Recent Advances in the Synthesis of Polymer-Grafted Low-K and High-K Nanoparticles for Dielectric and Electronic Applications

molecules

Review Recent Advances in the Synthesis of Polymer-Grafted Low-K and High-K Nanoparticles for Dielectric and Electronic Applications

Bhausaheb V. Tawade 1 , Ikeoluwa E. Apata 1 , Nihar Pradhan 2 , Alamgir Karim 3 and Dharmaraj Raghavan 1,*

1 Department of Chemistry, Howard University, Washington, DC 20059, USA; [email protected] (B.V.T.); [email protected] (I.E.A.) 2 Department of Chemistry, Physics and Atmospheric Science, Jackson State University, Jackson, MS 39217, USA; [email protected] 3 Department of Chemical and Biomolecular Engineering, University of Houston, Houston, TX 77204, USA; [email protected] * Correspondence: [email protected]

Abstract: The synthesis of polymer-grafted nanoparticles (PGNPs) or hairy nanoparticles (HNPs) by tethering of polymer chains to the surface of nanoparticles is an important technique to obtain nanostructured hybrid materials that have been widely used in the formulation of advanced polymer nanocomposites. Ceramic-based polymer nanocomposites integrate key attributes of polymer and ceramic nanomaterial to improve the dielectric properties such as breakdown strength, energy density and dielectric loss. This review describes the “grafting from” and “grafting to” approaches commonly adopted to graft polymer chains on NPs pertaining to nano-dielectrics. The article also covers various surface initiated controlled radical polymerization techniques, along with templated approaches for Citation: Tawade, B.V.; Apata, I.E.; grafting of polymer chains onto SiO2, TiO2, BaTiO3, and Al2O3 nanomaterials. As a look towards Pradhan, N.; Karim, A.; Raghavan, D. applications, an outlook on high-performance polymer nanocomposite capacitors for the design of Recent Advances in the Synthesis of high energy density pulsed power thin-ﬁlm capacitors is also presented. Polymer-Grafted Low-K and High-K Nanoparticles for Dielectric and Electronic Applications. Molecules Keywords: polymer-grafted nanoparticles; dielectric properties; energy density; SiO2; TiO2; BaTiO3; 2021, 26, 2942. https://doi.org/ Al2O3; reversible deactivation radical polymerization; ATRP; RAFT; NMP; click chemistry 10.3390/molecules26102942

Academic Editor: Bhanu P. S. Chauhan 1. Introduction The growing demand for power electronics and energy storage serves as an ex- Received: 14 April 2021 cellent motivation for developing next generation dielectrics and electrical insulation Accepted: 10 May 2021 materials [1,2]. Dielectric polymers and polymer nanocomposites stand out as next gener- Published: 15 May 2021 ation dielectric materials for many electrical insulation and energy storage applications owing to their high dielectric strength, high voltage endurance, low dielectric loss, low Publisher’s Note: MDPI stays neutral equivalent series resistance, a gradual failure mechanism, light weight, low cost and ease with regard to jurisdictional claims in of processability [3–10]. The use of polymer-based dielectric capacitors in various sectors published maps and institutional affil- is summarized in Figure1A. As a result of numerous emerging potential applications of iations. polymer-based dielectric materials and capacitors, research on strategies for enhancing capacitive energy storage methods has experienced significant growth. Figure1B shows the number of yearly publications in the last 25 years on the topic of “dielectric polymer capacitor” as found in the Sci-Finder database. Clearly, over the years the research interest Copyright: © 2021 by the authors. in the field of polymer dielectric capacitors has grown exponentially. Licensee MDPI, Basel, Switzerland. Apart from the use of polymers in nanocomposites, inorganic materials such as This article is an open access article ceramics are critical components for nanocomposite capacitors due to their extremely large distributed under the terms and dielectric constants, often times >1000. Despite their high dielectric constants, inorganic conditions of the Creative Commons materials suffer from a low breakdown strength and non-graceful failure mode. Polymer Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ nanocomposites integrate key attributes of polymer and ceramic nanomaterial to improve 4.0/). the overall dielectric properties [11,12].

Molecules 2021, 26, 2942. https://doi.org/10.3390/molecules26102942 https://www.mdpi.com/journal/molecules Molecules 2021Molecules, 26, x 2021FOR, 26PEER, 2942 REVIEW 2 of 44 2 of 43

Figure 1. (FigureA) The 1. (emergingA) The emerging applications applications of of dielectric dielectric capacitors capacitors (B) Number(B) Number of publications of publications on “polymer on dielectric “polymer capacitor” dielectric capacitor” perper year year in thethe last last 25 25 years. years.

Several comprehensive review articles including a couple of review articles from Apartour group from have the been use publishedof polymers in the in field nanoco of polymermposites, and polymer inorganic nanocomposite materials di-such as ce- ramicselectrics are critical [13–26]. components Our first review for article nanocompos dealt withite coverage capacitors of the nanoscaledue to their strategies extremely in large dielectricthe field constants, of polymeric often and polymertimes >1000. nanocomposites Despite fortheir use inhigh emerging dielectric dielectric constants, capacitor- inorganic materialsbased suffer energy from storage a low applications breakdown [13]. Somestreng ofth the and strategies non-graceful to address failure permittivity mode. Polymer nanocompositescontrast between integrate nanofillers key and attributes the polymer of matrix polymer including and potentialceramic for nanomaterial developing to im- gradient permittivity structured nanofillers were presented. Additionally, we had described proveapproaches the overall to improvedielectric the properties compatibility [11,12]. of nanofiller with polymer, minimize nanofiller Severalaggregation, comprehensive and mitigate the review permittivity articles contrast including between a couple nanofiller of review and polymer, articles In from our groupour have second been review published article, we in discussedthe field different of polymer chemical and routes polymer for surface nanocomposite functional- dielec- trics [13–26].ization of Our ceramic first nanoparticles review article [14]. dealt For instance, with coverage the article of dealt the with nanoscale the synthesis strategies of in the low-k and high-κ nanomaterials [19–24] as well as surface functionalization of nanoma- field terialsof polymeric including and treatment polymer with hydrogen nanocomposites peroxide, silane for use coupling in emerging agents, phosphonic dielectric capacitor-basedacid and energy dopamine storage moieties applicat thations improved [13]. theSome interaction of the betweenstrategies nanomaterials to address and permittivity contrastpolymer between matrix. nanofillers and the polymer matrix including potential for developing gradient Inpermittivity the review article structured published nanofillers in Nanotechnology were presented.[14], it was pointedAdditionally, out that thewe had de- scribedselection approaches of the surface to improve modifying the coupling compatibility agent on theof surfacenanofiller of nanoparticles/layer with polymer, minimize dictate the dielectric properties of the nanocomposites as well as the performance of nanofillerthe bilayer aggregation, as it relates and to gate mitigate dielectrics. the Although, permittivity functionalization contrast ofbetween nanomaterials nanofiller and polymer,with chemicalIn our second agents isreview less cumbersome article, we and di lessscussed equipment different intensive chemical there are routes several for surface functionalizationshortcomings toof adopting ceramic this nanoparticles method viz., (i) [14]. the structureFor instance, of the chemical the article modifying dealt with the synthesisagent of is distinctlylow-k and different high-κ from nanomaterials the long chain [19–24] of polymer as well matrix as (ii) surface side reaction functionalization of the chemical agent could lead to multilayer formation and (iii) physical adsorption of the of nanomaterialsmodifying agent. including Unlike the surfacetreatment modification with hydrogen of nanoparticles peroxide, with chemical silane agents, coupling the agents, phosphonicpolymer graftingacid and of thedopamine nanoparticles moieties yield nanoparticles that improved with surface the energyinteraction which closelybetween nano- materialsmatches and with polymer that of thematrix. polymer matrix. The improved compatibility of polymer-grafted Innanoparticles the review with article polymer published matrix often in yieldsNanotechnology nanocomposites [14], with it superiorwas pointed properties out that the compared to nanocomposites with chemical agent-modified nanoparticles. For instance, selection of the surface modifying coupling agent on the surface of nanoparticles/layer maximum energy density and extraction efficiency values for polymethylmethacrylate dictate(PMMA) the dielectric grafted BaTiO properties3 filled PMMA of the nanocompositesnanocomposites was as found well to as be the two performance fold higher of the bilayerthan as that it relates of coupling to gate agent dielectrics. surface-modified Although, BaTiO3 functionalizationfilled PMMA nanocomposites of nanomaterials [27]. with chemical Thereagents are manyis less approaches cumbersome to improve and theless compatibility equipment of theintensive nanoparticles there with are several shortcomingspolymer matrix, to adopting the nanoparticles this method spatial viz dispersion., (i) the in thestructure matrix andof the decrease chemical the per- modifying mittivity contrast between polymer and nanoparticles. Approaches could be based on agent is distinctly different from the long chain of polymer matrix (ii) side reaction of the chemical agent could lead to multilayer formation and (iii) physical adsorption of the modifying agent. Unlike the surface modification of nanoparticles with chemical agents, the polymer grafting of the nanoparticles yield nanoparticles with surface energy which closely matches with that of the polymer matrix. The improved compatibility of polymer-grafted nanoparticles with polymer matrix often yields nanocomposites with superior properties compared to nanocomposites with chemical agent-modified nanoparticles. For instance, maximum energy density and extraction efficiency values for polymethylmethacrylate (PMMA) grafted BaTiO3 filled PMMA nanocomposites was

Molecules 2021, 26, x FOR PEER REVIEW 3 of 43

found to be two fold higher than that of coupling agent surface-modified BaTiO3 filled PMMA nanocomposites [27]. There are many approaches to improve the compatibility of the nanoparticles with polymer matrix, the nanoparticles spatial dispersion in the matrix and decrease the permittivity contrast between polymer and nanoparticles. Approaches could be based on the use of by external triggers such as a simple control of the film processing conditions (controlling % loading of filler) [28] or, of the electrostatic repulsion (tuning by change pH) [29] or with a magnetic field (tuning based on magnetic field) [30] or an internal

Molecules 2021, 26, 2942 trigger such as chemical/polymer grafting approach [31,32]. This 3article of 44 only deals with internal trigger (by synthesis of polymer-grafted nanoparticles) to address the compatibility of nanoparticles and polymer. Several recent reviews have comprehensively cov- theered use ofthe by topic external of triggers polymer such grafting as a simple of control nanopa of therticles film processing[33–40]. For conditions example, the review by (controllingAmeduri % et loading al., [35] of filler)dealt [primarily28] or, of the with electrostatic grafting repulsion of polymers (tuning byon change high-K NPs (BaTiO3) for pH)use [29 in] or the with formulation a magnetic field of (tuning high based energy on magnetic storage field) fluorinated [30] or an internal polyme triggerr nanocomposites. In such as chemical/polymer grafting approach [31,32]. This article only deals with internal triggerthe present (by synthesis review, of polymer-grafted we cover the nanoparticles) synthesis of to addresspolymer-grafted the compatibility high-K of and low-K nano- nanoparticlesparticles for and the polymer. fabrication Several of recent nanocomposites reviews have comprehensively for electronics covered and thedielectric application. topicUnlike, of polymer Yang grafting et al.’s of [38] nanoparticles review [which33–40]. Fordisc example,usses only the review the synthesis by Ameduri of polymer-grafted ethigh al., [35 and] dealt low primarily K-nanoparticles with grafting of polymersusing surface on high-K initiated-polymerization NPs (BaTiO3) for use in the approaches, our formulation of high energy storage fluorinated polymer nanocomposites. In the present review,review we will cover cover the synthesis the broad of polymer-grafted gamut of approach high-K andes available low-K nanoparticles to synthesize for polymer-grafted thesilicon fabrication dioxide of nanocomposites (SiO2), titanium for electronics dioxide and (TiO dielectric2), barium application. titanate Unlike, (BaTiO Yang 3), and aluminum etoxide al.’s [38 (Al] review2O3) nanoparticles which discusses onlyand thetheir synthesis applicatio of polymer-graftedns as dielectrics high andand low electronics. K-nanoparticles using surface initiated-polymerization approaches, our review will cover the broadThe gamut grafting of approaches of polymeric available chai to synthesizens to nanoparticles polymer-grafted can silicon generally dioxide be achieved by four (SiOapproaches2), titanium dioxidenamely (TiO (i)2 ),“grafting barium titanate to”; (ii) (BaTiO “grafting3), and aluminum from”; (iii) oxide templated (Al2O3) and (iv) in situ nanoparticlespolymerization and their or applications encapsulatio as dielectricsn. Figure and electronics.2 presents pictorially the various approaches commonlyThe grafting adopted of polymeric to prepare chains to nanoparticlespolymer-grafted can generally nanoparticles. be achieved byAll four the four approaches approaches namely (i) “grafting to”; (ii) “grafting from”; (iii) templated and (iv) in situ polymerizationyield polymer-grafted or encapsulation. nanoparticles Figure2 presents of pictoriallyvarying theshell various architecture. approaches The polymer graft commonlyconformation adopted on to preparethe nanoparticles polymer-grafted is nanoparticles.a result of the All covalent the four approaches bond formation that com- yieldpensates polymer-grafted for the entropy nanoparticles loss of resulting varying shell from architecture. the polymer The polymer chains graft stretching con- away from the formationsurface. on If the the nanoparticles polymer ischains a result on of thethe covalent grafted bond nanoparticles formation that have compensates molecular weight lower for the entropy loss resulting from the polymer chains stretching away from the surface. Ifthan the polymer the entanglement chains on the grafted molecular nanoparticles weight, have th molecularen the harvested weight lower nanoparticles than the are commonly entanglementblended with molecular virgin weight, polymer then the to harvestedform polymer nanoparticles nanocomposite. are commonly blendedOn the other hand, if the withmolecular virgin polymer weight to form of polymerthe polymer nanocomposite. chains On on the the other polymer-grafted hand, if the molecular nanoparticles is far weight of the polymer chains on the polymer-grafted nanoparticles is far greater than the entanglementgreater than molecular the entanglement weight, a nanocomposite molecular could beweight, formed withouta nanocomposite the addition of could be formed anwithout external polymerthe addition matrix. Theof an former external is called polyme multi componentr matrix. system The whileformer the latteris called multi compo- isnent called system single component while the system latter [37 is,41 called–43]. single component system [37,41–43].

FigureFigure 2. Approaches 2. Approaches for grafting for grafting polymer polymer chains on thechains surface on of the nanomaterials. surface of (nanomaterials.A) Polymer- (A) Poly- nanocompositemer-nanocomposite formed from formed polymer-grafted from polyme NPs andr-grafted polymer matrix. NPs (andB) Polymer polymer nanocomposite matrix. (B) Polymer nano- formedcomposite from only formed polymer-grafted from only NPs). polymer-grafted NPs).

Molecules 2021, 26, 2942 4 of 44

As represented in Figure2, encapsulation or in situ polymerization approach is based on monomers being initially adsorbed on the NPs surface, and initiation of polymerization of the adsorbed monomer layer, yielding polymer-coated NPs. Sometimes the encapsulation approach could be termed as in situ grafting through approach because the monomers adsorbed on the NPs undergo polymerization in the presence of initiator in the bulk [44,45]. The second approach uses block copolymer-based micelle-template in the synthesis of hairy nanoparticles (HNPs). In this method, a precursor, commonly a metal salt or an organometallic compound, is loaded into the core of polymer micelles based on either multi-molecular block copolymer or unimolecular star block copolymer. The reduction of (complex) metal ions in the micelle core yields core–shell NPs [46,47]. The third approach is based on grafting-to which involves the attachment of end-functionalized polymer chains on the surface of NPs via suitable chemical reactions. A variety of reactions such as es- teriﬁcation, silylation, click reactions including thiol-ene, alkyne-azide cycloaddition, etc. have generally been utilized in the grafting to approach. The fourth approach is based on grafting-from/SI-CRP which consists of growing polymer chains directly from the surface of nanoparticles functionalized with suitable initiator/CTA functionalities. There have been remarkable developments in the surface-initiated controlled radical polymerization (SI-CRP) route for the synthesis of polymer-grafted nanoparticles [33,48–52]. Pioneering work from Matyjaszewski [53], Mueller [54], Benicewicz [55], Takahara [56], Hawker [57] and coworkers have paved the road for progress in SI-CRP methods. SI-CRPs (ATRP, SI-RAFT and SI-NMP) have been successfully employed for the generation of plethora of polymer grafted nanoparticles (PGNPs) because of its tolerance towards various functional groups [48]. Table1 summarizes the advantages and disadvantages of the four approaches outlined in the synthesis of PGNPs. Among the various approaches, grafting from approach is widely employed in the polymer functionalization of nanoparticles because of its ability to synthesize well-deﬁned polymer architectures of desired composition and molecular weight, and a shell of controlled thickness on the nanoparticle surface. Given the enor- mous data available on grafting from technique, this article will predominantly cover this approach. Examples of other approaches in the polymer functionalization of ceramic oxide NPs are also covered.

Table 1. Comparison of advantages and disadvantages of polymer grafting methods.

Grafting Methods Advantages Disadvantages Due to the steric hindrance high grafting A number of coupling reactions and click reaction are density could not be achieved. available. The approach is limited to polymer grafts Grafting to Well-defined end-functionalized polymers can be with defined end groups. obtained from CRPs. The surface of nanoparticles may have Clean approach, less labor intensive [33] unreacted functionality High grafting density, tuning of thickness with The stringent reaction conditions have to Grafting from molecular weight of growing chain is possible [48] be maintained. Scalability is difficult. Templated Well-defined size of nanoparticles can be obtained [58] Not cost effective Difficulty in controlling grafting density The technique is scalable and similar to conventional and molecular weights. In situ polymerizations free radical polymerization [59] Well defined structures such as block copolymers cannot be synthesized. Molecules 2021, 26, x FOR PEER REVIEW 5 of 43

2. Grafting from Approach Grafting from approach may entail the use of anionic or cationic or free radical polymerization in the functionalization of NPs. SI-anionic and cationic polymerizations are excellent routes in providing polymer grafted nanoparticles (PGNPs) with predetermined molecular weights of narrow dispersity [60–63]. However, the complexity of the experimental techniques limits their broad use [64–67]. Alternatively, initiator im- Moleculesmobilized2021, 26, 2942NPs have been subjected to free radical polymerization to yield graft5 ofNPs 44 [68,69]. Conventional free radical polymerization suffers from poor control of molecular weights, chain-end functionality, and polydispersity [31]. Therefore, surface initiated controlled radical polymerization2. Grafting from Approach techniques such as atom transfer radical polymeriza- Grafting from approach may entail the use of anionic or cationic or free radical poly- tion (ATRP), reversiblemerization addition in the− functionalizationfragmentation of NPs.chain-transfer SI-anionic and polymerization cationic polymerizations (RAFT) are and nitroxide mediatedexcellent polymerization routes in providing (NMP polymer) have grafted been nanoparticles pursued (PGNPs) for with the predetermined synthesis of well-defined PGNPs.molecular Controlled weights radical of narrow polymerization dispersity [60–63]. However, (CRP) thetechnique complexity involves of the experi- re- mental techniques limits their broad use [64–67]. Alternatively, initiator immobilized NPs versible activation–deactivationhave been subjected equilibrium to free radical between polymerization active to yield chain graft NPspropagating [68,69]. Conventional species and dormant species freewhich radical lower polymerization the rate suffersof chain from propagation poor control of than molecular that weights, of conventional chain-end functionality, and polydispersity [31]. Therefore, surface initiated controlled radical poly- free radical polymerization.merization techniquesThus, CRP such aspoly atommerization transfer radical offers polymerization a route (ATRP), to synthesize reversible PGNPs with well-definedaddition molecular−fragmentation weights chain-transfer and low polymerization dispersity. (RAFT) Figure and nitroxide3 gives mediateda general scheme for the variouspolymerization SI-CRPs methods. (NMP) have beenTypically, pursued forthe the synthesis synthesis of of well-defined PGNPs PGNPs.is based Con- on trolled radical polymerization (CRP) technique involves reversible activation–deactivation the surface modificationequilibrium of NPs, between then active anchoring/immobilization chain propagating species and dormant of initiator/chain species which lower trans- the fer agent attachmentrate on of the chain surface-modifi propagation than thated ofNPs conventional and finally free radical polymerization polymerization. Thus,using CRP the surface initiator attachedpolymerization to NPs offersto obtain a route PGNPs. to synthesize Surface PGNPs modification with well-defined of molecular NPs is often weights ac- and low dispersity. Figure3 gives a general scheme for the various SI-CRPs methods. complished with couplingTypically, agents the synthesis such ofas PGNPssilane, is basedphosphonic on the surface acids, modification dopamine, of NPs, etc. then More anchor- de- tails about the surfaceing/immobilization modification of of NPs initiator/chain with reagents transfer agent such attachment as silane on agent, the surface-modified phosphonic NPs and finally polymerization using the surface initiator attached to NPs to obtain PGNPs. acid, and dopamine canSurface be found modification in our of NPs recent is often review accomplished article with[14]. coupling The second agents such step as is silane, introducing initiator functionalityphosphonic on acids, the dopamine, surface etc. agent More modified details about NPs. the surface Alternatively, modification initiator of NPs functionality and couplingwith reagents agen sucht are as pre-reacted silane agent, phosphonic to form acid,initiator and dopamine functionalized can be found coupling in our recent review article [14]. The second step is introducing initiator functionality on the agent which is then subsequentlysurface agent modified reacted NPs. with Alternatively, NPs [70]. initiator The merits functionality and anddemerits coupling of agent various are surface initiated controlledpre-reacted radical to form polymerization initiator functionalized techniques coupling agenthave which been is presented then subsequently in Ta- ble 2. reacted with NPs [70]. The merits and demerits of various surface initiated controlled radical polymerization techniques have been presented in Table2.

Figure 3. Scheme Figurefor surface 3. Scheme initiated for surface controlled initiated controlled radical radical polymerization polymerization methods. methods.

Table 2. Comparison of advantages and disadvantages of grafting from methods.

Grafting from Methods Advantages Disadvantages Small amount of copper persists along with Control of molecular weights and polymer, its removal is difficult and affects dispersity. the properties of the final product. ATRP Variation to ATRP technique broaden the Not suitable for acidic monomers applicability of the technique to a range of Difficulty in synthesizing high molecular surface initiated polymer grafting [48] weight grafts [71] Adaptability of RAFT to a range of Because of the presence of sulfur containing RAFT polymerization conditions high degree of moiety RAFT polymers are often colored

Molecules 2021, 26, 2942 6 of 44

Table 2. Comparison of advantages and disadvantages of grafting from methods.

Grafting from Methods Advantages Disadvantages Small amount of copper persists along Control of molecular weights and dispersity. with polymer, its removal is difficult and Variation to ATRP technique broaden the affects the properties of the final product. ATRP applicability of the technique to a range of surface Not suitable for acidic monomers initiated polymer grafting [48] Difficulty in synthesizing high molecular weight grafts [71] Because of the presence of sulfur Adaptability of RAFT to a range of polymerization containing moiety RAFT polymers are conditions high degree of fidelity, ability to work RAFT often colored and have foul odor and the in the presence of oxygen, compatibility with a synthesis of RAFT agents involves broad range of functional groups [48] multiple steps [48] However, it is not applicable for most of monomers and functional groups [48] It requires high temperatures and longer NMP is one of the successfully used SI-CRP NMP time due to slow polymerization kinetics. techniques for polymer grafting [72] There are difficulties associated with synthesis and stability of nitroxide and alkoxy amine [73]

3. Atom Transfer Radical Polymerization (ATRP) Atom transfer radical polymerization (ATRP) is one of the most versatile polymerization techniques adopted towards the synthesis of PGNPs because the technique can be used under broad experimental conditions and can be adapted to synthesis of polymers with a wide range of functional groups [74,75]. The polymerization of activated vinyl monomer by ATRP process generally requires alkyl halide initiator and a transition metal complex as catalyst (e.g., CuBr/ligand). ATRP involves reversible activation—deactivation equilibrium between a metal-ligand complex and halide end-capped chain to form radical species which propagates the polymerization. Mechanistic details of ATRP can be found in the literature [71,74,76–78]. Several modiﬁcations to ATRP have been studied such as, activator regenerated by electron transfer ATRP (ARGET ATRP), reverse ATRP, UV Light mediated ATRP, and elec- trochemical mediated ATRP, etc. In ARGET ATRP a reducing agent viz., 2-ethylhexanoate or ascorbic acid or glucose is employed to regenerate the active transition metal complex via reduction of the higher oxidation state transition metal complex [79]. On the other hand, “reverse” ATRP consists of the addition of transition metal complexes in the higher oxidation state and the generation of the lower oxidation state activator by reaction with a conventional free radical initiator [76,80,81]. Initially, alkyl halide initiators are immobilized onto the NP surface. Using CuBr/ligand system, the polymerization proceeds like the classical ATRP polymerization in bulk or solution and monomers are polymerized on the surface of the NPs in a controlled manner.

3.1. SI-ATRP Polymerization to Prepare Polymer-Grafted SiO2 Nanoparticles ATRP reactions have been extensively used to grow polymer/block copolymer brushes from the surface of silica with controlled graft densities [82–84]. For example, polymer/copolymer brushes of PMMA [83,85], polystyrene (PS) [86,87], poly(glycidyl methacrylate) (PGMA) [88,89], poly(2-hydroxyethyl methacrylate) (PHEMA [90], poly(4-vinylpyridine) (PVP) [91], poly(N-isopropylacrylamide) (PNiPAAm) [92], poly(sodium 4-styrene sulfonate) (PSS) [93], poly((ethylene glycol)methyl ether methacrylate) (POEGMA) [94], poly(2- (dimethylamino)ethyl methacrylate) (PDMAEMA) [95,96], etc. have been successfully grafted on SiO2 surface via SI-ATRP. Pinto et al. [97] employed SI-ATRP for grafting of PMMA brushes thinner than 50 nm on SiO2 substrate for tunnel emitter transistor application at operating voltage below 5 V (which is an important requirement for industrial Molecules 2021, 26, 2942 7 of 44

adoption). Hwang et al. [98] employed SI-ATRP for grafting PS brushes on silica surface with controlled molecular weight (24,600–135,000 g/mol) as well as grafting density Molecules 2021, 26, x FOR PEER REVIEW 2 7 of 43 (0.34–0.54 chains/nm ). The performance of pentacene-based thin-ﬁlm transistor fabricated from PS-grafted SiO2 as a gate dielectric was evaluated as a function of polymer brush thickness viz. 12.4, 47.5 and 113.1 nm. The device fabricated from 47 nm thickness 2 of PS brush exhibited highest mobility (µFET = 0.099 cm /V·s) indicating that optimum molecularbare SiO weight2 layer polymer (µFET = brushes 0.05 cm need2/V·s). to be grownThe electrode/active from the surface oflayer dielectric interface for showed en- achievinghanced mobility best performance. which Thecould OTFTs be attributed with the PS-grafted to grafted SiO2 PSlayer influencing showed 2 times the morphology of 2 2 higherpentacene mobility by (µ FETenhancing= 0.099 cm the/V ·s)crystalline than that of stru barecture SiO2 layer [98]. (µ FETLi =and 0.05 cmcoworkers/V·s). synthesized The electrode/active layer interface showed enhanced mobility which could be attributed PMMA-g-SiO2 NPs with ~10 nm PMMA brush onto the SiO2 layer (~9 nm) via SI-ATRP. to grafted PS inﬂuencing the morphology of pentacene by enhancing the crystalline struc- PMMA brush/SiO2 bilayer dielectrics showed the lowest leakage compared to bare SiO2 ture [98]. Li and coworkers synthesized PMMA-g-SiO2 NPs with ~10 nm PMMA brush ontoand the spin SiO coated2 layer (~9PMMA/SiO nm) via SI-ATRP.2 dielectrics PMMA which brush/SiO could2 bilayer be attributed dielectrics to showed improved interfacial themorphology, lowest leakage a comparedsmaller tonumber bare SiO of2 and pinholes spin coated at the PMMA/SiO interface2 dielectrics due to whichthe close packing of could be attributed to improved interfacial morphology, a smaller number of pinholes at polymer brush (Figure 4). The surface-grafted PMMA brush (10 nm)/SiO2 (9nm) on sili- the interface due to the close packing of polymer brush (Figure4). The surface-grafted con wafer exhibited lower leakage and higher breakdown strength than that of sur- PMMA brush (10 nm)/SiO2 (9nm) on silicon wafer exhibited lower leakage and higher breakdownface-grafted strength PMMA than thatbrush of surface-grafted(20 nm) on PMMAsilicon brushwafer (20 (free nm) onof silicon9 nm wafer SiO2 layer) (Figure (free4A,B). of 9 The nm SiOauthors2 layer) attributed (Figure4A,B). the The enhancement authors attributed in thethe enhancement breakdown in strength the of PMMA breakdownbrush (10 strengthnm)/SiO of2 PMMA (9 nm) brush on silicon (10 nm)/SiO wafer2 (9 over nm) PMMA on silicon brush wafer over(20 nm) PMMA grafted on silicon brush (20 nm) grafted on silicon wafer (free of 9 nm SiO2 layer) due to the presence of wafer (free of 9 nm SiO2 layer) due to the presence of bilayer and improved interaction bilayer and improved interaction between polymer brush and SiO2 layer [99,100]. The 2 2 PMMA-betweeng-SiO polymer2 nanodielectric brush exhibited and SiO goodlayer operational [99,100]. stability, The PMMA- and goodg compatibility-SiO nanodielectric exhib- withited organic good semiconductors,operational stability, which enabled and OFETs good to compatibility work at high performance with organic and low semiconductors, voltagewhich [ 101enabled]. OFETs to work at high performance and low voltage [101].

FigureFigure 4. 4.(A )(A Leakage) Leakage characteristics characteristics and (B) and breakdown (B) breakdown electric ﬁeld electric characteristics field characteristics for different for different

dielectricsdielectrics measured measured with thewith structure the structure of Au/dielectric/Si of Au/dielectric/Si capacitor. Curve capacitor. a, SiO2 (9Curve nm); curvea, SiO b,2 (9 nm); curve b, spin-coatedspin-coated PMMA PMMA (10 nm)/SiO (10 nm)/SiO2 (9 nm);2 (9 curve nm); c, surface-graftingcurve c, surface-grafting PMMA (20 nm); PMMA curve d, (20 surface- nm); curve d, sur- graftingface-grafting PMMA PMMA (10 nm)/SiO (10 2nm)/SiO(9 nm). (2C (9) Capacitance-voltagenm). (C) Capacitance-voltage characteristics characteristics for PMMA/SiO2 for PMMA/SiO2 dielectrics.dielectrics.C-V Ccurves-V curves were measuredwere measured at an ac signal at an frequency ac signal of frequency 1 MHz. (D) Schematicof 1 MHz. diagram (D) Schematic of diagram distribution of pinhole defect in the dielectrics, indicating the reason why PMMA brush/SiO bilayer of distribution of pinhole defect in the dielectrics, indicating the reason why2 PMMA brush/SiO2 dielectrics show the lowest the leakage compared bare SiO2 and spin coated PMMA/SiO2 dielectrics. bilayer dielectrics show the lowest the leakage compared bare SiO2 and spin coated PMMA/SiO2 Reproduced with permission from Ref. [101]. dielectrics. Reproduced with permission from Ref [101].

Similar observations were also made by Li and coworkers by operating copper phthalocyanine (CuPc) transistors at an operational voltages of 2.0 V using surface-grafted ~10 nm PMMA brush on silica [70]. Additionally, it was noted that the thickness of the polymer brush on silica could be modulated based on the activity of the catalyst, the reactant concentration and reaction time. The PMMA brushes on silica showed high-quality dielectric property, including excellent insulating characteristics, large capacitance, and low charge-trapping density. Field-effect transistors with PMMA brush as the dielectric layer demonstrate excellent charge transport. Table 3 summarizes dielectric and electronic properties of transistors fabricated from surface-grafted polymer brushes.

Molecules 2021, 26, 2942 8 of 44

Similar observations were also made by Li and coworkers by operating copper phthalocyanine (CuPc) transistors at an operational voltages of 2.0 V using surface-grafted ~10 nm PMMA brush on silica [70]. Additionally, it was noted that the thickness of the polymer brush on silica could be modulated based on the activity of the catalyst, the reactant concentration and reaction time. The PMMA brushes on silica showed high-quality dielectric property, including excellent insulating characteristics, large capacitance, and low charge-trapping density. Field-effect transistors with PMMA brush as the dielectric layer demonstrate excellent charge transport. Table3 summarizes dielectric and electronic properties of transistors fabricated from surface-grafted polymer brushes.

Table 3. Summary of dielectric and electronic properties of polymer brushes grafted from SiO2 using ATRP.

