Chemistry in Polymer and Material

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‘Click’ Chemistry in Polymer and Material

Science: An Update

Wolfgang H. Binder,* Robert Sachsenhofer

Introduction

Who will be reading a review on a topic that was reviewed

only 15 months previously[1] 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 certainlyranks 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 involvingmultiple

functional groups (i.e.: at graft-, star-, and block copoly-

mers, 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 approa-

ches the level of small-molecule organic chemistry in

terms of functional broadness, structural integrity, and

molecular addressability.

Review

W. H. Binder, R. Sachsenhofer

Martin-Luther University Halle-Wittenberg, Faculty of Natural

Sciences II (Chemistry and Physics), Institute of Chemistry/

Division Technical and Macromolecular Chemistry, Heinrich-

Damerowstr. 4/12; 06120 Halle (Saale), Germany

E-mail: [email protected]

The metal catalyzed azide/alkyne ‘click’ reaction (a variation of the Huisgen 1,3-dipolarcycloaddition reaction between terminal acetylenes and azides) has vastly increased inbroadness and application in the field of polymer science. Thus, this reaction representsone of the few universal, highly efficient functionalization reactions, which combines bothhigh efficiency with an enormously high tolerance of functional groups and solvents underhighly moderate reaction temperatures

(25–70 8C). The present review assembles anupdate of this reaction in the field of polymerscience (linear polymers, surfaces) with a focus onthe synthesis of functionalized polymeric archi-tectures and surfaces.

952

Macromol. Rapid Commun. 2008 , 29 , 952–981

2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/marc.200800089


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Briefly, the azide/alkyne click reaction[12–14] represents a

metal-catalyzed variant of the Huisgen 1,3-dipolar

cycloaddition reaction[15,16] between C–C triple, C –N triple

bonds[17] 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 materi-

als[20] 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 mol1, 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 2006to 2008 (deadline: 15th March 2008) will draw a line to a

previous review in this journal[1] and other reviews

describing the azide/alkyne click reaction in general,[12,22]

for application in polymer chemistry,[1,23–28] dendri-

mers,[25,29] carbohydrate chemistry,[30–32] materials chem-

istry,[1] and organic chemistry,[28,33] as well as for

peptides[34] 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 reac-

tions (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

Briefly, the basic process of the Huisgen 1,3-dipolar

cycloaddition[2,10,11] generates 1,4- and 1,5-triazoles

respectively (Scheme 1). Nearly all functional groups are

compatible with this process, except those that are a) eitherself 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 poly-

mers[38] and surfaces,[39] thus enabling the use of free

thiols despite the often interfering azide/amine reduction

by the free thio-moiety.

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 nm[40] or4 nm[41]), metallic Cu0 clusters [41–43])

as well as copper/charcoal[44] 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

‘Click’ Chemistry in Polymer and Material Science: An Update

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

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.

Scheme 1. Azide/alkyne - ‘‘click’’ - reaction.

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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 systems[48]

often used include amino bases,[49]

(triethylamine (TEA),2,6-lutidine, N , N -diisopropylethylamine (DIPEA), N , N , N 0, N 0,

N 00-pentamethylethylenetetramine (PMDETA), hexame-

thyltriethylenetetramine (HMTETA), tris[(2-pyridyl)-

methyl]amine (TPMA), tris[(2-dimethylamino)ethyl]amine

(Me6TREN), 2,20-bipyridines (bpy), 2,20:20,600-terpyridine

(tpy), ammonium salts,[42] and mono- and multivalent

triazoles[49]) but also phosphines such as tris(carboxy-

ethyl)phosphine (TCPE). A detailed investigation[50] 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 cou-

plings) or dimerization reactions of the finally formed

triazoles.[51] The latter dimerization reaction is a very

important one, generating 5,50-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 Rucomplexes[53,54] such as (CpRuCl(PPh3), [Cp

RuCl2]2,

CpRuCl(NBD), and CpRuCl(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] deter-

mined 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


Scheme 2. Proposed mechanism of the azide/alkyne - ‘‘click’’ - reaction.

