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Keywords:

  • 1,3-dipolar cycloaddition;
  • azide/alkyne ‘click’ reaction;
  • polymerization (general);
  • surfaces

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Mechanistic Details/Catalysts
  5. Click Reactions on Linear Polymers
  6. Complex Polymeric Architectures
  7. Click Reactions on Surfaces
  8. Conclusion and Outlook
  9. Acknowledgements
  10. Biographical Information
  11. Biographical Information
Thumbnail image of graphical abstract

The metal catalyzed azide/alkyne ‘click’ reaction (a variation of the Huisgen 1,3-dipolar cycloaddition reaction between terminal acetylenes and azides) has vastly increased in broadness and application in the field of polymer science. Thus, this reaction represents one of the few universal, highly efficient functionalization reactions, which combines both high efficiency with an enormously high tolerance of functional groups and solvents under highly moderate reaction temperatures (25–70 °C). The present review assembles an update of this reaction in the field of polymer science (linear polymers, surfaces) with a focus on the synthesis of functionalized polymeric architectures and surfaces.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Mechanistic Details/Catalysts
  5. Click Reactions on Linear Polymers
  6. Complex Polymeric Architectures
  7. Click Reactions on Surfaces
  8. Conclusion and Outlook
  9. Acknowledgements
  10. Biographical Information
  11. Biographical Information

Who will be reading a review on a topic that was reviewed only 15 months previously1 by the very same author? According to a SciFinder search in January 2008, the azide/alkyne ‘click’ reaction (also termed CuAAc) has had enormous impact within the field of polymer science. Thus, ≈220 original papers have been published in the context of click chemistry and polymer science, more than 20 reviews and at least 10 patents have appeared, altogether stressing the importance of this reaction. Given the short timeline for discovery of CuI catalysis (≈2001–2002 by Meldal et al.2, 3 and ≈2002 by Sharpless et al.4) and the first published applications in polymer science (≈2004),5–10 someone may ask the question ‘where does it stem from’, and - in the same line - find a quick answer: a high efficiency reaction, coupled with a high functional group tolerance and solvent insensitivity (also highly active in water), working equally well under homogeneous and heterogeneous conditions certainly ranks high on the polymer scientists' wish list. Therefore, this reaction is a solution to many problems that have been encountered in polymer science for a long time, such as: a) a poor degree of functionalization with many conventional methods, especially when involving multiple functional groups (i.e.: at graft-, star-, and block copolymers, dendrimers, as well as on densely packed surfaces and interfaces); b) purification problems associated with the often emerging partially functionalized mixtures; c) incomplete reaction on surfaces and interfaces; and d) harsh reaction conditions of conventional methods, which often lead to the break-up of associates and assemblies, in particular in the newly emerging supramolecular sciences. As a main surplus, the click reaction combines excellently with many controlled polymerization reactions developed during the past decades,11 thus opening the way to a nearly unlimited investigation of new functionalized polymeric architectures, hitherto unreachable by the polymerization methods themselves. With azide/alkyne click chemistry in hand, polymer chemistry now approaches the level of small-molecule organic chemistry in terms of functional broadness, structural integrity, and molecular addressability.

Briefly, the azide/alkyne click reaction12–14 represents a metal-catalyzed variant of the Huisgen 1,3-dipolar cycloaddition reaction15, 16 between C[BOND]C triple, C[BOND]N triple bonds17 and alkyl-/aryl-/sulfonyl azides. The relevant outcomes of this reaction are a) tetrazoles,13, 18 b) 1,2,3-triazoles,2–4, 19 or c) 1,2-oxazoles, respectively. In addition, classical Diels–Alder-type reactions have been used extensively for the functionalization of polymeric materials20 and surfaces.21 According to the definition of Sharpless et al.,12 a ‘click reaction’ is defined by a gain of thermodynamic enthalpy of at least 20 kcal · mol−1, thus opening the way to a high yielding and thus nearly substrate-insensitive reaction. The present review focuses entirely on the azide/alkyne reaction catalyzed by CuI species, and the now more widely used purely thermal (‘Huisgen-type’) processes in polymer science and on surfaces. A survey of the most recent literature related to the polymer science and materials field in the years 2006 to 2008 (deadline: 15th March 2008) will draw a line to a previous review in this journal1 and other reviews describing the azide/alkyne click reaction in general,12, 22 for application in polymer chemistry,1, 23–28 dendrimers,25, 29 carbohydrate chemistry,30–32 materials chemistry,1 and organic chemistry,28, 33 as well as for peptides34 and drug discovery.35 In addition, this whole special issue of Macromolecular Rapid Commununication is dedicated to this topic, including many more specialized reviews and original papers. Thus special reviews will cover topics such as the role of the copper species on polymer ‘clicking’ (by M. Meldal), the generation of polymeric architectures (by Turro et al.), the combination of biodegradable polymers and azide/alkyne click reactions (by R. Jerome et al.); the synthesis of multistep reactions in combination with azide/alkyne click reactions (by M. Malkoch et al.); click chemistry for the synthesis of macromolecular chimeras (polymer/biopolymer hybrids, by K. Velonia); and reversible addition fragmentation transfer (RAFT) from silica nanoparticles (by W. Brittain et al.). The present review will briefly and concisely update the topic azide/alkyne click chemistry in polymer science, including literature up to March 2008.

