Elsevier

Polymer

Volume 158, 5 December 2018, Pages 297-307
Polymer

A versatile method for preparing well-defined polymers with aggregation-induced emission property

https://doi.org/10.1016/j.polymer.2018.10.050Get rights and content

Highlights

  • ROMP was explored to prepare varied well-defined AIE polymers.

  • Fluorescence wavelength of AIE polymers could be manipulated by varying the AIE fluorogens.

  • Self-assembly of amphiphilic AIE block copolymers produced fluorescent nanoparticles with varied structures.

Abstract

The living ring-opening metathesis polymerization (ROMP) was demonstrated as a versatile method for preparing the well-defined polymers with aggregation-induced emission (AIE) property. In this approach, the norbornene-based monomers were prepared containing side groups of varied typical AIE fluorogens such as distyrenneanthracene (M1), 1,2,3,4,5-pentaphenylsiole (M2), and tetraphenylethene derivative (M3). Initiating by the Grubbs third generation catalyst (G3), ROMP could consume all of the exemplified monomers in less than 30 min and produce the corresponding well-defined AIE polymers of poly(M1), poly(M2), and poly(M3) with controlled molecular weight and narrow polydispersity. By copolymerizing with the norbornene-based monomer (M4) having poly(ethylene glycol) (PEG) side chain, ROMP could also produce the well-defined amphiphilic AIE block copolymers such as poly(M1)-b-poly(M4) and poly(M2)-b-poly(M4). Compared to norbornene-based AIE monomers, the resultant polymers showed enhanced AIE property, in which the same fluorescence intensity was obtained from the lower molar concentration of AIE fluorogen inside polymers than that inside monomers. In addition, the self-assembly of amphiphilic AIE block copolymers in selective solvents produced the fluorescent nanoparticles with varied morphologies and structures including spherical micelles, cylindrical micelles, and vesicles.

Introduction

Aggregation-induced emission (AIE) is an important fluorescence phenomenon and gains great attentions since it was discovered and named by Tang and co-workers in 2001 [1]. Different from the traditional fluorophores with aggregation-caused quenching (ACQ) property, the AIE fluorogens are non-emissive in dilute solution and emit intense fluorescence in the aggregated state [[2], [3], [4], [5], [6]]. The mechanism of AIE phenomenon has been clarified and ascribed to the restricted intramolecular rotation of AIE fluorogens in their aggregated state. Based on this, a variety of AIE fluorogens with different fluorescent wavelength has been developed such as tetraphenylethene [7,8], siloles [9,10], triphenylethene [11,12], distyrylanthracene [13,14], and cyano-substituted diarylethene [15,16]. These AIE fluorogens with different chemical structures and fluorescent properties have shown promising applications in organic light-emitting diodes, fluorescent bioprobes, chemosensors, mechanochromic materials, etc. [[17], [18], [19], [20], [21]].

Although much progress has been achieved on the research of small AIE molecules, AIE polymers have been less explored up to now [22]. When AIE fluorogens are incorporated into polymer chains, the long-chain segments of AIE polymers and the nearby fluorogens should inherently impose steric hindrance on the free rotation of the embedded fluorescent units [2,23,24]. In this case, the intramolecular rotation of AIE fluorogens are more easily restricted in polymer systems leading to the higher fluorescent emission yield, compared to the small AIE molecules [[24], [25], [26]]. In addition, the structure, composition, topology, morphology, and functionalities of AIE polymers can be feasibly tuned to meet the needs of applications, which are difficult to achieve in small AIE molecules [27,28]. As a result, the combination of AIE property and the polymer characteristics should produce attractive materials with unique properties and applications.

