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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 Ha¨ussler,†^ Jianzhao Liu,†^ Yongqiang Dong,†^ Dehai Liang,‡^ Erqiang Chen,‡
Guochen Jia,†^ and Ben Zhong Tang*,†,§
Department of Chemistry, The Hong Kong Uni V ersity of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China; Department of Polymer Science and Engineering, Peking Uni V ersity, Beijing 100871, China; and Department of Polymer Science & Engineering, Zhejiang Uni V ersity, Hangzhou 310027, China Recei V ed March 10, 2008
ABSTRACT: Synthesis of hyperbranched poly(1,2,3-triazole)s ( hb -PTAs) has been a challenge: the AB 2 monomers were inclined to self-oligomerize, and their Cu(I)-catalyzed click polymerizations failed 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 A 2 + B 3 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 -P 1 ) with high molecular weights in high yields. Metal-mediated regioselective polymerizations afforded soluble 1,4- and 1,5-linked polymers ( hb -1,4-P 1 and hb -1,5-P 1 ), presenting the first examples of regioregular hb -PTAs with macroscopic processability. The reactions were affected by substrate and catalyst: electron-rich alkyne generally slowed down the cycloaddition reaction, while ruthenium catalysts (CpRu(PPh 3 ) 2 Cl and [CpRuCl 2 ] n ) exhibited higher substrate tolerance than copper catalyst [Cu(PPh 3 ) 3 Br]. Regiostructures and regioregularities of the hb -PTAs were determined spectroscopically. Degrees of branching of hb - r -P 1 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-P 1 , hb -1,5-P 1 and hb - r -P 1 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 develop- ment 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 molecules with heteroatom links from reactive modular building blocks in high efficiency under benign conditions through simple isolation procedures. A number of click reactions have been explored and identified, with the metal-catalyzed Huisgen [3 + 2] azide-alkyne cycloaddition^3 being hailed as the cream of the crop. The Cu(I)- and Ru(II)-mediated 1,3-dipolar cycload- ditions proceed steadily, affording 1,4- and 1,5-disubstituted 1,2,3-triazoles, respectively, in high yields.1, 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, biocon- jugate chemistry, surface modification, and materials develop- ment.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 azido- and 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 reaction- based “graft-to” and “graft-from” approaches, linear polymer chains decorated by dendritic grafts (or dendronized linear polymers) have been successfully prepared.6, 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 (N 3 - Ar-N 3 ) and arylene- diynes (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 Notwith- standing 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 AB 2 -type monomers of azidoarylenediynes with a general formula of N 3 - Ar-(CtCH) 2 have been prepared by two groups in Germany and Belgium.^11 One of the monomers was subjected to Cu(I)-catalyzed polymerization, but the product soon “precipitated” from the reaction mixture, “which is no
- To whom correspondence should be address: Ph +852-2358-7375; Fax +852-2358-1594; e-mail [email protected].†
‡ The Hong Kong University of Science & Technology. § Peking University. Zhejiang University.
3808 Macromolecules 2008 , 41 , 3808- 3822
10.1021/ma800538m CCC: $40.75 2008 American Chemical Society Published on Web 05/15/
longer soluble in any organic solvent”.^11 AB 2 -type monomers of ethynylenediazides (N 3 - R-CtC-R-N 3 ) have also been prepared. The internal alkyne monomers, however, could only be polymerized by thermally activated 1,3-dipolar polycycload- dition, producing hyperbranched regiorandom poly(1,2,3-tria- zole)s ( hb-r- PTAs). In addition, the AB 2 monomers were difficult to prepare and purify and suffered from self-oligomer- ization during storage under ambient conditions.^11 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 reactions and as a natural extension of our alkyne-based research 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) and diazide (N 3 - R-N 3 ) 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 F 1,4 ratio up to ∼92%) in a high yield (up to ∼98%).^13 In this work, we intended to synthesize hyper- branched polymers ( hb -PTAs) by click polymerization. We took an A 2 + B 3 approach, using easy-to-make and stable-to-keep diazide (A 2 ) and triyne (B 3 ) as monomers, in an effort to circumvent the self-oligomerization problem met by the AB 2 systems discussed above. The A 2 /B 3 monomers were readily polymerized by metal-mediated click reactions and thermally activated Huigen cycloaddition (Scheme 1). The Cu- and Ru- catalyzed click polymerizations afforded hyperbranched poly- mers with regular 1,4- 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 triazole rings affected the electronic communications 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. A 2 - and B^3 - type monomers of diazides 2 and triyne 3 were designed to realize the planned A 2 + B 3 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 transformed to 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 (B 3 ) monomer of tris(4-ethynylphenyl)amine ( 3 ), from which hb -PTAs with useful photonic properties may be derived. The monomer was prepared in a high yield (∼90%) by iodination of triphenylamine, followed by coupling with (tri- methylsilyl)acetylene and base-catalyzed disilylation.14f, 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 mono- azides were prepared by the same synthetic routes as 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 standard click reaction conditions^1 (Scheme 4). 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(PPh 3 ) 2 Cl, however, produced 1,5-disubstituted 1,2,3-triazoles 10 in higher yields (∼ 79 - 84%), revealing a higher substrate tolerance of the Ru(II) complex over the Cu(I) catalyst. The model reactions confirm that triyne 3 can be used as a B 3 monomer in the planned A 2 + B 3 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 regio- regularity 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 (A 2 ) and Triyne (B 3 ) Monomers
Scheme 2. Preparation of Diazide Monomers (2)
Scheme 3. Preparation of Monoazide Model Compounds (8)
Macromolecules, Vol. 41, No. 11, 2008 Synthesis and Properties of Hyperbranched Polytriazoles 3809
whether and how it would perform as a monomer in its thermal polymerizations with diazides 2.