Polymer Active Polymer Mean Molecular Capacitance Eb µ Diameter/Graft Semiconductor VT FET Grafted Filler Diameter Weight (nF/cm2) (MV/cm) cm2/(V·s) Density Layer PS-g-SiO (WF) 135,000 2 300 nm 113 nm Pentacene 7.5 @ 100 Hz NA −38 0.094 [98] g/mol PMMA-g-SiO 2 ~9 nm ~10 nm Pentacene NA 142 @ 1 MHz 7 −1 ~0.2 [101] PMMA-g-SiO 2 2–3 nm 10 nm CuPc NA 220 @ 1 MHz NA −0.75 0.12 [70]

3.2. SI-ATRP Polymerization to Prepare Polymer-Grafted TiO2 Nanoparticles ATRP has also been widely used to grow PMMA [102–106], PS [107–110], poly(styrene sulfonic acid) (PSSA) [111,112], poly(oxyethylene methacrylate) (POEM) [113,114], PNI- PAAm, [115,116], PHEMA [117] on the surface of TiO2. For example, Krysiak et al.; [118] performed the SI-ATRP grafting of poly(di (ethylene glycol) methyl ether methacrylate) on the surface of TiO2 (rutile) so as to yield polymer brushes with thickness of 10–15 nm (as measured by TEM) and molecular weight, Mn of ~60,000 g/mol. Similarly, Park et al. [114] utilized ATRP for the synthesis of TiO2 nanoparticles grafted with POEM and PSSA. In the first step, the -OH groups on the surface of TiO2 nanoparticles were converted to -Cl groups by the reaction of TiO2 with 2-chloropropionyl chloride (CPC) (ATRP initiator) which was used to initiate POEM and PSSA grafting on the surface of the TiO2 nanoparticles. The modified TiO2 nanoparticles showed better dispersion in alcohol than unmodified nanoparticles. X-ray diffraction (XRD) studies of polymer-grafted-TiO2 nanoparticles revealed that there was no significant change in the crystalline structure of the TiO2 nanoparticles. There are number of reports on utilization of SI-ATRP for grafting of polymer on TiO2 nanoparticles, however no significant studies have been reported on the dielectric properties of SI-ATRP polymer grafted TIO2 nanoparticles filled polymer nanocomposites.

3.3. SI-ATRP Polymerization to Prepare Polymer-Grafted BaTiO3 Nanoparticles

The initial reporting about the use of SI-ATRP approach to graft polymer on BaTiO3 nanoparticles was based on performing hydroxylation, sialylation, grafting of the anchoring group, followed by chain growth polymerization [119]. Table4 summarizes the conditions used to synthesize various polymer-grafted BaTiO3 nanoparticles. Figure5 presents the scheme for synthesis of PMMA-grafted BaTiO3 nanoparticles. This study showed that the thickness of the PMMA shell could be varied by changing the feed ratio of BaTiO3 (76% to 0%) to MMA resulting in grafted nanoparticles with dielectric constant ranging from 14.6 to 3.49 (pure PMMA). The PMMA-grafted BaTiO3 nanoparticles showed dielectric loss below 0.04, which was slightly lower than that of PMMA. MoleculesMolecules 20212021, 26, 26, ,x 2942 FOR PEER REVIEW 9 of 44 9 of 43

FigureFigure 5. 5.(A ()A Schematic) Schematic diagram diagram illustrating illustrating ATRP ATRP approach approach of growing of growing PMMA PMMA from BaTiO from3 BaTiO3 (B) (BTEM) TEM images images of of PMMA-BaTiO 3 (3a (),a PMMA3-BaTiO), PMMA3-BaTiO3 (b)3PMMA2-BaTiO (b) PMMA2-BaTiO3 (c), PMMA1-BaTiO3 (c), PMMA1-BaTiO3 3 (d). (d().C) ( CSEM) SEM of of the the cross-sectional cross-sectional imagesimages of of composite composite ﬁlms: films: PMMA1-BaTiO PMMA1-BaTiO3 (a) and3 (a) PMMA2- and PMMA2-BaTiO3 BaTiO(b). (3D(b)TGA). (D)TGA curves curves for forthe the pure pure PMMA PMMA and and PMMA-BaTiOPMMA-BaTiO3. Reproduced3. Reproduced with with permission permission from fromRef Ref.[119]. [119 ].

Likewise, You et al. [120] demonstrated an approach to tune the dimension of BaTiO Likewise, You et al. [120] demonstrated an approach to tune the dimension3 of Ba- nanoparticles and vary the polymer shell thickness using ATRP method in the absence TiO3 nanoparticles and vary the polymer shell thickness using ATRP method in the ab- of metal catalyst. Initially, the BaTiO3 nanoparticles were formed by polycondensation of precursorssence of (bariummetal catalyst. hydroxide Initially, (Ba(OH) 2the) and BaTiO titanium(IV)3 nanoparticles tetraisopropoxide were formed (Ti(OiPr) by4) polyconden- andsation HBPA) of followed precursors by calcination. (barium (Figurehydroxide6) The NPs(Ba(OH) were then2) and modiﬁed titanium(IV) by bi-functional tetraisopropoxide ligands(Ti(OiPr) (12-hydroxydodecanoic4) and HBPA) followed acid and by 2-bromophenylacetyl calcination. (Figure bromide) 6) The followed NPs bywere MMA then modified polymerizationby bi-functional using ligands white light(12-hydroxydodecanoic and photocatalyst. Using acid this and approach, 2-bromophenylacetyl the authors bromide) demonstrated that the dimensions of BaTiO nanoparticles could be adjusted based on followed by MMA polymerization using3 white light and photocatalyst. Using this ap- the molar ratio of HBPA and precursors, while the thickness of polymeric shell could be adjustedproach, based the authors upon the demonstrated duration of white that LED the irradiation. dimensions The of dielectric BaTiO properties3 nanoparticles of could be core/shelladjusted BaTiO based3/PMMA on the hybrid molar nanoparticles ratio of HBPA were foundand precursors, to depend upon while the dimensionthe thickness of poly- ofmeric BaTiO shell3 core could and the be molecularadjusted weightbased ofupon PMMA the shell.duration For of example, white theLED dielectric irradiation. The di- constantelectric of properties core/shell BaTiOof core/shell3/PMMA BaTiO hybrid3/PMMA nanoparticles hybrid with nanoparticles larger core size were (core found to de- size: ~39 nm, ε = 22.23 ± 1.09, shell thickness: 6 nm) was found to be higher than that of pend upon the dimension of BaTiO3 core and the molecular weight of PMMA shell. For smaller core size sample (core: ~17 nm, ε = 17.06 ± 0.58, shell thickness: 6 nm). This is example, the dielectric constant of core/shell BaTiO3/PMMA hybrid nanoparticles with due to the increased contribution of BaTiO3 to the overall dielectric constant with increase inlarger the core core size size of BaTiO (core3 size:and changes~39 nm, in εthe = 22.23 crystallinity ± 1.09, fromshell cubic thickness: (paramagnetic) 6 nm) was to found to be tetragonalhigher than (ferromagnetic). that of smaller Similarly, core thesize dielectric sample constant (core: ~17 of core/shell nm, ε = 17.06 BaTiO ±3 /PMMA0.58, shell thickness: hybrid6 nm). nanoparticles This is due withto the varying increased molecular contribution weight of of PMMA BaTiO shell3 to the were overall studied dielectric and constant itwith showed increase an inverse in relationshipthe core size to theof thicknessBaTiO3 ofand the changes PMMA shell. in the For example,crystallinity the from cubic dielectric(paramagnetic) constant ofto core/shell tetragonal BaTiO (ferromagnetic).3/PMMA hybrid nanoparticles Similarly, withthe smallerdielectric shell constant of thickness (shell thickness: 6 nm core size ~39 nm, ε = 22.23 ± 1.09) was found to be higher thancore/shell that of largerBaTiO shell3/PMMA thickness hybrid (shell nanoparticles thickness: 8 nm with core varying size ~39 nm,molecularε~13). This weight is of PMMA becauseshell were larger studied shell thickness and it corresponds showed an to theinverse higher relationship proportion of to PMMA the th contributionickness of the PMMA shell. For example, the dielectric constant of core/shell BaTiO3/PMMA hybrid nanoparticles with smaller shell thickness (shell thickness: 6 nm core size ~39 nm, ε = 22.23 ± 1.09) was found to be higher than that of larger shell thickness (shell thickness: 8 nm core size ~39 nm, ε~13). This is because larger shell thickness corresponds to the higher proportion of PMMA contribution to the overall dielectric constant of core-shell nanoparticles, especially given that PMMA has lower dielectric constant than that of core BaTiO3.

Molecules 2021, 26, 2942 10 of 44

Molecules 2021, 26, x FOR PEER REVIEW 10 of 43 to the overall dielectric constant of core-shell nanoparticles, especially given that PMMA has lower dielectric constant than that of core BaTiO3.

FigureFigure 6. 6.Scheme Scheme for for the the preparation preparation of core/shell of core/shell ferroelectric ferroelectric BaTiO BaTiO3/PMMA3/PMMA hybrid hybrid nanopar- nanoparticles ticlesby metal-free by metal-free ATRP ATRP process process driven driven by by visible visible li lightght based onon novelnovel hyperbranched hyperbranched aromatic aromatic pol- polyamidesyamides (HBPA) (HBPA) asas functionalfunctional matrix. matrix. Reproduced Reproduced with with permission permission from Ref.from [120 Ref]. [120].

ApartApart from from BaTiO BaTiO3 core3 size,core polymersize, polymer shell thickness, shell thickness, also the composition also the composition of polymer of pol- shellymer can shell influence can the influence dielectric propertiesthe dielectric of nanocomposites. properties Inof this nanocomposites. regard, Zhang et al.In [121 this] regard, studied core-shell structured PMMA@BaTiO3 (brush thickness, 7–12 nm) and PTFEMA@BaTiO3 Zhang et al. [121] studied core-shell structured PMMA@BaTiO3 (brush thickness, 7–12 (brush thickness, ~5 nm) nanoparticles that were synthesized by reacting (3-aminopropyl) nm) and PTFEMA@BaTiO3 (brush thickness, ~5 nm) nanoparticles that were synthesized trimethoxysilane (APTMS) and α-bromoisobutyrylbromide (BIBB) with BaTiO3 nanoparti- clesby followedreacting by (3-aminopropyl) reaction with methyl trimethoxysilane methacrylate (MMA) (APTMS) or 1,1,1-trifluoroethyl and α-bromoisobutyrylbromide methacrylate(BIBB) (TEFMA). with BaTiO At 1:13 weightnanoparticles feed ratio, followed (BaTiO by3 and reaction MMA with or TFEMA), methyl methacrylate the polymer (MMA) brushor 1,1,1-trifluoroethyl thickness for PMMA@BaTiO methacrylate3 and PTFEMA@BaTiO(TEFMA). At 1:13 was weight found feed to be ratio, 7 nm and(BaTiO3 and 4.5MMA nm, respectivelyor TFEMA), with graftingthe densitypolymer of 5.5%brush and 1.5%,thickness respectively. for MMAPMMA@BaTiO formed 3 and largerPTFEMA@BaTiO shell due to its3 was enhanced found reactivity to be 7 nm than and TFEMA. 4.5 nm, The re studyspectively of the with dielectric grafting prop- density of erties5.5% of and PMMA@BaTiO 1.5%, respectively.3 and PTFEMA@BaTiO MMA formed larger3 exhibited shellsignificant due to its improvementenhanced reactivity in than dispersity of polymer-grafted BaTiO3 nanoparticles in polyvinylidene fluoride (PVDF) TFEMA. The study of the dielectric properties of PMMA@BaTiO3 and PTFEMA@BaTiO3 matrix leading to decreased dielectric loss. Furthermore, PMMA@BaTiO3/PVDF and exhibited significant improvement in dispersity of polymer-grafted BaTiO3 nanoparticles PTFEMA@BaTiO3/PVDF nanocomposites exhibited attenuation of dielectric constant of 16.6%in polyvinylidene and 5.5% at grafted fluoride density (PVDF) of 5.5% matrix and 1.5%, leading respectively to decreased compared dielectric to controls. loss. A Further- 3 3/ comparisonmore, PMMA@BaTiO of the performance/PVDF of PTFEMA@BaTiOand PTFEMA@BaTiO3 nanoparticlesPVDF in nanocomposites PVDF matrix showed exhibited at- 90%tenuation decrease of indielectric dielectric constant loss as compared of 16.6% toand BaTiO 5.5%3/PVDF at grafted while density PMMA@BaTiO of 5.5% 3and 1.5%, nanoparticles/PVDFrespectively compared nanocomposites to controls. showed 80%A decreasecomparison in dielectric of lossthe as comparedperformance of toPTFEMA@BaTiO BaTiO3/PVDF. This3 nanoparticles could be attributed in PVDF to the matrix stronger showed interaction 90% betweendecrease PFTEMA in dielectric loss with PVDF matrix resulting in an enhancement in the interfacial polarization and stabiliza- as compared to BaTiO3/PVDF while PMMA@BaTiO3 nanoparticles/PVDF nanocompo- tion of electric field (Figure7). sites showed 80% decrease in dielectric loss as compared to BaTiO3/PVDF. This could be attributed to the stronger interaction between PFTEMA with PVDF matrix resulting in an enhancement in the interfacial polarization and stabilization of electric field (Figure 7).

Molecules 2021, 26, x FOR PEER REVIEW 11 of 43 Molecules 2021, 26, 2942 11 of 44

Molecules 2021, 26, x FOR PEER REVIEW 11 of 43

Figure 7. Fluorine-fluorine and hydrogen-fluorine interactions in the nanocomposites of PVDF and BaTiO3 grafted with PTFEMA and PMMA, respectively. Reproduced with permission from Ref [121].FigureFigure 7. Fluorine-fluorine 7. Fluorine-fluorine and hydrogen-fluorine and hydrogen-fl interactionsuorine in the nanocomposites interactions of PVDF in the and nanocomposites of PVDF and BaTiO grafted with PTFEMA and PMMA, respectively. Reproduced with permission from Ref. [121]. BaTiO3 3 grafted with PTFEMA and PMMA, respectively. Reproduced with permission from Ref Alternatively, PMMA can be grafted on BaTiO3 nanoparticles by coating of a highly polarizable[121].Alternatively, tetrameric PMMA metallophthalocyanine can be grafted on BaTiO (TMPc)3 nanoparticles as ATRP initiator by coating on of the a highly surface of BaTiOpolarizable3 nanoparticles tetrameric instead metallophthalocyanine of conventional (TMPc) ATRP asinitiator ATRP initiatorfollowed on by the polymerization surface of BaTiO3 nanoparticles instead of conventional ATRP initiator followed by polymerization of MMA (Figure 8). As control, R2-PMMA@BaTiO3 nanoparticles without3 TMPc interfa- of MMAAlternatively, (Figure8). As control, PMMA R2-PMMA@BaTiO can be graftednanoparticles on withoutBaTiO TMPc nanoparticles interfacial by coating of a highly cial layer were synthesized via phosphonate 3coupling of (R2-Br) followed by ATRP layerpolarizable were synthesized tetrameric via phosphonate metallophthalocyanine coupling of (R2-Br) followed (TMPc) by ATRP as polymer- ATRP initiator on the surface of polymerizationization of MMA. of DueMMA. to theDue high to polarizabilitythe high polarizability of the TMPc of interfacial the TMPc layer interfacial and the highlayer and BaTiO3 nanoparticles instead of conventional ATRP initiator followed by polymerization thedielectric high dielectric constant constant of TMPc [of122 TMPc,123], poly(vinylidene[122,123], poly(vinylidene fluoride-co- hexafluoropropylene) fluoride-co- hexafluoro- propylene)(PVDF-HFP)/PMMA-TMPc@BaTiOof MMA (PVDF-HFP)/PMMA-TMPc@BaTiO (Figure 8). As control,3 films exhibited R2-PMMA@BaTiO3 films higher exhibited dielectric higher constant3 nanoparticlesdielectric (26% higher constant without TMPc interfa- (26%thancial higher nanocomposite layer than werenanocomposite without synthesized TMPc), without and improvedvia TMPc), phosphonat and higher improved energye densitycoupling higher (20% energy higherof density(R2-Br) followed by ATRP than neat (PVDF-HFP)) at nanofiller filling ratios of 4.69 vol% [124]. (20%polymerization higher than neat (PVDF-HFP)) of MMA. atDue nanofiller to the filling high ratios polarizability of 4.69 vol% [124].of the TMPc interfacial layer and the high dielectric constant of TMPc [122,123], poly(vinylidene fluoride-co- hexafluoro- propylene) (PVDF-HFP)/PMMA-TMPc@BaTiO3 films exhibited higher dielectric constant (26% higher than nanocomposite without TMPc), and improved higher energy density (20% higher than neat (PVDF-HFP)) at nanofiller filling ratios of 4.69 vol% [124].

FigureFigure 8. (A 8.) Schematic(A) Schematic illustration illustration of of the the preparatio preparationn ofof three-three- and and two-phase two-phase P(VDF-HFP)/BaTiO P(VDF-HFP)/BaTiO3 nanocomposites,3 nanocomposites, respectively;respectively; (B) Chemical (B) Chemical structure structure of ofthe the TMPc-Br TMPc-Br ATRP ATRP initiator.initiator. Reproduced Reproduced with with permission permission from from Ref. [Ref124]. [124].

Xie et al. [125] synthesized a core@double-shell structured PMMA@HBP@BT nanocomposite via a two-step process as depicted in Figure 9. In the first step, the hyperbranched aromatic polyamide was grafted on the surface of BaTiO3 nanoparticles, and in the second step, the hyperbranched amine was used for grafting of PMMA shell via SI-ATRP. The thickness of the second shell was controlled by adjusting the ratio of MMA and macro initiator, BT@HBP-Br. The SEM morphology of PMMA@HBP@BT revealed improved adhesion between BT nanoparticles and polymers (HPB and PMMA, cova- Figure 8. (A) Schematic illustration of the preparation of three- and two-phase P(VDF-HFP)/BaTiO3 nanocomposites, lently attached) as compared to BT@HBP/PMMA nanocomposite. The respectively; (B) ChemicalPMMA@HBP@BT/PMMA structure of the TMPc-Br nanocomposite ATRP initiato exhibitedr. Reproduced high dielectric with constantpermission (39.3, from 10 Ref [124]. times higher than that of PMMA) as well as low dielectric loss (0.0276). The nanocompo- Xie et al. [125] synthesized a core@double-shell structured PMMA@HBP@BT nanocomposite via a two-step process as depicted in Figure 9. In the first step, the hyperbranched aromatic polyamide was grafted on the surface of BaTiO3 nanoparticles, and in the second step, the hyperbranched amine was used for grafting of PMMA shell via SI-ATRP. The thickness of the second shell was controlled by adjusting the ratio of MMA and macro initiator, BT@HBP-Br. The SEM morphology of PMMA@HBP@BT revealed improved adhesion between BT nanoparticles and polymers (HPB and PMMA, covalently attached) as compared to BT@HBP/PMMA nanocomposite. The PMMA@HBP@BT/PMMA nanocomposite exhibited high dielectric constant (39.3, 10 times higher than that of PMMA) as well as low dielectric loss (0.0276). The nanocompo-

Molecules 2021, 26, x FOR PEER REVIEW 13 of 43 Molecules 2021, 26, x FOR PEER REVIEW 13 of 43 Molecules 2021, 26, x FOR PEER REVIEW 13 of 43

single component nanocomposites (various core-shell nanoparticles were formed by changingsingle component the feed rationanocomposites of PFOMA and(various BaTiO core-shell3) were evaluated nanoparticles over a werebroad formed frequency by changing the feed ratio of PFOMA and BaTiO3) were evaluated over a broad frequency singlechanging component the feed rationanocomposites of PFOMA and(various BaTiO core-shell3) were evaluated nanoparticles over a werebroad formed frequency by fromchangingsingle 40 component Hz the to feed 30 MHz rationanocomposites at of room PFOMA temperature. and(various BaTiO Thcore-shell3)e were results evaluated revealednanoparticles over that a the werebroad dielectric formed frequency con- by changingsingle component the feed rationanocomposites of PFOMA and(various BaTiO core-shell3) were evaluated nanoparticles over a werebroad formed frequency by stantfrom 40(k) Hzincreased to 30 MHz and atdielectric room temperature. loss reduced Th significantlye results revealed with the that addition the dielectric of BaTiO con-3. changingstant (k) increased the feed ratioand dielectricof PFOMA loss and reduced BaTiO 3significantly) were evaluated with overthe addition a broad offrequency BaTiO3. fromchanging 40 Hz the to feed 30 MHz ratio at of room PFOMA temperature. and BaTiO Th3)e were results evaluated revealed over that athe broad dielectric frequency con-3 Thestant k (k) of increasedthe composite and dielectricwas up to loss 7.4 reducedat 100 kHz significantly at room temperature with the addition when theof BaTiOBaTiO33. fromThe k 40 of Hz the to composite 30 MHz atwas room up temperature.to 7.4 at 100 kHzThe resultsat room revealed temperature that the when dielectric the BaTiO con-3 3 stantThefrom k (k)40 of Hzincreasedthe to composite 30 MHz and atdielectricwas room up temperature.to loss 7.4 reducedat 100 kHzTh significantlye resultsat room revealed temperature with the that addition thewhen dielectric theof BaTiOBaTiO con-3. loadingstant (k) was increased up to 70and wt% dielectric which isloss almost reduced three significantly times greater with that the of additionpure PPFOMA of BaTiO (k =3. Thestant k (k) of theincreased composite and dielectricwas up to loss 7.4 reducedat 100 kHz significantly at room temperature with the addition when theof BaTiOBaTiO33. 2.6).loading However, was up the to 70dielectric wt% which loss (0.01) is almost of PPFOMA@BaTiO three times greater3 composite that of pure of one PPFOMA component (k = The2.6). kHowever, of the composite the dielectric was lossup to (0.01) 7.4 atof 100PPFOMA@BaTiO kHz at room temperature3 composite of when one componentthe BaTiO3 loading2.6).The kHowever, of was the upcomposite the to 70dielectric wt% was which lossup to (0.01)is 7.4 almost atof 100PPFOMA@BaTiO three kHz times at room greater temperature3 composite that of pure of when one PPFOMA componentthe BaTiO (k =3 polymer2.6).loading However, was nanocomposite up the to 70dielectric wt% for which loss 70 (0.01) wt%is almost ofloading PPFOMA@BaTiO three was times much greater 3lower composite that than of pure ofthat one PPFOMA of component the pure (k = 2.6).polymerloading However, was nanocomposite up the to dielectric70 wt% for which loss 70 (0.01) wt%is almost ofloading PPFOMA@BaTiO three was times much greater 3lower composite that than of pure ofthat one PPFOMA of component the pure (k = PPFOMA2.6). However, (0.04). the It isdielectric interesting loss to (0.01) highlight of PPFOMA@BaTiO that the nanocomposite3 composite despite of one high component loading polymer2.6).PPFOMA However, nanocomposite (0.04). the It dielectricis interesting for loss 70 to (0.01)wt% highlight loadingof PPFOMA@BaTiO that was the muchnanocomposite 3lower composite than despite ofthat one highof component the loading pure ofPPFOMApolymer nanofiller nanocomposite (0.04). exhibited It is interesting low for loss 70 even towt% highlight less loading than that thatwas the of nanocompositemuch pure lowerpolymer than despiteat 70%that highnanoparticleof the loading pure PPFOMAofpolymer nanofiller nanocomposite(0.04). exhibited It is interesting low for loss 70 evento wt% highlight less loading than that thatwas the of nanocompositemuch pure lowerpolymer than despiteat 70%that highnanoparticleof the loading pure loading.ofPPFOMA nanofiller (0.04). exhibited It is interesting low loss evento highlight less than that that the of nanocomposite pure polymer despiteat 70% highnanoparticle loading ofPPFOMA nanofiller (0.04). exhibited It is interesting low loss evento highlight less than that that the of nanocomposite pure polymer despiteat 70% highnanoparticle loading loading.The structure-property relationship study of polymer-grafted BaTiO3 nanoparticles of nanofillerThe structure-property exhibited low loss relationship even less studythan that of polymer-grafted of pure polymer BaTiO at 70%3 nanoparticles nanoparticle loading.of nanofillerThe structure-property exhibited low loss relationship even less studythan that of polymer-grafted of pure polymer BaTiO at 70%3 nanoparticles nanoparticle (synthesizedloading.The structure-property by ATRP technique) relationship filled studypolymer of polymer-graftednanocomposites BaTiO[27,119–121,124–126]3 nanoparticles (synthesizedloading.The structure-property by ATRP technique) relationship filled studypolymer of polymer-graftednanocomposites BaTiO[27,119–121,124–126]3 nanoparticles clearlyThe indicates structure-property that several relationship factors influe studynce ofthe polymer-grafted dielectric performance BaTiO3 nanoparticlesof the nano- (synthesizedclearlyThe indicates structure-property by ATRPthat several technique) relationship factors filled influe studypolymernce ofthe polymer-graftednanocomposites dielectric performance BaTiO[27,119–121,124–126]3 nanoparticlesof the nano- compositeclearly(synthesized indicates including by ATRPthat the several technique) thickness factors of filled the influe corepolymernce and the the nanocomposites dielectric shell of the performance core-shell [27,119–121,124–126] nanoparticlesof the nano- clearly(synthesizedcomposite indicates including by ATRPthat the several technique) thickness factors offilled theinflue corepolymernce and the the nanocomposites dielectric shell of theperformance core-shell [27,119–121,124–126] nanoparticlesof the nano- andcompositeclearly the indicatestype including of polymer-grafted that the several thickness factors on of the the influe nanopa corence andrticles, the the dielectric shellinterfacial of the performance separation core-shell nanoparticlesbetweenof the nano- core compositeclearlyand the indicatestype including of polymer-grafted that the several thickness factors on of the the influe nanopa corence andrticles, the the dielectric shellinterfacial of the performance separationcore-shell nanoparticlesbetweenof the nano- core NPsandcomposite theand type polymer including of polymer-grafted shell, the thicknessthe composition on of the the nanopa core of nanocompositeandrticles, the shellinterfacial of the(single separation core-shell or multicomponent nanoparticlesbetween core andNPscomposite theand type polymer including of polymer-grafted shell, the thicknessthe composition on of the the nanopa core of nanocompositeandrticles, the shellinterfacial of the(single separation core-shell or multicomponent nanoparticlesbetween core typeNPsand theandof nanocomposite),type polymer of polymer-grafted shell, the typecomposition on of the interfac nanopa ofial nanocomposite rticles,layer and interfacial double (single separation shell or coverage multicomponent between of nano- core Molecules 2021, 26, 2942 NPstypeand the andof nanocomposite),type polymer of polymer-grafted shell, the typecomposition on of the interfac nanopa ofial nanocomposite rticles,layer and interfacial double (single separation shell or coverage multicomponent between12 of 44of nano- core particles.typeNPs andof nanocomposite), polymer shell, the typecomposition of interfac ofial nanocomposite layer and double (single shell or coverage multicomponent of nano- typeparticles.NPs ofand nanocomposite), polymer shell, thethe typecomposition of interfac ofial nanocomposite layer and double (single shell or coverage multicomponent of nanoparticles.type of nanocomposite), the type of interfacial layer and double shell coverage of nano- Tableparticles.type 4. Details of nanocomposite), about various polymer-gr the type aftedof interfac nanoparticlesial layer using and ATRP.double shell coverage of nano- Tableparticles. 4. Details about various polymer-grafted nanoparticles using ATRP. Tableparticles. 4. Details about various polymer-grafted nanoparticles using ATRP. Table 4. Details about various polymer-grafted nanoparticles using ATRP. Polymer-Grafted NanomaterialTable 4. Details about various Anchoring polymer-gr Moietyafted nanoparticles Polymerization using ATRP. Conditions Ref. Polymer-Grafted NanomaterialTable 4. Details about various Anchoring polymer-gr Moietyafted nanoparticles Polymerization using ATRP. Conditions Ref. Polymer-Grafted NanomaterialTable 4. Details about various Anchoring polymer-gr Moietyafted nanoparticles Polymerization usingWhite ATRP. Light, Conditions Ref. Polymer-GraftedPolymer-Grafted Nanomaterial Nanomaterial Anchoring Anchoring Moiety Polymerization PolymerizationWhite Conditions Light, Conditions Ref. Ref. Polymer-Grafted Nanomaterial Anchoring Moiety Photocatalyst5,10-di(1-naphthyl)-5,10-d PolymerizationWhite Light, Conditions Ref. Polymer-GraftedPPMA NanomaterialBaTiO3 Anchoring Moiety Photocatalyst5,10-di(1-naphthyl)-5,10-d PolymerizationWhite Light, Conditions [120] Ref. PPMA BaTiO3 Photocatalyst5,10-di(1-naphthyl)-5,10-dWhite Light, [120] PPMA BaTiO3 ihydrophenazineWhite Light, [120] PPMA BaTiO3 Photocatalyst5,10-di(1-naphthyl)-5,10-dPhotocatalyst5,10-di(1-ihydrophenazineWhite Light, [120] ihydrophenazineDMF, RT PPMAPPMA BaTiOBaTiO3 3 Photocatalyst5,10-di(1-naphthyl)-5,10-dnaphthyl)-5,10-DMF, RT [120] [120] PPMA BaTiO3 Photocatalyst5,10-di(1-naphthyl)-5,10-dihydrophenazineDMF, RT [120] Poly(2- hydroxylPPMA ethyl BaTiO3 dihydrophenazineihydrophenazineDMF, RT [120] Poly(2- hydroxyl ethyl Poly(2- hydroxyl ethyl ihydrophenazineDMF,DMF, RT RT methacrylate)-b-poly DMF, RT Poly(2-methacrylate)-b-poly hydroxyl ethyl DMF, RT Poly(2-(methylmethacrylate)-b-poly hydroxyl methacrylate); ethyl CuBr/CuBr2, PMDETA (methylPoly(2- hydroxyl methacrylate); ethyl CuBr/CuBr2, PMDETA methacrylate)-b-poly(methylPoly(2-methacrylate)-b-poly hydroxylmethacrylate); (methyl ethyl CuBr/CuBr2, PMDETA (methylSodium methacrylate); BaTiO3 CuBr/CuBrH2O/DMF,2, PMDETA [127] methacrylate)-b-polySodium BaTiO3 CuBr/CuBrH22,O/DMF, PMDETA [127] (methylmethacrylate);methacrylate)-b-polySodium methacrylate); BaTiO3 CuBr/CuBrH2O/DMF,2, PMDETA [127] polyacrylate-b-poly(2-h(methylSodium methacrylate); BaTiOBaTiO3 3 CuBr/CuBrH2O/DMF,H602 O/DMF,°C,2, 24PMDETA h [127] [127] Sodiumpolyacrylate-b-poly(2-h polyacrylate-b- ◦ 60 °C, 24 h polyacrylate-b-poly(2-h(methylSodium methacrylate); BaTiO3 CuBr/CuBr60 C,H602 24O/DMF,°C, h2, 24PMDETA h [127] poly(2-hydroxylydroxylSodium ethyl ethyl BaTiO3 H2O/DMF, [127] polyacrylate-b-poly(2-hSodium BaTiO3 60H2 O/DMF,°C, 24 h [127] methacrylate)ydroxyl ethyl polyacrylate-b-poly(2-hmethacrylate) 60 °C, 24 h polyacrylate-b-poly(2-hmethacrylate)ydroxyl ethyl 60 °C, 24 h Poly(1H,1H,2H,2H-Poly(1H,1H,2H,2H-perydroxyl ethyl Poly(1H,1H,2H,2H-permethacrylate)ydroxyl ethyl CuBr,CuBr, PMDETA, PMDETA, DMF DMF Poly(1H,1H,2H,2H-per 3 CuBr, PMDETA, DMF perﬂuorooctylmethacrylate)fluorooctyl BaTiOBaTiO3 3 CuBr,◦ PMDETA, DMF [126] [126] Poly(1H,1H,2H,2H-permethacrylate)fluorooctyl BaTiO CuBr,70 C, 70PMDETA, 24°C, h 24 h DMF [126] methacrylate)methacrylate)fluorooctyl BaTiO3 70 °C, 24 h [126] Poly(1H,1H,2H,2H-permethacrylate) CuBr, 70PMDETA, °C, 24 h DMF Poly(1H,1H,2H,2H-permethacrylate)fluorooctyl BaTiO3 CuBr, PMDETA, DMF [126] fluorooctyl BaTiO3 CuBr, 70PMDETA, °C, 24 h DMF [126] methacrylate)fluorooctylPMMA BaTiO3 CuBr,CuBr, PMDETA, 70PMDETA, °C, DMF24 h DMF [119][126] PMMAmethacrylate)PMMA BaTiOBaTiO3 3 CuBr,◦ 6070PMDETA, °C, 24 h DMF [119] [119] methacrylate)PMMA BaTiO3 60 C,60 24°C, h 24 h [119] CuBr, 60PMDETA, °C, 24 h DMF PMMA BaTiO3 CuBr, PMDETA, DMF [119] PMMA BaTiO3 CuBr, 60PMDETA, °C, 24 h DMF [119] PMMAPMMA BaTiO3 CuBr, 60PMDETA, °C, 24 h DMF [119] Poly(Trifluoroethyl BaTiO3 CuBr, PMDETA, DMF [121] Poly(TrifluoroethylPMMA BaTiO CuBr,CuBr, PMDETA, 7060PMDETA, °C, DMF1224 h DMF [121] Poly(Triﬂuoroethylmethacrylate)Poly(Trifluoroethyl PTFEMA BaTiOBaTiO3 3 ◦ 70 °C, 12 h [121] [121] methacrylate)PMMA PTFEMA CuBr,70 C, 70PMDETA, 12°C, h 12 h DMF methacrylate)methacrylate)Poly(TrifluoroethylPMMA PTFEMA PTFEMA BaTiO3 CuBr, PMDETA, DMF [121] methacrylate) PTFEMA 3 3 2 6 Poly(TrifluoroethylPMMA BaTiO3 BaTiO3@TMPc-Br CuCl/CuClCuBr,270 ,PMDETA, Me °C,6TREN, 12 h DMF 60 °C, 24h [124][121] methacrylate)Poly(TrifluoroethylPMMA PTFEMA BaTiO3 BaTiO @TMPc-Br CuCl/CuCl 70, Me °C,TREN, 12 h 60 °C, 24h [124][121] PMMA BaTiO3 BaTiO3@TMPc-Br CuCl/CuCl2, Me6TREN, 60 °C, 24h [124] methacrylate) PTFEMA CuCl/CuClCuBr,702, Me °C,PMDETA,6 TREN,12 h methacrylate)PMMAPMMA PTFEMA BaTiOBaTiO3 3 BaTiOBaTiOBaTiO3@APS@HBP-Br3@TMPc-Br CuCl/CuCl◦2, Me6TREN, 60 °C, [24h124] [124][125] PMMA BaTiO3 BaTiO3@APS@HBP-Br 60CuBr,C, 24 PMDETA, h [125] PMMA BaTiO3 BaTiOBaTiO3@APS@HBP-Br3@TMPc-Br CuCl/CuCl260, Me °C,6TREN, 24 h 60 °C, 24h [124][125] PMMA BaTiO3 BaTiOBaTiO3@APS@HBP-Br3@TMPc-Br CuCl/CuClCuBr,260, Me °C,PMDETA,6TREN, 24 h 60 °C, 24h [125] [124] PMMA BaTiO3 BaTiO3@APS@HBP-Br CuBr,CuBr, PMDETA,60 °C,PMDETA, 24 h [125] PMMA BaTiO3 BaTiO3@APS@HBP-Br CuCl2◦, Me6TREN, Tin(II) [125] PS/PMMAPMMA BaTiO3 BaTiO3@APS@HBP-Br CuCl60CuBr,2, C,60Me 24°C, 6PMDETA,TREN, h 24 h Tin(II) [125][27] PS/PMMAPMMA BaTiO3 BaTiO3@APS@HBP-Br ethylhexanoate,CuCl2, Me60 °C,6TREN, Anisol,24 h Tin(II) 110 °C [125][27] PS/PMMA BaTiO3 ethylhexanoate,60 °C, Anisol,24 h 110 °C [27] CuClethylhexanoate,CuCl, Me2, MeTREN,6TREN, Anisol, Tin(II) Tin(II) 110 °C PS/PMMA BaTiO3 ethylhexanoate,CuCl2 2,6 Me6TREN, Anisol, Tin(II) 110 °C [27] Poly(lauryl 3 CuBr/CuBr2 6 2, HMTETA PS/PMMAPoly(laurylPS/PMMA BaTiOBaTiO3 ethylhexanoate,ethylhexanoate,CuClCuBr/CuBr, Me TREN, Anisol,2, Anisol,HMTETA Tin(II) 110 °C[27 ] [27] PS/PMMA BaTiOAl2O3 3 ethylhexanoate,◦ Anisol, 110 °C [128][27] methacrylate)Poly(lauryl Al2O3 ethylhexanoate,CuBr/CuBrToluene,110 C 1002, Anisol,HMTETA °C, 16 110h °C [128] methacrylate) Al2O3 Toluene, 100 °C, 16 h [128] methacrylate)Poly(lauryl CuBr/CuBrToluene, 1002, HMTETA °C, 16 h Poly(lauryl Al2 O3 CuBr/CuBr2, HMTETA [128] methacrylate)Poly(lauryl Al2 O3 CuBr/CuBrCuBr/CuBrToluene,2, HMTETA 1002, HMTETA °C, 16 h [128] Poly(lauryl methacrylate) methacrylate) AlAlO2 O3 Toluene, 100 °C, 16 h [128] [128] methacrylate) 2 3 Toluene,Toluene, 100 ◦ 100C, 16 °C, h 16 h

