Recent progress in polymerization‐induced self‐assembly: From the perspective of driving forces

Polymerization‐induced self‐assembly (PISA) enables the simultaneous growth and self‐assembly of block copolymers in one pot and therefore has developed into a high‐efficiency platform for the preparation of polymer assemblies with high concentration and excellent reproducibility. During the past decade, the driving force of PISA has extended from hydrophobic interactions to other supramolecular interactions, which has greatly innovated the design of PISA, enlarged the monomer/solvent toolkit, and endowed the polymer assemblies with intrinsic dynamicity and responsiveness. To unravel the important role of driving forces in the formation of polymeric assemblies, this review summarized the recent development of PISA from the perspective of driving forces. Motivated by this goal, here we give a brief overview of the basic principles of PISA and systematically discuss the various driving forces in the PISA system, including hydrophobic interactions, hydrogen bonding, electrostatic interactions, and π‐π interactions. Furthermore, PISA systems that are driven and regulated by crystallization or liquid crystalline ordering were also highlighted.


INTRODUCTION
The solution self-assembly of block copolymers (BCPs) has attracted intense interest since the first report on the aqueous self-assembly of BCPs by Eisenberg. [1]14][15][16][17][18] These assemblies have played an important role in the field of drug and gene delivery, [19] artificial cells, [4] colloidal self-assembly, [20] nano-motors, [21] organic/inorganic hybrid nanomaterials, [22] etc.In general, the polymeric assemblies were prepared via the nanoprecipitation method, where the solution of BCP is slowly added into a block-selective solvent to induce the self-assembly of BCP into polymeric assemblies. [23]However, it has been suffering from complex influential factors and poor reproducibility because of the slow rearrangement kinetics of the copolymers.Besides, polymer assemblies were produced at a quite low concentration (generally < 1 wt %), which largely restrained their scale-up.
From the perspective of the driving force, the majority of PISA systems are constructed based on hydrophobic interactions, where the self-assembly and re-organization of the amphiphilic copolymers are dependent on the chain length of the hydrophobic block.Besides the hydrophobic interactions, other supramolecular interactions, including hydrogen bonding, [93][94][95][96] electrostatic interactions, [97][98][99][100][101][102][103][104] and π-π interactions, [105][106][107] have also been exploited as the driving force for PISA (Figure 1B).Besides, some systems that involve multiple supramolecular driving forces, such as crystallization, [60,62,[108][109][110] and liquid crystallization, [111] have also been used to direct the PISA.Owing to the different self-assembly mechanisms of these driving forces, polymer assemblies stabilized by these supramolecular interactions have shown distinct structures, dynamicity, responsiveness, and functionality.For example, the hydrogenbonded polymer assemblies have intrinsic temperature-and pH-responsiveness, [96] while the electrostatic interactiondriven PISA brings ionic monomers for the monomer arsenal of PISA, resulting in polymer assemblies with saltresponsiveness. [103]Therefore, the exploration of new PISA formulations that are driven by new supramolecular interactions has attracted more and more attention.This review intends to summarize the recent development of PISA with different driving forces, including hydrophobic interactions, hydrogen bonding, electrostatic interactions, and π-π interactions.Furthermore, PISA systems that are driven and regulated by multiple supramolecular interactions, such as crystallization and liquid crystallization, are also discussed.The polymer assemblies with different driving forces in this review and their polymerization types and techniques are summarized in Table 1.

PISA DRIVEN BY HYDROPHOBIC INTERACTIONS
In most PISA systems, hydrophobic interactions underlie the in-situ self-assembly of BCPs and determine the morphology evolution of the polymer assemblies.[129][130][131] In other words, the hydrophobic interaction energy increases with the increasing DP of the TA B L E 1 A summary of all the driving forces of polymerization-induced self-assembly (PISA) discussed in this review.

Driving force
References Insights PISA type

Hydrophobic interactions
Yuan et al. [112] Fluorinated monomers with general solvent applicability

Dispersion polymerization RAFT
Yuan et al. [113] Host-guest interaction assisted aqueous dispersion polymerization of hydrophobic monomers

Dispersion polymerization RAFT
O'Reilly et al. [114] Predicting monomers for aqueous PISA by quantifying the hydrophobicity of their oligomers

Emulsion polymerization RAFT
Hydrogen bonding O'Reilly et al. [95] The first example of hydrogen bonding-driven PISA

Dispersion polymerization RAFT Electrostatic interactions
Cai et al. [98] The first example of PIESA with an anionic template

Dispersion polymerization ROMP
Choi et al. [118,119] The morphology of the π-conjugated assemblies could be regulated by adjusting the intensity of π-π interactions

Dispersion polymerization Coordination polymerization
Choi et al [121] PI-CDSA strategy that enabled the precise synthesis of 1D nanofibers without the need for preformed seeds

Dispersion polymerization ROP Liquid Crystallization
Yuan et al. [123] The influence of temperature and solvent on the liquid crystallization-driven PISA

Dispersion polymerization RAFT
Chen et al. [124] Photo-responsive liquid crystalline assemblies via PISA

