Aggregation-induced emission polymers via reversible-deactivation radical polymerization

Aggregation-induced emission (AIE) is a unique phenomenon whereby aggregation of molecules induces ﬂuorescence emission as opposed to the more commonly known aggregation-caused quenching (ACQ). AIE has the potential to be utilized in the large-scale production of AIE-active polymeric materials because of their wide range of practical applications such as stimuli-responsive sensors, biological imaging agents, and drug delivery systems. This is evident from the increasing number of publications over the years since AIE was ﬁrst discovered. In addition, the ever-growing interest in this ﬁeld has led many researchers around the world to develop new and creative methods in the design of monomers, initiators and crosslinkers, with the goal of broadening the scope and utility of AIE polymers. One of the most promising approaches to the design and synthesis of AIE polymers is the use of the reversible-deactivation radical polymerization (RDRP) techniques, which enabled the production of well-controlled AIE materials that are often difﬁcult to achieve by other methods. In this review, a summary of some recent works that utilize RDRP for AIE polymer design and synthesis is presented, including (i) the design of AIE-related monomers, initiators/crosslinkers; the achievements in preparation of AIE polymers using (ii) reversible addition–fragmentation chain transfer (RAFT) technique; (iii) atom transfer radical polymerization (ATRP) technique; (iv) other techniques such as Cu(0)-RDRP technique and nitroxide-mediated polymerization (NMP) technique; (v) the possible applications of these AIE polymers, and ﬁnally (vi) a summary/perspective and the future direction of AIE polymers.

In stark contrast to this phenomenon, in 2001, Tang and co-workers discovered a type of special fluorescent molecule that emits poorly in the molecular or diluted state, but emits strongly upon radiative excitation in the aggregated state.10][11][12] At that time, only a peculiar class of silole compounds where a molecule, 1-methyl-1,2,3,4,5-pentaphenylsilole (Figure 1A), fluoresces strongly only upon aggregation was discovered. [8]Subsequently, this new phenomenon attracted a significant amount of research attention, leading to the discovery of a series of new molecules with AIE property in the next few years.Meanwhile, the mechanistic understanding of this new phenomenon also became a hot topic in this field.
The competing effect of ACQ and AIE for any given luminophores depends on multiple factors including (but not limited to) molecular structure and composition, molecular behavior when isolated and when in close proximity to other molecules (i.e., aggregated state).[15] This mechanism comprises two parts: restriction of intramolecular rotation (RIR) (Figure 1B) and restriction of intramolecular vibration (RIV) (Figure 1C), as exemplified by 1,1,2,2tetraphenylethene (TPE) and 10,10′,11,11′-tetrahydro-5,5′bidibenzo[a,d] [7]annulenylidene (THBDBA), respectively.This theory assumes most molecules that underwent ACQ instead of AIE, possess highly coplanar aromatic rings in them, while AIE molecules adopt a "propeller-shaped" structure, where the aromatic rings represent the "rotors," able to rotate freely in the molecular state and promote energy transfer among molecules, hence generating a new path for non-radiative decay. [16]In the aggregated form, RIM imposed onto the molecules forces energy dissipation to occur via the radiative pathway instead of the standard mechanical energy dissipation pathway, with fluorescence emission.For example as shown in Figure 2A, a classic ACQ luminophore N,N-dicyclohexyl-1,7-dibromo-3,4,9,10perylenetetracarboxylic diimide (DDPD) shows an intense color when dissolved in tetrahydrofuran (THF) solution, but forms insoluble aggregates when water was added due to the solubility (free volume) effect, [3] thus quenching the PL. [17]ontrary to this observation, AIE luminogens (AIEgens) reverses the effect of ACQ (Figure 2B). [18]A solution of hexaphenylsilole (HPS) in THF displayed extremely low PL owing to the freely rotatable peripheral rings, but showed an intense color when water was added, by forming insoluble aggregates.
Since its first discovery, AIE molecules were believed to have many new applications that cannot be achieved by conventional fluorescent molecules.However, AIE molecules alone have only limited applications due to their poor mechanical and film-forming properties.Therefore, the need for incorporating AIE components into polymers is considered necessary in many applications, such as in optoelectronic and biomedical applications where luminescent materials are commonly employed as films and aggregates, with properties vastly different from single isolated molecules. [19]In addition, these AIE molecules can be used as probes when incorporated into aggregates by monitoring their PL intensities, which is especially useful in the field of material science and engineering, where information on reaction mechanisms and processes are of paramount importance.
In 2003, Chen et al. reported the world's first AIEactive polymer and set the stage for many researchers globally to follow this research pathway in understanding the mysteries of the AIE phenomenon. [20]AIE polymers overall provide more benefits compared to small AIE molecules, such as ease of processing, good ability to form films, and structural diversity.Since then, AIE polymers have found various applications such as AIE-active polytriazole-based explosive chemosensors synthesized via click polymerization, [21] high performance polymeric lightemitting diodes with low-cost wet fabrication, high fluorescence quantum nanoparticles with excellent thermal and film-forming stability, [22] and fluorescent polymeric nanoparticles (FPNs) synthesized from a "one-pot" multicomponent Mannich reaction as bioimaging agents for L929 cells. [23]Some reviews have already explored the structure, design, reaction pathways, and applications of AIE polymers, [16,18,[24][25][26] while other reviews explored the area of AIE polymers for biomedical-related applications, [27] chirality, [28] supramolecular AIE polymers, [29] AIE click polymerization, [30,31] one-component AIE polymerization, two-component AIE polymerization, and multicomponent polymerization. [30,32]However, the AIE polymers that were synthesized till date with predetermined molecular weights (M n ), low dispersity (Ð) values, and well-defined structures via reversible-deactivation radical polymerization (RDRP) specifically has not been systematically summarized.
Moreover, these AIE polymers synthesized via RDRP with well-designed structure, chain length, well-controlled molecular weights and molecular weight distributions (dispersity, Ð) are of great importance in certain applications such as theranostics, FPNs, and environmental variation detection.For example, Doncom et al. explained on the importance of polymer dispersity on final polymer nanoparticle morphologies, where such molecular weight distributions can affect the polymer chain length and relative volume fractions of the different blocks in a block copolymer system. [33]As relative volume fraction is related to the packing parameter, varying the chain length in an amphiphilic block copolymer will result in different polymer chains preferentially adopting different interfacial curvatures, thus varying the final predicted particle morphology.In order for AIE polymers to be used in F I G U R E 2 Fluorescence photographs of solutions/suspensions and chemical structure of (A) DDPD and (B) HPS in THF-water mixtures, with different water contents demonstrating aggregation-caused quenching (ACQ) and aggregation-induced emission (AIE), respectively.Adapted with permission [18] : copyright 2014, Royal Society of Chemistry.
such advanced applications, it needs to have low dispersity values and uniform sizes, so that the morphology obtained can be well-predicted, and to achieve better functionalities.
A similar phenomenon was observed by Terreau et al., where they noticed that the self-assembly of poly(styreneb-acrylic acid) (PS-b-PAA) di-block copolymer synthesized using THF/DMF (N,N-dimethylformamide) solvent system, in water, formed large vesicles with a broad range of hydrodynamic diameter at low molecular weight distributions (Ð < 1.05). [34]However, upon mixing PS-b-PAA di-block copolymers with similar PS chain length but different PAA chain length to artificially broaden the dispersity (1.1 < Ð < 2.2), smaller vesicles with a narrower range of hydrodynamic diameter combined with the appearance of spheres were observed.The dispersity affects the overall morphology of the different polymers, and in turn, affects the AIE characteristics.This relationship was exemplified by He and co-workers in 2019, where they synthesized poly(N-(2-methacryloyloxyethyl)pyrrolidone)-b-poly(lauryl methacrylate-co-1-ethenyl-4-(1,2,2-triphenylethenyl) benzene), PNMP-b-P(LMA-co-TPE) AIE-active amphiphilic block copolymers that self-assembles when dissolved in different solvents. [35]The AIE characteristics of the polymer is determined by the location of the TPE units and by the polymer morphology; dissolving the polymers in n-dodecane causes the P(LMA-co-TPE) segment to inhibit π-π interactions, causes low quantum yields (QY).A change in morphology from simple spheres to worm-like micelles causes greater degree of aggregation, leading to greater RIM effect and hence higher QY.These examples demonstrated that a loss of control over the dispersity of the polymer product can significantly affect the final morphology or the functionality of these AIE polymers.Therefore, this review is dedicated to highlighting and summarizing some of these recent works that utilized RDRP to design and produce AIE polymers, including the different types of RDRP methods, synthetical strategies, and their potential applications.

