5. AIE polymers via other RDRP methods

Of the many types of RDRP, Cu(0)-RDRP and NMP were the other methods 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.[168] In 2006, Percec, Sahoo and co-workers termed this unique polymerization technique as Single-Electron Transfer Living Radical Polymerization (SET-LRP),[169] which helped differentiated it from the standard ATRP technique.
NMP was first reported and patented by Solomon, Rizzardo and Cacioli of CSIRO Australia in 1986.[35] Similar to RAFT and ATRP, NMP is a technique that bears resemblance to an ideal living polymerization: (1) the ability to control desired polymer product molecular weight with low dispersity (Ð ) values, (2) has realistic industrial potential with simple implementation steps that often requires only a single unimolecular initiator to produce the desired product, and (3) no need for use of transition metal catalysts. NMP uses alkoxyamine initiators which can undergo homolysis of the C-O bond to yield a stable nitroxide radical that is characteristic of a persistent radical, leading to favoured generation of one product over all others.[170-173] Given the benefits Cu(0)-RDRP and NMP can offer, they are considered suitable methods for synthesizing AIE polymers.
In 2021, Haddleton, Zhang and co-workers synthesized cationic glycopolymers structurally similar to poly(ionic liquids) (PILs) using Cu(0)-RDRP technique.[174] 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:H2O 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 adsorbs 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, detection by fluorescence emission of bacteria became 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, Hudson and co-workers in 2018, prepared three different acrylic monomers using organic semiconductors as motifs, and employed them as electron-transporting (n-type) materials.[175] 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 benzimidazole-containing monomers due to the large induction period prior to onset of polymerization attributed to slow coordination of benzimidazole groups to CuBr2. 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 andN,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 broaderÐ , 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),[176] organic photovoltaics (OPVs),[177] organic thin-film transistors (OTFTs),[178] organic electrochemical transistors (OECTs) and organic thermoelectric (OTE) generators.[179]
AIE molecules can also include transition metals such as iridium (Ir) which allows tuning of the color of fluorescence emitted. In 2019, Hudson and co-workers 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 fibres with similar structures to nanoscale RGB pixels (Figure 9A ).[180]The different blocks were then combined to give di- and tri-block luminescent BBCPs, which displays 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) were 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.
An initiator containing a norbornene moiety and a carbazole-based acrylic monomer (CzBA) were used to copolymerize with 8 wt% of different luminescent material, yielding materials with tunable colors via Cu(0)-RDRP. In this study, acrylic-based monomers containing Ir(piq)2(acac), Ir(ppy)2(acac), (piq = 2-phenylisoquinoline, ppy = 2-phenylpyridine) (IrPIQ and IrPPY), and donor-acceptor type monomer 4-(5-([1,1′-biphenyl]-4-yl)-1,3,4-oxadiazol-2-yl)-N,N -di-p -tolylaniline (tBuODA) were employed to produce polymers that emit red, green, and blue light respectively. Grafting through approach was used to give tBuODA75-b -IrPPY75-BB, tBuODA60-b -IrPIQ90-BB, and IrPPY100-b -IrPIQ50-BB diblock copolymers in a way that emits colors intermediate to both constituent homopolymers. Well-defined two-color interface was observed upon combining the chromophores with a BBCP controllable solvent polarity. As the water fraction increases from 0 to 98% in the solution of tBuODA75-b -IrPPY75-BB, tBuODA60-b -IrPIQ90-BB, phosphorescence was observed to increase significantly for the red and green iridium fluorophores, while blue fluorescence decreases gradually. Due to significant spectral overlap between tBuODA and IrPPY emission profiles, energy transfer efficiency for tBuODA75-b -IrPPY75-BB could not be accurately determined. In addition, BBCPs can be used to prepare nanofibers with multiple compartments. A linear “pentablock” nanofiber mimicking a white OLED design with discrete RGB emissions was prepared, possessing unique characteristics such as high energy efficiency and diffuse lighting for use as a potential next-generation solid state lighting.
In addition, Cu(0)-RDRP can be employed to synthesize polymers capable of behaving as drug carriers as exemplified by Jia, Tang and co-workers of which they synthesized a brush-like polymer with AIE features for drug delivery and intracellular drug tracking.[181] 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 TPE bromoisobutyrate (TPEBIB) and anticancer drug doxorubicin (DOX),[182] it could potentially lead to a compound capable of real-time monitoring of cell targeting, drug release and cancer cell viability. TPEBIB was synthesized and used as an initiator in the copolymerization of poly(ethylene glycol) acrylate (PEGA) and hydrazine (Hyd) monomers via Cu(0)-RDRP, and subsequently conjugating DOX to the centre carrier through the hydrazone bonds to form the complex carrier TPE-PEGA-Hyd-DOX smart prodrug containing approximately 11 wt% DOX.[181] DOX is released in a controlled manner when exposed to cancer cells due to hydrazine bond cleavage in acidic conditions due to improved cellular uptake levels. 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 hours under normal cell conditions compared with 40% after 24 hours under cancer cell conditions helps confirmed 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 Wang, Yang and co-workers for the study of multi-arm star polymers with TPE-functionalized core,[183] and the study of through-space charge-transfer thermally activated delayed fluorescence (TSCT-TADF) phenomenon using AIE-functionalized monomers.[184-186]
NMP can also be used for the synthesis of AIE polymers with a unique morphology as demonstrated by Nicolas and co-workers in 2017, whom employed carbodiimide chemistry to link 4-(N -methylpiperazine)-1,8-naphthalimide-based AIE dye,[187] with 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-(N -methylpiperazine)-1,8-naphthalimide-polyisoprene (Napht-PI) and subsequently, co-nanoprecipitated with cladribine-diglycolate-polyisoprene (CdA-digly-PI) to form Napht-PI CdA-digly-PI prodrug nanoparticles (Figure 9B ).[188] 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 incubation time. Confocal laser scanning spectroscopy (CLSM) on Napht-PI CdA-digly-PI prodrug nanoparticles were carried out though incubation with A549 human lung carcinoma cells for intracellular imaging. The low cytotoxicity of these nanoparticles combined with the sharp fluorescence signal from the AIE-active 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 Nicolas’ work,[188] were used previously to conjugate with different chemical species due to their versatile chemical structures.[189] These dyes exhibit AIE properties due to a twisted intramolecular charge transfer (TICT) process originating from RIM.
The use NMP in AIE polymer synthesis was also demonstrated by Qiao, Pang and co-workers in 2021 where TPE-functionalized 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (NH2-TEMPO) and 3-(((2-cyanopropan-2-yl)oxy)(cyclohexyl)amino)-2,2-dimethyl-3-phenylpropanenitrile (Dispolreg 007) were used to study reaction kinetics in homogenous and heterogenous polymerization systems respectively.[37]