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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 m-bridge,[67]

c) a significant autoaccelera-tion 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 transi-

tion 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

(p-complexation, multiple copper species) and b) because

of the many known (highly different) structures of

copper-acetylides. However, the basic feature (i.e., low-

ering of the p K a value of the Cu-acetylide by up to 9.8 units

as determined[66] by DFT calculations) is the most

important contribution towards the rate acceleration.

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 experiments[9,39,73–81], 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

The enormous interest in linear polymers lies primarily in

the combination of click reactions with controlled poly-

merization 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 anexemplary 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 poly-

merization see ref.[82].

Atom Transfer Radical Polymerization (ATRP)

As indicated in Table 1 (entries 1–18), a variety of click

reactions in conjunction with ATRP have beendescribed.[45,50,83–115] The reaction has been investigated

with polymers such as polystyrene, various polyacrylates

and polymethacrylates, polyacrylnitrile, and poly(ethy-

lene 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 initiator[115] 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 /N 3 approach[84,85,88,90,94,102,103,107–109,113,114,116]

takes advantage of the terminal bromine-moiety, inher-

ently present within the ATRP reaction. A final Br/

N3 -nucleophilic reaction (usually performed in NaN3/

N , N -dimethylformamide (DMF)) then completes the intro-

duction 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 spec-

trometry (MALDI MS) experiments of the final ‘clicked’

products.

Side-chain modified polymers [86,92,93,97,102,109,111] 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 reac-

tion is observed, which leads to a high density of func-

tional side chains within the main-chain polymer.




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Table 1. Overview of ‘click’-reactions with linear- or graft-polymers.

Entry Polymer/substrate Polymerization

method

Catalyst/

conditions

Ref.

1 ATRP CuBr/r.t. [86]

2 ATRP N -alkyl-2-

pyridylmethanimine-

CuBr/70 -C

[115]

3 ATRP CuBr/THF/r.t. 4,4(-

di(5-nonyl:)-

2,2(-bipyridine

[84,90]

4a ATRP NaN3/ZnCl2/120-C [10]

4b ATRP NaN3/ZnCl2/120 -C [10]

5 ATRP CuBr/PMDETA/r.t. [107]

6 ATRP CuBr/DMF/r.t. [106]

7 ATRP CuI/DBN/THF/35 -C [85]

8 ATRP CuBr/PMDETA/THF/

35 -C

[108]

9 ATRP CuBr/PMDETA [103]

10 ATRP CuBr/DMF/r.t [102]

( Continued )

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method

Catalyst/

conditions

Ref.

11 ATRP CuBr/PMDETA/r.t [94]

12 ATRP CuBr/PMDETA/50 -C [98]

13 ATRP CuBr/DMF [97]

14 ATRP CuBr/bipyridine/DMF/120 -C

[89]

15 ATRP Cu0/CuBr or CuBr/

PMDETA/DMF/r.t.

[87]

16 ATRP CuBr/PMDETA/sodium

ascorbate/DMF

[88]

17a ATRP Cu(PPh3)3Br/DIPEA [109]

17b ATRP Cu(PPh3)3Br/DIPEA [109]

18 ATRP CuBr/bipyridine/

THF/r.t.

[113]

19 NMP NaN3/ZnCl2/DMF/

120 -C

[119]

Table 1. (Continued)



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method

Catalyst/

conditions

Ref.

20 NMP Cu(PPh3)3Br/DIPEA/

dioxane

[120]

21 NMP Cu(PPh3)3Br/DIPEA/

THF

[123]

22 NMP [(CH3CN)4Cu]PF6/

TBTA/DIPEA/DMF

[81]

23 NMP CuBr/PMDETA/

DMF/r.t.

[96]

24 NMP Cu(PPh3)3Br/DIPEA [122]

25 NMP toluene [121]

( Continued )


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method

Catalyst/

conditions

Ref.