Reversible-deactivation radical polymerization
Since the early 1980s, researchers from around the world have realized that the addition of certain chemical compounds into a polymerization mixture allows reversible reaction with chain carrier molecules. [36]Many terms have been used to describe these polymerization reactions, including (but not limited to): controlled/living radical polymerization, "controlled" and "living" polymerization, and radical polymerization with minimal termination. [37]In 2010, the International Union of Pure and Applied Chemistry (IUPAC) stepped in to generalize all such polymerization reactions by coining the term: reversible-deactivation radical polymerization (RDRP). [36,37]RDRP can be defined as a polymerization reaction where side reactions, such as chain transfer reactions and termination reactions, are considered trivial or negligible throughout the polymerization process, and the molecular weight of the growing polymer increases linearly with monomer conversion.This revolutionary polymerization method sparked possibilities in synthesizing complex, well-defined polymer architectures and morphologies with multifunctionalities, which otherwise will not be possible by conventional methods. [36,37]DRP polymerization techniques include: nitroxidemediated polymerization (NMP), [38][39][40] atom transfer radical polymerization (ATRP), [41][42][43][44][45][46][47][48] reversible additionfragmentation chain transfer (RAFT), [49][50][51][52] iodine-transfer polymerization (ITP), [53] reverse iodine-transfer polymerization (RITP), [54] reversible chain transfer catalyzed polymerization (RTCP), [55] reversible complexation-mediated polymerization (RCMP), [56] organotellurium-mediated radical polymerization (TERP), [57] cobalt-mediated radical polymerization (CMRP) and catalytic chain transfer (CCT), [58,59] iniferter polymerization, [60,61] selenium-centered radicalmediated polymerization (SRMP), [62] and organostibinemediated radical polymerization (SBRP). [63]RDRP polymerization techniques based on reversible deactivation of radicals were among the first to be employed such as in NMP where 2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) stable radical captures propagating radicals to form dormant species reversibly, CMRP that uses cobalt transition metal, SRMP and SBRP that uses selenium and antimony species, respectively, to form stable radical species that behaves similarly to the radicals in NMP (Scheme 1A).Similar to reversible deactivation processes, catalytic reversible deactivation processes such as ATRP employs metal catalysts to reduce the activation energy for radical generation and activate alkyl halides or halogen-capped propagating chains, and then deactivate the radicals formed (Scheme 1B).In this manner, ATRP is able to control radical formation and chain propagation through the metal catalysts.The degenerative transfer process pioneered by RAFT uses a range of dithioesters, trithiocarbonates, xanthates, and dithiocarbamates that reversibly adds to monomers and propagating radicals to form chain transfer agents (CTA), led to the discovery of many other fascinating polymerization techniques such as iniferter, TERP, and the polymerization series that revolves around the use of iodine species such as ITP, RITP, RCMP, and RTCP (Scheme 1C).Even though a range of different RDRP techniques has been developed over the past few decades, the more widely used methods for designing AIE polymers via RDRP are RAFT and ATRP owing to their applicability to a wide range of monomers and reaction conditions, including the robustness of both techniques.
The use of RDRP as a polymerization technique stems from the fact that it is fundamentally more versatile and powerful compared to conventional radial polymerization: (i) the ability to synthesize polymer chains with predetermined molar masses and narrow molar mass distribution (dispersity, Ð); (ii) the ability to continue polymerization by adding more monomers owing to the better stability of the dormant propagating chain; (iii) high-chain end fidelity and ease of attaching functional groups to polymer chain ends; (iv) lower probability of side reactions such as termination reactions occurring; and (v) ease of fabricating various polymer shapes and morphologies. [64]All these benefits of using RDRP over other types of polymerization led many researchers to search for and invent unique ways to synthesize polymers with AIE properties in a controlled manner, resulting in a plethora of morphologies discovered and produced over time.
Given the success and advantages of RDRP, this polymerization technique is capable of producing a multitude of polymer morphologies such as single block and block copolymers spanning a huge range of topological morphologies such as homopolymer, di-/tri-/multi-block, star-shaped, sequencedefined, (hyper)branched, dendritic, graft and brush type, cyclic (ring), network, single-chain nanoparticles (NPs), [65] bearing unique properties such as stimuli responsiveness to mechanical stress, [66,67] temperature and pH changes, [68,69] and light irradiation. [70][79] Ever since the discovery of the AIE phenomenon in 2001, there are over 10,000 publications till date detailing the different aspects of AIE (Figure 3).Specifically, to AIE polymers synthesized using the RDRP techniques aforementioned, there exists more than 140 publications, and the number is projected to increase given the multitude of benefits in using RDRP to synthesize AIE polymers.In this review, we describe firstly, a brief introduction to RDRP, the AIE phenomenon, and AIE polymers.We will then elaborate on the design of AIE monomers and provide a list of some polymers synthesized via RDRP with the incorporation of AIE moieties.Next, we explore how RAFT can be used to design AIE polymers, including the design, the different types of process and polymerization mechanisms involved.Afterwards, similar to RAFT polymerization, we explore ATRP polymerization.Then, some elaborations on the other types of RDRP for AIE polymers, and potential applications of these AIE polymers.Finally, we present a summary and our perspective on the current progress of the AIE-active polymers.

THE STRATEGIES TO INCORPORATE AIE MOLECULES INTO POLYMERS BY RDRP
By exploiting the versatility of RDRP, AIE molecules can be facilely incorporated into the final polymer product through multiple ways to ultimately synthesize AIE-active polymers; block copolymer containing separate blocks of non-AIE monomers and AIE monomers (Scheme 2A), surface-grafted block copolymer (Scheme 2B), AIE monomers as crosslinkers for two block copolymers (Scheme 2C), AIE moieties as pendent groups in block copolymer (Scheme 2D), hyperbranched with random distribution of AIE monomers linked by non-AIE monomer backbone (Scheme 2E), four-arm star polymer with AIE moiety as the central core (Scheme 2F), end-functionalized AIE moiety chain extended with non-AIE monomers (Scheme 2G), direct linkage of AIE monomers (Scheme 2H), and unusual AIE behavior of non-AIE monomers after surface-grafted direct linkage (Scheme 2I).
Many different combinations of monomers, initiators, linkers, and catalysts type and amount were discovered and experimented to synthesize AIE polymers since the advent of AIE discovery.A few of the more interesting reaction types as examples are listed in Table 1 that incorporate AIE characteristics into the final polymeric product.
For the reaction types categorized as "RAFT" above, AIBN is typically used as the standard initiator and includes one type of RAFT agent as the CTA.Most of the polymerization reactions listed are considered to be multi-component reactions (MCRs) where more than one type of monomer is involved.For example, Entries 1-4 (Table 1) involves three different monomers and is considered a random copolymer reaction, where Entry 4 shows a special case of one of the monomer block (PEG) coupled to the RAFT agent to form a macro-CTA.Entries 5 and 6 (Table 1) are considered a four-component and two-component reactions, respectively.Entries 1-6 (Table 1) are classified as block copolymers containing AIE monomers (Scheme 2A).A two-component surface-initiated reaction with the RAFT agent anchored onto a solid surface (Entry 7, Table 1) allows for rapid functionalization through surface-grafted block copolymers of non-AIE and AIE monomers (Scheme 2B).Further expansion of this concept enabled the successful synthesis of a hyperbranched polymer variant using a non-AIE vinylfunctionalized diamide as a crosslinking backbone (Entry 8, Table 1) to incorporate the AIE monomers as pendent groups randomly distributed across the entire polymer molecule (Scheme 2E).Functionalization of symmetrical AIE monomers with vinyl bonds imparts crosslinking capabilities in a two-component reaction (Entries 9 and 10, Table 1) to crosslink two block copolymers made up of the other non-AIE monomer (Scheme 2C).Similar to a hyperbranched polymer mentioned before, the incorporation of an AIE monomer as a pendent group in an MCR (Entry 11, Table 1) also endows the block copolymer with AIE properties (Scheme 2D).
For all reaction types categorized as "ATRP" above, most of the examples use AIE molecule-functionalized macro-initiator to produce the final polymeric product possessing AIE characteristics.For example, ATRP initiator core-functionalized with AIE moiety (Entries 12 and 13, Table 1) generates four-arm star polymer after polymerizing with non-AIE monomers endows it with unique AIE fluorescence properties (Scheme 2F).Another popular choice for incorporating AIE groups into polymers is through end-functionalized AIE moieties (Entries 14-19, Table 1), with subsequent chain extension using non-AIE monomers (Scheme 2G).Polymerizing directly vinyl-functionalized AIE monomers (Entry 20, Table 1) provides ease in handling the reaction via direct linkages of the AIE monomers (Scheme 2H).Surface-initiated ATRP of AIE-inactive acrylonitrile monomers (Entry 21, Table 1) yielded unusual AIE fluorescence behavior in the final product without presence of any phenyl groups or aromatic rings (Scheme 2I).
For all reaction types categorized as "Others" above, although Cu(0) polymerizations bear similarities to ATRP, they use different oxidation states of Cu and are considered different to ATRP.Similar to Entry 1, AIE monomers are attached to the polymeric product through ionic bonding with N-containing non-AIE monomers as backbone (Entry 22, Table 1) to form block copolymers with the AIE moieties as pendent groups (Scheme 2D).Similar to Entry 20, unique designs of AIE monomers are polymerized separately (Entry 23, Table 1) through direct linkages (Scheme 2G).Similar to Entries 1-6, complex AIE active monomers functionalized with vinyl bonds (Entry 24, Table 1) allow for  [177]   b Others (Cu(0)) [178]   b Others (Cu(0)) [183]   (Continues)

AIE POLYMERS VIA RAFT
The RAFT polymerization was first reported and invented by Moad, Rizzardo, and Thang in 1998 from the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia, as one of the most powerful and versatile polymerization techniques to synthesize uniquely complex polymer architectures. [49,50,80,81]Coincidentally within a short period of time, Rhodia's chemists patented xanthates and coined the term "Macromolecular Design by Interchange of Xanthate" (MADIX). [82]Due to this coincidence, while both RAFT and MADIX master patents are based on identical polymerization mechanisms and similarly use thiocarbonylthio compounds (RAFT agents) such as trithiocarbonates, dithioesters, xanthates, and dithiocarbamates, the slight difference lies in MADIX only covering xanthates as RAFT agents. [83]ith over 15,000 papers on RAFT polymerization presently, RAFT is considered a technique that emulates an ideal living polymerization due to its ability to continue polymerization after adding more monomers, has good control over end-product polymer molecular weight, generates low dispersity (Ð) values, excellent tolerance to wide range of monomers bearing functional groups, and the ability to synthesize complex architectures (i.e., brush-shaped, star, hyperbranched, network, etc.) [65] for various applications such as diagnostic components and biomedical implants, [84] environmentally sustainable materials, [85] and other materials science and medicinerelated applications. [86]Hence, RAFT is suitable for use as a polymerization technique to synthesize AIE polymers.In the following sections, we are going to introduce the synthesis of AIE amphiphilic polymers, AIE interface/surfacebound polymers, and AIE branched polymers using RAFT polymerization.

