Supramolecular chiral polymeric aggregates: Construction and applications

Inspired by the mystery of natural homochirality, the design and construction of artificial chiral systems with special functions including mechanics, chiroptical devices, and chiral separation have aroused the great interest of scientists. Due to its diverse conformations and components, polymers become an excellent candidate to construct artificial chiral aggregates, which provides an opportunity to fabricate advanced chiral materials. Herein, in this review, we summarized not only the different strategies for construction and regulation of supramolecular chirality in chiral or achiral polymer systems, mainly including post‐assembly and polymerization‐induced chiral self‐assembly in chiral polymer systems and several chirality transfer and induction strategies in achiral polymer systems, but also the formation mechanisms of chiral aggregates by the chirality amplification from the molecular level to supramolecular level. Furthermore, we present the typical applications of chiral polymeric aggregates in circularly polarized light emission, chiral recognition and separation, and chiral elastomer. Finally, the current challenges and future directions of chiral polymeric aggregates are also elaborated.

F I G U R E 1 Chiral architectures in nature at various scales, from enantiomeric molecule at sub-nanometer scale to DNA and enzyme at nanometer scale, further to living system and galaxy at macroscopic scale And the tertiary chirality is chiral helical monodomains or single crystals, which are produced by noncovalent bond interactions between molecules, always called phase chirality. The quaternary chiral structure is the macroscopic chiral object with a higher order and more sophisticated structure, which can be obtained by the further interaction between the secondary or tertiary chiral structures. [11,12] At present, chiral substances have been generally regarded as the material basis of life, which is of great significance to the origin and development of life, and are widely used in chemistry, physics, biomedicine, and other fields currently. [13] In polymeric system, the current research on chirality is mainly focused on the second-level helical conformation (conformational chirality) and the third-level chiral supramolecular aggregates (phase chirality). Although lots of exquisite chiral supramolecular systems have been reported recently, [14][15][16][17][18][19] some challenge are still faced in developing effective approaches to achieve effective chiral assembly, chiral amplification, chiral transfer, chiral memory, and even fabrication of chiral optical devices in polymer.

Supramolecular chemistry and chirality
In 1978, Lehn put forward the concept of "Supramolecule" for the first time beyond the research scope of subject-object chemistry. [20] Supramolecule refers to the combination of two or more units relying on the interaction of noncovalent bonds in self-processes, such as self-assembly and self-organization. [21,22] It possesses a complex but ordered structure and maintains certain integrity with specific micro/macro-structural characteristics. That is, supramolecular chemistry can be expressed as "chemistry beyond the molecule". The concept of supramolecule has made scientists realize that the smallest basic unit of function is no longer a molecule but a supramolecular unit, and the physicochemical function can be achieved by the supramolecular assembly. Generally, configurational chirality at the molecular level is governed by covalent bonds between atoms, or the conformational chirality of the asymmetric arrangement of specific groups in the molecule is determined by the rotation of the bonds. Supramolecular chirality is usually formed by the helical arrangement of the building blocks. The driving forces are always dynamic and weak noncovalent interactions. Therefore, the obtained supramolecular chiral assemblies can be rationally regulated by changing external stimuli (such as pH, temperature, light, and solvent). [23] The supramolecular chirality generally reflects the chirality of ordered aggregates formed by noncovalent interactions between building blocks, which are usually arranged in a helical arrangement. This structure enables molecules or chromophores that originally have chirality to be amplified in a nonlinear manner, or even enables those that are originally achiral to exhibit chiral information through a certain aggregation form. [24] The building blocks of chiral supramolecular units can be chiral or achiral inorganic molecules, organic molecules, polymers, and even biological macromolecules (Figure 2A,B). Through the supramolecular assembly, chiral building blocks can be used to construct a supramolecular chiral system with different functions. In general, the chirality of supramolecular assemblies is not maintained by a single driving force, but by a synergistic effect of the multiple noncovalent forces such as π-π stacking, hydrogen bonding, metal-ligand complexation, acid-base interaction, van der Waals forces, and so on. [5,[25][26][27][28][29][30] These noncovalent bond forces are generally reversible, providing good dynamic reversibility and controllability. Furthermore, the dynamic interactions enable the assemblies a rich variety of suprastructures and multifunctionality. Supramolecular assembly not only has the properties of each assembled element, but also a significantly higher overall performance than the simple superposition of each assembled element, which is known as synergy and amplification effect. [31] In recent years, the supramolecular assembly strategy has become one of the effective ways to design and synthesize novel chiral nanomaterials with special structures and excellent properties. [32][33][34][35][36][37] According to whether chiral units are involved in the supramolecular assembly process, the construction methods of supramolecular chiral assemblies can be roughly divided into the following types: (1) "chiral + chiral" means that only chiral units are used to form ordered supramolecular structure, always exhibiting novel chiral properties beyond a single chiral molecule; [40,41] (2) "chiral + achiral" means that achiral units and chiral units are co-assembled to induce achiral assembly units to show chiral signals in a chiral environment; (3) "achiral + achiral" means complete assembly by achiral units through spontaneous symmetry breakage, leading to exhibiting chirality through their own structural characteristics. [42] Usually, there are two typical amplification effects of chiral induction, namely "Sergeants-and-Soldiers Principle" and "Majority-Rules". [43][44][45] The "Sergeants-and-Soldiers Principle" refers to the addition of small amounts of chiral units ("Sergeants") that can directly or indirectly interact with achiral counterparts ("Soldiers") through the intermolecular interaction and induce the formation of supramolecular chiral helical structures with amplified chiral signals. In 1989, Green F I G U R E 2 (A) The supramolecular co-assembly between chiral peptide amphiphiles and metalloporphyrin forming highly ordered peptide amphiphile metalloporphyrin arrays. Reproduced with permission. [38] Copyright 2012, American Chemical Society. (B) Chirality-dependent coordination assembly process between diphenylalanine molecules and Cu 2+ , forming different supramolecular chiral structures. Reproduced with permission. [37] Copyright 2022, American Chemical Society. (C) Scanning electron microscopy images of gold thiolate hierarchically organized particles with different chiral cysteine surface ligands. Reproduced with permission. [39] Copyright 2020, American Association for the Advancement of Science et al. first discovered the "Sergeants-and-Soldiers Principle" when studying chiral/achiral isocyanate copolymers. [46] At low content of chiral units, the strong optical activity of the copolymer solution can exhibit very well due to the chirality transfer and amplification. This rule is not only applicable to isocyanate copolymers, but also to chiral amplification phenomenon in many helical polymers or supramolecular chiral aggregates. [47] For example, Meijer et al. reported that the "Sergeants-and-Soldiers Principle" existed in the supramolecular chiral aggregate system when a small amount of chiral L-or D-dibenzoyl tartaric acid (sergeant) is added to a large number of achiral oligo-p-phenylenevinylene derivative units (soldier). The chiral induction was driven by the hydrogen bond interaction between the diaminotriazine group and the chiral tartaric acid molecule, and the π-π interaction between the benzene rings in the chain structures. [48] The strategy was subsequently widely used in polymeric chiral supramolecular systems to achieve chiral aggregates with amplification effect. In polymer system, a nonlinear increasing style of optical activity with the concentration of chiral building blocks ("normal") and the composition driven helicity inversion ("abnormal") manner are the two most common amplification modes. Deng et al. fabricated optically active helical copolymers derived from an achiral monomer and a chiral monomer, and the emulsion copolymerization process followed the rule. [49] The morphology of the chiral aggregates was nanoparticle, which showed nonlinear increase of optical activity with the increase of chiral monomers concentration in copolymer emulsions. Wan et al. reported a macromolecular acid-base complex of highly isotactic poly(2-vinylpyridine), (+)-camphorsulfonic acid, and dodecylbenzenesulfonic acid to form helical structures. [45] Four distinct types of "Sergeants-and-Soldiers Principle" mode chiral amplification were achieved by the change of acid ratio, the chiral acid content, and the solvent nature. The "Majority-Rules" suggested that a slightly enantiomeric excess (e.e) in a mixture of enantiomers can induce supramolecular assembly with the same helical direction through the synergistic effect of weak intermolecular interactions. In other words, the supramolecular assembly can also exhibit high chiral purity when the enantiomeric mixtures contained a lower e.e value. Liu's research group designed and synthesized L-type and D-type glutamate enantiomers, and then successfully prepared chiral supramolecular gels by the solgel assembly method. [50] Meanwhile, adjusting the doping ratio of the two enantiomers can change the internal helical structure of the gel and achieve the chiral reverse of the assembly. When the two enantiomers of L-type and D-type were doped in equal proportions, a smooth sheetlike assembly was obtained. However, left helical ribbons are formed when the D-type enantiomer was excessive. Meanwhile, the excess L-type enantiomer led to right spiral ribbons. These phenomena fully indicate that the chiral characteristics or helix direction of the assembled structure are determined by the chirality of the excess enantiomer in the system. Recently, Kotov et al. synthesized various chiral structures, including twisted spikes and flat nanoribbons with high complexity, based on the assembly of gold thiolate nanoplatelets with cysteine surface ligands by changing the ratio of L-type and D-type enantiomers. [39] When the two enantiomers of L-type and D-type were equal, the flat Au nanoribbons were obtained ( Figure 2C). In polymer system, Suginome et al. synthesized Majority-Rules-Type helical poly(quinoxaline-2,3-diyl)s, which was attributed to large ∆G h values (the energy difference between P-and M-helical conformations). [51] According to the theory developed by Green et al. for polymers on helix-sense-reversal conformations, the maximum value of the screw-sense excess is determined by ∆G h values. The formed single-handed helical structure could be used as a highly enantioselective chiral ligand in asymmetric reactions. In summary, "Sergeants-and-Soldiers Principle" and "Majority-Rules" are widely applied to the majority of supramolecular chiral systems built from small molecules or polymers.