Xie et al. [125] synthesized a core@double-shell structured PMMA@HBP@BT nanocom-

posite via a two-step process as depicted in Figure9. In the ﬁrst step, the hyperbranched aromatic polyamide was grafted on the surface of BaTiO nanoparticles, and in the second 3 step, the hyperbranched amine was used for grafting of PMMA shell via SI-ATRP. The thickness of the second shell was controlled by adjusting the ratio of MMA and macro initiator, BT@HBP-Br. The SEM morphology of PMMA@HBP@BT revealed improved adhesion between BT nanoparticles and polymers (HPB and PMMA, covalently attached) as compared to BT@HBP/PMMA nanocomposite. The PMMA@HBP@BT/PMMA nanocomposite exhibited high dielectric constant (39.3, 10 times higher than that of PMMA) as well as low dielectric loss (0.0276). The nanocomposite of BT@HBP in PMMA matrix (56.7% loading) resulted in high dielectric constant of 113 while loss was increased to 0.485 (16.6 times higher than that of PMMA@HBP@BT). Thus, double core-shell structured PMMA@HBP@BT provides another approach for preparing nanocomposites with higher dielectric constant and low dielectric loss. Molecules 2021, 26, x FOR PEER REVIEW 12 of 43

Molecules 2021, 26, x FOR PEER REVIEW 12 of 43

site of BT@HBP in PMMA matrix (56.7% loading) resulted in high dielectric constant of site of BT@HBP113 while in PMMA loss was matrix increased (56.7% to 0.485loading) (16.6 resultedtimes higher in high than dielectric that of PMMA@HBP@BT constant of ). Thus, double core-shell structured PMMA@HBP@BT provides another approach for 113 while loss was increased to 0.485 (16.6 times higher than that of PMMA@HBP@BT). preparing nanocomposites with higher dielectric constant and low dielectric loss. Molecules 2021Thus,, 26, 2942 double core-shell structured PMMA@HBP@BT provides another approach13 of 44 for preparing nanocomposites with higher dielectric constant and low dielectric loss.

Figure 9. Schematic illustrating the preparation of PMMA@HBP@BT. Reproduced with permission from Ref [125].

FigureFigure 9. Schematic 9. Schematic illustratingThe illustrating attachment the preparation the of preparation of phosphonic PMMA@HBP@BT. of acid-basedPMMA@HBP@BT. Reproduced ATRP with permission initiatorReproduced from on Ref. BaTiOwith [125]. permission3 nanoparticles followed by growth of PMMA on BaTiO3 nanoparticles via activated regenerated by from Ref [125]. The attachment of phosphonic acid-based ATRP initiator on BaTiO nanoparticles electron transfer (AGRET) ATRP approach (Figure 10) was reported3 to compare and followed by growth of PMMA on BaTiO3 nanoparticles via activated regenerated by The attachmentcontrastelectron the of dielectric transfer phosphonic (AGRET) performance acid-based ATRP approach of sing ATRPle- (Figure and initiator multi-component10) was reportedon BaTiO to nanocomposites compare3 nanoparticles and [27]. A comparisoncontrast the dielectricof PMMA@BaTiO performance3 ofone single- component and multi-component nanocomposite nanocomposites and phosphonic [27]. ac- followed by growth of PMMA on BaTiO3 nanoparticles via activated regenerated by id-modifiedA comparison BaTiO of3 PMMA@BaTiOmixed PMMA3 one two component component nanocomposite nanocomposite, and phosphonic at same acid-loading of 16 electron transfer (AGRET) ATRP approach (Figure 10) was reported to compare and vol%,modiﬁed showed BaTiO that3 themixed two-component PMMA two component nanoco nanocomposite,mposite has at energy same loading density of 16 of vol%, ~1.9 J/cm3 at showed that the two-component nanocomposite has energy density of ~1.9 J/cm3 at contrast the dielectric performance of single- and multi-component nanocomposites 3[27]. 256 V/µm256 V/ whileµm while one one component component nanocompositenanocomposite has has energy energy density density of ~2 J/cm of 3~2at aJ/cm 25% at a 25% A comparison of PMMA@BaTiO3 one component nanocomposite and phosphonic ac- lowerlower field ﬁeld strength strength (220 (220 V/µm) V/µm) whichwhich implies implies a 2-fold a 2-fold enhancement enhancement in energy indensity energy density id-modified BaTiO3 mixed PMMA two component nanocomposite, at same loading of 16 due todue the to covalent the covalent attachment attachment of PMMAPMMA to to BaTiO BaTiO3 nanoparticles.3 nanoparticles. vol%, showed that the two-component nanocomposite has energy density of ~1.9 J/cm3 at 256 V/µm while one component nanocomposite has energy density of ~2 J/cm3 at a 25% lower field strength (220 V/µm) which implies a 2-fold enhancement in energy density due to the covalent attachment of PMMA to BaTiO3 nanoparticles.

Figure 10.Figure Surface-initiated 10. Surface-initiated polymerization polymerization of ofstyrene styrene or methylmethyl methacrylate methacrylate via ARGET via ARGET ATRP. Reproduced ATRP. Reproduced with permis- with permission fromsion Ref from [27]. Ref. [27].

Zhang et al. [126] synthesized core-shell structured poly(1H,1H,2H,2H-perfluorooctyl methacrylate) (PPFOMA )@BaTiO 3 nanoparticles via SI-ATRP and was used for the formulation of single component nanocomposite. One of Figure 10. Surface-initiated polymerization of styrene or methyl methacrylate via ARGET ATRP. Reproduced with per- the distinct advantages of single component nanocomposite is the ability to load high % mission from Ref [27]. of ceramic nanofiller with minimal effect on dispersibility. The dielectric properties of Zhang et al. [126] synthesized core-shell structured poly(1H,1H,2H,2H-perfluorooctyl methacrylate) (PPFOMA )@BaTiO3 nanoparticles via SI-ATRP and was used for the formulation of single component nanocomposite. One of the distinct advantages of single component nanocomposite is the ability to load high % of ceramic nanofiller with minimal effect on dispersibility. The dielectric properties of

Molecules 2021, 26, 2942 14 of 44

Zhang et al. [126] synthesized core-shell structured poly(1H,1H,2H,2H-perfluorooctyl methacrylate) (PPFOMA )@BaTiO3 nanoparticles via SI-ATRP and was used for the formulation of single component nanocomposite. One of the distinct advantages of single component nanocomposite is the ability to load high % of ceramic nanofiller with minimal effect on dispersibility. The dielectric properties of single component nanocomposites (various core-shell nanoparticles were formed by changing the feed ratio of PFOMA and BaTiO3) were evaluated over a broad frequency from 40 Hz to 30 MHz at room temperature. The results revealed that the dielectric constant (k) increased and dielectric loss reduced significantly with the addition of BaTiO3. The k of the composite was up to 7.4 at 100 kHz at room temperature when the BaTiO3 loading was up to 70 wt% which is almost three times greater that of pure PPFOMA (k = 2.6). However, the dielectric loss (0.01) of PPFOMA@BaTiO3 composite of one component polymer nanocomposite for 70 wt% loading was much lower than that of the pure PPFOMA (0.04). It is interesting to highlight that the nanocomposite despite high loading of nanofiller exhibited low loss even less than that of pure polymer at 70% nanoparticle loading. The structure-property relationship study of polymer-grafted BaTiO3 nanoparticles (synthesized by ATRP technique) filled polymer nanocomposites [27,119–121,124–126] clearly indicates that several factors influence the dielectric performance of the nanocomposite including the thickness of the core and the shell of the core-shell nanoparticles and the type of polymer-grafted on the nanoparticles, interfacial separation between core NPs and polymer shell, the composition of nanocomposite (single or multicomponent type of nanocomposite), the type of interfacial layer and double shell coverage of nanoparticles.

3.4. SI-ATRP Polymerization to Prepare Polymer-Grafted Al2O3 Nanoparticles Sanchez et al., [128] reported the modification of aluminum oxide nanoparticles by poly(lauryl methacrylate) (PLMA) using surface-initiated ATRP (SI-ATRP) technique. The molecular weight of grafted polymer ranged between 23,000 and 83,000 g/mol. PLMA- grafted nanoparticles filled LDPE matrix resulted in lower dielectric loss-tangent (~0.0008 to ~0.0003 with 1 wt% at 100 Hz) compared to LDPE filled with bare Al2O3. This may be a result of the enhanced adhesion between LDPE and the lauryl chains of the grafted polymer on the nanoparticles. Table5 summarizes of dielectric properties of polymer nanocomposites fabricated from polymer brushes-grafted ceramic nanoparticles obtained using different grafting techniques. Molecules 2021, 26, 2942 15 of 44

Table 5. Summary of dielectric properties of polymer nanocomposites fabricated from polymer brushes-grafted ceramic nanoparticles.

Shell Thickness Grafting Energy Density U Polymer@ﬁller Mean Diameter % Loading Matrix εr tan δ E (kV/mm) (nm) Approach b (J/cm3)

PS@BaTiO3 [129] ~7 nm NA 22% v/v PS Grafting to 5.8 NA 143 NA

PTFMPCS@BaTiO3 [130] 100 nm 11 nm 5 vol% PVDF-TrFE-CTFE SI-RAFT ~58 NA 459 36.6 @514 kV/mm

PVDF@BaTiO3 [131] ~100 nm NA 30 vol% PVDF Grafting to 27.9 0.08872 117.3 NA

PS@Al2O3 [132] 50 nm 0.13 25 wt% PS Grafting to 2.63 NA NA NA

PS@Al2O3 [132] 50 nm 0.13 25 wt% PMMA Grafting to 3.19 NA NA NA

PS@BaTiO3 [131] ~100 nm NA 30 vol% PVDF Grafting to 23.6 0.0866 107 NA

P(VDF-HFP)@BaTiO3 [133] 100 nm NA 50 vol% NA Grafting to 34.8 0.128 20 MV/m 0.3 @20 MV/m

PGMA@BaTiO3 [134] <100 nm ~20 nm NA PGMA SI-ATRP 54 0.039 ~3 MV/m ~21.51 @3 MV/m

PHEMA@PMMA @BaTiO3 [127] 100 nm 10 nm 38 vol% NA SI-ATRP NA ~0.025 NA ~0.061 @70 kV/cm

PANa@PHEMA@BaTiO3 [127] 100 nm 10 nm 21 vol% NA SI-ATRP NA ~0.022 NA ~0.09 @70 kV/cm

PMMA@ BaTiO3 [27] 50 nm NA 22 vol% NA SI-ATRP 11.4 NA 218 3 @~220 V/µm

PTTEMA@BaTiO3 [135] ~50 nm 14–15 nm 20 vol% PTTEMA SI-RAFT ∼20 <0.02 ~220 ~3.4 @210 V/µm

PMMA@BaTiO3 [119] 100 nm 10 nm 76 wt% NA SI-ATRP 14.6 0.0372 NA NA

PMMA@BaTiO3 [121] ~200 nm 7 nm 80 wt% PVDF SI-ATRP ~28.5 0.025 @100 kHz NA NA

PTFEMA@BaTiO3 [121] ~200 nm 4.5 nm 80 wt% PVDF SI-ATRP ~35 0.022 NA NA

PPFOMA@BaTiO3 [126] 30–50 nm 5 nm 70.70 wt% NA SI-ATRP 7.4 0.01 NA NA

PMMA@TiO2 [136] 50 to 100 nm 5 nm 1 vol% PVDF-HFP In situ 10.5 <0.04 560 14.2 @500 V/µm

PS@TiO2 [137] 40–50 nm NA 27 wt% NA Grafting to 6.4 0.04 NA NA

PS@TiO2 [129] 18 nm NA 39% v/v NA Grafting to 12.8 0.1 114 NA

PS@TiO2 [138] 25–30 nm NA 36.9 vol% PS SI-RAFT ~65 ~0.03 NA NA

PS@Al2O3 [139] 50–200 nm 0.12 30 wt% iso-Al NPs@PS Grafting to 9.50 0.01 175 1.70

PEB@Al2O3 [140] 100 nm 2–5 nm 25.0 vol% PP Grafting to 5.7 NA 37.5 NA

HBP@Al2O3 [141] 30 nm NA 20 wt% Epoxy In situ 5.0 <0.025 32.83 NA

PP@Al2O3 [142] 140 nm NA 10.4 vol% NA In situ 10.5 0.24 120 14.4 @120 V/µm Molecules 2021, 26, 2942 16 of 44

4. Reversible Addition−Fragmentation Chain-Transfer Polymerization (RAFT) Living free-radical polymerization by reversible addition−fragmentation chain transfer (RAFT), is one of the most versatile and powerful technique for controlled radical polymerization which was invented in 1998 by Moad and co-workers [143] RAFT polymerization involves a degenerative chain transfer method to control polymerization [144] unlike ATRP and NMP which has a persistent radical in the system [76,145–147]. The control in RAFT polymerization is derived from the chain transfer agent (CTA) and the details about RAFT mechanism can be found in the following references [144,148–150]. In comparison to other CRPs, RAFT polymerization has number of advantages such as being adaptable to almost all free radical polymerizable monomers, the ability to synthesize multi-block copolymers with a high degree of ﬁdelity, the ability to work in the presence of oxygen, no need of inorganic catalysts and mild polymerization conditions, similar to that of conventional free radical polymerization [151–156]. SI-RAFT has been widely used for the preparation of polymer-grafted nanoparticles by attaching CTA functionality to the surface of nanoparticles. In SI-RAFT polymerization, the attachment of the CTA moiety to the NP surface could be done via “Z” group or “R” group. If the NP is attached to the “Z” group of the CTA then growing polymer chains will detach propagate, and then reattach to the NP surface, just like a “graft to” approach [157,158]. Thus, “Z” group attachment of CTA lead to decreased graft density because of the bulky nature of the polymer chains being grafted to nanoparticles using graft to approach. However, if the NP is attached to the “R” group of the CTA then the monomer gets sequentially added to the propagating polymer radicals present on the NP surface. This approach is the preferred pathway to synthesize core-shell NPs using SI-RAFT. Table6 summarizes some examples of the anchored CTA structures and polymerization conditions used to synthesize various polymer grafted nanoparticles via SI-RAFT.

4.1. SI-RAFT Polymerization to Prepare Polymer-Grafted SiO2 Nanoparticles A variety of polymers such as PMMA, PS, PNiPAAm, PAA, PHEMA, P4VP, polyisoprene have been grown from the surface of silica nanoparticles through ”grafting from” approach via SI-RAFT polymerization [48,159–165]. For example, the amino-functionalized SiO2 (SiO2–NH2) nanoparticles served as the precursor for RAFT polymerization and were synthesized by reacting amino propyl triethoxysilane (APTES) with the bare SiO2 nanoparticles. Subsequently, the RAFT-CTA viz, 4-cyano-4-(dodecylsulfanylthiocarbonyl) sulfanyl pentanoic acid (CDP) agent was immobilized on the surface of SiO2–NH2 nanoparticles by amide forming reaction. The CDP immobilized SiO2 nanoparticles were then used in the surface-initiated RAFT polymerization of HEMA with AIBN as the free radical initiator, to form PHEMA-g-SiO2 nanoparticles [164]. There are several other examples of immobilization of RAFT-CTA on the surface of SiO2 nanoparticles. For example, dopamine is reacted with silanized nanoparticles followed by dicyclohexyl carbodiimide (DCC) coupling, [160] or silanization of nanoparticles with modiﬁed RAFT-CTA agent where the RAFT-CTA agent was precoupled with silane agent [166–168], or silanization of nanoparticles with chloro functionality so as to eventually react with sodium/potassium ethyl xanthate to form xanthate [162,163]. Molecules 2021, 26, x FOR PEER REVIEW 16 of 43 Molecules 2021, 26, x FOR PEER REVIEW 16 of 43 MoleculesMolecules 2021Molecules, 26 20212021, x 2021 FOR,, 2626,,, 26PEERxx FORFOR, x FOR REVIEW PEERPEER PEER REVIEWREVIEW REVIEW 16 of 431616 16ofof of4343 43 Molecules 2021, 26, x FOR PEER REVIEW 16 of 43 Molecules 2021, 26, x FOR PEER REVIEW 16 of 43 Molecules 2021, 26, x FOR PEER REVIEW 16 of 43 Molecules 2021, 26, x FOR PEER REVIEW 16 of 43

no-functionalized SiO2 (SiO2–NH2) nanoparticles served as the precursor for RAFT no-functionalized SiO2 (SiO2–NH2) nanoparticles served as the precursor for RAFT no-functionalizedno-functionalized SiO2 (SiOSiO222 –NH(SiO(SiO2 22)–NH 2nanoparticles22)) 2nanoparticlesnanoparticles served servedserved as the asas precursor thethe precursorprecursor for RAFT forfor RAFTRAFT no-functionalizedpolymerizationno-functionalized and SiO wereSiO2 (SiO synthesized(SiO2–NH–NH2) nanoparticles) bynanoparticles reacting amino served served propyl as asthe triethoxysilanethe precursor precursor for (APTES)for RAFT RAFT polymerizationno-functionalized and SiO were2 (SiO synthesized2–NH2) nanoparticles by reacting amino served propyl as the triethoxysilane precursor for (APTES) RAFT polymerizationpolymerizationwithno-functionalizedpolymerization the and barewere and and SiO synthesizedwereSiO 2were (SiO 2synthesized synthesized2nanoparticles.–NH by2 )reacting nanoparticles by by reacting reactingaminoSubsequently, amino propyl servedamino propyl triethoxysilane propylas the thetriethoxysilane triethoxysilane precursorRAFT-CTA (APTES) for (APTES) (APTES)RAFT viz, polymerizationwithno-functionalized the bare and SiO wereSiO2 (SiO 2synthesized 2nanoparticles.–NH2) nanoparticles by reacting Subsequently, amino served propyl as the thetriethoxysilane precursorRAFT-CTA for (APTES) RAFTviz, with withthe thebare bareSiO 2 SiOnanoparticles.22 2 nanoparticles.nanoparticles. Subsequently, Subsequently,Subsequently, the thetheRAFT-CTA RAFT-CTARAFT-CTA viz, viz,viz, with4-cyano-4-(dodecylsulfanylthiocarbonyl)polymerizationwith thethe bare andbare were SiOSiO2 synthesized nanoparticles.nanoparticles. by sulfanylreacting Subsequently,Subsequently, aminopentanoic propyl acid the triethoxysilane the(CDP) RAFT-CTARAFT-CTA agent (APTES)was viz,im-viz, 4-cyano-4-(dodecylsulfanylthiocarbonyl)withpolymerization the bare and wereSiO 2synthesized nanoparticles. by sulfanylreacting Subsequently, aminopentanoic propyl acidthe triethoxysilane (CDP) RAFT-CTA agent (APTES)was viz,im- 4-cyano-4-(dodecylsulfanylthiocarbonyl)4-cyano-4-(dodecylsulfanylthiocarbonyl)mobilizedwith4-cyano-4-(dodecylsulfanylthiocarbonyl) the on thebare surface SiO of2 SiOnanoparticles.2–NH sulfanyl2 nanoparticles sulfanyl sulfanylpentanoicSubsequently, pentanoic by pentanoic amideacid (CDP) acidforming acidthe (CDP) agent(CDP) reaction.RAFT-CTA agentwas agent im-Thewas was CDP viz,im- im- 4-cyano-4-(dodecylsulfanylthiocarbonyl)mobilizedwith the on thebare surface SiO of2 SiOnanoparticles.2–NH2 nanoparticles sulfanyl Subsequently, pentanoic by amide acidformingthe (CDP) RAFT-CTAreaction. agent Thewas CDP viz,im- mobilizedmobilized on the on surface the surface of SiO of2–NH SiO22 –NHnanoparticles22 nanoparticles by amide by amide forming forming reaction. reaction. The CDP The CDP mobilizedimmobilized4-cyano-4-(dodecylsulfanylthiocarbonyl)mobilized on on SiOthe the 2surface nanoparticles surface of ofSiO SiO2 –NHwere2–NH2 thennanoparticlesnanoparticles2 nanoparticles sulfanyl used in pentanoicthe byby by amideamidesurface-initiated amide acidformingforming forming (CDP) reaction.reaction. RAFT reaction. agent polymeri- TheThewas The CDPCDP im- CDP mobilizedimmobilized4-cyano-4-(dodecylsulfanylthiocarbonyl) on SiOthe 2surface nanoparticles of SiO2 –NHwere2 thennanoparticles sulfanyl used in pentanoicthe by amidesurface-initiated acidforming (CDP) reaction. RAFT agent polymeri- Thewas CDP im- immobilizedimmobilized SiO2 nanoparticles SiO22 nanoparticles were werethen usedthen inused the in surface-initiated the surface-initiated RAFT RAFT polymeri- polymeri- immobilizedzationmobilizedimmobilizedimmobilized of HEMA on SiOSiOthe SiO 2surface withnanoparticles2 nanoparticles AIBN of SiO as2 –NHwerethe were free2 thennanoparticles then radical used used ininitiator, inthe by the surface-initiatedamide surface-initiatedto form forming PHEMA- reaction. RAFT RAFTg-SiO polymeri- The polymeri-2 nano- CDP immobilizedzationmobilized of HEMA on SiOthe 2surface withnanoparticles AIBN of SiO as2 –NHwerethe free2 thennanoparticles radical used ininitiator, the by amidesurface-initiated to form forming PHEMA- reaction. RAFTg-SiO polymeri- The2 nano- CDP zationzation of HEMA of HEMA with AIBNwith AIBN as the as free the radical free radical initiator, initiator, to form to formPHEMA- PHEMA-g-SiO2g -SiOnano-22 nano- zationparticlesimmobilizedzation of [164].ofHEMA HEMA SiO 2with nanoparticles with AIBN AIBN as aswerethe the free then free radical usedradical ininitiator, initiator,the surface-initiated to toform form PHEMA- PHEMA- RAFTg-SiO gpolymeri--SiO2 nano-nano-2 nanoparticles [164]. 2 particlesparticleszationimmobilized [164]. of [164]. HEMA SiO 2with nanoparticles AIBN as werethe free then radical used ininitiator, the surface-initiated to form PHEMA- RAFTg-SiO polymeri- nano- particleszationparticlesThere of [164]. HEMA are[164]. several with otherAIBN examples as the free of immobradical ilizationinitiator, of to RAFT-CTA form PHEMA- on theg-SiO surface2 nano- of particleszationThere of [164]. HEMA are several with otherAIBN examples as the free of immobradical ilizationinitiator, of to RAFT-CTA form PHEMA- on theg-SiO surface2 nano- of ThereSiOparticles2 Therenanoparticles. areThere [164].several are are several otherseveral For otherexamples example, other examples examples of dopamine immob of ofimmobilization immob is reactedilizationilizationilization of RAFT-CTA with ofof ofRAFT-CTARAFT-CTA silanized RAFT-CTA on the nanoparticles onon surface on thethe the surfacesurface ofsurface fol- ofof of SiOparticles2 Therenanoparticles. [164]. are several For other example, examples dopamine of immob is reactedilization with of RAFT-CTA silanized nanoparticles on the surface fol- of SiO2 nanoparticles.SiO22 nanoparticles.2 For example, For example, dopamine dopamine is reacted is reactedreacted with silanizedwithwith silanizedsilanized nanoparticles nanoparticlesnanoparticles fol- fol-fol- SiOlowedSiO2 nanoparticles.There bynanoparticles. dicyclohexyl are several For For other carbodiimideexample, example, examples dopamine dopamine ( DCC)of immob coupling,is isreactedilization reacted [160]with ofwith RAFT-CTA orsilanized sisilanizedlanization nanoparticles on nanoparticles theof nanoparti- surface fol- of fol- lowedSiO2 nanoparticles.There by dicyclohexyl are several For other carbodiimideexample, examples dopamine ( DCC)of immob coupling,is reactedilization [160] with of RAFT-CTA orsilanized silanization nanoparticles on theof nanoparti- surface fol- of lowedlowedclesSiOlowed bylowed 2 dicyclohexylwithnanoparticles. byby by modifieddicyclohexyl dicyclohexyl carbodiimide RAFT-CTAFor carbodiimideexample, carbodiimide agent(DCC) dopamine (where DCC)coupling, (DCC) coupling,theis coupling,reacted RAFT-CTA[160] or[160] with [160]silanization oragentsilanized orsilanization lanizationsi lanizationwas of nanoparticles precouplednanoparti- ofof ofnanoparti-nanoparti- nanoparti- with fol- Molecules 2021, 26, 2942 lowedclesSiO 2 withnanoparticles. by modifieddicyclohexyl RAFT-CTAFor carbodiimideexample, agent dopamine (whereDCC) coupling,isthe reacted RAFT-CTA [160] with oragentsilanized silanization was nanoparticles precoupled17 of of 44nanoparti- with fol- cles withclessilanelowedcles modifiedwith agentwithby modifieddicyclohexyl modified[166–168], RAFT-CTA RAFT-CTA RAFT-CTA orcarbodiimide agentsilanization agentwhere agent ( whereDCC)of the wherenano RAFT-CTA coupling,thetheparticles the RAFT-CTARAFT-CTA RAFT-CTA [160]withagent orchloro agentagentwas siagentlanization precoupled waswasfunctionality was precoupledprecoupled precoupled of nanoparti-with so aswithwith withto Molecules 2021, 26, x FOR PEER REVIEWclessilanelowed with agentby modifieddicyclohexyl [166–168], RAFT-CTA orcarbodiimide silanization agent ( whereDCC)of nano coupling,theparticles RAFT-CTA [160]with orchloroagent silanization wasfunctionality precoupled of nanoparti- so 8aswith of to8 silanesilaneeventually clesagentsilane with agent[166–168], agent modified react [166–168], [166–168], with or RAFT-CTA silanization sodium/potassium or orsilanization silanization agent of nano whereof particlesofethynano nano thel particlesxanthate particles RAFT-CTAwith chloro withto with form chloroagent functionality chloro xanthate wasfunctionality functionality precoupled [162,163]. so as soto so aswith asto to silaneeventuallycles with agent modified react [166–168], with RAFT-CTA sodium/potassium or silanization agent whereof ethynano thel particlesxanthate RAFT-CTA withto form chloroagent xanthate wasfunctionality precoupled [162,163]. so aswith to eventuallyeventuallysilaneeventually react agent withreact [166–168], react sodium/potassiumwith with sodium/potassium or sodium/potassium silanization ethy ofl xanthateethynano ethyll particlesxanthatexanthatel xanthate to form withtoto xanthatetoformform formchloro xanthatexanthate xanthate [162,163].functionality [162,163].[162,163]. [162,163]. so as to eventuallysilane agent react [166–168], with sodium/potassium or silanization of ethynanol particlesxanthate withto form chloro xanthate functionality [162,163]. so as to Table 6. DetailsTable about6. Details various about polymer various grafted polymer nanoparticles grafted nanoparticles using SI-RAFT. using SI-RAFT. Tableeventually 6. Details about react various with sodium/potassium polymer grafted nanoparticles ethyl xanthate using toSI-RAFT. form xanthate [162,163]. Table Table6. Detailseventually 6. Details about variousabout react various withpolymer sodium/potassium polymer grafted graf nanoparticlested nanoparticles ethy usingl xanthate SI-RAFT.using toSI-RAFT. form xanthate [162,163]. TableTable 6. Details6. Details about about various various polymer polymer graf graftedtedted nanoparticlesnanoparticles nanoparticles usingusing using SI-RAFT.SI-RAFT. SI-RAFT. Table 6. Details about various polymer grafted nanoparticlesPolymerizationPolymerization using SI-RAFT. GraftedGrafted Polymer Polymer Table Nanoparticle 6. Details about various Anchoring Anchoring polymer graf CTACTAted nanoparticlesPolymerization using SI-RAFT. Ref. Ref. Grafted Polymer Nanoparticle Anchoring CTA PolymerizationPolymerization Ref. Grafted Polymer TableTable Nanoparticle 6.6. DetailsDetails aboutabout variousvarious Anchoring polymerpolymer graf grafCTAtedted nanoparticlesnanoparticlesConditionsPolymerization usingusingConditionsPolymerization SI-RAFT.SI-RAFT. Ref. GraftedGrafted GraftedPolymer Polymer Polymer Nanoparticle Nanoparticle Nanoparticle Anchoring Anchoring Anchoring CTA CTA CTA PolymerizationConditions Ref. Ref.Ref. Grafted Polymer Nanoparticle Anchoring CTA ConditionsPolymerizationConditionsConditions Ref. GraftedPoly(vinylidene Polymer Nanoparticle Anchoring CTA PolymerizationPolymerizationTBPPi,Conditions DMC Ref. GraftedGraftedPoly(vinylidene PolymerPolymer Nanoparticle NanoparticleBaTiO3 Anchoring Anchoring CTACTA TBPPi,TBPPi,Conditions DMC DMC [169]Ref.Ref. Poly(vinylidenePoly(vinylidenePoly(vinylidenePoly(vinylidenefluoride) ﬂuoride) BaTiOBaTiO3 3 TBPPi,TBPPi,Conditions◦Conditions 65DMCTBPPi, °C, DMC15 DMC h [169] [169] Poly(vinylidenefluoride) BaTiOBaTiO3 BaTiO33 3 65TBPPi,65C, °C, 15 h DMC15 h [169] [169][169][169] fluoride) BaTiO3 65 °C, 15 h [169] fluoride)Poly(vinylidenefluoride)fluoride) BaTiO3 65 °C,TBPPi,65 15 65°C, h °C, DMC15 15h h [169] Poly(vinylidenefluoride) BaTiO3 TBPPi,65 °C, DMC15 h [169] Poly(vinylidene 3 TBPPi, DMC fluoride) BaTiOBaTiO3 AIBN,65 °C, THF15 h [169][169] fluoride) BaTiO3 AIBN,65 °C, THF15 h [170] fluoride) BaTiO3 AIBN,AIBN,AIBN,6580 THFAIBN, °C, THF 15THF THFh [170] BaTiOBaTiOBaTiO3 33 AIBN,80 °C, 15THF h [170[170]] [170] BaTiOBaTiO3 3 3 80 ◦AIBN,80C, °C, 15 h 15THF h [170][170] BaTiO3 80 °C,AIBN,80 15 80°C, h °C, 15THF 15h h [170] Poly{2,5-bis[(4-methox BaTiO3 AIBN,80 °C, 15THF h [170] Poly{2,5-bis[(4-methox 3 AIBN, THF BaTiO3 AIBN,80 °C, THF15 h [170] Poly{2,5-bis[(4-methoxPoly{2,5-bis[(4-methoxPoly{2,5-bis[(4-methox BaTiO3 AIBN, THF [170] Poly{2,5-bis[(4-yphenyl)oxycarbonyl]sPoly{2,5-bis[(4-methox BaTiO3 80 °C, 15 h [171] yphenyl)oxycarbonyl]sPoly{2,5-bis[(4-methox BaTiO3 AIBN,AIBN,AIBN,80 80THFAIBN, °C, THF°C, 15THF 6 hTHFh [171] yphenyl)oxycarbonyl]syphenyl)oxycarbonyl]sPoly{2,5-bis[(4-methox BaTiOBaTiO3 33 3 80 °C, 6 h [171] [171][171] methoxyphenyl)oxycarbonyl]styrenes}yphenyl)oxycarbonyl]syphenyl)oxycarbonyl]styrenes} (PMPCS) BaTiOBaTiO3 3 AIBN,◦80 °C, THF6 h [171] [171][171] yphenyl)oxycarbonyl]sPoly{2,5-bis[(4-methoxPoly{2,5-bis[(4-methoxtyrenes} (PMPCS) BaTiO3 80 80°C,AIBN,80 C,6 80h 6°C, h °C, THF6 h 6 h [171] tyrenes}yphenyl)oxycarbonyl]s(PMPCS)tyrenes}tyrenes} tyrenes}(PMPCS) (PMPCS)(PMPCS) (PMPCS) BaTiO3 AIBN,80 °C, THF6 h [171] tyrenes} (PMPCS) 3 AIBN, DMFTHF yphenyl)oxycarbonyl]styrenes} (PMPCS) BaTiO3 AIBN,80 °C, DMF 6 h [171] yphenyl)oxycarbonyl]styrenes}Polystyrene (PMPCS) BaTiO3 AIBN,80 °C, DMF 6 h [171][172] Polystyrene BaTiO3 AIBN,AIBN, 80DMFAIBN, °C, DMF 6 hDMF [172] tyrenes} (PMPCS) 3 AIBN,AIBN,80 DMF°C, DMF12 h Polystyrenetyrenes}PolystyrenePolystyrene (PMPCS) BaTiOBaTiO3 BaTiO3 3 AIBN,80 °C, DMF12 h [172] [172][172][172] PolystyrenePolystyrene BaTiOBaTiO3 3 80 °C,◦80 12 °C, h 12 h [172] [172] Polystyrene BaTiO3 80 AIBN,80C, 80°C, 12 °C, h DMF12 12h h [172] Poly(2-(2,2Polystyrene′:5′,2″-terthie BaTiO3 AIBN,80 °C, DMF12 h [172] Poly(2-(2,2′:5′,2″-terthie 3 AIBN, DMF Poly(2-(2,2Poly(2-(2,2′:5Polystyrene′,2″-terthie′:5′,2″-terthie BaTiO3 80 °C, 12 h [172] Poly(2-(2,2Poly(2-(2,20 0Polystyrenen-5-yl)ethyl′′:5:5′′,2,2:5″′,2-terthie ″-terthie BaTiO AIBN,80 °C, Dioxane 12 h [172] Poly(2-(2,2Poly(2-(2,2:5 n-5-yl)ethyl,2”-terthien-5-′:5′,2″-terthie BaTiO3 AIBN,80 °C, Dioxane 12 h [173,174] n-5-yl)ethylPoly(2-(2,2methacrylate)n-5-yl)ethyln-5-yl)ethyl′:5 ′,2″-terthie BaTiO3 AIBN,AIBN,AIBN, DioxaneAIBN,90 Dioxane °C, Dioxane Dioxane3 h [173,174] yl)ethylPoly(2-(2,2 methacrylate)methacrylate)n-5-yl)ethyl′:5′,2″-terthie BaTiOBaTiOBaTiO3 BaTiO333 3 AIBN,◦90 °C, Dioxane 3 h [173[173,174,174][173[173] [173 ,,174],174] methacrylate)Poly(2-(2,2methacrylate)′:5′ ,2″-terthie BaTiO3 90 90°C,90 C,3 h°C, 3 h 3 h [173,174] (PTTEMA)methacrylate)n-5-yl)ethyl(PTTEMA)methacrylate) BaTiO3 AIBN,90 90°C, Dioxane °C, 3 h 3 h [173,174] methacrylate)n-5-yl)ethyln-5-yl)ethyl(PTTEMA) BaTiO3 AIBN,AIBN,90 °C, DioxaneDioxane 3 h [173,174] Poly(1H,1H,2H,2H-hep(PTTEMA)methacrylate)(PTTEMA)(PTTEMA)(PTTEMA) BaTiO3 90 °C, 3 h [173,174] Poly(1H,1H,2H,2H-Poly(1H,1H,2H,2H-hepmethacrylate)(PTTEMA) BaTiO3 90 °C, 3 h [173,174] Poly(1H,1H,2H,2H-hepPoly(1H,1H,2H,2H-hepmethacrylate) AIBN,AIBN,90 DMF°C, DMF 3 h heptadecaﬂuorodecylPoly(1H,1H,2H,2H-hepPoly(1H,1H,2H,2H-heptadecafluorodecyl(PTTEMA) acrylate) BaTiOBaTiO3 3 AIBN,◦ DMF [175] [175] Poly(1H,1H,2H,2H-heptadecafluorodecyl(PTTEMA)(PTTEMA) BaTiO3 AIBN,60AIBN, DMF60C,AIBN, 6°C, h DMF 6 DMFh [175] tadecafluorodecylPoly(1H,1H,2H,2H-hep(PHFDA)tadecafluorodecyltadecafluorodecyltadecafluorodecyl BaTiOBaTiO3 BaTiO33 3 AIBN,60 °C, DMF 6 h [175] [175][175][175] acrylate)tadecafluorodecyl (PHFDA) BaTiO3 60 °C, 6 h [175] Poly(1H,1H,2H,2H-hepPoly(1H,1H,2H,2H-hepacrylate)tadecafluorodecyl (PHFDA) BaTiO3 60 °C,AIBN,60 6 60h°C, °C,DMF 6 h 6 h [175] acrylate)acrylate)tadecafluorodecylacrylate) (PHFDA) (PHFDA) (PHFDA) BaTiO3 AIBN,60 °C, DMF 6 h [175] acrylate)tadecafluorodecyl (PHFDA) BaTiO3 AIBN,AIBN,60 DMF, °C, DMF,DMF 6 h [175] Polystyrenetadecafluorodecyl BaTiO 33 AIBN, DMF, [176] [175] acrylate)Polystyrene (PHFDA) BaTiO3 3 AIBN,60◦ °C, DMF, 6 h [176] acrylate)Polystyrene (PHFDA) BaTiO3 AIBN,AIBN,80 DMF,60AIBN,80 C°C, °CDMF, 6 DMF,h [176] Polystyreneacrylate)Polystyrene (PHFDA) BaTiOBaTiO3 33 3 80 °C [176] [176] PolystyrenePolystyrene BaTiOBaTiO3 AIBN, DMF, [176][176] Polystyrene BaTiO3 80AIBN, °C 80 80°C DMF, °C [176] Poly(stearylPolystyrene BaTiO3 AIBN,AIBN,80 °C DMF,THF, [176] Poly(stearylPoly(stearyl 2 AIBN,AIBN,AIBN, THF, DMF,THF, Poly(stearylPolystyrene BaTiOSiO2 3 AIBN,80 °CTHF, [177,178][176] Poly(stearylmethacrylate)PolystyrenePoly(stearylPoly(stearyl BaTiOSiOSiO22 3 AIBN,AIBN, THF,AIBN,◦60 °C THF, THF, [177,178[177,178]] [176] methacrylate)methacrylate) SiO2 SiO22 60 6080C °C [177,178][177,178] methacrylate)Poly(stearyl SiOSiO2 2 AIBN,8060 °C THF, [177,178][177,178] methacrylate)methacrylate)Poly(stearylmethacrylate) SiO2 60 AIBN,°C 60 60°C THF, °C [177,178] Poly(2-hydroxyethylmethacrylate)Poly(stearyl SiO2 AIBN,60 °C THF, [177,178] Poly(2-hydroxyethylPoly(stearyl 2 AIBN, THF, Poly(2-hydroxyethylmethacrylate) SiO2 AIBN,60 °CTHF, [177,178][164] Poly(2-hydroxyethylPoly(2-hydroxyethylPoly(2-hydroxyethylPoly(2-hydroxyethylmethacrylate) SiO22 AIBN,AIBN,AIBN, THF,AIBN,7060 THF, °C THF, THF, [177,178][164] Poly(2-hydroxyethylmethacrylate) SiO2 SiOSiOSiO22 2 AIBN,6070 °C THF, [164[164]] [164][164][164] methacrylate)methacrylate)methacrylate) SiO22 70 70°C◦ 70C °C [164] Poly(2-hydroxyethylmethacrylate)methacrylate) SiO2 AIBN,70 70°C THF, °C [164] Poly(2-hydroxyethylPoly(2-hydroxyethylmethacrylate) SiO2 AIBN,AIBN,70 °C THF,THF, [164] methacrylate) SiO2 AIBN,70 °C DMF [164] Poly(acrylicmethacrylate) acid) SiO 22 AIBN,70 °C DMF [164][162] Poly(acrylicmethacrylate) acid) SiO2 AIBN,AIBN, DMF70AIBN,70 °C, °C DMF 3 DMFh [162] Poly(acrylicPoly(acrylic acid) acid) SiO2 SiO22 2 70 °C, 3 h [162] [162][162] Poly(acrylicPoly(acrylic acid) acid) SiO SiO2 AIBN,AIBN,70 DMF °C, DMF 3 h [162][162] Poly(acrylicPoly(acrylic acid) acid) SiO SiO22 70 °C,AIBN,◦70 3 70h°C, °C,DMF 3 h 3 h [162] [162] Poly(acrylic acid) SiO2 70AIBN,70C, 3°C, h DMF 3 h [162] Poly(acrylicPoly(stearyl acid) SiO2 AIBN, DMFTHF, [162] Poly(acrylicPoly(stearyl acid) SiO22 AIBN,70 °C, THF, 3 h [177,178][162] Poly(stearylmethacrylate)Poly(stearylPoly(stearyl SiO2 AIBN,AIBN, 70THF,70AIBN,60 °C,°C, °C THF, 33 hTHF,h [177,178] methacrylate) SiO2 SiO22 2 60 °C [177,178][177,178][177,178] methacrylate)Poly(stearyl SiOSiO2 AIBN,60 °C THF, [177,178][177,178] methacrylate)Poly(stearylmethacrylate)Poly(stearylmethacrylate) SiO2 AIBN,60 AIBN,°C 60 THF, 60°C THF, °C [177,178] methacrylate)Poly(stearyl SiOSiO22 AIBN,◦60 °C THF, [177,178[177,178]] methacrylate)methacrylate)Poly(stearylPolystyrene TiOSiO2 AIBN,6011060C °C, °CTHF, 96 h [177,178] [138] methacrylate)Polystyrene SiOTiO22 11060 °C, °C 96 h [177,178] [138] Polystyrenemethacrylate)Polystyrene TiO2 TiO22 2 110 °C,110 9660 °C, h °C 96 h [138] [138] PolystyrenePolystyrene TiOTiO2 110110 °C, °C, 96 96h h [138] [138] 2 Polystyrene TiO 110 °C, 96 h [138] Polystyrene TiO2 110◦ °C, 96 h [138] PolystyrenePolystyrene TiOTiO2 110110C, 96°C, h 96 h [138] [138] Polystyrene TiO22 110 °C, 96 h [138]