AIE amphiphilic polymers via RAFT
As previously mentioned, RAFT allows the preparation of polymers with well-designed structure, well-controlled molecular weights and low dispersity, and these polymer characteristics are critical in the preparation of polymer nanoobjects via different types of self-assembly methods, such as solution self-assembly and polymerization-induced selfassembly (PISA).With the more commonly known applications of AIE, the AIE component were also incorporated into block copolymers via RAFT to prepare nano-objects with AIE property for practical applications.For example, in 2019, He and co-workers used the RAFT technique to synthesize a new class of AIE amphiphilic block copolymers, namely, poly(N-(2-methacryloyloxyethyl)pyrrolidone)-b-poly(lauryl methacrylate-co-1-ethenyl-4-(1,2,2-triphenylethenyl) benzene), PNMP-b-P(LMA-co-TPE) that can self-assemble into various polymer morphologies, such as spheres, worms, and vesicles in water and n-dodecane solvents. [35]The degree of polymerization (DP) of the PNMP block was kept constant at 35 while varying the DP of the P(LMA-co-TPE) block.At 1 wt% aqueous solution, spherical micelles of 30-40 nm were formed for PNMP 35 -b-P(LMA 9 -co-TPE 0.9 ), while worms become dominant for PNMP 35 -b-P(LMA 24co-TPE 2.7 ) and PNMP 35 -b-P(LMA 55 -co-TPE 6.3 ).These AIE-active amphiphilic copolymers could act as luminescent probes, and were applied in bioimaging using HeLa cells as substrates.Interestingly, quantum yields (QY) of PNMP 35 -b-P(LMA 38 -co-TPE 4.7 ) and PNMP 35 -b-P(LMA 55 -co-TPE 6.3 ) were found to be greatly enhanced compared to others due to higher DP of the AIE-active TPE moiety.According to the authors, polymer morphology and the location where the TPE moieties are located played a role in enhancing QY; when the polymers were dissolved in n-dodecane, the solvated P(LMA-co-TPE) segment of PNMP 35 -b-P(LMA 9co-TPE 0.9 ) vesicles inhibited π-π interactions, along with the low DP of the TPE moiety, caused the overall QY to be weak.A morphological phase change from vesicles to PNMP 35 -b-P(LMA 18 -co-TPE 1.9 ) worm-like micelles encourages greater degree of aggregation of the TPE moieties (higher QY).However, a further increase in DP of the TPE moieties led to formation of small spheres with less degree of aggregation (lower QY) of the TPE moieties.When the polymers were dissolved in water, the P(LMA-co-TPE) segment was located in the hydrophobic core of the polymer morphologies, as such QY becomes dependent mainly on the formed polymer morphologies and DP of the TPE moiety.PPNMP 35 -b-P(LMA 9 -co-TPE 0.9 ), PNMP 35 -b-P(LMA 18 -co-TPE 1.9 ), and PNMP 35 -b-P(LMA 24 -co-TPE 2.7 ) formed only simple spheres and rods, while polymers with higher DP of the TPE moiety formed worm-like micelles and vesicles with greater degree of aggregation, thus leading to enhancement of QY.The authors noted that biotoxicity of the polymers increases at higher solid content; however, even at a high 40 wt% solid content of PNMP 35 -b-P(LMA 55 -co-TPE 6.3 ), cell viability of HeLa cells is still greater than 85%.This trend was also observed upon increasing copolymer concentration from 10 to 100 μg mL −1 at a constant 40 wt% solid content.Under appropriate conditions, these polymers are expected to serve as excellent and highly efficient bioimaging probes.
Variations in the chemical structure of the TPE moiety were also explored by Li and co-workers in 2019 by synthesizing poly(ethylene glycol mono methyl ether amphiphilic block copolymer, where RAFT agent was first coupled to the hydrophilic PEG moiety, with CO 2 -responsive PDEAEMA block and AIE-active PTPEMA (Figure 4A). [87]The unique reversible transformation process from vesicles to micelles was achieved by reducing the interfacial tension between the hydrophobic blocks and the aqueous solution through adding non-selective co-solvents or upon exposure to CO 2 .This transformation also showed an approximate decrease in fluorescence by 30%, which could be attributed to the higher degree of intramolecular rotation, resulting in higher probability for nonradiative decay.The reverse transformation from micelles to vesicles can be achieved by bubbling the same solution with argon gas, promoting the release of any encapsulated hydrophilic molecules inside the vesicular compartments, mainly due to the protonation and deprotonation of the DMAEMA block in the polymersomes.
Another variation of the commonly known TPE moiety chemical structure was explored by Huang et al. in 2019, [122] by combining a novel AIE dye tetraphenylethenefunctionalized distyrene (TPES) with poly(ethylene glycol mono methyl ether methacrylate) (PEGMA) to form poly(ethylene glycol mono methyl ether methacrylate)- PEG-TS1 and PEG-TS2 self-assembled into FONs with measured diameters of 150 and 400 nm, respectively, in aqueous solutions with strong fluorescence emission peak at 515 nm due to the AIE phenomenon.External UV-light excites the ground state (S 0 ) energy levels in PEG-TS1 to different excitation state energy levels of S 1 and S 2 singlet states, which can relax back to S 0 via various pathways.When PEG-TS1 is dissolved in a poor solvent such as H 2 O rather than a good solvent such as THF, it aggregates into FONs, causing the nonradiative relaxation pathway via intramolecular rotation to be blocked due to intramolecular interactions such as C-H⋅⋅⋅π and C-H⋅⋅⋅H-C bonds, resulting in the radiative decay relaxation pathway and fluorescence emissions.Similar to R-PEG-20 and R-PEG-40 reported by Zhang et al., [88] PEG-TS series polymers showed greater than 90% cellular viability with HepG2 cells at a concentration of 80 μg mL  [87] (B) RAFT polymerization of R-PEG-20 and R-PEG-40 completed with schematic illustration showing the cell imaging applications of R-PEG FPNs.Reproduced with permission: copyright 2013, Royal Society of Chemistry. [88]ermo-responsive NIPAm blocks and AIE-responsive TPE crosslinking blocks via RAFT technique. [96]The polymers displayed an emission wavelength of approximately 485 nm and the highest PL intensity when the water fraction (f w ) reached 90% in a water/THF mixture solvent system.It was also discovered that HepG2 liver cancer cells at a concentration of 2 μg mL −1 absorbed over 80% of the polymers.Such polymers can find applications in controlled/target drug delivery, cell imaging, and tracking.
The incorporation of AIE components into amphiphilic block copolymers enables the preparation of photoluminescent nano-objects with different morphologies.More recently, with the rising of PISA, AIE-active nano-objects can be prepared directly during polymerization.As opposed to the previous section on self-assembly, PISA emphasizes on the self-assembly of these morphologies induced/triggered by in situ polymerization and not by adding any external agents or stimuli.
Recently in 2022, our group expanded the scope of this area by using the RAFT technique to perform PISA to synthesize photoluminescent polymer assemblies with rarely achieved inverse mesophases such as spongosomes, cubosomes, and hexosomes (Figure 5A). [125]The resultant polymer PDMA-b-(PTBA-r-PTPE) x -CDPA possesses both F I G U R E 5 (A) Synthetic scheme of the block copolymer PDMA 41 -b-P(TBA-r-TPE) x -based fluorescent nano/micro-objects via PISA, and the structure of each type of morphologies achieved via PISA with increasing of the hydrophobic chain length.Reproduced with permission: copyright 2022, Royal Society of Chemistry. [125](B) RAFT dispersion polymerization PISA of CO 2 -Responsive P(HEO 2 MA) 40 -b-P(MAEBA-co-DMAEMA-co-TPEMA) polymeric nanoobjects, exhibiting morphology evolution and fluorescence variation induced by CO 2. Reproduced with permission: copyright 2019, American Chemical Society. [126]2 O 2 responsiveness from the boronic moiety and AIE PL properties from the TPE moiety, allowing the polymer to be stimuli-responsive in addition to luminescence.These higher order morphologies bear high specific surface area and ability to load hydrophilic and hydrophobic chemicals for drug delivery systems and targeted drug release applications.
Similarly, in 2020, Qiu et al. also realized the unique behavior of DMAEMA when exposed to changes in pH levels and CO 2 presence.They prepared CO 2 -responsive polymer morphologies endowed with AIE properties using alcohol RAFT dispersion polymerization to synthesize poly(2-(2-hydroxyethoxy) ethyl methacrylate)-poly(methacryloxyethoxy) benzaldehyde)-poly(2-(dimethylamino)ethyl methacrylate)-poly(4-(1,2,2-triphenylvinyl)phenyl methacrylate) (P(HEO2MA)-b-P(MAEBA-co-DMAEMA-co-TPEMA)) (Figure 5B). [126]These nano-objects formed via PISA, transformed from spheres to vesicles following F I G U R E 6 (A) Fabrication of Fe 3 O 4 @SiO 2 @P(DMAEMA-co-TPEE) YS-NPs via sol-gel deposition and SI-RAFT polymerization.Reproduced with permission: copyright 2021, Elsevier. [130](B) Synthesis of HPEAM-TPEAH from EBA, TPEAH, and BDAAT.Reproduced with permission: copyright 2018, Royal Society of Chemistry. [5] increase in PL intensity.Upon CO 2 bubbling, existing spheres can also transform into a mixture of hemispherical "jellyfish"-like structures and vesicles, while existing vesicles can transform into higher order complex vesicles.The authors also discovered that treatment with CO 2 caused an increase in nano-object sizes from 142 to 314 nm (dissolved in methanol) and from 146 to 358 nm (dissolved in water) over a period of 60 min.A few other similar examples include RAFT-PISA processes in nonsynchronous synthesis of raspberry-like nanoparticles, [127] in drug delivery systems of where in situ drug loading of doxorubicin (DOX) via PISA with azoreductase-responsive PEG-b-P(BMA-co-TPE-AZO-MMA), [128] and for in situ monitoring and understanding of the photo-PISA process mechanism. [129]