To date, several polymerization reactions have been adopted to prepare AIE polymers, which could be classified into two categories: the step-growth polymerization [[29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39]] and the chain-growth polymerization [40,41]. The step-growth polymerization produced AIE polymers with relatively lower molecular weight and broad polydispersity index (PDI). In addition, the AIE fluorogens were usually arranged along the resultant polymer main chains. The chain-growth polymerization produced AIE polymers with higher molecular weight containing the AIE fluorogens as side groups. In past a few years, various well-defined AIE polymers with controlled molecular weight and narrow PDI have been prepared by the living radical polymerization techniques including reversible addition fragmentation chain transfer (RAFT) polymerization [[42], [43], [44]] and atom transfer radical polymerization (ATRP) [45]. As a powerful living polymerization technique, however, ring-opening metathesis polymerization (ROMP) has been seldom employed to prepare the AIE polymers. To date, only one AIE fluorogen of tetraphenylethene has been successfully used to prepare the well-defined AIE polymers with controlled molecular weight and narrow PDI (<1.1) based on ROMP [[46], [47], [48]].

Herein, we explored the ROMP for the formation of well-defined AIE polymers containing varied AIE fluorogens such as distyrenneanthracene, 1,2,3,4,5-pentaphenylsiole, and tetraphenylethene derivative. By varying the AIE fluorogens, the fluorescence of the AIE polymers could be easily manipulated in a broad range of light wavelength, corresponding to the different light colors. As shown in Scheme 1, the norbornene-based monomers were prepared containing side groups of varied typical AIE fluorogens such as distyrenneanthracene (M1), 1,2,3,4,5-pentaphenylsiole (M2), and tetraphenylethene derivative (M3). With the Grubbs third generation catalyst (G3) as initiator, ROMP could produce the corresponding well-defined AIE polymers of poly(M1), poly(M2), and poly(M3) with controlled molecular weight and narrow polydispersity under mild polymerization condition. In addition, by sequentially copolymerizing the AIE monomers and the norbornene-based monomer (M4) with poly(ethylene glycol) (PEG) side chain, ROMP could also produce the well-defined amphiphilic AIE block copolymers such as poly(M1)-b-poly(M4) and poly(M2)-b-poly(M4) (Scheme 2). Furthermore, the AIE properties of the well-defined AIE polymers and the self-assembly behavior of the amphiphilic diblock copolymers were systematically investigated.

Section snippets

Materials

N-(Glycine)-cis-5-norbornene-exo-dicarboximide, diphenylmethane, 4-methybenzophenone, 4-methylbenzophenone, 4-bromo-benzophenone, titanium (VI) chloride, N,N-dimethlbenzamide, malononitrile, naphthalene, boron tribramide, 3-bromo-1-propanol, n-butyllithium (n-BuLi, 2.4 M in hexane), lithium, ammonium chloride (NH4Cl), p-toluenesulfonic (p-TSA), N-bromosuccinimide (NBS), benzoyl peroxide (BPO), N-potassium phthalimide, hydrazine hydrate (80 wt% solution in H2O) (N2H4), polyethylene glycol

Synthesis of well-defined AIE homopolymers of Poly(M1), Poly(M2), and Poly(M3) based on ROMP

As shown in Scheme 1, the well-defined AIE homopolymers of Poly(M1), Poly(M2), and Poly(M3) were synthesized by ROMP with a similar protocol at room temperature using G3 as the initiator. For the formation of Poly(M1)100, the initial feed ratio of [M1]0/[G3]0 was used as 100/1 and [M1]0 was used as 86 mmol/L in DCM. After 3 min polymerization, EVE was added into the polymerization solution to quench the polymerization reaction and detach the Ru catalyst from the polymer chain end. Fig. 1B shows

Conclusions

ROMP was demonstrated as a versatile method to prepare the well-defined AIE polymers. In this approach, the norbornene-based monomers were designed to have side groups of varied typical AIE fluorogens including distyrenneanthracene, 1,2,3,4,5-pentaphenylsiole, and tetraphenylethene derivative. Using G3 catalyst as initiator, ROMP could efficiently produce the well-defined AIE homopolymers containing varied AIE fluorogen side groups. In addition, by sequentially polymerizing the hydrophobic AIE

Acknowledgment

Generous support was primarily provided by National Science Foundation of China (21504007).