Dispersion polymerization RAFT
Zhang et al. [125,126] Chiral liquid crystalline assemblies via PISA Dispersion polymerization RAFT core-forming block, which drives the in-situ self-assembly of the BCPs.During the past years, numerous PISA formulations have been designed via the "living"/controlled dispersion polymerization in water, [25,42] alcohols, [27,132] and other green solvents. [32]However, due to the principle of the PISA process, a selected solvophilic monomer is required to form a solvophobic block in a selected solvent.The strict selection rule of monomer and solvent has severely restrained the development of the PISA based on dispersion polymerization.Therefore, new strategies that enable to design and even prediction of monomers suitable for PISA are urgently needed.In this section, instead of a comprehensive review of the existing PISA formulations, we mainly focus on recent new insights about the design of hydrophobic interaction-driven PISA formulations.
An important limitation of a typical dispersion polymerization-mediated PISA formulation is a monomer could only be performed in one or a few solvents in the PISA process.Therefore, designing new monomers that have general solvent applicability is necessary.Fluorinated monomers are soluble in common organic solvents while the corresponding fluoropolymers are amphiphobic.Based on this feature, Yuan et al. developed a series of semi-fluorinated methacrylates with general solvent serviceability (Figure 2A).RAFT dispersion polymerization of 2-(perfluorobutyl)ethyl methacrylate (FBEMA), 2-(perfluorohexyl)ethyl methacrylate (FHEMA), and 2-(perfluorooctyl)ethyl methacrylate (FOEMA) were evaluated in ethanol with poly [2-(dimethylamino)ethyl methacrylate] (PDMA) as the macro chain-transfer agent (macro-CTA). [112]The PISA behaviors of these semi-fluorinated monomers were correlated to their fluoroalkyl sidechain length.For the PISA of FBEMA with a short fluoroalkyl sidechain, spherical micelles, WLMs, and vesicles evolved in sequence as the DP of PFBEMA increased, whereas only spherical micelles were produced for the PISA of FHEMA.Interestingly, cylindrical micelles were generated regardless of the DP of PFOEMA because of the liquid crystalline ordering of PFOEMA.The generalizability of these semifluorinated monomers in different solvents was demonstrated by the PISA of PDMA-b-PFHEMA in toluene, 1,4-dioxane, isopropanol, and dimethylformamide.Taking advantage of the superior solvent applicability of the fluorinated polymer assemblies, An et al. developed robust Pickering emulsifiers, which could stabilize several types of oil/water and oil/oil emulsions. [133]136][137][138][139] Undoubtedly, aqueous PISA is the most intriguing PISA system.On one hand, water enables fast polymerization and meets the requirement of green chemistry.On the other hand, polymer assemblies produced through aqueous PISA preclude the use of organic solvents, which is of great importance for their use in biomedical applications.However, monomers suitable for aqueous dispersion PISA were still rare.Yuan's group tried to expand the monomer toolbox for aqueous dispersion polymerization by solubilizing the hydrophobic monomers into water through the host-guest interaction.They elegantly introduced randomly methylated β-cyclodextrin (RMCD) to form a hydrophilic host-guest complex with hydrophobic monomers in an aqueous solution and realized aqueous RAFT dispersion polymerization of hydrophobic monomers with poly(ethylene glycol) (PEG) as the macro-CTA (Figure 2B). [113]The key to the success of this strategy is that RMCD would detach from the polymer sidechain as the polymerization proceeds because of the steric effect.Consequently, amphiphilic copolymers were produced and self-assembled into polymer assemblies in situ.With this strategy, aqueous RAFT dispersion polymerization of styrene was successfully implemented, with the morphology of the poly(ethylene glycol)-b-polystyrene (PEG-b-PS) assemblies evolving from lamellae to ribbon, and eventually to tubular vesicles.This strategy was also proved effective for aqueous RAFT dispersion polymerization of butyl acrylate, [113] ferrocenylmethyl acrylate, [140] t-butyl acrylate, [141] and 2,3,4,5,6pentafluorostyrene [142] (Figure 2C).Besides, a large number of hydrophobic molecules have been reported to have hostguest interaction with cyclodextrins. [143]Therefore, it can be predicted that the monomer toolkit for aqueous dispersion PISA will be greatly enlarged with this strategy.Considering the large family of host-guest chemistry, [144,145] this work may also inspire the host-guest chemistry-mediated PISA in both aqueous and non-aqueous mediums.
O'Reilly et al. proposed to predict whether a monomer could be used for PISA by quantifying the hydrophobicity of its oligomers. [114]They determined the change in hydrophobicity during polymerization by calculating the octanol-water partition coefficient divided by surface area (LogP oct /SA), where a positive LogP oct /SA value indicates a hydrophobic oligomer and polymer with higher LogP oct /SA value should be more hydrophobic (Figure 2D).With this criterion, they successfully predicted and identified five new monomers for aqueous dispersion PISA (Figure 2E).Besides, the LogP oct /SA was also related to the self-assembly behaviors of the amphiphilic copolymers in PISA, where a higher LogP oct /SA value suggests lower DP at the onset of assembly (Figure 2F).Their work provided a crucial tool for predicting suitable monomers in aqueous PISA, and would largely promote the enrichment of aqueous PISA monomer library.
Besides the search for new monomer/solvent pairs, several new strategies for PISA were proposed to avoid the strict selection rule of monomer/solvent pairs for PISA. [115,146,147]or example, Zhao et al. reported the polymerizationinduced interfacial self-assembly of the hydrophobic methyl methacrylate (MMA) in a toluene/water emulsion using a hydrophilic while oleophobic macro-CTA. [115]The interfacial chain extension of the macro-CTA yielded amphiphilic BCPs that self-assembled into microcapsules at the oil/water interface.Besides MMA, this strategy may apply to other hydrophobic monomers, and would provide a new way to prepare microcapsules, providing the stability issue and the morphology regulation of the microcapsules were addressed.