The attempted thermal polymerization of 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 F 1, ratio of 54% was obtained. These data indicate that the thermal polymerization of the electron-rich triyne in the nonpolar solvent 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 polymeri- zation 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 F 1,4 ratios of the polymers, however, remained unchanged within experimental error. The thermal polymerizations of 3 with 2a were further studied in an effort to optimize the process (Figure S4; Supporting Information). Under optimal conditions, 3 was polymerized with 2b , affording an hb - r -P 1b in ∼76% yield (Table 2, no. 2). Its M w,r and PDI values were estimated by GPC to be 11 400 and 2.7, respectively. It should be noted that the GPC calibrated by linear polystyrene standards can significantly underestimate M w,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 M w,a value of the polymer, which was found to be 177 500, about 14-fold higher than its M w,r value.
It became clear that triyne 3 could undergo thermal polym- erization 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 prepare regioregular polymers at a fast rate, we paid attention to the metal catalysts used in the azide-alkyne click reactions.1, 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 CuSO 4 /sodium ascorbate under the standard click reaction conditions^1 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.^11 The CuSO 4 /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(PPh 3 ) 3 Br^17 to initiate the click polymerization of triyne 3 with diazides 2. The polymerization of 3 and 2a conducted in the presence of Cu(PPh 3 ) 3 Br in DMF at 60 °C for 80 min produced an hb -1,4-P 1a 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-P 1b 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 CpRu(PPh 3 ) 2 Cl proceeded even faster, in com- parison to the Cu(I) system, thanks to the higher substrate tolerance of the Ru(II) complex. The Ru(II)-catalyzed polym- erization of 3 and 2b furnished a soluble hb -1,5-P 1b in ∼75% yield in as short as 30 min (Table 2, no. 7). The preparation of the Ru(II) complex, however, is a nontrivial job that requires high synthetic skills.1e^ Dichloro(pentamethylcyclopentadienyl)- ruthenium(III) oligomer {[CpRuCl 2 ] n } is a precursor to CpRu(PPh 3 ) 2 Cl and can be facilely prepared in high yield by refluxing RuCl 3 · n H 2 O and CpH in ethanol for a few hours.^18 Although we worried that the stable Ru(III) precursor may not work well as a click catalyst, it smoothly catalyzed the polycycloaddition of 3 and 2 at a moderate temperature (40 °C), producing soluble hb -1,5-P 1 polymers in high yields (>83%). Processability, Stability, and Solubility. All the freshly prepared samples of hb -PTAs, including the regiorandom hb- r- P 1 and regioregular hb -1,4-P 1 and hb -1,5-P 1 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 differential scanning calorimetry (DSC) measurements when the polymers were heated up to 200 °C. The polymers prepared by the Cu(PPh 3 ) 3 Br catalyst ( hb -1,4- P 1 ), however, gradually became partially insoluble upon 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-P 1 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 CuSO 4 /sodium ascorbate with an hb -1,5-P 1 polymer prepared from the ruthenium-catalyzed click 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-dipolar cycloaddition reaction 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-P 1 polymers with amine solvents, but the results were not satisfac- tory because of the poor solubility of the polymers in the hydrophilic solvents. Another possible reason for the solubility change of hb -1,4- P 1 with storage is the aggregate formation in the solid state.
Table 1. Effect of Solvent on Thermal Polycycloaddition of Diazide 2a and Triyne 3 a no. solvent time (h) yield (%) M w,r b^ PDI c^ F 1,4 (%) d 1 toluene 90 trace 1900 1.3 54 2 chlorobenzene 77 39.2 3900 1.7 52 3 o -dichlorofbenzene 77 50.1 5200 1.9 53 4 dioxane 72 79.7 5800 2.3 53 a (^) Polymerization reactions carried out at 110 °C under nitrogen; [ 3 ] 0 / [ 2a ] 0 ) 2:3; [ 3 ] 0 ) 0.067 M (for nos. 1 and 2), 0.1 M (for nos. 3 and 4). b^ Relative weight-average molecular weight estimated by gel permeation chromatography (GPC) in THF on the basis of a linear polystyrene calibration. c^ PDI ) polydispersity index ( M w / M n ). d^ Molar fraction of 1,4- regioisomeric unit.