Poly(methyl Poly(methyl AIBN,AIBN, DMF, DMF, methacrylate)-b-polyst TiO 2 [179] [179] methacrylate)-b-polystyrene 2 90 ◦C, 6 h 90 °C, 6 h yrene

Literature presents several examples of the use of unimodal polymer grafted SiO2 nanoparticles to enhance the breakdown strength of polymer grafted SiO2 filled nanocomposites [177,178,180]. For example, SI-RAFT technique has been used in the synthesis of poly(stearyl methacrylate) (PSMA) (Mn = 45 kg/mol and the graft density = 0.04 chain/nm2) grafted SiO2 (10–15 nm diameter) nanoparticles [177]. The dielectric performance of PSMA-grafted SiO2 nanoparticles/XLPE was compared with XLPE, pure PSMA/XLPE and unmodified SiO2/XLPE. Among the systems evaluated, the unmodified SiO2 nanoparticles filled/XLPE exhibited lowest dielectric breakdown strength while PSMA grafted

Molecules 2021, 26, x FOR PEER REVIEW 17 of 43

Poly(methyl AIBN, DMF, methacrylate)-b-polyst TiO2 [179] 90 °C, 6 h yrene

Literature presents several examples of the use of unimodal polymer grafted SiO2 nanoparticles to enhance the breakdown strength of polymer grafted SiO2 filled nanocomposites [177,178,180]. For example, SI-RAFT technique has been used in the synthesis of poly(stearyl methacrylate) (PSMA) (Mn = 45 kg/mol and the graft density = 0.04 chain/nm2) grafted SiO2 (10–15 nm diameter) nanoparticles [177]. The dielectric performance of PSMA-grafted SiO2 nanoparticles/XLPE was compared with XLPE, pure Molecules 2021, 26, 2942 18 of 44 PSMA/XLPE and unmodified SiO2/XLPE. Among the systems evaluated, the unmodified SiO2 nanoparticles filled/XLPE exhibited lowest dielectric breakdown strength while PSMA grafted SiO2 nanoparticles dispersed in XLPE showed the highest dielectric breakdownSiO2 nanoparticles strength. dispersed (Figure in11A) XLPE The showed intern theal highestfield distortion dielectric breakdown of PSMA strength.grafted SiO2 nanoparticles(Figure 11A) in The XLPE internal was ﬁeld found distortion to be of the PSMA least grafted (less than SiO2 nanoparticles10.6%) among in XLPEthe nanocom- was positesfound (Figure to be the 11B) least studied (less than over 10.6%) a wide among range the nanocompositesof DC fields from (Figure −30 11 kV/mmB) studied to −100 − − kV/mmover a indicating wide range oftremendous DC ﬁelds from potential30 kV/mm for improving to 100 kV/mm HVDC indicating power cable tremendous insulation. potential for improving HVDC power cable insulation. The long alkyl chain of PSMA The long alkyl chain of PSMA present on nanoparticles appears to have enhanced the present on nanoparticles appears to have enhanced the interaction of nanoparticles with interactionXLPE matrix of nanoparticles hence the improved with breakdownXLPE matrix strength hence of the nanocomposite improved breakdown [177]. strength of nanocomposite [177].

FigureFigure 11. (A 11.) Weibull(A) Weibull distribution distribution of ofDC DC breakdown breakdown strength, strength, ((BB)) Maximum Maximum internal internal field field distortion distortion under under external external DC DC electricelectric fields. fields. Reproduced Reproduced with with permission permission from from Ref Ref. [177]. [177]. The use of bimodal polymer grafted nanoparticles in polymer nanocomposite offers anThe attractive use of alternative bimodal approachpolymer forgrafted achieving nanoparticles improved breakdownin polymer strength nanocomposite and better offers an nanoparticleattractive alternative dispersion approach in polymer for nanocomposites. achieving improved The synthesis breakdown of bimodal strength polymer and better nanoparticlegrafted nanoparticles dispersion was in polymer explored nanocomposites. by Benicewicz, Schadler The synthesis and coworkers of bimodal [181–184 polymer]. graftedBimodal nanoparticles polymer grafted was SiOexplored2 nanoparticles by Benic wereewicz, synthesized Schadler by and sequential coworkers attachment [181–184]. Bimodalof electroactive polymer conjugated grafted SiO surface2 nanoparticles ligands followed were bysynthesize surface-initiatedd by sequential RAFT polymer- attachment of electroactiveization of GMA conjugated (Figure 12 )surface to form ligands PGMA. followed The electroactive by surface-initiated functionality (anthracene, RAFT polymer- izationthiophene, of GMA and terthiophene)(Figure 12) to was form also PGMA. grafted on The the electroactive nanoparticles. functionality Grafting of conjugated (anthracene, molecules (anthracene, thiophene and terthiophene) to the nanoparticle surface offers an thiophene, and terthiophene) was also grafted on the nanoparticles. Grafting of conju- approach to promote electron trapping at isolated regions of the composite while restricting gatedthe formationmolecules of (anthracene, conductive pathway thiophene [185 and], while terthiophene) the grafted PGMAto the chainsnanoparticle promoted surface offers an approach to promote electron trapping at isolated regions of the composite improved dispersion of the multifunctional SiO2 nanoparticles in epoxy resin. Bimodal whileterthiophene-PGMA restricting the formation functionalized of SiOconductive2 nanoparticles pathway filled [185], composites while showed the grafted the high- PGMA chainsest enhancement promoted improved in dielectric dispersi breakdownon of strength the multifunctional followed by bimodal SiO2 nanoparticles anthracene-PGMA in epoxy functionalized nanoparticles filled epoxy sample and the least was for thiophene-PGMA functionalized nanoparticles filled epoxy sample. The role of substituted aromatics grafted on nanoparticles in improving the dielectric breakdown strength of nanocomposite was explained on the basis of the Hammett relationship [186]. Molecules 2021, 26, x FOR PEER REVIEW 18 of 43

resin. Bimodal terthiophene-PGMA functionalized SiO2 nanoparticles filled composites showed the highest enhancement in dielectric breakdown strength followed by bimodal anthracene-PGMA functionalized nanoparticles filled epoxy sample and the least was for Molecules 2021, 26, 2942 thiophene-PGMA functionalized nanoparticles filled epoxy sample. The role of substi-19 of 44 tuted aromatics grafted on nanoparticles in improving the dielectric breakdown strength of nanocomposite was explained on the basis of the Hammett relationship [186].

Figure 12. (A) Synthesis of bimodal anthracene-PGMA silica nanoparticles (B) calculated summation of ionization energy Figure 12. (A) Synthesis of bimodal anthracene-PGMA silica nanoparticles (B) calculated summation of ionization energy and electron affinity for electroactive ligands V·s. experimentally determined dielectric breakdown strength of several and electron afﬁnity for electroactive ligands V·s. experimentally determined dielectric breakdown strength of several bimodal composites. Reproduced with permission from Ref [187]. bimodal composites. Reproduced with permission from Ref. [187].

Similarly, bimodal anthracene-PSMA grafted SiO2 nanoparticles were dispersed in Similarly, bimodal anthracene-PSMA grafted SiO2 nanoparticles were dispersed in polypropylene. The dispersion of bimodal modified SiO2 nanoparticles in polypropylene polypropylene. The dispersion of bimodal modiﬁed SiO2 nanoparticles in polypropylene resulted in in the the enhancement enhancement of of dielectric dielectric perm permittivityittivity by by 20% 20% and and an animprovement improvement in the in dielectricthe dielectric breakdown breakdown strength strength under under both both AC AC and and DC DC test test conditions conditions by by about about 15% compared to neat polypropylene [[178].178]. As note notedd earlier, the long alkyl chain of PSMA on grafted nanoparticles appears to have improved the compatibility of nanoparticlesnanoparticles with XLPE matrix hence the improved dielectric properties ofof nanocomposites.nanocomposites. Alternatively, thethe bimodal bimodal functionalized functionalized nanoparticles nanoparticles can can be synthesizedbe synthesized with with long longbrushes brushes of PS chainsof PS chains and short and P2VP short chains P2VP using chains SI-RAFT using technique.SI-RAFT technique. The combined The effectcom- binedof interaction effect of of interaction PS brushes of of PS the brushes grafted of nanoparticles the grafted nanoparticles with the matrix with and the the matrix reduction and thein silica reduction core-core in silica NPs core-core interaction NPs because interactio of then because dense short of the grafts dense of short P2VP grafts present of inP2VP the presentgrafted nanoparticles,in the grafted contributednanoparticles, to thecontribu improvedted to dispersion the improved of nanoparticles dispersion inof PSnanopar- matrix. ticlesUnlike in the PS earliermatrix. papers Unlike on the bimodal earlier graftedpapers nanoparticleson bimodal grafted which nanoparticles dealt with dielectric which dealtproperties with dielectric of nanocomposites, properties theof nanocomp emphasisosites, of Kumar the etemphasis al. publication of Kumar [188 et] al. was publica- on the tiondispersion [188] was of nanoparticles on the dispersi inon polymer of nanoparticles matrix and in the polymer impact matrix of microstructure and the impact on the of microstructuremechanical properties on the mechanical of nanocomposites. properties of nanocomposites.

4.2. SI-RAFT Polymerization to Prepare Polymer Grafted TiO22 Nanoparticles SI-RAFT hashas been been employed employed for growingfor growing polymers polymers such as such PMMA as [189PMMA,190], PS[189,190], [138], poly- PS [138],acrylic acidpolyacrylic (PAA) [191 ,192acid], poly(n-vinylpyrrolidone) (PAA) [191,192], [193poly(n-vinylpyrrolidone)], poly(chloromethyl styrene) [193], [194], poly(chloromethylpoly(2-hydroxyethyl styrene) acrylate) [[194],195], PMMA-poly(2-hydroxyethylb-PS, [179,196 ],acrylate) etc. on the[195], surface PMMA- TiO2b.-PS, PS [179,196],(Mn = 4800 etc. g/mol on the) was surface grown TiO from2. PS rutile (Mn TiO= 48002 nanoparticles g/mol) was viagrown SI-RAFT from polymerizationrutile TiO2 na- noparticlesand dispersed via in SI-RAFT PS matrix polymerization at various concentrations and dispersed to investigate in PS matrix the dielectricat various properties concen- trationsof nanocomposites. to investigate The the PS dielectric chains attachedproperties to of the nanocomposites. surfaces of TiO 2The(PS@TiO PS chains2) nanoparti- attached tocles the maintained surfaces of a TiO “brush-like”2 (PS@TiO structure2) nanoparticles and resulted maintained in chestnut-burr a “brush-like”(Figure structure 13C) self-and resultedassembled in NPchestnut-burr aggregates. (Figure With increase 13C) self-ass in theembled amount NP of PS@TiO aggregates.2-, the With composite increase showed in the a higher dielectric constant (~65) which could be attributed to the self-assembled chestnut- amount of PS@TiO2-, the composite showed a higher dielectric constant (~65) which couldburr aggregates be attributed of the to nanoparticlesthe self-assembled where ch a numberestnut-burr of rutile aggregates crystals of shared the nanoparticles lateral faces and formed capacitive microstructures. The crystals in these aggregates are separated by a polymer thin layer and allow a high percolation threshold, 41% v/v of ﬁller amount, before the formation of a continuous network responsible for the sudden change of the dielectric characteristics, (from random orientation to conductive pathways to conductive network) as depicted in Figure 13. Despite the high content of inorganic ﬁller, the dissipation factor remained low, even approaching the lower frequencies. Molecules 2021, 26, x FOR PEER REVIEW 19 of 43

where a number of rutile crystals shared lateral faces and formed capacitive microstructures. The crystals in these aggregates are separated by a polymer thin layer and allow a high percolation threshold, 41% v/v of filler amount, before the formation of a continuous network responsible for the sudden change of the dielectric characteristics, (from random Molecules 2021, 26, 2942 orientation to conductive pathways to conductive network) as depicted 20in ofFigure 44 13. Despite the high content of inorganic filler, the dissipation factor remained low, even approaching the lower frequencies.

Figure 13. (A) Schematic diagram illustrating the process of RAFT polymerization on the surface of rutile nanoparticles; Figure 13. (A) Schematic diagram illustrating the process of RAFT polymerization on the surface of rutile 4nanoparticles; (B) Varia- (B) Variation of the dielectric constant of the PS@TiO2/PS composite V·s. the volume fraction of TiO2 at 10 Hz; (C) TEM tion of the dielectric constant of the PS@TiO2/PS composite V·s. the volume fraction of TiO2 at 104 Hz; (C) TEM micrograph of the micrograph of the TiO2–PS composites (at 36.9% TiO2 content). Reproduced with permission from Ref. [138]. TiO2–PS composites (at 36.9% TiO2 content). Reproduced with permission from Ref [138].

4.3. SI-RAFT Polymerization to Prepare Polymer Grafted BaTiO3 Nanoparticles 4.3. SI-RAFT Polymerization to Prepare Polymer Grafted BaTiO3 Nanoparticles Ming Zhu et al. [197] synthesized core-shell structured polymer@BaTiO3 nanoparticles ofMing varying Zhu polymer et al. composition[197] synthesized (PMMA@BT, core-shell PGMA@BT, structured and PHEMA@BT) polymer@BaTiO and constant3 nanoparticles shellof varying thickness polymer (Figure 14 )composition using SI-RAFT (PMMA@BT, technique. The PGMA@BT, synthesized nanoparticles and PHEMA@BT) were and constantused toshell study thickness the compositional (Figure effect14) using of the SI-RAFT polymeric technique. shells of PGNPs The onsynthesized the dielectric nanopar- properties of the nanocomposites i.e., breakdown strength, leakage currents, energy storage ticlescapability, were used and to energy study storage the compositional efficiency of the effect nanocomposites. of the polymeric The differences shells of PGNPs in the on the dielectricdielectric properties properties of thethe various nanocomposites core-shell NPs i.e., (with breakdown PHEMA, PGMA strength, and PMMA leakage shell) currents, energywere storage attributed capability, to the differences and energy in the dipole storage moment efficiency of pendant of the groups nanocomposites. in the shell. The The dif- ferenceshydroxyethyl in the dielectric pendant group properties in PHEMA of the was various responsible core-shell for the NPs larger (with dipole PHEMA, moment PGMA andand PMMA higher shell) moisture were absorption, attributed resulting to the in thedifferences higher dielectric in the constant dipole andmoment higher lossof pendant groupsas compared in the shell. to PGMA The hydroxyethyl and PMMA. Among pendant the group systems in studied, PHEMA PHEMA@BT/PVDF was responsible for the nanocomposite exhibited highest storage energy density due to the high dielectric constant largerof PHEMA@BTdipole moment while and the PGMA@BT/PVDF higher moisture nanocomposite absorption, resulting exhibited highestin the dischargehigher dielectric constantdensity and due higher to the highloss breakdownas compared strength to PGMA and low and dielectric PMMA. loss Among of PGMA@BT the systems (20% studied, loading),PHEMA@BT/PVDF while PMMA@BT/PVDF nanocomposite (20% loading) exhibit nanocompositeed highest storage exhibited energy highest density energy due to the storagehigh dielectric efficiency constant with moderate of PHEMA@BT dielectric constant while and the moderate PGMA@BT/PVDF breakdown strength. nanocomposite exhibitedSome highest studies discharge have systematically density due studied to th thee rolehigh of breakdown pendant groups strength in the polymeric and low dielec- shell of encapsulated nanoparticles on the dielectric properties of nanocomposite. For ex- tric loss of PGMA@BT (20% loading), while PMMA@BT/PVDF (20% loading) nanocom- ample, Zhang and workers [170] varied the number of fluorine substituents present on the posite exhibited highest energy storage efficiency with moderate dielectric constant and molecular structure of polymer shell of core shell structured rigid-fluoro-polymer@ BaTiO3 moderatenanoparticles breakdown by performing strength. RAFT polymerization with styrenic monomers containing different number of fluorine (M-3F, M-5F and M-7F) (Figure 15A), Evaluation of dielectric performance of the nanocomposites of rigid-fluoro-polymer nanoparticles@ BaTiO3 (P-3F, P-5F and P-7F) and poly (Vinylidene fluoride-trifluororethylene-chlorotrifluoroethylene

Molecules 2021, 26, x FOR PEER REVIEW 20 of 43

Molecules 2021, 26, 2942 21 of 44

(PVDF-TrFE-CTFE) indicated a strong dependance of permittivity and energy densities on the molecular structure of ﬂuorinated styrenic monomer. For 5% loading, the nanocomposite formulated with ﬂuorinated styrenic monomer containing 3F exhibited the highest Molecules 2021, 26, x FOR PEER REVIEW −1 −3 20 of 43 breakdown strength (542 kV mm ) and highest energy density (14.5 J cm ) which could be attributed to compact interfacial interactions of P-3F with PVDF-TrFE-CTFE matrix.

Figure 14. BaTiO3 core-shell nanoparticles by surface-initiated RAFT polymerization. Reproduced with permission from Ref [197].

Some studies have systematically studied the role of pendant groups in the polymeric shell of encapsulated nanoparticles on the dielectric properties of nanocomposite. For example, Zhang and workers [170] varied the number of fluorine substituents present on the molecular structure of polymer shell of core shell structured rigid-fluoro-polymer@ BaTiO3 nanoparticles by performing RAFT polymerization with styrenic monomers containing different number of fluorine (M-3F, M-5F and M-7F) (Figure 15A), Evaluation of dielectric performance of the nanocomposites of rigid-fluoro-polymer nanoparticles@ BaTiO3 (P-3F, P-5F and P-7F) and poly (Vinylidene fluoride-trifluororethylene-chlorotrifluoroethylene (PVDF-TrFE-CTFE) indicated a strong dependance of permittivity and energy densities on the molecular structure of fluorinated styrenic monomer. For 5% loading, the nanocomposite formulated with fluorinated styrenic monomer containing 3F exhibited the highest breakdown strength −1 −3 FigureFigure(542 14.kV 14.BaTiO mm BaTiO3 core-shell) 3 andcore-shell nanoparticleshighest nanoparticles energy by surface-initiated density by surface-initiated (14.5 RAFT polymerization.J cm RAFT) which polymerization. Reproduced could be attributed Reproduced to withwithcompact permission permission interfacial from from Ref. [ 197 interactionsRef]. [197]. of P-3F with PVDF-TrFE-CTFE matrix.

Some studies have systematically studied the role of pendant groups in the polymeric shell of encapsulated nanoparticles on the dielectric properties of nanocomposite. For example, Zhang and workers [170] varied the number of fluorine substituents present on the molecular structure of polymer shell of core shell structured rigid-fluoro-polymer@ BaTiO3 nanoparticles by performing RAFT polymerization with styrenic monomers containing different number of fluorine (M-3F, M-5F and M-7F) (Figure 15A), Evaluation of dielectric performance of the nanocomposites of rigid-fluoro-polymer nanoparticles@ BaTiO3 (P-3F, P-5F and P-7F) and poly (Vinylidene fluoride-trifluororethylene-chlorotrifluoroethylene (PVDF-TrFE-CTFE) indicated a strong dependance of permittivity and energy densities on the molecular structure of fluorinated styrenic monomer. For 5% loading, the nanocomposite formulated with fluorinated styrenic monomer containing 3F exhibited the highest breakdown strength (542 kV mm−1) and highest energy density (14.5 J cm−3) which could be attributed to compact interfacial interactions of P-3F with PVDF-TrFE-CTFE matrix.

FigureFigure 15. 15.(A )(A Synthetic) Synthetic route route for formation for format of TFMPCSion of TFMPCS monomer monomer (B) schematic (B) illustration schematic of illustration of

rigid-fluoropolymer@rigid-fluoropolymer@ BaTiO 3BaTiO/PVDF-TrFE-CTFE3/PVDF-TrFE-CTFE dielectric nanocomposite dielectric nanocomposite films (C) The permittivity films (C) The permittiv- ofity PVDF-TrFE-CTFE of PVDF-TrFE-CTFE nanocomposites nanocomposites films with BT-3F0,films with BT-3F1, BT-3F0, BT-3F2, BT-3F1, and BT-3F3 BT-3F2, at 1 and kHz. BT-3F3 at 1 kHz. (D(D) Variation) Variation of characteristicof characteristic breakdown breakdown strength strength from Weibull from distribution Weibull distribution for samples withfor samples with variousvarious volume volume fractions fractions of fillers. of Reproducedfillers. Repr withoduced permission with frompermission Ref. [130 ].from Ref [130].

Figure 15. (A) Synthetic route for formation of TFMPCS monomer (B) schematic illustration of rigid-fluoropolymer@ BaTiO3/PVDF-TrFE-CTFE dielectric nanocomposite films (C) The permittivity of PVDF-TrFE-CTFE nanocomposites films with BT-3F0, BT-3F1, BT-3F2, and BT-3F3 at 1 kHz. (D) Variation of characteristic breakdown strength from Weibull distribution for samples with various volume fractions of fillers. Reproduced with permission from Ref [130].