AIE interface/surface-bound polymers via surface-initiated RAFT
AIE molecules can also be polymerized with surfacegrafted polymers to impart fluorescent properties to the resulting particles via surface-initiated RAFT polymerization, which improves the overall colloidal stability of the AIE moieties.In 2021, Huo et al. prepared a novel multistimuli-responsive, multifunctional polymeric nanoparticle poly(2-(dimethylamino)ethyl methacrylate)-co-poly((4vinylphenyl)ethene-1,1,2-triyl)tribenzene) with PDMAEMA as the organic carrier, grafted from SiO 2 surface as the inorganic carrier with asymmetrical encapsulation of Fe 3 O 4 nanoparticles (Fe 3 O 4 @SiO 2 @P(DMAEMA-co-TPEE)) (Figure 6A). [130]The resulting composite nanoparticle possesses a yolk-shell (YS) morphology with the AIE-active TPEE block to form yolk-shell nanoparticles (YS-NPs), allowing for real-time monitoring of any changes to environmental magnetic field, temperature, and pH levels, with the added ability to detect CO 2 presence in aqueous solution.In addition to the commonly known pH/thermoresponsiveness of the PDMAEMA block, the incorporation of Fe 3 O 4 endows the polymer with superparamagnetism, where higher PL intensity was observed for shorter distance to the source of magnetism.The fluorescence behavior of these YS-NPs depends on the extent of aggregation of the AIE molecules, which in turn depends on the extent of aggregation of the polymer brushes.When these YS-NPs are dissolved in a good solvent such as THF, the polymer brushes are highly stretched out compared to when a poor solvent such as H 2 O was used, which caused the contraction and overlapping of the polymer brushes leading to higher fluorescence emissions.Similarly, in research works by Zhang et al. [87] and Qiu et al., [126] the authors observed reversible CO 2 detection ability where the PL intensity decreased gradually approximately from pH 9.5 to pH 5.5 after bubbling CO 2 for 10 min, and returned to the original pH after bubbling with N 2 gas for the same amount of time.The relative "free" TPE units allowed the polymer brush to respond sensitively and accurately toward these external environmental changes through fluorescence variation.Notably, the solution of YS-NPs exhibited high colloidal stability during the changes, and surface aggregation-induced emission (SAIE) process was proposed for the aggregation of TPE units on the surface of YS-NPs.6B) with impressive Zn 2+ detection ability as measured directly from fluorescence intensity in the [Zn 2+ ] range of 4-18 μmol L −1 . [5]Upon interaction with Zn 2+ ions, the temperature at which HPEAM-TPEAH aggregates increases, which makes HPEAM-TPEAH more soluble, thereby reduces the RIM effect and causes a decrease in fluorescence intensity, which was considered as a "turn-off" response.The rationale of using Zn 2+ as opposed to other metal ions such as Na + , K + , Mg 2+ , Mn 2+ , Ca 2+ , and Fe 2+ is due to the significant effect on the LCST of the hyperbranched copolymer that Zn 2+ caused, even at concentrations less than 1 × 10 −5 M. HPEAM-TPEAH also showed greater than 95% cell viability in HeLa cells within 24 h of incubation time for concentration range of 1.0 × 10 −6 M to 5.0 × 10 −5 M.

AIE branched polymers via RAFT
Another form of branched polymers is dendritic polymers synthesized by Li and Gao in 2013 for the investigation into the cage effect imposed by these polymers on AIE pendent groups, affording the rarely observed solid-stateemissive blue light for such dendrimers. [131]The authors synthesized dendritic molecular brushes (DMBs) based on multifunctional segmented hyperbranched backbones via a grafting from approach, such as DMB functionalized with poly(glycidyl methacrylate) (GMA) side chains (DMB-1), DMB functionalized with poly(2-hydroxyl-3-azidopropyl methacrylate) (DMB-2) derived from DMB-1, DMB functionalized with pyrene moieties (DMB-Py) derived from DMB-2, and DMB functionalized with TPE units (DMB-TPE).DMB-TPE exhibited typical AIE effects when TPE units are incorporated into DMBs to form a hyperbranched macromolecule, where QY increased exponentially from 5.69% to 36.6% upon increasing f w from 60 vol% to 90 vol%.Due to the unique structure of the "spring-like cage" DMB-TPE, it confines and imposes greater RIM effect on the TPE moieties when dissolved in a poor solvent such as H 2 O.In addition, the cage structure of DMB-TPE also exerts an amplifying effect (cage effect) on the TPE units in the aggregated state, which increased QY by approximately two-folds when compared to mono-ethynyl TPE monomer, allowing the fabrication of a prototype ink paint containing DMB-TPE, which emits blue light strongly under UV irradiation.

AIE POLYMERS VIA ATRP
The ATRP technique was coincidentally invented and discovered separately by three different groups of researchers around the world in 1995 by (i) Kato et al., [41] (ii) Wang and Matyjaszewski, [42] and (iii) Percec and Barboiu. [43]After ATRP was patented in 1998 by Matyjaszewski and Wang as one of the most successful RDRP process, [44] numerous other US patents, applications, and publications worldwide also featured this polymerization technique. [46]ATRP is based on a process termed atom transfer radical addition (ATRA) developed in 1945, [132] involving the anti-Markonikov addition of alkyl halide radicals to alkenes in the Kharasch addition reactions. [133] ] to form a ternary initiating system, it is able to polymerize methyl methacrylate (MMA) via a radical pathway, thus behaving similarly to the ATRA process. [41]n the other hand, Wang and Matyjaszewski discovered that polymerizing styrene using an alkyl chloride initiator and a CuCl/2,2′-bipyridine (Cu(bpy)Cl) catalyst complex yielded well-defined high molecular weight homopolymers with low Ð values. [42]Later on the same year, Percec and Barboiu discovered that styrene polymerization can also be carried out using arenesulfonyl chloride initiators catalyzed by CuCl/bpy catalyst complex, producing homopolymers with good conversions but with relatively high Ð values (Ð > 1.50). [43]n general, for a typical ATRP mechanism, a redox reaction happens between an initiator bearing at least one transferable atom(s) or group(s) and a transition metal complex bearing a transition metal salt at a lower oxidation state and ligands attached to it.The metal catalyst cleaves the initiator homolytically and itself is oxidized in the process, enabling monomer addition to take place.The homolytic atom or group then transfers between the growing polymer chain end and the metal catalysts, causing the metal center to cycle between lower and higher oxidation states, thus establishing a dynamic equilibrium.
ATRP has over 19,000 papers till date, covering many areas ranging from synthesis to real-life applications of polymers synthesized from ATRP.ATRP shares the same advantages with RAFT and NMP, as it provides a simple route to synthesize polymers with good control of molecular weight, low dispersity (Ð) values, good tolerance against many functional groups, and the ability to produce welldefined polymer architectures.The main drawback of this technique is the presence of trace amounts of metal ions such as Cu in the end-polymer product, which is difficult to remove and can pose problems for certain applications.However, this problem can be circumvented by using UV-mediated metal-free catalysts such as the use of phenothiazine [134] and perylene. [135,136]Nevertheless, the versatility of ATRP enabled it to be used as a technique to synthesize AIE polymers with a slightly different design to the monomers and initiators involved compared to RAFT.In the following sections, we are going to introduce various methods to incoporate AIE components into polymers, namely the synthesis of AIE core-funtionalized polymers, AIE end-functinalized polymers, AIE monomer-component-functionalized polymers, and AIE interface/surface-bound polymers using ATRP polymerization.

AIE core-functionalized polymers via ATRP
The incorporation of AIE components into the core of polymers to form star-shaped block copolymers is an interesting way to impart fluorescence properties to polymers.A good example of AIE core-functionalized polymer produced via ATRP is reported by Guan et al. in 2016, where they synthesized a novel polyelectrolyte tetraphenylethenegraft-poly[2-(methacryloyloxy)-ethyltrimethylammonium F I G U R E 7 (A) Illustration of the self-assembly behavior of TPE-PMETAC in pure water after adding different types of counterions.Reproduced with permission: copyright 2016, Royal Society of Chemistry. [48](B) Illustration showing the preparation of fluorescent PTH-P(BzMA-MPC) copolymers and their self-assembly behavior into FPNs in aqueous solution, where the resulting AIE-active FPNs were used for cell imaging.Reproduced with permission: copyright 2020, Elsevier. [153]loride] (TPE-PMETAC) using ATRP from a TPE-derived four-arm macro-initiator, tetraphenylethylene-2-bromo-2methylpropionate (TPE-BMP).This polymer is capable of self-assembly into a core-shell microsphere structure in an aqueous solution where the TPE block forms the core and PMETAC forms the shell (Figure 7A). [48]The AIE feature comes from polymer chain aggregation at high concentrations, and is induced by simple exchange of counterions.It was discovered that TPE-PMETAC fluorescence intensity increases nonlinearly with increasing THF volume fraction similar to AIEE, [137] giving a bright blue emission at approximately 465 nm in two of 98 v/v water/THF solvent system.In addition, it was observed that the fluorescence intensity of cationic microspheres containing quaternary ammonium groups increases according to the series Cl − < (perchlorate) ClO 4 − < (hexafluorophosphate) PF 6 − < (bis-(trifluoromethylsulfonyl)imide) TFSI − , through ion-pairing interactions leading to "hydrophobic-induced collapse" of PMETAC block. [48]Reducing the size of microspheres, reduces the electrostatic repulsion forces between each microsphere and induces aggregation, evident in the size of the microspheres ranked from largest to smallest; TFSI − , PF (TPE-tetraPDEAEAM) possessing inherent AIE properties using ATRP technique and TPE-BMP as the macro-ATRP initiator containing AIE-active TPE. [138]The main difference lies in the stimuliresponsiveness of the side group where the former is electrically charged, while the latter is electrically neutral.These polymers respond to changes in temperature, pH levels and CO 2 levels, with obvious soluble-to-insoluble phase transition at the lower critical solution temperature (LCST).[140] To study the effect of polymer concentration on the cloud point in aqueous solution, when the polymer concentration increases from 0.5 to 2.0 g L −1 , the LCST decreases from 41.5 • C to 34.5 • C, along with aggregation of TPE moieties at the critical solution temperature (cloud point temperature) of 37.5 • C, resulting in enhanced fluorescence.TPE-tetraPDEAEAM were incubated with HeLa cells for 48 h at a concentration range of 50-400 μg mL −1 with cell viability of greater than 95%.The polymers are not cytotoxic to the cells at a concentration of 200 μg mL −1 for 48 h, which allowed for tracking of the cells for as long as nine passages.Incorporating AIE moieties to functionalize polymer cores were also exemplified by other groups, [47,[141][142][143][144][145][146][147] where they have been used as stimuli-responsive materials, cellular tracking agents, and advanced drug delivery systems.