Macromolecules, Vol. 41, No. 11, 2008 Synthesis and Properties of Hyperbranched Polytriazoles 3811
We “created” a simple molecule of tris[4-(1-methyl-1,2,3-triazol- 4-yl)phenyl]amine ( 15 ) that resembles the cyclic unit of hb - 1,4-P 1 and subjected it to theoretical stimulation, using AM Hamiltonian in the MOPAC package embedded in the Chem3D Ultra program. In the optimized conformation, the phenyl and triazole rings of the 1,4-isomer experience little steric interaction and can locate almost in the same plane (Chart 1A). During storage, the cyclic plates of the hb -1,4-P 1 polymers may gradually pack, with the aid of the π-π stacking attractions between their aromatic units. This “physical cross-linking”
process progressively knits more polymer molecules together and widens the three-dimensional networks, hence gradually decreasing the polymer solubility. On the other hand, there exists steric repulsion between the phenyl and triazole rings in the 1,5-isomer 16. The rings are therefore twisted out of coplanarity to alleviate the involved steric effect (Chart 1B). This twisted, nonplanar structure makes the cyclic units of hb -1,5-P 1 polymers difficult to pack in the solid state. The 1,5-regioregular polymers thus can maintain their good solubility for a long period of time. The hb - r -P 1 polymers
Scheme 6. Syntheses of hb-r- P1 Polymers by Thermal Polycycloadditions of Diazides 2 and Triyne 3
Table 2. Syntheses of Regiorandom and Regioregular hb -PTAs by Thermally Initiated and Metal-Catalyzed Polycycloadditions of Diazides 2 and Triyne 3 a no. monomers catalyst solvent temp (°C) time (h) polymer yield (%) M w,r b^ PDI b^ M w,a c 1 2a + 3 (thermal) dioxane 101 72.00 hb-r- P 1a d^ 64.0 5500 2. 2 2b + 3 (thermal) dioxane 101 72.00 hb-r- P 1b e^ 75.7 11400 2.7 177500 3 2a + 3 CuSO 4 /SA THF/H 2 O f^ rt inst. g^ hb- 1,4 - P 1a gel g 4 2b + 3 CuSO 4 /SA THF/H 2 O f^ rt inst. g^ hb- 1,4 - P 1b gel g 5 2a + 3 Cu(PPh 3 )3 Br DMF 60 1.33 hb- 1,4-P 1a 46.3 2600 1. 6 2b + 3 Cu(PPh 3 )3 Br DMF 60 1.33 hb- 1,4 - P 1b 51.6 5000 2. 7 2a + 3 CpRu(PPh 3 )2 Cl THF 60 0.33 hb- 1,5 - P 1a 62.5 h^ 5400 2.4 72800 8 2b + 3 CpRu(PPh 3 )2 Cl THF 60 0.50 hb- 1,5 - P 1b 74.9 9400 2.7 32200 9 2a + 3 [CpRuCl 2 ] n THF 40 2.00 hb- 1,5 - P 1a 85.4 5000 1. 10 2b + 3 [CpRuCl 2 ] n THF 40 2.00 hb- 1,5 - P 1b 83.2 7700 2. a (^) Polymerization reactions carried out under nitrogen; [ 3 ] 0 /[ 2 ] 0 ) 2:3, [ 3 ] 0 ) 0.12 M. Abbreviations: SA ) sodium ascorbate; rt ) room tempera- ture. b^ Relative (r) value estimated in THF by GPC on the basis of a linear polystyrene calibration. c^ Absolute (a) value measured in THF (for no. 2) or DMF (for nos. 7 and 8) by the LLS technique. d^ F 1,4 ) 53%. e^ F 1,4 ) 50%. f^ Volume ratio: 5:1. g^ Instant gel formation (once the monomer and catalyst solutions were mixed). h^ Soluble fraction (total yield: 84.5%).
3812 Qin et al. Macromolecules, Vol. 41, No. 11, 2008
comparison with the spectra of the monomers and model compounds, the peaks of the polymer are readily assigned. Of great value is the peak at δ 8.60 arising from the resonance of the proton of the 1,4-isomeric unit (no. 47) of the polymer (Figure 3E). This peak is well separated from the peak at δ 7.89, which is associated with the resonances of the protons of the 1,5-isomeric unit of the polymer (no. 42) and the phenyl ring adjacent to its 1,4-isomeric unit (no. 46). On the basis of above spectral analysis, the F 1,4 ratio of hb - r -P 1b can be correlated with its NMR data as follows:
F 1, F 1,
)
A 47 A 42,46 - 2 A 47
(1)
where F 1,5 is the fraction of its 1,5-isomeric unit, and A 47 and A 42,46 are the integrated areas of peaks 47 and (42, 46), respectively. Because F 1,4 + F 1,5 ) 1, eq 1 can be transformed to the following equation:
F 1,4 )
A 47 A 42,46 - A 47
(2)
The F 1,4 ratio for hb - r- P 1b was calculated to be 50% from eq 2 and the spectral data given in Figure 3. A similar equation was derived for calculating the regioregularity of hb - r- P 1a [ F 1, ) A 39 /( A 34,38 - A 39 )], from which its F 1,4 ratio was determined to be 53% (cf. Figure S2, Supporting Information). The F 1,4 ratios of around 50% for the hb - r- P 1 polymers confirm that the thermal polymerizations of diazides 2 and