F I G U R E 3 (A)
Helical polymer types and representative macromolecular structures. Reproduced with permission. [60] Copyright 2009, American Chemical Society. (B) Schematic illustration of the mechanism of chiral transfer from molecular chirality to phase chirality in the self-assembly of chiral block polymers. Reproduced with permission. [61] Copyright 2012, American Chemical Society. (C) Chiral structures constructed in polymer system and their unique properties. Reproduced with permission. [62] Copyright 2021, Royal Society of Chemistry

Supramolecular chirality in polymer aggregates
As an important part of chiral materials, chiral polymers have a wide range of application prospects in biology, medicine, communication, information and national defense, and so on due to their unique optical, electrical, magnetic and molecular recognition, asymmetric catalysis, and mechanical properties. However, the underlying mechanism of forming well-structured chiral polymers is particularly complex and still in the early stages of research compared with the well-established asymmetric synthesis of chiral small molecules.
Macromolecular helicity in polymer system was first found in isotactic polypropylenes, which were synthesized by the famous Ziegler-Natta catalyst in 1955. [52] Although the helical conformation was relatively unstable in solution state and even existed in a mixture of different handedness, it pioneered the synthesis of helical polymers. Subsequently, helical polymers including poly(tertbutyl isocyanide), [53] triphenylmethyl methacrylate (TrMA), [54] isotactic polychloral, [55] polyisocyanates, [56] polysilanes, [57,58] polyacetylenes, [59] and others have been successfully synthesized by the helix-sense-selective polymerization using chiral catalysts or initiators under kinetic or thermodynamic control. As shown in Figure 3A, the helical polymers can be categorized into three types: helical polymer with high helix inversion barriers, helical polymer with low helix inversion barriers, and helical polymer with folded conformation (foldamers). [60] The obtained helical polymers have been successfully applied in the field of chiral catalysis and chiral resolution. However, both the traditional chiral monomer polymerization and the asymmetric polymerization of achiral monomers have certain limitations. The former has relatively complex and time-consuming synthesis procedures, and always needs the use of expensive chiral catalysts, while the latter has to use chiral catalysts/initiators that generally lack universality. Meanwhile, it is essential but difficult to control the stereoregularity of all units in the polymer chain. The previous research mainly focused on the macromolecular helicity in solution phase. From a practical application perspective, it is more important to regulate the chiral properties of polymer in an aggregation state.
The supramolecular chirality constructed from functionality-oriented polymers combines features of "bottom-up" molecular self-assembly strategy and polymer chemistry, exhibiting more sophisticated microstructures ( Figure 3B,C). Recently, polymeric chiral supramolecules with rational controlled handedness, not just limited to synthetic helical polymers, more similar to the biological hierarchical helical structures, aroused growing interest due to the development of supramolecular chemistry. Chiral aggregates with ordered asymmetrical suprastructures can be formed via intermolecular noncovalent interactions such as van der Waals forces, CH/π, π-π stacking, dipole interactions, and hydrogen bonding. Compared with the traditional method of asymmetric polymerization to prepare chiral polymers, the construction of polymeric supramolecular chirality by assembly strategy has the advantages of simplicity, flexibility, high efficiency, and facile regulation. Furthermore, polymeric aggregated materials always exhibit broader application prospects than polymeric solutions due to the practical application environment limitations in solution.
Supramolecular chirality in polymer aggregates has attracted tremendous attention because of its unique molecular weight feature and predominant application prospects ( Figure 3C). [62,63] Polymeric chiral supramolecular chemistry can induce chiral arrangement of molecules or chromophores in the polymer chain in a nonlinear way, further forming a certain helical stacking arrangement. More importantly, chiral structures can be obtained through chirality transfer in polymers that do not originally possess chiral information. This not only avoids the use of expensive chiral monomers, but also eliminates the need for complex synthesis processes, such as asymmetric catalytic polymerization and enantioselective polymerization. Meanwhile, the structural range of chiral polymers has been significantly expanded in chiral induction, which is of great value to understanding the underlying mechanism of the transfer of asymmetric tendency. Over the years, scientists have reported many chiral induction methods in this field, mainly including chiral solvent, [64][65][66][67] chiral additives, [68,69] and other methods (circularly polarized light [CPL], [70][71][72][73] interfacial interaction, [74] gelation, [75,76] and liquid-crystal field [77,78] ).
In recent years, the special characterization methods have been rapidly developed to assist the investigation on the polymeric supramolecular chiral assembles. It is well known that a beam of plane-polarized light can be decomposed into two CPLs with the same amplitude and frequency but opposite rotation directions. Among them, the electric vector rotating in the clockwise direction is called right-handed CPL (r-CPL), and the electric vector rotating in the counterclockwise direction is called left-handed CPL (l-CPL). The absorption coefficient (ε) of the optically active material for l-and r-CPL is not equal, ε L ≠ ε R . The l-and r-CPL will become elliptically polarized light after passing through the optically active materials. This phenomenon is called circular dichroism (CD). CD spectroscopy is the most commonly used and effective method for characterizing supramolecular chirality. [79][80][81][82] The CD graph is drawn by taking the wavelength λ as the abscissa and the difference in absorption coefficient Δε = (ε L − ε R ) as the ordinate. The resulting spectrum is the circular dichroic spectrum. Since ε L ≠ ε R , the transmitted light is no longer plane-polarized light, but elliptically polarized light. The relationship between the molar ellipticity [θ] and Δε is: [θ] = 3300Δε. Since Δε has a positive value and a negative value, the circular dichroism also has a positive signal with peaks and a negative signal with valleys. When the external environment induces achiral units to take chiral arrangements, the CD at this time is called induced circular dichroism (ICD). [83] If the optically active substance absorbs in the ultraviolet-visible (UV-vis) light region, its characteristic left and right polarized light signal (CD signal) can be obtained in the CD spectrum. Compared with the absorption spectrum, the CD signal is relatively weak, but subtle changes in the molecular configuration or molecular arrangement of the chromophore can be sensitively captured by the CD spectrum. In supramolecular chiral systems, the CD signal of supramolecular aggregates is generally produced in the following situations: (1) The obvious change of ordered aggregation of molecules with chiral chromophores into supramolecular aggregates can be observed in the CD spectrum, but sometimes only a simple enhancement of peak intensity; (2) CD signal was observed in the UV-vis region corresponding to the achiral chromophore attached to the chiral group, which was transmitted through the chirality of the chiral group. The chiral signal intensity is related to the distance between the chiral group and the chromophore, and generally decreases sharply with the increase of the distance; (3) The chromophore is far away from the chiral center, but due to the orderly twisted packing of the whole aggregate. Therefore, the CD signal induced by chirality can also be observed; (4) The chromophore of the assembly unit is achiral but co-assembles with other chiral units which will lead to the generation of CD signal in the UV-vis region corresponding to the chromophore. This type of CD signal is completely induced by the chiral assembly. In particular, while a pair of enantiomers is involved in supramolecular assembly, the enantiomers are each assembled into a helical structure with opposite chirality, and the CD signal intensity is linearly related to the e.e value of the enantiomer, indicating that chiral self-sorting occurred during the assembly process. Vibrating circular dichroism (VCD) can provide the polarization absorption signal of the additional vibration zone of the molecule, which is helpful for studying the way of molecular accumulation without chromophore. [84,85] In addition, CD spectroscopy reveals the chiral characteristics of chiral materials in the ground state, while CPL spectroscopy reveals the chiral characteristics of chiral materials in the excited state. [86] These characterization techniques can be used to study the stereochemistry, conformation, and three dimensionality of chiral polymeric aggregates.
Hence, in this review, we highlighted the self-assembly strategy to regulate the polymeric chirality and then rational morphological chirality in the aggregated state with innovative and fascinating functions in detail. First, the various approaches of chirality regulation in polymer aggregates were described, which mainly consisted of the directed assembly of chiral polymers ( Figure 4, yellow background 1-3) with intrinsic chirality transfer and amplification, and the chiral surroundings-assisted assembly of achiral polymers ( Figure 4, green background 4 and 5) with external chirality transfer and amplification. [62] Second, the representative examples of chiral polymeric aggregates were discussed, mainly including CPL emission, chiral recognition and separation, and chiral liquid-crystalline elastomer (LCE). Finally, the challenges and future outlooks including the preparation methods and potential of chiral polymeric aggregates were outlined.