PISA DRIVEN BY HYDROGEN BONDING
Hydrogen bonding serves as an important driving force for structural stabilization in biological systems, such as the formation of DNA double helix and 3D folding of proteins. [148,149]The formation of a DNA double helix is a precise self-assembly process that involves the entwining of two mutually complementary strands via the cooperative hydrogen bonding between the constitutional nucleotides.The self-assembly process is a two-stage process consisting of a nucleation phase followed by a cascade propagation sequence. [150]The nucleation phase is dominated by the unfavorable entropy penalty, although the formation of hydrogen bonding between a few base pairs contributes to the favorable enthalpy term.However, once the nucleation has occurred, the favorable enthalpy term continuously increases with the number of base pairs, while the entropy penalty changes little, leading to a decrease in the Gibbs free energy and the stabilization of the DNA double helix.
The hydrogen bonding between man-made polymers also obeys similar thermodynamic rules.Kříž et al. studied the complexation between poly(4-vinylpyridine) (PVP) and poly(4-vinylphenol) (PVF) in tetrahydrofuran by using 1 H/ 13 C nuclear magnetic resonance (NMR) spin-diffusion experiments and quantum calculations. [151]Because of the  [112] (B) Reproduced with permission: Copyright 2017, John Wiley and Sons. [113]D,F) Reproduced with permission: Copyright 2018, John Wiley and Sons. [114]operative hydrogen bonding, PVP and PVF formed a stable complex that gradually precipitated from tetrahydrofuran.The dependence of the binding degree on the number of binding sites has a sigmoidal shape, indicative of higher-order cooperative binding.
Taking advantage of the cooperativity of the hydrogen bonding between polymeric hydrogen bonding donor/acceptor, O'Reilly et al. reported the first example of hydrogen bonding-driven PISA in 2015 (Figure 3A). [95]They prepared two nucleobase derivatives, 2-(2-(adenine-9-yl)acetoxyl) ethyl methacrylate (AMA) and 2-(2-(thymine-1-yl)acetoxyl) ethyl methacrylate (TMA), and systematically evaluated their PISA behaviors in chloroform and 1,4-dioxane, respectively.It was revealed that the strength of hydrogen bonding has a significant influence on the morphology of the polymer assemblies.PISA of AMA in chloroform generated nanospheres with rough surfaces, while in 1,4-dioxane the morphology of polymer assemblies evolved from WLMs to vesicles as the DP of PAMA increased (Figure 3B).The different PISA behaviors in chloroform and 1,4-dioxane were because the hydrogen bonding interaction between PAMA blocks was stronger in CHCl 3 , which restricted the mobility of polymer chains and their re-organization.Consequently, the morphology of the copolymer assemblies was frozen to rough spheres.Compared with AMA, TMA has weaker hydrogen bonding.Therefore, vesicles and nano-discs were obtained via the PISA of TMA in chloroform and 1,4-dioxane, respectively (Figure 3C). [94]Moreover, the copolymerization of AMA and TMA was prone to generate an alternating sequence distribution, because of the hydrogen bonding between AMA and TMA.Correspondingly, RAFT dispersion copolymerization of AMA and TMA gave more complex morphology evolution due to the complex hydrogen bonding and polymer chain sequence distribution.
Cai et al. revisited the RAFT dispersion polymerization of diacetone acrylamide (DAAm) and pointed out that hydrogen bonding was a key driving force that drove the F I G U R E 4 Hydrogen bonding-regulated polymerization-induced self-assembly (PISA).(A) Nanofibers prepared via reversible addition-fragmentation chain-transfer (RAFT) polymerization mediated by a bis-urea-containing chain-transfer agent (CTA).(B) PISA of 2-methoxyethyl acrylate (MEA) with a bis-urea-containing PDMAc macro-CTA as the hydrophilic block.(A) Reproduced with permission: Copyright 2018, American Chemical Society. [116](B) Reproduced with permission: Copyright 2019, John Wiley and Sons. [117]SA of DAAm. [93]Successful photo-initiated RAFT dispersion polymerization of DAAm was achieved by using poly(2-hydroxypropyl methacrylamide) (PHPMA) as the macro-CTA (Figure 3D).As the DP of PDAAm increased, the morphology of the polymer assemblies evolved from silk/film to ribbons and vesicles, which is distinct from the thermo-initiated PISA behaviors of DAAm. [152]The hydrogen bonding between PDAAm blocks was investigated using 1 H NMR and infrared spectroscopy (IR). 1 H NMR characterization revealed a downfield shift for the peak of -COCH 3 , whereas IR indicated that both the ketonic C=O and amide II vibrations shifted to lower wavenumbers, suggesting the presence of hydrogen bonding.They thus attributed the abnormal morphology transformation to the hydrogen bonding-enhanced internal stress and the agitation-induced shear force.
Hua et al. designed a water-soluble monomer, N-(2-methylpyridine)-acrylamide (MPA), with selfcomplementary double hydrogen bonding, and successfully achieved a hydrogen bonding-driven aqueous PISA system (Figure 3E). [96]The RAFT dispersion polymerization of MPA was performed in water at 70 • C, with PEG as the macro-CTA.As the DP of PMPA increased, the morphology of the assemblies evolved from micelles to WLMs and vesicles.The driving force of this PISA system was carefully investigated by temperature-variable IR and 1 H NMR. IR revealed that as the temperature increased from 25 to 85 • C, the amide II peak shifted from 1655 to 1669 cm −1 , and the C=N of pyridine shifted from 1595 to 1590 cm −1 , suggesting the weakening of the hydrogen bonding at high temperatures.Temperature-variable NMR indicated that the signals of PMPA in deuterated water strengthened, accompanied by a peak shift from 8.08 to 8.28 ppm, as the temperature increased from 25 to 60 • C.These characterizations verified the hydrogen bonding between pyridine and the amide groups.As a hydrogen bond breaker, urea was added to the polymer assemblies.The hydrodynamic size of the dispersion decreased from 24 to 13 nm, and the scattering intensity declined progressively, as the concentration of urea increased from 0 to 11 M. Transmission electron microscopy (TEM) indicated the micelles disassembled completely at 11 M of urea, thus verifying the hydrogen bonding as the driving force for PISA.
Besides the hydrogen bonding-driven formation of polymer assemblies, because of its directionality, hydrogen bonding has also been used to direct the formation of nanofibers, which generally have a quite narrow experiment window in traditional PISA systems. [153]Rieger et al. designed a bis-urea-containing CTA and prepared a series of hydrophilic macro-CTAs by RAFT polymerization of the commonly used hydrophilic monomers, such as N,Ndimethylacrylamide (DMAc), acrylic acid (AAc), acrylamide (AAm), and 2-(N,N-dimethylamino)ethyl acrylate (DMAEA) (Figure 4A). [116]Because of the strong hydrogen bond-ing ability of the bis-urea, this hydrophilic macro-CTA all underwent self-assembly into nanofibers in water.Taking advantage of this merit, they performed aqueous PISA of 2-methoxyethyl acrylate (MEA) using the bis-urea terminated PDMAc as the macro-CTA, and obtained nanofibers in a broad range of solids contents and chain lengths (Figure 4B). [117]As a control, spheres or WLMs were generated for the PISA of PDMAc-b-PMEA without a bisurea sticker, which confirmed the peculiar role of hydrogen bonding in regulating the morphology of the assemblies.
As the strength of hydrogen bonding can be readily regulated by varying the nature and number of the hydrogen bonding unit, solvent polarity, pH, and temperature, polymer assemblies stabilized with hydrogen bonding generally feature stimuli-responsiveness and adaptive properties. [154]However, the polar protic solvent, especially water, would weaken the hydrogen bonding among polymers by the competitive binding, [155] whereas monomers with strong hydrogen bonding generally suffer from solubility issues. [156]Therefore, to develop a hydrogen bondingdriven PISA system, it is needed to balance the hydrogen bonding strength of monomers and their corresponding polymers.