electron-rich triyne 3 have proceeded in a regiorandom fashion. In our previous work, we found that the diazides and the electron-deficient diynes thermally polymerized regioselec- tively.^13 Evidently, the regioselectivity of the thermal polym- erization is dramatically affected by the electronic structure of the alkyne monomer, offering a molecular engineering means for tuning the regioregularity of PTA. It is well-known that the Cu(I)- and Ru(II)-catalyzed click reactions1,2^ yield 1,4- and 1,5-disubstituted 1,2,3-triazoles, respectively. As expected, the polymers obtained from the click polymerizations of 2b and 3 initiated by the Cu(PPh 3 ) 3 Br and CpRu(PPh 3 ) 2 Cl catalysts exhibit strong and nil signals at δ 8.63, respectively (panels A and B of Figure 4). Different from CpRu(PPh 3 ) 2 Cl complex, [Cp*RuCl 2 ] n oligomer possesses a Ru(III) center. The polymer prepared from this catalyst also gives a spectrum free of the diagnostic peak for the 1,4-isomeric unit at δ 8.63 (Figure 4C). This confirms that the 1,5- regioregular polymer can be prepared by the “simple”, stable, and “cheap” Ru(III) complex. This also suggests the possibility that the Ru(III) center may have been in situ reduced to a Ru(II) active species during the click polymerization reaction. Similar results were obtained from the spectral analyses of the hb -1,4- and -1,5-P 1a polymers prepared from the copper- and ruthenium- catalyzed click polymerizations of 2a and 3 (Figure S8, Supporting Information). Determination of Degree of Branching. An important structural parameter for a hyperbranched polymer is its degree of branching (DB), which is often determined by 1 H NMR spectral analysis.^10 It can been seen from Chart 2 that there exist six structural units in an hb - r -P 1 : one dendritic unit ( D ), two linear units [one with an unreacted ethynyl group ( L e ) and another with an unreacted azido group ( L a )], and three terminal units [one with two ethynyl groups ( T ee ), one with two azido groups ( T aa), and one with an ethynyl group and an azido group ( T ea )]. We scrutinized with great care the NMR spectra of hb - r -P 1b measured in different solvents, especially its peaks associated with the resonances of the triple bond and the azido group (actually its adjacent methylene unit), and found with delight that the spectrum taken in chloroform- d could be used for the calculation of DB value of the polymer. To simplify the calculation, we assume that there are no side reactions such as loop formations in the azide-alkyne 1,3- dipolar polycycloaddition reactions. By comparing the NMR spectrum of hb - r -P 1b with those of its monomers ( 2b and 3 ; cf. Figure 2), it is found that the following relationships hold for the molar fractions of the six structural units ( f D , f Le , f La , f Tee , f Taa , and f Tea ):
Chart 1. Simulated Conformations of (A) Tris[4-(1-methyl-1,2,3-triazol-4-yl)phenyl]amine (15) and (B) Tris[4-(1-methyl-1,2,3-triazol-5-yl)phenyl]amine (16)
Figure 1. IR spectra of monomers 3 (A) and 2a (B) and hyperbranched polymers hb-r- P 1a (C) and hb-r- P 1b (D).
3814 Qin et al. Macromolecules, Vol. 41, No. 11, 2008
f Le + 2 f Tee + f Tea 2 f La + 4 f Taa + 2 f Tea
)
A 1 A 10
(3)
f Le + 2 f La + 2 f Tee + 4 f Taa + 3 f Tea 3 f D + 2 f Le + 3 f L + f Tee + 3 f T + 2 f Tea
)
A 1 + A 10 A 41,48 /
(4)
6 f D + 4 f Le + 8 f La + 2 f Tee + 10 f Taa + 6 f Tea 12 f D + 12 f Le + 12 f La + 12 f Tee + 12 f Taa + 12 f Tea
)
A 35 A 2,3,42- 47 - A 41,48 /
(5)
f D + f Le + f La + f Tee + f Taa + f Tea ) 1 (6)
where A 1 , A 10 , A 41,48, A 35,54, and A 2,3,42- 47 are the integrated areas of the resonance peaks 1, 10, (41, 48), (35, 54), and (2, 3, 42 - 47) as labeled in Figure 2 and Chart 3. From the integrated areas of the resonance peaks in the 1 H NMR spectrum, we get
A 1 A 10
) 0.
(7)
A 1 + A 10 A 41,48 /
)
0.077 + 0. 0.785/
(8)
A 35, A 2,3,42- 47 - A 41,48 /
) 1. 2.493 - 0.785/
(9)
From eqs 3–9, we can derive the following equation:
f Le + 2 f Tee + f Tea ) 0.473 (10) Closer examination of the peak at δ 3.07 reveals that the peak contains three components, as shown in the inset of Figure 2C. Using a curve-fitting program and considering the electron- withdrawing effect of the triazole ring, we deconvoluted the resonance peak into three small peaks and assigned them from downfield to upfield to L e , T ea , and T ee , with the ratio of their integrals calculated to be 0.14:1.00:1.21. Equations 11 and 12 are thus established:
Chart 2. Chemical Structures of Dendritic ( D ), Linear ( L ), and Terminal ( T ) Units of hb-r- P1 Polymers
Figure 2.^1 H NMR spectra of chloroform- d solutions of monomers 3 (A) and 2b (B) and their polymer hb - r- P 1b (C) measured at room temperature. Labels of the resonance peaks of the polymer correspond to those given in Chart 3. The solvent and water peaks are marked with asterisks.