26 free radical CuSO4 5H2O/

sodium ascorbate/

H2O/DMSO

[82]

27 RAFTRNMP Cu(PPh3)3Br/DIPEA/

THF/H2O/r.t./3d

[118]

28 RAFT CuI/DBU/DMAc/40 -C [129]

29 RAFT CuBr/PMDETA/

DMF/r.t

[130]

30 RAFT CuSO4/sodium

ascorbate/

H2O/t BuOH

[128]

31 ATRP CuBr/bipyridine/

120 -C

[104]

32 RAFT CuSO4/sodium

ascorbate/H2O

[117]

33 RAFT CuSO4 5H2O/sodium

ascorbate/DMSO/50-C

[101]




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method

Catalyst/

conditions

Ref.

34 RAFT Cu(PPh3)3/DIPEA/

DMF/r.t.

[132]

35 RAFT CuBr/PMDETA/

DMF/r.t.

[127]

36 RAFT Cu(PPh3)3Br/DIPEA/

THF/H2O

[133]

37 RAFT CuI/DBU/THF [131]

38 RAFT [134]

39 RAFT CuI/DBU/THF/40 -C [135]

( Continued )


960




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method

Catalyst/

conditions

Ref.

40 ROMP Cu(I)/DIPEA/DMF/

toluene/H2O

[76]

41 ROMP Cu(PPh3)3Br/DIPEA/

DMF/50 -C

[9,75]

[74]

42 ROMP [137]

43 ROMP CuBr/PMDETA/

DMF/50 -C

[136]

44 [150]

45 living anionic [38]

46 living cationic

ring opening

CuSO4 5H2O/

water/t BuOH

[145]

47 living cationic

polymerization

of isobutene

Cu(PPh3)3Br/DIPEA/

toluene

[77]

48 living anionic

polymerization

CuBr/DMF/60 -C [149]




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method

Catalyst/

conditions

Ref.

49 polyaddition Cu(I) [153,152]

50 polyaddition 100 -C [154]

51 polyaddition CuSO4.5H2O/

sodium ascorbate/

H2O/tBuOH

[155]

52 polyaddition CuSO4.5H2O/

sodium ascorbate/H2O/

tBuOH

[156]

53 polyaddition [151]

54 polyaddition CuSO4 5H2O/sodium

ascorbate H2O:t BuOH (1: 1)/r.t.

[6]

55 polyaddition Cu/Cu(OAc)2/

TBTA THF/

CH3CN

[161]

56 polyaddition CuSO4 5H2O/sodium

ascorbate

[157]

57 CVD CuSO4 5H2O/sodium

ascorbate H2O:tBuOH (2: 1)

[70]

( Continued )


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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 macro-

cyclic polymers by click reactions,[89] which was first

described by Grayson et al. (Table 1, entry 14) with an a ,v-

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



method

Catalyst/

conditions

Ref.

58 topological CuI/CH3CN [164]

59a/b polyaddition CuSO4 5H2O/H2O:tBuOH (2: 1)

[163]

60 anionic ring

opening

CuSO4 5H2O/sodium

ascorbate/100 -C

[143,141]

[140]

61 ROP CuSO4 5H2O/Cu wire/

37 -C

[146]

62 sequential

stepwise

solid phase

synthesis

CuI/ascorbic acid/

DIPEA butan-2-ol/

DMF/pyridine

[172]

63 CuIIBr/ascorbic acid/

propylamine DMSO/r.t.

[176]

64 DNA-synthesis DMSO/H2O/80 -C/72 h (no CuI!!) [188]

65 solid phase synthesis or

DNA-polymerase

CuI [186]

66 solid phase synthesis CuSO4 5H2O, sodium

ascorbate H2O/MeOH

[61,182]




13/30

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 function-

alized 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 sty-

renes,[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)- N 3 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 sty-

renes,[118] hydroxylated methacrylates,[132] and glycosy-

lated methacrylates[129]) 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 monomers[128,131,133] 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 ter-minal acetylenes within the monomers[131,133] or initia-

tors[129] 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 agent[117,127,130] as well as within the sidechain[128]

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 polymeri-

zation.[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 trans-

formation of the terminal isobutylsulfanylthiocarbonyl-

sulfanyl moiety into the propargylic moiety by a one-pot

aminolysis/Michael addition sequence is reported.

Although no yield was provided, the publication repre-

sents the first example for the preparation of cyclic

poly( N -isopropylacrylamide).