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      In many cases the monomers noted were copolymerized with simpler norbornene structures; in these cases, only the most structurally exotic monomer has been noted. Numerous substituted norbornenes and other bicyclo[2.2.1]heptene systems have been subjected to ROMP using metal carbene complexes (see Fig. 4), including those possessing the following structural features: (1) norbornene [318], including end-capping strategies with 5-hexenyl acetate [319], tetraallyl-functionalized zinc complexes (end-capping cross-linking) [320], and norbornene in a microemulsion system using a photoactivated catalyst system [321]; (2) norbornenes covalently-linked to α-chloroester groups (e.g. 70) [322], to thiolactone groups [323], to groups that can undergo RAFT polymerizations (e.g. trithiocarbonates) [324], to BODIPY groups [325], to luminescent iridium pyridylcarbene complexes [326,327], to benzobismole groups (e.g. 71) [328], to gold surface-bound imidazolium moieties (e.g. 72) [329], to polyglycidol groups [330], to polyacrylate groups [331,332], to polylactone groups [333], to polystyrenes [334], and to decaborane groups [335]; (3) norbornenes fused to fluorinated cyclobutane ring systems (e.g. 73) [336]; (4) silylated norbornenes [337–339]; (5) silacyclobutane-fused norbornene [340–343]; (6) bis(norbornene)s linked through siloxane groups (to crosslink norbornenedicarboxylate ester ROMP polymers) [344]; (7) 5-ethylidenenorbornene [345]; (8) dicyclopentadiene [346–351], including frontal ROMP [352], and an ester-functionalized dicyclopentadiene derivative (e.g. 74) [353]; (9) norbornadiene [354]; (10) 1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo-endo-dimethano-naphthalene (75) (and use as a cross-linker in polymerization reactions) [355,356]; (11) tricyclo[6.4.0.19,12]-tridec-10-ene (76) [357]; (12) exo-1,4,4a,9,9a,10-hexahydro-9,10(1′,2′)-benzeno-l,4-methanoanthracene (the norbornadiene-anthracene Diels-Alder adduct) [358]; (13) a dendrimeric nonakis(norbornene) connected through a cyanuric acid core [359]; (14) norbornenepyrrolidines [360]; (15) bis(norbornenepyrroline)s linked through a perylene bis(imide) system [361]; (16) ammonium salts derived from norbornenepyrrolidine [362]; (17) ammonium salts from norbornenepyrrolidine linked to a perylene bis(imide) system [363]; (18) norbornenecarboxylate esters, including those that feature covalent linkages to benzophenone units [364], to BODIPY fluorophores (and study of the ROMP kinetics by fluorescence microscopy) [365], to rhodamine B (and norbornenecarboxamides linked to polypeptides) (e.g. 77) [366] and other fluorophores [367], and to polythiophene or polylactate systems in a copolymerization [368]; (19) norbornene-anthracene Diels-Alder polymers (e.g. 78) [369]; (20) norbornenedicarboxylate esters [370], including those connected to triterpenoid groups [371] or to a gold surface through thiolate groups [372]; (21) norbornenesuccinimides, including those connected to amine groups [373], to amino acid and oligopeptide systems [374], to 3,4,5-trihydroxybenzamide derivatives [375], to dendrimeric 3,4,5-trihydroxybenzamide systems [376], to 3,4,5-trihydroxybenzamide systems connected to ferrocene moieties [377,378], to aromatic aldehydes through a triazole linkage [379], to doxorubicin [380], to dendrimeric polyaromatic systems [381], to α-bromoester groups [382–384], to groups that can initiate a RAFT polymerization process (e.g. 79) [385], to perylenediimide groups [386]; to a terpyridine system [387], to copper-ligating pyridine ligands (e.g. 80) [388], to ferrocene groups [389], to iron and cobalt sandwich complexes (e.g. 81) [390], to alkylpalladium complexes [followed by cross-linking with bis(norbornenesuccinimide) derivatives] [391], to monosaccharide units (e.g. 82) [392], to dendrimeric polyols or dendrimeric polyols connected to fluorescent groups) [393], to an iminoacylpalladium complex [and cross-linking