Hyperbranched Polytriazoles

Hyperbranched Polytriazoles

(Parte 1 de 4)

Hyperbranched Polytriazoles: Click Polymerization, Regioisomeric Structure, Light Emission, and Fluorescent Patterning

Anjun Qin,† Jacky W. Y. Lam,† Cathy K. W. Jim,† Li Zhang,† Jingjing Yan,‡ Matthias Haussler,† Jianzhao Liu,† Yongqiang Dong,† Dehai Liang,‡ Erqiang Chen,‡ Guochen Jia,† and Ben Zhong Tang*,†,§

Department of Chemistry, The Hong Kong UniVersity of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China; Department of Polymer Science and Engineering, Peking UniVersity, Beijing 100871, China; and Department of Polymer Science & Engineering, Zhejiang UniVersity, Hangzhou 310027, China

ReceiVed March 10, 2008

ABSTRACT: Synthesis of hyperbranched poly(1,2,3-triazole)s (hb-PTAs) has been a challenge: the AB2 monomers were inclined to self-oligomerize,and their Cu(I)-catalyzedclick polymerizationsfailed to yield soluble polymers. We tackled the challenge in this work and succeeded in generating hb-PTAs with regioregularity, processability, and functionality. We took an A2 + B3 approach and used diazides 2 and triyne 3 as monomers, which are free of self-oligomerization concerns. Thermal polymerizations of 2 and 3 produced regiorandom polymers (hb-r-P1) with high molecular weights in high yields. Metal-mediated regioselective polymerizations afforded soluble 1,4- and 1,5-linked polymers (hb-1,4-P1 and hb-1,5-P1), presenting the first examples of regioregular hb-PTAs with macroscopic processability. The reactions were affected by substrate and catalyst:

electron-richalkyne generallyslowed down the cycloadditionreaction,while rutheniumcatalysts(Cp*Ru(PPh3)2Cl and [Cp*RuCl2]n) exhibited higher substrate tolerance than copper catalyst [Cu(PPh3)3Br]. Regiostructures and regioregularities of the hb-PTAs were determined spectroscopically. Degrees of branching of hb-r-P1 were calculated to be ∼90%. Conformations of the hb-PTAs were affected by the steric effects between their aromatic units, which in turn affected their solubility, conjugation, luminescence, and aggregation. While the polymer solutions all emitted deep blue light, the films of hb-1,4-P1, hb-1,5-P1 and hb-r-P1 emitted blue, yellow, and white light, respectively, due to the difference in the aggregation of their chromophoric units in the solid state. Fluorescent photoresist patterns with various emission colors were readily generated from photo-cross-linking of the polymers through a nitrene-mediated photolysis mechanism.

Introduction

Chemists have worked enthusiastically toward the development of new reactions with efficient atom economy, strong functionality tolerance, and high stereo- and regioselectivity. A remarkable progress along this line has been the development of “click chemistry”1,2 for effective and selective chemical transformations. A “click reaction” can create functional moleculeswith heteroatomlinks from reactivemodular building blocksin high efficiencyunder benignconditionsthroughsimple isolation procedures. A number of click reactions have been explored and identified, with the metal-catalyzed Huisgen [3 + 2] azide-alkyne cycloaddition3 being hailed as the cream of the crop. The Cu(I)- and Ru(I)-mediated 1,3-dipolar cycloadditions proceed steadily, affording 1,4- and 1,5-disubstituted 1,2,3-triazoles, respectively, in high yields.1,2

The azide-alkyne click reaction is modular, regioselective, and efficient and requires mild reaction conditions and simple purification procedures. These appealing features have enabled the reaction to evolve into a powerful synthetic tool, finding applications in diverse areas including drug discovery, bioconjugate chemistry, surface modification, and materials development.2,4 The click reaction has also been utilized in polymer science, mainly in the area of functionalization of preformed polymers through postpolymerization reactions.5,6 The azidoand ethynyl-functionalized polymers have been used as the branching points and chain extenders for the syntheses of graft and block copolymers, respectively. Taking the click reactionbased “graft-to” and “graft-from” approaches, linear polymer chains decorated by dendritic grafts (or dendronized linear polymers) have been successfully prepared.6,7

