Light-Mediated Polymerization Induced by Semiconducting Nanomaterials: State-of-the-Art and Future Perspectives

Abstract

Direct capture of solar energy for chemical transformation via photocatalysis proves to be a cost-effective and energy-saving approach to construct organic compounds. With the recent growth in photosynthesis, photopolymerization has been established as a robust strategy for the production of specialty polymers with complex structures, precise molecular weight, and narrow dispersity. A key challenge in photopolymerization is the scarcity of effective photomediators (photoinitiators, photocatalysts, etc.) that can provide polymerization with high yield and well-defined polymer products. Current efforts on developing photomediators have mainly focused on organic dyes and metal complexes. On the other hand, nanomaterials (NMs), particularly semiconducting nanomaterials (SNMs), are suitable candidates for photochemical reactions due to their unique optical and electrical properties, such as high absorption coefficients, large charge diffusion lengths, and broad absorption spectra. This review provides a comprehensive insight into SNMs’ photomediated polymerizations and highlights the roles SNMs play in photopolymerizations, types of polymerizations, applications in producing advanced materials, and the future directions.

1. Introduction

Harvesting light as a trigger for polymerizations provides unprecedented advantages including abundant solar energy sources, low energy consumption, and mild reaction conditions and therefore represents advanced techniques for macromolecular synthesis.^1,2 The earliest examples of photopolymerization mainly focused on nonliving radical and cationic polymerizations,^3,4 while nonliving polymerizations usually suffer from low initiation efficiency and broad molecular weight (MW) distribution. In contrast, recent progress in controlled/living polymerization techniques, especially controlled radical polymerization (CRP), has revolutionized the synthesis of macromolecules by offering polymers with complex architectures, tunable molecular weights, and narrow MW distributions.⁵⁻⁷ The integration of photomediated methodology with controlled polymerizations provides additional advantages such as high chain end fidelity and temporal/spatial control over polymerization.^2,8−13 The polymerization process could be simply controlled by switching the light “on” and “off”. Indeed, the light-mediated process has been successfully incorporated into various polymerization systems including atom transfer radical polymerization (ATRP),¹⁴⁻¹⁸ cationic polymerizations,¹⁹⁻²¹ nitroxide-mediated radical polymerization (NMP),²² ring-opening polymerization,^13,23 and reversible addition–fragmentation chain-transfer (RAFT) polymerization.²⁴⁻³⁰

One of the key components in photopolymerization systems is the photoresponsive species, which we named photomediators. Photomediators can be mainly divided into two categories according to their functions in the reaction, which are photoinitiators (PIs) and photocatalysts (PCs).² PIs are usually responsible for initiating polymerization with little control over the propagation and termination process and usually result in nonliving polymerizations. On the other hand, PCs could mediate the activating/deactivating process in the living polymerization via photocatalytic cycles and thus have emerged as a powerful tool to synthesize polymers with high precision of chemical composition and to manufacture complex polymeric materials.^8,31−34 Due to the recent explosion of interest in photoinduced polymerizations, a large number of photomediators, including organic small molecules^{8,15,16,20,21,27,30,35−37} and metal complexes,^17,38−41 have been demonstrated to establish efficient photopolymerization systems and have revolutionized light-mediated polymerization by exploring oxygen tolerance,⁴² lowering the catalyst loading,⁴³ expanding the scope of monomers,⁴⁴⁻⁴⁶ and others.⁴⁷⁻⁴⁹ Therefore, the development of easy-to-produce photomediators that could afford polymer products with complex architectures and targeted MW is of general interest.

In the past 30 years, the development of semiconducting nanomaterials (SNMs) has demonstrated numerous breakthroughs in many fields such as bioimaging,⁵⁰ light-emitting diodes,⁵¹ and solar cells.⁵² With a typical size ranging from 1 to 100 nm, the shape of SNMs can also vary from one dimension to three dimensions. Starting from inexpensive raw materials, SNMs can easily be synthesized and purified with different surface chemistry, sizes, crystal structures, and compositions, exhibiting tunable band gaps and energy levels.⁵³ Moreover, SNMs also have large extinction coefficients (>10⁵ M^–1 cm^–1),⁵³ broad absorption spectra,⁵⁴ and multiple molecular binding points.^55,56 These characteristics make SNMs a promising class of photomediators. Indeed, SNMs have been reported as robust PIs/PCs for CO₂ reduction,⁵⁷ water splitting,⁵⁸ and small-molecule transformation.^59,60 Although SNMs appear to be an excellent alternative to metal complexes and organic PCs, little attention has been paid to the SNM-catalyzed photopolymerization.⁶¹ Furthermore, because nanotechnology and polymerization science are two different research areas, a broad scope of knowledge and experiment skills are required for conducting the study in SNM-induced photopolymerization, which also restricts its development.

Currently, although there are a number of excellent reviews on photopolymerization,^{2,4,8,9,11,37,62−64} a few specific reviews focus on SNM-induced photopolymerization.^61,65,66 Therefore, the aim of this review is to highlight the state-of-the-art progress of using SNMs in regulating photopolymerizations, especially radical polymerizations, summarize its applications in controlled polymerization, and provide some future perspectives. We also included some polymerization examples of recently developed photoresponsive NMs such as noble metal nanoparticles (NPs). We elucidated the roles of SNMs in photopolymerization by focusing on mechanistic aspects, classified the types of SNMs and photopolymerization methodologies that have been reported in this field, and demonstrated their applications in the preparation of advanced polymeric materials.

2. Role of SNMs in the Reaction

As we discussed in the Introduction, photomediators are divided into two categories including PIs and PCs in the past reviews of photopolymerization.² In this review, we further classify the function of SNMs in photoinduced polymerizations into the following three categories: PIs, photoactivators/photosensitizers, and PCs, based on the polymerization mechanisms.

2.1. SNMs as Photoinitiators

When the excited-state SNMs directly activate the monomer and initiate the polymerization process without the assistance of other co-initiation species, we can define the SNMs herein as PIs. Notably, the definition of photoinitiators in this review is slightly different from the previous report,² as we do not include the SNMs which could photoinitiate polymerization in the presence of co-initiators. We argue that the polymerization in the presence of co-initiators is actually initiated by radical species from the co-initiators instead of SNMs, and the role of SNMs in the polymerization system is actually photoactivating/sensitizing the co-initiators and thus should be considered as photoactivators, which will be thoroughly demonstrated in section 2.2. The currently proposed initiation mechanism using SNMs as PIs mainly involves three possible pathways upon photoexcitation (Scheme 1): (A) direct reduction of a monomer via electron transfer from SNMs’ conduction band (CB);⁶⁷ (B) direct oxidation of a monomer via hole transfer from SNMs’ valence band (VB);^68,69 and (C) radical generation on the SNMs’ surface to initiate the monomers.⁷⁰ Mechanisms (A) and (B) require the SNMs with suitable band gap energy to ensure the charge transfer is thermodynamically favorable. For example, the reduction of methyl methacrylate requires a redox potential of <−2.1 V vs SCE, whereas oxidation could occur with a valence band of SNMs more positive than +2.0 V vs SCE.⁷⁰ Mechanism (C) occurs via a charge transfer from the anion-localized surface of SNMs to the monomer, which requires the coordination between SNMs’ surface and monomers.⁷¹ Generally, most polymerizations conducted by direct activation of the monomers using SNMs are relatively inefficient, with monomer conversion usually less than 20%.^67,68,70

Scheme 1. SNMs as Photoinitiators in Photopolymerizations: (a) Reduction of Monomer via Electron Transfer from Conduction Band, (b) Oxidation of Monomer via Hole Transfer from Valence Band, and (c) Radical Generation on the SNMs’ Surface (CB Is the Conduction Band and VB Is the Valence Band).