PISA DRIVEN BY ELECTROSTATIC INTERACTIONS
Electrostatic interactions are ubiquitous long-range supramolecular interactions with adjustable strength depending on the choice of solvents, the concentration, and the chemical nature of the counterions. [157]Driven by electrostatic interactions, the complementary polyions can bind to form polyion complexes (PICs) with varying morphologies, including molecular complexes, nanoparticles, and hydrogels. [158]In 1995, Harada and Kataoka first reported the formation of PIC micelles through the electrostatic complexation of poly(ethylene glycol)-b-poly(L-lysine) and poly(ethylene glycol)-b-poly(a,β-aspartic acid). [159]y regulating the chemical composition of the BCPs and the self-assembly temperature, they successfully achieved micelles, cylinders, vesicles, connected cylinders, and connected-plane networks. [160,161]Besides polymers with opposite charges, a variety of charged compounds, including DNA and proteins, were also exploited to construct PIC nanoparticles, which have shown great potential in gene and protein delivery. [162,163]166] Compared with the polyion complexation, the complexation between polyions and small-molecule electrolytes usually has a lower binding constant.Therefore, soluble complexes can be obtained by choosing proper polyions and ionic monomers.The binding constant would increase prominently when the ionic monomers convert to polymers, owing to the cooperativity of the polyion complexation.1][102][103]167] Photo-initiated aqueous RAFT dispersion polymerization of the cationic monomer 2aminoethylacrylamide hydrochloride (AEAM) was successfully performed with the anionic poly(sodium 2-acrylamido-2-methylpropanesulfonate) (PAMPS) as the template and PHPMA as the macro-CTA (Figure 5A). [98]The initial charge stoichiometry (n + /n -) was 2.18.Interestingly, the PIESA exhibited different polymerization kinetics from the traditional PISA, where the hydrophobic association enriches the nonionic monomers into the micellar cores, and speeds up the polymerization.On the contrary, the polymerization kinetics of the PIESA were divided into two distinct regimes by the isoelectric point (IEP) of the system (Figure 5B).Before the IEP, the PHPMA-b-PAEAM copolymers formed water-insoluble PIC with PAMPS and drove the in-situ selfassembly of the complex.Correspondingly, the morphology of the resulting assemblies evolved from spheres to networks.As the monomer conversion further increased, the charge of the assemblies reversed from negative to positive (Figure 5C).Because of the electrostatic repulsion between the positively charged nanoparticles and AEAM, the polymerization was soothed, and the network disintegrated into spheres.They further optimized the chain length of the PHPMA stabilizer chain, the solids content, and the solvent composition, and successfully fabricated various polymer assemblies, ranging from vesicles to lamellae, WLMs, and nanoporous films, thus establishing a reliable and scalable platform for controlled synthesis of low-dimensional PICs. [99]Theoretically, this strategy is feasible for all the hydrophilic ionic monomers, exhibiting great potential in expanding the monomer library for aqueous dispersion PISA.
Recently, they further developed this strategy by iterative polymerization of ionic monomers with opposite charges. [97]Following this strategy, PHPMA was sequentially chain-extended by AMPS and histamine acrylamide hydrochloride (HisAM) via photo-initiated RAFT polymerization (Figure 5D).During the polymerization of HisAM, the diblock copolymer PHPMA-b-PAMPS acted as both the macro-CTA and PIC template.With this strategy, they synthesized a series of triblock copolymers with equal charge stoichiometry (n + /n -= 1).As the DP of PHisAM increases from 30 to 100, the morphology of the assemblies evolved from spherical micelles to films and eventually to microsized vesicles.Furthermore, by regulating the n + /n -, they successfully prepared positively-charged vesicles, which is promising as a gene carrier.
Besides the polyelectrolytes, PIESA accommodates a broad library of templates, such as 2D lamellae, [101] ionic micelles, [102] and siRNA. [104]For example, Cai et al. synthesized the anionic PAMPS-b-PDAAm micelles by PISA and further used them as the template for the PIESA of PHPMAb-PHisAM. [102]The electrostatic interactions drove the hierarchical co-assembly of the PAMPS-b-PDAAm micelles with the PHPMA-b-PHisAM BCPs to form multicompartmental assemblies (Figure 5E).As the DP of PHisAM increased from 100 to 200, the morphology of the assemblies evolved from multicompartment micelles to monolayer colloidal nanosheets, and eventually to colloidal nanocages.Recently, Shen et al. successfully prepared short interfering RNA (siRNA) encapsulated nanoparticles, lamellae, and nanotubes by the PIESA of poly(ethylene glycol)-b-poly(3acrylamidopropyl trimethylammonium chloride) (PEG-b-PAPTAC) with siRNA as the anionic template. [104]These  B,C) Reproduced with permission: Copyright 2015, American Chemical Society. [98](D) Adapted with permission: Copyright 2018, American Chemical Society. [97](E) Adapted with permission: Copyright 2020, American Chemical Society. [102]RNA-encapsulated assemblies showed enhanced stability against enzymatic degradation compared with the bare siRNA.Considering the mild polymerization conditions and the facile design, this strategy would provide a promising platform for siRNA delivery.
With a large monomer library, rich self-assembly morphologies, and a peculiar self-assembly mechanism, PIESA has shown great potential and represents one of the most intriguing aspects of PISA.Nevertheless, because of the complicated and obscure influential factors, such as the charge stoichiometry and the association constant of the polyion complexes, PIESA with predictable morphology evolution has not been realized.It will be an interesting topic for the polymer community to clarify and control the morphology evolution of PIESA.

