3. 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.[46, 47, 77, 78] Coincidentally within a short period of time, Rhodia’s chemists, patented xanthates and coined the term “Macromolecular Design by Interchange of Xanthate” (MADIX).[79] Due to this coincidence, while both RAFT and MADIX master patents are based off 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.[80]
With over 15,000 papers on RAFT polymerization presently, RAFT is considered 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.)[62] for various applications such as diagnostic components and biomedical implants,[81]environmentally-sustainable materials,[82] and other materials science and medicine-related applications.[83] Hence, RAFT is suitable for use as a polymerization technique to synthesize AIE polymers.

3.1 RAFT AIE amphiphilic block copolymers

As mentioned in early sections, 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 nano-objects via different types of self-assembly methods, such as solution self-assembly and polymerization-induced self-assembly (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, Li, Dong and co-workers, used the RAFT technique to synthesize a new class of AIE amphiphilic 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 andn -dodecane solvents.[84] The degree of polymerization (DP) of the PNMP block was kept constant at 35 while varying the P(LMA-co -TPE) block. At 1 wt% aqueous solution, spherical micelles of 30-40 nm were formed for PNMP35-b -P(LMA9-co -TPE0.9), while worms become dominant for PNMP35-b -P(LMA24-co -TPE2.7) and PNMP35-b -P(LMA55-co -TPE6.3). 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 PNMP35-b -P(LMA38-co -TPE4.7) and PNMP35-b -P(LMA55-co -TPE6.3) were found to be greatly enhanced compared to others due to higher DP of the AIE-active TPE moiety. Polymer morphology also played a role in enhancing QY where worms were found to increase QY more than spheres. The authors noted that biotoxicity of the polymers increases at higher solid content however, even at a high 40 wt% solid content of PNMP35-b -P(LMA55-co -TPE6.3), cell viability of HeLa cells is still greater than 85%. This trend was also observed upon increasing copolymer concentration from 10 µg mL-1 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 was also explored by Li and coworkers in 2019 by synthesizing poly(ethylene glycol mono methyl ether)-b -poly[(2-(diethylamino)ethyl methacrylate)-co -3-(4-(1,2,2-triphenylvinyl)phenoxy)propyl methacrylate)] (PEG-b -P(DEAEMA-co -TPEMA)) amphiphilic block copolymer where RAFT agent was first coupled to the hydrophilic PEG moiety, with CO2-responsive PDEAEMA block and AIE active PTPEMA (Figure 4A ).[85] 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 CO­2. 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.
The design of AIE molecules need not be limited to molecules bearing only a single vinyl group, it also applies to molecules bearing two or more of such groups. Zhang, Wei and co-workers in 2013, facilely incorporated a symmetrical cross-linkable AIE dye termed R-E with a vinyl end group on both sides into poly(ethylene glycol mono methyl ether methacrylate) to synthesize R-PEG-20 and R-PEG-40 AIE-based FPNs (Figure 4B ).[86] The obtained amphiphilic FPNs is capable of self-assembly in aqueous solution which produces nanoparticles of uniform size, high water dispersibility, strong red fluorescence and excellent biocompatibility, enabling them to serve in cell imaging applications. Cell-counting kit-8 (CCK8) assays were performed to determine cell viability of these FPNs, and the authors determined excellent cellular uptake levels by A549 cells greater than 90%, even at high concentrations of 80 µg mL-1. These FPNs can also be produced from a wide range of monomers and impart greater stability compared to those nanoparticles formed from self-assembly, as FPNs prepared via self-assembly are often unstable in physiological solution due to the weak interactions among these amphiphilic fluorescent molecules. Similar studies have also been conducted by the same group and other groups on preparing AIE dyes via RAFT polymerization capable of self-assembly with similar characteristics since 2014 such as AIE crosslinkers,[87-94] AIE pendent groups with non-AIE monomers block copolymers,[95-103] AIE end-functionalized block copolymers,[48, 49, 104-107] and AIE-functionalized monomers,[108-119] which helped to expand the library of AIE-functionalized polymers.