Molecules 2021, 26, 2942 22 of 44

Other studies have explored the effect of shell thickness on the dielectric properties of nanocomposite while maintaining similar composition of the polymer shell of core-shell nanoparticles. Zhang et al. [130] synthesized core shell structured rigid-fluoro-polymer@BaTiO3 nanoparticles via SI-RAFT polymerization of 2,5-bis[(4-trifluoromethoxyphenyl)oxycarbonyl] styrene (TFMPCS) with RAFT agent anchored to BaTiO3 nanoparticles. TFMPCS was synthesized starting from 2-vinylterephthalic acid as depicted in Figure 15A. The PGNPs were incorporated in PVDF-TrFE-CTFE matrix to study the dielectric properties of the nanocomposite. A careful analysis of the results revealed that the dielectric constant, breakdown strength and energy density of the polymer nanocomposites were significantly affected by the thickness of rigid-fluoro-polymer shell around the BaTiO3 nanoparticles. For instance, nanocomposite with higher shell thickness (i.e., obtained from BT-3F3) exhibited higher breakdown strength while dielectric permittivity showed an inverse relationship with shell thickness (Figure 15). This is expected because polymers in general have higher breakdown strength while bare nanoparticles have higher permittivity. The energy density for 5 vol% BT-3F3/PVDF-TrFE-CTFE nanocomposite (36.6 J cm−3 at the electric field of 514 kV mm−1) was significantly higher compared to pure PVDF-TrFE-CTFE (15.4 J cm−3 at the electric field of 457 kV mm−1). Similarly, Yang et al. [176] studied the effect of shell thickness of PGNPs on the dielectric properties of nanocomposite while keeping the polymer composition of core-shell NPs constant. The RAFT agent (EDMAT) was initially immobilized on the surface of silanized BaTiO3 nanoparticles by conducting reaction with n-hydroxysuccinimide activated ester of EDMAT (NHS-EDMAT). A series of PS @BaTiO3 nanoparticles were prepared by RAFT polymerization where, the shell thickness was tuned by changing the feed ratios of styrene and BaTiO3-EDMAT. The dielectric constant of single component core-shell (shell thickness varying from 7 to 12 nm) nanocomposite ranged from 14–24 depending upon the shell thickness (7 to 12 nm) and the dielectric loss ranged from 0.009–0.13. Additionally, the dielectric constant as well as the dielectric loss of all the nanocomposites showed a weak frequency dependence over a wider range of frequencies (1 Hz to 1 MHz). BaTiO3-EDMAT nanoparticles have not only been utilized for surface-initiated RAFT polymerization of styrene but also for the polymerization of fluoroalkyl acrylates viz., 1H,1H,2H,2H-heptadecafluorodecyl acrylate (HFDA) and trifluoroethyl acrylate (TFEA) [175]. The surface energies of poly(fluoroalkyl acrylate)are generally lower than those of hydro- genated polymers e.g., PS. Several fluoroalkyl acrylate monomers with different structures were grafted on BaTiO3 nanoparticles and surface-initiated RAFT polymerization was conducted so as to synthesize polymer grafted BaTiO3 nanoparticles with the least surface energy. Dielectric evaluation of fluoro-polymer@BaTiO3/PVDF-HFP nanocomposites revealed that the energy density of 50% PTFEA@BaTiO3/PVDF-HFP/nanocomposites (6.23 J.cm−3) was 150% greater than that of the pure PVDF-HFP (~4.10 J.cm−3). Fur- ther, nanocomposite derived from PTFEA@BaTiO3 exhibited slightly better dielectric performance over that derived from poly(1H,1H,2H,2H-heptadecafluorodecyl acrylate) PHFDA@BaTiO3 because PTFEA has pendent trifluoroethyl group which promotes a more compact interface compared to PHFDA which has long perfluoroalkyl pendant group. The structure-property relationship study of polymer-grafted BaTiO3 nanoparticles (synthesized by RAFT technique) filled polymer nanocomposites clearly indicates that several factors influence the dielectric performance of the nanocomposite including the polymer composition of the shell, the pendant groups of the polymer shell, and the shell thickness of the core-shell nanoparticles.

5. SI-Nitroxide-Mediated Polymerization (SI-NMP) to Prepare Polymer Grafted Nanoparticles Nitroxide-mediated polymerization (NMP) involves reversible activation−deactivation of propagating polymer chains by a nitroxide radical [198]. NMP polymerization has been widely used for grafting styrenic monomers however, other monomers methyl methacrylate, n-butyl acrylate, N-isopropylacrylamide, acrylic acid, etc. have also been grafted on the surface of NPs [32,199–205]. The initiators used for NMP polymerization include Molecules 2021, 26, x FOR PEER REVIEW 22 of 43

5. SI-Nitroxide-Mediated Polymerization (SI-NMP) to Prepare Polymer Grafted Nanoparticles Nitroxide-mediated polymerization (NMP) involves reversible activation−deactivation of propagating polymer chains by a nitroxide radical [198]. NMP polymerization has been widely used for grafting styrenic monomers however, other monomers methyl methacrylate, n-butyl acrylate, N-isopropylacrylamide, acrylic acid, etc. have also been grafted on the surface of NPs [32,199–205]. The initiators used for NMP polymerization include 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) [57], N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] nitroxide (DEPN) [206] and TIPNO [207]. The, SI-NMP of grafting polymer chains on NPs involves initially immobi- Moleculeslizing2021, 26TEMPO, 2942 or DEPN or TIPNO initiator functionalities on the NP surface [48]. The23 of 44 beauty of NMP polymerization is in that the nitroxide radical endcaps the polymer chain to form a persistent radical effect without the need for a separate initiator or catalyst (The 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) [57], N-tert-butyl-N-[1-diethylphosphono-(2,2- propagating speciesdimethylpropyl)] are formed via nitroxide dissociation (DEPN) of [206 a nitroxide] and TIPNO radical). [207]. The, During SI-NMP polymer- of grafting ization, the equilibriumpolymer between chains ondormant NPs involves and initiallyactive immobilizingspecies shifts TEMPO towards or DEPN the ordormant TIPNO ini- species and limits thetiator number functionalities of active on theradi NPcal surface species [48 ].present The beauty and of also NMP restrict polymerization possible is in termination reactions.that the nitroxide radical endcaps the polymer chain to form a persistent radical effect without the need for a separate initiator or catalyst (The propagating species are formed via dissociation of a nitroxide radical). During polymerization, the equilibrium between 5.1. SI-NMP Polymerizationdormant to and Prepare active species Polymer shifts Grafted towards SiO the2 dormantNanoparticles species and limits the number of active radical species present and also restrict possible termination reactions. Yang et al. [208] utilized SI-NMP polymerization for the preparation of polystyrene

2 5.1. SI-NMP Polymerization2 to Prepare Polymer Grafted SiO2 Nanoparticles grafted SiO nanoparticles. The SiO nanoparticles were initially treated with thionyl chloride, and the modifiedYang nanoparticles et al. [208] utilized were SI-NMP then polymerization reacted with for thetertiary preparation butyl of hydrop- polystyrene eroxide (TBHP) to graftedintroduce SiO2 nanoparticles.peroxide groups The SiO on2 nanoparticles the surfaces were of initially nanoparticles. treated with Then thionyl chloride, and the modified nanoparticles were then reacted with tertiary butyl hydroper- NMP polymerizationoxide was (TBHP) initiated to introduce in the peroxidepresence groups of TEMPO on the surfaces agent of to nanoparticles. graft polystyrene Then NMP 2 on the surface of SiOpolymerization particles [208]. was initiated Alternatively, in the presence Chevigny of TEMPO and agent coworkers to graft polystyrene employed on the APTMS modified SiOsurface2 nanoparticles of SiO2 particles and [208 grafted]. Alternatively, MAMA-SG1 Chevigny (BlocBuilder), and coworkers employed (NMP initi- APTMS modified SiO2 nanoparticles and grafted MAMA-SG1 (BlocBuilder), (NMP initiator) for ator) for subsequent SI-NMP grafting of PS to SiO2 nanoparticles. The SI-NMP polymer- subsequent SI-NMP grafting of PS to SiO2 nanoparticles. The SI-NMP polymerization ization of styrene wasofstyrene carried was out carried in the out presence in the presence of free of free MAMA-SG1 MAMA-SG1 asas a a sacrificial sacrificial initiator ini- to tiator to ensure a betterensure control a better of control the polymerization of the polymerization (Figure (Figure 16). 16).

Figure 16. 16.Scheme Scheme for anchoring for anchoring of the MAMA-SG1 of the MAMA-SG1 initiator and SI-NMP initiator polymerization and SI-NMP of styrene polymerization on the SiO2 NP of surface. sty- reneReproduced on the with SiO permission2 NP surface. from Ref. Reproduced [32]. with permission from Ref [32].

5.2. SI-NMP Polymerization to Prepare Polymer Grafted TiO2 and BaTiO3 Nanoparticles 5.2. SI-NMP Polymerization to Prepare Polymer Grafted TiO2 and BaTiO3 Nanoparticles SI-NMP has also been employed for grafting of PS [209,210] and poly(4-chloromethyl SI-NMP has styrene-also g-4-vinylpyridine)been employed (PCMSt- gfor-P4VP) grafting [211] on TiO 2ofas wellPS as poly(4-hydroxystyrene)[209,210] and poly(4-chloromethyl(PVP) styrene- [212]g and-4-vinylpyridine) poly(styrene-co-maleic (PCMSt-g-P4VP) anhydride) (PSMA) [211] copolymers on TiO2 [ 213as ]well on BaTiO as 3 poly(4-hydroxystyrene)nanoparticles (PVP) to[212] obtain and surface poly(styrene-co-maleic modiﬁed NPs. However, anhydride) there has been no(PSMA) dielectric co- data reported of polymer grafted nanoparticles synthesized using SI-NMP technique. polymers [213] on BaTiO3 nanoparticles to obtain surface modified NPs. However, there has been no dielectric6. Graftingdata reported to Method of to po Preparelymer Polymer grafted Grafted nanoparticles Nanoparticles synthesized using SI-NMP technique. The grafting to method is based on the use of polymer chain with functional groups that is randomly distributed along the chain or attached at the end of the polymer chain. The attachment of the graft polymer on nanoparticle surface requires coupling reaction of the functionalized backbone or the end-group functionalized polymer chain with the surface functionalized nanoparticles. Common reaction techniques used to synthesize functional polymers for grafting to method include free-radical polymerization, anionic polymerization, ATRP, and RAFT. The coupling reactions generally used in grafting to

Molecules 2021, 26, 2942 24 of 44

methods are click reactions, silanization, phophonate coupling, esteriﬁcation, etherﬁca- tion, etc.

6.1. Grafting to Method to Prepare Polymer-Grafted SiO2 Nanoparticles The combination of CRP techniques (ATRP, [214] RAFT, [215] etc.) and coupling reactions has been useful in grafting PMMA, PS, PNiPAAm, poly(N-vinylcarbazole), poly(7-(6-(acryloyloxy) hexyloxy) coumarin), etc. on SiO2 nanoparticles. Initially, silane- terminated polymer or phosphonate-terminated polymer is synthesized so as to graft polymer on nanoparticles [216–218]. For example, PS samples with end-functionalized dimethylchlorosilane of different molecular weights (8 kDa, 26 kDa, 108 kDa, and 126 kDa) were grafted on SiO2 bilayer. The bilayer was then used as organic-oxide hybrid gate dielectrics to fabricate solution-processed triethylsilylethynyl anthradithiophene (TES-ADT) organic ﬁeld-effect transistors (OFETs). The molecular weights of PS chains signiﬁcantly altered the areal grafting densities (due to steric hindrance), the interfacial structure and the dielectric properties as well as the performance of the OFETs. The lower molecular weight PS-g-SiO2 surface exhibited smoother brush like structure while higher molecular weight PS-g-SiO2 surface exhibited pancake like structure. The smoother surface of 8 kDa 2 −1 −1 PS-g-SiO2 surface showed the highest mobility (2.12 cm ·V ·s ) whereas the pancake 2 −1 −1 surface of 135 kDa PS-g-SiO2 showed the lowest mobility (0.85 cm ·V ·s )[219].

6.2. Grafting to Method to Prepare Polymer Grafted TiO2 Nanoparticles Phosphonic ester end capped PS has been synthesized using ATRP technique (Figure 17). The phosphonic acid end-functionalized PS was then coupled with oleic acid stabilized cylindrical shaped titanium oxide nanoparticles (TiO2-OLEIC) to obtain PS@TiO2. The PS@TiO2 nanoparticles thus prepared were used for the fabrication of capacitors as well as pentacene thin ﬁlm transistors. The dielectric constant of single component PS@TiO2 nanocomposite was ~9 (which is nearly 3.6 times higher than that of polystyrene) at 18.2 vol- Molecules 2021, 26, x FOR PEER REVIEW ume % loading of PS @TiO2, while the mobilities of PS@TiO2/ITO (bilayer) approached24 of 43 2 0.2 cm /V·s. [220] showing the importance of synthesized PS grafted TiO2 nanoparticles via grafting to approach in electronics and dielectric applications.

O O POEt O 3 OEt Br + Br Br Br HO 5 Br O Br 5 O 5 P OEt O

Cu(I)Br O O 1) TMSBr OH PMDETA OH Br Br O 5 P OH n O 5 P OH 2) Dioxane / Water O 100 oC O

PhosphonicAcid end capped PS

100 oC Phosphonic Acid OH OH end capped PS O O Br O 24 h O P 5 O n - Oleic Acid O OH O C17H33

FigureFigure 17. Synthesis 17. Synthesis of phosphonate of phosphonate end capped end polystyrene capped polyst and ligandyrene exchange and ligand reaction exchange of diethyl reaction phosphonate of di- end cappedethyl phosphonate polystyrene with end oleic capped acid terminated polystyren TiOe2 withnanoparticles oleic acid to generateterminated polystyrene TiO2 nanoparticles coated TiO2 (PS@TiO to gen-2) Reproducederate polystyrene with permission coated from Ref.TiO [2202 (PS@TiO]. 2) Reproduced with permission from Ref [220].

Using “grafting to” approach, block copolymer has been grafted to TiO2 nanoparticles with hydroxyl group as the end functionality of the anchoring block copolymer. Hailu and coworkers [221] demonstrated using “grafting-to” approach the ability to graft PMMA-b-PS-OH to silylated TiO2 nanoparticles to form block copolymer grafted nanoparticle (Figure 18). It was observed that the dispersion of PMMA-b-PS-g-TiO2 nanoparticles in PMMA and PS-PMMA BCP films was far better compared to that in PS films which could be attributed to the improved interactions of the outer corona of the PMMA-b-PS-g-TiO2 NPs with the PMMA component of BCP. The addition of 2.6 vol% of BCP-g-TiO2 NPs resulted in 18% enhancement in the permittivity and lower dielectric loss compared to the bare TiO2 nanoparticles filled BCP nanocomposite [222].

Figure 18. (A) Scheme for synthesis of PMMA-b-PS -g-TiO2 nanoparticles; (B) TEM (no stain) images of (a) oven annealed pristine TiO2 (10 wt%)-BCP (57 k-b-25 k) composite (b) As-cast PS (60 k) with 10 wt% BCP-g-TiO2 (c) As-cast PMMA (33 k) with 10 wt% BCP-g-TiO2, and (d) Oven annealed PMMA (33 k) with 10 wt% BCP-g-TiO2. (scale bar: 0.5 µm). Reproduced with permission from Ref [221].

Similarly, using “grafting to” approach, block copolymer was grafted onto TiO2 nanoparticles with dopamine as the anchoring group. Obata et al. synthesized copolymer containing dopamine as pedant groups via RAFT technique and subsequently coupled it with TiO2 nanoparticles to yield block copolymer grafted TiO2 nanoparticles [223]. Al- ternatively, silylation approach can also be used to graft PMMA on TiO2 nanoparticles by coupling of TiO2 with preformed trimethoxysilyl functionalized PMMA that was synthesized via ATRP technique [224].

Molecules 2021, 26, x FOR PEER REVIEW 24 of 43

O O POEt O 3 OEt Br + Br Br Br HO 5 Br O Br 5 O 5 P OEt O

Cu(I)Br O O 1) TMSBr OH PMDETA OH Br Br O 5 P OH n O 5 P OH 2) Dioxane / Water O 100 oC O

PhosphonicAcid end capped PS

100 oC Phosphonic Acid OH OH end capped PS O O Br O 24 h O P 5 O n - Oleic Acid O OH O C17H33

Figure 17. Synthesis of phosphonate end capped polystyrene and ligand exchange reaction of diethyl phosphonate end capped polystyrene with oleic acid terminated TiO2 nanoparticles to gen- erate polystyrene coated TiO2 (PS@TiO2) Reproduced with permission from Ref [220].

Using “grafting to” approach, block copolymer has been grafted to TiO2 nanoparticles with hydroxyl group as the end functionality of the anchoring block copolymer. Molecules 2021, 26, 2942 25 of 44 Hailu and coworkers [221] demonstrated using “grafting-to” approach the ability to graft PMMA-b-PS-OH to silylated TiO2 nanoparticles to form block copolymer grafted nano- Using “grafting to” approach, block copolymer has been grafted to TiO nanoparticles particle (Figure 18). It was observed that the dispersion of PMMA-b-PS-2 g-TiO2 nanopar- with hydroxyl group as the end functionality of the anchoring block copolymer. Hailu and ticles in PMMAcoworkers and PS-PMMA [221] demonstrated BCP using films “grafting-to” was approach far better the ability compared to graft PMMA- to that in PS films b-PS-OH to silylated TiO2 nanoparticles to form block copolymer grafted nanoparticle which could be(Figure attributed 18). It was observed to the that theimproved dispersion of PMMA-b-PS-interactionsg-TiO 2 nanoparticlesof the outer in corona of the PMMA-b-PS-g-TiOPMMA2 andNPs PS-PMMA with BCPthe films PMMA was far better component compared to thatof inBCP. PS films The which addition could of 2.6 vol% of be attributed to the improved interactions of the outer corona of the PMMA-b-PS-g-TiO2 BCP-g-TiO2 NPsNPs resulted with the PMMA in 18% component enhancement of BCP. The addition in ofthe 2.6 vol%permittivity of BCP-g-TiO2 NPsand lower dielectric resulted in 18% enhancement in the permittivity and lower dielectric loss compared to the loss compared to the bare TiO2 nanoparticles filled BCP nanocomposite [222]. bare TiO2 nanoparticles filled BCP nanocomposite [222].

FigureFigure 18. ( A18.) Scheme (A) forScheme synthesis offor PMMA-b-PS synthe -gsis-TiO 2ofnanoparticles; PMMA-b-PS (B) TEM (no-g-TiO stain) images2 nanoparticles; of (a) oven annealed (B) TEM (no stain) im- pristine TiO2 (10 wt%)-BCP (57 k-b-25 k) composite (b) As-cast PS (60 k) with 10 wt% BCP-g-TiO2 (c) As-cast PMMA (33 k) 2 withages 10 wt%of ( BCP-a) oveng-TiO2, andannealed (d) Oven annealed pristine PMMA TiO (33 k) with(10 10wt%)-BCP wt% BCP-g-TiO (572. (scale k-b-25 bar: 0.5 µk)m). composite Reproduced (b) As-cast PS (60 k) withwith permission 10 wt% from BCP- Ref. [221g].-TiO2 (c) As-cast PMMA (33 k) with 10 wt% BCP-g-TiO2, and (d) Oven annealed

PMMA (33 k) with 10Similarly, wt% BCP- using “graftingg-TiO2.to” (scale approach, bar: block 0.5 µm). copolymer Reproduc was grafteded ontowith TiO permission2 from Ref [221]. nanoparticles with dopamine as the anchoring group. Obata et al. synthesized copolymer containing dopamine as pedant groups via RAFT technique and subsequently coupled it with TiO2 nanoparticles to yield block copolymer grafted TiO2 nanoparticles [223]. Al- ternatively, silylation approach can also be used to graft PMMA on TiO nanoparticles by Similarly, using “grafting to” approach, block copolymer2 was grafted onto TiO2 coupling of TiO2 with preformed trimethoxysilyl functionalized PMMA that was synthe- nanoparticles withsized viadopamine ATRP technique as [ 224the]. anchoring group. Obata et al. synthesized copolymer

containing dopamine6.3. Grafting as to Methodpedant to Prepare groups Polymer via Grafted RAFT BaTiO3 Nanoparticlestechnique and subsequently coupled it with TiO2 nanoparticlesThe silylation to routeyield has alsoblock been used copolymer in the grafting tografted approach ofTiO polymer2 nanoparticles grafted [223]. Al- BaTiO3 nanoparticles. Xie et al. [133] formed PVDF-HFP@BaTiO3 nanocomposites by ternatively, silylationinitially synthesizing) approach P(VDF-HFP) can also with be glycidyl used methacrylate to graft (GMA) PMMA functionality on TiO via 2 nanoparticles by ATRP (Fluorine atom of the PVDF-HFP was utilized to initiate the ATRP of the GMA) 2 coupling of TiOtechnique. with The preformed functionalized polymertrimethoxysilyl was then reacted withfunctionalized the APTMS-functionalized PMMA that was synthesized via ATRPBaTiO 3techniquenanoparticles. The[224]. coupling reaction between PVDF-HFP-GMA (epoxy functionality) and amino-functionalized BaTiO3 yielded PVDF-HFP@BaTiO3 nanocomposite with superior dielectric properties. For example, the nanocomposite with 50% nanoparticle

Molecules 2021, 26, x FOR PEER REVIEW 25 of 43

6.3. Grafting to Method to Prepare Polymer Grafted BaTiO3 Nanoparticles The silylation route has also been used in the grafting to approach of polymer grafted BaTiO3 nanoparticles. Xie et al. [133] formed PVDF-HFP@BaTiO3 nanocomposites by initially synthesizing) P(VDF-HFP) with glycidyl methacrylate (GMA) functionality via ATRP (Fluorine atom of the PVDF-HFP was utilized to initiate the ATRP of the GMA) technique. The functionalized polymer was then reacted with the APTMS-functionalized BaTiO3 nanoparticles. The coupling reaction between PVDF-HFP-GMA (epoxy functionality) and amino-functionalized BaTiO3 yielded PVDF-HFP@BaTiO3 nanocomposite with superior dielectric properties. For example, the nanocomposite with 50% nanoparticle loading exhibited dielectric constant of 34.8 at 1 MHz, about 3.9 times greater than that of Molecules 2021, 26, 2942 26 of 44 pristine PVDF-HFP while dielectric loss observed was 0.128 at 1 MHz. Alternatively Yang and coworkers [225] synthesized core-shell structured polymer@BaTiO3 nanoparticles for dielectric applications via “grafting to” route using a loading exhibited dielectric constant of 34.8 at 1 MHz, about 3.9 times greater than that of combinationpristine PVDF-HFP of “thiol-ene” while dielectric and silylation loss observed chemistry. was 0.128 atThiol-terminated 1 MHz. PS or PMMA (molecular weight of PS1 and PMMA1~10K, PS2 and PMMA2~40K and PS3 and Alternatively Yang and coworkers [225] synthesized core-shell structured polymer@BaTiO3 PMMA3~80K)nanoparticles werefor dielectric prepared applications by RAFT via “graftingpolymerization to” route usingand awas combination allowed of to “thiol- react with vinyl-functionalizedene” and silylation chemistry.(methacryloxyp Thiol-terminatedropyltrimethoxy) PS or PMMA silanized (molecular BaTiO weight3 ofnanoparticles PS1 and to formPMMA1~10K, a series of polymer PS2 and PMMA2~40K grafted nanoparticles and PS3 and PMMA3~80K)PS@BaTiO3 and were PMMA@BaTiO prepared by RAFT3 (Figure 19). polymerizationIt was observed and wasthat allowed the graft to react densit withy vinyl-functionalizeddecreased with increase (methacryloxypropy- in the molecular ltrimethoxy) silanized BaTiO nanoparticles to form a series of polymer grafted nanoparti- weight of the grafted polymer.3 The dielectric constant of PS@BT (k = 30–33) and cles PS@BaTiO3 and PMMA@BaTiO3 (Figure 19). It was observed that the graft density PMMA@BTdecreased (k with = 34–38) increase single in the molecularcomponent weight nanoco of themposites grafted polymer. was greater The dielectric than that con- of pure polymersstant of (k PS@BT for PS (k = = 2.74 30–33) and and k PMMA@BTfor PMMA (k = =3.69) 34–38) while single the component low dielectric nanocomposites loss (for PS@ BT = 0.013was and greater for than PMMA@ that of pureBT polymers= 0.032) was (k for main PS = 2.74tained and over k for PMMAa wider = 3.69range) while of frequency. the Comparedlow dielectric to PMMA@BT loss (for PS@ nanocomposites, BT = 0.013 and for PMMA@PS@BT nanocomposites BT = 0.032) was maintained exhibited over higher a en- ergywider efficiency range ofdue frequency. to lower Compared remnant to polarizat PMMA@BTion. nanocomposites, Furthermore, PS@BT the energy nanocompos- efficiency of ites exhibited higher energy efficiency due to lower remnant polarization. Furthermore, boththe PS@BT energy and efficiency PMMA@BT of both PS@BTnanocomposites and PMMA@BT exhibited nanocomposites a strong dependence exhibited a strong on the mo- leculardependence weight onof thethe molecular grafted polymer weight of thechains grafted and polymer the grafting chains and density the grafting indicating density that the designindicating of core-shell that the nanoparticle design of core-shell filled nanoparticle polymer nanocomposites filled polymer nanocomposites with high energy with density highand energyhigh energy density efficiency and high energy is intric efficiencyately related is intricately to the related shell tostructure. the shell structure.

Figure Figure19. Schematic 19. Schematic illustration illustration for for (A (A)) synthesis of of thiol-terminated thiol-terminated PS and PS thiol-terminated and thiol-terminated PMMA via PMMA RAFT poly- via RAFT 3 polymerizationmerization and and (B ()B preparation) preparation of of core-shell structured structured polymer@BaTiO polymer@BaTiO3 nanocomposites nanocomposites by thiol by− thiolene click−ene reaction. click reaction. (C) The relationship between the molecular weight of grafting polymer and grafting density of the core-shell structured nanoparticles (D) Energy efﬁciency of the core-shell structured polymer@BT nanocomposites under the electric ﬁeld of 10 kV/mm. Reproduced with permission from Ref. [225].

Molecules 2021, 26, x FOR PEER REVIEW 26 of 43

(C) The relationship between the molecular weight of grafting polymer and grafting density of the core-shell structured nanoparticles (D) Energy efficiency of the core-shell structured polymer@BT nanocomposites under the electric field of 10 kV/mm. Reproduced with permission from Ref [225].

Similarly, Ma et al. [131] synthesized core-shell structured PVDF@BT and PS@BT Molecules 2021, 26, 2942 27 of 44 nanoparticles via thiol-ene coupling. Where thiol-terminated poly(vinylidene fluoride) (PVDF-SH) and thiol-terminated polystyrene (PS-SH), was reacted with γ-methacryloxypropyltrimethoxysilane (MPS) functionalized BaTiO3 nanoparticles (as Similarly, Ma et al. [131] synthesized core-shell structured PVDF@BT and PS@BT nanopar- depictedticles viain thiol-eneFigure coupling.20). It Wherewas thiol-terminatedobserved that poly(vinylidene the dielectric fluoride) permittivity (PVDF-SH) and and Eb of PVDF@BT/PVDFthiol-terminated (117 polystyrene kV/mm) (PS-SH), and was PS@BT/PV reacted withDFγ-methacryloxypropyltrimethoxysilane composites (107 kV/mm) was better than (MPS)that of functionalized unmodified-BT/PVDF BaTiO3 nanoparticles comp (asosites depicted (58.5 in Figure kV/mm). 20). It was The observed superior that theεr of PVDF compareddielectric to permittivity PS resulted and in Eb higher of PVDF@BT/PVDF dielectric (117constant kV/mm) of and PVDF@BT/PVDF PS@BT/PVDF composites (εr = 33 at 30% (107 kV/mm) was better than that of unmodified-BT/PVDF composites (58.5 kV/mm). The loading)superior overεr PS@BT/PVDF of PVDF compared composites to PS resulted (ε inr higher= 25 at dielectric 30% loading). constant of Furthermore, PVDF@BT/PVDF PVDF@BT NPs (exhibitedεr = 33 at 30% better loading) compatibilityover PS@BT/PVDF with compositesPVDF matrix (εr = compared 25 at 30% loading). to PS@BT Further- resulting in improvedmore, PVDF@BTbreakdown NPs exhibitedstrength better of nanocomposit compatibility withe. PVDFIn other matrix words, compared PVDF to PS@BT shell act as a bufferresulting layer and in improved reduced breakdown the electrical strength mismat of nanocomposite.ch between In the other matrix words, and PVDF core shell nanofillers act as a buffer layer and reduced the electrical mismatch between the matrix and core comparednanoﬁllers to the compared PS hell. to the PS hell.

Figure 20.Figure (A) Synthesis 20. (A) Synthesis of SH-terminated of SH-terminated PS PSand and SH-terminated SH-terminated PVDF. PVDF. (B )(B Grafting) Grafting to approach to approach for synthesis for synthesis of hybrid of hybrid nanoparticles.nanoparticles. Reproduced Reproduced with permission with permission from from Ref Ref. [131]. [131].

6.4. Grafting6.4. Grafting to Method to Method to to Prepare Prepare PolymerPolymer Grafted Grafted Al2 OAl3 2NanoparticlesO3 Nanoparticles The The“grafting “grafting to” to” techniques techniques hashas also also been been used used to graft to graft PS with PS -COOH with -COOH end groups end groups by reacting with –OH groups on the surface of Al2O3 nanoparticles (Al NPs) [139]. The PS by reactingwith -COOH with end–OH group groups was initiallyon the synthesizedsurface of byAl free2O3 radicalnanoparticles polymerization (Al NPs) initiated [139]. The PS with by-COOH 4,40 -azobis end (4-cyanovaleric group was initially acid) (ACVA) synthesized in toluene. by The free grafting radical of high polymerization surface energy initiated by 4,4Al′ -azobis NPs with (4-cyanovaleric PS having -COOH acid) end group (ACVA) greatly in reducedtoluene. the The aggregation grafting of of Al high NPs insurface energy comparisonAl NPs with to the PS bare having Al NPs -COOH in PS matrix. end When group the Algreatly NPs and reduced PS grafted the Al aggregation NPs were of Al mixed with PS to form PS nanocomposite films, the results showed larger voids for agg-Al NPs inNPs comparison filled PS composite to the filmbare but Al a NPs more in homogeneous PS matrix. composite When the film Al for NPs PS graftedand PS Al grafted Al NPs NPs.were The mixed dielectric with constant PS to of form the pristine PS nanocomposite PS film, the PS films films, doped the with results 30 wt% aggshowed-Al larger voidsNPs for andagg PS-Al grafted NPs filled Al NPs PS at 10composite5 Hz were foundfilm but to be a 2.80,more 4.75 homogeneous and 9.50, respectively. composite film for PSThe grafted breakdown Al NPs. strength The and dielectric energy density constant of the of PS the film pristine doped with PS PSfilm, grafted the AlPS NPs films doped (211–175 kVmm−1 and 1.70 J/cm3 at 1000 Hz) was greater than PS film doped with agg-Al with 30 wt% agg-Al NPs and PS grafted Al NPs at 105 Hz were found to be 2.80, 4.75 and NPs (183.77 to 30 kVmm−1 and 0.26 J/cm3 at 1000 Hz) and this was ascribed to the good 9.50, compatibilityrespectively. and The good breakdown dispersion ofstrength the PS grafted and energy Al NPs indensity the PS film.of the PS film doped with −1 3 PS graftedPS- Alg-Al NPs2O3 nanoparticles(211–175 kVmm were also and synthesized1.70 J/cm byat silanization 1000 Hz) of was Al2 Ogreater3 NPs with than PS film dopeddimethylchlorosilane-end-capped with agg-Al NPs (183.77 to polystyrene30 kVmm− (PS)1 and to 0.26 obtain J/cm grafted3 at nanoparticles1000 Hz) and with this was as- 2 cribedgraft to the density good of compatibility 0.13 chains/nm and. The good different dispersion wt% of PS-Alof the2O PS3 nanoparticles grafted Al wereNPs in the PS blended with PS to fabricate nanocomposites with dielectric constant in the range 2.59 film. to 7.79. The nanocomposite film was found to be an efficient surface passivator for the PS-oxideg-Al dielectric2O3 nanoparticles layer in organic were field-effect also synthesized transistors (OFETs). by silanization The field-effect of mobilityAl2O3 NPs with –3 2 dimethylchlorosilane-end-capped(1.4 × 10 cm /V·s) and threshold polystyrene voltage (4.4 V) (PS) of OFETs to obtain with PS-Algrafted2O3 nanoparticlesnanoparti- with cles were found to be significantly better than that of nanocomposite with bare Al O graft density of 0.13 chains/nm2. The different wt% of PS-Al2O3 nanoparticles2 3 were nanoparticles (field-effect mobility = 1.7 × 10–4 cm2/V·s threshold voltage = 6.7 V) [132]. blended with PS to fabricate nanocomposites with dielectric constant in the range 2.59 to 7.79. The nanocomposite film was found to be an efficient surface passivator for the oxide dielectric layer in organic field-effect transistors (OFETs). The field-effect mobility (1.4 ×

Molecules 2021, 26, 2942 28 of 44

7. In Situ Polymerization to Prepare Polymer Grafted Nanoparticles In situ polymerization has been widely used for producing well-dispersed metal oxide nanoparticle/polymer composite. In this technique, nano-sized metal oxide particles are mixed with organic monomers either in the presence or absence of a solvent followed by polymerization of the respective monomers. The nanoparticles are encapsulated in polymer shell via physical or chemical adsorption by taking advantage of the reactive functionality (e.g., acrylate functionality, it can be termed as “grafting through” or “grafting onto”) on NP surface. Both emulsion and suspension polymerization methods or in situ synthesis of both NPs (sol-gel synthesis) and polymers (free radical polymerization) have been employed [45,59,226–230]. Morales-Acosta and coworkers performed sol–gel and in situ polymerization using tetraethyl orthosilicate (TEOS) as SiO2 precursor, methyl methacrylate (MMA) as monomer, and 3-(trimethoxysilyl)propyl methacrylate (TMSPM) as coupling agent to improve the compatibility between PMMA and SiO2. Various core-shell nanoparticles with equimolar proportion of TEOS and MMA and varying concentrations of coupling agent, TMSPMA were prepared so as to study the effect of coupling agent concentration on the properties of fabricated nanocomposite films. All of the PMMA–SiO2 hybrid films exhibited higher dielectric constant (5.7 to 14) than that of PMMA (κ = 3.2 at 1 MHz) and bare SiO2 (κ = 3.9 at 1 MHz). The enhancement in the permittivity was attributed to residual solvents (-OH groups) and MMA (-C=C-groups, due to incomplete conversion into PMMA) present in the nanocomposite films [230]. Morales-Acosta and coworkers utilized low-temperature sol-gel and in situ polymerization to obtain PS- or PMMA-grafted-metal oxide (SiO2, TiO2, ZrO2) hybrid films for gate dielectric applications in the thin film transistors [231–234]. Similarly, Sánchez-Ahumada et al. synthesized PS-TiO2 hybrid dielectric films by performing sol-gel process with titanium butoxide (TB) as precursor and in situ polymerization of styrene in presence of the coupling agent, 3-trimetoxy-silyl-propyl-methacrylate (TMSPM) simultaneously. The dielectric constant of the hybrid film was 5.2 at 1 MHz, which is higher than that of pristine PS −6 2 (2.74). PS-TiO2 hybrid dielectric films exhibited leakage current of 1 × 10 A/cm which is low enough to qualify the hybrid material as a dielectric gate in electronic devices [235]. PMMA embedded TiO2 nanoparticles were also synthesized via in situ free radical polymerization of methyl methacrylate using benzyl peroxide as an initiator in aqueous solution of polyvinyl alcohol (PVA) and sodium phosphate along with preformed TiO2 nanoparticles. The dielectric properties of PMMA embedded TiO2 nanoparticle filled PMMA nanocomposite showed high dielectric constant with low dielectric loss, [236]. Wang and coworkers reported the synthesis of PMMA-g-TiO2 via in situ emulsion polymerization technique. The dielectric study of PMMA-g-TiO2/PVDF-HFP nanocomposite film showed that the permittivity of the nanocomposite was enhanced by 13.9% compared to the pristine PVDF-HFP film whereas the breakdown field strength of the nanocomposite was nearly doubled compared to bare TiO2/PVDF-HFP nanocomposite. The enhanced dielectric performance of the nanocomposite resulted the improvement in the energy density of the PMMA-g-TiO2/PVDF-HFP nanocomposite (at 1 vol.% nanoparticle loading) by 14.4% w.r.t pristine PVDF-HFP (from 12.4 to 14.2 J/cm3) and an improvement in charge-discharge energy efficiency of 47% below 500 MV/m electric field [136]. Recently,Zhou et al. synthesized polyurea-grafted core-shell nanoparticles (PUA@BaTiO3) via in situ polymerization using 4,40-methylene diphenyl diisocyanate and 4,4’-oxydianiline as monomers. The PUA@BaTiO3 nanoparticles were subsequently blended with PVDF- CTFE to fabricate nanocomposite films for evaluation of dielectric properties. The incorporation of PUA@BaTiO3 in PVDF-CTFE matrix resulted in 1.65 times higher energy density (8.94 J/cm3) than that of pristine PVDF-CTFE (5.41 J/cm3). Further, the energy density of PUA@ BaTiO3/PVDF-CTFE nanocomposite was also 1.45 times higher than that of pristine BaTiO3/PVDF-CTFE nanocomposite [237]. Similarly, Jinhong et al., [141] reported the grafting of hyperbranched aromatic polyamide on Al2O3 nanoparticles (HBP@Al2O3) and the use of functionalized nanoparticles to en- Molecules 2021, 26, 2942 29 of 44

hance the dielectric properties of epoxy nanocomposite. The incorporation of HBP@Al2O3 nanoparticles to epoxy matrix resulted in an enhancement in the glass transition temperatures (176.3 to 208.1 ◦C with 20 wt% of the ﬁller). Furthermore, the dielectric constant of the HBP@Al2O3/epoxy nanocomposite was reported to be 5.0, compared to neat epoxy (3.5) and that of composite formed with 20 wt% bare Al2O3 nanoparticles (4.75). It was concluded that the improvement of Tgs (176.3 to 208.1 ◦C), dielectric strength (29.40 to 32.83 KV/mm) and the reduction of dielectric loss (0.024 to 0.020) were due to the good dispersion of the grafted NPs in the polymer matrix and also because of good interfacial adhesion of the grafted hyperbranched aromatic polyamide Al2O3 nanoparticles with the epoxy matrix.