Ring-Opening Metathesis Polymerization (ROMP) and

Ring-Opening Polymerization (ROP)

ROMP/click methodologies[9,39,75] (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(oxynorbor-

nenes).[9,74–76] Using oxynorbornenes with pendant azido

and acetylenic units, the copolymerization into either

homo-,[9] block-,[74,76] and statistical copolymers[75] 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


964




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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 cyclooc-

tene moieties is provided by ref.[136]. Polymerization using

a bis-bromoalkene initiator furnished the bistelechelic

poly(norbornene). Subsequent exchange against the azideand 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 strategies[113,140–148] (see Table 1, i.e.:

entries 46, 60, and 61) have been described recently, either

using an alkyne moiety[143–145] or an a-chlorocapro-

lacton[140–142] as functional monomers or an appropriate

alkyne endgroup.[147] In this way, functionalized poly-

(glycolides),[148] poly(lactides),[146] poly(caprolac-

tones),[140–144,147] poly(oxazolines),[145] and poly(L-valine)-

block-poly(acrylic acid) copolymers[113] have been

successfully prepared.

Anionic and Cationic Polymerization

Few examples that combine living anionic[38,149] and

cationic[45,73,77,145] polymerization reactions with click

reactions have been described (Table 1, entries 45–48).

As shown in Table 1, entry 45, living anionic polymeriza-tion 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 ( M n ¼3 100 g mol1, M w= M n ¼

1.10).[77] The reaction has been performed in biphasic

reaction systems, featuring toluene/water solvent mix-

tures 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 iso-

cyanides has been described.[150]

Polyaddition/Polycondensation

A variety of examples have been reported (see Table 1,

entries 49–59), that demonstrate polyaddition[6,70,151–160]

or polycondensation processes in junction with the azide/

alkyne click reaction. In principle, two approaches should

be discerned: a) chain-growth or network-formation bydirect azide/alkyne click reaction[6,152–156,158,161] or b) the

introduction of azide or alkyne groups into the growing

chains[70,151,157] or endgroups by the respective monomer

units for further attachment. Using strategy ‘a’ a variety of

functional polymers such as high glass transition tem-

perature (T g) polymers,[152] metal adhesive polymers,[6,153]

polymers with optical non-linearity,[154,160] organic semi-

conductors,[158] or high temperature-stable polymers[155]

have been prepared. Strategy ‘b’ has been used to

prepare side-chain-functionalized polyurethanes[151]

poly( p-phenylene vinylenes),[157] poly[(4-ethynyl- p-

xylylene)-co-( p-xylylene)]s,[70]

poly(fluorenes),[160]

andpoly(pyrroles).[162]

Gels and networks[6,82,88,136,152,153,159,163–166] 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] carbohy-

drates,[30,31,34,109,111,137,173–184] cellulose,[185] and oligonu-

cleotides,[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

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 archi-

tecture. 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




15/30


Table 2. Overview of ‘click’-reactions for the synthesis of complex polymer architectures.

Entry Polymer/substrate Type Catalyst/conditions Ref.

1 star polymers CuI/TBTA/DIPEA [77]

2 graft-polymer CuBr/PMDETA/THF/DMF [210]

3 star polymers CuSO4.5H2O/sodium

ascorbate/70 -C

[105]

4 tadpole-shaped

polymers

CuI/N(Et)3/THF/35 -C [142]

5 star polymers CuSO4 5H2O/sodium

ascorbate/H2O/r.t.

[203]

6 graft-polymer CuSO4 5H2O/sodium

ascorbate/H2O/MeOH/60 -C

[211]

( Continued )

966




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7 capsule CuSO4 5H2O/sodium

ascorbate

[225]

8 terpolymers CuBr/Me6TREN/DMF/50 -C [100]

9 copolymer Cu(PPh3)3Br/DIPEA/CH2Cl2 [144]

10 graft-polymer [212]

11 rod-coil block

polymers

CuBr/PMDETA/r.t [171]

12 star polymers CuBr/PMDETA [91]

13 block-copolymer CuBr/bipyridine/NMP/r.t [207]

14 CuSO4 5H2O/sodium

ascorbate/70 -C

[92]

15 triblock copolymers CuBr/PMDETA/

DMF/120 -C

[208]




17/30



16 block-copolymer CuBr/bipyridine/THF/r.t. [209]

17 polymer brushes CuBr/PMDETA/DMF [93]

18 multisegmented

block-copolymers

CuBr/PMDETA/DMF/r.t. [95]

19 graft-polymer CuSO4.5H2O/sodium

ascorbate/H2O/CH2Cl2/r.t.