PISA REGULATED BY π-π INTERACTIONS
The π-π interactions are a kind of non-covalent interaction prominent in aromatic systems, with great significance in both biological and artificial self-assembly systems.For example, π-π interactions are vital in the stabilization of DNA double helix and the folding of proteins. [168]Besides, they are frequently used in the design of artificial light-harvesting Copyright 2012, American Chemical Society. [54](C) Adapted with permission: Copyright 2014, the Royal Society of Chemistry. [118](D) Adapted with permission: Copyright 2021, the Royal Society of Chemistry. [107]mplexes and photovoltaic devices. [169,170]Although still in debate, one popular theory elaborated the origin of π-π interactions as the electronic attraction between two opposite quadruple moments. [171]In this theory, the aromatic faces of a benzene-type molecule manifest partial nega-tive electrostatic potential, while the periphery manifests a partial positive electrostatic potential.Consequently, the aromatic rings with π-π interactions adopt an edge-to-face geometry or a parallel displaced geometry, with the substituted and large multiring aromatic compounds preferring the parallel displaced geometry.The strength of π-π interactions increases with the size and overlapping degree of the aromatic compounds, while it decreases as the solvent polarity increases. [172]-π interactions act as the main driving force in the selfassembly of π-conjugated polymers. [173]Owing to the rigid nature and the strong π-π interactions, the π-conjugated polymers tend to stack regularly to form anisotropic nanostructures with great potential in optoelectronic devices.In 2012, Choi et al. for the first time achieved the PISA of poly (N-cyclohexyl-exo-norbornene-5,6-dicarboximide)b-polyacetylene (PNB-b-PA) through sequential ROMP of NB and cyclooctatetraene (COT) in one pot (Figure 6A). [54]wing to the strong π-π interactions among the PA blocks, the generated PNB-b-PA self-assembled into nanospheres at a low [COT]/[NB] ratio.As the [COT]/[NB] ratio increased from 10/50 to 50/50, the morphology of the assemblies gradually evolved from nanospheres to undulated "nanocaterpillars".Interestingly, as NB has a much higher reactivity than COT in ROMP, quasi-diblock copolymers were favored when NB and COT were copolymerized in one pot.Taking advantage of this attribute, the above PISA protocol was further simplified to one-shot polymerization. [174]iven the rich variety of conjugated polymers, the π-π interaction-driven PISA of π-conjugated BCPs should have a large monomer arsenal.Besides the PNB-b-PA assemblies, assemblies containing other π-conjugated blocks, including polythiophene (PT), [118,121,175] poly(3-methylthiophene) (P3MT), [119] poly(p-phenylenevinylene) (PPV), [176] poly(cyclopentenylene-vinylene) (PCPV), [177][178][179][180] and poly (phenylacetylene) (PPA), [181] were also successfully fabricated via PISA (Figure 6B).For instance, Choi et al. achieved the PISA of fully conjugated poly(2,5dihexyloxy-1,4-phenylene)-b-polythiophene (PPP-b-PT) in tetrahydrofuran (THF) via a quasi-living Grignard metathesis polymerization, with PPP as the corona-forming block, and PT as the core-forming block (Figure 6C). [118]As the DP of the PT block increased, the morphology of the assemblies transformed from nanospheres to nano caterpillars, whose length and diameter could be regulated by varying the DP of PT.Ultraviolet-visible spectra showed two vibronic peaks of PT at 556 and 598 nm, and X-ray diffraction revealed the ( 100) and ( 200) reflections of PT with a d-spacing of 0.38∼0.44nm, both suggesting the strong π-π interactions among PT blocks.
The morphology of the π-conjugated assemblies could be regulated by adjusting the intensity of π-π interactions.For example, by changing the PT block to the P3MT block, the stacking mode of the thiophene rings converted from edgeto-face to parallel displaced geometry, resulting in a decrease in the π-π interactions.Consequently, the corresponding PPP-b-P3MT copolymers underwent a peculiar morphology transformation from single-line to multi-line nanocaterpillars as the DP of the P3MT block increased (Figure 6D). [107,119]ndoubtedly, the development of the PISA of πconjugated polymers would be beneficial to the practical applications of conjugated polymers in sensors, imaging agents, photovoltaics, etc.However, the metallic catalysts in the synthesis of π-conjugated polymers may have a deteriorative influence on the performance of the polymers. [107]ore efforts are needed to develop metal-free PISA of π-conjugated polymers in the future.
For the non-conjugated coil-coil BCPs, the π-π interactions are generally weak and work cooperatively with the hydrophobic interactions.In 2021, Hong et al. reported the regulative role of π-π interactions during the PISA of the coumarin-derived monomer, 7-(2-methacryloyloxyethoxy-4methyl-coumarin) (CMA) (Figure 7). [105]With the polymerization of CMA, the chemical shifts of the coumarin protons moved upfield in the 1 H NMR spectrum, and its fluorescence emission peak underwent a red shift from 376 to 401 nm, exhibiting a polymerization-induced π-π interaction enhancement.The authors claimed that the strong π-π interactions would lead to compact stacking of the hydrophobic PCMA blocks, which favored small vesicles with high surface curvature.Therefore, the vesicular size can be rationally regulated by the π-π interactions (Figure 7A).For example, the π-π interactions could be impaired by copolymerization CMA with 2-(diisopropylamino)ethyl methacrylate (DIPEMA) (Figure 7B).As the molar fraction of DIPEMA increased from 0 to 0.4, the chemical shift of the coumarin protons moved downfield, while the fluorescence emission of the vesicles exhibits continuous blue-shift, suggesting the impairment of the π-π interactions (Figure 7C,D).Consequently, the vesicular diameter increased from 76.8 to 334 nm (Figure 7E).On the contrary, the π-π interactions could be enhanced by copolymerizing CMA with 2-(methacryloyloxy) ethyl anthracene-9carboxylate (MAEAC), because of a larger π-conjugated system.In a follow-up study, the strong π-π interactions between anthracene groups were exploited to provide directionality for the PISA of MAEAC, yielding polymeric nanotubes with adjustable lengths depending on the solids content of the PISA formulation. [106]