AIE end-functionalized polymers via ATRP
Another way for incorporating AIE components into polymers is converting one end of the polymer into an AIE-active moiety.Such end-functionalized polymers with AIE-active moieties can exhibit fluorescence properties, and were explored by Jiang and Hadjichristidis in 2019. [148]In this example, the authors synthesized a TPE-terminated linear polyethylene (PE) using Tris(3-(4-(1,2,2-triphenylvinyl)phenoxy)propyl)borane, synthesized from hydroboration of (2-(4(allyloxy)phenyl)ethene-1,1,2triyl)tribenzene with BH 3 , as an initiator for the polyhomologation of dimethylsulfoxonium methylide to afford well-defined α-TPE-ω-OH linear polyethylenes (PE).All polymeric products showed AIE fluorescence either in the bulk phase or the solution phase, due to self-assembly behavior of the PE-based block copolymers in DMF solvent.The fluorescence intensity of the solutions can be determined from the block copolymer compositions and micelle size.At 90% v/v n-hexane fraction in a 0.1 g L −1 toluene/nhexane solvent system, the highest PL intensity was observed, which was 4.5-fold higher than pure toluene solvent system.For TPE-PE-b-PtBuA polymers, the critical micelle concentration (CMC) values are in the range of 0.5 − 1.5 × 10 −2 mg mL −1 , with the highest CMC value recorded to be 1.47 × 10 −2 mg mL −1 for the polymer with the highest PtBA content.The authors then extended their work to synthesize amphiphilic block copolymers TPE-PE-b-PAA by treating TPE-PE-b-PtBA with TFA to hydrolyze the tBu group to COOH group. [149]The synthesized polymer is responsive to pH changes, and it can emit fluorescence when exposed to certain ions.Change in fluorescence intensity was attributed to pH responsivity of the PAA block, causing dif-ferent degrees of aggregation of the TPE block.In addition, the influence of different cations at different pH levels on the fluorescence of TPE-PE-b-PAA was also investigated.The authors found that for the cations Li + , Na + , K + , Cs + , electron cloud polarizability is the dominant factor in determining fluorescence intensity, and therefore ranked them in increasing fluorescence order Li + < Na + < K + .Cs + has the largest polarizable electron cloud; however, due to the secondary factor electron repulsion, it was not ranked after K + .PhotoATRP can also be a viable option to synthesize polymers, which was exploited by Liu et al. in 2021, to produce poly(methyl methacrylate)s (TPE-PMMA) with AIE properties by combining methyl methacrylate monomers with 4-(1,2,2-triphenylvinyl)benzyl 2-bromo-2-phenylacetate (TPE-BPA) AIE-functionalized initiator, and catalyzed by air-stable copper(II) bromide/Tris(2pyridylmethyl)amine (Cu II Br 2 /TPMA) photocatalyst under benign conditions. [150]Polymerization reaction was conducted using LED light of wavelength 405 nm, and the introduction of the TPE moiety did not affect the polymerization kinetics and temporal control.AIEE effect was observed for TPE-PMMA solutions with higher molecular weights and with increased viscosity.
In 2016, Yang et al. prepared AIE-active amphiphilic tetraphenylthiophene (TP)-terminated poly(acrylic acid) (TP-PAA) using ATRP technique. [151]The resulting polymer self-assembled, primarily through hydrogen bond among carboxylic acid moieties at concentrations above the critical aggregation concentration (CAC) to form aggregates.The tBA pendant groups can be hydrolyzed by acids to the final AA pendant groups.In water, when TP-PAA concentration exceeds the CAC value (5.25 × 10 −6 M), the polymers aggregate into small micelles and fluoresces.At pH 2 to 9, there is almost negligible fluorescence as the fraction of aggregate emission is less than monomer emission.In contrast, at pH 9 to 12, the polymer fluoresces strongly.The authors tested TP-PAA as a potential bovine serum albumin (BSA) detector, where the aggregate emissions were more pronounce when mixed with BSA than the monomer emissions.Expanding on the application aspect of AIE-active polymers, in 2018, Zhuang et al. prepared polymeric micelles based on TPE-conjugated poly(N-6-carbobenzyloxy-Llysine)-b-poly(2-methacryloyloxyethyl phosphorylcholine) (TPE-PLys-b-PMPC) copolymer, which contains AIE-active TPE block in the micelle core. [152]The polymers were then loaded with an anticancer drug, DOX, for triggered intracellular drug release traced by fluorescent imaging of the micelles, which was made possible through hydrophobic interaction between DOX and PLys blocks in the polymer.The AIE effect stems from the fluorescent behavior of these polymers during the formation of micelles, when TPE located in the micellar core has profound RIM effect.Blank TPE-PLys-b-PMPC showed insignificant toxicity while DOX-loaded micelles showed excellent growth inhibition against HeLa cells and 4T1 cells that can be traced by their respective fluorescence upon cellular endocytosis, making such polymers a good candidate for antitumor and anticancer treatments.
In 2012, Gu et al. synthesized a pyrazoline-based AIE dual-active initiator 4- 2-bromo-2methylpropanoate (TPP-NI) possessing an electron donor group (dimethyl-amino) and an electron acceptor group (1,8-napthalimide) capable of intramolecular charge transfer (ICT) and AIE effects.TPP-NI was then used as the intiator to polymerize styrene (St), methyl methacrylate (MMA), and 2-hydroxyethyl methacrylate (HEMA) separately. [154]ith reference to the PL intensity of pure DMF solution, TPP-NI-PS showed 155-fold increase in PL intensity when dissolved in DMF-ethanol solvent system, while TPP-NI-PMMA showed 65-fold increase when dissolved in DMF-water system, and TPP-NI-PHEMA showed 10-fold increase when dissolved in DMF-water system with 70-fold increase when the solvent was acidified.The AIE effect of TPP-NI-PHEMA was much weaker than for TPP-NI-PS and TPP-NI-PMMA due to profound emission quenching caused by the polar PHEMA block promoting initiator ICT, and also due to the strong interaction between PHEMA block with the polar solvent molecules, leading to the lowest degree of aggregation.PHEMA amplifies the pH value effect as it causes more dimethylamino groups of TPP-NI to be exposed, which made it possible for PHEMA to serve as an optical sensor and drug-delivery agent via effective encasement of hydrophobic drug molecules.The fluorophore displays AIEE effect and increased quantum yields in strong polar solvents due to greater restriction of intramolecular motion of the individual AIE units after covalent linkage, leading to lower ICT effect.StTPP-NI shows almost negligible QY in DMF solution (0.16%), while bearing a high QY of 27% in cyclohexane solution.PStTPP-NI produced larger QY values in DMF and THF solutions than StTPP-NI in the same solvents, possibly due to the larger steric effects limiting intramolecular rotations of the AIE moiety.In another example where Wang et al. in 2020 synthesized P(tBA-r-TPEA)-b-PCholMA) (BCP-1) from acrylate-functionalized TPE units (TPEA), where these AIE-active units are found in the corona forming part of the block copolymer. [161]Quantum yield after micellization of BCP-1 was found to greatly increase from 0.38% (before micellization) to 9.36%, which can be used to monitor the micellization process and to study the effects of solvents on the process.The micellization process caused overcrowding and stretching of the coronal chains, which in turn caused significant RIM effect on nearby TPE moieties, resulting in the AIE effect.An interesting discovery was also made when TPE moieties located on the outer end of the corona chains did not show AIE effect upon forming micellization, which indicated that RIM of AIE moieties does not necessarily require their spatial aggregation.Furthermore, some variations of AIE-functionalized components include AIE moieties as pendent groups, [162] as monomers, [163,164] and as part of hyperbranched polymers, [165] were exemplified by other groups using ATRP.

AIE interface/surface-bound polymers via ATRP
Surface-initiation methods impart different unique properties onto the solid surface, and enable good customization of these surfaces.In 2017, Mao et al. incorporated AIE functionalities onto silica nanoparticles (SNPs) via the Stöber method to prepare luminescent silica nanoparticles (LSNPs), which were then converted into a macro-initiator where zwitterionic 2-(methacryloyloxy)ethyl phosphorylcholine (MPC) monomers were polymerized via surface-initiated ATRP (SI-ATRP) to form SNPs-AIE-pMPC (Figure 8A). [166]Similar to the work by Dong et al. in 2020, [153] Huang et al. had extended their work from randomly dispersed PTH-P(BzMA-MPC)-20 (40) FPNs to PTH-functionalized mesoporous silica nanoparticles (MSNs) with surface grafted block copolymer PTH@MSNs-poly(PEGMA-co-IA) from poly(ethylene glycol)methyl acrylate (PEGMA) and itaconic acid (IA) as monomers using light irradiation. [167]Some interesting properties of this polymer include the ability to conjugate with the anticancer drug cis-diammineplatinum dichloride (CDDP) with pH-responsive behaviors for sophisticated controlled drug delivery systems with high water dispersibility, low cytotoxicity, and excellent candidate as a cell imaging agent.
The scope of AIE molecules can also be expanded to include molecules with no perceivable aromatic rings or AIE-like features.An unusual case of AIE fluorescence was reported by Kopeć et al. in 2020, when ATRP was used to synthesize and graft well-defined low molecular weight (M n < 10,000 g mol −1 ) polyacrylonitrile (PAN) from silicon (Si) wafers (Figure 8B). [168]PAN is a non-conjugated polymer that does not contain any phenyl ring structures, yet it is still capable of AIE fluorescence behavior.The reason can be explained by the clustering of the nitrile groups in PAN, causing an overlap of π and lone-pair electron clouds, resulting in similar RIM effects as TPE groups. [169]These PAN brushes F I G U R E 8 (A) Illustration showing the synthesis of SNPs-AIE and SNPs-AIE functionalized with pMPC to form SNPs-AIE-pMPC. Reproduced with permission: copyright 2017, Elsevier. [166](B) Illustration of the synthetic route to PAN brushes by photoinduced SI-ATRP.Reproduced with permission: copyright 2020, Royal Society of Chemistry. [168]re prepared via photoinduced ATRP, allowing for significant reduction in catalyst amounts.A review put together by Yuan and Zhang in 2017 [170] elaborates in more detail the beauty of nonconventional macromolecular luminogens with AIE characteristics, which can be attributed to the similar clustering behavior as PAN, resulting in fluorescence of the polymeric product.

