janus-type polymers with new functionalities, Janus-type miktoarm star copolymers comprising helical poly(phenyl isocyanide) (PPI) and a vinyl polymer were designed and synthesized via a combination of Pd(II)-initiated isocyanide polymerization and atom transfer radical polymerization (ATRP). A functional β-cyclodextrin bearing 7 Pd(II) complexes at one side and 14 bromine groups at the other side ((Pd(II))7-CD-(Br)14) was prepared and used as an initiator for the one-pot polymerization of phenyl isocyanide and the ATRP of vinyl monomers in a living and controlled manner. A variety of Janus-type copolymers with different structures and tunable compositions were facilely obtained by using this method. Thus, Janus-type copolymers composed of helical PPIs and tetraphenylethylene-modified vinyl polymers exhibited a significant circularly polarized luminescence performance in both soluble and aggregated states. Meanwhile, Janus-type copolymers containing PPIs and hydrophilic vinyl polymers presented amphiphilicity and self-assembled into diverse morphologies.

Recent advances of optically active helical polymers as adsorbents and chiral stationary phases for chiral resolution

Chiral resolution is very important and still a big challenge due to different biological activity and same physicochemical property of one pair (R)- and (S)-isomer. There is no doubt that chiral selectors are essentially needed for chiral resolution, which can stereoselectively interact with a pair of isomers. To date, a large amount of optically active helical polymers as chiral selectors have been synthesized via two strategies. First, the target helical polymers are derived from natural polysaccharide such as cellulose and amylose. Second, they can be synthesized by polymerization of chiral monomers. Alternatively, an achiral polymer is prepared first followed by static or dynamic chiral induction. Furthermore, a part of them is harnessed as chiral stationary phases for chromatographic chiral separation and as chiral adsorbents for enantioselective adsorption/crystallization, resulting in good enantioseparation efficiency. In summary, the present review will focus on recent progress of the polymers with optical activity for chiral resolution, especially the literature published in the past 10 years. In addition, development prospects and future challenges of optically active helical polymers will be discussed in detail.

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

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.Using a chiral alkyne-Pd(II) catalyst, single-handed helical polyisocyanides bearing naphthalene and pyrene probes were produced from achiral isocyanide monomers. These polymers showed excellent self-sorting properties as revealed using a fluorescence resonance energy transfer (FRET) investigation and were self-assembled into two-dimensional (2D) smectic nanostructures driven by both helicity and chain length. Incorporating helical poly(phenyl isocyanide) (PPI) onto semiconducting poly(3-hexylthiophene) (P3HT) induced the asymmetric assembly of the resulting P3HT-b-PPI copolymers into single-handed cylindrical micelles with controlled dimensions and tunable photoluminescence.

Facile Synthesis of Cyanide and Isocyanides from CO

The reaction of K[N(SiMe₃ )₂ ] with ¹³ CO proceeds in C₆ D₆ or THF affording K¹³ CN and O(SiMe₃ )₂ under mild conditions as confirmed by crystallographic characterization of K(18-crown-6)CN. Similarly reaction of the alkali metal amides, M[N(SiR₃ )R'] (M=Li, K; R=Ph, Me; R'=alkyl, aryl) provides the corresponding ¹³ C labeled isocyanide RN¹³ C and MOSiR₃ , generally in high yields. In some instances, the use of the sterically bulky Ph₃ Si-substituent is required to preclude 1,2-silyl migration affording the silylcarbamoyl salt M[Me₃ SiC(O)NR']. These reactions have been used to obtain 19 examples of ¹³ C labelled isocyanides, and several examples of gram scale reactions are reported. The mechanism of the reactions is probed via reliable DFT calculations.

Electrospinning chiral fluorescent nanofibers from helical polyacetylene: preparation and enantioselective recognition ability

Chirality is ubiquitous in nature and closely related to the pharmacological effects of chiral drugs. Therefore, chiral recognition of molecular enantiomers becomes an important research theme. Fluorescence detection is highly sensitive and fast but has achieved only limited success in enantiomeric detection due to the lack of powerful chiral fluorescence detection materials. In this paper, a novel chiral fluorescent probe material, i.e. a chiral fluorescent nanofiber membrane, is prepared from chiral helical substituted polyacetylene by the electrospinning technique. The SEM images demonstrate the success in fabricating continuous, uniform nanofibers with a diameter of about 100 nm. Circular dichroism spectra show that the nanofibers exhibit fascinating optical activity. One of the enantiomeric chiral fluorescent membranes has chiral fluorescence recognition effects towards alanine and chiral phenylethylamine, while the other enantiomeric membrane does not. The prepared chiral fluorescent nano-materials are expected to find various applications in chirality-related fields due to their advantages such as chirality, fluorescence, and a high specific surface area. The established preparation approach also promises a potent and versatile platform for developing advanced nanofiber materials from conjugated polymers.

Chiral, thermal-responsive hydrogels containing helical hydrophilic polyacetylene: preparation and enantio-differentiating release ability

Chiral hydrogels constructed from helical hydrophilic polyacetylene demonstrate chirality, thermo-responsivity, biocompatibility and enantio-selective release ability towards chiral drugs.