POLYMERIZATION-INDUCED CRYSTALLIZATION-DRIVEN SELF-ASSEMBLY
For the amphiphilic crystalline-coil BCPs containing a semicrystalline core-forming block, their self-assembly proceeds in a distinct pathway compared with amorphous BCPs.Besides the hydrophobic interactions that favor minimal interfacial energy, the crystallization contributes to self-assembly by driving the anisotropic packing of the core-forming blocks. [182]Owing to the strong tendency of anisotropic packing, the crystallization-driven self-assembly (CDSA) of amphiphilic crystalline-coil BCPs favors anisotropic nanocrystals, such as 1D nanofibers and 2D nanosheets. [66,183,184]Furthermore, the nanostructure of these nanocrystals, such as the length of fibers and the area of nanosheets, can be precisely controlled by regulating the crystallization process of the copolymers. [185]aking advantage of PISA and CDSA, in 2017, Manners et al. achieved the PI-CDSA of polyisoprene-bpoly(ferrocenyldimethylsilane) (PIP-b-PFDMS) via anionic dispersion polymerization of dimethylsila [1]ferrocenophane (FP) monomer in tetrahydrofuran/n-hexane with polyisoprene (PIP) as the macro-initiator (Figure 8A). [120]ecause of the crystallinity of the PFDMS block in the tetrahydrofuran/n-hexane mixture, the PISA of PIP-b-PFDMS affords cylindrical micelles and platelet depending on the ratio of PIP/PFDMS.Through a seeded growth  A-E) Adapted with permission: Copyright 2021, American Chemical Society. [105]chanism, monodisperse cylindrical micelles were prepared at 10 wt% solids by dispersion polymerization of FP in the presence of PIP-b-PFDMS short cylindrical micelles, whose contour length manifested a linear relationship with the PIPto-seed ratio.Moreover, by changing the corona-forming and/or core-forming block of the seeds, block co-micelles with low dispersity could also be prepared with a similar procedure (Figure 8B,C). [108]ecently, Reuther et al. exploited the nickel-catalyzed coordination polymerization of chiral aryl isocyanide monomers to enable the scalable and controllable synthesis of chiral nano-objects of variable shapes and sizes. [110]The controlled synthesis of poly(aryl isocyanide)-b-poly(ethylene glycol)-b-poly(aryl isocyanide) (PAIC-b-PEG-b-PAIC) was achieved in 40 vol% ethanol/1,4-dioxane with nickelmodified PEG as the macro-initiator (Figure 8D).As the DP of each PAIC block increased from 30 to 50, the morphology of the assemblies evolved from nanofiber to nanoribbon, and eventually to nanosheet.Furthermore, using a 1D seeded growth process, they successfully achieved the controlled preparation of spiral nanofibers with controlled counter length, whereas spirangle assemblies composed of M helically stacked nanosheets were obtained using a 2D seeded growth process (Figure 8E-G).Interestingly, both the number-averaged height and area of the spirangles can be precisely regulated by the unimer-to-seed ratio.The authors argued that the liquid crystalline nature of PAIC blocks dictated the hierarchical self-assembly of the BCPs, and thus led to the amplification in the chiroptical activity of the spirangles, with a g-factor of −0.030 (Figure 8H).
Although the seeded growth strategy has enabled the precise fabrication of 1D, 2D, and even 3D nanostructures, preformed seeds were needed.Choi et al. argued that it was the rapid polymerization that resulted in kinetically trapped morphologies via an isodesmic step-growth manner.They reported a PI-CDSA strategy that allowed length-controlled and narrow-dispersed 1D nanofibers without the need for preformed seeds. [121]This strategy exploited the living Suzuki-Miyaura catalyst-transfer polymerization to synthesize the poly(3,4-dihexylthiophene)-b-polythiophene (P34DHTb-PT), which underwent autonomous self-assembly into undulated "nanocaterpillars" because of the crystallization tendency of the PT block in THF/H 2 O (Figure 8I,J).Surprisingly, the length of the nanocaterpillars was proportional to the DP of PT, with their number-averaged length increasing from 15 to 228 nm as the DP of PT increased from 5 to 40 (Figure 8K).A kinetic study revealed that the growth of these nanocaterpillars followed a nucleation-elongation mechanism, the key to which is the highly solubilizing P34DHT shell and the slow polymerization kinetics (Figure 8L).The  [108] (E-H) Reproduced with permission: Copyright 2023, American Chemical Society. [110](B) Reproduced with permission: Copyright 2020, Springer Nature Limited. [60](C) Adapted with permission: Copyright 2022, American Chemical Society. [62]D) Reproduced with permission: Copyright 2023, American Chemical Society. [122]4DHT shell suppressed the isodesmic step-growth mechanism, while the slow polymerization restrained successive seed formation and favored the seeded growth process.Their work has shed new light on the precise fabrication of polymer assemblies by regulating the interplay of the polymerization kinetics and the crystallization kinetics.
Despite the great potential, anionic polymerization has a stringent requirement on water and oxygen, and a relatively narrow tolerance to the functional groups, while living coordination polymerization and catalyst-transfer polymerization entail complicated syntheses. [186,187]Besides, for biomedical applications, the biocompatibility and biodegradability of the copolymers are important issues that need to be taken into consideration.In 2020, Patterson's group achieved the first ring-opening polymerization-induced CDSA (ROPI-CDSA) in toluene taking advantage of the organocatalyzed ROP of L-lactide (Figure 9A). [60]With PEG as the initiator, and triazabicyclodecene (TBD) as the catalyst, the polymerization finished within 90 s, yielding a dispersion with a bluish appearance.Nevertheless, the turbidity of the dispersion increased progressively after the polymerization, indicating the occurrence of post-polymerization self-assembly.Correspondingly, TEM of the PEG-b-PLLA assemblies revealed a morphology evolution from sphere to lamellae as the aging time prolonged from 1 to 24 h.The crystallization kinetics of the copolymer was monitored by wide-angle X-ray scattering, which revealed that metastable crystalline intermediates with low crystallinity formed in 1 h, while the crystallinity increased rapidly to 51% in 3 h, and then slowly to 81% in 24 h (Figure 9B).The crystallization kinetics of PLLA agreed well with the turbidity profiles, suggesting that the crystallization of PLLA block was the driving force during the self-assembly.Manners et al. realized the living ROPI-CDSA of poly(ethylene glycol)b-poly(fluorenetrimethylenecarbonate) (PEG-b-PFTMC) in 20 vol% dichloromethane/acetonitrile using a seeded-growth process, yielding narrowly dispersed nanofibers with precisely tunable counter length (Figure 9C). [62]emperature plays a significant role in a typical PI-CDSA system, as it concurrently alters the polymerization rate and the crystallization kinetics, and thus affects the competition between the formation of BCPs and the crystallization, which eventually affects the self-assembly kinetics. [61]Taking the PI-CDSA of polyisobutylene-b-poly(ε-caprolactone) (PIB-b-PCL) in cyclohexene as an example, the polymerization temperature has a pronounced influence on the polymerization rate, the crystallization ability, and the morphology of the assemblies (Figure 9D).When the temperature is lower than the crystallization temperature (T c ) of PCL (for example, 20 • C), the polymerization rate of ε-caprolactone (ε-CL) is slow, and the crystallization of PCL predominates as the driving force of the self-assembly, leading to 1D fibers. [122]t a temperature higher than the melting temperature (T m ) of PCL, the PCL block is in an amorphous state, and the morphology of the assemblies observed by TEM is the nonequilibrium morphology during the cooling process.For the PI-CDSA with ε-CL and δ-valerolactone as the monomers, temperature an obvious influence on both the chain sequence and the crystallization temperature of the coreforming block, resulting in a more complex correlation to the morphology of the assemblies.
Despite these exciting progresses, the study on PI-CDSA is still in its infancy, and there are still several limitations that need to be addressed.For example, all the PI-CDSA are performed in organic solvents by far, while PI-CDSA performed in water or other green solvents are highly needed.Besides, the PI-CDSA often manifests the characteristics of non-equilibrium assembly because of the large discrepancy between the rate of polymerization and crystallization, which makes it challenging to control the morphology of the assemblies and maintain their long-term stability.