AIE POLYMERS VIA OTHER RDRP METHODS
Apart from the well-known RAFT and ATRP polymerization methods to incorporate AIE functionalities into polymers, Cu(0)-RDRP and NMP were also used to synthesize AIE polymers.Cu(0)-RDRP was first reported by Matyjaszewski and co-workers in 1997 when they discovered that the addition of zerovalent copper metal powder into standard ATRP polymerizations of styrene and (meth)acrylates dramatically improved the rate of reaction by as much as 10-fold compared to without any addition of the powder via simple electron transfer process to remove excess Cu(II) deactivator species. [171]In 2006, Percec et al. termed this unique polymerization technique as single-electron transfer living radical polymerization (SET-LRP), [172] which helped differentiate it from the standard ATRP technique.
NMP was first reported and patented by Solomon, Rizzardo, and Cacioli of CSIRO Australia in 1986. [38]Similar to RAFT and ATRP, NMP is a technique that bears resemblance to an ideal living polymerization: (i) the ability to control the desired polymer product molecular weight with low dispersity (Ð) values, (ii) has realistic industrial potential with simple implementation steps that often requires only a single unimolecular initiator to produce the desired product, and (iii) no need for use of transition metal catalysts.[175][176] Given the benefits Cu(0)-RDRP and NMP can offer, they are considered suitable methods for synthesizing AIE polymers.
In 2021, Li et al. synthesized cationic glycopolymers structurally similar to poly(ionic liquids) (PILs) using Cu(0)-RDRP technique. [177]Post-polymerization modifications were performed on the poly(4-vinyl pyridine) (P4VP) groups with halogen-functionalized D-mannose and TPE units, thus imparting AIE properties onto the resulting polymer.The resulting polymer can be viewed as cationic glycopolymers, which is a hybrid material possessing both PIL and glycopolymer properties, which includes specific carbohydrate-protein recognition and antibacterial activities in bacteria such as gram-positive Staphylococcus aureus and gram-negative Escherichia coli.The TPE units have the ability to improve the interaction between PILs and bacteria surface biomacromolecules, causing further aggregation of PILs and concentration of TPE units in the bacteria leading to AIE fluorescence for fluorescence imaging.The combination of AIE-active TPE units with glycopolymers and PILs enables the tracking, killing, and detection of bacteria.For the synthesis of PILs, Cu(0)-RDRP was used to polymerize 4VP to obtain P4VP at high conversion of 94% at room temperature conditions in DMSO (dimethyl sulfoxide):H 2 O solvent system (v/v = 1:1).Then, quaternization reaction was employed to modify P4VP with organobromides such as TPEBr, mannose-Br, or 2-bromopropanol, yielding PILs of P4VP-ManTPE and P4VP-BPTPE.The PILs adsorb onto the bacteria surface via electrostatic interactions between positively charged pyridine rings and negatively charged bacterial membranes, while the hydrophilic parts may insert into the hydrophobic membrane parts to kill the bacteria.The authors noted that cationic glycopolymers have better bactericidal effects on gram-positive bacteria than gram-negative bacteria due to the difference in cell membrane structures.Concanavalin A was used to show that sugar-containing PILs can recognize proteins, leading to aggregation and significant AIE effect.It is also worthwhile to note that detection by fluorescence emission of bacteria becomes more sensitive when bacteria concentration increases due to the AIE effect.
Similar to RAFT and ATRP, Cu(0)-RDRP need not be limited to the commonly known AIE-active TPE molecule, other variations were explored by many research groups, for example, Tonge et al. in 2018 prepared three different acrylic monomers using organic semiconductors as motifs, and employed them as electron-transporting (n-type) materials. [178]Cu(0)-RDRP method was used to prepare triazine-, oxadiazole-, and benzimidazole-containing polymers from room-temperature reaction using Cu(0) wires to give low dispersity (Ð = 1.14) and high conversions of up to 97%.The authors faced a problem with benzimidazolecontaining monomers due to the large induction period prior to onset of polymerization attributed to slow coordination of benzimidazole groups to CuBr 2 .Due to the limited solubility of these hydrophobic monomers in polar solvents such as DMSO, DMF, and isopropanol, difficulties were faced when selecting appropriate solvents.N-methyl-2-pyrrolidone and N,N-dimethylacetamide (DMAc) solvents were found to be effective in dissolving the monomers and assisting in the catalysts activities.Higher molecular weights of the resulting polymers were also successfully achieved at a lower conversion percentage and broader Ð, which could be due to poorer overall polymer solubility.Nevertheless, Cu(0)-RDRP was successfully employed by the authors to synthesize polymers with optical properties from challenging monomers containing N-donor groups with low dispersity values (Ð = 1.14-1.39)and conversions higher than 92%.All polymers were found to be thermally stable, as determined from only a single-step decomposition at 275 • C, making them ideal for processing into organic devices such as organic light-emitting diodes (OLEDs), [179] organic photovoltaics (OPVs), [180] organic thin-film tran-sistors (OTFTs), [181] organic electrochemical transistors (OECTs), and organic thermoelectric (OTE) generators. [182]IE molecules can also include transition metals such as iridium (Ir), which allows tuning of the color of fluorescence emitted.In 2019, Sauve et al. synthesized a series of bottlebrush copolymers (BBCPs) from red (IrPIQ-MM), green (IrPPY-MM), and blue (tBuODA-MM) (RGB) luminescent macromonomers using a carbazole-based host, which was then used to prepare multiblock organic fibers with similar structures to nanoscale RGB pixels (Figure 9A). [183]The different blocks were then combined to give di-and tri-block luminescent BBCPs, which display AIE effects between blocks as the solvent polarity changes.The authors elaborated on solvent-responsive luminescent-encoded patterns by quantifying the changes in energy transfer efficiency and interchromophore distance among the different blocks after aggregation.White LED mimicking pentablock nanofibers were then synthesized containing multiple discrete emission zones by combining the different building blocks with charge-transporting materials.Well-defined interfaces in BBCPs can be used to regulate energy transfer between the segments.Förster resonance energy transfer (FRET) was observed with significant color change when BBCPs aggregate.Multicomponent nanofibers with increasing complexity can be prepared using this method to conduct studies on optoelectronic interaction between and within BBCPs.
In addition, Cu(0)-RDRP can be employed to synthesize polymers capable of behaving as drug carriers as exemplified by Naghibi et al., of which they synthesized a brush-like polymer with AIE features for drug delivery and intracellular drug Reproduced with permission: copyright 2019, American Chemical Society. [183](B) Synthesis of 4-(N-methylpiperazine)-1,8-naphthalimide-polyisoprene (Napht-PI) conjugate via NMP and its co-nanoprecipitation with cladribine-diglycolate-polyisoprene (CdA-digly-PI) to form AIE-active polymer prodrug nanoparticles (Napht-PI CdA-digly-PI).Reproduced with permission: copyright 2017, Royal Society of Chemistry. [191]acking. [184]The study aims to improve the loading capacity of drug carriers by using a brush-like polymer with many functional groups capable of holding the target drug compound.By combining 1,1,2,2-tetraphenylethene functional ethyl α-bromoisobutyrate (TPEBIB) initiator and anticancer drug doxorubicin (DOX), [185] it could potentially lead to a compound capable of real-time monitoring of cell targeting, drug release, and cancer cell viability through real-time visualization of the AIE fluorescence effect.TPEBIB was synthesized and used as an initiator in the copolymeriza-tion of poly(ethylene glycol) acrylate (PEGA) and hydrazine (Hyd) monomers via Cu(0)-RDRP, and subsequently conjugating DOX to the center carrier through the hydrazone bonds to form the complex carrier TPE-PEGA-Hyd-DOX smart prodrug containing approximately 11 wt% DOX. [184]OX is released in a controlled manner after cellular endocytosis of TPE-PEGA-Hyd-DOX micelles when exposed to cancer cells due to hydrazine bond cleavage in acidic conditions due to improved cellular uptake levels, which can be monitored via fluorescence imaging.This novel block copolymer is completely biocompatible with normal and cancer cells, with the cytotoxicity depending on the local pH levels.A comparison study was performed between pristine DOX solution as the control and TPE-PEGA-Hyd-DOX solutions at the same concentration, where drug release reached only 10% after 96 h under normal cell conditions compared with 40% after 24 h under cancer cell conditions helps confirm that conjugation of DOX to the drug carrier controls the release of DOX and protects normal cells against DOX.Many other studies also employed the Cu(0)-RDRP methods to synthesize AIE polymers such as Cu(0)-catalyzed SET-LRP reported by Wei et al. for the study of multi-arm star polymers with TPE-functionalized core, [186] and the study of throughspace charge-transfer thermally activated delayed fluorescence (TSCT-TADF) phenomenon using AIE-functionalized monomers.[187][188][189] NMP can also be used for the synthesis of AIE polymers with a unique morphology as demonstrated by Bao et al. in 2017, who employed carbodiimide chemistry to link 4-(N-methylpiperazine)-1,8-naphthalimide-based AIE dye (Napht) [190] with 2-((tert-butyl(1-(diethoxyphosphoryl)-2,2-dimethylpropyl)amino)oxy)propanoic acid (AMA-SG1) alkoxyamine, yielding Napht-AMA-SG1 in 82% yield via the grafting from or "drug-initiated" method.Isoprene monomers were then added to produce the AIE-active polymer 4-(Nmethylpiperazine)-1,8-naphthalimide-polyisoprene (Napht-PI) and subsequently, co-nanoprecipitated with cladribinediglycolate-polyisoprene (CdA-digly-PI) to form Napht-PI CdA-digly-PI prodrug nanoparticles (Figure 9B).[191] Cytotoxicity of the polymers synthesized were also determined by incubating with murine leukemia (L12210) cells, with cell viability reaching approximately 100%, up to a concentration of 250 μg mL −1 after 72-h incubation time.Confocal laser scanning spectroscopy (CLSM) on Napht-PI CdA-digly-PI prodrug nanoparticles was carried out through incubation with A549 human lung carcinoma cells for intracellular imaging.The low cytotoxicity of these nanoparticles, combined with the sharp fluorescence signal from the AIEactive part of the prodrug, provides excellent imaging and tracking abilities in living cells.It is worth noting that the 1,8-naphthalimide-based fluorescent dyes studied in Bao and co-workers' work [191] were used previously to conjugate with different chemical species due to their versatile chemical structures.[192] These dyes exhibit AIE properties due to a twisted intramolecular charge transfer (TICT) process originating from RIM.
The use of NMP in AIE polymer synthesis was also demonstrated by Ma et al. in  2021  where TPE-functionalized 4-amino-2,2,6,6tetramethylpiperidine-1-oxyl (NH 2 -TEMPO) and 3-(((2-cyanopropan-2-yl)oxy)(cyclohexyl)amino)-2,2dimethyl-3-phenylpropanenitrile (Dispolreg 007) were used to study reaction kinetics in solution NMP (homogenous) and miniemulsion NMP (heterogenous polymerization) systems, respectively. [40]The AIE mechanism for both solution NMP and miniemulsion NMP were found to be viscosity-dependent, meaning as polymerization continues, the growing polymer chains lead to chain entanglements, thereby increasing the viscosity of the system and imposing a greater RIM effect on the TPE moieties.This phenomenon occurred in the emulsion droplets for miniemulsion NMP, whereas for solution NMP, it occurred in the bulk phase.