through using a bis(norbornenesuccinimide)] [394], to a cobalt selenide molecular cluster [395], to fluorescent moieties (e.g. 83) [396,397], to polystyrene systems [398], to a polystyrene-polyacrylate assembly [399], to polystyrene or polylactate chains in a copolymerization [400], to polystyrene or polysiloxane systems in a copolymerization [401], to polylactate chains [402], to polyhedral oligomeric silsesquioxane (POSS) [403–405], to PEG groups [406]; to PEG groups or ammonium salts (e.g. 84) (aqueous phase ROMP) [407], to polydimethylsiloxane (PDMS) [408], and to UiO-66 metal organic framework systems [409]; (22) bis(norbornenesuccinimide)s for crosslinking [410]; (23) norbornenesuccinimide ROMP followed by termination of living polymer with single addition of a cyclopropene [411]; (24) development of selective chain transfer agents (cinnamyl alcohol derivatives) for end-capping of norbornenesuccinimides ROMP polymers [412]; (25) 7-isopropylidenenorbornenesuccinimides connected to monosaccharide units [413]; (26) furfural-derived oxanorbornene systems (e.g. 85) [414,415]; (27) norbornenemaleic anhydrides (and oxanorbornene analogs, e.g. 86) [416]; (28) oxanorbornenesuccinimides [417,418], including those connected polyacrylate chains [419], to phosphonate groups [420], to thiolactone units (e.g. 87) [421], and to bromopropyl groups [422]; and (29) vince lactam derivatives (e.g. 88 and stereoselective ROMP via MAP catalysts) [423]. Other ring systems that have been subjected to ROMP reactions are depicted in Fig. 5 and include: (1) cyclohexane-fused cyclobutenes conjugated to carbonyl or nitrile groups (alternating copolymerization with cyclohexene) (e.g. 89) [424–426]; (2) bromoladderenes (e.g. 90) (followed by mechanochemical conversion to polyacetylenes) [427]; (2) cyclopentene and cyclopentene copolymers with norbornene [428]; (3) cyclopentenes and analogous molecules containing multiple cyclopentenes for formation of covalent networks (e.g. 91) [429]; (4) cyclopentenes connected to groups that can initiate a radical polymerization (e.g. 92) [430]; (5) chiral cyclopentenol derivatives (e.g. 93) (ROMP using either racemic or optically active monomers) [431]; (6) cyclooctene [432–436] or substituted cyclooctene derivatives (e.g. 94) [437]; (7) cyclooctenes fused to imidazolium cations (e.g. 95) and cyclization of the final products through capping with cyclooctene or 1,9-decadiene, or bis(cycloocteneimidazolium) analogs [438]; (8) cyclooctenes or 1,5,9-cyclododecatriene and end-capping through cross metathesis with alkenes that have polymerizable groups (e.g. epoxides, oxetanes) [439]; (9) cyclooctenes substituted by alkenic side chains (e.g. 96) for hybrid ROMP and cross metathesis for the preparation of polyolefins of different topologies [440]; (10) cyclooctenes connected to α-bromoester groups [441,442]; (11) 1,5-cyclooctadiene and end-capping through metathesis with 1,4-diacetoxy-2-butene [443]; (12) cyclooctene, 1,5,9-cyclododecatriene (e.g. 97), and norbornene and termination with azlactone alkenes (e.g. 98) [444]; (13) a cyclic tetraether (e.g. 99) [445]; (14) oxadiazabicyclooctenones (e.g. 100) [446]; (15) paracyclophanedienes (e.g. 101) in REMP [447], in copolymerization with norbornenesuccinimide derivatives [448], in copolymerizations with a norbornene carboxylate ester ROMP polymer and a polyisocyanide for a helical triblock copolymer [449], and using C13 labeled systems for a mechanistic study [450]; (16) cyclophanedienes and naphthalenophanedienes (e.g. 102) [451]; (17) paracyclophanedienes connected to tetraphenylethylene (e.g. 103) [452]; (18) cyclopentadithiophene-vinylene trimer (e.g. 104) [453]; (19) bridged ferrocenes (e.g. 105, the monomer was prepared by RCM) [454]; (20) a macrocyclic octakis(lactone) (e.g. 106, the monomer was prepared through Z selective RCM with Z selective catalyst 33) [455]; and (21) ω-6-hexadecenelactone (e.g. 107) [456].

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