Mechanistic Details/Catalysts

  1. Top of page
  2. Abstract
  3. Introduction
  4. Mechanistic Details/Catalysts
  5. Click Reactions on Linear Polymers
  6. Complex Polymeric Architectures
  7. Click Reactions on Surfaces
  8. Conclusion and Outlook
  9. Acknowledgements
  10. Biographical Information
  11. Biographical Information

Briefly, the basic process of the Huisgen 1,3-dipolar cycloaddition2, 10, 11 generates 1,4- and 1,5-triazoles respectively (Scheme 1). Nearly all functional groups are compatible with this process, except those that are a) either self reactive or b) able to yield stable complexes with the CuI metal under catalyst deactivation. Main interfering functional groups are terminal azides and alkynes,36 strongly activated cyanides,13, 14, 18 free (accessible) thiol-moieties (R–SH) via the Staudinger reaction, as well as strained or electronically activated alkenes.16, 37 However, the possibility to use free-thiols prior to an azide/alkyne click reaction has been demonstrated recently on polymers38 and surfaces,39 thus enabling the use of free thiols despite the often interfering azide/amine reduction by the free thio-moiety.

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Scheme 1. Azide/alkyne - “click” - reaction.

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In addition to the use of CuI salts (amounts of approx. 0.25–2 mol-%, also coupled to regenerative systems with ascorbic acid), copper clusters (Cu/Cu-oxide nanoparticles, sized 7–10 nm40 or ≈4 nm41), metallic Cu0 clusters 41–43) as well as copper/charcoal44 have been described. A new and highly innovative approach towards a polymeric-bound CuI catalyst has been described by Bergbreiter et al.,45 attaching a bipyridyl ligand to a polyisobutylene, subsequently ligating the CuI species to the polymer. This generates a CuI species with a high solubility in hexane solvents for catalytic applications. Recently, the use of a CuI-free variant using the ring-strain of substituted cyclooctynes to promote the dipolar cycloaddition process has been described, enabling mild reactions on living (cellular) systems.46, 47

Most known solvents and biphasic reaction systems (mixtures of water/alcohol to water/toluene) can be applied with excellent results. Cocatalytic systems48 often used include amino bases,49 (triethylamine (TEA), 2,6-lutidine, N,N-diisopropylethylamine (DIPEA), N,N,N′,N′,N″-pentamethylethylenetetramine (PMDETA), hexamethyltriethylenetetramine (HMTETA), tris[(2-pyridyl)methyl]amine (TPMA), tris[(2-dimethylamino)ethyl]amine (Me6TREN), 2,2′-bipyridines (bpy), 2,2′:2′,6″-terpyridine (tpy), ammonium salts,42 and mono- and multivalent triazoles49) but also phosphines such as tris(carboxyethyl)phosphine (TCPE). A detailed investigation50 of several amines has demonstrated a relative kinetic effect of the added ligands in the order: PMDETA (230) > HMTETA (55) > Me6-TREN (50) > tpy (8.6) > TPMA (1.7) > no ligand > bpy (0.43). The increase in reaction rate is mostly explained by promoting the formation of the CuI-acetylide, reducing the oxidation of the CuI-species, but also by preventing side reactions of the acetylenes (Ullman couplings, Cadiot–Chodkiewizc couplings) or dimerization reactions of the finally formed triazoles.51 The latter dimerization reaction is a very important one, generating 5,5′-coupled dimeric triazoles when using carbonates as bases instead of the usual amine bases. Moreover, the copper-catalyzed hydrolysis of O-propargylic-carbamates has recently been described.52 Besides copper, other metals employed include Ru complexes53, 54 such as (CpRuCl(PPh3), [Cp*RuCl2]2, Cp*RuCl(NBD), and Cp*RuCl(COD) favoring not only the formation of 1,4-addition (e.g., with Ru(OAc)2(PPh3)2), but also the formation of 1,5-adducts by other Ru catalysts. In addition, the use of AuI-,55 Ni-, Pd-,56 and Pt-salts, has been described, although with significantly less catalytic activity.50

Strong effects of alternative synthetic methodologies have been observed under microwave irradiation.57–62 As it turns out, the click reaction can be strongly accelerated under microwave irradiation, however, favoring both the 1,4-adduction as well as the side reactions.

The mechanism (first experimentally proposed by Sharpless et al.4 and changed by Finn et al.,63, 64 determined by computational methods,65, 66 and finally revised by Bock et al.22) involves the following main features (Scheme 2): a) up to a ×105 rate acceleration and an absolute 1,4-regioselectivity of the CuI catalyzed process, b) a kinetic feature of the reaction that indicates at least second-order kinetics with respect to the concentration of the copper species,64 thus proposing at least two copper centers involved in this reaction, probably linking two acetylenes by a µ-bridge,67 c) a significant autoacceleration if multiple triazoles are formed,48 which reveals intermolecular ligand effects, d) a significant rate-reduction with strongly increasing amount of copper, and e) the formation of a copper-acetylide, whose primary structure and, therefore, direct activity within the transition state cannot be exactly predicted. In particular, the exact structure of the copper-acetylide is difficult to predict a) because of the large number of possible interactions (π-complexation, multiple copper species) and b) because of the many known (highly different) structures of copper-acetylides. However, the basic feature (i.e., lowering of the pKa value of the Cu-acetylide by up to 9.8 units as determined66 by DFT calculations) is the most important contribution towards the rate acceleration.

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Scheme 2. Proposed mechanism of the azide/alkyne - “click” - reaction.

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If the two reaction partners (azide and alkyne) are brought into close (enforced) spatial relationship, the reaction can be fast and efficient, as demonstrated in the case of proteins and enzymes (affinity based protein profiling (ABPP)),19, 36, 68, 69 microcontact printing,70, 71 or atomic force microscopy (AFM)-based techniques.72 All three methods generate a closer-than-usual distance between the two reaction partners, thus linking reactivity in this reaction to molecular distance. The absolute value of the necessary minimal distance at which the azide/alkyne click reaction occurs spontaneously has, however, not been specified in the literature.