8. Templated Approach to Prepare Polymer Grafted Nanoparticles Template-assisted method involves the formation of nanoparticles within the specific area of the template and the method can be efficiently used for fabrication of well-defined core-shell nanomaterials. Especially, template-assisted polymer grafting approach has been employed to control the size and shape (spherical, cylindrical, nanotubes, etc.) of core nanostructure as well as the structure of graft present on the surface of the nanoparticles [58,238–250]. Template-assisted polymer grafting which offers an easier way to synthesize nanoparticles (in situ) through micelle formation is a relatively straightforward technique. However, Gou et al. noticed bimodal distribution of PS/PMMA-g-CdS quantum dots on the core of the self-assembly of PS-b-PAA-b-PMMA triblock copolymer micelles [46]. This aspect was addressed by the selection of unimolecular star block copolymer micelles which often yields hairy nanoparticles with uniform sizes, various shapes, and sometimes unusual morphologies [47]. Matyjaszewski and coworkers demonstrated the utilization of poly(styrene-co-acrylonitrile)- b-poly (acrylic acid)-poly(divinylbenzene) (PSAN-b-PAA-PDVB) star-shaped copolymers obtained via activator regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP) (as depicted in Figure 21) as template for the synthesis of TiO2 nanoparticles. PMMA gate dielectrics layers fabricated with 0.4% wt. of the hybrid TiO2 nanoparticles was used in the measurement of organic field effect transistors (OFETs). The efficiency of OFETs was significantly better than OFETs based of pure PMMA as gate dielectric (charge carrier mobility has increased nearly 10-fold from ~0.06 to ~0.5 cm2/V·s). The improved performance of OFET could be ascribed to a significant decrease of roughness of dielectric layer (root mean square roughness was reduced from 15.3 nm to 0.43 nm) and Molecules 2021, 26, x FOR PEER REVIEW 29 of 43 changes to the surface energy (from 32.4 to 45.5 mN/m) of the gate dielectric layer after incorporation of hybrid nanoparticles [251].

Figure 21. FigureSynthesis 21. Synthesis of PSAN- of PSAN-b-PAA-PDVBb-PAA-PDVB star-shaped star-shaped polymer polymer templates. templates. Reproduced Reproduced with permission with permissi from Ref.on from [252]. Ref [252].

Guo et al. synthesized PS-grafted BaTiO3 nanoparticles with sizes of 11 nm and 27 nm using amphiphilic star-like poly(acrylic acid)-b-polystyrene (PAA-b-PS) diblock copolymer templates. PAA-b-PS was obtained by sequential atom transfer radical polymerization [252,253]. The dielectric performance with respect to temperature was studied for PS-BaTiO3 nanoparticles of 11 nm and 27 nm. Lin and coworkers also prepared PS-functionalized BaTiO3 NPs with different sizes (~27 nm and ~11 nm) by exploiting amphiphilic unimolecular star-like PAA-b-PS diblock copolymer as template. The synthesized nanoparticles were dispersed in low molecular weight PS-b-PMMA (MPS = 45,900 and MPMMA = 138,000) and high molecular weight PS-b-PMMA (MPS = 315,000 and MPMMA = 785,000) to fabricate PS@BaTiO3/PS-b-PMMA nanocomposite thin film. The incorporation of PS@BaTiO3 NPs into PS-b-PMMA, resulted in the preferential location of the BaTiO3 NPs in the PS nanocylinders. PS grafting to BaTiO3 NPs not only prevented aggregation by van der Waals forces, but also offered selective chemical affinity to the PS block of BCP. The measurements of dielectric properties of nanocomposite thin film revealed that the dielectric performance of the film was dependent upon the molecular weight of PS-b-PMMA and the size of PS@BaTiO3 NPs. BCP nanocomposite of 27 nm PS@BaTiO3 NP exhibited higher permittivity than that of 11 nm PS@BaTiO3 NP due to higher dielectric constant of large sized 27 nm NPs. Moreover, it was noticed that the nanocomposites of low molecular weight BCP exhibit higher dielectric constant than that of nanocomposite of high molecular weight due to lower permittivity of high molecular weight BCP. The low permittivity of higher molecular weight polymers could be attributed to the higher degree of chain coiling of longer polymer grafts than the low molecular weight polymer grafts [254]. Jiang and coworkers [58] synthesized PVDF-functionalized BaTiO3 nanoparticles by template-assisted approach. Firstly, they synthesized amphiphilic star diblock copolymer, by ATRP technique (Figure 22). PAA-b-PVDF (PAA as inner hydrophilic block while PVDF as outer hydrophobic block with well-controlled molecular weight of narrow dispersity) was dissolved in a mixture of DMF and benzyl alcohol followed by the addition of BaCl2.2H2O and TiCl4 as precursor and NaOH. Precursors assemble in the space of PAA blocks and PVDF chains serve as the arm of the self-assembled structure. The size of the nanoparticles was tuned based on the molecular weight of the PAA and PVDF blocks of the star copolymer. Notably, PVDF-BaTiO3 nanocomposites (single component) displayed not only high dielectric constant (~80 at 100 Hz) but also low dielectric loss (<0.2) over broad frequency range as compared to PVDF.

Molecules 2021, 26, 2942 30 of 44

Guo et al. synthesized PS-grafted BaTiO3 nanoparticles with sizes of 11 nm and 27 nm using amphiphilic star-like poly(acrylic acid)-b-polystyrene (PAA-b-PS) diblock copolymer templates. PAA-b-PS was obtained by sequential atom transfer radical polymerization [252,253]. The dielectric performance with respect to temperature was studied for PS-BaTiO3 nanoparticles of 11 nm and 27 nm. Lin and coworkers also prepared PS-functionalized BaTiO3 NPs with different sizes (~27 nm and ~11 nm) by exploiting amphiphilic unimolecular star-like PAA-b-PS diblock copolymer as template. The synthesized nanoparticles were dispersed in low molecular weight PS-b-PMMA (MPS = 45,900 and MPMMA = 138,000) and high molecular weight PS-b-PMMA (MPS = 315,000 and MPMMA = 785,000) to fabricate PS@BaTiO3/PS-b-PMMA nanocomposite thin film. The incorporation of PS@BaTiO3 NPs into PS-b-PMMA, resulted in the preferential location of the BaTiO3 NPs in the PS nanocylinders. PS grafting to BaTiO3 NPs not only prevented aggregation by van der Waals forces, but also offered selective chemical affinity to the PS block of BCP. The measurements of dielectric properties of nanocomposite thin film revealed that the dielectric performance of the film was dependent upon the molecular weight of PS-b-PMMA and the size of PS@BaTiO3 NPs. BCP nanocomposite of 27 nm PS@BaTiO3 NP exhibited higher permittivity than that of 11 nm PS@BaTiO3 NP due to higher dielectric constant of large sized 27 nm NPs. Moreover, it was noticed that the nanocomposites of low molecular weight BCP exhibit higher dielectric constant than that of nanocomposite of high molecular weight due to lower permittivity of high molecular weight BCP. The low permittivity of higher molecular weight polymers could be attributed to the higher degree of chain coiling of longer polymer grafts than the low molecular weight polymer grafts [254]. Jiang and coworkers [58] synthesized PVDF-functionalized BaTiO3 nanoparticles by template-assisted approach. Firstly, they synthesized amphiphilic star diblock copolymer, by ATRP technique (Figure 22). PAA-b-PVDF (PAA as inner hydrophilic block while PVDF as outer hydrophobic block with well-controlled molecular weight of narrow dispersity) was dissolved in a mixture of DMF and benzyl alcohol followed by the addition of BaCl2.2H2O and TiCl4 as precursor and NaOH. Precursors assemble in the space of PAA blocks and PVDF chains serve as the arm of the self-assembled structure. The size of the nanoparticles was tuned based on the molecular weight of the PAA and PVDF blocks of the star copolymer. Notably, PVDF-BaTiO3 nanocomposites (single component) displayed not only high dielectric constant (~80 at 100 Hz) but also low dielectric loss (<0.2) over Molecules 2021, 26, x FOR PEER REVIEW 30 of 43 broad frequency range as compared to PVDF.

Figure 22. Synthesis of amphiphilic 21-arm, star-like PAA-b-PVDF diblock copolymer and Figure 22. Synthesis of amphiphilic 21-arm, star-like PAA-b-PVDF diblock copolymer and PVDF@BaTiO3 nanoparticles. Reproduced with permission from Ref [58]. PVDF@BaTiO3 nanoparticles. Reproduced with permission from Ref. [58]. However, one of the challenges of unimolecular star block copolymer micelles using template approach in formulating core shell nanoparticles is the low graft density of the polymer grafted nanoparticles. Like the grafting to method, template assisted polymer grafting is not as widely sought-after technique for the synthesis of core-shell nanoparticles.

Molecules 2021, 26, 2942 31 of 44

However, one of the challenges of unimolecular star block copolymer micelles using template approach in formulating core shell nanoparticles is the low graft density of the polymer grafted nanoparticles. Like the grafting to method, template assisted polymer grafting is not as widely sought-after technique for the synthesis of core-shell nanoparticles.

9. Discussion Current technologies for pulsed power applications utilize polymers as the dielectric material of choice due to their high electrical resistance, low dielectric loss, self-healing capability, formability and flexibility. However, the most widely used polymeric system namely metallized biaxially oriented polypropylene (BOPP) and its variants do not meet the demands of next generation film dielectrics. Composite dielectrics offer an unique opportunity to combine the high εr of inorganic fillers with the high Ebd of a polymer matrix to achieve high energy density capacitors. For significant gains in permittivity in polymer composites so as to achieve higher energy density, loadings of dispersed fillers in the composite need to be above 20% v/v. At these loading levels, achieving good NP dispersion—especially in non-polar polymer matrices—is challenging due to particle agglomeration during film preparation. Aggregated nanoparticles of high permittivity act as electrical field expulsion defect centers in filled polymers. Such defect centers effectively distort the distribution of electric field, making the local electrical field in the matrix much higher than the average electric field and also lower the overall energy storage of the nanocomposite. The extent of the field distortion is adversely influenced by the discontinuous (sharp and large) permittivity contrast between the NPs and the polymer matrix. An approach to address the field distortion is to consider high permittivity nanoparticles with core-shell architectures so that the nanoparticles permittivity gradually approaches that of the polymer matrix. We discuss various approaches to synthesize polymer grafted nanoparticles. Among the three commonly used SI-CRPs, SI-ATRP has been shown to be one of the most versatile polymerization techniques because it can be used under broad experimental conditions and can be adapted to synthesize nanoparticles with polymer grafts having a wide range of functional groups. Additionally, ATRP can be used to synthesize core@ double-shell structured nanoparticle via a two-step process. It was noted that the thickness of the second shell can be controlled by adjusting the ratio of monomer and macro initiator. The polymerization of activating monomer by ATRP process requires the use of alkyl halide initiator and a transition metal complex as catalyst (e.g., CuBr/ligand). However, the persistence of small amount of copper catalyst in the grafted nanoparticle can pose challenges because of the potential adverse effect the copper ions could have on the dielectric properties of the nanocomposite. In this regard, ATRP techniques with extremely low amounts of copper have been investigated for the synthesis of polymer grafted nanoparticles. For example, PS and PMMA were grafted from phosphonic acid functionalized BaTiO3 NPs via activated regenerated by electron transfer (AGRET) ATRP approach using only ppm amount of the copper catalyst [27]. Alternatively, efforts have focused on conducting ATRP using light without the addition of any metal catalyst and the thickness of polymeric shell was tunable based on the duration of white LED irradiation. These approaches to conduct ARGET ATRP with limited Cu species or light mediated ATRP offer novel opportunities and new routes for engineering surfaces and interfaces of nanoparticles with polymer grafts without significant copper residues. In contrast to ATRP, RAFT can be considered as a conventional radical polymerization with the addition of a chain transfer agent (CTA), which mediates the polymerization. The RAFT CTAs can be bound on nanoparticle surfaces via two main approaches. In the first approach, the CTA is synthesized with a reactive anchoring group (chlorosilyl group, phosphonic acid group) and then covalently bound to an unmodified NP. The second approach relies on grafting a functional RAFT agent to pre-modified nanoparticles. There are several examples in the literature where RAFT polymerization has been successfully used to synthesize unimodal or bimodal brush modified nanoparticles so as to Molecules 2021, 26, 2942 32 of 44

achieve optimum dielectric performance of nanocomposite [177,178,186,187]. Using RAFT technique, the shell thickness, polymer composition and the aerial density of polymer brush in the core-shell nanoparticles have been successfully tuned and used for energy storage applications. Despite SI-ATRP being the most predominant and sought after technique for polymer grafting on nanoparticle surface, RAFT technique too has steadily gained popularity over the years. This is because of the adaptability of RAFT to a range of polymerization conditions. Further advances in RAFT polymerization include the ability to synthesize multi-block copolymers with a high degree of fidelity, conducting reactions in the presence of oxygen and its compatibility with a broad range of functional groups and the absence of copper residues after polymer grafting have clearly contributed to the recent drive for use of RAFT technique in the synthesis of core-shell nanoparticles. The “grafting to” approach, on the other hand, involves surface modification of nanomaterials with functionality which is complimentary to the end-group functionality of polymer followed by coupling via suitable conjugation or click chemistry. In particular, chloro silyl-terminated polymer or phosphonate-terminated polymer or click chemistry has been used to graft polymer on nanoparticles. Because of the ability to precisely control the molecular weight of grafts in the grafting to technique, a study of low molecular weight grafts on NP revealed a relatively smooth surface while high molecular weight grafts on NP revealed a pancake like structure suggesting the ability of grafting to technique to control microstructure of the polymer shell at the expense of polymer graft molecular weight. Although “grafting-to” approach is easy and efficient, it presents challenges such as decreased graft density with increase in the molecular weight of the grafted polymer. Therefore, grafting to technique is not as widely sought after method for the synthesis of core-shell nanoparticles. Template-assisted polymer grafting which offers an easier way to synthesize nanoparticles (in situ) through micelle formation is a relatively straightforward technique. An issue that has been observed is the formation of multiple NPs in the core of multi-molecular micelles [46]. To overcome this challenge, the use of unimolecular star block copolymer micelles has been tried out with a greater success in the synthesis of hairy nanoparticles with uniform sizes, various shapes, and sometimes unusual morphologies [47]. However, one of the challenges of unimolecular star block copolymer micelles using template approach is the low graft density of the polymer grafted nanoparticles. Like the grafting to method, template assisted polymer grafting is not as widely sought after graft technique for the synthesis of core-shell nanoparticles. In in situ polymerization method, metal oxide nanoparticles are usually mixed with organic monomers, either in the presence or absence of a solvent, and then the monomers are polymerized. There is a thermodynamic compatibility at the polymer matrix-nanoparticle reinforcement interface and thus provide stronger matrix dispersion bond with very good miscibility of the nanocomposite. Several structure-property relationship studies of polymer grafted nanoparticles filled polymer nanocomposites [27,119–121,124–126] using SI-CRP technique have been conducted and they clearly indicate that a number of factors influence the dielectric performance of the nanocomposite including the thickness of the core and the shell of the core-shell nanoparticles and the type of polymer grafted on the nanoparticles, interfacial separation between core NPs and polymer shell, the composition of nanocomposite (single or multicomponent type of nanocomposite), the presence of polarizable interfacial layer and double shell coverage of nanoparticles. For establishing clear structure-property relationships, an efficient initiator and control of polymer brush graft density is important. At present, the techniques for facile determination of polymer brush grafting density and the initiator efficiency are scarce. More importantly, a simple, versatile and accurate technique for determining the number of initiator units per square nanometer present on the nanoparticle surface is not available. This information would be highly relevant for the Molecules 2021, 26, 2942 33 of 44

development of composition-structure-property relationship paradigm of single or multi component nanocomposite. Another interesting application of core-shell structure is in the pursuit of all-polymer field-effect transistors in the generation of polymer brush-based gate dielectrics. Especially SI-CRP technique has drawn significant attention for design of polymer field effect transistors. The attractiveness in the use of SI-CRP technique is in the ease of device preparation and the avoidance of expensive fabrication facilities. The performance of pentacene-based thin-film transistor fabricated from PS-grafted SiO2 and PMMA-grafted SiO2 using ATRP technique as a gate dielectric showed the importance of interfacial material and its structure in the design of OFET [97,98]. The device fabricated from 47 nm thickness of PS brush 2 exhibited highest mobility (µFET = 0.099 cm /V·s) indicating that optimum molecular weight polymer brushes need to be grown from surface of dielectric for achieving desirable performance. On the other hand, the surface-grafted PMMA brush (10 nm)/SiO2 (9 nm) on silicon wafer exhibited lower leakage than that of surface-grafted PMMA brush (20 nm) on silicon wafer (free of 9 nm SiO2 layer). Additionally, it was noted that the thickness of the polymer brush on silica could be modulated based on the activity of the catalyst, the reactant concentration and reaction time. The PMMA brushes on silica showed excellent insulating characteristics, large capacitance, and low charge-trapping density. Field-effect transistors with PMMA brush as the dielectric layer demonstrate excellent charge transport. However, the field of polymer brush-based hybrid materials in OFET is still in its infancy stage [43] and it needs further exploration.

10. Summary and Future Outlook In this review article, we described various synthetic approaches for preparation of core-shell structures of polymer grafted nanoparticles. The grafting of polymeric chains to nanoparticles can generally be accomplished by four approaches namely (i) ‘grafting to’; (ii) ‘grafting from’; (iii) templated and (iv) in situ polymerization or encapsulation. All the four approaches yield polymer grafted nanoparticles of varying shell architectures. Unlike grafting to method, grafting from method allows to synthesize nanoparticles with high grafting density and polymer shells of varied composition. The “grafting from” method is also termed the surface-initiated controlled radical polymerization where the initiator functionality (SI-ATRP) or CTA functionality (SI-RAFT) or alkoxy amine functionality (NMP) is anchored to the surface of nanomaterials followed by growth of polymer chains. Recent progress in the various synthetic strategies for formulation of core-shell nanoparticles has created a myriad of architectures of core-shell nanoparticles. Inter- esting polymer architectures with unique features such as polymer loops, bottlebrushes have made the polymers synthesized by SI-CRP technique a valuable toolkit that can be used for a broad range of applications. Notably, the opportunity to synthesize BCP-g-NPs in formulating single-component hybrid materials has largely been unexplored and needs to be tapped for the design of nano-dielectrics. A high-performance core-shell hybrid material in which the polymer is directly grown from the nanoparticles provides an opportunity to synthesize single component nano-dielectrics. This subject need further exploration because it is possible to have high ceramic loading in the one component polymer-ceramic system with minimal negative effect of ceramics towards electrical discharge due to controlled minimal aggregation, i.e., high degree of dispersion. From an academic perspective, little is known about the structure and dynamics of self-assembling of single component BCP-g-NP system, i.e., whether a block copolymer tagged to nanoparticle can microphase- separate and self-assemble. There is also signiﬁcant interest for developing molecular level understanding of non-centrosymmetric materials from fundamental perspective, and the parameter space it presents in terms of grafting density, and copolymer length and composition is wide open. Under the appropriate processing conditions, symmetric BCPs can microphase separate to form parallel lamellae [255–257]. This arrangement presents a unique opportunity to advance the ﬁeld of core-shell nanoparticles in the formulating next generation nano-dielectrics of unprecedented performance. Molecules 2021, 26, 2942 34 of 44

Author Contributions: Conceptualization, B.V.T. and D.R.; writing—original draft preparation, B.V.T. and I.E.A.; writing—review and editing, B.V.T., I.E.A. and D.R.; supervision, D.R.; project administration, D.R. N.P. and A.K.; funding acquisition, D.R. N.P. and A.K. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Science Foundation (NSF), grant number DMR- 1901127. Data Availability Statement: No new data were created or analyzed in this study. Data sharing is not applicable to this article. Conﬂicts of Interest: The authors declare no conﬂict of interest. Sample Availability: Samples of the compounds are not available from the authors.

References 1. Zhang, Q.M.; Li, H.; Poh, M.; Xia, F.; Cheng, Z.Y.; Xu, H.; Huang, C. An all-organic composite actuator material with a high dielectric constant. Nature 2002, 419, 284–287. [CrossRef][PubMed] 2. Ortiz, R.P.; Facchetti, A.; Marks, T.J. High-k organic, inorganic, and hybrid dielectrics for low-voltage organic ﬁeld-effect transistors. Chem. Rev. 2010, 110, 205–239. [CrossRef] 3. Chauhan, A.; Patel, S.; Vaish, R.; Bowen, C. Anti-Ferroelectric Ceramics for High Energy Density Capacitors. Materials 2015, 8, 8009–8031. [CrossRef] 4. Dang, Z.-M.; Yuan, J.-K.; Yao, S.-H.; Liao, R.-J. Flexible Nanodielectric Materials with High Permittivity for Power Energy Storage. Adv. Mater. 2013, 25, 6334–6365. [CrossRef][PubMed] 5. Hao, X. A review on the dielectric materials for high energy-storage application. J. Adv. Dielectr. 2013, 03, 1330001. [CrossRef] 6. Palneedi, H.; Peddigari, M.; Hwang, G.-T.; Jeong, D.-Y.; Ryu, J. High-Performance Dielectric Ceramic Films for Energy Storage Capacitors: Progress and Outlook. Adv. Funct. Mater. 2018, 28, 1803665. [CrossRef] 7. Ho, J.; Jow, T.R.; Boggs, S. Historical introduction to capacitor technology. IEEE Electr. Insul. Mag. 2010, 26, 20–25. [CrossRef] 8. Kishi, H.; Mizuno, Y.; Chazono, H. Base-Metal Electrode-Multilayer Ceramic Capacitors: Past, Present and Future Perspectives. Jpn. J. Appl. Phys. 2003, 42, 1–15. [CrossRef] 9. Du, H.; Lin, X.; Zheng, H.; Qu, B.; Huang, Y.; Chu, D. Colossal permittivity in percolative ceramic/metal dielectric composites. J. Alloys Compd. 2016, 663, 848–861. [CrossRef] 10. Hong, K.; Lee, T.H.; Suh, J.M.; Yoon, S.-H.; Jang, H.W. Perspectives and challenges in multilayer ceramic capacitors for next generation electronics. J. Mater. Chem. C 2019, 7, 9782–9802. [CrossRef] 11. Tan, D.Q. Review of Polymer-Based Nanodielectric Exploration and Film Scale-Up for Advanced Capacitors. Adv. Funct. Mater. 2019, 1808567. [CrossRef] 12. Dang, Z.M.; Yuan, J.K.; Zha, J.W.; Zhou, T.; Li, S.T.; Hu, G.H. Fundamentals, processes and applications of high-permittivity polymer-matrix composites. Prog. Mater. Sci. 2012, 57, 660–723. [CrossRef] 13. Singh, M.; Apata, I.E.; Samant, S.; Wu, W.; Tawade, B.V.; Pradhan, N.; Raghavan, D.; Karim, A. Nanoscale Strategies to Enhance the Energy Storage Capacity of Polymeric Dielectric Capacitors: Review of Recent Advances. Polym. Rev. 2021, 1–50. [CrossRef] 14. Tawade, B.V.; Apata, I.E.; Singh, M.; Das, P.; Pradhan, N.; Al-Enizi, A.M.; Karim, A.; Raghavan, D. Recent developments in the synthesis of chemically modiﬁed nanomaterials for use in dielectric and electronics applications. Nanotechnology 2021, 32, 142004. [CrossRef][PubMed] 15. Luo, H.; Zhou, X.; Ellingford, C.; Zhang, Y.; Chen, S.; Zhou, K.; Zhang, D.; Bowen, C.R.; Wan, C. Interface design for high energy density polymer nanocomposites. Chem. Soc. Rev. 2019, 48, 4424–4465. [CrossRef] 16. Baer, E.; Zhu, L. 50th Anniversary Perspective: Dielectric Phenomena in Polymers and Multilayered Dielectric Films. Macro- molecules 2017, 50, 2239–2256. [CrossRef] 17. Maier, G. Low dielectric constant polymers for microelectronics. Prog. Polym. Sci. 2001, 26, 3–65. [CrossRef] 18. Wang, Q.; Zhu, L. Polymer nanocomposites for electrical energy storage. J. Polym. Sci. Part B Polym. Phys. 2011, 49, 1421–1429. [CrossRef] 19. Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M.A. Chemistry and Properties of Nanocrystals of Different Shapes. Chem. Rev. 2005, 105, 1025–1102. [CrossRef][PubMed] 20. Nikam, A.V.; Prasad, B.L.V.; Kulkarni, A.A. Wet chemical synthesis of metal oxide nanoparticles: A review. CrystEngComm 2018, 20, 5091–5107. [CrossRef] 21. Walton, R.I. Subcritical solvothermal synthesis of condensed inorganic materials. Chem. Soc. Rev. 2002, 31, 230–238. [CrossRef] 22. Park, J.; Joo, J.; Kwon, S.G.; Jang, Y.; Hyeon, T. Synthesis of Monodisperse Spherical Nanocrystals. Angew. Chemie Int. Ed. 2007, 46, 4630–4660. [CrossRef][PubMed] 23. Hyeon, T. Chemical synthesis of magnetic nanoparticles. Chem. Commun. 2003, 927–934. [CrossRef][PubMed] Molecules 2021, 26, 2942 35 of 44

24. Hulkoti, N.I.; Taranath, T.C. Biosynthesis of nanoparticles using microbes—A review. Colloids Surfaces B Biointerfaces 2014, 121, 474–483. [CrossRef] 25. Wei, J.; Zhu, L. Intrinsic polymer dielectrics for high energy density and low loss electric energy storage. Prog. Polym. Sci. 2020, 106, 101254. [CrossRef] 26. Huan, T.D.; Boggs, S.; Teyssedre, G.; Laurent, C.; Cakmak, M.; Kumar, S.; Ramprasad, R. Advanced polymeric dielectrics for high energy density applications. Prog. Mater. Sci. 2016, 83, 236–269. [CrossRef] 27. Paniagua, S.A.; Kim, Y.; Henry, K.; Kumar, R.; Perry, J.W.; Marder, S.R. Surface-Initiated Polymerization from Barium Titanate Nanoparticles for Hybrid Dielectric Capacitors. ACS Appl. Mater. Interfaces 2014, 6, 3477–3482. [CrossRef] 28. Jouault, N.; Vallat, P.; Dalmas, F.; Said, S.; Jestin, J.; Boué, F. Well-Dispersed Fractal Aggregates as Filler in Polymer−Silica Nanocomposites: Long-Range Effects in Rheology. Macromolecules 2009, 42, 2031–2040. [CrossRef] 29. Oberdisse, J.; El Harrak, A.; Carrot, G.; Jestin, J.; Boué, F. Structure and rheological properties of soft–hard nanocomposites: Influence of aggregation and interfacial modification. Polymer 2005, 46, 6695–6705. [CrossRef] 30. Jestin, J.; Cousin, F.; Dubois, I.; Ménager, C.; Schweins, R.; Oberdisse, J.; Boué, F. Anisotropic Reinforcement of Nanocomposites Tuned by Magnetic Orientation of the Filler Network. Adv. Mater. 2008, 20, 2533–2540. [CrossRef] 31. Radhakrishnan, B.; Ranjan, R.; Brittain, W.J. Surface initiated polymerizations from silica nanoparticles. Soft Matter 2006, 2, 386–396. [CrossRef] 32. Chevigny, C.; Gigmes, D.; Bertin, D.; Jestin, J.; Boué, F. Polystyrene grafting from silica nanoparticles via nitroxide-mediated polymerization (NMP): Synthesis and SANS analysis with the contrast variation method. Soft Matter 2009, 5, 3741–3753. [CrossRef] 33. Yan, J.; Bockstaller, M.R.; Matyjaszewski, K. Brush-modified materials: Control of molecular architecture, assembly behavior, properties and applications. Prog. Polym. Sci. 2020, 100, 101180. [CrossRef] 34. Ma, S.; Zhang, X.; Yu, B.; Zhou, F. Brushing up functional materials. NPG Asia Mater. 2019, 11, 24. [CrossRef] 35. Bouharras, F.E.; Raihane, M.; Ameduri, B. Recent progress on core-shell structured BaTiO3@polymer/fluorinated polymers nanocomposites for high energy storage: Synthesis, dielectric properties and applications. Prog. Mater. Sci. 2020, 113, 100670. [CrossRef] 36. Chancellor, A.J.; Seymour, B.T.; Zhao, B. Characterizing Polymer-Grafted Nanoparticles: From Basic Defining Parameters to Behavior in Solvents and Self-Assembled Structures. Anal. Chem. 2019.[CrossRef][PubMed] 37. Lenart, W.R.; Hore, M.J.A. Structure–property relationships of polymer-grafted nanospheres for designing advanced nanocomposites. Nano-Struct. Nano-Objects 2018, 16, 428–440. [CrossRef] 38. Yan, J.; Pietrasik, J.; Wypych-Puszkarz, A.; Ciekanska, M.; Matyjaszewski, K. Synthesis of High k Nanoparticles by Controlled Radical Polymerization. In Solution-Processable Components for Organic Electronic Devices; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2019; pp. 181–226. 39. Francis, R.; Joy, N.; Aparna, E.P.; Vijayan, R. Polymer Grafted Inorganic Nanoparticles, Preparation, Properties, and Applications: A Review. Polym. Rev. 2014, 54, 268–347. [CrossRef] 40. Wang, Z.; Bockstaller, M.R.; Matyjaszewski, K. Synthesis and Applications of ZnO/Polymer Nanohybrids. ACS Mater. Lett. 2021, 599–621. [CrossRef] 41. Fernandes, N.J.; Koerner, H.; Giannelis, E.P.; Vaia, R.A. Hairy nanoparticle assemblies as one-component functional polymer nanocomposites: Opportunities and challenges. MRS Commun. 2013, 3, 13–29. [CrossRef] 42. Williams, G.A.; Ishige, R.; Cromwell, O.R.; Chung, J.; Takahara, A.; Guan, Z. Mechanically Robust and Self-Healable Superlattice Nanocomposites by Self-Assembly of Single-Component “Sticky” Polymer-Grafted Nanoparticles. Adv. Mater. 2015, 27, 3934– 3941. [CrossRef][PubMed] 43. Sato, T.; Morinaga, T.; Marukane, S.; Narutomi, T.; Igarashi, T.; Kawano, Y.; Ohno, K.; Fukuda, T.; Tsujii, Y. Novel Solid-State Polymer Electrolyte of Colloidal Crystal Decorated with Ionic-Liquid Polymer Brush. Adv. Mater. 2011, 23, 4868–4872. [CrossRef] [PubMed] 44. Zhang, M.; Gao, G.; Li, C.-Q.; Liu, F.-Q. Titania-Coated Polystyrene Hybrid Microballs Prepared with Miniemulsion Polymeriza- tion. Langmuir 2004, 20, 1420–1424. [CrossRef][PubMed] 45. Sondi, I.; Fedynyshyn, T.H.; Sinta, R.; Matijević,E. Encapsulation of Nanosized Silica by in Situ Polymerization of tert -Butyl Acrylate Monomer. Langmuir 2000, 16, 9031–9034. [CrossRef] 46. Guo, Y.; Moffitt, M.G. Semiconductor quantum dots with environmentally responsive mixed polystyrene/poly(methyl methacrylate) brush layers. Macromolecules 2007, 40, 5868–5878. [CrossRef] 47. Chen, Y.; Yoon, Y.J.; Pang, X.; He, Y.; Jung, J.; Feng, C.; Zhang, G.; Lin, Z. Precisely Size-Tunable Monodisperse Hairy Plasmonic Nanoparticles via Amphiphilic Star-Like Block Copolymers. Small 2016, 12, 6714–6723. [CrossRef][PubMed] 48. Zoppe, J.O.; Ataman, N.C.; Mocny, P.; Wang, J.; Moraes, J.; Klok, H.-A. Surface-Initiated Controlled Radical Polymerization: State-of-the-Art, Opportunities, and Challenges in Surface and Interface Engineering with Polymer Brushes. Chem. Rev. 2017, 117, 1105–1318. [CrossRef] 49. Hui, C.M.; Pietrasik, J.; Schmitt, M.; Mahoney, C.; Choi, J.; Bockstaller, M.R.; Matyjaszewski, K. Surface-initiated polymerization as an enabling tool for multifunctional (Nano-)engineered hybrid materials. Chem. Mater. 2014, 26, 745–762. [CrossRef] 50. Sakellariou, G.; Priftis, D.; Baskaran, D. Surface-initiated polymerization from carbon nanotubes: Strategies and perspectives. Chem. Soc. Rev. 2013, 42, 677–704. [CrossRef] Molecules 2021, 26, 2942 36 of 44