Apart from SNMs, noble metal NPs, such as gold (Au) and silver (Ag),^72,73 can also directly activate monomers via surface plasmon resonance (SPR)-induced hot electron transfer. Upon irradiation, a high concentration of energetic electrons whose energy levels are above Fermi level and strong electric fields are formed on the surface of the metal NPs.⁷⁴ These energetic electrons can then be transferred and activate the monomers close to the surface and induce photopolymerization.^75,76 Limitations of this method include the following: (1) the polymerization is restricted to the surface, thus hindering scale-up reactions, and (2) direct excitation of hot electrons usually requires large incident light intensities.⁷⁴

2.2. SNMs as Photoactivators

When the polymerization system consists of multiple initiating components (tertiary amines, RAFT agents, etc.) besides SNMs, photoexcited SNMs are used to transfer charges to those species, which could further initiate the polymerizations. We can thus define SNMs in this polymerization system as photoactivators or photosensitizers. Scheme 2a illustrates the general mechanism of SNM-photoinduced polymerization by sensitizing co-initiators. Based on their redox potentials, co-initiators can accept either excited electrons or holes from SNMs to provide radical sources for initiating monomers, followed by a free-radical chain-growth polymerization. Typical co-initiators include tertiary amines,^70,77 iodonium salts,⁷⁸⁻⁸⁰ and alcohols,^79,81−86 which are molecules that can be easily oxidized or reduced to generate radical sources. On the other hand, redox mediators, such as RAFT agents⁸⁷ and the Cu(II) complex,⁸⁸ are usually compounds used for regulating controlled polymerizations under thermal or photoinitiation.⁸⁹ Particularly for photopolymerization, RAFT agents could be initiated via a photointerfier process, reacting with active radicals originating from photoinitiation or directly being reduced by PCs.⁹⁰ Similarly, the copper catalyst could be reduced to the low oxidation state by the excited photosensitizer,⁹¹ which could further react with alkyl bromide initiators. For example, Scheme 2b presents a typical mechanism of photo-ATRP in the presence of SNMs and CuBr₂. Generally, the excited-state SNM could reduce Cu(II) to the active species Cu(I), which could react with alkyl halides and initiate the polymerization (Scheme 2b). In contrast to the uncontrolled propagation process induced by co-initiators, precise mediation of reversible activation/deactivation process could be achieved using redox mediators, operating polymerization in a controlled fashion and resulting in polymers with narrow molecular weight distribution (around 1.20).⁸⁸ However, since SNMs do not directly participate in the activation–deactivation cycle, residual catalysts may lead to the loss in temporal control.⁹²

Scheme 2. SNMs Serve as Photoactivators in the Presence of (a) Co-initiators for Conventional Free Radical Photopolymerization and (b) Redox-Mediator-Controlled Photopolymerization.

2.3. SNMs Serve as Photocatalysts

Photocatalysts are capable of donating or accepting electrons to/from other substrates under light irradiation, and they can also be relaxed to the ground state by accepting back charge transfer from the reaction systems in order to form a catalytic cycle.⁸ The typical mechanism of photocatalyzed polymerization is shown in Scheme 3. Starting with photoexcitation, SNMs directly reduce the initiators or polymer chain ends via photoinduced electron transfer, forming active radicals. The radicals can either propagate with monomers or transfer electrons back to SNMs to complete the catalytic cycle. The charge transfer is restricted between SNMs and initiators/polymer chain ends, and therefore, SNMs can directly participate in mediating the reversible deactivation equilibrium. This newly established methodology⁹³ aims to conduct photocontrolled/living radical polymerizations, as the photocatalytic cycle induces reversible deactivation equilibrium,^1,17 providing well-defined polymeric structures and excellent temporal control over polymerization. Notably, sacrificial agents (electron/hole donors) could also be used to help with the regeneration of catalysts.²⁸

Scheme 3. SNMs Serve as Photocatalysts in Photopolymerizations.

3. SNMs Used in Photopolymerization

We highlighted in this section representative classes of SNMs which have been explored for photopolymerization. We also discuss the main characteristics of different materials including absorption wavelength, extinction coefficient, typical size, band gap energy, photoluminescence (PL) lifetime, and energy levels of the CB/VB. Some important examples are summarized in Table 1.

Table 1. Examples of Optical and Electronic Properties of SNMs.

category	examples	absorption onset (nm)	band gap (eV)	emission (nm)	PL lifetime (ns)	CB (eV)	VB (eV)
metal oxide	ZnO94	380	3.2	a	a	–4.30	–7.50
	Bi2O394,95	440	2.8	400–44096	0.9497	–4.60	–7.40
metal chalcogenide	CdSe (3.3 nm)93	565	2.3	575	16–2298	–3.21	–5.55
	CdS (3.7 nm)99	420	2.95	550	15–20100	–3.80	–6.80
perovskite nanocrystal	CsPbX3101 (X = Cl,Br, I)	420–665	1.99–2.97	425–680	1–29102	–3.26 to −3.45	–6.24 to −5.44
metal-free SNM	carbon QDs103−105	380–420	3.1–3.5	510–560b	2.81–48.7	∼2.0	∼−5.2

^aEmission varies with different luminescence mechanisms; please see the reviews for further detail.^106,107

^bExcitation wavelength of 458 nm.

3.1. Metal Oxides

Metal oxide SNMs including titanium dioxide (TiO₂), zinc oxide (ZnO), iron oxide (Fe₂O₃), and tungsten oxide (WO₃) have shown excellent photocatalytic performance in water splitting and carbon dioxide reduction due to their excellent photosensitivity and stability.⁶¹ However, as metal oxides are usually wide-band-gap semiconductors,⁶¹ the absorption of metal oxide SNMs is limited mainly in the UV region. For example, the band gaps of TiO₂ and ZnO are both 3.2 eV.^108,109 However, UV light does not represent a significant part of the solar spectrum and usually leads to undesired side reactions such as monomer self-initiation.¹² On the other hand, Bi₂O₃ has been demonstrated to be an efficient PC for controlled polymerization under visible light^95,110 due to the small band gap around 2.8 eV.⁹⁴ Moreover, Bi₂O₃ also offers additional advantages such as inexpensive raw materials, high stability, and low toxicity.⁹⁴

3.2. Metal Chalcogenides

Metal chalcogenides are composed of at least one chalcogen anion and at least one electropositive metal element. Changing the chemical components of metal elements renders the band gap energies spanning from the near-infrared (PbS, PbSe, etc.) to the ultraviolet (ZnS and ZnSe, etc.) regions. Particularly, metal chalcogenide quantum dots (QDs), solution-dispersible nanocrystals, represent an emerging class of PCs for photoinduced organic synthesis.^99,111−115 The size of metal chalcogenide QDs could be controlled within 1–10 nm with a narrow distribution, providing large surface area to volume ratios for chemical transformation to happen.¹¹⁶ High extinction coefficients (around 10⁶ M^–1 cm^–1)¹¹⁷ of the materials facilitate the reactions happening at extremely low catalyst loadings (usually less than 10^–3 mol %).¹¹¹ Moreover, due to the quantum confinements, the reductive potential of metal chalcogenide QDs can be easily tuned in the range of 2 eV via changing the size of QDs,¹¹⁸ enabling efficient photoinduced electron transfer to organic compounds.^119,120 For instance, the reduction potential of CdSe QDs can be tuned to −1.59 V vs SCE¹¹¹ with a diameter of 3.3 nm, which is sufficient to reduce the commonly employed ATRP initiators such as alkyl bromide which has an oxidation potential around 0.8 V vs SCE¹⁵).

3.3. Hybrid Semiconductor–Metal Nanoparticles

Hybrid semiconductor–metal NPs (HSMNPs) usually combine two or more disparate materials into a single semiconducting nanosystem, conjugated with at least a metal island (Au, Ag, etc.) growing on a semiconductor NP.¹²¹ The synthetic control of HSMNPs offers a high degree of control over the shapes, sizes, compositions, and morphologies of HSMNPs,^121,122 which allows the easy control of the semiconductor band gap, adjustment of the relative energetics level to optimize the charge separation, and alternation of the dispersibility of HSMNPs in diverse matrices.¹²³ Another advantage of HSMNPs as a photomediator is that the presence of the metal domain highly enhances efficient spatial charge separation, thus reducing charge recombination and significantly enhancing the photocatalytic ability of HSMNPs.¹²⁴ For example, cadmium sulfide hybrid nanorods with gold tips (CdS-Au) (5.1 ± 0.6 nm diameter of the nanorods and a total length of 30 ± 3 nm) have a first excitonic peak at 460 nm, with an extremely large molar extinction coefficient at 385 nm (10⁷ M^–1 cm^–1).⁸⁵ The metal domain was aimed to enhance the photoinduced charge separation by accepting electrons from nanorods and therefore preventing the charge recombination.¹²⁴ The enhanced electron–hole charge separation in HSMNPs facilitates effective photoinduced charge transfer, which leads to a faster polymerization rate and higher monomer conversion than that in bare NP counterparts.¹²⁴