Synthetic polymer chemists have attempted to develop the click reaction into a new polymerization technique. The effort, however, has met with limited success. The Cu(I)-catalyzed polymerizations of arylenediazides (N3-Ar-N3) and arylenediynes (HCtC-Ar-CtH) (Ar ) phenyl, pyridyl, fluorenyl, etc.) were sluggish, taking as long as 7-10 days to finish.8 The products often precipitated from the reaction mixtures even at the oligomer stage or became insoluble in common organic solvents after purification, unless very long alkyl chains, such as n-dodecyl groups, were attached to the arylene rings. All the polymers were nonfluorescent in the solid state, although their dilute solutions emitted UV light, suggesting that the polymer luminescence was quenched by aggregate formation.

Triazole dendrimers have been elegantly synthesized from the Cu(I)-catalyzed ligation of azides and alkynes.9 Notwithstanding the powerfulness of click reaction, it still takes about 10 steps to prepare a dendrimer comprising of third-generation dendrons with an absolute molecular weight of about 6000. Hyperbranched polymer is a structural congener of dendritic polymer. Although structurally imperfect, a hyperbranched polymer can be facilely synthesized via a single-step reaction by a one-pot procedure in large scale and quantity.10 To explore the utility of click reaction in the synthesis of hyperbranched polymers, two AB2-type monomers of azidoarylenediynes with a general formula of N3-Ar-(CtCH)2 have been prepared by two groups in Germany and Belgium.1 One of the monomers was subjectedto Cu(I)-catalyzedpolymerization,but the product soon “precipitated” from the reaction mixture, “which is no

* To whom correspondenceshould be address:Ph +852-2358-7375;Fax +852-2358-1594; e-mail tangbenz@ust.hk.

† The Hong Kong University of Science & Technology. ‡ Peking University. § Zhejiang University.

10.1021/ma800538m C: $40.75 2008 American Chemical Society Published on Web 05/15/2008

longer soluble in any organic solvent”.1 AB2-type monomers of ethynylenediazides (N3-R-CtC-R-N3) have also been prepared. The internal alkyne monomers, however, could only be polymerized by thermally activated 1,3-dipolar polycycloaddition, producing hyperbranched regiorandom poly(1,2,3-tria- zole)s (hb-r-PTAs). In addition, the AB2 monomers were difficult to prepare and purify and suffered from self-oligomer- ization during storage under ambient conditions.1

Our research groups have been interested in the exploration of alkyne-based polymerization reactions, with the aim of developing alkyne monomers into versatile building blocks for the construction of new macromolecules with linear and hyperbranched structures and advanced functional properties.

Using monoynes (RCtCH), diynes [R(CtCH)2] and triynes

[R(CtCH)3] as monomers, we have successfully synthesized a variety of polyacetylenes, polyarylenes, and polydiynes by metathesis, cyclotrimerization, and coupling polymerizations, respectively.12 Because of the key role of alkynes in the click reactionsand as a naturalextensionof our alkyne-basedresearch program, we have recently embarked on the syntheses of functional PTAs by click polymerizations.

In our previous work, we have developed a metal-free, thermally initiated, regioselective 1,3-dipolar polycycloaddition process: simply heating a mixture of bis(aroylacetylene)

(HCtC-OCArCO-CtCH)anddiazide(N3-R-N3) in a polar solvent, such as a N,N-dimethylformamide (DMF)/toluene mixture, readily furnished a linear PTA with a high regioregu- larity (1,4-content or F1,4 ratio up to ∼92%) in a high yield (up to ∼98%).13 In this work, we intended to synthesize hyper- branchedpolymers(hb-PTAs) by click polymerization.We took an A2 + B3 approach, using easy-to-make and stable-to-keep diazide (A2) and triyne (B3) as monomers, in an effort to circumvent the self-oligomerization problem met by the AB2 systems discussed above. The A2/B3 monomers were readily polymerized by metal-mediated click reactions and thermally activated Huigen cycloaddition (Scheme 1). The Cu- and Rucatalyzed click polymerizations afforded hyperbranched polymers with regular1,4- and 1,5-linkages(hb-1,4- and -1,5-PTAs), respectively. Both polymers are soluble in common solvents, such as dichloromethane (DCM), tetrahydrofuran (THF), and dimethyl sulfoxide (DMSO), representing the first examples of hb-PTAs with regioregular structures and macroscopically processability. The steric interactions between the aromatic phenyl and triazolerings affectedthe electroniccommunications in the hb-PTAs, which in turn influenced their packing and luminescence behaviors in the solid state. Fluorescent images with different emission colors were readily generated by UV photolysis of the polymer films.