Chiral polymer aggregates constructed by chiral polymers
In order to construct supramolecular chirality to further form chiral aggregates in bulk from chiral polymers, different strategies including post-assembly of chiral polymers and in situ polymerization-induced chiral self-assembly (PICSA) were proposed. [90][91][92] Both of them are the bottom-up synthesis of chiral supramolecular aggregates by the chirality amplification process of intrinsic chiral monomers, from molecules to supramolecules and aggregates in bulk.

2.1.1
Post-assembly of chiral polymers by intrinsic chirality transfer Polymers containing the chiral center in the main chain or side chain could self-assemble into supramolecular chiral aggregates through the transfer and amplification of chirality  [87] (2) PFAB*: chiral center in the side-chain; [88] (3) PMAA-b-PAzo*: typical amphiphilic structure for polymerization-induced chiral self-assembly (chiral center in the side-chain). [14] (B) Chemical structure of typical achiral polymer. (4) PAzo x -r-AzoOH y : achiral random copolymer; [89] (5) PEO-b-PMMA(azo): achiral block copolymer; [68] (6) F8AZO: achiral main-chain alternating copolymer [64] from molecules to supramolecules and further to phases or aggregates. [61,93] Especially, the chiral block copolymers (BCP*) are excellent candidates to construct supramolecular chirality and phase chirality in polymer systems due to the spontaneous microphase separation, which has attracted myriad attention because of the intriguing abilities to form chiral structures and potential applications. [16,32,94,95] One typical example is enantiomeric poly(lactide acid) PLAcontaining BCPs*, which could assemble into diverse chiral morphologies under different assembly conditions. [96,97] In 2012, Ho et al. reported that enantiomeric chiral block copolymers, polystyrene-b-poly(L-lactide acid) (PS-b-PLLA) and polystyrene-b-poly(D-lactide acid) (PS-b-PDLA), could self-assemble into left-and right-handed helical nanostructures, respectively. The process of chiral transfer was clearly demonstrated by CD and transmission electron microscope (TEM). Supramolecular chirality and morphological chirality (phase chirality) were determined by the molecular chirality of PLA block. [61] The results provided insights into the morphological evolution from the molecular level to phase level. Recently, Ho's group further broadened the chiral phase structure with curved multilayered lamellae containing concentric lamellar and roll-cake textures. This can be ascribed to the spiraling of the twisted ribbons by the self-assembly of PLLA-rich PS-b-PLLA under the kinetic control of the nucleation and growth in the microphase separation process. [18] It was found that the formation of the twisted ribbon was the synergistic effects of conformational asymmetry, liquid-crystal behavior, and steric hindrance ( Figure 5A). In addition to the spiral hierarchical superstructures, twisted aggregates with single-sided strip could also be fabricated by the self-assembly of PS-b-PLA in a distinct fashion. Zhu et al. reported a twisted and single-sided strip morphology, called Moebius strips, generated by the chiral block copolymer PS-b-PDLA through the similar postassembly strategy. The fine tuning between the crystallization process of PDLA and the microphase separation was the key factor in the formation of the Moebius strips. [87] Moreover, while the PDLA block was removed, the mesoporous chiral channels could be constructed within the Moebius strips.
Kinetical control and confinement assembly (assembly in an ultra-thin film, cylindrical pores, and emulsion droplets) both have an important effect on the formation process of chiral suprastructures. Herein, Zhu's group further systematically studied the self-assembly behavior of the chiral block polymer PS-b-PDLA in emulsion droplets to delicately control the assembly structures. [15] The results indicated that three different morphologies could be obtained by changing the solvent evaporation rate under 3D confinement. The results indicated that the self-assembly process possessed a kinetically dependent morphological evolution. The irreversible transitions of morphology from I → III and II → III were further realized, indicating that morphology I and II were kinetically trapped metastable state ( Figure 5B). Furthermore, the crystallization mechanism of PLLA in lamellae-forming PS-b-PLLA was further discussed by Steinhart et al. [17] They found that the fastest PLLA crystal growth was preferentially aligned with the nanopore axes to the same degree compared to the PLLA homopolymer, independent of whether PS was glassy or soft. The results provided insights into the diverse assembly features of chiral block copolymer PS-b-PLLA. The delicate balance of chiral interaction, crystallization of PLA, and microphase separation behavior played a significant role in regulating the chiral suprastructures of PS-b-PLLA block copolymer. These examples demonstrated that the block polymer PS-b-PLA is an excellent candidate to construct chiral suprastructures and the subtle condition changes will lead to the change of helical phase structure under the thermodynamic-/kinetic-controlled assembly conditions. Apart from typical morphologies formed by chiral PS-b-PLA, some special three-dimensional topological structures, including rod-like micelles and helical toroids, and so on, could be obtained by post-assembly of other chiral polymers. Lin et al. reported the successful preparation of uniform toroids by chiral poly(γ-benzyl-L-glutamate)-F I G U R E 5 (A) Spiral hierarchical superstructures from twisted ribbons by self-assembly of chiral block copolymers (PS-b-PLLA). Reproduced with permission. [18] Copyright 2020, American Chemical Society. (B) Self-assembly of chiral block copolymer (PS-b-PDLA) in emulsion droplets under kinetical control. Reproduced with permission. [15] Copyright 2020, American Chemical Society. (C) Structure of the helical nanotoroids assembled by homopolymer PBLG and chiral block copolymers (PBLG-b-PEG). Reproduced with permission. [99] Copyright 2020, Wiley-VCH. (D) Induction and regulation of supramolecular chirality in chiral domino-type polymer. Reproduced with permission. [100] Copyright 2021, Springer graft-poly(ethylene glycol) (PBLG-g-PEG) copolymers via the post-assembly strategy in the ternary mixed solvents THF/DMF/H 2 O, where the chiral arrangement of the phenyl substituents on PBLG backbone was formed. [98] As shown in Figure 5C, they further fabricated a novel nano-aggregates composed of the toroidal and helical structures. [99] The chiral homopolymer PBLG can self-assemble into toroid-like aggregates first, and then the chiral block copolymers (PBLGb-PEG) assembled on the surface of toroids to form helical toroid-like aggregates. The morphological chirality could be regulated by the inherent chirality of the chiral block polymers.
In addition to the self-assembly of chiral block copolymers to regulate the chiral suprastructures driven by the balance of chiral interactions, phase separation, and crystallization, the polymer with chiral α-end group could also be used to construct supramolecular chirality through chiral transfer and chiral amplification. [100,101] In particular, recently our group demonstrated that chiral domino effect could be used to fabricate polymer-based chiral aggregates by the post-assembly method in mixed solvent. The azobenzene (Azo) polymers were rationally designed and prepared by atom transfer radical polymerization (ATRP) including a chiral α-end Azo moiety as one chiral terminus and achiral Azo repeating units as building blocks. [100] The chiral terminus can regulate the helical orientation of the achiral Azo stacks in the aggregation state ( Figure 5D). Moreover, the spacer length between the chiral residue and the achiral repeating units could control the supramolecular chirality of the side-chain Azo groups. The chiral-achiral interaction was further analyzed by theoretical simulations. Different from the former examples, this method provided a low cost approach for the preparation of chiral aggregates, as it required only one chiral unit in the polymer chain.
Previous reports have shown that the post-assembly of chiral polymers is an effective strategy to construct chiral aggregates with the chirality transfer from repeating units to aggregates. The special chiral polymeric aggregates with various morphologies have broad potential applications in chiroptical metamaterials, chiral plasmonics and photonics. Owing to the rapid development of assembly technology and the living polymerization strategy simultaneously, we will introduce the production of chiral aggregates by the in situ assembly strategy during the polymerization process of chiral monomers in detail in the following section.