A final remark should be made with respect to the optimal system for succeeding in an azide/alkyne click reaction. Honestly spoken, despite reviewing the literature as well as many of our own experiments9, 39, 73–81, I cannot tell. The number of variables and requirements is simply too high as to provide a general answer to this certainly important question. The tables and examples below may provide a hint and explanation in themselves, and thus eventually lead to an answer with respect to a specific system. The only, scientifically unsatisfactory, but truly honest answer I can provide is: check it out, it's simple.

Click Reactions on Linear Polymers

  1. Top of page
  2. Abstract
  3. Introduction
  4. Mechanistic Details/Catalysts
  5. Click Reactions on Linear Polymers
  6. Complex Polymeric Architectures
  7. Click Reactions on Surfaces
  8. Conclusion and Outlook
  9. Acknowledgements
  10. Biographical Information
  11. Biographical Information

The enormous interest in linear polymers lies primarily in the combination of click reactions with controlled polymerization processes. This chapter lists examples published till the midst of January 2008, where known polymerization processes have been combined with the azide/alkyne processes, mostly relating to the chemical possibilities and the chemical realization of this endeavor.

Table 1 lists the known click reactions on various linear- or graft-polymers, according to the polymerization method and the final chemical structure of the polymer, either before the click reaction, or after. Cases are shown in an exemplary manner, as to indicate the chemistry required in order to conduct a click reaction within or after a specific polymerization process. In general, as shown in Table 1, more than 60 entries are described, which indicates the enormous broadness of the investigations. For an example of click chemistry in conjunction with free-radical polymerization see ref.82.

Table 1. Overview of ‘click’-reactions with linear- or graft-polymers.
EntryPolymer/substratePolymerization methodCatalyst/conditionsRef.
1
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ATRPCuBr/r.t.86
2
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ATRPN-alkyl-2-pyridylmethanimine-CuBr/70 °C115
3
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ATRPCuBr/THF/r.t. 4,4′-di(5-nonyl)- 2,2′-bipyridine84, 90
4a
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ATRPNaN3/ZnCl2/120 °C10
4b
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ATRPNaN3/ZnCl2/120 °C10
5
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ATRPCuBr/PMDETA/r.t.107
6
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ATRPCuBr/DMF/r.t.106
7
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ATRPCuI/DBN/THF/35 °C85
8
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ATRPCuBr/PMDETA/THF/35 °C108
9
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ATRPCuBr/PMDETA103
10
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ATRPCuBr/DMF/r.t102
11
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ATRPCuBr/PMDETA/r.t94
12
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ATRPCuBr/PMDETA/50 °C98
13
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ATRPCuBr/DMF97
14
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ATRPCuBr/bipyridine/DMF/120 °C89
15
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ATRPCu0/CuBr or CuBr/PMDETA/DMF/r.t.87
16
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ATRPCuBr/PMDETA/sodium ascorbate/DMF88
17a
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ATRPCu(PPh3)3Br/DIPEA109
17b
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ATRPCu(PPh3)3Br/DIPEA109
18
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ATRPCuBr/bipyridine/THF/r.t.113
19
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NMPNaN3/ZnCl2/DMF/120 °C119
20
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NMPCu(PPh3)3Br/DIPEA/dioxane120
21
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NMPCu(PPh3)3Br/DIPEA/THF123
22
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NMP[(CH3CN)4Cu]PF6/TBTA/DIPEA/DMF81
23
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NMPCuBr/PMDETA/DMF/r.t.96
24
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NMPCu(PPh3)3Br/DIPEA122
25
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NMPtoluene121
26
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free radicalCuSO4 · 5H2O/sodium ascorbate/H2O/DMSO82
27
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RAFT + NMPCu(PPh3)3Br/DIPEA/THF/H2O/r.t./3d118
28
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RAFTCuI/DBU/DMAc/40 °C129
29
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RAFTCuBr/PMDETA/DMF/r.t130
30
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RAFTCuSO4/sodium ascorbate/H2O/tBuOH128
31
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ATRPCuBr/bipyridine/120 °C104
32
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RAFTCuSO4/sodium ascorbate/H2O117
33
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RAFTCuSO4 · 5H2O/sodium ascorbate/DMSO/50 °C101
34
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RAFTCu(PPh3)3/DIPEA/DMF/r.t.132
35
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RAFTCuBr/PMDETA/DMF/r.t.127
36
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RAFTCu(PPh3)3Br/DIPEA/THF/H2O133
37
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RAFTCuI/DBU/THF131
38
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RAFT 134
39
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RAFTCuI/DBU/THF/40 °C135
40
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ROMPCu(I)/DIPEA/DMF/toluene/H2O76
41
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ROMPCu(PPh3)3Br/DIPEA/DMF/50 °C9, 75, 74
42
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ROMP 137
43
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ROMPCuBr/PMDETA/DMF/50 °C136
44
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  150
45
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living anionic 38
46
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living cationic ring openingCuSO4 · 5H2O/water/tBuOH145
47
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living cationic polymerization of isobuteneCu(PPh3)3Br/DIPEA/toluene77
48
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living anionic polymerizationCuBr/DMF/60 °C149
49
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polyadditionCu(I)153, 152
50
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polyaddition100 °C154
51
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polyadditionCuSO4.5H2O/sodium ascorbate/H2O/tBuOH155
52
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polyadditionCuSO4.5H2O/sodium ascorbate/H2O/tBuOH156
53
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polyaddition 151
54
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polyadditionCuSO4 · 5H2O/sodium ascorbate H2O:tBuOH (1: 1)/r.t.6
55
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polyadditionCu/Cu(OAc)2/TBTA THF/CH3CN161
56
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polyadditionCuSO4 · 5H2O/sodium ascorbate157
57
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CVDCuSO4 · 5H2O/sodium ascorbate H2O:tBuOH (2: 1)70
58
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topologicalCuI/CH3CN164
59a/b
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polyadditionCuSO4 · 5H2O/H2O:tBuOH (2: 1)163
60
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anionic ring openingCuSO4 · 5H2O/sodium ascorbate/100 °C143, 141, 140
61
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ROPCuSO4 · 5H2O/Cu wire/37 °C146
62
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sequential stepwise solid phase synthesisCuI/ascorbic acid/DIPEA butan-2-ol/DMF/pyridine172
63
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 CuIIBr/ascorbic acid/propylamine DMSO/r.t.176
64
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DNA-synthesisDMSO/H2O/80 °C/72 h (no CuI!!)188
65
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solid phase synthesis or DNA-polymeraseCuI186
66
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solid phase synthesisCuSO4 · 5H2O, sodium ascorbate H2O/MeOH61, 182