51. Advincula, R.C. Surface Initiated Polymerization from Nanoparticle Surfaces. J. Dispers. Sci. Technol. 2003, 24, 343–361. [CrossRef] 52. Edmondson, S.; Osborne, V.L.; Huck, W.T.S. Polymer brushes via surface-initiated polymerizations. Chem. Soc. Rev. 2004, 33, 14–22. [CrossRef][PubMed] 53. Matyjaszewski, K.; Miller, P.J.; Shukla, N.; Immaraporn, B.; Gelman, A.; Luokala, B.B.; Siclovan, T.M.; Kickelbick, G.; Vallant, T.; Hoffmann, H.; et al. Polymers at Interfaces: Using Atom Transfer Radical Polymerization in the Controlled Growth of Homopolymers and Block Copolymers from Silicon Surfaces in the Absence of Untethered Sacriﬁcial Initiator. Macromolecules 1999, 32, 8716–8724. [CrossRef] 54. Cheng, G.; Böker, A.; Zhang, M.; Krausch, G.; Müller, A.H.E. Amphiphilic Cylindrical Core−Shell Brushes via a “Grafting From” Process Using ATRP. Macromolecules 2001, 34, 6883–6888. [CrossRef] 55. Li, C.; Benicewicz, B.C. Synthesis of Well-Deﬁned Polymer Brushes Grafted onto Silica Nanoparticles via Surface Reversible Addition−Fragmentation Chain Transfer Polymerization. Macromolecules 2005, 38, 5929–5936. [CrossRef] 56. Matsuno, R.; Yamamoto, K.; Otsuka, H.; Takahara, A. Polystyrene-Grafted Magnetite Nanoparticles Prepared through Surface- Initiated Nitroxyl-Mediated Radical Polymerization. Chem. Mater. 2003, 15, 3–5. [CrossRef] 57. Husseman, M.; Malmström, E.E.; McNamara, M.; Mate, M.; Mecerreyes, D.; Benoit, D.G.; Hedrick, J.L.; Mansky, P.; Huang, E.; Russell, T.P.; et al. Controlled Synthesis of Polymer Brushes by “Living” Free Radical Polymerization Techniques. Macromolecules 1999, 32, 1424–1431. [CrossRef] 58. Jiang, B.; Pang, X.; Li, B.; Lin, Z. Organic-Inorganic Nanocomposites via Placing Monodisperse Ferroelectric Nanocrystals in Direct and Permanent Contact with Ferroelectric Polymers. J. Am. Chem. Soc. 2015, 137, 11760–11767. [CrossRef] 59. Bhanvase, B.A.; Sonawane, S.H. Ultrasound assited In-situ Emulsion polymerization for polymer nanocomposite: A review. Chem. Eng. Process. Process Intensif. 2014.[CrossRef] 60. Sato, M.; Kato, T.; Ohishi, T.; Ishige, R.; Ohta, N.; White, K.L.; Hirai, T.; Takahara, A. Precise Synthesis of Poly(methyl methacrylate) Brush with Well-Controlled Stereoregularity Using a Surface-Initiated Living Anionic Polymerization Method. Macromolecules 2016, 49, 2071–2076. [CrossRef] 61. Advincula, R.; Zhou, Q.; Park, M.; Wang, S.; Mays, J.; Sakellariou, G.; Pispas, S.; Hadjichristidis, N. Polymer Brushes by Living Anionic Surface Initiated Polymerization on Flat Silicon (SiOx) and Gold Surfaces: Homopolymers and Block Copolymers. Langmuir 2002, 18, 8672–8684. [CrossRef] 62. Zhao, B.; Brittain, W.J. Synthesis of Polystyrene Brushes on Silicate Substrates via Carbocationic Polymerization from Self- Assembled Monolayers. Macromolecules 2000, 33, 342–348. [CrossRef] 63. Zhao, B.; Brittain, W.J. Synthesis, Characterization, and Properties of Tethered Polystyrene- b -polyacrylate Brushes on Flat Silicate Substrates. Macromolecules 2000, 33, 8813–8820. [CrossRef] 64. Hadjichristidis, N.; Iatrou, H.; Pispas, S.; Pitsikalis, M. Anionic polymerization: High vacuum techniques. J. Polym. Sci. Part A Polym. Chem. 2000, 38, 3211–3234. [CrossRef] 65. Jordan, R.; Ulman, A.; Kang, J.F.; Rafailovich, M.H.; Sokolov, J. Surface-Initiated Anionic Polymerization of Styrene by Means of Self-Assembled Monolayers. J. Am. Chem. Soc. 1999, 121, 1016–1022. [CrossRef] 66. Li, Z.; Baskaran, D. Surface-Initiated Anionic Polymerization from Nanomaterials. In Anionic Polymerization; Springer: Tokyo, Japan, 2015; pp. 495–537. 67. Advincula, R. Polymer Brushes by Anionic and Cationic Surface-Initiated Polymerization (SIP). In Surface-Initiated Polymerization I; Jordan, R., Ed.; Springer: Berlin/Heidelberg, Germany, 2006; pp. 107–136. ISBN 978-3-540-30247-6. 68. Prucker, O.; Rühe, J. Synthesis of poly(styrene) monolayers attached to high surface area silica gels through self-assembled monolayers of azo initiators. Macromolecules 1998, 31, 592–601. [CrossRef] 69. Prucker, O.; Rühe, J. Mechanism of Radical Chain Polymerizations Initiated by Azo Compounds Covalently Bound to the Surface of Spherical Particles. Macromolecules 1998, 31, 602–613. [CrossRef] 70. Li, Q.; Zhang, Y.; Li, H.; Tang, Q.; Jiang, L.; Chi, L.; Fuchs, H.; Hu, W. Battery drivable organic single-crystalline transistors based on surface grafting ultrathin polymer dielectric. Adv. Funct. Mater. 2009, 19, 2987–2991. [CrossRef] 71. Patten, T.E.; Matyjaszewski, K. Atom Transfer Radical Polymerization and the Synthesis of Polymeric Materials. Adv. Mater. 1998, 10, 901–915. [CrossRef] 72. Bartholome, C.; Beyou, E.; Bourgeat-Lami, E.; Chaumont, P.; Lefebvre, F.; Zydowicz, N. Nitroxide-Mediated Polymerization of Styrene Initiated from the Surface of Silica Nanoparticles. In Situ Generation and Grafting of Alkoxyamine Initiators. Macromolecules 2005, 38, 1099–1106. [CrossRef] 73. Grubbs, R.B. Nitroxide-Mediated Radical Polymerization: Limitations and Versatility. Polym. Rev. 2011, 51, 104–137. [CrossRef] 74. Matyjaszewski, K.; Xia, J. Atom transfer radical polymerization. Chem. Rev. 2001, 101, 2921–2990. [CrossRef] 75. Kamigaito, M.; Ando, T.; Sawamoto, M. Metal-Catalyzed Living Radical Polymerization. Chem. Rev. 2001, 101, 3689–3746. [CrossRef][PubMed] 76. Matyjaszewski, K. Atom Transfer Radical Polymerization (ATRP): Current Status and Future Perspectives. Macromolecules 2012, 45, 4015–4039. [CrossRef] 77. Konkolewicz, D.; Wang, Y.; Zhong, M.; Krys, P.; Isse, A.A.; Gennaro, A.; Matyjaszewski, K. Reversible-Deactivation Radical Polymerization in the Presence of Metallic Copper. A Critical Assessment of the SARA ATRP and SET-LRP Mechanisms. Macromolecules 2013, 46, 8749–8772. [CrossRef] Molecules 2021, 26, 2942 37 of 44

78. Konkolewicz, D.; Wang, Y.; Krys, P.; Zhong, M.; Isse, A.A.; Gennaro, A.; Matyjaszewski, K. SARA ATRP or SET-LRP. End of controversy? Polym. Chem. 2014, 5, 4396–4417. [CrossRef] 79. Jakubowski, W.; Min, K.; Matyjaszewski, K. Activators Regenerated by Electron Transfer for Atom Transfer Radical Polymerization of Styrene. Macromolecules 2006, 39, 39–45. [CrossRef] 80. Wang, J.-S.; Matyjaszewski, K. “Living”/Controlled Radical Polymerization. Transition-Metal-Catalyzed Atom Transfer Radical Polymerization in the Presence of a Conventional Radical Initiator. Macromolecules 1995, 28, 7572–7573. [CrossRef] 81. Corrigan, N.; Jung, K.; Moad, G.; Hawker, C.J.; Matyjaszewski, K.; Boyer, C. Reversible-deactivation radical polymerization (Controlled/living radical polymerization): From discovery to materials design and applications. Prog. Polym. Sci. 2020, 111, 101311. [CrossRef] 82. Wu, L.; Glebe, U.; Böker, A. Surface-initiated controlled radical polymerizations from silica nanoparticles, gold nanocrystals, and bionanoparticles. Polym. Chem. 2015, 6, 5143–5184. [CrossRef] 83. Wang, Z.; Yan, J.; Liu, T.; Wei, Q.; Li, S.; Olszewski, M.; Wu, J.; Sobieski, J.; Fantin, M.; Bockstaller, M.R.; et al. Control of Dispersity and Grafting Density of Particle Brushes by Variation of ATRP Catalyst Concentration. ACS Macro Lett. 2019, 8, 859–864. [CrossRef] 84. Wang, Z.; Lee, J.; Wang, Z.; Zhao, Y.; Yan, J.; Lin, Y.; Li, S.; Liu, T.; Olszewski, M.; Pietrasik, J.; et al. Tunable Assembly of Block Copolymer Tethered Particle Brushes by Surface-Initiated Atom Transfer Radical Polymerization. ACS Macro Lett. 2020, 9, 806–812. [CrossRef] 85. Farmer, S.C.; Patten, T.E. Photoluminescent Polymer/Quantum Dot Composite Nanoparticles. Chem. Mater. 2001, 13, 3920–3926. [CrossRef] 86. Zhang, L.; Zhou, G.; Sun, B.; Chen, F.; Zhao, M.; Li, T. Tunable Shell Thickness in Silica Nanospheres Functionalized by a Hydrophobic PMMA-PSt Diblock Copolymer Brush via Activators Generated by Electron Transfer for Atom Transfer Radical Polymerization. Macromol. Chem. Phys. 2013, 214, 1602–1611. [CrossRef] 87. Wang, Z.; Fantin, M.; Sobieski, J.; Wang, Z.; Yan, J.; Lee, J.; Liu, T.; Li, S.; Olszewski, M.; Bockstaller, M.R.; et al. Pushing the Limit: Synthesis of SiO 2–g -PMMA/PS Particle Brushes via ATRP with Very Low Concentration of Functionalized SiO 2 –Br Nanoparticles. Macromolecules 2019, 52, 8713–8723. [CrossRef] 88. Bayramoglu, G.; Ozalp, V.C.; Oztekin, M.; Arica, M.Y. Rapid and label-free detection of Brucella melitensis in milk and milk products using an aptasensor. Talanta 2019, 200, 263–271. [CrossRef] 89. Bayramoglu, G.; Arica, M.Y. Star type polymer grafted and polyamidoxime modified silica coated-magnetic particles for adsorption of U(VI) ions from solution. Chem. Eng. Res. Des. 2019, 147, 146–159. [CrossRef] 90. Nguyen, D.T.; Phu Nguyen, T.N.; Nguyen, D.C.; Thanh Ho, V.T.; Islam, M.R.; Lim, K.T.; Bach, L.G. A Robust Modification of SiO2 Nanoparticles by Poly(2-hydroxyethylmethacrylate) via Surface-Initiated Atom Transfer Radical Polymerization. Asian J. Chem. 2019, 31, 337–342. [CrossRef] 91. Ma, A.; Zhang, J.; Wang, N.; Bai, L.; Chen, H.; Wang, W.; Yang, H.; Yang, L.; Niu, Y.; Wei, D. Surface-Initiated Metal-Free Photoinduced ATRP of 4-Vinylpyridine from SiO 2 via Visible Light Photocatalysis for Self-Healing Hydrogels. Ind. Eng. Chem. Res. 2018, 57, 17417–17429. [CrossRef] 92. Wu, T.; Zhang, Y.; Wang, X.; Liu, S. Fabrication of hybrid silica nanoparticles densely grafted with thermoresponsive poly(N- isopropylacrylamide) brushes of controlled thickness via surface-initiated atom transfer radical polymerization. Chem. Mater. 2008, 20, 101–109. [CrossRef] 93. Xing, L.; Guo, N.; Zhang, Y.; Zhang, H.; Liu, J. A negatively charged loose nanofiltration membrane by blending with poly (sodium 4-styrene sulfonate) grafted SiO2 via SI-ATRP for dye purification. Sep. Purif. Technol. 2015, 146, 50–59. [CrossRef] 94. Du, Z.; Sun, X.; Tai, X.; Wang, G.; Liu, X. Synthesis of hybrid silica nanoparticles grafted with thermoresponsive poly(ethylene glycol) methyl ether methacrylate via AGET-ATRP. RSC Adv. 2015, 5, 17194–17201. [CrossRef] 95. Saigal, T.; Dong, H.; Matyjaszewski, K.; Tilton, R.D. Pickering Emulsions Stabilized by Nanoparticles with Thermally Responsive Grafted Polymer Brushes. Langmuir 2010, 26, 15200–15209. [CrossRef][PubMed] 96. Zhou, L.; Yuan, W.; Yuan, J.; Hong, X. Preparation of double-responsive SiO2-g-PDMAEMA nanoparticles via ATRP. Mater. Lett. 2008, 62, 1372–1375. [CrossRef] 97. Pinto, J.C.; Whiting, G.L.; Khodabakhsh, S.; Torre, L.; Rodríguez, A.; Dalgliesh, R.M.; Higgins, A.M.; Andreasen, J.W.; Nielsen, M.M.; Geoghegan, M.; et al. Organic Thin Film Transistors with Polymer Brush Gate Dielectrics Synthesized by Atom Transfer Radical Polymerization. Adv. Funct. Mater. 2008, 18, 36–43. [CrossRef] 98. Hwang, D.H.; Nomura, A.; Kim, J.; Kim, J.H.; Cho, H.; Lee, C.; Ohno, K.; Tsujii, Y. Synthesis and characterization of polystyrene brushes for organic thin film transistors. J. Nanosci. Nanotechnol. 2012, 12, 4137–4141. [CrossRef] 99. Facchetti, A.; Yoon, M.-H.; Marks, T.J. Gate Dielectrics for Organic Field-Effect Transistors: New Opportunities for Organic Electronics. Adv. Mater. 2005, 17, 1705–1725. [CrossRef] 100. Veres, J.; Ogier, S.; Lloyd, G.; de Leeuw, D. Gate Insulators in Organic Field-Effect Transistors. Chem. Mater. 2004, 16, 4543–4555. [CrossRef] 101. Li, L.; Hu, W.; Chi, L.; Fuchs, H. Polymer brush and inorganic oxide hybrid nanodielectrics for high performance organic transistors. J. Phys. Chem. B 2010, 114, 5315–5319. [CrossRef][PubMed] Molecules 2021, 26, 2942 38 of 44

102. Kopeć,M.; Spanjers, J.; Scavo, E.; Ernens, D.; Duvigneau, J.; Julius Vancso, G. Surface-initiated ATRP from polydopamine-modified TiO2 nanoparticles. Eur. Polym. J. 2018, 106, 291–296. [CrossRef] 103. Maeda, S.; Fujita, M.; Idota, N.; Matsukawa, K.; Sugahara, Y. Preparation of Transparent Bulk TiO2/PMMA Hybrids with Improved Refractive Indices via an in Situ Polymerization Process Using TiO2 Nanoparticles Bearing PMMA Chains Grown by Surface-Initiated Atom Transfer Radical Polymerization. ACS Appl. Mater. Interfaces 2016, 8, 34762–34769. [CrossRef] 104. Liu, L.; Chen, H.; Yang, F. Enhancing membrane performance by blending ATRP grafted PMMA-TiO 2 or PMMA-PSBMA-TiO2 in PVDF. Sep. Purif. Technol. 2014, 133, 22–31. [CrossRef] 105. Fan, X.; Lin, L.; Messersmith, P.B. Surface-initiated polymerization from TiO2 nanoparticle surfaces through a biomimetic initiator: A new route toward polymer-matrix nanocomposites. Compos. Sci. Technol. 2006, 66, 1198–1204. [CrossRef] 106. Cui, W.W.; Tang, D.Y.; Gong, Z.L. Electrospun poly(vinylidene fluoride)/poly(methyl methacrylate) grafted TiO 2 composite nanofibrous membrane as polymer electrolyte for lithium-ion batteries. J. Power Sources 2013, 223, 206–213. [CrossRef] 107. Xia, R.; Ruan, Z.; Zhang, Y.; Zhu, H.; Cao, M.; Chen, P.; Miao, J.; Qian, J. Click polymerization and characterization of TiO2 nanoparticles to one-dimensional nanochains. Chem. Phys. Lett. 2017, 687, 312–316. [CrossRef] 108. Kumar, A.; Bansal, A.; Behera, B.; Jain, S.L.; Ray, S.S. Ternary hybrid polymeric nanocomposites through grafting of polystyrene on graphene oxide-TiO2 by surface initiated atom transfer radical polymerization (SI-ATRP). Mater. Chem. Phys. 2016, 172, 189–196. [CrossRef] 109. Vergnat, V.; Roland, T.; Pourroy, G.; Masson, P. Effect of covalent grafting on mechanical properties of TiO 2/polystyrene composites. Mater. Chem. Phys. 2014, 147, 261–267. [CrossRef] 110. Wang, W.; Cao, H.; Zhu, G.; Wang, P. A facile strategy to modify TiO 2 nanoparticles via surface-initiated ATRP of styrene. J. Polym. Sci. Part A Polym. Chem. 2010, 48, 1782–1790. [CrossRef] 111. Tae Park, J.; Soo Lee, C.; Hun Park, C.; Hak Kim, J. Preparation of TiO2/Ag binary nanocomposite as high-activity visible-light- driven photocatalyst via graft polymerization. Chem. Phys. Lett. 2017, 685, 119–126. [CrossRef] 112. Park, J.T.; Koh, J.H.; Roh, D.K.; Shul, Y.G.; Kim, J.H. Proton-conducting nanocomposite membranes based on P(VDF-co-CTFE)-g- PSSA graft copolymer and TiO2-PSSA nanoparticles. Int. J. Hydrogen Energy 2011, 36, 1820–1827. [CrossRef] 113. Park, J.T.; Koh, J.H.; Seo, J.A.; Cho, Y.S.; Kim, J.H. Synthesis and characterization of TiO 2 /Ag/polymer ternary nanoparticles via surface-initiated atom transfer radical polymerization. Appl. Surf. Sci. 2011, 257, 8301–8306. [CrossRef] 114. Park, J.T.; Koh, J.H.; Koh, J.K.; Kim, J.H. Surface-initiated atom transfer radical polymerization from TiO2 nanoparticles. Appl. Surf. Sci. 2009, 255, 3739–3744. [CrossRef] 115. Gong, Z.; Tang, D.; Guo, Y. The fabrication and self-flocculation effect of hybrid TiO2 nanoparticles grafted with poly(N- isopropylacrylamide) at ambient temperature via surface-initiated atom transfer radical polymerization. J. Mater. Chem. 2012, 22, 16872. [CrossRef] 116. Chen, H.; Pan, S.; Xiong, Y.; Peng, C.; Pang, X.; Li, L.; Xiong, Y.; Xu, W. Preparation of thermo-responsive superhydrophobic TiO 2 /poly(N- isopropylacrylamide) microspheres. Appl. Surf. Sci. 2012, 258, 9505–9509. [CrossRef] 117. Zhang, G.; Lu, S.; Zhang, L.; Meng, Q.; Shen, C.; Zhang, J. Novel polysulfone hybrid ultrafiltration membrane prepared with TiO2-g-HEMA and its antifouling characteristics. J. Memb. Sci. 2013, 436, 163–173. [CrossRef] 118. Krysiak, E.; Wypych-Puszkarz, A.; Krysiak, K.; Nowaczyk, G.; Makrocka-Rydzyk, M.; Jurga, S.; Ulanski, J. Core-shell system based on titanium dioxide with elevated value of dielectric permittivity: Synthesis and characterization. Synth. Met. 2015, 209, 150–157. [CrossRef] 119. Xie, L.; Huang, X.; Wu, C.; Jiang, P. Core-shell structured poly(methyl methacrylate)/BaTiO3 nanocomposites prepared by in situ atom transfer radical polymerization: A route to high dielectric constant materials with the inherent low loss of the base polymer. J. Mater. Chem. 2011, 21, 5897–5906. [CrossRef] 120. You, N.; Zhang, C.; Liang, Y.; Zhang, Q.; Fu, P.; Liu, M.; Zhao, Q.; Cui, Z.; Pang, X. Facile Fabrication of Size-Tunable Core/Shell Ferroelectric/Polymeric Nanoparticles with Tailorable Dielectric Properties via Organocatalyzed Atom Transfer Radical Polymerization Driven by Visible Light. Sci. Rep. 2019, 9, 1869. [CrossRef] 121. Zhang, X.; Zhao, S.; Wang, F.; Ma, Y.; Wang, L.; Chen, D.; Zhao, C.; Yang, W. Improving dielectric properties of Ba- TiO3/poly(vinylidene fluoride) composites by employing core-shell structured BaTiO3@Poly(methylmethacrylate) and BaTiO3@Poly(trifluoroethyl methacrylate) nanoparticles. Appl. Surf. Sci. 2017, 403, 71–79. [CrossRef] 122. Bobnar, V.; LeV·stik, A.; Huang, C.; Zhang, Q.M. Intrinsic dielectric properties and charge transport in oligomers of organic semiconductor copper phthalocyanine. Phys. Rev. B 2005, 71, 041202. [CrossRef] 123. Nalwa, H.S.; Dalton, L.R.; Vasudevan, P. Dielectric properties of copper-phthalocyanine polymer. Eur. Polym. J. 1985, 21, 943–947. [CrossRef] 124. Wang, J.; Guan, F.; Cui, L.; Pan, J.; Wang, Q.; Zhu, L. Achieving high electric energy storage in a polymer nanocomposite at low filling ratios using a highly polarizable phthalocyanine interphase. J. Polym. Sci. Part B Polym. Phys. 2014, 52, 1669–1680. [CrossRef] 125. Xie, L.; Huang, X.; Huang, Y.; Yang, K.; Jiang, P. Core@Double-Shell Structured BaTiO 3 –Polymer Nanocomposites with High Dielectric Constant and Low Dielectric Loss for Energy Storage Application. J. Phys. Chem. C 2013, 117, 22525–22537. [CrossRef] Molecules 2021, 26, 2942 39 of 44

126. Zhang, X.; Chen, H.; Ma, Y.; Zhao, C.; Yang, W. Preparation and dielectric properties of core–shell structural composites of poly(1H,1H,2H,2H-perfluorooctyl methacrylate)@BaTiO3 nanoparticles. Appl. Surf. Sci. 2013, 277, 121–127. [CrossRef] 127. Huang, Y.; Huang, X.; Schadler, L.S.; He, J.; Jiang, P. Core@Double-Shell Structured Nanocomposites: A Route to High Dielectric Constant and Low Loss Material. ACS Appl. Mater. Interfaces 2016, 8, 25496–25507. [CrossRef][PubMed] 128. Cobo Sánchez, C.; Wåhlander, M.; Taylor, N.; Fogelström, L.; Malmström, E. Novel Nanocomposites of Poly(lauryl methacrylate)- Grafted Al2O3 Nanoparticles in LDPE. ACS Appl. Mater. Interfaces 2015, 7, 25669–25678. [CrossRef][PubMed] 129. Grabowski, C.A.; Fillery, S.P.; Koerner, H.; Tchoul, M.; Drummy, L.; Beier, C.W.; Brutchey, R.L.; Durstock, M.F.; Vaia, R.A. Dielectric performance of high permitivity nanocomposites: Impact of polystyrene grafting on BaTiO3 and TiO2. Nanocomposites 2016, 2, 117–124. [CrossRef] 130. Chen, S.; Lv, X.; Han, X.; Luo, H.; Bowen, C.R.; Zhang, D. Significantly improved energy density of BaTiO 3 nanocomposites by accurate interfacial tailoring using a novel rigid-fluoro-polymer. Polym. Chem. 2018, 9, 548–557. [CrossRef] 131. Ma, J.; Azhar, U.; Zong, C.; Zhang, Y.; Xu, A.; Zhai, C.; Zhang, L.; Zhang, S. Core-shell structured PVDF@BT nanoparticles for dielectric materials: A novel composite to prove the dependence of dielectric properties on ferroelectric shell. Mater. Des. 2019, 164, 107556. [CrossRef] 132. Kim, K.; Park, M.S.; Na, Y.; Choi, J.; Jenekhe, S.A.; Kim, F.S. Preparation and application of polystyrene-grafted alumina core-shell nanoparticles for dielectric surface passivation in solution-processed polymer thin film transistors. Org. Electron. 2019, 65, 305–310. [CrossRef] 133. Xie, L.; Huang, X.; Yang, K.; Li, S.; Jiang, P. “Grafting to” route to PVDF-HFP-GMA/BaTiO3 nanocomposites with high dielectric constant and high thermal conductivity for energy storage and thermal management applications. J. Mater. Chem. A 2014, 2, 5244. [CrossRef] 134. Ejaz, M.; Puli, V.S.; Elupula, R.; Adireddy, S.; Riggs, B.C.; Chrisey, D.B.; Grayson, S.M. Core-shell structured poly(glycidyl methacrylate)/BaTiO3 nanocomposites prepared by surface-initiated atom transfer radical polymerization: A novel material for high energy density dielectric storage. J. Polym. Sci. Part A Polym. Chem. 2015, 53, 719–728. [CrossRef] 135. Qiao, Y.; Islam, M.S.; Wang, L.; Yan, Y.; Zhang, J.; Benicewicz, B.C.; Ploehn, H.J.; Tang, C. Thiophene Polymer-Grafted Barium Titanate Nanoparticles toward Nanodielectric Composites. Chem. Mater. 2014, 26, 5319–5326. [CrossRef] 136. Wang, C.; Zhang, J.; Gong, S.; Ren, K. Significantly enhanced breakdown field for core-shell structured poly(vinylidene fluoride- hexafluoropropylene)/TiO2 nanocomposites for ultra-high energy density capacitor applications. J. Appl. Phys. 2018, 124, 154103. [CrossRef] 137. Tchoul, M.N.; Fillery, S.P.; Koerner, H.; Drummy, L.F.; Oyerokun, F.T.; Mirau, P.A.; Durstock, M.F.; Vaia, R.A. Assemblies of titanium dioxide-polystyrene hybrid nanoparticles for dielectric applications. Chem. Mater. 2010, 22, 1749–1759. [CrossRef] 138. Crippa, M.; Bianchi, A.; Cristofori, D.; D’Arienzo, M.; Merletti, F.; Morazzoni, F.; Scotti, R.; Simonutti, R. High dielectric constant rutile-polystyrene composite with enhanced percolative threshold. J. Mater. Chem. C 2013, 1, 484–492. [CrossRef] 139. Yang, C.; Marian, C.; Liu, J.; Di, Q.; Xu, M.; Zhang, Y.; Han, W.; Liu, K. Polymer grafted aluminum nanoparticles for percolative composite films with enhanced compatibility. Polymers 2019, 11, 638. [CrossRef] 140. Zhang, G.; Li, Y.; Tang, S.; Thompson, R.D.; Zhu, L. The Role of Field Electron Emission in Polypropylene/Aluminum Nanodi- electrics Under High Electric Fields. ACS Appl. Mater. Interfaces 2017, 9, 10106–10119. [CrossRef] 141. Yu, J.; Huo, R.; Wu, C.; Wu, X.; Wang, G.; Jiang, P. Influence of interface structure on dielectric properties of epoxy/alumina nanocomposites. Macromol. Res. 2012, 20, 816–826. [CrossRef] 142. Fredin, L.A.; Li, Z.; Lanagan, M.T.; Ratner, M.A.; Marks, T.J. Substantial recoverable energy storage in percolative metallic aluminum-polypropylene nanocomposites. Adv. Funct. Mater. 2013, 23, 3560–3569. [CrossRef] 143. Chiefari, J.; Chong, Y.K.; Ercole, F.; Krstina, J.; Jeffery, J.; Le, T.P.T.; Mayadunne, R.T.A.; Meijs, G.F.; Moad, C.L.; Moad, G.; et al. (Living Free-Radical Polymerization by Reversible Addition−Fragmentation Chain Transfer: The RAFT Process. Macromolecules 1998, 31, 5559–5562. [CrossRef] 144. Goto, A.; Fukuda, T. Kinetics of living radical polymerization. Prog. Polym. Sci. 2004, 29, 329–385. [CrossRef] 145. Fischer, H. The Persistent Radical Effect: A Principle for Selective Radical Reactions and Living Radical Polymerizations. Chem. Rev. 2001, 101, 3581–3610. [CrossRef][PubMed] 146. Ouchi, M.; Terashima, T.; Sawamoto, M. Transition Metal-Catalyzed Living Radical Polymerization: Toward Perfection in Catalysis and Precision Polymer Synthesis. Chem. Rev. 2009, 109, 4963–5050. [CrossRef] 147. Nicolas, J.; Guillaneuf, Y.; Bertin, D.; Gigmes, D.; Charleux, B. Nitroxide-Mediated Polymerization. In Polymer Science: A Comprehensive Reference; Matyjaszewski, K., Möller, M., Eds.; Elsevier: Amsterdam, The Netherland, 2012; Volume 3, pp. 277–350; ISBN 9780080878621. 148. Moad, G.; Rizzardo, E.; Thang, S.H. Living Radical Polymerization by the RAFT Process—A Third Update. Aust. J. Chem. 2012, 65, 985–1076. [CrossRef] 149. Perrier, S. 50th Anniversary Perspective: RAFT Polymerization—A User Guide. Macromolecules 2017, 50, 7433–7447. [CrossRef] 150. Moad, G.; Rizzardo, E.; Thang, S.H. Radical addition–fragmentation chemistry in polymer synthesis. Polymer (Guildf). 2008, 49, 1079–1131. [CrossRef] 151. Gody, G.; Maschmeyer, T.; Zetterlund, P.B.; Perrier, S. Rapid and quantitative one-pot synthesis of sequence-controlled polymers by radical polymerization. Nat. Commun. 2013, 4, 2505. [CrossRef] Molecules 2021, 26, 2942 40 of 44