APPLICATIONS OF AIE POLYMERS
][195][196] Compared to AIE small molecules, AIE polymers have multiple advantages such as high fluorescence emission efficiency in the aggregated and solid forms, good processability, good film-forming ability, ability to perform post-polymerization modification on functional groups, and also for improved cellular uptake in drug delivery systems. [197]For example, Wu et al. in 2018 synthesized a series of conjugated polymer-based photosensitizers, which showed higher efficiency of between 1.73-fold to 5.07-fold in singlet ( 1 O 2 ) species production than their monomer counterparts, via enhanced intersystem crossing and improved light-harvesting ability, which is used to kill cancer cells or bacterial cells in photodynamic therapy due to the strong oxidation ability of singlet ( 1 O 2 ) species. [198]The ability for AIE polymers to form films via spin-coating, static casting, and ink-jet printing, [199] allows for further study into solid state luminescent materials, and fabrication of large-area thin solid films, [24,200] which is otherwise very difficult to achieve or requiring expensive techniques for AIE small molecules.In this section, some examples will be highlighted with more emphasis on theranostic applications, which briefly describes the importance of AIE polymers and how AIE polymers can improve our daily activities.

Theranostics
Theranostics is a novel concept that combines diagnosis (cell imaging) and therapy treatment (targeted drug release) in a full drug delivery system where AIE polymers are utilized as drug trackers to monitor the release of drug molecules after entering the targeted cells.Biological imaging techniques have played an important role in the field of biomedical application such as in guiding drug carriers for targeted cell treatments, cancer cell detection, and stem cell transplantation.Fluorescence imaging garnered worldwide interest as the "next-generation" technology in high-precision imaging at the subcellular level, with strong PL, high sensitivity, and versatility in the designing of the fluorescent nanoparticles.
Zhang et al. reported red R-PEG series and red R-F127 series fluorescent organic nanoparticles (FONs) in 2013 and 2014, respectively, [88,201] with excellent cell compatibility of at least 90% against A549 cells.Dong et al. in 2020 reported the facile preparation of AIE-active PTH-P(BzMA-MPC) FPNs with good water dispersibility and similar cell compatibility percentage against L02 cells as the previously mentioned FONs. [153]Similar trend patterns in cell penetration ability of both types of FONs and FPNs were confirmed by confocal laser scanning spectroscopy (CLSM).
Drug delivery systems containing self-guiding carrier molecules for anticancer drug treatment became popular in recent years.The use of anticancer drugs alone led to an increased possibility of drug resistance development in cancer cells, [202] and the lack of real-time monitoring of the entire delivery system of drug contributed to the limited application in cancer cell treatments.In 2018, F I G U R E 1 0 (A) Illustration of self-assembly behavior of DOX-loaded TPE-PLys-b-PMPC micelles, along with particle size distribution determined by DLS, the typical morphology of the polymer micelles detected by TEM, and CLSM images of DOX-loaded micelles after incubation with HeLa cells for 2, 4, 6, and 8 h.The DOX concentration is 10 μg mL −1 (scale bar in images was 10 μm).Reproduced with permission: copyright 2017, Wiley-VCH. [152](B) Illustration of preparing TPE-TPA-DCM-loaded BSA NPs, along with in vivo noninvasive fluorescence imaging of H 22 -tumor-bearing mice after intravenous injection of TPE-TPA-DCM-loaded BSA NPs (with TPE-TPA-DCM loading of 0.86 wt%) (upper picture) and bare TPE-TPA-DCM NPs (lower picture) at same TPE-TPA-DCM concentration (white circles mark the tumor sites).Ex vivo fluorescence imaging on tumor tissue and major organs of mice treated with fluorogen-loaded BSA NPs, which were sacrificed at 24 h post injection.Reproduced with permission: copyright 2011, Wiley-VCH. [193](C) The pHresponsive AIE process assisted by the self-assembly of block copolymers to form PEG-b-P(Lys-TPE). Reproduced with permission: copyright 2021, Royal Society of Chemistry. [4]uang et al. prepared poly(N6-carbobenzyloxy-L -lysine)b-poly(2-methacryloyloxyethyl phosphorylcholine) (TPE-PLys-b-PMPC) capable of spherical core-shell self-assembly with encapsulation of DOX in the micelle core via hydrophobic interactions for intracellular release and tracking (Figure 10A). [152]CLSM images were taken after incubating DOX-loaded TPE-PLys-b-PMPC with HeLa cells, where the images are taken at timestamps of 2, 4, 6, and 8 h.Red fluorescence pattern trends revealed that DOX entered the cytoplasm during the first 4 h after incubation, and slowly diffused into the cell nuclei from the 6 h mark onwards, while blue fluorescence pattern trends suggest that TPE-PLys-b-PMPC remains only in the cell cytoplasm.TPE-PLys-b-PMPC improves the endocytosis of DOX and degrades during the first 4 h after incubation through which DOX is released, that ultimately diffuses into the cell nuclei and inhibits cancer cell growth.This mode of mechanism is one of the ways for designing smart drug delivery systems with triggered biodegradable drug carriers.
Based on similar principles of intracellular drug release, Naghibi et al. in 2022 synthesized TPE-PEGA-Hyd-DOX with a slightly different drug release mechanism, where the hydrazone bond acts as the linker and conjugates with the anticancer drug DOX. [184]Confocal images of TPE-PEGA-Hyd-DOX, TPE-PEGA-Hyd, and pristine DOX were taken after incubation with HeLa cells and NIH3T3 cells, respectively, with a much stronger fluorescence registered for TPE-PEGA-Hyd-DOX in both cases.The DOX channel image represents the extent of release of the drug into the cells.In both cases, pristine DOX uptake was relatively less readily done than TPE-PEGA-Hyd-DOX, where the merged images confirmed the excellent performance of this drug carrier delivery system.In addition, Naghibi et al. found that drug release was limited in healthy cells compared to cancer cells, as the environment in the cancer cells encouraged the hydrazone bond cleavage and subsequent release of DOX in the cell cytoplasm, where it migrated into the cell nuclei to kill the cells.The combined benefits of AIE and targeted drug delivery enabled this drug delivery system to monitor drug movement and in vivo cellular responses in real-time, which can help to revolutionize traditional methods in direct administration of medicinal drugs.
In  10B). [193]These FPNs were then examined for their cell compatibilities against MCF-7 breast cancer cells and murine hepatoma-22 (H 22 )-tumor-bearing mouse models.in vivo imaging of BSA-loaded FPNs were determined via noninvasive fluorescence imaging of the live animals after injection of the FPNs with images taken at the 3-, 8-, 28-h mark for BSA-loaded FPNs and bare FPNs, respectively, where fluorescence intensity was twice as high for the mice with tumors than for the mice without any tumors.Ex vivo imaging on the different parts of the mice when it was sacrificed 24 h post-injection helps to confirm the accumulation of the BSA-loaded FPNs in the tumor areas through visualization of the intense coloration in that particular area.Similar work was performed by Yang et al. in 2016 when they synthesized AIE-active amphiphilic tetraphenylthiophene (TP)-terminated poly(acrylic acid) (TP-PAA), capable of self-assembly into small micelle aggregates. [151]TP-PAA has the ability to denature BSA and uncoil the structure to expose the two main fluorescent units in it, namely Trp-212 and Tyr-263. [203]TP-PAA preferentially bind to BSA through the Trp-212 unit of BSA by diffusing into the large BSA chains and undergoes RIR effect.When the concentration of BSA is kept constant, the Trp emission at 350 nm after excitation with 280-nm wavelength decreased as concentration of TP-PAA increases, which indicates probable FRET from BSA to TP-PAA, as Trp emission profile overlaps with TP-PAA absorption profile.Thus, TP-PAA can be employed as a sensor for detecting the presence of BSA and possibly be used to quantify the amount of BSA in a sample.

pH level fluctuation sensor
By incorporating stimuli-responsiveness into AIE polymers, it can respond to environmental variations such as temperature, light, pH, and so forth.One of the important modifications performed to AIE polymers is the ability to detect environmental pH changes, which becomes important when dealing with biological applications such as intracellular drug delivery and carrier systems. [204] 2016, Yang et al. employed ATRP to synthesize AIEactive amphiphilic tetraphenylthiophene (TP)-terminated poly(acrylic acid) (TP-PAA) by hydrolyzing the tBA pendant groups of TPP-PtBA, which have the ability to self-assemble into aggregates via intermolecular hydrogen bonding of the carboxylic acid moieties. [151]These small micelle aggregates contain hydrophobic TP units as the core and hydrophilic PAA chains as the shell, which reacts with the addition of bases to produce varying fluorescence emission.It was observed that there were negligible variations in fluorescence emission for the pH range 2-9; however, significant fluorescence emission changes were detected for the pH range 9-12, where these aggregates fluoresce stronger than the corresponding monomer.As the pH of the system increases, higher fraction of the carboxylic acid groups deprotonates to form negatively charged carboxylate ions that interacts strongly with the solvent molecules, such as the hydrophobic TP core of the micelles pack more closely together to compensate for the unfavorable interactions with the solvent molecules, resulting in significant RIR effect and AIE.This explanation was also supported by the fact that there was a reduction in average hydrodynamic diameter of these micelles from 250 nm at pH 9 to 75 nm at pH 12.
A study was conducted by Wang et al. in 2021, where block copolymer poly(ethylene glycol)-b-poly(L-lysine) (PEG-b-PLys) was synthesized and modified with TPE-CHO group to form PEG-b-P(Lys-TPE) bearing reversible pH-responsive fluorescence properties. [4]PEG-b-P(Lys-TPE) forms spheres with a core-shell structure when added to a solvent system comprising DMF/H 2 O, and forms vesicles when added to a solvent system comprising THF/H 2 O.For the THF/H 2 O solvent system, fluorescence intensity dropped drastically upon reducing pH level from 10.7 to 1.4, postulated to be the detaching of the TPE moiety from the imine bond, causing the polymer to lose the AIE characteristics, while regaining strong fluorescence after increasing the pH level to 12.6, indicating the re-attachment of the TPE moiety to the polymer via the imine bond (Figure 10C).The authors also found that this reversible behavior is only possible in a mixed solvent system, as the polymer exhibited irreversible pH fluorescence behavior when pH variations were performed in pure water solvent systems due to the precipitation of TPE residues after detachment from the polymer.Nevertheless, such a polymer can find potential use as a pH probe in mixed-solvent systems, but has limited applications in single-solvent systems.