Atom Transfer Radical Polymerization (ATRP)

As indicated in Table 1 (entries 1–18), a variety of click reactions in conjunction with ATRP have been described.45, 50, 83–115 The reaction has been investigated with polymers such as polystyrene, various polyacrylates and polymethacrylates, polyacrylnitrile, and poly(ethylene oxides). As the main methods for conducting ATRP and click reactions has been described in the previous review,1 this issue will not be discussed here in more detail. In principle the following strategies can be applied to achieve a combination between the click reaction and ATRP:

  • The initiator approach,85, 89, 91, 103, 106, 110 which relies on the use of an alkyne-functionalized initiator or azide-functionalized initiator115 as shown in Table 1, entries 6, 7, 8, 14 or 2. Relevant to this point is the fact that the acetylenic/azido initiator-moiety is not cross-reactive within the subsequent ATRP process.

  • The Br/Nmath imageapproach84, 85, 88, 90, 94, 102, 103, 107–109, 113, 114, 116 takes advantage of the terminal bromine-moiety, inherently present within the ATRP reaction. A final Br/Nmath image-nucleophilic reaction (usually performed in NaN3/N,N-dimethylformamide (DMF)) then completes the introduction of an azido-moiety to the terminus of the polymer (see Table 1, entries 3, 5, 6, 8, 10, 11, 12, 16, and 18). The full conversion of this reaction has been demonstrated by NMR methods and subsequent characterization by matrix-assisted laser desorption ionization mass spectrometry (MALDI MS) experiments of the final ‘clicked’ products.

  • Side-chain modified polymers86, 92, 93, 97, 102, 109, 111 with pendant azido- or acetylenic moieties for the generation of graft-polymers (e.g., Table 1, entry 1, 10, 13, 17a, and 17b) have been reported. Again, the full compatibility of the terminal azido or acetylenic moieties with the ATRP reaction is observed, which leads to a high density of functional side chains within the main-chain polymer.

As the main structural issues are solved within the combination of click reactions and ATRP, this method is a fully established reaction sequence that leads to all kinds of chain-end, head-end, and side-chain modified polymers by the ATRP process.

An important point concerns the formation of macrocyclic polymers by click reactions,89 which was first described by Grayson et al. (Table 1, entry 14) with an α,ω-bifunctional polystyrene, and has now been extended to the generation of cyclic poly(N-isopropylacrylamides)104 (Table 1, entry 31) in yields that range from 60 to 80%. This verifies the prediction made in the previous review,1 that the click reaction will be an efficient tool for the generation of polymeric macrocycles in the near future. In a similar manner (see Table 1, entry 32) the generation of cyclic poly(N-isopropylacrylamides) by RAFT/click methods has been described in high yields.117

Nitroxide-Mediated Polymerization (NMP)

As time has gone by, many NMP methods have been conducted in junction with the click reaction.81, 96, 118–125 As with ATRP, the polymerization of side-chain functionalized monomers bearing azido-moieties is simple, and leads to copolymers onto which additional functional groups can be easily attached. Thus photolabile moieties (entry 20)120 or potential crosslinking sites (entry 24) can be introduced by the corresponding acetylenes or azides.124 The initiator approach (Table 1, entries 21–23) has been tested using an azide/alkyne-modified Hawker-type nitroxide to initiate the polymerization of styrenes,96, 123 acrylates,81, 96 and N-isopropylacrylamide.81 The attachment site can be easily functionalized by the azide/alkyne click reaction, to furnish the corresponding monofunctionalized, telechelic polymers. Furthermore, the simplicicity of the generation of a large structural variety of Hawker-type nitroxide initiators could be used to demonstrate remote effects between the end-group on the nitroxide and the growing radical.81 A very fine example of a combination of a modified NMP initiator (prepared by the click approach) and ATRP has been reported (Table 1, entry 23).96, 126 Besides the nitroxide moiety, an ATRP-initiating site as well as a terminal alkyne moiety are bound to the initiator, which generates three possible sites for the attachment of three different polymers: initiation of polystyrene by the NMP method, followed by the attachment of poly(ethylene glycol) (PEG)-N3 through the terminal alkyne, followed by ATRP of methyl methacrylate (MMA) was used to generate multivalent, three-arm star polymers with three different polymers on each arm.

Reversible Addition Fragmentation Transfer (RAFT)

Since the last review, an enormous increase in publications combining RAFT with the azide/alkyne click reaction have been described.25, 101, 105, 117, 118, 127–135 As many monomers (such as N-isopropylacrylamides,117, 130 substituted styrenes,118 hydroxylated methacrylates,132 and glycosylated methacrylates129) are easier to combine with RAFT than with ATRP or NMP, the RAFT/click methodology seems to be the method of choice for several polymers in this context.