Results and Discussion

Monomer Syntheses and Model Reactions. A2- and B3- type monomers of diazides 2 and triyne 3 were designed to realize the planned A2 + B3 approach to hb-PTAs. Taking into consideration that click polymerizations of diazide and diyne monomers with rigid structures were inclined to produce insoluble gels,8 flexible alkyl chains were introduced into the molecular structures of the diazide monomers. The azides, namely 1,4-bis(n-azidoalkoxy)benzenes, were facilely prepared by etherization of hydroquinone 4 with R,ω-dibromoalkanes in a basic medium, followed by substitution reaction with sodium azide in DMSO (Scheme 2).

Triphenylamine has been widely used in the development of new organic light-emitting materials and devices due to its excellent solubility and stability as well as high luminescence and hole-transporting efficiencies.14 Synthetically, it can be readily transformedto a triyne with each of its aryl ring carrying one triple bond at the para position. In this work, we chose triphenylamine as a building block to prepare an amine-cored triyne (B3) monomer of tris(4-ethynylphenyl)amine (3), from which hb-PTAs with useful photonic propertiesmay be derived.

The monomer was prepared in a high yield (∼90%) by iodination of triphenylamine, followed by coupling with (trimethylsilyl)acetylene and base-catalyzed disilylation.14f,15

As mentioned above, triphenylamine is a photoresponsive molecule. The amine-cored triyne 3, however, may show low reactivity toward diazides 2 because of its electron-rich nature. To address this concern, cycloadditions of triyne 3 with monoazides 8 were conducted as model reactions. The monoazideswere preparedby the same syntheticroutesas the diazides 2, using phenol 6, instead of hydroquinone 8, as a staring material (Scheme 3).

The aminotriyne 3 was subjected to the reaction with monoazides 8 in the presence of the copper catalyst under the standardclick reactionconditions1 (Scheme4). 1,4-Disubstituted 1,2,3-triazoles 9 were isolated in ∼38-54% yields, indicative of a moderate reactivity of triyne 3 in the Cu(I)-catalyzed reaction. The click reactions catalyzed by Cp*Ru(PPh3)2Cl, however, produced 1,5-disubstituted1,2,3-triazoles10 in higher yields (∼79-84%), revealing a higher substrate tolerance of the Ru(I) complex over the Cu(I) catalyst. The model reactions confirm that triyne 3 can be used as a B3 monomer in the planned A2 + B3 approach to hb-PTAs. An added bonus of these reactions is the provision of model compounds 9 and 10 as authentic standards of 1,4- and 1,5-disubstituted 1,2,3- triazoles for the structural characterization, especially regioregularity elucidation, of the polymer products (hb-PTAs) that will be synthesized by the polycycloadditions of triyne 3 with diazides 2 (vide post).

Scheme 1. Syntheses of hb-PTAs by Metal-Catalyzed Click Polymerizations and Thermally Activated 1,3-Dipolar

Polycycloadditions of Diazide (A2) and Triyne (B3) Monomers

Scheme 2. Preparation of Diazide Monomers (2)

Scheme 3. Preparation of Monoazide Model Compounds (8)

Macromolecules, Vol. 41, No. 1, 2008 Synthesis and Properties of Hyperbranched Polytriazoles 3809

Triazole derivatives with 1 and without azido moieties 12 were designed and prepared as model compounds for the study of photolysis mechanisms of hb-PTAs. The reaction of an equimolar mixture of 4-ethynyl-N,N-diphenylbenzenamine(13) and 1,4-bis(6-azidohexyloxy)benzene(2b) affordedazidotriazole 1 in 28% yield, while the reaction of phenylacetylene (14) with 1-(6-azidohexyloxy)benzene(8b) produced 1-(6-phenoxyhexyl)-4-phenyl-1,2,3-triazole (12) in a yield as high as 8% (Scheme 5). Aminomonoyne 13 is more electron rich than phenylacetylene (14). The results of these model reactions once again confirm the electronic effect involved in the Cu(I)- catalyzed click reaction.

