1. Introduction

1.1 AIE and its incorporation into polymers

Fluorescent molecules have shown their great potential in a diverse range of applications since their first discovery, such as for digital technologies, fluorescence molecular tagging, staining biological cells, and probes for detecting environmental variations.[1-5] Conventionally, fluorescent molecules emit strongly in the molecular or solution state, but experience appreciable effects of photoluminescence (PL) quenching in the aggregated state, which is a well-known concept discovered and termed by Förster as Aggregation-Caused Quenching (ACQ) in 1954.[6, 7]
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. They coined this phenomenon as Aggregation-Induced Emission (AIE).[8-12] 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). Researchers have attempted to rationalize such observations and among the many proposed explanations, the restriction of intramolecular motion (RIM) theory took the throne.[13-15] This mechanism comprises two parts: restriction of intramolecular rotation (RIR) (Figure 1B ) and restriction of intramolecular vibration (RIV) (Figure 1C ). 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 inFigure 2A , a classic ACQ luminophoreN,N -dicyclohexyl-1,7-dibromo-3,4,9,10-perylenetetracarboxylic 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] Contrary 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 shows 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, Tang et. al . reported the world’s first AIE-active 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, provides more benefits than 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 light-emitting 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 bio-imaging agents for L929 cells.[23] Some reviews have already explored the structure, design, reaction pathways, and applications of AIE polymers,[16, 18, 24-26] 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 multi-component polymerization.[30, 32] However, the AIE polymers that were synthesized till date with pre-determined molecular weights, low dispersity 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 are of great importance in certain applications such as theranostics, FPNs, and environmental variation detection. Therefore, this review is dedicated to highlight and summarize 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.

1.2 RDRP

Since the early 1980s, researchers from around the world have realised that the addition of certain chemical compounds into a polymerization mixture allows reversible reaction with chain carrier molecules.[33] 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’.[34] 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).[33, 34] 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 multi-functionalities, which otherwise will not be possible by conventional methods.[33, 34]
RDRP polymerization techniques include, Nitroxide-Mediated Polymerization (NMP),[35-37] Atom Transfer Radical Polymerization (ATRP),[38-45] Reversible Addition-Fragmentation Chain Transfer (RAFT),[46-49] Iodine-Transfer Polymerization,[50] Reverse Iodine-Transfer Polymerization (RITP),[51] Reversible Chain Transfer Catalyzed Polymerization (RTCP),[52]Reversible Complexation Mediated Polymerization (RCMP),[53] Organotellurium-Mediated Radical Polymerization (TERP),[54] Cobalt Mediated Radical Polymerization and Catalytic Chain Transfer (CCT),[55, 56] Iniferter Polymerization,[57, 58] Selenium-Centred Radical-Mediated Polymerization,[59] and Organostibine-Mediated Radical Polymerization (SBRP).[60] Even though a range of different RDRP technique have been developed over the past decades, the most popular 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: (1) the ability to synthesize polymer chains with predetermined molar masses and narrow molar mass distribution (dispersity, Ð ); (2) the ability to continue polymerization by adding more monomers owing to the better stability of the dormant propagating chain; (3) high-chain end fidelity and ease of attaching functional groups to polymer chain ends; (4) lower probability of side reactions such as termination reactions occurring; and (5) ease of fabricating various polymer shapes and morphologies.[61] 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 co-polymers spanning a huge range of topological morphologies such as homopolymer, di-/tri-/multi-block, star-shaped, sequence-defined, (hyper)branched, dendritic, graft and brush type, cyclic (ring), network, single-chain nanoparticles (NPs),[62] bearing unique properties such as stimuli responsiveness to mechanical stress,[63, 64] temperature and pH changes,[65, 66] and light irradiation.[67] Such polymers found potential applications in the field of therapeutics such as nanomedicine, nanotechnology and materials science,[68-73] energy production and efficiency optimisation, and electronics.[74-76] 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.