Another variation of the commonly known TPE moiety chemical structure was explored by Huang, Liu, Wei and co-workers in 2019,[120] by combining a novel AIE dye tetraphenylethene-functionalized distyrene (TPES) with poly(ethylene glycol mono methyl ether methacrylate) (PEGMA) to form poly(ethylene glycol mono methyl ether methacrylate)-b -poly((Z)-3-(4-(1,2,2-triphenylvinyl)phenyl)-2-(4’-vinyl-[1,1’-biphenyl]-4-yl)acrylonitrile)) (PEG-b -TS) polymers. PEG-TS1 and PEG-TS2 self-assembled into FPNs with measured diameters of 150 nm and 400 nm respectively in aqueous solutions. Similar to R-PEG-20 and R-PEG-40 reported by Zhang, Wei and co-workers,[86] PEG-TS series polymers showed greater than 90% cellular viability with HepG2 cells at a concentration of 80 µg mL-1.
A less bulky symmetrical TPE-based poly(N -isopropylacrylamide-co­ -(E )-1,2-diphenyl-1,2-bis(4-((4-vinylbenzyl)oxy)phenyl)ethene) (P(NIPAm-co -TPE2St)) was synthesized by Yang, Lin and co-workers in 2022, using thermo-responsive NIPAm blocks and AIE-responsive TPE cross-linking blocks via RAFT technique.[94] The polymers displayed an emission wavelength of approximately 485 nm and the highest PL intensity when the water fraction 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-1absorbed over 80% of the polymers. Such polymers can find applications in controlled/target drug delivery, cell imaging and tracking.
In 2017, Liu, Zhang, Wei and co-workers successfully incorporated AIE molecules into polymer particles by combining all multiple reactants together in a multicomponent reaction (MCR) termed the ‘three-component mercaptoacetic acid locking imine (MALI) reaction, with RAFT technique in a “one-pot” reaction to synthesize poly(polyethylene glycol mono methyl ether methacrylate)-co -poly(10-undecenal)-poly(((Z )-3-(4-aminophenyl)-2-(10-hexadecyl-10H-phenothiazin-3-yl)acrylonitrile) (PPEGMA-co -PUCL-Phe1 and PPEGMA-co -PUCL-Phe2) (Figure 4C ).[121] Luminescent organic nanoparticles (LONs) are capable of self-assembly when dispersed in aqueous medium and are found to possess multiple traits such as AIE features, good brightness, good water dispersibility and excellent biocompatibility. Incubation of HeLa cells with PPEGMA-co -PUCL-Phe2 LONs at a concentration of 120 µg mL-1 resulted in over 95% cell viability within 24 h, and excellent staining ability at a concentration of 20 µg mL-1 believed to be a result of cellular endocytosis.
The incorporation of AIE components into amphiphilic block copolymers enables the preparation of photoluminescent nanoobjects 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 situpolymerization and not by adding any external agents or stimuli.
For example, in 2017, Wang, Wei, Yuan and co-workers employed the RAFT-PISA process to synthesize poly(N,N -dimethylaminoethyl)-b -poly[benzyl methacrylate-co -1-ethenyl-4-(1,2,2-triphenylethenyl)benzene] (PDMA-b -P(BzMA-TPE)) capable of self-assembling into different morphologies such as spheres, worms and vesicles during polymerization (Figure 5A ).[122] The authors found that PL intensity and QY increases in the order: PDMA39-P(BzMA-TPE)-120 (micelles) < PDMA39-P(BzMA-TPE)-240 (worms) < PDMA39-P(BzMA-TPE)-360 (vesicles), and that all polymer samples displayed stronger PL intensities when dissolved in water than in ethanol.
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 5B ).[123]The resultant polymer PDMA-(PTBA-r -PTPE)-CDPA possesses both H2O2 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, Xing and co-workers also realized the unique behavior of DMAEMA when exposed to changes in pH levels and CO2 presence. They prepared CO-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 5C ).[124] These nano-objects formed via PISA, transformed from spheres to vesicles following an increase in PL intensity. Upon CO2 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 CO2 caused an increase in nano-object sizes from 142 nm to 314 nm (dissolved in methanol) and 146 nm 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,[125] 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),[126]and for in situ monitoring and understanding of the photo-PISA process mechanism.[127]