PISA REGULATED BY LIQUID CRYSTALLINE ORDERING
Liquid crystals are intermediate phases between solid and liquid. [150]The liquid crystal-forming molecules, termed mesogens, generally have rodlike molecular shapes and are aligned along a specific direction in the liquid crystalline phase. [188]To maintain the stability of the ordered orientation of the mesogens, supramolecular interactions, such as hydrogen bonding, π-π interactions, and dipole-dipole interactions, are generally involved. [150]During solution self-assembly, the liquid crystalline ordering drives the amphiphilic BCPs to anisotropic morphologies, [189] especially 1D nanofibers, [190,191] while the hydrophobic interactions favor spherical assemblies. [192]6][193][194][195][196][197] Similar to the crystallization behaviors, the liquid crystalline ordering is also highly temperature-dependent, with the mesogens rearranging into less ordered mesophases, and eventually to isotropic liquid as the temperature increases. [198]Consequently, the morphology of the assemblies produced via PISA of the mesogenic monomers is closely related to the polymerization temperature.Yuan et al. achieved the RAFT dispersion polymerization of FOEMA with PDMA as the macro-CTA and studied the influence of polymerization temperature on the morphology of the PDMA-b-PFOEMA assemblies in DMF (Figure 11A). [123]ue to the decrement in the liquid crystalline ordering degree with the increment in temperature, ellipsoidal micelles were obtained at 50 • C, while spherical micelles were generated at 70 • C. Besides, the polymerization solvent could also affect the morphology of the assemblies by affecting the steric repulsion among the hydrophilic chains. [199]herefore, instead of ellipsoidal micelles, the PISA of PDMA-b-PFOEMA afforded nanofibers at 50 • C in ethanol.Taking advantage of the competitive contributions from liquid crystallization and steric repulsion, the morphology of the assemblies can be further programmed.When the temperature was higher than the liquid crystal-to-isotropic phase transition temperature, the disappearance of liquid crystalline ordering led to a morphology transition from ellipsoids to spheres, whereas the spherical micelles could reversibly transform into ellipsoids as the temperature slowly decreased to room temperature (Figure 11B-D).
Compared with crystallization, liquid crystallization has a lower degree of order owing to the weak supramolecular interactions between the liquid crystalline mesogens.Consequently, the morphology of the liquid crystalline assemblies can be readily modulated via the molecular shape of mesogens.Chen et al. reported an interesting photo-responsive liquid crystalline monomer, 11-[4-(4-butylphenylazo)phenoxy]undecyl methacrylate (MAAz), which was copolymerized with PMAA macro-CTA in ethanol, producing liquid crystalline assemblies with various anisotropic morphologies, including cuboids, short belts,  [123] (E) Reproduced with permission: Copyright 2018, American Chemical Society. [124](F) Reproduced with permission: Copyright 2022, American Chemical Society. [126]mellae, cylinders, and ellipsoids (Figure 11E). [124]Under UV irradiation, both the ellipsoids and cuboids transformed into spheres, because the photo-induced isomerization of the azobenzene groups destructed the liquid crystalline ordering of the trans-azobenzene mesogens.
Besides the role of driving force during PISA, the liquid crystalline ordering also connects the molecular information with the macroscopic properties of the polymer assemblies by transferring and amplifying the molecular information to the supramolecular and macroscopic levels. [200]Zhang et al.
successfully fabricated a series of liquid crystalline assemblies with supramolecular chirality via polymerizationinduced chiral self-assembly (PICSA) of the chiral azocontaining monomers. [125]During the PICSA process, the chirality evolved from the terminal alkyl sidechain to the azobenzene mesogens, and eventually induced the expression of the supramolecular chirality.Through systematical investigation, they revealed that the synergistic effects of the DP of the hydrophobic block and the spacer length had a significant influence on the chiral expression of the assemblies by modulating the strength of the liquid crystalline ordering (Figure 11F). [126]As the DP of the azo-block increased, the mesogens stacked more ordered and tended to be planar, which was antagonistic to the formation of helical structures.Consequently, the chiral expression first increased and then decreased as the DP of the azo-block increased.The spacer length exerted an influence on the chiroptical properties of the assemblies by affecting the stacking modes of the azo mesogens.When the spacer length x > 2, the azo mesogens stacked by the H-aggregates, whereas for x = 2, the stacking mode evolved from intra-chain π-π stacking to inter-chain Haggregation, and eventually to J-type aggregation depending on the DP of the azo-block, which lead to multiple chiroptical inversions.