3.4. Perovskite Nanocrystals

Perovskite semiconducting materials have revolutionized the fabrication of advanced materials such as photovoltaics¹²⁵ and light-emitting diodes¹²⁶ due to their excellent optoelectronic properties including narrow emission band (full width at half-maximum <20 nm),¹²⁷ high PL quantum yields (up to 90%),^128,129 large extinction coefficients (around 10⁶ M^–1 cm^–1),¹³⁰ and long charge diffusion lengths (>175 μm).¹³¹ These outstanding optoelectronic properties also make perovskite SNMs good candidates for PCs.^132,133 For example, the CB of lead halide (Cl, Br, I) nanocrystals (NCs) ranges from −1.24 to −1.54 V vs SCE and VB ranges from 0.71 to 1.47 V vs SCE,¹⁰¹ making the charge transfer between NCs and most organic molecules thermodynamically favorable. Moreoever, the PL lifetime of CsPbBr₃ NCs ranges from 10 to 100 ns in various solvents,¹⁰² which are sufficient for photoinduced charge transfer.¹³⁴ Currently, examples of perovskite photoinduced polymerization are very rare,^54,135−137 which might be caused by their instability toward water and air.^138−140

3.5. Metal-Free SNMs

Developing metal-free catalysts for chemical transformations has attracted numerous attention in the past few years because of their low toxicity and cost efficiency.^{10,37,141−143} Metal-free SNMs are mainly carbon-based materials with excellent photostability, low toxicity, and sustainable raw materials.¹⁰³ Metal-free SNMs such as carbon QDs,^103,144 nanostructured conducting polymers,¹⁴⁵ polymer nanofibers,¹⁴⁶ and microsized carbon nitride particles¹⁴⁷ have been developed for both controlled and free-radical polymerizations. Metal-free SNMs also process fascinating optical and electrical properties. For example, carbon dots (2–10 nm) exhibit an emission centered at 510 nm with a quantum yield of 8.82% upon irradiation at 458 nm.¹⁰³ Relatively strong absorption is usually observed in the UV region due to the n−π* (C=O) transition in the core and on the surface of carbon QDs.¹⁴⁸ The fluorescence lifetime of carbon dots usually ranges from 1 to 10 ns,¹⁰³ which is sufficient for an efficient charge transfer to happen.¹⁴⁹ Furthermore, doping with heteroatoms such as phosphorus and sulfur could promote the photogeneration of electrons,¹⁰⁴ which is beneficial to the photocatalytic performance of carbon dots.

4. Type of Polymerizations Photoinduced by SNMs

4.1. Nonliving/Conventional Free-Radical Polymerization

In this section, we introduce the free-radical polymerizations photoinduced by SNMs based on their charge transfer process and mechanisms.

4.1.1. Free-Radical Polymerization Directly Initiated by Photoinduced Charge Transfer from SNMs to Monomers

The earliest example of semiconducting material initiated free-radical polymerization can be traced back to the 1960s. Oster and Markham separately reported a photopolymerization initiated by ZnO under UV (λ = 365 nm) irradiation.^150,151 Methyl methacrylate (MMA) was successfully polymerized in the presence of oxygen. Authors proposed that light could produce an excited-state oxygen and form a radical anion on the surface of ZnO, which could initiate the monomer and propagate with a free-radical chain mechanism. The anion-initiated mechanism was further confirmed by electron paramagnetic resonance (EPR) studies. This hypothesis was also supported by the fact that polymerization could proceed with electrophilic monomers rather than with styrene.¹⁵¹ In 1985, Kamat reported an n-type GaAs electrode photopolymerization of 1-vinylpyrene under visible light.¹⁵² Due to the small band gap of GaAs electrodes (∼1.4 eV), polymerization could be operated under visible light. The authors claimed that a hole could migrate from excited GaAs electrodes to monomer 1-vinylpyrene, forming an oxidized 1-vinylpyrene which could subsequently initiate the radical polymerization at the electrode surface.

Two of the disadvantages of the aforementioned bulk semiconductor materials are their small surface area and strong light scattering, which hinder the light absorption and consequentially limit the polymerization efficiency. Therefore, quantum-sized SNMs, which have a surface area to volume ratio higher than that of the bulk materials, were developed for photopolymerization to overcome these limitations. In 1992, Hoffmann and co-workers reported a radical polymerization of vinylic monomers photoinitiated by quantum-sized NPs including TiO₂, ZnO, and CdS in alkyl alcohol solvents upon the UV irradiation.^68,153 Significantly enhanced monomer conversion was observed for quantum-sized NPs (19%)¹⁵³ compared to that in their bulk counterparts (9%).¹⁵⁰ An anionic initiation mechanism followed by a free-radical propagation step was proposed (Figure 1a). The authors suggested that monomers could be reduced by photoexcited electrons at the CB on the surface of NPs, forming a radical anion which further initiated polymerization and propagated the monomers. This hypothesis was further confirmed by the fact that negligible polymerization was observed in the presence of oxygen or acetonitrile, which are known to interfere with radical anions. It is worthwhile to mention that the direct electron transfer from ZnO to monomers is not feasible because the CB of ZnO (−0.44 V vs SCE) is less negative than the reduction potential of monomers (−2.1 V vs SCE). The authors proposed that methyl methacrylates could be reduced by the electrons stored at the surface traps, and the surface traps possessed a very negative redox potential (<−2.24 V vs SCE). Further kinetics study by Mills on polymerization photoinitiated by annealed ZnO NPs also pointed out that reactions could preferentially take place at the NP surface.¹⁵⁴

Figure 1.

(a) Proposed mechanism of CdS NP photoinitiated polymerization via directly electron transfer from NPs to monomers. (b) Proposed mechanism of NP photoinitiated polymerization assisted by dyes under visible light. (c) DTG curves of PMMA prepared using CdS photoinitiated polymerization (left) and AIBN (right). Adapted with permission from ref (156). Copyright 1997 Springer Nature.

By introducing organic dyes as photosensitizers for SNMs, the chemisorbed or physisorbed dyes on the SNMs’ surface could prolong the electron–hole separation¹⁵⁵ and thus significantly enhance electron transfer efficiency to the monomers. For example, eosin was reported to photosensitize CdS NPs under visible light.¹⁵⁵ Photoexcited electrons could be transferred from the eosin dye to the CB of semiconductors and subsequently relocate to the monomer for initiation polymerization (Figure 1b). A higher polymer yield was observed in the presence of eosin and CdS NPs (22%) compared to that in the CdS NPs (2%) alone, which was attributed to more free radicals generated by the dye-sensitized system.

Apart from anionic initiation induced by the electron transfer from excited-state SNMs to monomers, Popovic demonstrated a different initiation mechanism involving hole transfer from the VB to monomers.¹⁵⁶ Instead of reacting with the carbon double bonds on monomer molecules, the photoexcited CdS could oxidize MMA on the β-position through the hole transfer, abstracting a hydrogen atom and forming a stable acrylic radical which acted as an initiator and propagated monomers in a free-radical manner. This mechanism was supported by differential thermogravimetry (DTG) results (Figure 1c), where an additional peak and stronger thermal stability were observed in the poly(methyl methacrylate) (PMMA) samples obtained from CdS photoinitiated polymerization (Figure 1c, left) compared to sample thermal initiated by 2,2′-azobisisobutyronitrile (AIBN) (Figure 1c, right). The additional peak was attributed to the polymer with unsaturated chain ends, suggesting that polymerization might be initiated by acrylic radicals. A similar VB–hole initiation mechanism was reported by Ni and co-workers,¹⁵⁷ where polymerization of methyl methacrylate photoinitiated by nanosized TiO₂ semiconductors was established. Furthermore, because polymerization was conducted in the presence of oxygen, the authors also claimed that hydroxyl radical (OH^•) generated from the reaction between oxygen and electron–hole pairs could act as an initiation species.

4.1.2. Free-Radical Polymerizations Photoinduced by SNMs in the Presence of Co-initiators

One of the challenges for directly reducing monomers is that the reduction potential of acrylate-type monomers is very negative (around −2.1 V vs SCE),^70,153 which requires strongly reductive excited states of the SNMs and makes the reduction of monomers thermodynamically unfavorable. Therefore, co-initiators are introduced into polymerization systems to promote the generation of radical species and consequently enhance the initiator efficiency. Furthermore, co-initiators frequently act as electron donors/acceptors, which could suppress the charge recombination and improve the charge separation.