2.1.2
In situ polymerization-induced chiral self-assembly Controlled assembly of chiral polymer with different sizes and shapes into high-level structures including collective functionalities is an important pursued goal in chiral polymer chemistry. However, the post-assembly of chiral polymers usually exhibits low solid content and cumbersome processing operation steps. In recent years, as a convenient F I G U R E 6 (A) Schematic illustration of polymerization-induced chiral self-assembly (PICSA) formed by a chiral block polymer containing Azo group in the side chain. Reproduced with permission. [14] Copyright 2020, Wiley-VCH. (B) The multiple chiroptical inversion in the PICSA process using a Azocontaining block polymer without external stimulus. Reproduced with permission. [121] Copyright 2021, Wiley-VCH. (C) The change of stacking mode of Azo group in PICSA process. Reproduced with permission. [122] Copyright 2021, Wiley-VCH. (D) The precise synthesis and PICSA process of poly(phenyl isocyanide)-b-poly(phenyleneethylene) copolymers into various chiral structures. Reproduced with permission. [123] Copyright 2021, American Chemical Society method for fabricating the desired block copolymer morphology, polymerization-induced self-assembly (PISA) has been extensively studied. [102][103][104][105][106][107][108][109][110][111][112][113][114][115] PISA facilitates the formation of a myriad of macromolecular nanostructures depending on the solvophilic/solvophobic volume ratio at a very high solid contents (up to 50 wt%). Living radical polymerization techniques, including ATRP [116,117] , nitroxide-mediated polymerization (NMP) [118][119][120] , and reversible additionfragmentation chain transfer (RAFT) polymerization have been widely used in PISA process to enable the controllable assembly. [104,105,107,108] Based on the previous reports on PISA, recently, our group reported a simple, efficient, and in situ strategy, termed PICSA, for the precise and large-scale preparation of hierarchical and multiple-scale supramolecular chiral aggregates with various morphologies. [124] PICSA approach integrated the features of supramolecular self-assembly, liquid-crystalline ordering and simultaneous polymerization reaction, and achieved the hierarchical supramolecular chiral assemblies in situ based on the stacking of chiral Azo units ( Figure 6A). [14] The precise induction, transfer, and amplification of chirality in polymer systems were realized during the PICSA process. Subsequently, we further demonstrated that multiple supramolecular chiral inversions of the Azo moieties can be regulated by the degree of polymerization (DP) and achiral tail lengths, which were driven by the transformation of intra-chain π-π stacking, inter-chain H and J aggregation of Azo stacks. These stacking mode transitions were demonstrated to be controlled by the phase state of liquid crystalline. These studies demonstrated that the supramolecular chirality could be regulated in polymeric system without external stimulus and stereocenter switching ( Figure 6B). [121] Further photoisomerization and thermal recovery experiments revealed that the helix inversion proceeds via a thermodynamically and kinetically controlled pathway-dependent manners. [122] Further research on polymerization-induced helicity inversion (PIHI) can lead to a deeper understanding of the regulation of self-assembly process, stacking modes, and supramolecular chirality in polymer systems ( Figure 6C).
Very recently, Wu et al. introduced the macromolecular chirality into the PICSA process. The in situ mainchain helical polymer assemblies were obtained when the poly(phenyl isocyanide)-b-poly(phenyleneethylene) (PPI-b-PPE) copolymers were controlled synthesized. As the PPE block length increased, different morphologies including helical nanofibers with a controlled size and defined helicity and nanoparticles were obtained. Moreover, the CD and CPL data demonstrated that the chirality in both ground state and excited state could be induced ( Figure 6D). [123] In another example, the achiral guest molecule (Nile Red) was introduced into the PICSA system of helical poly(aryl isocyanide) amphiphilic diblock copolymers, which were encapsulated into the core of the spheres. [92] The successful chirality transfer from copolymer to Nile Red was confirmed by the chiral signal of Nile Red in the PICSA process. These studies demonstrated that the PICSA strategy was an effective tool to fabricate chiral functional materials for the controlled release of drugs.
Shen et al. found a special morphological evolution process in PISA system. [125] The achiral BCP with a fluorinated stabilizer underwent a morphology transition from spheres to helical nanowires during aging, which consists of the twist of thinner nanowires. A fusion mechanism was then proposed, revealing that spheres were formed in the earlier PISA process and then underwent a fusion into helical nanowires after aging for days. The marginally dissolved 2,2,2-triuoroethyl methacrylate (TFEMA) units within the stabilizer block functioned as a glue between different nanowires. Thus, the nanowires aggregated and twisted into multiple helices so as to reduce the contact between TFEMA units and ethanol solvent.
These studies showed that both monomeric chirality of molecular configuration and macromolecular chirality in the main chain in PICSA can be transferred to supramolecular and macroscopic chirality. Furthermore, the very high solid contents could be obtained by the facile operation in the PICSA system. It provided a new strategy for fabricating optically active polymeric aggregates with well-defined morphologies.

Assembly of achiral polymers by external chirality transfer
Based on the discussions above, it can be obtained that the chiral supramolecular assembly of polymers can enable the originally chiral molecules or chromophores in the polymer chain to be chiral amplification in a nonlinear way, forming a certain ordered chiral aggregate. And more importantly, chirality information can be also induced through chiral transfer in polymers that do not originally possess chiral property. Under the interaction of noncovalent bonds or other external stimulus, chirality transfer between achiral polymers and chiral guests imparts chirality information to achiral polymeric aggregates. Chiral induction not only avoids the use of expensive chiral sources, but also simplifies synthesis processes such as asymmetric catalytic polymerization and enantioselective polymerization. It also expands the structural diversity of chiral polymeric aggregates, which is very important for the practical applications. Over the years, scientists have reported many methods in this field, mainly including the induction of chiral solvent, chiral additives, CPL, interfacial interaction, and liquid-crystal field. [73,77,78,126,128]

Chiral Solvent
Compared to small molecules, polymers have the advantages of film formation, thermodynamic stability, chemical modifiability, and special photoelectric properties, so the production of polymer aggregates with optical activity is of greater significance for practical application. Many studies have shown that when a chiral solvent is introduced into an achiral polymer system, the van der Waals forces, CH/π interactions, dipole interactions, metal-ligand coordination, etc., can induce the formation of optically active polymer aggregates. [79,128,129] In 2010, our group presented the synthesis of achiral polymer poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-4,4ʹazobenzene] (F8AZO). It could assemble into optically active aggregates in mixed solvent containing chiral limonene, which showed obvious aggregation ICD. Moreover, the dynamic disaggregation process occurred due to the reversible trans-cis photoisomerization of Azo groups in the polymer main chain ( Figure 7A). Subsequently, our group demonstrated the achiral π-conjugated polymers, poly(9,9dioctylfluorene) (PF8), could assemble into chiral gel in neat limonene under relatively low temperature. Atomic force microscopy (AFM) data revealed that the gels were composed of lots of entangled helical nanofibers with defined handedness. The chiral PF8 aggregates showed better chiral memory and chiral sensing properties than the chiral polyfluorene, and remarkable circularly polarized luminescence was also observed in aggregation state ( Figure 7B). The results revealed that helically distorted PF8 aggregate was responsible for the CD and CPL functionality.
Limonene, as the "magic" solvent molecule, not only could interplay with alkyl chains through CH/π interaction, but also induce polymers with Azo units in the side chain to form the supramolecular chiral structure. By rationally designing the side-chain Azo polymer structures via RAFT polymerization, our group synthesized an achiral nematic polymer system including Azo units and terminal hydroxyl groups in the polymer side chain. Here the chiral limonene solvent was used as chiral source in the vapor form. The chirality could transfer from limonene vapor to Azo building blocks, and the Azo units bearing hydroxyl groups could then be crosslinked via acetal reaction. After completely removing the chiral source (chiral limonene), the partially cross-linked Azo polymer chains could store the chiral information and trigger the complete chiral self-recovery when the supramolecular chiral structure was destroyed by light, solvent, and heat ( Figure 7C). [89] This is the first example on the dynamic chirality switch based on an achiral polymeric film without any chiral source.
In addition to the formation of side-chain polymer chiral aggregates induced by the chiral solvent, some examples of solvent-to-polymer chirality transfer for constructing mainchain helical structures were also reported. For example, Fujiki et al. reported that the intra-chain axial chirality was formed when the achiral polymer, PDPAs, was dissolved in limonene, which could transfer molecular chirality to the side phenyl stacks. [66] Moreover, the chain length and substitution position have a significant effect on the chirality transfer process. Sterically congested and highly stable side-chain structures were unfavorable for the chirality transfer. The macromolecular chirality in an achiral polymer was successfully constructed by the chiral solvent induction ( Figure 7D). These results indicated that both side-chain and main-chain polymers could be induced into helical structures by appropriate chiral solvent, which provided a facile way to construct chiral structures from achiral polymer systems.