CONCLUSION
PISA has developed into an efficient and reliable platform for the design and fabrication of polymer assemblies with customized chemical composition, nanostructure, and functionality.During the past decade, an in-depth understanding of the principle of PISA has largely broadened its scope.The driving force of PISA has extended from the hydrophobic interactions to other supramolecular interactions, including hydrogen bonding, electrostatic interactions, π-π interactions, crystallization, and liquid crystallization.The introduction of these supramolecular interactions as the driving force of PISA has greatly innovated the design principles of PISA, enlarged the monomer/solvent toolkit, and endowed the polymer assemblies with intrinsic dynamicity and responsiveness.The limited monomer toolkit has greatly expanded by hydrogen bonding monomers and hydrophilic ionic monomers via PISA driven by hydrogen bonding and electrostatic interactions, respectively.Besides, the supramolecular interactions with directionality, such as hydrogen bonding, π-π interactions, crystallization, and liquid crystallization, have provided new insights for the preparation of polymer assemblies with anisotropic morphologies.The ordered stacking of the copolymers in the assemblies also provided an important bridge for the expression of the molecular information at the meso-and macroscopic levels.Despite these achievements, the exploration of the driving force of PISA is still in its infancy, and in our opinion, key achievements to be envisaged in the future are as below: 1.The rapid development of supramolecular chemistry has provided an opportunity for the design and construction of unconventional PISA systems exploiting new supramolecular interactions as the driving force.
For instance, supramolecular interactions that were less explored in polymer chemistry, including the dipoledipole interactions, [201] cation-π interactions, [202] halogen bonding, [203] and chalcogen bonding, [204] will endow the corresponding assemblies with fascinating properties and functionality.2. High-efficiency methodology for screening feasible monomer/solvent pairs for PISA is still needed.With the rapid development of machine learning in chemistry and materials science, [205,206] we anticipate that soon machine learning may be used to accelerate the prediction and mining of monomer/solvent pairs for PISA.3. Most assemblies stabilized by the supramolecular interactions are dynamic and stimuli-responsive.For example, the assemblies stabilized by hydrogen bonding are thermo-and pH-responsive, whereas the assemblies stabilized via electrostatic interactions are salt-responsive.Therefore, more potential applications of these assemblies are expected taking advantage of the dynamicity and responsiveness of these assemblies.

A C K N O W L E D G M E N T S
National Natural Science Foundation of China (Project No. 21905171) is acknowledged for financial support.Ms. X.Ma is acknowledged for her help in the artwork.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.

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I G U R E 1 (A) Typical polymerization-induced self-assembly (PISA) processes based on the "living"/controlled dispersion polymerization.(B) The driving forces of PISA.

F I G U R E 2
Polymerization-induced self-assembly (PISA) driven by hydrophobic interactions.(A) PISA behaviors of the semi-fluorinated methacrylates.(B,C) Host-guest interaction-assisted aqueous dispersion polymerization of (B) styrene and (C) other hydrophobic monomers.(D-F) The LogP oct /SA value as a criterion for the choice of monomers for aqueous PISA.(D) Dependence of the LogP oct /SA value on the degree of polymerization (DP) of the polymers.(E) Five monomers suitable for the aqueous PISA as predicted by the LogP oct /SA value.(F) The correlation of the critical DP of self-assembly to the LogP oct /SA value.(A) Reproduced with permission: Copyright 2018, John Wiley and Sons.

F I G U R E 7
Polymerization-induced self-assembly (PISA) regulated by π-π interactions.(A) Preparation of sub-100 nm vesicles via PISA of 7-(2methacryloyloxyethoxy-4-methyl-coumarin) (CMA), a monomer with strong π-π interactions.(B) Reversible addition-fragmentation chain-transfer (RAFT) dispersion copolymerization of CMA and 2-(diisopropylamino)ethyl methacrylate (DIPEMA) as an approach to regulating the π-π interactions in the assemblies.(C) The change in the chemical shifts of H b and H d (as shown in Figure 6B with the molar fraction of DIPEMA.(D) The fluorescence emission spectra and (E) the hydrodynamic diameters of the PEO 45 -b-P(CMA 1−x -co-DIPEMA x ) 80 vesicles.(

F
I G U R E 8 (A) Schematic representation of the polymerization-induced crystallization-driven self-assembly (PI-CDSA) of polyisoprene-bpoly(ferrocenyldimethylsilane) (PIP-b-PFDMS).(B) Atomic force microscopy (AFM) height micrograph of the block co-micelles fabricated by the seeded PI-CDSA of PIP-b-PFDMS with PDMS-b-PFDMS micelles as the seeds.(C) AFM height micrograph of the block co-micelles fabricated by the seeded PI-CDSA of PIP-b-PFDMG with PDMS-b-PFDMS micelles as the seeds.(D-I) PI-CDSA of PAIC-b-PEG-b-PAIC for chiral nanostructures.(D) Synthetic route for PAIC-b-PEG-b-PAIC via nickel-catalyzed living polymerization.(E) Transmission electron microscopy (TEM) images of the chiral nanofibers obtained via the 1D-seeded PI-CDSA of PAIC-b-PEG-b-PAIC.(F) TEM and (G) Scanning electron microscopy (SEM) images of the spirangle nanostructures fabricated by the 2D seeded PI-CDSA of PAIC-b-PEG-b-PAIC.(H) CD spectra of the PAIC-b-PEG-b-PAIC assemblies.(I) Synthetic route for P34DHT-b-PT via living Suzuki-Miyaura catalyst-transfer polymerization.(J) AFM height micrograph of the undulated 1D "nanocaterpillars" fabricated via the PI-CDSA.(K) A plot of the length of the nanocaterpillars versus the degree of polymerization (DP) of the PT block.(L) Kinetic study of the length of the nanocaterpillars during PI-CDSA.(B,C) Reproduced with permission: Copyright 2018, American Chemical Society.

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I G U R E 1 0 (A) Schematic representation and (B) the applicable monomers of polymerization-induced self-assembly (PISA) regulated by the liquid crystalline ordering.

F
I G U R E 1 1 (A) Preparation of liquid crystalline assemblies via the reversible addition-fragmentation chain-transfer (RAFT) dispersion polymerization of 2-(perfluorooctyl)ethyl methacrylate (FOEMA) with poly[2-(dimethylamino)ethyl methacrylate] (PDMA) as the macro chain-transfer agent (macro-CTA) at 50 and 70 • C. (B) Temperature-programmed small angle X-ray scattering of the PDMA-b-PFOEMA assemblies.(C) Transmission electron microscopy (TEM) image of the PDMA-b-PFOEMA assemblies quenched in liquid nitrogen from 95 • C. (D) The spherical assemblies in Figure 10C recovered to an ellipsoidal shape after a heating-slow cooling cycle.(E) PISA of PMAA-b-PMAAz for anisotropic nanoparticles.(F) RAFT dispersion polymerization of the chiral azo-containing monomers for chiral liquid crystalline assemblies.(B-D) Reproduced with permission: Copyright 2018, American Chemical Society.