Alcohols are usually considered as the hole scavengers because of their small and negative redox potential (around −0.24 V vs NHE⁸³). Indeed, many examples have employed alcohols as co-initiators to generate OH^• for initiating chain propagation.^79,81−86 Kuchmii showcased a photopolymerization of butyl methacrylate (BMA) facilitated by hydrated ferric oxide (Fe₂O₃) NPs in the presence of 2-propanol.⁸³ Upon either UV or visible light irradiation, 2-propanol could donate an electron to the VB of excited Fe₂O₃ NPs (1.6 V vs NHE), generating active isopropoxy radicals capable of initiating a chain polymerization of BMA (Figure 2a). The fate of the excited electron was not probed in the study, but the authors suggested that it might be accepted by the proton formed during the oxidation of isopropyl alcohol. Moreover, OH^• could also be yielded from photolysis of semiconductor NPs in water. In 2014, Yagci and co-workers reported a UV-light-induced polymerization of acrylamides initiated by both ZnO and iron-doped ZnO NPs in the aqueous environment.⁷⁹ A photoinduced electron–hole pair from NP-reduced water and molecular oxygen resulted in active OH^• and thus initiated polymerization (Figure 2b). This mechanism is further evidenced by capturing OH^• using an excess of 1,1-diphenylethylene in a nonpolymerizable model reaction (Figure 2c).

Figure 2.

(a) Energy diagram and proposed charge transfer processes of the Fe₂O₃ photoinduced polymerization of BMA in the presence of 2-propanol. Adapted with permission from ref (83). Copyright 2001 Springer Nature. (b) Photopolymerization using ZnO NPs in aqueous phase. (c) Capturing OH^• in a nonpolymerizable model reaction. Panels (b) and (c) are adapted with permission from ref (79). Copyright 2014 John Wiley and Sons.

Although OH^• could serve as an efficient co-initiator, performing polymerization in the typical OH^• sources such as alcohols or water limits the scope of monomers polymerized via the methodology. Tertiary amines (TEAs), on the other hand, are the well-known sacrificial additives in photoinduced organic transformation.^38,93,158 TEA could form a radical cation by donating the lone pair electron on the N atom to the photogenerated hole, prevent back electron transfer from the CB to the VB, and meanwhile render the polymerization process in the organic medium. In 2012, Yagci reported a polymerization of MMA photoinitiated by mesoporous g-C₃N₄ (mpg-C₃N₄) in conjunction with tertiary amines.⁷⁷ Silica NPs are used as templates for C₃N₄, providing large surface areas as well as accessible surface sites for the photocatalysis process. The metal-free colloidal structure of mpg-C₃N₄ not only overcame the toxicity problem of metal oxide catalysts but also could be recovered and reused without losing catalytic performance. Amine was proposed to be oxidized by the photoexcited hole, form a radical cation, and subsequently abstract hydrogen from another amine to yield the initiating radical (Figure 3a). Another report by Stucky employed various amines (Figure 3b) in CdS semiconductor QDs to induce neat polymerization.⁷⁰ Amines with different structure parameters including the number and size of substituents and functional groups were examined. Fourier transform infrared spectroscopy (FTIR) transmission of different amine-treated QDs displayed that the native oleic acid ligands on CdS QDs could be replaced by amines (Figure 3c), where primary amines coordinate to semiconductor surfaces while secondary and tertiary amines possibly only remain weakly bound to the surface because of the low affinity and steric effect. Combining with the photoinitiation efficiency assisted by different amines (Figure 3d), the authors hypothesized that the removal of surface ligands increased the accessibility of the QDs’ surface, consequently promoting photocatalytic activity of CdS QDs.^99,116 The highest initiation efficiency exhibited by the triethylamine system probably came from the faster electron-donating nature of triethylamine, and a more open QDs’ surface resulted from the complete displacement of original ligands on the surface.

Figure 3.

(a) Proposed initiation mechanism using mpg-C₃N₄ in the presence of amines. Adapted from ref (77). Copyright 2012 American Chemical Society. (b) Chemical structures of the co-initiators employed CdS QD-photoinduced neat polymerization. (c) FTIR transmission spectra of ligand-exchanged QD films. (d) Conversion of monomer versus time in the presence of different amines. Panels (b–d) are adapted from ref (70). Copyright 2008 American Chemical Society.

Surface ligands play an important role to confer solubility and prevent agglomeration for NPs.¹⁵⁹ Surface ligands such as lauric acid (LA) were found to interact with the photogenerated holes on magnetic iron oxide NPs upon visible light radiation (Figure 4a),¹⁶⁰ followed by decarboxylation and formation of initiating radicals. Decarboxylation of LA was further confirmed by carbon dioxide evolution in a model reaction without the monomer (Figure 4b). Schmitt and co-workers developed a photoinitiated bulk polymerization of multifunctional resins and acrylic esters using nanoscaled ZnO that are surface-functionalized with carboxylic acids.^161−163 The same decarboxylation reaction of carboxylic acids induced by the photoexcited electron–hole pair was proposed and confirmed by EPR and gas chromatography–mass spectrometry measurements. Successful radical photopolymerizations could be operated even under weak simulated illumination (365 nm light intensity ≈0.5 mW cm^–2).¹⁶³ Very recently, Ni’s group demonstrated a free-radical polymerization of vinyl acetate photoinitiated by a binary mixture of TiO₂ NPs and carboxylic diacid.¹⁶⁴ Polymerization in aqueous conditions approached over 90% monomer conversion, which is much higher than that of bulk polymerization. The authors ascribed the superior performance to the high decarboxylation efficiency in water. Moreover, the polymerization rate was much more rapid using diacids with an even number of carbon atoms than odd number of carbon atoms (Figure 4c), as the former one is prone to coordinate with the NPs’ surface in a bidentate pattern and thus accelerate the decarboxylation process. Such a decarboxylation–initiation process was also developed for utilizing butyric acid and TiO₂ NP as the co-initiator and photosensitizer, respectively.¹⁶⁵

Figure 4.

(a) Proposed mechanism for Fe₃O₄ nanoparticles photoinitiated polymerization of vinyl monomers with the LA surface agent. (b) Decarboxylation of LA using sodium carbonate with phenolphthalein in model reactions. Panels (a) and (b) are adapted with permission from ref (160). Copyright 2015 Royal Society of Chemistry. (c) Photopolymerization rate in the presence of different diacids. Adapted with permission from ref (164). Copyright 2017 Springer Nature.

Apart from aforementioned SNMs’ photoinitiated polymerization assisted by hole scavengers, electron acceptors such as onium salts (iodonium, sulfonium, etc.) could also function as the co-initiators to trigger polymerizations.⁷⁸⁻⁸⁰ For example, iodonium salt could accept photoexcited electrons and separate into iodobenzenes and phenyl radicals, which subsequently initiate the chain propagation (Figure 5a). Kawai and co-workers reported a photosensitized polymerization of ionic liquid-based monomers in the presence of CdTe NCs and an onium salt, diphenyliodonium hexafluorophosphate (DPI).⁸⁰ The photoluminescence quenching of CdTe NCs by DPI gave almost a linear Stern–Volmer relationship (Figure 5b,c), revealing a photoinduced electron transfer (PET) process from the excited NCs to co-initiators. Polymerization photosensitized by CdTe was so rapid that full conversion could be achieved within 1 min, which benefited from the large molar extinction coefficient and the long excitation lifetime of CdTe NCs. Notably, the polymerization was preferable to be conducted under visible light in order to avoid direct excitation of onium salts. Apart from SNMs, Au NPs were also developed as the photoinitiator for radical polymerizations of acrylic monomers in the presence of [4-[(octyloxy)phenyl]phenyl]iodonium hexafluoroantimonate as the co-initiator.⁷⁸ Thiol-stabilized Au NPs acted as electron donors upon excitation at the plasmon resonance band (λ > 450 nm), transferring electrons to iodonium salts which then decomposed into free radicals. After the plasmonic resonance electron was donated, the electron-poor Au core could accept another electron from stabilizing the ligand via intramolecular electron transfer.

Figure 5.

(a) Proposed photoinitiation mechanism in the presence of NPs and iodonium salts. (b) PL spectral change of CdTe NCs with the addition of onium salts. (c) Stern–Volmer plot of CdTe NCs emission quenching with onium salts. Adapted from ref (80). Copyright 2007 American Chemical Society.

4.1.3. Free-Radical Polymerizations Using SNMs via Surface-Induced Photoinitiation

Besides the redox process mentioned above, surface-coordination-induced charge transfer is also a facile route to initiate polymerization.¹¹⁶ The strong Lewis acid metal atoms/cations at the surface are prone to coordinate with the double bond of monomers and donate electrons from the anion localized state. For instance, Tan’s group reported recently a perovskite NC-initiated photopolymerization of vinyl monomers under white light illumination via a surface initiation mechanism.¹³⁶ The authors postulated that the olefin of the monomer could coordinate with an electron-deficient lead (Pb) site on the perovskite surface, giving rise to an active radical under irradiation and triggering a chain propagation process. Termination of polymerization could happen through the combination of propagating radicals or chain transfer, and the polymer might still attach to the NCs’ surface after the reaction. High molecular weight polymers of around 200 kDa, dispersity around 2.0, and conversion over 12% were obtained after 14 h illumination. The free-radical chain-growth polymerization mechanism was also evidenced by a total restraint of polymerization in the presence of radical inhibitors.