Chiral additive
In addition to chiral solvent, the construction of chiral nanostructure with ICD is a simple, flexible, green, and efficient method by introducing chiral small molecules into achiral systems. [130,131] Especially, lots of interestingly chiral structures with microphase separative phenomenon could be achieved by the chiral additives owing to the delicate balance of enthalpic driving and entropic penalty in block copolymer system. For example, the use of chiral dopants in achiral amphiphilic polymers could trigger the chirality transfer from dopants to multi-scale chiral microphases, which has very important applications in the fields of chiral separation and chiral template induction. By adjusting the repeating units of the polymer, the volume ratio of the  (Figures 4 and 8A). [68] Various morphologies including particles, spheres, worms, helical cylinders, and spirals were obtained by the multiple driven forces, mainly including self-organization of liquid-crystalline side-chain and hydrogen bonding interaction between TA molecules and LCBCPs. Moreover, the results indicated that the strength of the liquid-crystalline related to the Azo side-chain structures played a decisive role in the morphological transition. In 2022, Lu's group further used a similar Azo polymer and induced micron-scale macroscopic helical structures through chiral doping of TA molecules. Further mimicking spiral dynamics of plant vines in nature, the photoisomerization of Azo groups in the achiral BCP was applied to change the shape of the helical structures, which would be a smart material as the dynamic chiral switch ( Figure 8B). [69] The photo-responsive Azo groups in the polymer facilitate the photo-controllable modulation of the self-assembled helical morphology, enabling the construction and contactless regulation of dynamic complex nanostructures. Meanwhile, Yu et al. reported the fabrication and control of helical structures formed by the hierarchical assembly between an achiral Azo-containing copolymer (PEO 114 -b-PM 11 AZn) and chiral dopant TA. [127] The chirality transfer and amplification in films could be realized through hydrogen bonds among the PEO blocks, mesogenic blocks, and TA molecules. Helical structures in thin films could be easily constructed by inducing aggregated chirality under optimized annealing conditions. Apart from the self-organization of liquid-crystalline polymers, some examples without mesogens were also reported in recent years. For example, Watkins et al. reported the addition of TA enantioisomer into achiral diblock copolymers, poly(ethylene oxide)-b-poly(tert-butyl acrylate) (PEO-b-PtBA), to form the helical superstructures. This suggested the successful chirality transfer from the chiral small molecule to achiral polymer backbone with the occurrence of phase separation ( Figure 8C). [132] Moreover, the handedness of helical aggregates could be easily regulated by the chirality of TA. In another example, Lu et al. designed and fabricated a novel double-helical structure through the co-assembly of poly(1,4-butadiene)-b-poly(ethylene oxide) (PBd-b-PEO) diblock copolymer with the additive TA. [133] Supramolecular chirality in PBd-b-PEO was achieved through intermolecular hydrogen bonding, and the induced helix orientation depended on the chirality of the TA. After the removal of TA, this helical structure could act as a chirality-induced template to achieve the chiral arrangement of different achiral building blocks such as nanoparticles and organic molecules ( Figure 8D). The gold nanoparticles (AuNPs) and AIE molecule TPEOH could be induced to adopt the chiral helical arrangement. But unfortunately, there was no fluorescence or CPL emission due to the isomerization of TPEOH after thermal annealing.
Previous studies have shown that the self-organization of liquid crystalline of the Azo groups and phase separation strength of the PEO-based block copolymer are also key driving forces to regulate the various aggregated structures. The addition of chiral molecules into the achiral polymers could lead to the formation of polymeric chiral aggregates F I G U R E 8 (A) Chiral additive-induced achiral liquid-crystalline block copolymers (LCBCPs) into macro aggregates with morphological structural chirality. Reproduced with permission. [68] Copyright 2022, American Chemical Society. (B) The assembly and reversible change of chiral to achiral morphology in LCBCP film doped with TA. Reproduced with permission. [69] Copyright 2021, Wiley-VCH. (C) Chiral additive-induced achiral block copolymers into macro aggregates with helical phases. Reproduced with permission. [132] Copyright 2014, American Chemical Society. (D) The construction of double-helical nanostructures by the co-assembly between an achiral diblock copolymer and TA. Reproduced with permission. [133] Copyright 2018, Wiley-VCH with controlled morphologies. It could be an effective way to construct the hierarchically ordered chiral structure by rationally introducing chiral interaction site. Apart from chiral TA molecule, chiral binaphthyl derivatives and phenyl lactic acid, could also be used as chiral dopants to induce supramolecular chirality of polymers. [134,135]

Other chirality induction methods
Recently, other special methods including CPL irradiation, [71,73,139] interfacial interaction, [74,136] gelation, [76,86] and liquid-crystal field [78] have been widely introduced to induce the chiral structures in achiral polymer systems. The generation of homochirality in nature and organisms has long been thought to be caused by CPL. [137] Hence, the transfer of spin angular momenta from CPL to the chromophore in the chemical structure may cause asymmetric structural changes of chemical bonds, further leading to the molecular or supramolecular chirality in the photochemical synthesis. [138] In 2017, our group and Fujiki et al. systematically investigated the chiral induction behavior of achiral side-chain Azo polymers by CPL with different handedness, irradiation wavelength, and irradiation time in an optofluidic medium ( Figure 9A). [138] The results indicated that supramolecular chirality of Azo units was successfully induced with a high dissymmetry ratio (±1.3 × 10 −2 at 313 nm). Subsequently, the inverted and switched behaviors of supramolecular chirality were further realized in several cycles irradiated by CPL with the same handless but different wavelengths. The experimental demonstration provided a new insight into the supramolecular chirality in achiral polymer aggregates induced by CPL. [70] Subsequently, Liu et al. reported that the achiral Azo polymers could be triggered photo-alignment into the supramolecular helical structure by CPL from different forms, including transmitted CPL and emitted CPL. It is found that Azo polymers with longer methylene groups were more sensitive to the chiral induction by CPL. In addition, the Azo polymers after annealing showed a very high absorptive dissymmetry factor (g abs ) after CPL irradiation. Both the transmitted and emitted CPL can be used to regulate the chiral alignment in the achiral Azo polymers systems ( Figure 9B). Meijer et al. reported an alternating copolymer poly(9,9ʹdialkylfluorene-alt-azobenzene) (PFAB*) including fluorene and photoisomerized Azo units forming cholesteric liquidcrystalline phases in thin films with controllable chiroptical properties. [88] Then the supramolecular helicity could be in situ switched by alternating irradiation with l-and r-CPL stimuli. The supramolecular helicity of the aggregate films could also be reversibly regulated by the trans-cis-trans isomerization process of Azo units ( Figure 9C).
In 2013, Liu et al. constructed a novel superhelical structure by an achiral random copolymer containing poly(Nvinylcarbazole) (PVK) bearing Azo chromophores and nitro moieties (PVK-AZO-NO 2 ) at air-water two-phase interface ( Figure 9D). [136] The compression in the two-phase interface, as an external stimulus, played an important role in the formation of helices in the achiral polymeric film, which further self-assembled into large twisted fibrillar bundles. This method provides a facile way to construct supramolecular chiral structure based on an achiral polymer system.