Metal ion selective sensor
Metal ion pollution is a major environmental concern, and it is crucial that these metal ions can be readily detected through the use of probes that interact with them and provide sensing capabilities.Fluorogenic probes have the ability to interact with the metal ions via complexation and other chemical reactions to change their fluorescence properties, which can be detected by fluorescence measurements. [205]IE-based polymer probes can be designed to take advantage of the metal ion-induced aggregation effect to detect a single type or multiple types of metal ions by registering a change in fluorescence intensity.Metal ion probes can also be designed to detect a single type of metal ion instead of multiple metal ions.Liu et al. in 2018 facilely constructed a hyperbranched AIE poly(acrylamide) HPEAM-TPEAH to be used as a probe for the detection of Zn 2+ specifically. [5]An aqueous mixture of fluorescence HPEAM-TPEAH and different metal ions were prepared to determine which metal ion is responsive toward HPEAM-TPEA, and the authors discovered amongst the many metal ions tested such as Zn 2+ , Mn 2+ , Na + , Ca 2+ , Mg 2+ , Fe 2+ , and K + , only Zn 2+ ion provided a significant decrease in fluorescence intensity when mixed with HPEAM-TPEAH in water and in simulated body fluid.Zn 2+ ion remained detectable even at low concentration of 2 × 10 −5 M, indicating the highly selective and sensitive "turn-off" response of HPEAM-TPEAH toward Zn 2+ ion.

in situ monitoring of reaction process
The PISA process is a fascinating process whereby growing polymer chain starts to fold on itself upon reaching a critical DP and causes immediate phase separation leading to spherical micelles formations, which mainly depends on the length of the hydrophobic chain segments. [206,207]As the DP of the hydrophobic chain segments continue to increase, a fusion process occurs to transform these spherical micelles into more complicated morphologies such as worms, vesicles, and so forth. [208,209]AIE-functionalized polymers provide a non-invasive and facile way for in situ tracking of the reaction process.) by reacting POEGA macro-CTA with St and TPEE via photo-RAFT dispersion copolymerization. [129]Fluorescence intensity increases as particle sizes increase, but showed little increase when monomer conversion goes beyond 39%, which can be explained by the aggregation of the different particles to form aggregates rather than core growth where the TPEE moiety is located in.The morphological transformation from spheres to worms to hollow particles and then to vesicles showed lower rate of increase of fluorescence intensity compared to monomer conversion.The point at which there was a discernible change in the rate of fluorescence intensity increase when graphed against average hydrodynamic diameter of the particles can be correlated to the point at which there is a morphological transformation from spheres to worms or vesicles.

Nitroaromatic pollutants sensor
Nitroaromatic compounds such as picric acid, 1,3,5trinitrotoluene, nitrobenzene, and p-nitrotoluene are widely used as raw materials and intermediates in the manufacturing industry.Sudden large-scale accidental discharge of such compounds during the production process can cause widespread damage to the environment and to the human body due to their high toxicity and high persistency. [210,211]s such, there exists a need for a rapid way to detect nitroaromatic compounds and pollutants.and Polymer 2 formed nanoparticles in a solvent system of THF/water with f w at 99% interacting with 1,3-dinitrobenzene, 4-nitrotoluene, nitrobenzene and p-nitrofluorobenzene separately to cause significant fluorescence quenching almost instantaneously, allowing for the rapid detection of these compounds with quenching efficiencies of 97%, 95%, 83%, and 80%, respectively. [118]An important property of these sensors is the limit of detection, which is measured by how low the concentration of the compounds or pollutants can go before it becomes undetectable, and both Polymer 1 and Polymer 2 are able to detect the presence of these compounds below 0.25 ppm, which is deemed to be highly sensitive.Subsequently, Polymer 1 and Polymer 2 were tested for their sensitivities toward aliphatic nitro-compounds such as nitroethane, 1-nitro-propane, 2nitropropane, and non-nitroaromatic compounds such as toluene, phenol, and halogen-containing aromatics, where both Polymer 1 and Polymer 2 showed only slight fluorescence quenching effect, hence proving their highly selective and sensitive behavior toward nitroaromatic compounds.Recently, Xue et al. in 2023 reported the successful synthesis of AIE type non-traditional intrinsic luminescent (NTIL) polymers such as polytriphenylmethyl azide (PTPMA), polyfluorotriphenylmethyl azide (PFTPMA), polydifluorotriphenylmethyl azide (P2FTPMA), polychlorotriphenylmethyl azide (PCTPMA), and polydichlorotriphenylmethyl azide (P2CTPMA) via RAFT polymerization-induced emission (PIE). [200]An interesting discovery revealed the nonemissive behavior of the triphenylmethyl azide moiety only becoming emissive when polymerized. [200]Significant fluorescence emission was observed when f w increases from 70% to 90% for PTPMA, with the solution emitting strong cyan luminescence.FRET was observed between electron-deficient 2,4,6-trinitrotoluene (TNT) and electronrich PTPMA, leading to fluorescence quenching, with fluorescence quenching ratios reaching 80% just after addition of 40 ppm of TNT, indicating the ability of PTPMA to detect explosives quickly and with high sensitivity.In addition, the ACQ luminescent dye, Nile red can be dispersed in the PTPMA matrix by capitalizing on the good film-forming property of PTPMA to form solid state luminescent materials, which enhances the fluorescence property of Nile red in the solid state via effective resonance energy transfer from PTPMA to Nile red.

Potential application in optoelectronic
Optoelectronics is an area that concerns the study between light and electronic devices.Over the years, researchers have put in a lot of effort to improve the performance of optoelectronic devices.One of the more successful breakthroughs in this area is the invention of efficient OLEDs with high quantum efficiency and low efficiency roll-off values such as lens-free OLEDs with over 50% external quantum efficiency developed by Kim, Yoo and co-workers, and single-layer blue OLEDs with low quantum efficiency roll-off by Sachnik et al., respectively. [212]Photopatterning is another area within optoelectronics that uses photomasks to create 2D photopatterns that can be incorporated into advanced electronic devices.Ma et al. in 2022 employed photo-RDRP to create a library of hydrophilic-hydrophobic AIE polymers synthesized in both batch and flow reactors, using 1,1,2,2-tetraphenylethene functional ethyl α-bromoisobutyrate (TPEBIB) initiator. [213]hotopatterning was performed using negative copper photomask on a thin film of TPE-PMMA 100 to generate a 2D fluorescent photopattern, with only the exposed regions showing fluorescence quenching ("turn-off" response), which could possibly be due to photo-oxidation.Such TPE-terminated polymers can have potential applications in optoelectronics after being fabricated into a film; however, more research needs to be done before these AIE polymers can be included in optoelectronic devices.

SUMMARY AND PERSPECTIVE
This review summarizes some of the many interesting AIE polymer end product designs from a wide range of monomers and some important applications that AIE polymers can bring about.The unique discovery of the AIE phenomenon manages to solve problems associated with the ACQ phenomenon, as aggregation is highly encouraged for AIE polymers to be useful.Due to the versatility of RDRP, various strategies can be used to incorporate AIE components into polymers such as direct polymerization of non-AIE monomers and AIE monomers, surface-initiated polymerization, AIE monomers containing more than one vinyl bond acting as crosslinkers, AIE components as pendent groups that can be found in hyperbranched-type polymers, AIE core-functionalized multiarm star polymers, AIE endfunctionalized polymers, direct linkage of AIE monomers, and through the unusual AIE fluorescence behavior exhibited after polymerizing non-AIE monomers.More efforts are being invested in discovering other possible combinations of monomers and initiators/crosslinkers to produce unique AIE polymers possessing multistimuli-responsive properties for high-throughput new applications or improving upon currently known applications, including their use as cell imaging agents and drug delivery systems in theranostics applications, pH sensors, metal ion selective sensors, in situ reaction process monitoring and tracking, nitroaromatic pollutants sensor, and potential applications in optoelectronics.An emerging trend in the AIE polymer field is the shift toward simpler fabrication processes where multicomponent reactions and one-pot reactions assisted by microwave or ultrasonic irradiation are favored over tedious multistep preparations.Another exciting area of AIE polymers is the use of carbohydratebased monomers and unusual monomers without phenyl groups but still able to possess AIE characteristics after polymerization such as acrylonitrile, and epoxide-containing branched monomers, which may find application for imaging and biological-related purposes.The possibility to combine artificial intelligence (AI) and machine learning (ML) to AIE polymers fabrication and application opens up exciting future directions for high-end technologies such as incorporation of AIE polymers into AI systems with complex logic gates as multisensors, advanced ML models that can rapidly predict structure-property rela-tionships (SPRs) of AIE/ACQ polymers, ML tools with the ability to generate fast and accurate information on pathogens through AIE responsiveness to environmental variations, and so forth.In addition, AI and ML can also be applied to automate polymerization techniques on the benchtop to quickly screen and identify different SPRs in a large chemical space for high-throughput experiments and high-throughput screening, which would otherwise require laborious work by researchers.Even though AIE polymers became popular more than a decade ago, it can only be considered in the infancy stage of development as many of the applications are being constantly developed and improved upon.With the unwavering efforts of many researchers around the world, AIE polymers will become even better and more useful in the future.

S C H E M E 1
Various simplified reversible-deactivation radical polymerization (RDRP) mechanisms broadly categorized into: (A) reversible deactivation, (B) catalytic reversible deactivation, and (C) degenerative transfer, with other reactions such as propagation, transfer, and termination-related reactions omitted for clarity (adapted from Ref.[132]).

S C H E M E 2
Possible polymerization reactions via reversible-deactivation radical polymerization (RDRP) to synthesize AIE polymers.

TA B L E 1
Overview of different reversible-deactivation radical polymerization (RDRP) techniques utilized with selected monomers, RAFT agents/ATRP initiators to synthesize AIE polymers.

TA B L E 1 (Continued) Entry Polymerization type Monomer RAFT agent/ATRP initiator Reference
Non-AIE-active monomer that can be polymerized via ATRP to yield AIE-active polymer with unusual AIE fluorescence behavior.