Banin and co-workers recently developed a CdS–Au hybrid NP-photoinitiated solvent-free polymerization through a surface charge transfer mechanism.⁷¹ Without any additives, inorganic ligand Sn₂S₆^4–-capped hybrid NPs could initiate the photopolymerization of hydroxyethyl acrylates (HEA), achieving almost full conversion within 300 s, showing efficiency higher than that of the bare CdS (Figure 6a). Two possible surface initiation mechanisms were proposed. The authors suggested, in the first pathway, charge transfer between excited HNPs and monomers could happen via surface coating/ligand mediation, where surface ligands could act as hole traps/acceptors to transfer the excited holes to the monomers (Figure 6b). The second pathway involved an interaction between the excited NPs and double bonds on monomers, followed by a charge transfer from the anion-localized surface of NPs to monomers (Figure 6c). Although further investigations are still needed, the surface-induced initiation mechanism provides a cornerstone for future study.

Figure 6.

(a) Conversion of HEA monomers versus time. Polymerization photoinitiated by Sn₂S₆^4–-coated bare NPs (red) and HNPs (black). (b) Proposed mechanism 1: charge transfer via surface coating mediation. (c) Proposed mechanism 2: charge transfer via anion-localized state. Adapted with permission from ref (71). Copyright 2019 Royal Society of Chemistry.

4.2. Living/Controlled Radical Polymerizations (CRPs) Photoinduced by SNMs

In this section, we explicitly highlight the current developed SNMs for mediating photo-CRPs, particularly for ATRP and RAFT polymerization.

4.2.1. Photoinduced ATRP Catalyzed by SNMs

For traditional ATRP, the reversible activation/deactivation process highly depends on the redox cycle of Cu(I)/Cu(II).^6,62,89 Introducing photosensitizers such as SNMs into polymerization systems appear to be a promising way to mediate the equilibrium of Cu(I)/Cu(II) species and achieve a high degree of photocontrol. Indeed, graphitic carbon nitride,¹⁶⁶ carbon QDs,¹⁴⁴ TiO₂NPs,¹⁶⁷ ZnO/Fe-doped ZnO NPs,⁸⁸ and titania/reduced graphene oxide nanocomposites¹⁶⁸ have been reported to work as photosensitizers in copper-mediated ATRP. The photoexcited SNMs could reduce the copper catalysts from Cu(II) to Cu(I), and the resulting Cu(I)-active species could further react with alkyl halides and return back to the high valence state, while reduced alkyl radicals initiate the ATRP process (Figure 7a). In general, a good correlation between experimental and theoretical molecular weights was observed in the all of the above cases, and a variety of monomers including methacrylates^88,144,167 and acrylamides¹⁶⁷ were successfully polymerized through SNM-photosensitized ATRP. However, UV light was used as the primary irradiation source,^88,166,167 which requires high energy consumption and might lead to the monomer self-initiation.¹⁶⁹ Moreover, temporal control could not completely be achieved in some cases, which might be because of the residual Cu(I) catalyst after photoactivation.⁹² For example, when ZnO was used as the photosensitizers for Cu(II)Br₂-mediated polymerization of MMA,⁸⁸ the monomer conversion still slightly increased even when the light was “off” (Figure 7b).

Figure 7.

(a) Proposed mechanism of UV-light-induced ATRP polymerization using NMs (TiO₂). Adapted from ref (167). Copyright 2013 American Chemical Society. (b) Monomer conversion versus time when light is “on” and “off ”. Adapted with permission from ref (88). Copyright 2014 John Wiley and Sons.

In 2015, Liu’s group reported a copper-free photoinitiated ATRP using niobium (Nb) NPs as a new class of recyclable PCs in the presence of ethyl 2-bromoisobutyrate (EBiB) initiators.¹⁷⁰ The resulting polymer showcased low dispersity ranging from 1.16 to 1.30 and controlled molecular weights. EBiB was proposed to be activated by the carbon-centered radical on the benzyl alcohol ligand originated from photoinduced ligand to metal charge transfer. However, chain extension was not performed in this study to explore the chain end fidelity of the resulting products.

Apart from serving as photosensitizers, SNMs could also work as PCs for ATRP. In 2018, Egap’s group reported the very first example of visible-light-mediated ATRP polymerization using CdSe QDs as PCs.⁹³ A wide variety of functional monomers (Figure 8a) were successfully polymerized with high yield (48.3–99.0%) and moderate dispersity (Đ = 1.25–1.83) using 3.3 nm CdSe QDs in the presence of external sacrificial donor N,N-diisopropylethylamine (DIPEA). Due to the sufficiently negative reduction potential of CdSe QDs (−1.59 vs SCE)¹¹¹ and large absorption coefficient efficiency¹¹⁷ (over 10⁶ M^–1 cm^–1), polymerization could be conducted in the presence of only 0.4 mM catalyst concentration. Excellent spatial and temporal control of the polymer architectures was achieved, and block copolymer was obtained via macroinitiator chain extension, indicating the strong living characteristic of the polymerization. A TEA-assisted oxidative quenching cycle was proposed to describe the mechanism (Figure 8b). First, excited-state QDs* were generated under irradiation and subsequently reduced the dormant bromide chain end to a radical under irradiation, which then participated in propagating monomers. The QD radical cation forming from the PET process could then accept an electron from DIPEA to complete the catalyst cycle. A Stern–Volmer plot of CdSe QDs PL quenching at various concentrations of alkyl bromide initiators or DIPEA showed that initiators are far more effective than DIPEA in quenching the PL of CdSe (Figure 8c), suggesting alkyl bromide was more likely to react with excited QDs. However, one drawback of this polymerization system is that the amine radical anion intermediate forming from catalyst step turnover^28,158 could probably end-cap the radicals and lead to undesirable termination.²⁸ Moreover, due to nonpolar ligand capping on CdSe QDs, polymerization could not be operated in polar solvents, which are known for facilitating electron transfer and stabilizing charge-separated species.¹⁷¹

Figure 8.

(a) Monomers and (b) proposed mechanisms of photo-ATRP polymerization catalyzed by CdSe QDs. (c) Stern–Volmer plot of CdSe QD fluorescence quenched by EBP and DIPEA. Adapted from ref (93). Copyright 2018 American Chemical Society.

4.2.2. Photoinduced RAFT Polymerization Catalyzed by SNMs

An essential factor to achieve a successful RAFT polymerization is utilizing an appropriate amount of external initiation species such as azobisisobutyronitrile to initiate chain transfer agents.¹⁷² SNM photoinitiators, which could efficiently produce active radicals under photoexcited states, appear to be good candidates for RAFT polymerization initiation. In 2014, Liu’s group developed a well-controlled visible-light-mediated RAFT polymerization enabled by surface-modified Nb(OH)₅NPs.¹⁷³ The authors proposed a photoinitiated RAFT polymerization mechanism^42,174,175 where excited electrons could transfer from surface ligands to the metal atom core, forming a radical cation at the surface (Figure 9a). The radical cation then lost one α-hydrogen, giving rise to a −O–CH–Ph radical, which further attacked the RAFT agent and started the RAFT process. Small dispersity (Đ = 1.17) could still be achieved even at 86% monomer conversion, where chain termination and side reactions often happen at such high conversion. Polymerization could be started or stopped at will simply by turning the light on and off, and dormant species were still able to be activated even after six cycles of the light “on–off” (Figure 9b). As no polymerization was observed without Nb(OH)₅ NPs, the ability to control the activity of polymerization is due to the incorporation of SNMs rather than the photolysis of RAFT agents. NPs could be recovered from the reaction mixture and still efficiently initiate the RAFT polymerization, giving more than 70% monomer conversion even after six recycle times. However, polymer yield gradually decreased and Đ increased after every recycle (Figure 9c), which is probably due to partially losing grafted groups on the NP surface.¹⁷³

Figure 9.

Photoinitiated RAFT polymerization triggered by Nb(OH)₅ NPs. (a) Proposed mechanism. (b) Monomer conversion versus time while turning the light on and off. (c) Đ and yields of polymer versus recycle times. Adapted with permission from ref (173). Copyright 2014 John Wiley and Sons.