152. Gody, G.; Maschmeyer, T.; Zetterlund, P.B.; Perrier, S. Pushing the Limit of the RAFT Process: Multiblock Copolymers by One-Pot Rapid Multiple Chain Extensions at Full Monomer Conversion. Macromolecules 2014, 47, 3451–3460. [CrossRef] 153. Chapman, R.; Gormley, A.J.; Herpoldt, K.-L.; Stevens, M.M. Highly Controlled Open Vessel RAFT Polymerizations by Enzyme Degassing. Macromolecules 2014, 47, 8541–8547. [CrossRef] 154. Gody, G.; Barbey, R.; Danial, M.; Perrier, S. Ultrafast RAFT polymerization: Multiblock copolymers within minutes. Polym. Chem. 2015, 6, 1502–1511. [CrossRef] 155. Gody, G.; Rossner, C.; Moraes, J.; Vana, P.; Maschmeyer, T.; Perrier, S. One-Pot RAFT/“Click” Chemistry via Isocyanates: Efficient Synthesis of α-End-Functionalized Polymers. J. Am. Chem. Soc. 2012, 134, 12596–12603. [CrossRef] 156. Moraes, J.; Maschmeyer, T.; Perrier, S. “Pseudo-star” Copolymers Formed by a Combination of RAFT Polymerization and Isocyanate-Coupling. Aust. J. Chem. 2011, 64, 1047. [CrossRef] 157. Zhao, Y.; Perrier, S. Synthesis of Well-Defined Homopolymer and Diblock Copolymer Grafted onto Silica Particles by Z-Supported RAFT Polymerization. Macromolecules 2006, 39, 8603–8608. [CrossRef] 158. Stenzel, M.H.; Zhang, L.; Huck, W.T.S. Temperature-Responsive Glycopolymer Brushes Synthesized via RAFT Polymerization Using the Z-group Approach. Macromol. Rapid Commun. 2006, 27, 1121–1126. [CrossRef] 159. Zhu, L.-J.; Zhu, L.-P.; Zhang, P.-B.; Zhu, B.-K.; Xu, Y.-Y. Surface zwitterionicalization of poly(vinylidene fluoride) membranes from the entrapped reactive core–shell silica nanoparticles. J. Colloid Interface Sci. 2016, 468, 110–119. [CrossRef] 160. Le-Masurier, S.P.; Gody, G.; Perrier, S.; Granville, A.M. One-pot polymer brush synthesis via simultaneous isocyanate coupling chemistry and “grafting from” RAFT polymerization. Polym. Chem. 2014, 5, 2816–2823. [CrossRef] 161. Cai, Y.; Peng, W.; Demeshko, S.; Tian, J.; Vana, P. Silica-Coated Magnetite Nanoparticles Carrying a High-Density Polymer Brush Shell of Hydrophilic Polymer. Macromol. Rapid Commun. 2018, 39, 1800226. [CrossRef] 162. Qu, Z.; Hu, F.; Chen, K.; Duan, Z.; Gu, H.; Xu, H. A facile route to the synthesis of spherical poly(acrylic acid) brushes via RAFT polymerization for high-capacity protein immobilization. J. Colloid Interface Sci. 2013, 398, 82–87. [CrossRef][PubMed] 163. Liu, J.; Zhang, L.; Shi, S.; Chen, S.; Zhou, N.; Zhang, Z.; Cheng, Z.; Zhu, X. A Novel and Universal Route to SiO 2 -Supported Organic/Inorganic Hybrid Noble Metal Nanomaterials via Surface RAFT Polymerization. Langmuir 2010, 26, 14806–14813. [CrossRef] 164. Zhu, L.-J.; Zhu, L.-P.; Jiang, J.-H.; Yi, Z.; Zhao, Y.-F.; Zhu, B.-K.; Xu, Y.-Y. Hydrophilic and anti-fouling polyethersulfone ultrafiltration membranes with poly(2-hydroxyethyl methacrylate) grafted silica nanoparticles as additive. J. Memb. Sci. 2014, 451, 157–168. [CrossRef] 165. Tumnantong, D.; Rempel, G.; Prasassarakich, P. Polyisoprene-Silica Nanoparticles Synthesized via RAFT Emulsifier-Free Emulsion Polymerization Using Water-Soluble Initiators. Polymers 2017, 9, 637. [CrossRef] 166. Bach, L.G.; Quynh, B.T.P.; Thuong, N.T.; Ho, V.T.T. Synthesis and characterization of photoluminescent Eu(III) coordinated with poly(2-hydroxyethyl methacrylate) grafted SiO 2 nanoparticles via surface RAFT polymerization. Mol. Cryst. Liq. Cryst. 2017, 644, 175–182. [CrossRef] 167. Bach, L.G.; Bui, Q.T.P.; Cao, X.T.; Ho, V.T.T.; Lim, K.T. A new approach for synthesis of SiO2/poly(2-hydroxyethyl methacrylate):Tb3+ nanohybrids by combination of surface-initiated raft polymerization and coordination chemistry. Polym. Bull. 2016, 73, 2627–2638. [CrossRef] 168. Huang, G.; Xiong, Z.; Qin, H.; Zhu, J.; Sun, Z.; Zhang, Y.; Peng, X.; Ou, J.; Zou, H. Synthesis of zwitterionic polymer brushes hybrid silica nanoparticles via controlled polymerization for highly efficient enrichment of glycopeptides. Anal. Chim. Acta 2014, 809, 61–68. [CrossRef] 169. Bouharras, F.E.; Raihane, M.; Silly, G.; Totee, C.; Ameduri, B. Core–shell structured poly(vinylidene fluoride)- grafted -BaTiO 3 nanocomposites prepared via reversible addition–fragmentation chain transfer (RAFT) polymerization of VDF for high energy storage capacitors. Polym. Chem. 2019, 10, 891–904. [CrossRef] 170. Qian, K.; Lv, X.; Chen, S.; Luo, H.; Zhang, D. Interfacial engineering tailoring the dielectric behavior and energy density of BaTiO 3 /P(VDF-TrFE-CTFE) nanocomposites by regulating a liquid-crystalline polymer modifier structure. Dalt. Trans. 2018, 47, 12759–12768. [CrossRef] 171. Zhang, D.; Ma, C.; Zhou, X.; Chen, S.; Luo, H.; Bowen, C.R.; Zhou, K. High Performance Capacitors Using BaTiO 3 Nanowires Engineered by Rigid Liquid-crystalline Polymers. J. Phys. Chem. C 2017, 121, 20075–20083. [CrossRef] 172. Cao, X.T.; Showkat, A.M.; Lee, W.-K.; Lim, K.T. Luminescence of Terbium (III) Complexes Incorporated in Carboxylic Acid Functionalized Polystyrene/BaTiO 3 Nanocomposites. Mol. Cryst. Liq. Cryst. 2015, 622, 36–43. [CrossRef] 173. Qiao, Y.; Yin, X.; Wang, L.; Islam, M.S.; Benicewicz, B.C.; Ploehn, H.J.; Tang, C. Bimodal Polymer Brush Core–Shell Barium Titanate Nanoparticles: A Strategy for High-Permittivity Polymer Nanocomposites. Macromolecules 2015, 48, 8998–9006. [CrossRef] 174. Qiao, Y.; Islam, M.S.; Han, K.; Leonhardt, E.; Zhang, J.; Wang, Q.; Ploehn, H.J.; Tang, C. Polymers Containing Highly Polarizable Conjugated Side Chains as High-Performance All-Organic Nanodielectric Materials. Adv. Funct. Mater. 2013, 23, 5638–5646. [CrossRef] 175. Yang, K.; Huang, X.; Huang, Y.; Xie, L.; Jiang, P. Fluoro-Polymer@BaTiO 3 Hybrid Nanoparticles Prepared via RAFT Polymeriza- tion: Toward Ferroelectric Polymer Nanocomposites with High Dielectric Constant and Low Dielectric Loss for Energy Storage Application. Chem. Mater. 2013, 25, 2327–2338. [CrossRef] Molecules 2021, 26, 2942 41 of 44

176. Yang, K.; Huang, X.; Xie, L.; Wu, C.; Jiang, P.; Tanaka, T. Core-Shell Structured Polystyrene/BaTiO 3 Hybrid Nanodielectrics Prepared by In Situ RAFT Polymerization: A Route to High Dielectric Constant and Low Loss Materials with Weak Frequency Dependence. Macromol. Rapid Commun. 2012, 33, 1921–1926. [CrossRef][PubMed] 177. Zhang, L.; Khani, M.M.; Krentz, T.M.; Huang, Y.; Zhou, Y.; Benicewicz, B.C.; Nelson, J.K.; Schadler, L.S. Suppression of space charge in crosslinked polyethylene filled with poly(stearyl methacrylate)-grafted SiO 2 nanoparticles. Appl. Phys. Lett. 2017, 110, 132903. [CrossRef] 178. Krentz, T.; Khani, M.M.; Bell, M.; Benicewicz, B.C.; Nelson, J.K.; Zhao, S.; Hillborg, H.; Schadler, L.S. Morphologically dependent alternating-current and direct-current breakdown strength in silica-polypropylene nanocomposites. J. Appl. Polym. Sci. 2017, 134. [CrossRef] 179. Xiong, L.; Liang, H.B.; Wang, R.M.; Pang, Y. The effect of surface modification of TiO2 with diblock copolymers on the properties of epoxy nanocomposites. Polym. Plast. Technol. Eng. 2010, 49, 1483–1488. [CrossRef] 180. Li, C.; Han, J.; Ryu, C.Y.; Benicewicz, B.C. A versatile method to prepare RAFT agent anchored substrates and the preparation of PMMA grafted-nanoparticles. Macromolecules 2006, 39, 3175–3183. [CrossRef] 181. Schadler, L.S.; Kumar, S.K.; Benicewicz, B.C.; Lewis, S.L.; Harton, S.E. Designed Interfaces in Polymer Nanocomposites: A Fundamental Viewpoint. MRS Bull. 2007, 32, 335–340. [CrossRef] 182. Li, Y.; Krentz, T.M.; Wang, L.; Benicewicz, B.C.; Schadler, L.S. Ligand Engineering of Polymer Nanocomposites: From the Simple to the Complex. ACS Appl. Mater. Interfaces 2014, 6, 6005–6021. [CrossRef][PubMed] 183. Natarajan, B.; Neely, T.; Rungta, A.; Benicewicz, B.C.; Schadler, L.S. Thermomechanical Properties of Bimodal Brush Modified Nanoparticle Composites. Macromolecules 2013, 46, 4909–4918. [CrossRef] 184. Rungta, A.; Natarajan, B.; Neely, T.; Dukes, D.; Schadler, L.S.; Benicewicz, B.C. Grafting Bimodal Polymer Brushes on Nanoparti- cles Using Controlled Radical Polymerization. Macromolecules 2012, 45, 9303–9311. [CrossRef] 185. Tanaka, T. Dielectric nanocomposites with insulating properties. IEEE Trans. Dielectr. Electr. Insul. 2005, 12, 914–928. [CrossRef] 186. Siddabattuni, S.; Schuman, T.P.; Dogan, F. Dielectric properties of polymer-particle nanocomposites influenced by electronic nature of filler surfaces. ACS Appl. Mater. Interfaces 2013, 5, 1917–1927. [CrossRef] 187. Bell, M.; Krentz, T.; Keith Nelson, J.; Schadler, L.; Wu, K.; Breneman, C.; Zhao, S.; Hillborg, H.; Benicewicz, B. Investigation of dielectric breakdown in silica-epoxy nanocomposites using designed interfaces. J. Colloid Interface Sci. 2017, 495, 130–139. [CrossRef][PubMed] 188. Zhao, D.; Di Nicola, M.; Khani, M.M.; Jestin, J.; Benicewicz, B.C.; Kumar, S.K. Self-Assembly of Monodisperse versus Bidisperse Polymer-Grafted Nanoparticles. ACS Macro Lett. 2016, 5, 790–795. [CrossRef] 189. Hojjati, B.; Charpentier, P.A. Synthesis of TiO2-polymer nanocomposite in supercritical CO2 via RAFT polymerization. Polymer 2010, 51, 5345–5351. [CrossRef] 190. Hojjati, B.; Charpentier, P.A. Synthesis and kinetics of graft polymerization of methyl methacrylate from the RAFT coordinated surface of nano-TiO2. J. Polym. Sci. Part A Polym. Chem. 2008, 46, 3926–3937. [CrossRef] 191. Gregurec, D.; Politakos, N.; Yate, L.; Moya, S.E. Strontium confinement in polyacrylic acid brushes: A soft nanoarchitectonics approach for the design of titania coatings with enhanced osseointegration. Mol. Syst. Des. Eng. 2019, 4, 421–430. [CrossRef] 192. Hojjati, B.; Sui, R.; Charpentier, P.A. Synthesis of TiO2/PAA nanocomposite by RAFT polymerization. Polymer 2007, 48, 5850–5858. [CrossRef] 193. Rafiei, H.; Abbasian, M.; Yegani, R. Synthesis of well-defined poly(n-vinylpyrrolidone)/n-TiO2 nanocomposites by xanthate- mediated radical polymerization. Iran. Polym. J. (Engl. Ed.) 2020.[CrossRef] 194. Abbasian, M.; Masoumi, B.; Masoudi, A.; Rashidzadeh, B. Synthesis and Characterization of Poly Chloromethylstyrene TiO2- Nanocomposite Through a Simple Method via Reversible Addition-Fragmentation Transfer Polymerization. Polym. Plast. Technol. Eng. 2014, 53, 1150–1159. [CrossRef] 195. Che, X.C.; Jin, Y.Z.; Lee, Y.S. Preparation of nano-TiO2/polyurethane emulsions via in situ RAFT polymerization. Prog. Org. Coatings 2010, 69, 534–538. [CrossRef] 196. Xiong, L.; Wang, R.; Liang, H.; Pang, Y.; Guan, J. Synthesis and characterization of poly(methyl methacrylate)-b-polystyrene/ TiO2 nanocomposites via reversible addition-fragmentation chain transfer polymerization. J. Macromol. Sci. Part A Pure Appl. Chem. 2010, 47, 903–908. [CrossRef] 197. Zhu, M.; Huang, X.; Yang, K.; Zhai, X.; Zhang, J.; He, J.; Jiang, P. Energy Storage in Ferroelectric Polymer Nanocomposites Filled with Core–Shell Structured Polymer@BaTiO3 Nanoparticles: Understanding the Role of Polymer Shells in the Interfacial Regions. ACS Appl. Mater. Interfaces 2014, 6, 19644–19654. [CrossRef][PubMed] 198. Georges, M.K.; Veregin, R.P.N.; Kazmaier, P.M.; Hamer, G.K. Narrow molecular weight resins by a free-radical polymerization process. Macromolecules 1993, 26, 2987–2988. [CrossRef] 199. Goto, A.; Fukuda, T. Kinetic Study on Nitroxide-Mediated Free Radical Polymerization of tert -Butyl Acrylate. Macromolecules 1999, 32, 618–623. [CrossRef] 200. Guillaneuf, Y.; Gigmes, D.; Marque, S.R.A.; Astolfi, P.; Greci, L.; Tordo, P.; Bertin, D. First Effective Nitroxide-Mediated Polymerization of Methyl Methacrylate. Macromolecules 2007, 40, 3108–3114. [CrossRef] Molecules 2021, 26, 2942 42 of 44

201. Nicolas, J.; Charleux, B.; Guerret, O.; Magnet, S. Nitroxide-Mediated Controlled Free-Radical Emulsion Polymerization of Styrene andn-Butyl Acrylate with a Water-Soluble Alkoxyamine as Initiator. Angew. Chemie 2004, 116, 6312–6315. [CrossRef] 202. Couvreur, L.; Lefay, C.; Belleney, J.; Charleux, B.; Guerret, O.; Magnet, S. First Nitroxide-Mediated Controlled Free-Radical Polymerization of Acrylic Acid. Macromolecules 2003, 36, 8260–8267. [CrossRef] 203. Deleuze, C.; Delville, M.H.; Pellerin, V.; Derail, C.; Billon, L. Hybrid Core@Soft Shell Particles as Adhesive Elementary Building Blocks for Colloidal Crystals. Macromolecules 2009, 42, 5303–5309. [CrossRef] 204. Parvole, J.; Ahrens, L.; Blas, H.; Vinas, J.; Boissière, C.; Sanchez, C.; Save, M.; Charleux, B. Grafting polymer chains bearing an N -succinimidyl activated ester end-group onto primary amine-coated silica particles and application of a simple, one-step approach via nitroxide-mediated controlled/living free-radical polymerization. J. Polym. Sci. Part A Polym. Chem. 2010, 48, 173–185. [CrossRef] 205. Ostaci, R.; Celle, C.; Seytre, G.; Beyou, E.; Chapel, J.; Drockenmuller, E. Influence of nitroxide structure on polystyrene brushes “grafted-from” silicon wafers. J. Polym. Sci. Part A Polym. Chem. 2008, 46, 3367–3374. [CrossRef] 206. Benoit, D.; Grimaldi, S.; Robin, S.; Finet, J.-P.; Tordo, P.; Gnanou, Y. Kinetics and Mechanism of Controlled Free-Radical Polymerization of Styrene and n -Butyl Acrylate in the Presence of an Acyclic β-Phosphonylated Nitroxide. J. Am. Chem. Soc. 2000, 122, 5929–5939. [CrossRef] 207. Pitliya, P.; Butcher, R.J.; Karim, A.; Hudrlik, P.F.; Hudrlik, A.M.; Raghavan, D. 3-[1-(4-Bromophenyl)ethoxy]-2,2,5-trimethyl-4- phenyl-3-azahexane. Acta Crystallogr. Sect. E Struct. Reports Online 2013, 69, o1792–o1793. [CrossRef] 208. Ni, G.; Yang, W.; Bo, L.; Guo, H.; Zhang, W.; Gao, J. Preparation of polystyrene/SiO2 nanocomposites by surface-initiated nitroxide-mediated radical polymerization. Chinese Sci. Bull. 2006, 51, 1644–1647. [CrossRef] 209. Matsuno, R.; Otsuka, H.; Takahara, A. Polystyrene-grafted titanium oxide nanoparticles prepared through surface-initiated nitroxide-mediated radical polymerization and their application to polymer hybrid thin films. Soft Matter 2006, 2, 415–421. [CrossRef] 210. Abbasian, M.; Khakpour Aali, N. Nitroxide-Mediated Radical Polymerization of Styrene Initiated from the Surface of Titanium Oxide Nanoparticles. J. Nanostructures 2016, 6, 38–45. [CrossRef] 211. Jaymand, M. Synthesis and characterization of novel type poly (4-chloromethyl styrene-grft-4-vinylpyridine)/TiO2 nanocomposite via nitroxide-mediated radical polymerization. Polymer 2011, 52, 4760–4769. [CrossRef] 212. Lee, Y.J. Preparation of Poly(4-hydroxystyrene) Based Functional Block Copolymer Through Living Radical Polymerization and Its Nanocomposite with BaTiO3 for Dielectric Material. J. Nanosci. Nanotechnol. 2009, 9.[CrossRef] 213. Woo, J.H.; Park, M.; Lee, S.-S.; Hong, S.C. Preparation of Functional Polystyrene Copolymers Through Nitroxide Mediated Polymerization and Their Applications as Surface Modifiers for BaTiO3 Nanoparticles. J. Nanosci. Nanotechnol. 2009, 9, 1872–1880. [CrossRef][PubMed] 214. Mai, T.B.; Tran, T.N.; Rafiqul Islam, M.; Park, J.M.; Lim, K.T. Covalent functionalization of silica nanoparticles with poly(N- isopropylacrylamide) employing thiol-ene chemistry and activator regenerated by electron transfer ATRP protocol. J. Mater. Sci. 2014, 49, 1519–1526. [CrossRef] 215. Han, M.S.; Zhang, X.Y.; Li, L.; Peng, C.; Bao, L.; Ou, E.C.; Xiong, Y.Q.; Xu, W.J. Dual-switchable surfaces between hydrophobic and superhydrophobic fabricated by the combination of click chemistry and RAFT. Express Polym. Lett. 2014, 8, 528–542. [CrossRef] 216. Guo, C.; Wang, B.; Shan, J. Preparation of Thermosensitive Hollow Imprinted Microspheres via Combining Distillation Precipita- tion Polymerization and Thiol-ene Click Chemistry. Chinese J. Chem. 2015, 33, 225–234. [CrossRef] 217. Li, G.L.; Xu, L.Q.; Tang, X.; Neoh, K.G.; Kang, E.T. Hairy Hollow Microspheres of Fluorescent Shell and Temperature-Responsive Brushes via Combined Distillation-Precipitation Polymerization and Thiol−ene Click Chemistry. Macromolecules 2010, 43, 5797–5803. [CrossRef] 218. Zhang, Q.; Yang, S.; Zhu, T.; Oh, J.K.; Li, P. Soft-nanocoupling between silica and gold nanoparticles based on block copolymer. React. Funct. Polym. 2017, 110, 30–37. [CrossRef] 219. Lee, S.; Jang, M.; Yang, H. Optimized Grafting Density of End-Functionalized Polymers to Polar Dielectric Surfaces for Solution- Processed Organic Field-Effect Transistors. ACS Appl. Mater. Interfaces 2014, 6, 20444–20451. [CrossRef][PubMed] 220. Maliakal, A.; Katz, H.; Cotts, P.M.; Subramoney, S.; Mirau, P. Inorganic Oxide Core, Polymer Shell Nanocomposite as a High K Gate Dielectric for Flexible Electronics Applications. J. Am. Chem. Soc. 2005, 127, 14655–14662. [CrossRef] 221. Hailu, S.T.; Samant, S.; Grabowski, C.; Durstock, M.; Karim, A.; Raghavan, D. Synthesis of highly dispersed, block copolymer- grafted TiO2 nanoparticles within neat block copolymer films. J. Polym. Sci. Part A Polym. Chem. 2015, 53, 468–478. [CrossRef] 222. Hailu, S.T.; Samant, S.; Grabowski, C.; Durstock, M.; Karim, A.; Raghavan, D. Dielectric Study of PMMA-b-PS -g-TiO2/PS-PMMA BCP Nanocomposite Films. Unpublished data. 223. Obata, M.; Yamai, K.; Takahashi, M.; Ueno, S.; Ogura, H.; Egami, Y. Synthesis of an oxygen-permeable block copolymer with catechol groups and its application in polymer-ceramic pressure-sensitive paint. Polymer 2020, 191, 122281. [CrossRef] 224. Klaysri, R.; Wichaidit, S.; Piticharoenphun, S.; Mekasuwandumrong, O.; Praserthdam, P. Synthesis of TiO2-grafted onto PMMA film via ATRP: Using monomer as a coupling agent and reusability in photocatalytic application. Mater. Res. Bull. 2016, 83, 640–648. [CrossRef] 225. Yang, K.; Huang, X.; Zhu, M.; Xie, L.; Tanaka, T.; Jiang, P. Combining RAFT Polymerization and Thiol–Ene Click Reaction for Core–Shell Structured Polymer@BaTiO3 Nanodielectrics with High Dielectric Constant, Low Dielectric Loss, and High Energy Storage Capability. ACS Appl. Mater. Interfaces 2014, 6, 1812–1822. [CrossRef] Molecules 2021, 26, 2942 43 of 44

226. Bourgeat-Lami, E.; Lang, J. Encapsulation of Inorganic Particles by Dispersion Polymerization in Polar Media. J. Colloid Interface Sci. 1998, 197, 293–308. [CrossRef][PubMed] 227. Guo, J.; Zhang, H.; Li, C.; Zang, L.; Luo, J. In situ synthesis of poly(methyl methacrylate)/SiO 2 hybrid nanocomposites via grafting onto strategy based on UV irradiation in the presence of iron aqueous solution. J. Nanomater. 2012, 2012.[CrossRef] 228. Landfester, K. Synthesis of colloidal particles in miniemulsions. Annu. Rev. Mater. Res. 2006, 36, 231–279. [CrossRef] 229. Tang, E.; Dong, S. Preparation of styrene polymer/ZnO nanocomposite latex via miniemulsion polymerization and its antibacterial property. Colloid Polym. Sci. 2009, 287, 1025–1032. [CrossRef] 230. Morales-Acosta, M.D.; Alvarado-Beltrán, C.G.; Quevedo-López, M.A.; Gnade, B.E.; Mendoza-Galván, A.; Ramírez-Bon, R. Adjustable structural, optical and dielectric characteristics in sol–gel PMMA–SiO2 hybrid films. J. Non. Cryst. Solids 2013, 362, 124–135. [CrossRef] 231. Alvarado-Beltrán, C.G.; Almaral-Sánchez, J.L.; Mejia, I.; Quevedo-López, M.A.; Ramirez-Bon, R. Sol–Gel PMMA–ZrO 2 Hybrid Layers as Gate Dielectric for Low-Temperature ZnO-Based Thin-Film Transistors. ACS Omega 2017, 2, 6968–6974. [CrossRef] 232. Morales-Acosta, M.D.; Quevedo-López, M.A.; Ramírez-Bon, R. PMMA–SiO2 hybrid films as gate dielectric for ZnO based thin-film transistors. Mater. Chem. Phys. 2014, 146, 380–388. [CrossRef] 233. Alvarado-Beltrán, C.G.; Almaral-Sánchez, J.L.; Ramírez-Bon, R. Synthesis and properties of PMMA-ZrO 2 organic-inorganic hybrid films. J. Appl. Polym. Sci. 2015, 132, n. [CrossRef] 234. Alvarado-Beltrán, C.G.; Almaral-Sánchez, J.L.; Quevedo-López, M.A.; Ramirez-Bon, R. Dielectric Gate Applications of PMMA- TiO2 Hybrid Films in ZnO-Based Thin Film Transistors. Int. J. Electrochem. Sci 2015, 10, 4068–4082. 235. Sánchez-Ahumada, D.; Verastica-Ward, L.J.; Gálvez-López, M.F.; Castro-Beltrán, A.; Ramirez-Bon, R.; Alvarado-Beltrán, C.G. Low-temperature synthesis and physical characteristics of PS TiO2 hybrid films for transparent dielectric gate applications. Polymer 2019, 172, 170–177. [CrossRef] 236. Kandulna, R.; Choudhary, R.B.; Singh, R.; Purty, B. PMMA–TiO2 based polymeric nanocomposite material for electron transport layer in OLED application. J. Mater. Sci. Mater. Electron. 2018, 29, 5893–5907. [CrossRef] 237. Zhou, Y.; Liu, Q.; Chen, F.; Zhao, Y.; Sun, S.; Guo, J.; Yang, Y.; Xu, J. Significantly enhanced energy storage in core–shell structured poly(vinylidene fluoride-co-chlorotrifluoroethylene)/BaTiO3@polyurea nanocomposite films. J. Mater. Sci. 2020, 55, 11296–11309. [CrossRef] 238. Pang, X.; Zhao, L.; Han, W.; Xin, X.; Lin, Z. A general and robust strategy for the synthesis of nearly monodisperse colloidal nanocrystals. Nat. Nanotechnol. 2013, 8, 426–431. [CrossRef] 239. Müllner, M.; Yuan, J.; Weiss, S.; Walther, A.; Förtsch, M.; Drechsler, M.; Müller, A.H.E. Water-Soluble Organo−Silica Hybrid Nanotubes Templated by Cylindrical Polymer Brushes. J. Am. Chem. Soc. 2010, 132, 16587–16592. [CrossRef] 240. Zheng, Z.; Daniel, A.; Yu, W.; Weber, B.; Ling, J.; Müller, A.H.E. Rare-Earth Metal Cations Incorporated Silica Hybrid Nanoparticles Templated by Cylindrical Polymer Brushes. Chem. Mater. 2013, 25, 4585–4594. [CrossRef] 241. Müllner, M.; Lunkenbein, T.; Breu, J.; Caruso, F.; Müller, A.H.E. Template-Directed Synthesis of Silica Nanowires and Nanotubes from Cylindrical Core–Shell Polymer Brushes. Chem. Mater. 2012, 24, 1802–1810. [CrossRef] 242. Yuan, J.; Lu, Y.; Schacher, F.; Lunkenbein, T.; Weiss, S.; Schmalz, H.; Müller, A.H.E. Template-Directed Synthesis of Hybrid Titania Nanowires within Core−Shell Bishydrophilic Cylindrical Polymer Brushes. Chem. Mater. 2009, 21, 4146–4154. [CrossRef] 243. Yuan, J.; Schacher, F.; Drechsler, M.; Hanisch, A.; Lu, Y.; Ballauff, M.; Müller, A.H.E. Stimuli-Responsive Organosilica Hybrid Nanowires Decorated with Metal Nanoparticles. Chem. Mater. 2010, 22, 2626–2634. [CrossRef] 244. Xu, H.; Xu, Y.; Pang, X.; He, Y.; Jung, J.; Xia, H.; Lin, Z. A general route to nanocrystal kebabs periodically assembled on stretched flexible polymer shish. Sci. Adv. 2015, 1, 1–12. [CrossRef][PubMed] 245. He, M.; Pang, X.; Liu, X.; Jiang, B.; He, Y.; Snaith, H.; Lin, Z. Monodisperse Dual-Functional Upconversion Nanoparticles Enabled Near-Infrared Organolead Halide PeroV·skite Solar Cells. Angew. Chemie Int. Ed. 2016, 55, 4280–4284. [CrossRef][PubMed] 246. Liu, Y.; Wang, J.; Zhang, M.; Li, H.; Lin, Z. Polymer-Ligated Nanocrystals Enabled by Nonlinear Block Copolymer Nanoreactors: Synthesis, Properties, and Applications. ACS Nano 2020, 14, 12491–12521. [CrossRef] 247. Xie, G.; Ding, H.; Daniel, W.F.M.; Wang, Z.; Pietrasik, J.; Sheiko, S.S.; Matyjaszewski, K. Preparation of titania nanoparticles with tunable anisotropy and branched structures from core–shell molecular bottlebrushes. Polymer 2016, 98, 481–486. [CrossRef] 248. Müllner, M.; Lunkenbein, T.; Schieder, M.; Gröschel, A.H.; Miyajima, N.; Förtsch, M.; Breu, J.; Caruso, F.; Müller, A.H.E. Template- Directed Mild Synthesis of Anatase Hybrid Nanotubes within Cylindrical Core–Shell–Corona Polymer Brushes. Macromolecules 2012, 45, 6981–6988. [CrossRef] 249. Yuan, J.; Xu, Y.; Walther, A.; Bolisetty, S.; Schumacher, M.; Schmalz, H.; Ballauff, M.; Müller, A.H.E. Water-soluble organo-silica hybrid nanowires. Nat. Mater. 2008, 7, 718–722. [CrossRef] 250. Wang, H.; Liu, Y.; Li, M.; Huang, H.; Xu, H.M.; Hong, R.J.; Shen, H. Multifunctional TiO2 nanowires-modified nanoparticles bilayer film for 3D dye-sensitized solar cells. Optoelectron. Adv. Mater. Rapid Commun. 2010, 4, 1166–1169. [CrossRef] 251. Budzalek, K.; Ding, H.; Janasz, L.; Wypych-Puszkarz, A.; Cetinkaya, O.; Pietrasik, J.; Kozanecki, M.; Ulanski, J.; Matyjaszewski, K. Star polymer–TiO2 nanohybrids to effectively modify the surface of PMMA dielectric layers for solution processable OFETs. J. Mater. Chem. C 2021, 9, 1269–1278. [CrossRef] Molecules 2021, 26, 2942 44 of 44

252. Pang, X.; Zhao, L.; Akinc, M.; Kim, J.K.; Lin, Z. Novel Amphiphilic Multi-Arm, Star-Like Block Copolymers as Unimolecular Micelles. Macromolecules 2011, 44, 3746–3752. [CrossRef] 253. Guo, H.Z.; Mudryk, Y.; Ahmad, M.I.; Pang, X.C.; Zhao, L.; Akinc, M.; Pecharsky, V.K.; Bowler, N.; Lin, Z.Q.; Tan, X. Structure evolution and dielectric behavior of polystyrene-capped barium titanate nanoparticles. J. Mater. Chem. 2012.[CrossRef] 254. Pang, X.; He, Y.; Jiang, B.; Iocozzia, J.; Zhao, L.; Guo, H.; Liu, J.; Akinc, M.; Bowler, N.; Tan, X.; et al. Block copolymer/ferroelectric nanoparticle nanocomposites. Nanoscale 2013, 5, 8695. [CrossRef] 255. Bates, C.M.; Bates, F.S. 50th Anniversary Perspective: Block Polymers—Pure Potential. Macromolecules 2017, 50, 3–22. [CrossRef] 256. Samant, S.P.; Grabowski, C.A.; Kisslinger, K.; Yager, K.G.; Yuan, G.; Satija, S.K.; Durstock, M.F.; Raghavan, D.; Karim, A. Directed Self-Assembly of Block Copolymers for High Breakdown Strength Polymer Film Capacitors. ACS Appl. Mater. Interfaces 2016, 8, 7966–7976. [CrossRef][PubMed] 257. Samant, S.; Hailu, S.; Singh, M.; Pradhan, N.; Yager, K.; Al-Enizi, A.M.; Raghavan, D.; Karim, A. Alignment frustration in block copolymer ﬁlms with block copolymer grafted TiO2 nanoparticles under soft-shear cold zone annealing. Polym. Adv. Technol. 2021, 32, 2052–2060. [CrossRef]