Macromolecules, Vol. 41, No. 11, 2008 Synthesis and Properties of Hyperbranched Polytriazoles 3815
f La ) 0.09 (18)
Eventually, under the assumptions we made above and within experimental errors, the DB value for hb - r -P 1b is deduced to be
DB ) 1 - f Le - f La ) 1 - 0.02 - 0.09 ) 0.89 (19) Taking the same approach, the DB value for hb - r -P 1a is calculated to be 0.90, using eqs S1-S6 and NMR data (Figure S10) given in the Supporting Information. The similar DB values for hb - r -P 1a (0.90) and hb - r -P 1b (0.89) support the conclusion drawn from the spectroscopic analyses that the alkyl chain length exerts little effect on the polymer structure (vide supra). The high DB values duly verify the hyperbranched nature of the
molecular structures of the polymers. The DB values of the hb - r -P 1 polymers are much higher than those of the “conventional” hyperbranched polymers (commonly ∼0.50)10,22^ but close to those of some of the hyperbranched polyphenylenes we previ- ously synthesized from the diyne polycyclotrimerizations (∼0.79^23 and 0.78-1.00^24 ). This is probably because the azide-alkyne polycycloaddition and the alkyne polycyclotri- merization share a similar growth mechanism of polymer species via aromatic ring formation. Photophysical Properties. Linear PTAs have previously been prepared by other groups from Cu(I)-catalyzed polycycloaddi- tions of conjugated arylenediazides and arylenediynes.^8 It was found that the absorption spectra of the polymers were “es-
Figure 3.^1 H NMR spectra of DMSO- d 6 solutions of monomers 3 (A) and 2b (B), model compounds 9b (C) and 10b (D), and polymer hb-r- P 1b (E) measured at room temperature. Labels of the resonance peaks of hb-r- P 1b correspond to those given in Chart 2. The solvent and water peaks are marked with asterisks.
Macromolecules, Vol. 41, No. 11, 2008 Synthesis and Properties of Hyperbranched Polytriazoles 3817
sentially a superposition of those of the monomers used” with “no ground state interaction between the different moieties of the polymers” and that the polymers were “nonfluorescent in the solid state” although their dilute solutions emitted UV light with emission maxima (λem ) of 355-379 nm.^8 This suggests that the solid-state luminescence has been efficiently quenched due to the π-π stacking aggregation of the polymer chains.
The hb -PTAs synthesized in this work contain numerous conjugated triphenylamine and triazole units. Is there any electronic communication between these two aromatic moieties? How do the 1,4- and 1,5-regiostructures affect electronic conjugation and supramolecular aggregation? Are the polymers light-emitting, especially in the solid state? To answer these questions, we measured absorption and emission spectra of the hb -PTAs in dilute solutions and solid films. To gain insights into involved photophysical and aggregative processes, we also measured the spectra of some model compounds.
Model compounds of 1-(6-phenoxyhexyl)-4-phenyl-1,2,3- triazole ( 12 ) and triphenylamine absorb in the UV spectral region, with absorption maxima (λab ) at 251 and 305 nm, respectively (Figure 5A). The absorption spectrum of 10b , a model compound with a 1,5-regiostructure, is peaked at 330 nm, which is 79 and 25 nm red-shifted from those of triazole 12 and triphenylamine, respectively. The spectrum of 9b , a model compound with a 1,4-regiostructure, is further red-shifted from that of 10b , indicative of a higher extent of electronic communication between triazole and triphenylamine units in the 1,4-isomer. This spectral data are in excellent agreement with the simulation results shown in Chart 1: the 1,4-isomer is conformationally more planar and hence electronically more conjugated.
The absorption spectra of hb -1,5- and -1,4-P 1b polymers are also bathochromically shifted from those of triazole and triphenylamine compounds and are located in the vicinity of those of their corresponding model compounds with 1,5- ( 10b ) and 1,4-regiostructures ( 9b ; Figure 5B). This reveals that there exist ground-state interactions between the triphenylamine and triazole moieties in the polymers and that the electronic communications are dictated by the regioisomeric structures of triazolyl units. As expected, the absorption of hb - r -P 1b occurs
in the spectral region between those of its regioregular coun- terparts of hb -1,5-P 1b and hb -1,4-P 1b because the regiorandom polymers contains ∼50% of the 1,5- and 1,4-regioisomeric units. Also as anticipated, similar spectral data were obtained in the hb -P 1a series (Figure S11, Supporting Information) because the length of alkyl chain does not affect the electronic communica- tion between the aromatic moieties in the polymers. The absorption spectrum of the thin film of model compound 9b is bathochromically shifted from that of its dilute solution (Figure 6A), suggestive of aggregate formation of its aromatic moieties through π-π stacking interaction in the solid state. The solution of 9b exhibits a symmetrically shaped emission spectrum with a λem of 399 nm, whereas its film displays a broad, red-shifted spectrum extending into long wavelength region (up to ∼650 nm), characteristic of the emission of aggregative species, such as dimers or excimers. Its regioiso- meric counterpart with a 1,5-linkage, viz. 10b , gives similar spectral data: both absorption and emission spectra of the film are bathochromically shifted from those of the solution. Although weaker in intensity in comparison to its dimer emission at 450 nm, the monomer emission of the film of 10b is still discernible at ∼400 nm as a shoulder peak. Moreover, the emission spectrum of 10b is narrower than that of 9b as a whole, with the full width at half-maximum (fwhm) of the former (61 nm) being only about half of the latter (114 nm). These spectral data suggest that the aggregation in 10b is not as severe as in 9b , probably because the more twisted conformation of the former has impeded the aggregate forma- tion. The solutions of the hb -PTAs emitted deep blue light of 409 - 416 nm, with fluorescence quantum yields of 14-43% (Figure 6 and Table 3). The emission spectra of their films are red-shifted from those of their solutions and are even broader than those of the films of their corresponding model compounds. As can be seen from Figure 6, the spectrum of the film of hb - 1,4-P 1b is broader than that of its model compound 9b. The film spectrum covers almost the entire visible spectral region, with an fwhm of 133 nm, which is more than 2-fold wider than that of its solution (64 nm; Table 3, no. 2). The “monomer” emission of hb -1,5-P 1b film becomes weaker, in comparison to its “dimer” or excimer emission (Figure 6E). The hb-r- P 1b film also shows a broad emission spectrum. Similar spectral data are obtained in the series of hb -P 1a polymers (Table 3 and Figure S12, Supporting Information). These results indicate that the aromatic moieties in the hb -PTAs are aggregated in the solid state. The extents of chromophore aggregation in the hb -PTAs, however, are not as severe as in the linear PTA system, in which the solid films became totally nonfluorescent.