[213]

20 star polymers CuBr/PMDETA/DMF/r.t. [204]

21 star polymers CuBr/PMDETA/DMF/r.t. [126]

( Continued )


968




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22 star polymers CuSO4/sodium ascorbate/

100 -C/mW-irradiation

[147]

23 comb polymers CuBr/PMDETA/THF/r.t. [214]

24 star polymers CuI/PMDETA/DMF/80-C [205]

25 graft polymers CuBr/PMDETA/DMF/r.t. [114]

26 dendrimers CuSO4 5H2O/sodium

ascorbate/H2O/THF (1: 4)/r.t.

[221]




19/30




ascorbate/H2O/THF (1: 4)/r.t.

[202]

28 hyperbranched

polymers

CuSO4 5H2O/sodium

ascorbate/H2O/t BuOH/

hexane (5: 5: 1)/r.t.

[222]

29 dendrimers Cu/CuSO4/TBTA DMF/r.t [223]

30 graft-polymer CuI/DMF/80 -C [215]

31 dendrimers CuBr/PMDETA/THF/r.t. [99]


ascorbate/H2O/THF/40 -C

[216]

( Continued )


970




20/30

dendrimers[59,99,112,175,189–202] and hyperbranched poly-

mers[5,7,130]

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 func

tional 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 capsules[225,226] 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.

Click Reactions on Surfaces

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/alkyneclick reaction within the pocket of enzymes (based protein

profiling (ABPP))[19,36,69,227] by direct microcontact print-

ing,[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]

polymersomes[240–242] and liposomes[243–245] have been

reported (see Table 3). In the case of SAMs the use

of appropriately azide-[39,101,228–231] or alkyne-

functionalized[132,173,174,232–234] surfaces by direct ligand-

adsorption have been described. Alternatively, in-situ

generation of terminal azides by bromide/azide exchange

directly on the v-bromoalkyl-functional monolayer can be

effected,[231,235] which eliminates the pressing instability

of v-azido-1-thioalkanes prior to the SAM-formation

process. Dynamic or labile assembly structures (such as




ascorbate/H2O/THF

[224]


ascorbate/H2O/THF/r.t.

[189]

35 dendrimers CuBr/PMDETA/DMF/80 -C [112]

36 block-copolymer CuBr/PMDETA/CH2Cl2/r.t. [241]




21/30


Table 3. Overview of ‘click’ reactions on surfaces, nanoparticles, polymersomes, vesicles, micelles, carbon nanotubes, and resins.

Entry Polymer/substrate Surface Catalyst/conditions Ref.

1 SAM on Au/planar CuSO4 5H2O/sodium

ascorbate/H2O/EtOH

[228]

2 SAM on SiO2/planar thermal/70 -C/neat [231]


ascorbate/H2O/EtOH

[232]


ascorbate and

Cu(Ph3)3Br/H2O/EtOH

[39]


ascorbate/H2O/EtOH

and DMSO/H2O

[229]

6 SAM on SiO2/planar no catalyst/r.t./m-contact

printing

[233]

7 SAM on Au/planar TBTA CuBF4/hydroquinone/

DMSO/H2O

[230]

8 porous Si CuSO4/ascorbic acid,MeCN/

tris-buffer/pH 8.0/r.t.

[234]

9 SAM on Au/planar CuSO4/sodium ascorbate/

H2O/EtOH

[174]

10 SAM on glass CuSO4 5H2O/TBTA/TCEP/

PBS-buffer/t BuOH/4 -C

[173]

11 SAM on Au-nanoparticles

1,8W0,4 nm

dioxane/hexane/r.t. [80]

12 SAM on SiO2/planar CuSO4 5H2O/sodium

ascorbate

[235]

13 SAM on SiO2/planar CuSO4 5H2O/sodium

ascorbate/DMSO/50 -C

[101]


ascorbate/r.t.