One of the challenges in photoinitiated RAFT polymerization is that the use of co-initiating radicals might cause undesired chain termination.² On the other hand, directly reducing RAFT agents using PCs could circumvent the formation of initiator-derived byproducts and meanwhile maintain the temporal control over polymerization.^2,8 Very recently, CdSe QDs were proven to be robust PCs for PET-RAFT polymerization in both organic^176,177 and aqueous¹⁷⁸ solutions. CdSe QDs have a strong reducing potential (−1.59 vs SCE for 3.3 nm QDs), which is sufficient to reduce typical RAFT agents (redox potential ranging from −0.3 to −0.8 V vs SCE). In 2019, Egap’s group¹⁷⁶ demonstrated an efficient PET-RAFT polymerization in various polar organic solvents including dimethylsulfoxide (DMSO), N,N-dimethylacetamide, and N,N-dimethylformamide (DMF). A broad scope of functional methacrylate and acrylate monomers was successfully polymerized, with Đ ranging from 1.09 to 1.24. The di- and trithio-CTAs were found to bind with the metal core of CdSe QDs surface via Lewis acid–base interaction¹⁷⁹ and partially substitute the original ligands, thus allowing in situ growth of polymer chains from the surface of CdSe QDs to form well-dispersed core–shell polymer–QD nanocomposites (Figure 10a). Weiss’ group¹⁷⁸ reported an aqueous PET-RAFT polymerization of a series of acrylamides and acrylates using CdSe QDs as PCs. The polymerization could be operated under ultralow (<0.5 ppm) QD loadings, resulting in more than 90% monomer conversion in 2.5 h and low dispersity (<1.10). Furthermore, using protein concentrators with large-pore (30 kDa) filters (Figure 10b), polymers could be isolated and QDs could selectively be retained in the concentrate. Moreover, the size effect of CdSe QDs over the RAFT polymerization has also been recently reported by Pang’s group.¹⁷⁷ The better controllability of polymerization was observed using larger-size QDs (6.8 nm) than using the 3.8 nm QDs.

Figure 10.

(a) Proposed mechanism of CdSe QD-catalyzed PET-RAFT polymerization in organic solvents and in situ growth of the polymer shell. Adapted with permission from ref (176). Copyright 2020 Royal Society of Chemistry. (b) Procedure for the separation of QDs from polymer products. Adapted from ref (178). Copyright 2020 American Chemical Society.

Although CdSe QDs have shown excellent catalytic performance in photomediated RAFT polymerization, the residual toxic metal Cd raises the concern of contamination of the polymer products. Therefore, utilizing metal-free SNMs such as carbon QDs,^103,180 silicon quantum dots,¹⁸¹ nanofibers,¹⁴⁶ and nanosize-conducting polymers¹⁴⁵ becomes an alterative approach for PET-RAFT polymerization. For example, Matyjaszewski and co-workers developed a robust metal-free PET-RAFT polymerization using heteroatom-doped carbon quantum dots (CDs) as a new class of PCs.¹⁰³ Well-defined polymers with corresponding molecular weights and small dispersity (Đ < 1.10) could be obtained from this approach under blue light-emitting diodes and natural sunlight. Formidable control over the polymerization process was revealed by excellent spatial control (Figure 11a, left), monomodal and symmetric GPC profiles (Figure 11a, middle), as well as successful chain extension (Figure 11a, right). The authors found that doping the heteroatom could promote the polymerization efficiency, and further screening different heteroatoms showed that P- and S-doped CDs were more effective PCs, likely due to their strong absorption in the blue light region and high degree of graphitization. A typical PET-RAFT polymerization mechanism via an oxidative quenching pathway was proposed (Figure 11b), and the authors also found that introducing sacrificial hole scavengers would increase the excited CD lifetimes by preventing charge recombination and therefore would enhance the overall polymerization efficiency.

Figure 11.

(a) Light “on/off” experiments catalyzed by CDs (left), GPC profile of the PMMA (middle), and GPC profiles of PMMA macroinitiator and the diblock copolymer after chain extension (right). (b) Proposed mechanism of CD-photocatalyzed RAFT polymerization. Adapted with permission from ref (103). Copyright 2018 John Wiley and Sons.

Metal oxide NPs have also proven to be efficient PCs for RAFT polymerization. Nano-ZnO (30 nm) was developed by He and co-workers as a PC for PET-RAFT polymerization of MMA in DMF at ambient temperature.¹⁸² UV light was used as the irradiation source due to the ZnO absorption spectrum, achieving around 15% of monomer conversion and Đ > 1.40 for 5 h. A similar strategy for PET-RAFT polymerization was reported by You and co-workers using TiO₂ (type P25) as a photoredox catalyst under UV light.¹⁸³ It is worthwhile to mention that the photolysis of RAFT agents is likely to happen under UV irradiation,⁹⁰ where RAFT agents could be directly photoinitiated and propagate with monomers. Therefore, the photolysis process may occur in parallel with PET-RAFT polymerization using ZnO and TiO₂ NPs with UV light. A control study without metal oxide NPs could be useful to distinguish two different polymerization pathways.

One of the major challenges in the aforementioned PET-RAFT polymerization is that the irradiation wavelength is limited to the blue and UV light regions. The high energy light not only may lead to undesired photolysis of RAFT agents^184,185 but also may not be cost-effective due to the high energy consumption. Therefore, the development of PCs upon far-red and near-infrared (NIR) irradiation for photoinduced polymerization is of general interest.^{76,169,186,187} In 2020, Egap’s group reported lead halide perovskite (CsPbBr₃) NCs as band-edge-tunable PCs for efficient photoinduced visible and NIR irradiation.⁵⁴ By simply modulating the ratio of iodine and bromine in the CsPbBr_xI_3–x NCs, the perovskite-catalyzed PET-RAFT polymerization of various functional acrylates and methyacrylates could be performed under a broad range of irradiation sources ranging from blue to red light (460–635 nm), yielding polymers with small dispersity (Đ = 1.02–1.13). More importantly, by leveraging the extremely large two-photon absorption (TPA) cross section of CsPbBr₃ (1.8 × 10⁵ GM), the authors performed a TPA-induced PET-RAFT polymerization with irradiation of an 800 nm laser. CsPbBr₃ was proposed to simultaneously absorb two 1.55 eV photons, giving one above-gap excitation and transferring excited electrons to RAFT agents (Figure 12). Encouragingly, the polymerization was conducted in a controlled fashion, achieving 60.2% monomer conversion and Đ = 1.07. The TPA-induced RAFT polymerization demonstrates the great potential of SNMs as NIR PCs for photopolymerizations.

Figure 12.

CsPbBr₃ NC-photocatalyzed RAFT polymerization via two-photon absorption. Adapted from ref (54). Copyright 2020 American Chemical Society.

Apart from the aforementioned nanostructures, microsized semiconducting materials such as graphitic carbon nitride (g-C₃N₄)¹⁴⁷ and bismuth oxide (Bi₂O₃)⁹⁵ were also introduced recently as reusable and recyclable PCs for PET-RAFT polymerization. In the case of g-C₃N₄, the robust catalytic nature was revealed by successfully conducting polymerization with nonpurified monomers and air, providing various opportunities for large-scale industrial applications.⁴² Bi₂O₃, on the other hand, was reported to mediate a macromolecular design by the interchange of xanthate (MADIX)/RAFT polymerization for both more activated monomers and less activated monomers using only a general household light bulb (Figure 13a). Polymerization could be operated in a variety of solvents including DMSO, hexafluoroisopropanol, and dioxane (Figure 13b) with a pseudo-first-order kinetics feature, offering polymers with precise control over MW and small dispersity less than 1.20. In addition, heterogeneous Bi₂O₃ could be removed from the reaction mixture and reused for at least three cycles with a negligible reduction in catalytic performance (Figure 13c).

Figure 13.

(a) MADIX/RAFT polymerization photomediated by Bi₂O₃. (b) Kinetic plot of the MADIX/RAFT polymerization in different solvents. (c) Recycling of the Bi₂O₃ catalyst and polymerization effect. Adapted with permission from ref (95). Copyright 2019 John Wiley and Sons.

In 2020, Müllner’s group reported a facile method to incorporate alkyl bromide redox chemistry into the RAFT process using Bi₂O₃ as the PC.¹¹⁰ Alkyl bromides were proposed to be reduced by photoexcited Bi₂O₃ and generating an active radical. The alkyl radicals could react with RAFT agents and promote a controlled photopolymerization. The presence of the alkyl substituent and trithiocarbonate from RAFT agents was further confirmed by nuclear magnetic resonance and electrospray ionization mass spectrometry. The control of polymerization was governed by the degenerative chain transfer process onto RAFT agents rather than the alkyl bromide redox equilibrium. A wide variety of functionalities including peptides could be installed into polymer using different alkyl bromides. Furthermore, varying the ratio of two different RAFT agents in the reaction mixture could tune the dispersity of resulting polymers from 1.18 to 1.64.