3.3 AIE block copolymers via Surface-initiated RAFT

AIE molecules can also be polymerized with surface grafted polymers to impart fluorescent properties to the resulting particles via surface-initiated RAFT polymerization. In 2021, Qiao, Pang and co-workers prepared a novel multi-stimuli responsive, multi-functional polymeric nanoparticle poly(2-(dimethylamino)ethyl methacrylate)-co -poly((4-vinylphenyl) ethene-1,1,2-triyl)tribenzene) with PDMAEMA as the organic carrier,grafted from SiO2 surface as the inorganic carrier with asymmetrical encapsulation of Fe3O4 nanoparticles (Fe3O4@SiO2@P(DMAEMA-co -TPEE)) (Figure 6A ).[128] The resulting composite nanoparticle possesses a yolk-shell (YS) morphology with the AIE-active TPEE block, allowing for real-time monitoring of any changes to environmental magnetic field, temperature and pH levels, with the added ability to detect CO2 presence in aqueous solution. In addition to the commonly known pH/thermo-responsiveness of the PDMAEMA block, the incorporation of Fe3O4 endows the polymer with superparamagnetism where higher PL intensity was observed for shorter distance to the source of magnetism. Similar to the works by Li et al .[85] and Xinget al. [124], the authors observed reversible CO2 detection ability where the PL intensity decreased gradually from pH ~9.5 to pH ~5.5 after bubbling CO2 for 10 min, and returned to the original pH after bubbling with N2 gas for the same amount of time. The relative “free” TPE units allowed the polymer brush to respond sensitively and accurately towards 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. Another similar study was conducted by Tian, Zhang, Wei and co-workers where fluorescent nanodiamonds (FNDs)-poly(2-methacryloyloxyethyl phosphorylcholine (MPC) (FNDs-polyMPC) composites was fabricated using surface-initiated photoRAFT technique and tested for their high water dispersibility and excellent cellular uptake as cell imaging agents.[129]

3.4 Hyperbranched AIE block copolymers via RAFT

To prove the versatility of AIE molecules, Bai, Zhang and co-workers in 2018, successfully employed RAFT technique to synthesize a thermo-responsive hyperbranched copoly(bis(N,N -ethyl acrylamide)/(N,N -methylene bisacrylamide)) (HPEAM-MBA) and copoly(bis(N,N -ethyl acrylamide)/4-(2-(4-(allyloxy)phenyl)-1,2-diphenylvinyl)phenol) (HPEAM-TPEAH) polymers (Figure 6B ) with impressive Zn2+ detection ability as measured directly from fluorescence intensity in the [Zn2+] range of 4 – 18 µmol L-1.[5] Upon interaction with Zn2+ ions, the RIM effect was induced on TPE moieties due to a change in the polymer lower critical solution temperature (LCST) and thereby results in fluorescence, which was considered as a “turn-off” response. The rationale of using Zn2+ as opposed to other metal ions such as Na+, K+, Mg2+, Mn2+, Ca2+, and Fe2+ is due to the significant effect on the LCST of the hyperbranched copolymer that Zn2+ caused, even at concentrations less than \(1\times 10^{-5}\ M\). HPEAM-TPEAH also showed greater than 95% cell viability in HeLa cells within 24 h of incubation time for concentration range of \(1.0\times 10^{-6}\ M\) to\(5.0\times 10^{-5}\ M\). Another form of hyperbranched polymers is dendritic polymers synthesized by Gao and co-worker in 2013 for the investigation into the cage effect imposed by these polymers on AIE pendent groups, affording the rarely observed solid-state-emissive blue light for such dendrimers.[130]