[116]

( Continued )

972




22/30



15 liposome CuSO4 5H2O/sodium

ascorbate/H2O

[243]

16 polymersomes CuSO4 5H2O/sodium

ascorbate/TBTA

[240]

17 bionanoparticle/virus CuBr/PCDS [257]

18 liposome CuSO4/sodium ascorbate/

HEPES-buffer/pH¼6.5

[245]

19 liposome CuBr/H2O [244]

20 polymer layer AFM-tip/225 -C [237]

21 responsive polymer

click capsules

CuSO4 5H2O/sodium

ascorbate

[238]

22 layer by layer (LbL) film

of polymer

CuSO4 5H2O/sodium

ascorbate/H2O

[239]




23/30



23 surface-functionalized

micelles

CuSO4.5H2O/sodium

ascorbate/H2O/r.t.

[125,124]

24 CdSe-NP CuBr [80]

25 CdSe-NP CuBr/TBTA/DIPEA or DT [78]

26 Fe2O3-NP DT /toluene [79]

27 Fe2O3-NP CuSO4 [256]

28 SAM on

Au-nanoparticles

dioxane/hexane/r.t. [254]

29 SAM on

Au-nanoparticles

CuI/r.t. [255]

30 Au-nanorods CuSO4/ascorbic acid/4 -C [258]

31 SWNT- nanocomposites CuI [247]

( Continued )


974




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32 SWNT- nanocomposites CuI [246]

33 self-separating

homogeneous

CuI catalysts

CuCl/heptane/EtOH [45]

34 CuI/NEt3/THF [198]

35 cotton surface CuBr/ N -(n-propyl)-2-

pyridylmethanimine/

toluene/70 -C

[110]

36 Wang resins Cu(PPh3)3Br/DIPEA/DMSO/60 -C [111]

37 enantioselectivecatalysts

on resins

CuI/DIPEA/

DMF:H2O (1: 1)/35 -C

[250]

38 Merrifield resins CuI/DIPEA/DMF/H2O/40 -C [251]

39 pybox resins CuI/DIEA/THF/35-

C [252]




25/30

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.

Grafting-to[101,110,236] and grafting-from[81,116] tech-

niques have been employed to effect the attachment of

polymers onto surfaces. Moreover, the reaction has been

extended to carbon nanotubes[246–248] and fullerenes,[249]

solid resins,[111,172,197,250–253] colloidal polymer parti-

cles,[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] SiO2

[101]) as well as viruses[257] and

Au-nanorods[258] 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 nanoparticles[78] 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

Click chemistry, in particular azide/alkyne click chemistry,

has advanced and found its way into the chemists’ mind.



40 functionalized

cross-linked solid

supports

CuBr/PMDETA/DMF/80-C [197]

41 5-substituted tetrazoles toluene/–40 to 120 -C [17]

42 pH sensitive releasing

systems

CuSO4 5H2O/sodium ascorbate/

t -BuOH:H2O (1: 1)

[52]

43 polysaccharides CuSO4 5H2O/sodium

ascorbate/DMSO/r.t.

[185]

44 polymersomes CuSO4.5H2O/sodium

ascorbate/

bathophenanthroline/4 -C

[241]


976




26/30

As with many novel and unconventional approaches (to

cite ‘combinatorial chemistry’ as one prominent example)

the reaction has had its ‘induction period’, and subse-

quently its royal uprise and present general acceptance in

chemistry. Given the short period between discovery to the

present, the reaction has been radically revolutionizing theway 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.

Acknowledgements: The authors are thankful for the grant FWF 18740 B03 for financial support.

Received: February 11, 2008; Revised: March 31, 2008; Accepted:March 31, 2008; DOI: 10.1002/marc.200800089

Keywords: 1,3-dipolar cycloaddition; azide/alkyne ‘click’ reac-

tion; polymerization (general); surfaces

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