^8 The aromatic units strung in the linear PTA chains were such planar plates as phenyl, pyridyl, fluorenyl, and triazolyl groups, which can be stacked or piled up via π-π interaction in the solid state. The triphenylamino units embedded in the hb -PTAs are non- planar and hence not easy to pack well. Different from the linear PTA chains, the hb -PTA spheres are also difficult to align well. These are probably the reasons why the aggregation is less developed in the hb -PTAs and why the polymers are still luminescent in the solid state. Fluorescent Photopatterning. Since the films of the hb -PTAs are emissive, we tried to utilize them to create fluorescent images. Luminescent polymers with good film-forming ability and high photolysis efficiency are promising candidate materials to be used in the photoresist processes for the generation of fluorescent patterns,^25 which have potential applications in liquid crystal displays, light-emitting diodes, medicinal diagnostic biochips, etc.26,27^ The hb -PTAs are soluble and film-forming. They contain many azido, ethynyl, and triazolyl functional
Figure 4.^1 H NMR spectra of (A) hb -1,4-P 1b and (B, C) hb -1,5-P 1b in DMSO- d 6 measured at room temperature. The peaks of water and solvent are marked with cross (×) and asterisk (), respectively. The polymers were prepared by the click polymerizations of 2b and 3 catalyzed by (A) Cu(PPh 3 ) 3 Br, (B) CpRu(PPh 3 ) 2 Cl, and (C) [Cp*RuCl 2 ] n.
3818 Qin et al. Macromolecules, Vol. 41, No. 11, 2008
why hb - r -P 1a emits a white light: the regiorandom polymer contains a roughly equal amount of 1,4- and 1,5-isomeric units, and the mixing of the complementary yellow and blue colors of the light emitted from the two units results in the light emission of a white color.^29 Similarly, the photopatterns generated from the films of hb -1,4-P 1b , hb -1,5-P 1b , and hb - r - P 1b emit yellow, blue, and white light, respectively, demon- strating that the emission colors of the hb -PTA films are hardly affected the lengths of their alkyl chains. The exposure of the hb-r- P 1a film to the UV irradiation results in a decrease in the intensity of its IR absorption band at 2098 cm-^1 (Figure 8). This band is assignable to the stretching vibrations of the ethynyl (CtC) and azido (N 3 ) groups, but the decrease in the band intensity is probably mainly due to the photolysis of the azido group because the intensity of the ≡C-H band at 3283 cm-^1 maintains almost unchanged. On the other hand, a new band associated with carbonyl (CdO) stretching appears at 1720 cm-^1 , indicating that the polymer is oxidized during the photolysis process. The studies of the structural changes of the polymers accompanying the photolysis processes by the wet spectroscopic methods such as NMR are hampered by the insolubility of the photo-cross-linking products. We thus conducted the photolysis reactions of some small model compounds, in the hope that their photolysis products can be caught by NMR spectroscopy. Model compound 11 contains triazolyl and azido groups but no ethynyl group. Its UV photolysis leads to the formation of an insoluble product, excluding the possibility that the ethynyl group is involved in the photolysis process of the azidotriazole compound. Model compound 12 contains a triazolyl group but no azido group. It remains soluble, and its NMR spectrum remains unchanged even after it has been irradiated by a UV lamp for 12 h, implying that the triazolyl group has not participated in the photo-cross- linking process. Since the two model reactions discussed above have excluded the possible involvement of the ethynyl and triazolyl groups, the azido group must have been responsible for the photo-cross- linking reaction. This conclusion is further substantiated by another model reaction, in which a compound containing an azido group ( 8b ) is irradiated by a UV lamp. The resonance peak of the protons of the methylene group adjacent to the azido group at δ 3.24 (no. 61) disappears after the photoirradiation due to the photolysis of the azido group (Figure 9). A new peak assignable to the resonance of an aldehyde proton appears at δ 9.78, indicating that the model compound is oxidized during its photolysis process. The above experimental results suggest that the photo-cross- linking process of the hb -PTA starts from the photolysis of its
azido group (Scheme 8). UV irradiation of hb -P 1 cleaves the N-^ - N+^ bond of the azido group, releases a molecule of nitrogen, and generates an active nitrene radical ( 17 ). 30 Com- bination of the nitrene radical in 17 with that in another denitrogenated polymer results in the formation of 18 , a cross- linking product. The azo group hooking up two hb -PTA polymers can be broken by photoinduced homolysis, releasing another nitrogen molecule and yielding another polymer radical ( 19 ).^30 Coupling of two radicals furnishes anther cross-linking product 20. Since the polymer contains numerous azido groups as verified by the spectral analysis and mathematical derivati- zation, the reaction routes of 17 f 18 and 19 f 20 will repeat, eventually leading to the formation of heavily cross-linked products that are insoluble in any organic solvents. The radical in 19 can also react with oxygen, thus transforming the methylene group adjacent to the azido group to an aldehyde species ( 21 ).