As in the case of ATRP, azido-/alkyne-modified RAFT initiators 25, 101, 105, 127, 129–131, 135 as well as side-chain modified monomers128, 131, 133 can be used in this strategy. Whereas azido-modified RAFT-initiators can be used without detriment to initiate a living polymerization, which leads to fully endgroup-functionalized telechelic polymers (proven by MALDI and NMR experiments), there is a divergence in the literature concerning the use of acetylene-RAFT initiators. Some authors definitely claim the necessity to use trimethylsilyl (TMS)-protected terminal acetylenes within the monomers131, 133 or initiators129 for a successful RAFT polymerization (otherwise observing crosslinking during the polymerization reaction with the free acetylenes), some authors use unprotected RAFT initiators for the polymerization of styrene,101, 135 acrylamides,101 and vinylacetate.135 With azido moieties within the initiator part, the interference is much lower, since they can be used both within the initiating RAFT agent117, 127, 130 as well as within the sidechain128 of the used monomer. In addition, the use of triazole units within the monomer, close to the growing radical centre, leads to good results for the final RAFT polymerization.134

An outstanding example for the generation of cyclic poly(N-isopropylacrylamides) by RAFT/click methods has been described in high yields (see Table 1, entry 32).117 The use of an azido-modified RAFT initiator, subsequent polymerization of N-isopropylacrylamide, and final transformation of the terminal isobutylsulfanylthiocarbonylsulfanyl moiety into the propargylic moiety by a one-pot aminolysis/Michael addition sequence is reported. Although no yield was provided, the publication represents the first example for the preparation of cyclic poly(N-isopropylacrylamide).

Ring-Opening Metathesis Polymerization (ROMP) and Ring-Opening Polymerization (ROP)

ROMP/click methodologies9, 39, 75 (see Table 1, entries 40–43) inherently offer an enormously broad aspect in polymer chemistry, since ROMP chemistry is a simple and highly efficient approach towards functionalized polymers, in particular towards block-copolymers. We have first developed efficient attachment strategies of various ligands, in particular of supramolecular receptors (hydrogen-bonding structures) onto poly(oxynorbornenes).9, 74–76 Using oxynorbornenes with pendant azido and acetylenic units, the copolymerization into either homo-,9 block-,74, 76 and statistical copolymers75 could be demonstrated. The important aspect of this synthetic approach lies in the fact that these polymers represent universal scaffolds for the attachment of supramolecular units onto the backbone, which allows modulation of the density, distribution, and thus stickiness on a molecular scale. Oxynorbornene monomers are advantageous over the corresponding norbornenes because of their reduced ring-strain, thus eliminating the concurring dipolar cycloaddition reaction onto the norbornene ring.16, 37 A very fine example to generate a three-arm star polymer, based on ROMP and side-chain liquid-crystalline cyclooctene moieties is provided by ref.136. Polymerization using a bis-bromoalkene initiator furnished the bistelechelic poly(norbornene). Subsequent exchange against the azide and coupling to a central core yielded the final polymeric star-architecture. Purely thermal strategies have also been employed also, e.g., see ref.137. Recently, these results were also demonstrated on other poly(norbornene)s using the same strategy.138 In addition, a paper on the preparation of N-heterocyclic carbenes from poly(p-azidomethylstyrenes) and click reactions have been described.139

Several ROP/click strategies113, 140–148 (see Table 1, i.e.: entries 46, 60, and 61) have been described recently, either using an alkyne moiety143–145 or an α-chlorocaprolacton140–142 as functional monomers or an appropriate alkyne endgroup.147 In this way, functionalized poly(glycolides),148 poly(lactides),146 poly(caprolactones),140–144, 147 poly(oxazolines),145 and poly(L-valine)-block-poly(acrylic acid) copolymers113 have been successfully prepared.

Anionic and Cationic Polymerization

Few examples that combine living anionic38, 149 and cationic45, 73, 77, 145 polymerization reactions with click reactions have been described (Table 1, entries 45–48). As shown in Table 1, entry 45, living anionic polymerization of ethylene oxide has been combined with subsequent hydroxyl/mesyl/bromide/azide exchange.38 Interestingly, the photochemical addition of 2-mercaptoethylamine onto an allylic bond is described in the presence of the azido moiety, which represents one of the examples of successful thiol chemistry in the presence of the azide moiety without concomitant reduction. Another example of living anionic polymerization and click chemistry has been reported with poly(p-propargyloxy styrenes).149 With living cationic polymerization, supramolecular ligands have been fixed onto mono-,73 bi-, and trivalent telechelic poly(isobutylenes), prepared by quasi-living cationic polymerization (equation image = 3 100 g · mol−1, equation image = 1.10).77 The reaction has been performed in biphasic reaction systems, featuring toluene/water solvent mixtures and CuIBr as the catalyst with yields above 94%, which demonstrates the high efficiency in heterogeneous reaction systems. Poly(1,3-oxazolines) using 2-(pent-4-ynyl)-2-oxazoline as monomer have also been polymerized by living cationic polymerization and functionalized by a subsequent click reaction.145 Furthermore, the Ni-catalyzed polymerization of alkyne-functionalized isocyanides has been described.150

Polyaddition/Polycondensation

A variety of examples have been reported (see Table 1, entries 49–59), that demonstrate polyaddition6, 70, 151–160 or polycondensation processes in junction with the azide/alkyne click reaction. In principle, two approaches should be discerned: a) chain-growth or network-formation by direct azide/alkyne click reaction6, 152–156, 158, 161 or b) the introduction of azide or alkyne groups into the growing chains70, 151, 157 or endgroups by the respective monomer units for further attachment. Using strategy ‘a’ a variety of functional polymers such as high glass transition temperature (Tg) polymers,152 metal adhesive polymers,6, 153 polymers with optical non-linearity,154, 160 organic semiconductors,158 or high temperature-stable polymers155 have been prepared. Strategy ‘b’ has been used to prepare side-chain-functionalized polyurethanes151 poly(p-phenylene vinylenes),157 poly[(4-ethynyl-p-xylylene)-co-(p-xylylene)]s,70 poly(fluorenes),160 and poly(pyrroles).162