All the monomers and model compounds were characterized by spectroscopicmethods, from which satisfactoryanalysis data were obtained [see Experimental Section and Supporting

Information (Figures S1-S3) for details]. Unlike the AB2 monomers used in the early studies,1 the A2 and B3 monomers prepared in the present work are stable and can be stored in a dark place at room temperature for a long period of time. No structural changes caused by such undesired reactions as selfoligomerization were observed after the monomers had been kept in our laboratories for more than 6 months.

1,3-Dipolar Polycycloadditions of Diazides 2 and Triyne 3. In our previous study, we found that thermal polymerizations bis(6-azidohexyloxy)benzene and 3,3′-(1,6-hexylenedioxy)- bis(benzoylacetylene), for example, in a DMF/toluene mixture for a short period (e.g., 30 min) resulted in the formation of polymeric products. The polymerization conducted in an N,N- dimethylacetamide/toluene mixture at 100 °C for 6 h produced a linear PTA with an Mw value of 25 300 and an F1,4 ratio of ∼92% in ∼96% yield.13 The triyne 3 we prepared in this work is, however, electron-rich. Although it reacted with monoazides 8 in the presence of the Cu(I) and Ru(I) catalysts, it failed to undergo thermal cycloaddition after its mixture with 8a was refluxed in dioxane for 24 h. It was thus intriguing to check

Scheme 4. Syntheses of Model Compounds of 1,4- and 1,5-Disubstituted 1,2,3-Triazoles 9 and 10 Scheme 5. Syntheses of Model Compounds of 1,4-Disubstituted 1,2,3-Triazoles 1 and 12

3810 Qin et al. Macromolecules, Vol. 41, No. 1, 2008

whether and how it would perform as a monomer in its thermal polymerizations with diazides 2.

The attempted thermal polymerizationof 3 with 2a in toluene was virtually failed: after refluxing in the solvent for as long as ∼4 days, only was a trace amount of hexane-insoluble product isolated (Table 1, no. 1). The regiostructure of the product was elucidated by NMR analysis (vide infra), from which an F1,4 ratio of 54% was obtained. These data indicate that the thermal polymerizationof the electron-richtriyne in the nonpolarsolvent is very sluggish and practically regiorandom. The thermal polymerization, however, could be accelerated by enhancing the solvent polarity (Scheme 6). When the solvent was changed from less polar toluene to more polar dioxane, the polymerization time was shortened (from ∼4 to 3 days) and the polymer yield was greatly increased (from ∼0% to ∼80%; cf. Table 1, nos. 1 and 4). The F1,4 ratiosof the polymers,however,remained unchanged within experimental error.

The thermalpolymerizationsof 3 with 2a were furtherstudied in an effort to optimize the process (Figure S4; Supporting Information).Under optimalconditions,3 was polymerizedwith 2b, affording an hb-r-P1b in ∼76% yield (Table 2, no. 2). Its

Mw,r and PDI values were estimated by GPC to be 1 400 and 2.7, respectively. It should be noted that the GPC calibrated by linear polystyrenestandardscan significantlyunderestimateMw,r values of hyperbranched polymers.16 Deffieux et al., for example, found that the relative molecular weights of their hyperbranched polystyrenes estimated by GPC were normally ∼7-fold, sometimes even ∼30-fold, lower than the absolute values determined by the laser light scattering (LLS) technique.16b We employed the LLS technique to measure the

Mw,a value of the polymer, which was found to be 177 500, about 14-fold higher than its Mw,r value.

It became clear that triyne 3 could undergo thermal polymerization with diazides 2 in a polar solvent to produce high molecular weight polymers with regiorandom structure and macroscopic processability in good yields, albeit at a slow rate. To prepareregioregularpolymersat a fast rate, we paid attention to the metal catalystsused in the azide-alkyne click reactions.1,2 As the model reactions had proved that triyne 3 cyclized with monoazides 8 in the presence of the Cu(I) catalyst, we tried to use it to synthesize hb-PTAs through its polycycloaddition with diazides 2 (Scheme 7). Mixing 3/2a with CuSO4/sodium ascorbate under the standard click reaction conditions1 caused instant formation of precipitates, which could not be dissolved in any common organic solvents (Table 2, no. 3). Similar results were obtained when 3 was subjected to the click polymerization with 2b: mixing the monomer and catalyst solutions resulted in immediate gel formation. The “standard recipe” for the click reaction is thus not suitable for the synthesis of processable hb- PTAs, confirming the early observations by other research groups.1