Concluding Remarks In this work, we established an effective A 2 + B 3 route to soluble hb -PTAs. The diazide 2 and triyne 3 monomers were easy to prepare and free of the self-oligomerization problem encountered in the AB 2 system.^11 The thermally activated and metal-catalyzed polycycloadditions afforded regiorandom and regioregular hb -PTAs with high molecular weights ( M w up to ∼ 180 × 103 ). The solubility problem of hb -1,4-P 1 was solved by using nonaqueous catalyst of Cu(PPh 3 ) 3 Br to initiate the click polymerizations in DMF. CpRu(PPh 3 ) 2 Cl and its inexpensive, stable precursor [CpRuCl 2 ] n were found to exhibit high catalytic activity to the electron-rich triyne monomer and smoothly catalyzed the click polymerizations in a regioselective manner, furnishing hb -1,5-P 1 in high yields (>83%). The hb - 1,4-P 1 and hb -1,5-P 1 polymers are soluble and film-forming, representing the first examples of regioregular hb -PTAs with macroscopic processability. The good solubility of the polymers enabled us to elucidate their structures spectroscopically. Using model compounds, we determined the regiostructures (1,4- and 1,5-linkages) and regioregularities ( F 1,4 ratios) of the hb -PTAs. Through math- ematic derivatization, the BD values of hb - r -P 1 were calculated to be ∼90%. The hb -1,5-P 1 polymers were found to exhibit better solubility, shorter conjugation, bluer luminescence, and milder aggregation compared to their hb -1,4-P 1 counterparts. This was rationalized on the basis of regiostructural and steric effects: the aromatic units in the former are conformationally
Figure 8. IR spectra of hb - r- P 1a (A) before and (B) after UV irradiation for 5 min.
Figure 9.^1 H NMR spectra of CDCl 3 solutions of 8b (A) before and (B) after UV irradiation for 10 h.
3820 Qin et al. Macromolecules, Vol. 41, No. 11, 2008
more twisted than the latter. We succeeded in generating fluorescent images with blue, yellow, and white emissions by irradiating hb -1,4-P 1 , hb -1,5-P 1 , and hb-r- P 1 films, respectively, demonstrating that photopatterns with different emission colors can be achieved by photolyses of the polymers with same chemical formulas but different regiostructures. This offers an attractive way to tune the emission color of a polymer film: simply changing the regiostructure of the same polymer without going through the trouble of synthesizing a new polymer with a different chemical structure.
Experimental Section
General information about materials and instrumentations, experimental procedures for the syntheses of the monomers and the model compounds, and structural characterization data for the monomers and the model compounds are given in the Supporting Information. All the polymerization reactions and manipulations were carried our under nitrogen using Schlenk techniques in a vacuum line system or an inert atmosphere glovebox, except for the purifications of the polymers, which were conducted in an open atmosphere. Numerical spectral data for all the polymers, including hb - r -P 1 , hb -1,4-P 1 , and hb -1,5-P 1 , are given in the Supporting Information. Thermally Activated Regiorandom 1,3-Dipolar Polycycload- dition (Scheme 6). Into a 20 mL Schlenk tube with a stopcock in the side arm were added 2 (0.6 mmol) and 3 (0.4 mmol). The tube was evacuated and refilled with nitrogen three times through the side arm. Freshly distilled dioxane (3.5 mL) was then injected into the tube to dissolve the monomers. The reaction mixture was refluxed for 72 h. After cooling to room temperature, the solution was diluted with a small amount of chloroform and then added dropwise to 300 mL of a hexane/chloroform mixture (10:1 by volume) through a cotton filter under stirring. The precipitate was allowed to stand overnight and was then collected and dried to a constant weight in a vacuum oven at room temperature. Copper-Catalyzed 1,4-Regioregular Click Polymerization (Scheme 7). Into a 20 mL Schlenk tube with a stopcock in the side arm were added 2 (0.3 mmol), 3 (0.2 mmol), and Cu(PPh 3 ) 3 Br (0.012 mmol) under nitrogen in a glovebox. Dry DMF (1.7 mL) was injected into the tube to dissolve the monomers and catalyst. The reaction mixture was stirred at 60 °C for 80 min. Afterward, the mixture was diluted with chloroform and then added dropwise to 100 mL of a hexane/chloroform mixture (10:1 by volume) via a cotton filter under