Gels and networks6, 82, 88, 136, 152, 153, 159, 163–166 have also been formed by azide/alkyne click reactions. This strategy has proven useful as a simple crosslinking strategy, but also for the formation of highly sensitive gel and network structures not accessible by other methods.164 As supramolecularily preorganized molecules often tend to disintegrate upon thermal treatment, the azide/alkyne click reactions represent an important step towards stable networks of defined crosslinking density, thus ‘freezing-in’ a specific supramolecular structure.88, 166

Click Reactions on Other Polymers

The field of ‘other’ polymers in click chemistry is large and can be hardly overseen putting all other polymers not presented in the previous groups into this category. Thus azide/alkyne click chemistry has been described vastly with peptides,59, 64, 143, 150, 167–172 carbohydrates,30, 31, 34, 109, 111, 137, 173–184 cellulose,185 and oligonucleotides,61, 186–188 Suffice to say that the broadness is enormous and exceeds the scope of this article. Details will be partly presented in special reviews within this special issue.

Complex Polymeric Architectures

  1. Top of page
  2. Abstract
  3. Introduction
  4. Mechanistic Details/Catalysts
  5. Click Reactions on Linear Polymers
  6. Complex Polymeric Architectures
  7. Click Reactions on Surfaces
  8. Conclusion and Outlook
  9. Acknowledgements
  10. Biographical Information
  11. Biographical Information

The beauty and usefulness of the azide/alkyne click reaction is best demonstrated in the build-up of larger polymeric structures, in particular the polymeric architecture. As already mentioned, the field of dendrimers has been reviewed recently,29 therefore putting a focus on the polymeric architecture itself, mentioning new publications on azide/alkyne click chemistry on dendrimers59, 99, 112, 175, 189–202 and hyperbranched polymers5, 7, 130 more on the side.

The main polymeric architectures available are shown in Table 2 (see selected formulas for details within Table 2). It is clearly visible that quite complex polymeric structures, which bear a large variety of different functional groups, can be accessed easily with azide/alkyne click chemistry. Thus star-polymers,77, 91, 105, 106, 147, 203–206 (block)-copolymers,92, 100, 114, 144, 207–209 multisegmented block-copolymers,95 rod-coil-block copolymers,171 graft-polymers,142, 210–215 dendrimers,1, 27, 29, 31, 59, 99, 112, 132, 133, 175, 177, 189, 191, 194–196, 198–202, 216–224 polymer-brushes,93 and crosslinked capsules225, 226 can be prepared, relying on the aforementioned living polymerization methods and subsequent transformations. Yields are often high, putting these reactions far above others in terms of yield, efficiency, and easiness. As can be easily judged, nearly every polymeric architecture is now available by proper planning and appropriate manpower.

Table 2. Overview of ‘click’-reactions for the synthesis of complex polymer architectures.
EntryPolymer/substrateTypeCatalyst/conditionsRef.
1
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star polymersCuI/TBTA/DIPEA77
2
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graft-polymerCuBr/PMDETA/THF/DMF210
3
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star polymersCuSO4.5H2O/sodium ascorbate/70 °C105
4
thumbnail image
tadpole-shaped polymersCuI/N(Et)3/THF/35 °C142
5
thumbnail image
star polymersCuSO4 · 5H2O/sodium ascorbate/H2O/r.t.203
6
thumbnail image
graft-polymerCuSO4 · 5H2O/sodium ascorbate/H2O/MeOH/60 °C211
7
thumbnail image
capsuleCuSO4 · 5H2O/sodium ascorbate225
8
thumbnail image
terpolymersCuBr/Me6TREN/DMF/50 °C100
9
thumbnail image
copolymerCu(PPh3)3Br/DIPEA/CH2Cl2144
10
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graft-polymer 212
11
thumbnail image
rod-coil block polymersCuBr/PMDETA/r.t171
12
thumbnail image
star polymersCuBr/PMDETA91
13
thumbnail image
block-copolymerCuBr/bipyridine/NMP/r.t207
14
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 CuSO4 · 5H2O/sodium ascorbate/70 °C92
15
thumbnail image
triblock copolymersCuBr/PMDETA/DMF/120 °C208
16
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block-copolymerCuBr/bipyridine/THF/r.t.209
17
thumbnail image
polymer brushesCuBr/PMDETA/DMF93
18
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multisegmented block-copolymersCuBr/PMDETA/DMF/r.t.95
19
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graft-polymerCuSO4.5H2O/sodium ascorbate/H2O/CH2Cl2/r.t.213
20
thumbnail image
star polymersCuBr/PMDETA/DMF/r.t.204
21
thumbnail image
star polymersCuBr/PMDETA/DMF/r.t.126
22
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star polymersCuSO4/sodium ascorbate/100 °C/µW-irradiation147
23
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comb polymersCuBr/PMDETA/THF/r.t.214
24
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star polymersCuI/PMDETA/DMF/80 °C205
25
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graft polymersCuBr/PMDETA/DMF/r.t.114
26
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dendrimersCuSO4 · 5H2O/sodium ascorbate/H2O/THF (1: 4)/r.t.221
27
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dendrimersCuSO4 · 5H2O/sodium ascorbate/H2O/THF (1: 4)/r.t.202
28
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hyperbranched polymersCuSO4 · 5H2O/sodium ascorbate/H2O/tBuOH/hexane (5: 5: 1)/r.t.222
29
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dendrimersCu/CuSO4/TBTA DMF/r.t223
30
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graft-polymerCuI/DMF/80 °C215
31
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dendrimersCuBr/PMDETA/THF/r.t.99
32
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dendrimersCuSO4 · 5H2O/sodium ascorbate/H2O/THF/40 °C216
33
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dendrimersCuSO4 · 5H2O/sodium ascorbate/H2O/THF224
34
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dendrimersCuSO4 · 5H2O/sodium ascorbate/H2O/THF/r.t.189
35
thumbnail image
dendrimersCuBr/PMDETA/DMF/80 °C112
36
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block-copolymerCuBr/PMDETA/CH2Cl2/r.t.241