4.3. Other Types of Polymerization Photoinduced by SNMs

We summarized herein other types of photopolymerization mechanisms induced by SNMs apart from chain-growth radical mechanism. For instance, Ravoo’s group reported a step-growth polymerization of ethanol amines driven by TiO₂ NPs upon UV irradiation to prepare microstructured polymer brushes.¹⁸⁸ The authors suggested that ethanol amine monomers could accept photogenerated holes from TiO₂ NPs, forming the corresponding aldehyde that reacted with another ethanol amine molecule to yield imine. Imines were then reduced to secondary amines by photoexcited electrons in the CB of TiO₂ to complete the catalytic cycle.

Conjugated polymers have received great interest due to their wide application in fabricating various materials such as optoelectronic devices¹⁸⁹ and batteries.¹⁹⁰ The mechanism of photoinduced polymerization of conjugated polymers usually begins with hole transfer from photosensitizer to monomer followed by the monomer radical cation coupling.¹⁹¹ We herein highlight the photoinduced methodology for synthesizing conjugated polymers using NMs, a technique that is rarely reported but possesses great potential. For example, Neckers and co-workers reported a photopolymerization of oligothiophene-decorated Au NPs.¹⁹² 5-Mercapto-2,2′-bithiophene (BTSH) was attached to the Au NPs’ surface through thiol–gold bonds. Upon UV irradiation, bithiophene was able to be oxidized by Au NPs, which have a strong electron affinity, via intramolecular electron-transfer reactions. The oxidized radical cation rapidly coupled with other bithiophene radical cations, leading to the formation of polythiophene and aggregation of Au NPs. The photopolymerization among Au NPs was further confirmed by UV absorption spectrum of Au NPs after irradiation, where the plasmon absorption band was broadened and red-shifted. Furthermore, femtosecond transient absorption showcased that charge separation rates between bithiophene ligands and Au NPs decreased with increasing NP sizes.

Bulk materials such as TiO₂ were also examined to polymerize pyrrole¹⁹³ and 3,4-ethylenedioxythiophene¹⁹⁴ in situ at their surface, which would form a conducting polymer layer. Electrophiles such as F-SnO₂ electrodes or oxygen were used to accept photoexcited electrons, whereas monomers react with surface-trapped holes, giving rise to the conducting polymer layer via the radical coupling. The formation of conjugated polymers was supported by FTIR spectroscopy and cyclic voltammetry results. Moreover, Ag NPs have also been reported to polymerize pyrrole via a similar mechanism.^195,196 However, these reports lack of comprehensive study on reaction yield, MW, as well as dispersity.

Recently, Tüysüz reported a photoinduced polymerization of synthesizing poly(3,4-ethylenedioxythiophene) (PEDOT), enabled by cesium lead iodide (CsPbI₃) perovskite QDs (Scheme 4).¹³⁵ Because the oxidation potential of 3,4-ethylenedioxythiophene (1.9 V vs NHE) is energetically lower than the VB of CsPbI₃ QDs (1.3 V vs NHE), the direct reduction of 3,4-ethylenedioxythiophene is energetically unfavorable. Thus, trimer 3,4-ethylenedioxythiophene (TerEDOT), whose oxidation potential located at 0.8 V vs NHE, was used instead as the starting monomer. Successful polymerization of TerEDOT was confirmed by the appearance of a broad peak around 750 nm in the UV absorption spectrum, and TEM images displayed that CsPbI₃ QDs were encapsulated into the polymer matrix. Furthermore, because oxygen could be used as electron acceptors for CsPbI₃ QDs, polymerization could be performed in air. However, the crystal structure of CsPbI₃ QDs transformed from cubic to orthorhombic after reacting in air. The change of crystal structure might be induced by the oxidation and removal of surfactants on the QDs. A recent study by Chen’s group¹³⁷ revealed that the polymerization efficiency of TerEDOT could be highly enhanced by simply treating CsPbI₃ QDs with methyl acetate. The use of methyl acetate could replace the original oleic acid ligands and introduce defects on the surface of QDs, which might accelerate the charge transfer and impede the radiative charge recombination.

Scheme 4. Proposed Mechanism for Polymerization of TerEDOT Using CsPbI₃ QDs as PCs.

Adapted from ref (135). Copyright 2017 American Chemical Society.

5. Application of NM-Induced Photopolymerization

5.1. Photoinduced Three-Dimensional (3D) Printing Using NPs as PIs

Photopolymerization-based 3D printing has attracted significant attention from both academic and industrial research in recent years.^197,198 This rapidly developing technology offers a facile route for fabricating functional materials with various optical, chemical, and mechanical properties.^197,199 A representative 3D printing system is composed of monomers, oligomers, and PIs. Beginning with the liquid state monomers/oligomers, PIs can be activated upon exposure to the light source and triggered a cured/photopolymerized process, leading to the solidification of liquid monomers. To achieve successful photopolymerization, the polymerization/cross-linking process should be fast enough (typically reaching full conversion in 1 min)¹⁹⁷ so that the ink could be rapidly transformed into the solid state.¹⁹⁸ Due to the large absorption coefficient, SNM-photoinduced polymerization provides new opportunities for conducting photo-3D printing under low catalyst loading.¹²³ The easily tunable solubility of SNMs even enables the photo-3D in the aqueous phase.²⁰⁰

In 2017, Magdassi and Banin reported a rapid water-phase 3D printing using hybrid semiconductor–metal NPs as a novel class of PIs (Figure 14a).⁸⁵ Excited electron–hole pairs origin from CdS/Au HNPs were proposed to react with water and oxygen, producing hydroxyl and superoxide radicals to initiate polymerization, and meanwhile HNPs could remain intact. This highly efficient polymerization was used for 3D printing of hydrogels by a digital light projector printer with UV light, giving rise to a spherical C180 buckyball hydrogel object (Figure 14b). Polymerization also showed good oxygen tolerance, and the printing process could be operated in the presence of only 0.2 nM HNPs. The 3D-printed buckyball hydrogel also exhibited strong green fluorescence under 365 nm excitation (Figure 14c), retaining delicate structures even after drying (Figure 14d).

Figure 14.

(a) Mechanism of polymerization using CdS–Au HNPs as photoinitiators. (b) Images of a 3D-printed buckyball under regular light and (c) under 365 nm excitation. (d) Scanning electron microscopy image of the dried structure. Adapted from ref (85). Copyright 2017 American Chemical Society.

Magdassi also established a rapid 3D printing method for fabricating complex hydrogels with UV-curable inks.²⁰⁰ 2,4,6-Trimethylbenzoyldiphenylphosphine oxide (TPO) was converted into water-dispersible NPs and used as the PIs, enabling building a stable woodpile-structured hydrogel within 25 min. TPO also showcased the superior performance in 3D printing over other common organic dye PIs under identical conditions. The excellent polymerization performance facilitated by TPO NPs was attributed to the good solubility and dispersion of the TPO in water.

5.2. Synthesis of Organic–Inorganic Nanocomposites via In Situ Photopolymerization

Polymer nanocomposites are a class of advanced materials that have been widely used in optical/electrical sensors,²⁰¹ batteries,^202,203 light-emitting diodes,²⁰⁴ absorption,^205,206 and biological imaging.²⁰⁷ With SNMs as photomediators, SNM-photoinduced polymerization emerges as a novel strategy to fabricate organic–inorganic nanocomposites.

In 2013, Aida and co-workers developed a facile synthesis method of photomodulable thermoresponsive hydrogels photocatalyzed by titania nanosheets (TiNSs).²⁰⁸ Upon irradiation, TiNSs produced hydroxyl radicals from water, which initiated polymerization and meanwhile cross-linked the polymer chains to form a 3D polymer network (Figure 15a), which could hold the water molecules. Various water-soluble vinyl monomers including acrylamide and N-isopropylacrylamide underwent such a gelation process. The yielded hydrogels showed sharp thermoresponses (Figure 15b,c) and an excellent resolution on pointwise modulations. A similar strategy was also employed for temperature-responsive poly(N-isopropylacrylamide) hydrogel using carbon nitride nanosheets (CNNs) as PCs.²⁰⁹ The hydroxyl radicals from water generated on CNNs were claimed as the initiating species. The nanocomposites could act as a temperature-controlled smart window around the lower critical solution temperature, reversibly altering the turbidity. This in situ polymerization/cross-linking route offers a facile synthesis of functional carbon nitride composite hydrogels with a promoted photocatalytic performance in removing organic pollutants, strong mechanical properties, and adjustable shapes.^210−212

Figure 15.