F I G U R E 9 (A) The supramolecular chiral stacking of Azo units induced by circularly polarized light (CPL) irradiation with different wavelength
and handedness in optofluidic medium. Reproduced with permission. [70] Copyright 2017, American Chemical Society. (B) The formation of chiral helix of Azo-based side-chain polymers with different spacers controlled by CPL produced by chiral cellulose nanocrystal films. Reproduced with permission. [138] Copyright 2022, Wiley-VCH. (C) The dynamic process of helicity switching of (S,S)-PFAB films and corresponding schematic illustration showing the change of supramolecular helicity under the alternating irradiation with l-and r-CPL. Reproduced with permission. [88] Copyright 2021, Wiley-VCH. (D) Supramolecular chirality induced by air-water interface in an achiral random copolymer system (PVK-AZO-NO 2 ). Reproduced with permission. [136] Copyright 2013, American Chemical Society. (E) The co-assembly of two achiral main-chain polymers with a gelator to form macroscopic supramolecular chirality. Reproduced with permission. [86] Copyright 2016, American Chemical Society. (F) Preparation of helical polythiophenes induced by asymmetric liquid-crystal fields. Reproduced with permission. [78] Copyright 2019, Wiley-VCH Recently, Liu's and our group proposed a new strategy for inducing achiral polymers into helical structures. [76,86] As shown in Figure 9E, a L-or D-glutamide gelator was chosen to co-assemble with two achiral conjugated polymers poly(9-(1-octylnonyl)-9H-carbazole-2,7-diyl) (PCz8) and poly (9,9-di-n-octylsila-fluorenyl-2,7-diyl) (PSi8), respectively. [86] The resultant gels exhibited macroscopic supramolecular chirality, which was determined by the molecular chirality of gelator. More importantly, the supramolecular chirality could be stored even after the gelator was removed.
Akagi et al. were committed to the asymmetric polymerization of achiral monomers induced by asymmetric liquid-crystal fields to produce optically active helical polymers. For example, a chiral nematic liquid-crystal field was prepared by adding a chiral cholesteric derivative as a chiral dopant to a mixture of two nematic liquid crystals. Then helical polyacetylene (H-PA) was prepared by polymerization of acetylene monomer catalyzed by the catalyst dissolved in the as-prepared chiral nematic solvent, realizing the chirality transfer from the external liquid crystal to the helical polymers. [139] Scanning electron microscopy (SEM) images revealed that the polyacetylene film was composed of fibers with a clockwise or counterclockwise helical structures. An obvious Cotton effect was observed in the π-π* transition region of the polyacetylene chain in the CD spectrum. This study showed that in the case of a right-handed helical N*-LC, the PA chain could grow in a left-handed manner, because the PA chain with the opposite twist direction to that of the N*-LC can propagate along the LC molecules. But when the PA chains with the same orientation as the N*-LC encountered the LC molecules, the propagating stereospecificity was hindered, thus causing the induced twist to be opposite to the actual liquid-crystal field. [77] Furthermore, as shown in Figure 9F, the asymmetric electrochemical polymerization in ionic liquids was achieved to synthesize helical conjugated polymers by the same group. [78] They prepared chiral liquid-crystalline ionic liquid (N*-LCIL) by adding axially chiral binaphthyl derivative as external chiral dopants to imidazolium cation-based achiral ionic liquid LCILs. Helical poly(3,4-ethylenedioxythiophene) (H-PEDOT) films were obtained by the polymerization of a dimer-or trimertype 3,4-ethylenedioxythiophene (EDOT) monomer in an N*-LCIL. Among them, N*-LCIL acted both as an asymmetric solvent and as a supporting electrolyte. The H-PEDOT films exhibited a helical π-stacked structure of conjugated chains and helical morphology of one-handed twisted fiber bundles. The fiber bundles were about 1 μm long, which was about half the helical pitch of N*-LCIL, but the helical direction was opposite to that of N*-LCIL. This method was different from the examples described above, because the chiral induction occurred during the polymerization process.

CPL emission
One of the fascinating advantages of chiral polymer aggregates is CPL emission, because it shows important potential application in optical information storage, 3D optical displays, bioencoding, and photoelectric devices. [140][141][142] Compared to the CPL materials formed by organic small molecules, helical polymer-based CPL materials are attracting more and more interest because of the merits of relatively high stability, good processability, and unique and controlled helical chirality of polymer backbones or side chains, which are more favorable to realize a high luminescence dissymmetry factor (g lum ). [143,144] Moreover, chiral polymeric aggregates are good templated materials and could be used as chiral matrix to induce other achiral luminescent units to realize CPL emission. [145] Here the CPL-active material could be divided into two categories according to the difference in fluorescent component: (1) The polymers could be chosen as the fluorescent component and hence the induced chirality in the polymeric system is the key factor to construct CPLactive materials; (2)  molecules) could be introduced into polymer system. [146][147][148][149][150][151] In addition, recently Deng et al. first demonstrated that chiral polymer film could be the CPL filter without the chemical or physical interaction between fluorescent components and chiral polymer when the CD of the chiral polymer film overlapped with the photoluminescence (PL) of the fluorescent components. [152,153] In 2012, Akagi et al. prepared a series of polythiophenes and their phenylene copolymers with chiral alkoxy carbonyl substituents in the side chains. The inter-chain helically π-stacked supramolecular structure was formed in both solutions and films, exhibiting RGB-colored fluorescence with high quantum yields and high g lum , due to the self-organizing of the N*-LC phase. Moreover, the white CPL emission could be obtained by the appropriate mixture of different types of polymers ( Figure 10A). The study demonstrated that the hierarchical chiral arrangement of conjugated polymers, such as chiral liquid crystalline, may be the key factor to enhance the luminescent dissymmetry factor of CPL. Subsequently, Lu et al. reported a general method for preparing polymeric helix with CPL emission property by the ternary co-assembly between a nonchiral block copolymer PB-b-PEO), a chiral additive, and nonchiral fluorescent molecules ( Figure 10B). They demonstrated that the helical handedness of BCP was able to regulate the chirality of CPL emission: left-handed CPL emission obtained by a right-handed helix and vice versa. [145] The results revealed that an antithetical effect was related to the length between the adjacent interacting points of nonchiral fluorescent molecules along with the helical structure.
Circularly polarized organic light emitting diodes (CP-OLEDs) have significant application prospect in 3D display and information storage. A variety of methods were discussed to prepare the polymeric CPL-active materials for the fabrication of CP-OLEDs. [157][158][159] Co-assembly between chiral polymer and achiral dye molecules resulted in the formation of CPL-active materials in which the chiral polymer aggregates could provide the chiral environment and even the initial emitter. In 2022, Cheng et al. synthesized two axial-chiral binaphthyl polymers with different dihedral angles, which co-assembled with achiral pyrene-naphthalimide (NPy) dye molecule. After the thermal annealing, the co-assembled film showed strong CPL emission due to the formation of more ordered helical nanofibers via intermolecular ππ interactions. Then, the CP-OLEDs device based on the annealed co-assembled film was prepared which showed high circularly polarized electroluminescence (CPEL) intensity ( Figure 10C). [155] This study will facilitate the polymeric CPL-active material into the area of 3D display.
Apart from the supramolecular chirality constructed by the polymer assembly in an ordered manner, polymers with main-chain helix handedness is another important type of CPL-active material. [160] In 2020, Maeda et al. reported that the PDPA bearing carboxy pendants could form onehanded helix by the induction of nonracemic amines in water. [156] When the chiral amines were removed, the induced main-chain helical structures could be retained. Moreover, the helical PDPA exhibited strong CPL emission in the homogeneous solution, which was different from the previously reported supramolecular chiral assemblies with CPL performance. This experiment provided a new idea to construct polymer-based CPL-memory materials by regulating the main-chain helicity, which has the potential applications in chiral recognition and enantioseparation materials ( Figure 10D).