The CuSO4/sodium ascorbate catalyst was used in a THF/ water mixture. The incompatibility between the growing hb-

PTA species and the aqueous medium may have induced the polymers to aggregate and hence precipitate. To avoid the use of aqueous medium, we employed a nonaqueous click catalyst of Cu(PPh3)3Br17 to initiate the click polymerization of triyne 3 with diazides 2. The polymerization of 3 and 2a conducted in the presence of Cu(PPh3)3Br in DMF at 60 °C for 80 min produced an hb-1,4-P1a in ∼46% (Table 2, no. 5), which was soluble in common organic solvents, including DCM, THF, DMF, and DMSO. Similarly, the polymerization of 3 and 2b carried out in the nonaqueous medium afforded a soluble hb- 1,4-P1b in ∼52% yield. The Cu(I) catalysts have greatly accelerated the polycycloaddition process (e.g., 80 min at 60 °C), in comparison to the thermally activated system (e.g., 72 h at 101 °C).

The 1,5-regioselective click polymerization of 3 and 2 catalyzed by Cp*Ru(PPh3)2Cl proceeded even faster, in comparison to the Cu(I) system, thanks to the higher substrate tolerance of the Ru(I) complex. The Ru(I)-catalyzed polymerization of 3 and 2b furnished a soluble hb-1,5-P1b in ∼75% yield in as short as 30 min (Table 2, no. 7). The preparation of the Ru(I) complex, however, is a nontrivial job that requires high synthetic skills.1e Dichloro(pentamethylcyclopentadienyl)- ruthenium(I) oligomer {[Cp*RuCl2]n} is a precursor to Cp*Ru(PPh3)2Cl and can be facilely prepared in high yield by refluxing RuCl3·nH2O and Cp*H in ethanol for a few hours.18 Although we worried that the stable Ru(I) precursor may not work well as a click catalyst, it smoothly catalyzed the polycycloadditionof 3 and 2 at a moderate temperature (40 °C), producing soluble hb-1,5-P1 polymers in high yields (>83%).

Processability, Stability, and Solubility. All the freshly prepared samples of hb-PTAs, including the regiorandom hbr-P1 and regioregular hb-1,4-P1 and hb-1,5-P1 polymers, are readily processable: thin solid films can be facilely prepared by static or spin casting of their 1,2-dichloroethane solutions onto solid substrates, such as silicon wafers, glass slides, and mica plates. All the polymers are thermally stable, irrespective of the polymerization processes by which they were prepared. As can be seen from the thermogravimetric analysis (TGA) curves shown in Figure S5 (Supporting Information), the hb- PTAs lose 10% of their original weights in the temperature region of 374-407 °C, indicative of a strong resistance to thermolysis. No glass transition temperatures were detected by the differentialscanningcalorimetry(DSC) measurementswhen the polymers were heated up to 200 °C.

The polymers prepared by the Cu(PPh3)3Br catalyst (hb-1,4- P1), however, graduallybecame partiallyinsolubleupon storage under ambient conditions.One possible reason for this solubility change is the postpolymerization reactions of the polymers catalyzed by the metallic residues trapped in the hb-1,4-P1 samples. The copper species may have coordinated with the “old” amino functional groups in the monomer repeat units and/ or the “new” triazole rings formed during the 1,3-dipolar polycycloaddition reactions.19

In a control experiment, we admixed a small amount of

CuSO4/sodium ascorbate with an hb-1,5-P1 polymer prepared from the ruthenium-catalyzedclick polymerization.The polymer became insoluble within a few minutes, although it remained soluble after storage for several months in the absence of the externally added copper catalyst. The copper species may have catalyzed the 1,3-dipolarcycloadditionreaction of the azido and ethynyl terminal groups on the peripheral surfaces of the polymer, making it cross-linked and hence insoluble. We tried to remove the catalyst residues by washing the hb-1,4-P1 polymers with amine solvents, but the results were not satisfactory because of the poor solubility of the polymers in the hydrophilic solvents.

(Parte 1 de 4)

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