stirring. The precipitate was allowed to stand overnight and was then collected and dried to a constant weight in a vacuum oven at room temperature. Ruthenium-Catalyzed 1,5-Regioregular Click Polymeriza- tion (Scheme 7). Into a 20 mL Schlenk tube with a stopcock in the side arm were added 2 (0.45 mmol), 3 (0.3 mmol), and Cp*Ru(PPh 3 ) 2 Cl (0.018 mmol) under nitrogen in a glovebox. Freshly distilled THF (2.5 mL) was then injected into the tube to dissolve the monomers and catalyst. The reaction mixture was heated at 60 °C for 20 or 30 min. The mixture was then diluted with a small amount of THF and added dropwise to 300 mL of a mixture of hexane/chloroform (10:1 by volume) through a cotton filter under stirring. The precipitate was allowed to stand overnight and was then collected and dried to a constant weight in a vacuum
oven at room temperature. Similarly, the polymerizations of 2 (0. mmol) and 3 (0.1 mmol) were carried out in the presence of [Cp*RuCl 2 ] n (6.7 μmol) at 40 °C for 2 h. Generation of Fluorescent Images via Polymer Photolyses. Photo-cross-linking reactions of the polymer films were initiated by the irradiation of a 365 nm UV light with an intensity of ∼ 30 mW/cm 2. The films were prepared by spin-coating the polymer solutions (∼2 wt % in 1,2-dichloroethane) at 800 rpm for 9 s and then at 2000 rpm for 60 s on precleaned silicon wafers. The films were dried in a vacuum oven at room temperature overnight. The photoresist patterns were prepared on the silicon wafers using copper negative photomasks. After UV irradiation, the films were developed in 1,2-dichloroethane for 40 s and then dried at room temperature overnight under reduced pressure. Fluorescent images of the patterns were taken on an Olympus BX41 fluorescent optical microscope with a 330-385 nm wide band UV excitation.
Acknowledgment. This work was partly supported by the Research Grants Council of Hong Kong (602707, 602706 and HKU2/05C), the National Natural Science Foundation of China (20634020), and the Ministry of Science & Technology of China (2002CB613401). B.Z.T. thanks the support from the Cao Guang- biao Foundation of Zhejiang University.
Supporting Information Available: Text describing general information about the materials and instrumentations and providing the experimental procedures and analysis data for the monomers, model compounds, and polymers; IR spectra of model compounds 9 and 10 and polymers hb -1,4-P 1 and hb -1,5-P 1 ; 1 H NMR spectra of monomers 3 and 2a , model compounds 9a and 10a , and polymers hb - r -P 1a , hb -1,4-P 1a , and hb -1,5-P 1a ; 13 C NMR spectra of model compounds 9 and 10 and polymers hb - r -P 1 , hb -1,4-P 1 , and hb - 1,5-P 1 ; chemical structure of hb - r -P 1a with labeling scheme for spectral analysis; figure showing the effect of concentration of triyne 3 on its thermally activated polycycloaddition with diazide 2a in dioxane; TGA thermograms of hb -P 1 polymers; and absorption and emission spectra of the solutions and films of the model compounds and polymers. This material is available free of charge via the Internet at http://pubs.acs.org.
References and Notes (1) (a) Tornøe, C. W.; Meldal, M. In Peptides: the Wa V e of the Future ; Lebel, M., Houghten, R., Eds.; American Peptide Society: San Diego, 2001; pp 263-264. (b) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001 , 40 , 2004. (c) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. Angew. Chem., Int. Ed. 2002 , 41 , 2596. (d) Tornøe, C. W.; Christensen, C.; Meldal, M. J. Org. Chem. 2002 , 67 , 3057. (e) Zhang, L.; Chen, X.; Xue, P.; Sun, H. H. Y.; Williams, I. D.; Sharpless, K. B.; Fokin, V. V.; Jia, G. J. Am. Chem. Soc. 2005 , 127 , 15998. (f) Majireck, M. M.; Weinreb, S. M. J. Org. Chem. 2006 , 71 , 8680. (2) For selected examples of reviews, see: (a) Wang, Q.; Chittaboina, S.; Barnhill, H. N. Lett. Org. Chem. 2005 , 2 , 136. (b) O’Reilly, R. K.; Joralemon, M. J.; Wooley, K. L.; Hawker, C. J. Chem. Mater. 2005 , 17 , 5976. (c) Bock, V. D.; Hiemstra, H.; van Maarseveen, J. H. Eur. J. Org. Chem. 2006 , 51. (d) Boldt, G. E.; Dickerson, T. J.; Janda, K. D. Drug Disco V ery Today 2006 , 11 , 143. (e) Sohma, Y.; Kiso, Y. Chem. Biol. Chem 2006 , 7 , 1549. (f) Braunschweig, A. B.; Dichtel,
Scheme 8. Proposed Photo-Cross-Linking Mechanism for hb -PTA
Macromolecules, Vol. 41, No. 11, 2008 Synthesis and Properties of Hyperbranched Polytriazoles 3821