Chiral recognition and resolution
Chiral recognition and enantioseparations is one of the most significant functions of chiral polymers for application in laboratory and industry. For some pharmaceuticals and agrochemicals, one enantiomer usually is positive but the other is useless or causes toxic side effects. Hence it is very important to measure the e.e values and separate chiral chemicals. Many kinds of materials were fabricated for enantioselective recognition and separation including aggregation-induced emission luminogens (AIEgens), [161] chiral covalent organic framework, [162] and helical polymers. [163] Among them, helical polymers show broad application prospects due to their excellent post-processing properties. [164] Substituted polyacetylenes are a typical type of dynamic helical polymers with variable conformation, which are good candidates for the chiral recognition and separation-active materials. Previous studies demonstrated that the substituted polyacetylenes could form helix structures by acid-base complexation or hydrogen bonding interaction, resulting in the strong induced CD signals, which could be used to chiral recognition for amines or amino acids. [165,166] Recently, the strategies of chiral recognition by naked-eye discrimination based on helical PAs materials have been thoroughly discussed. [167,168] In 2021, Maeda et al. reported the helical substituted polyacetylenes with static chiral structure, which showed visible color changes when the chirality of chiral amines changed. Changes in the effective π-conjugation length of polyene backbones can induce changes in nakedeye color, conferring their utility in direct colorimetric F I G U R E 1 0 CPL generated by chiral supramolecular polymer assemblies. (A) Schematic representation showing the inter-chain helically π-stacked structure of polythiophene-and thiophene-based copolymers and precise intermolecular distance calculated by X-ray diffraction data. Reproduced with permission. [154] Copyright 2012, American Chemical Society. (B) Scheme showing the antihelical self-assembly of achiral fluorescent molecules in the helix-formed block copolymer PBdEO/TA film and corresponding CPL spectrum. Reproduced with permission. [145] Copyright 2020, American Chemical Society. (C) Schematic illustration of the co-assembly between chiral polymer and achiral pyrene-naphthalimide and prepared CP-OLEDs device configuration. Reproduced with permission. [155] Copyright 2022, Wiley-VCH. (D) Scheme showing the helicity induction and memory of macromolecular chirality of poly(diphenylacetylene)s with CPL emission. Reproduced with permission. [156] Copyright 2020, American Chemical Society stereo-recognition. The chiral recognition system had a very high sensitivity to the naked eyes which could measure the difference of less than 2% in the e.e value. The on-site, naked-eye determination and quantitative determination of e.e values of nonracemic amines could be realized in helical polymer systems. [167] The mechanism was mainly due to the change of polymer helical pitch of π-conjugated polymer main-chain in various solvents ( Figure 11A). In 2022, Wan et al. reported a multichannel visual detection (multi-responses in absorption, CD, fluorescence, and CPL signals) of chiral amines by the self-reporting activated ester-amine reaction in the helical substituted polyacetylenes system according to the conformational change-induced emission. In this system, both achiral and chiral amine mixtures could be determined with fast and intense response and high accuracy. These researches suggested that chiral polymer-based materials had huge potential applications in the field of chiral recognition as smart responsive materials ( Figure 11B).
In addition to the chiral recognition and sensing, chiral enantioseparation is another important issue for chiral pharmaceuticals and agrochemicals. Enantioselective crystallization was an effective and convenient method to achieve optically pure chiral molecules by using chiral polymers. [163,[169][170][171][172] For example, Wu et al. synthesized the uniform organic/inorganic hybrid silica nanoparticles with the helical poly(phenyl isocyanide) (PPI) grafted on the surface with high density. The chiral hybrid nanoparticles (NPs) exhibited intensely optical activity due to the helical polymer chains on the surface. As a result, the chiral hybrid NPs showed high enantioselective during the crystallization of racemic Boc-alanine, in which the e.e value of the induced enantiomer can reach 95%. [170] Subsequently, Wu et al. further grafted the helical PPI onto Fe 3 O 4 magnetic nanoparticles. The formed NPs showed both magnetic character and optical activity. As a result, the enantioselective crystallization of racemic threonine could be realized and the used NPs could be further facilely recovered due to the magnetic character. [171] In 2019, Wan et al. expanded the strategy of enantioselective crystallization using the magnetic NPs. The chiral nanosplitters were fabricated by co-assembling amphiphilic poly(N 6 -methacryloyl-S-lysine)-blockpolystyrene ((S)-PMAL m -b-PSt n ) diblock chiral copolymers and hydrophobic Fe 3 O 4 @oelic acid magnetic NPs. [173] Then a facile and quantitative isolation of the racemic crystals in a single crystallization process can be achieved by the chiral polymeric nanosplitters with high reusability and separating capability. Moreover, it has been found that the aggregates with more contact area exhibited better chiral recognition ability. Therefore the worm-like micelles showed more effective chiral interaction than spherical micelle and vesicle.
Chiral polymeric structures were also applied as chiral stationary phases (CSPs) of high-performance liquid chromatography. [174][175][176] Helical polyacetylene-based derivatives are good candidates for CSPs. It was found that opposite helical senses could be obtained after thermal annealing compared with the original helical conformation despite bearing the same chiral center. As expected, the helical structure showed better chiral recognition ability than the structures with weak preferred-handed helicity. [174] Although chiral polymers as the chiral additives for enantioselective crystallization and CSPs of high-performance liquid chromatography have been reported, the used chiral polymers were mainly obtained by tedious synthesis procedures. The supramolecular polymeric assemblies formed by the facile assembly strategy are still in their infancy when used as chiral recognition reagents and CSPs. Exploring more polymeric chiral structures will be an important task in the future studies. And the relatively weak stability of noncovalent bonds F I G U R E 1 1 (A) Visual colorimetric detection of chiral amines based on main-chain helical polymer. Reproduced with permission. [167] Copyright 2021, American Association for the Advancement of Science. (B) Schematic illustration of enantioselective visual detection of chiral amines based on main-chain helical polymer. Reproduced with permission. [168] Copyright 2022, Wiley-VCH may be the major challenge for the wide application of polymer-based supramolecular helical assemblies as CSPs.

Chiral liquid-crystalline elastomer
In most previous examples, microscopic helical structures of small molecules, oligomers, and polymer assemblies can be induced by chiral sources (chiral solvents, chiral additives, CPL or asymmetric forces, etc.), in which the size is generally at the nanometer scale. This is crucial for the mechanistic studies of chirality transfer, regulation, and amplification. However, further transmitting and amplifying the molecular chirality information to the macroscopic helical structure changes (e.g., the helical shape of loofah and cucumber vines are derived from the chiral structure changes at the internal molecular level) remains a challenge. [177] This thorny problem comes from the preparation of materials in bulk with controllable chiral structure. Therefore, preparing polymer materials with good film-forming properties, ductility, and certain mechanical properties, as well as controllable chirality is an urgent problem to be addressed. [178][179][180] Liquid-crystal elastomers (LCEs) or networks (LCNs) have received great attention in recent years due to their ability to perform reversible shape changes and complex motions in response to external stimuli, which may be a good choice to construct macroscopic chiral materials.
In 2011, Selinger et al. investigated the effects of microscopic chiral mesogenic alignment on macroscopic helical deformations in twisted nematic elastomers. They synthesized liquid-crystal elastomer films with a left-handed 90 o orientation along the bottom to the top. This helical orientation was obtained by surface treatment with a chiral dopant and a mutually perpendicular orientation. The configuration of the chiral dopant determined the direction of the helical orientation. By photopolymerization, a cross-linked LCE film was obtained. They cut the film to obtain long strip samples, and the long axis of the strip was aligned parallel or perpendicular to the liquid crystal orientation of the intermediate layer. As shown in Figure 12A, it was found that for Sor L-type samples, the width determined the formed shape. That is to say, narrow splines formed helical surfaces, while wide splines formed columnar helical ribbons. Temperature changes could change torque, diameter, and helical direction and the entire deformation process was a thermally reversible process. [181] In addition to thermally driven helical shape changes, light-driven helical transitions or motions have also been successively reported, especially in the case of intercalation of Azo groups in polymer films. Katsonis et al. reported a chiral dopant induced a helical arrangement of mesogens with a left-handed or right-handed 90 o rotation along the film thickness direction. The cross-linking polymer contains a photo-responsive Azo group. They sheared along with F I G U R E 1 2 (A) Schematic diagram of the arrangement of helical nematic mesogens and various macroscopic helical morphologies. Reproduced with permission. [181] Copyright 2011, National Academy of Sciences. (B) Light-induced helical motion of Liquid-crystal networks using the polymer containing photo-responsive Azo group as cross-linker. Reproduced with permission. [182] Copyright 2014, Nature Publishing Group. (C) An anomalous reversible shape change based on a new Liquid-crystal elastomers containing Azo group. Reproduced with permission. [183] Copyright 2020, Wiley-VCH. (D) Light-induced twisting motion of polydomain film of chiral liquid-crystalline elastomer. Reproduced with permission. [184] Copyright 2021, American Chemical Society different angular directions, defined as the angle between the shearing direction and the orientation of the liquid crystal in the interlayer, and found that the shearing angle determined the direction and pitch of the helical shape. Under UV light irradiation, the pitch of the left-handed helical ribbon decreased, while that of the right-handed helical ribbon increased. The films obtained at larger shear angles even undergo helical inversion under UV light irradiation. Similar to the order-disorder phase transition mechanism of thermotropic liquid crystals, the photoisomerization transition destroyed the ordered arrangement of mesogens, causing them to shrink in the orientation direction and stretch in the vertical orientation. Through ingenious shearing paths, they obtained splines with both left-and right-handed helical ribbons with reversible piston-like telescopic motions features under UV light irradiation ( Figure 12B). [182] In an analogous manner, Katsonis et al. further designed a fluorine-containing Azo cross-linking agent with a longer half-life cis-structure. Upon photoisomerization-induced planar to helical shape transition, the helical deformation can be maintained for more than 8 days without reverting to the original planar shape. For the unsubstituted Azo cross-linking agent, the cisstructure will transform to a trans-form, and the anisotropic deformation will return to a short-term (less than 2 days) original transformation due to the poor thermal stability. This method of maintaining shape stability by prolonging the halflife of the cis-structure provides a reference for the design of molecular switches and light energy converters. [185] In 2020, our group and Zhao et al. reported a sidechain liquid-crystal polymer (SCLCP) elastomer based on a thermoplastic elastomer, styrene-butadiene-styrene (SBS) triblock copolymer and Azo units to obtain a novel SCLCP. The LCE showed an anomalous shape change, due to a subtle interaction of the opposing contributions to shape change from main-chains and side-group mesogens. [183] This LCE film had suitable processing temperature and liquid-crystal phase-isotropic transition temperature and extreme stretchability (500% strain at 50 • C). Interestingly, the mesogens were aligned perpendicular to the stretching direction, which was confirmed by X-ray diffraction (XRD) and polarized infrared spectroscopy. Different from most known LCEs, this LCE showed a pronounced auxetic-like shape change. In other words, the mechanically stretched monodomain splines were driven by the liquid-crystal ordered-disordered phase transition. The splines shrank along with the length and width directions simultaneously at the isotropic temperature, while in the liquid-crystal phase, the splines stretched in both directions at the same time. It was found that the thermally induced auxetic deformation-like behavior is due to a subtle interaction between the oriented main-chain and side-chains ( Figure 12C). This unusual auxetic-like reversible shape change expands the scope of application in liquidcrystal driver materials and auxetic materials in the field of smart drives. Although this LCE is achiral, the phenomenon inspires scientists to build novel chiral actuators. Subsequently, we further designed a new type of elastomer with SBS in the main-chain and chiral Azo, achiral Azo mesogens, and cross-linking sites in the polymer side chain. [184] Multidomain LCE films were obtained by solution casting and annealing under isotropic and LC phases, respectively.
Such films could realize plane-helix transition and dynamic reversible motion driven by LC phase transition. The configuration of the chiral Azo determined the macroscopic helical direction of the LCE film. Through thermally induced order-disorder transition of the helical arrangement of chiral Azo mesogens, they further amplified the molecular-level chiral asymmetry information to the macroscopic helical motion ( Figure 12D). The helical driving mechanism of this chiral molecule-involved multidomain LCE is significantly different from the previously reported single-domain LCE.
In addition to the application in CPL emission, chiral recognition and resolution and LCE, chiral polymers are excellent candidates for other applications including the asymmetric catalysis and biomedicine. Inspired by the biopolymers with catalytic function, unnatural polymers with controlled screw sense were fabricated to mimic the function. In recent years, many groups including Suginome, Yashima, Gellman, and others have reported helical polymers in the application of asymmetric catalysts. [51,[186][187][188][189][190][191][192][193] Suginome et al. reported helical poly(quinoxaline-2,3-diyl)s bearing metal-binding sites could be used as chiral ligands for asymmetric hydrosilylation of styrenes, due to the high purity for a one-handed screw sense and stability. [186] Subsequently, the same group synthesized a similar polymer-based catalyst by "Majority-Rules". [51] The enantioselectivity could be switched by solvent-dependent inversion of the helical sense. Yashima et al. reported a special example on highly enantioselective organocatalyst prepared by polymerization of a racemic monomer. [188] The helical polymer showed chiral memory after the removal of chiral resource. In addition to asymmetric catalysts based on helical polymers, Gellman's group is dedicated to the research of asymmetric catalysts on foldamers (special oligomers with a strong tendency to form a specific conformation). [190,191] The spatial chiral arrangement of functional groups could be predictable in the foldamers, which provided the basis of design for asymmetric catalysts. As one of the distinctive biochemical phenomena, chirality plays a crucial role in many physiological processes. Chiral polymers with good biocompatibility, tailorable ability, and ease of fabrication are also excellent candidates for diverse biomedical applications. As far as we know, chiral biomaterials have been widely investigated to study the interactions with cells, such as immune cells and the resulted immune responses. [194] The effects of chiral polymers on immune cells have attracted more and more attention. Regulating the chemical and physical properties of chiral polymers can lead to beneficial immune responses. [195,196]