Click Reactions on Surfaces

  1. Top of page
  2. Abstract
  3. Introduction
  4. Mechanistic Details/Catalysts
  5. Click Reactions on Linear Polymers
  6. Complex Polymeric Architectures
  7. Click Reactions on Surfaces
  8. Conclusion and Outlook
  9. Acknowledgements
  10. Biographical Information
  11. Biographical Information

An interesting aspect of the azide/alkyne click reaction lies in the fact that a reduced or enforced distance between the reaction partners leads to a strongly enhanced reaction rate. This effect has been demonstrated in the azide/alkyne click reaction within the pocket of enzymes (based protein profiling (ABPP))19, 36, 69, 227 by direct microcontact printing,70, 71 or by AFM tips,72 thus opening the chance for a sufficiently complete reaction at an interface. Moreover, since surfaces and interfaces are a chronic source of incomplete or insufficient chemical reactions, the azide/alkyne click reaction here definitely has changed the world of the interfacial scientist, enabling easy access to functionalized surfaces of reliable and reproducible surface densities. Thus a large variety of click reactions on self-assembled monolayers (SAMs),39, 80, 132, 173, 174, 228–236 polymeric surfaces,45, 116, 162, 185, 237, 238 layer by layer assemblies,238, 239 block copolymer (BCP) micelles,124, 125 polymersomes240–242 and liposomes243–245 have been reported (see Table 3). In the case of SAMs the use of appropriately azide-39, 101, 228–231 or alkyne-functionalized132, 173, 174, 232–234 surfaces by direct ligand-adsorption have been described. Alternatively, in-situ generation of terminal azides by bromide/azide exchange directly on the ω-bromoalkyl-functional monolayer can be effected,231, 235 which eliminates the pressing instability of ω-azido-1-thioalkanes prior to the SAM-formation process. Dynamic or labile assembly structures (such as polymersomes, BCP micelles, and liposomes) offer either a direct approach to modify the already existing surface of the assembly,124, 125, 240, 244 or to modify the molecule by click reaction before the assembly.241, 242, 245 The latter strategy is definitely less elegant, but sometimes more efficient.

Table 3. Overview of ‘click’ reactions on surfaces, nanoparticles, polymersomes, vesicles, micelles, carbon nanotubes, and resins.
EntryPolymer/substrateSurfaceCatalyst/conditionsRef.
1
thumbnail image
SAM on Au/planarCuSO4 · 5H2O/sodium ascorbate/H2O/EtOH228
2
thumbnail image
SAM on SiO2/planarthermal/70 °C/neat231
3
thumbnail image
SAM on Au/planarCuSO4 · 5H2O/sodium ascorbate/H2O/EtOH232
4
thumbnail image
SAM on Au/planarCuSO4 · 5H2O/sodium ascorbate and Cu(Ph3)3Br/H2O/EtOH39
5
thumbnail image
SAM on Au/planarCuSO4 · 5H2O/sodium ascorbate/H2O/EtOH and DMSO/H2O229
6
thumbnail image
SAM on SiO2/planarno catalyst/r.t./µ-contact printing233
7
thumbnail image
SAM on Au/planarTBTA CuBF4/hydroquinone/DMSO/H2O230
8
thumbnail image
porous SiCuSO4/ascorbic acid,MeCN/tris-buffer/pH 8.0/r.t.234
9
thumbnail image
SAM on Au/planarCuSO4/sodium ascorbate/H2O/EtOH174
10
thumbnail image
SAM on glassCuSO4 · 5H2O/TBTA/TCEP/PBS-buffer/tBuOH/4 °C173
11
thumbnail image
SAM on Au-nanoparticles 1,8 ± 0,4 nmdioxane/hexane/r.t.80
12
thumbnail image
SAM on SiO2/planarCuSO4 · 5H2O/sodium ascorbate235
13
thumbnail image
SAM on SiO2/planarCuSO4 · 5H2O/sodium ascorbate/DMSO/50 °C101
14
thumbnail image
SAM on Au/planarCuSO4 · 5H2O/sodium ascorbate/r.t.116
15
thumbnail image
liposomeCuSO4 · 5H2O/sodium ascorbate/H2O243
16
thumbnail image
polymersomesCuSO4 · 5H2O/sodium ascorbate/TBTA240
17
thumbnail image
bionanoparticle/virusCuBr/PCDS257
18
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liposomeCuSO4/sodium ascorbate/HEPES-buffer/pH = 6.5245
19
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liposomeCuBr/H2O244
20
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polymer layerAFM-tip/225 °C237
21
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responsive polymer click capsulesCuSO4 · 5H2O/sodium ascorbate238
22
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layer by layer (LbL) film of polymerCuSO4 · 5H2O/sodium ascorbate/H2O239
23
thumbnail image
surface-functionalized micellesCuSO4.5H2O/sodium ascorbate/H2O/r.t.125, 124
24
thumbnail image
CdSe-NPCuBr80
25
thumbnail image
CdSe-NPCuBr/TBTA/DIPEA or ΔT78
26
thumbnail image
Fe2O3-NPΔT/toluene79
27
thumbnail image
Fe2O3-NPCuSO4256
28
thumbnail image
SAM on Au-nanoparticlesdioxane/hexane/r.t.254
29
thumbnail image
SAM on Au-nanoparticlesCuI/r.t.255
30
thumbnail image
Au-nanorodsCuSO4/ascorbic acid/4 °C258
31
thumbnail image
SWNT- nanocompositesCuI247
32
thumbnail image
SWNT- nanocompositesCuI246
33
thumbnail image
self-separating homogeneous CuI catalystsCuCl/heptane/EtOH45
34
thumbnail image
 CuI/NEt3/THF198
35
thumbnail image
cotton surfaceCuBr/N-(n-propyl)-2-pyridylmethanimine/toluene/70 °C110
36
thumbnail image
Wang resinsCu(PPh3)3Br/DIPEA/DMSO/60 °C111
37
thumbnail image
enantioselectivecatalysts on resinsCuI/DIPEA/DMF:H2O (1: 1)/35 °C250
38
thumbnail image
Merrifield resinsCuI/DIPEA/DMF/H2O/40 °C251
39
thumbnail image
pybox resinsCuI/DIEA/THF/35 °C252
40
thumbnail image
functionalized cross-linked solid supportsCuBr/PMDETA/DMF/80 °C197
41
thumbnail image
5-substituted tetrazolestoluene/–40 to 120 °C17
42
thumbnail image
pH sensitive releasing systemsCuSO4 · 5H2O/sodium ascorbate/t-BuOH:H2O (1: 1)52
43
thumbnail image
polysaccharidesCuSO4 · 5H2O/sodium ascorbate/DMSO/r.t.185
44
thumbnail image
polymersomesCuSO4.5H2O/sodium ascorbate/bathophenanthroline/4 °C241