(a) In situ photoinduced polymerization and formation of nanocomposite hydrogelation catalyzed by TiNSs. (b) Formed thermal responsable hydrogel under different temperatures in test tubes and (c) in a patterned matrix. Adapted with permission from ref (208). Copyright 2013 Springer Nature.

Wang and Wu also demonstrated the synthesis of nanoparticle–hydrogel composites with the aid of semiconductor NPs and clay nanosheets.²¹³ NPs were used for generating initiating hydroxyl radicals under sunlight, and clay nanosheets were utilized for increasing cross-linking points, as the cross-linking points on the NP surfaces were insufficient. Various NPs including zinc oxide, titanium dioxide, iron(III) oxide, tin dioxide, zirconium dioxide, cadmium selenide, or cadmium telluride were established for successful photoinitiated polymerization of N,N-dimethylacrylamide. Formed nanocomposite hydrogels exhibit high toughness and elasticity (Figure 16a), and all of the hydrogels could tolerate a compression greater than 95%. The physicochemical properties of the semiconductor NPs were remained after polymerization, providing hydrogels with different fluorescence wavelengths via simply changing the size of the QDs (Figure 16b).

Figure 16.

(a) Images of the NC hydrogels during the tensile testing process. (b) Images of different fluorescence CdTe-NC hydrogels under UV light. Adapted with permission from ref (213). Copyright 2013 Springer Nature.

Very recently, Acar and co-workers fabricated luminescent ZnO/MMA nanocomposites utilizing methacrylic-acid-coated ZnO QDs as the photoinitiator.²¹⁴ Starting with Kolbe decarboxylation of surface-coated carboxylic acids by photoexcited ZnO, successful photopolymerization of MMA was performed in bulk and methylethyl ketone, giving rise to a homogeneous gel with strong yellow luminescence. No luminescence of ZnO was quenched during the polymerization. The same group also reported a facile method to form luminescent and stable CdS QD/polymer hybrids via the decarboxylation process of the oleic acid ligand.²¹⁵

Although the aforementioned examples could incorporate SNMs into the polymer matrix, incompatibility between inorganic nanobuilding blocks and organic polymers usually leads to phase separation and aggregation of SNMs.^176,216 On the other hand, the direct growth of polymers on the surface of SNMs (“grafting from”) enables both the polymer matrix and the shell of nanobuilding block to have the same chemical structure and hence to be miscible with each other.^176,217 By modifying the surface of photoactivated SNMs with initiation species, the SNMs could serve as both the PCs and inorganic nanocomposites, enabling the photopolymerization to happen in situ on the SNMs’ surface and forming well-dispersed polymer–SNM nanocomposites.^167,218,219 We also believe that shortening the distance between SNMs and initiators would enhance the electron transfer rate by increasing the collision frequency,²²⁰ consequently speeding up the rate of polymerization. For example, Yin and co-workers established a facile synthesis of inorganic–polymer hybrid nanocomposites using TiO₂ nanospheres as PIs (Figure 17a).²¹⁸ The TiO₂ NP surface was modified by 3-(trimethoxysilyl)propyl methacrylate (MPS) as the Si–O–Ti linkages. MPS moieties accepted the photogenerated radicals on the NPs’ surface, serving as initiators, and continuously propagated polymers, coating the polymer on the NPs’ surface uniformly, even in the presence of oxygen. TEM images showcased the successful formation of polymer shells on the NPs’ surface (Figure 17b). Faster polymerization was observed with higher crystallinity of the titania nanospheres. This technique could be generalized to other core materials such as SiO₂ and ZnS by simply coating the substrates with the TiO₂ thin layer.

Figure 17.

(a) Schematic illustration of surface-initiated polymerization using TiO₂ as PC and building block core for polymer grafted NPs. (b) TEM image of polymer grafted TiO₂ NPs. Panels (a) and (b) are adapted from ref (218). Copyright 2016 American Chemical Society. (c) Hot electron initiated photopolymerization. Adapted from ref (73). Copyright 2017 American Chemical Society.

Very recently, plasmon-induced polymerization appears to be an alternative method to coat polymer on the surface of the nanostructures.^73,75,221 Monomers are usually initiated by injection of hot electrons photogenerated by plasmons. Hot-electron initiation occurs close to the metal NPs (Au, Ag, etc.) surface, enabling polymerization happening within the nanogap. Monomers are initiated through the formation of [metal–C–C•] species near the NPs’ surface (Figure 17c). Nobel metal NPs, such as Au and Ag, were used as the nanobuilding block, and a blue shift of the plasmon resonances was usually observed after polymerization.⁷³ A large scope of vinyl monomers with different functional groups and conjugated polymers²²² can be produced via this methodology. More importantly, by changing the position of the hotspots, hot electron injection density can be optimized, leading to the growth control of the polymer shell. The plasmon-induced mechanism is also applied for controlled polymerizations such as NMP²²³ and RAFT^76,224 polymerization. Although surface modification of covalently attaching initiators on the gold particle surface was required, the living polymerization allowed the grafting block copolymers on the metal surface and control over polymer shell thickness.

6. Conclusion and Future Outlook

Utilizing SNMs as PIs/PCs has blossomed into an attractive field due to the recent resurgence of interest in photoinduced polymerization. Compared to currently developed metal complexes and organic PIs/PCs, SNMs provide added advantages such as simple synthesis procedures, abundant raw materials, and tunable band gaps and redox potentials. Various SNMs including metal oxides, metal chalcogenide QDs, and perovskite NCs as well as carbon-based metal-free QDs have been discovered for both conventional and living radical polymerizations, exhibiting performances similar or superior to those of the metal complexes and organic-based PIs/PCs. Furthermore, benefiting from the unique optical and electronic properties such as high extinction coefficient, giant TPA cross section, etc., SNM-induced photopolymerization methodology offers numerous opportunities in rapid photoinduced 3D printing, harvesting NIR irradiation sources, and fabrication of functional organic–inorganic nanocomposites.

Despite the great achievements in SNM-induced photopolymerization over the past decades, there are still many challenges and unknown potentials in the field. First, most SNMs contain heavy metals that may remain as the contamination in the yielding polymers, which may hinder the biomedical and electronic applications of the resulting polymers.¹⁵ Therefore, the development of metal-free PCs such as carbon nanodots¹⁴⁸ and semiconducting polymer NPs²²⁵ or substitution of high toxicity metal in SNMs with nontoxic metal atoms (Bi, Sb, Zn, etc.) is of general interest.^95,226

Moreover, expanding the scope of photopolymerization mediated by SNMs is also of particular interest. Currently, SNM-photoinduced polymerizations are limited to radical polymerizations such as RAFT and ATRP via the PET process. Leveraging the cation-rich surface and strong photoinduced hole transfer process of SNMs, we foresee that SNMs could work as PIs/PCs for other types of polymerizations such as cationic polymerizations¹⁹ and ROMP,²⁰ consequentially extending the monomer accessibility.

Moreover, doping the SNMs with catalytic active metals such as Ni,¹¹² Au,^227,228 Pd,^229,230 and Ru²³¹ could favorably influence their charge transfer efficiency, enhance the spatial charge separation, and suppress the radiative charge recombination,^121,123 thus improving the photocatalytic catalytic performance of SNMs.^71,85,232 We believe that heterostructures/nanohybrids forming from conjugating noble metal NPs with semiconductors could also be applied for photopolymerization systems with the rationally designed mechanism.

Furthermore, resolving structure–property–performance relationships²³³ would provide a guiding principle for design of efficient SNM PCs. For example, parameters such as energy level,¹³² crystal structure,⁷⁶ surface ligands,²³⁴ sizes, and shapes of SNMs¹⁷⁷ are determinative to their optoelectronic properties, electron distribution, and catalytic surface area and therefore could highly influence the photocatalytic performance of QDs. Meanwhile, high-throughput techniques²³⁵ and computational calculation²⁷ should also be robust tools for exploring the PCs’ structure–property relationship.

Last but not least, improving the oxygen tolerance^42,236,237 and scalability of SNM-photocatalyzed polymerization would bridge the gap between academic study and industrial applications. For example, building core–shell structures²³⁸ and adding sacrificial ligands¹¹⁶ would improve the stability of SNMs and prevent them from photobleaching. In addition, integrating flow systems²³⁹ with SNM-mediated photopolymerizations would provide numerous chances for performing large-scale reactions with an improved polymerization effect.

The authors declare no competing financial interest.