SUMMARY AND OUTLOOK
In summary, we have reviewed the strategies for constructing and regulating supramolecular chirality and main-chain helical chirality in polymer aggregates. Although the assembly process was more complicated than small molecules due to the intricate polymer structure (molecular weight, molecular weight distribution, and typical phase separation behavior), the diverse helical structures have been fabricated under strictly controlled conditions. The formation of supramolecular chirality in polymers can be understood as the chirality transfer and amplification from the chiral configuration of molecules: intrinsic chirality (chiral polymers) or external chirality (chiral solvent, chiral dopant, and external chiral stimulus). In achiral polymer system, external chiral source could be introduced into the assembly process to realize the chirality transfer to polymer aggregates, mainly including chiral solvents, chiral additives, and CPL. The diverse helical structures make supramolecular polymer assemblies as excellent candidates for chiral nanomaterials with superior functionalities. Therefore, we briefly described the typical applications of chiral polymeric materials in CPL emission, chiral recognition, and chiral elastomer. Although the great achievements with respect to the chiral assembly in polymer systems have been realized, the challenges on the chiral transfer mechanism and the relation between helical morphologies and practical performance still remain. Therefore, further investigation on the chiral assembly and chiral regulation of polymers are still necessary. First, hierarchical helical structures with various unique properties could be found in nature at different scales. Inspired by the nature, the design, construction, and further regulation of chiral structure in polymer systems at the microand macroscopic scales have attracted tremendous attention. In Section 2, we have reviewed several recent examples where diverse chiral morphologies by assembly of chiral or achiral polymers. However, the complexity of chiral transfer mechanism still needs to be elucidated. It is difficult to amplify the chirality at the macroscopic scale, and the morphology of the aggregates is difficult to control. Intrinsic chiral transfer and external chiral transfer are the two chiral transfer methods, but their chiral amplification mechanisms are still under debates. Furthermore, the scope of polymers needs to be further expanded. For the external chiral transfer style, it seems to be applicable to a wider range of polymers, but the underlying mechanism of chirality transfer is even more puzzling, which limits the rational design of more functional systems. For example, the study of the mechanism on CPL-induced mirror-symmetry breaking in achiral polymer aggregates is still in early stage. The relationship between the direction and wavelength of CPL and the electric dipole moment and the magnetic dipole moment of the polymer needs further study. Although the advantages of the chirality induction system in Section 2.2 are obvious, such as avoiding tedious synthesis of chiral polymers and expanding the structural diversity of chiral polymeric aggregates, the weak points still need to be solved, such as poor stability and difficult to prepare in large quantities. The poor stability of chiral structures is mainly due to the weak interaction of chiral source and achiral polymers and the chiral structures showed high dependence on the chiral sources. For addressing the issue, a chiral seed covalently linking to the achiral polymer chains may be a feasible strategy, which may overcome the poor stability caused by the change of chiral sources. The difficulty in preparing in large quantities is mainly due to the occurrence of precipitation. PICSA may be a good strategy to address the issue, which could prepare chiral supramolecular structures in onepot with high solid content. But now the scope of PICSA systems needs further expansion. The integration of complementary advantages between chirality induction strategy and PICSA will be a better way to construct chiral supramolecular polymeric aggregates.
Second, the dynamic chiral transition is a central feature of biological supramolecular systems. Design and preparation of novel dynamically supramolecular polymer systems are of great significance. And another feature related to dynamics is the stability of the structure. Even though our group has made some progress on the immobilization of chiral supramolecular structures by the covalent cross-linking strategy, the reports on the stability of chiral structures are still scarce. More novel dynamic polymer systems with high stability should be explored, which may allow us to design functional dynamic chiral materials, such as supramolecular catalysts with tunable enantioselectivity and catalytic activity, chiral optical materials with tailorable band gaps, and so on. The high stability guarantees the efficiency of the material during practical application.
Third, chirality plays a crucial role in biological and artificial systems. However, the relationship between chiral structure and material properties still needs to be explored. For example, in the CPL-related materials, the current design concept revolves around the coexistence of chiral structure and fluorescence, but the regulation mechanism and the essential connection are still at the exploratory stage. The g lum value is of significance for the CPL-active material application. In the chiral recognition area, the investigation on the chiral filler fabricated by polymers with supramolecular chirality is still in its infancy. The resolution stability and resolution efficiency of the polymeric chiral materials should also be improved. The chiral fixation by the synergistic effect of noncovalent interaction and covalent cross-linking may be the feasible idea, which could significantly improve material stability. [89] Finally, the construction of hybrid materials based on polymeric chiral supramolecular structures will be the new direction. By the rationally designed polymer chiral suprastructures, it is possible to construct co-assemblies with functional inorganic nanoparticles or biomacromolecules (such as protein and enzyme), which will open up new windows in selective catalysis (metal catalyst or enzyme catalyst), photothermal therapy with good biocompatibility and chiral optoelectronic devices, etc.