Grafting-to101, 110, 236 and grafting-from81, 116 techniques have been employed to effect the attachment of polymers onto surfaces. Moreover, the reaction has been extended to carbon nanotubes246–248 and fullerenes,249 solid resins,111, 172, 197, 250–253 colloidal polymer particles,226 and nanoparticles.78–81, 246, 254–256 Thus a large variety of nanoparticles (Au,80, 246, 254, 255 CdSe,78 Fe2O3,79, 81, 256 SiO2101) as well as viruses257 and Au-nanorods258 have been surface-functionalized by this method. Compared to conventional surface-modification methods, the azide/alkyne methodology enables an elegant, fast, and efficient approach to functionalized nanoparticles in a simple mode. An important point has been observed upon comparing the CuI-catalyzed reaction with the uncatalyzed, purely thermal click reaction on CdSe nanoparticles78 without the use of the CuI catalyst, the photoluminescence of the final, surface-modified CdSe nanoparticles remains nearly unchanged, whereas under CuI catalysis a significant drop in the quantum yield is observed. Therefore, the purely thermal azide/alkyne reaction may sometimes be advantageous over the metal-catalyzed click process.

Conclusion and Outlook

  1. Top of page
  2. Abstract
  3. Introduction
  4. Mechanistic Details/Catalysts
  5. Click Reactions on Linear Polymers
  6. Complex Polymeric Architectures
  7. Click Reactions on Surfaces
  8. Conclusion and Outlook
  9. Acknowledgements
  10. Biographical Information
  11. Biographical Information

Click chemistry, in particular azide/alkyne click chemistry, has advanced and found its way into the chemists' mind. As with many novel and unconventional approaches (to cite ‘combinatorial chemistry’ as one prominent example) the reaction has had its ‘induction period’, and subsequently its royal uprise and present general acceptance in chemistry. Given the short period between discovery to the present, the reaction has been radically revolutionizing the way organic, material, surface, and in particular polymer chemists will approach future projects and experiments.

Using azide/alkyne click chemistry, not only more, but also more complex molecules and materials can now be approached in cases where in earlier times longer experiments and planning had been required. With azide/alkyne click chemistry in hand, polymer chemistry now approaches the level of small-molecule organic chemistry in terms of functional broadness, structural integrity, and molecular addressability. This alone suffices as the outlook.

In the close future, however, another question will more urgently press us polymer chemists: “Useful or not Useful?”—It might be this change-in-mind, rather than the “New or not New” question that remains and will be posed for a longer period in our hastily changing scientific world.

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Biographical Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Mechanistic Details/Catalysts
  5. Click Reactions on Linear Polymers
  6. Complex Polymeric Architectures
  7. Click Reactions on Surfaces
  8. Conclusion and Outlook
  9. Acknowledgements
  10. Biographical Information
  11. Biographical Information

Wolfgang H. Binder is currently full professor of Macromolecular Chemistry at the Martin–Luther University Halle–Wittenberg. He studied chemistry at the University of Vienna and received a Ph.D. in organic chemistry (University of Vienna, 1995). Postdoctoral studies (1995–1997) with Prof. F. M. Menger at Emory University, Atlanta, USA, and with Prof. Mulzer (Vienna, Austria) completed his education. After Habilitation at the Vienna University of Technology (TU-Wien, 2004) and acting as an Associate Professor of Macromolecular Chemistry (TU-Wien, 2004–2007), he became full professor at the University Halle-Wittenberg (MLU) in 2007. His research interests include polymer synthesis, supramolecular chemistry, and nanotechnology

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Biographical Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Mechanistic Details/Catalysts
  5. Click Reactions on Linear Polymers
  6. Complex Polymeric Architectures
  7. Click Reactions on Surfaces
  8. Conclusion and Outlook
  9. Acknowledgements
  10. Biographical Information
  11. Biographical Information

Robert Sachsenhofer is currently a Ph.D. student at the Martin–Luther University Halle–Wittenberg, Germany, in the group of Prof. W. H. Binder. After finishing a diploma thesis under the supervision of Prof. Binder on the surface modification of luminescent cadmium selenide nanoparticles by ‘click’-type reactions at the Vienna University of Technology, Austria, he joined the group of Prof. Binder for his Ph.D. in the field of self-assembly of amphiphilic block copolymer.

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