Tandem Photoinduced Cationic Polymerization and CuAAC for

Article pubs.acs.org/Macromolecules

Tandem Photoinduced Cationic Polymerization and CuAAC for Macromolecular Synthesis Sean Doran,† Gorkem Yilmaz,† and Yusuf Yagci*,†,‡ †

Istanbul Technical University, Department of Chemistry, Maslak, Istanbul 34469, Turkey Center of Excellence for Advanced Materials Research (CEAMR) and Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia

‡

S Supporting Information *

ABSTRACT: A novel synthetic strategy involving sequential photoinduced cationic and copper-catalyzed azide−alkyne cycloaddition (CuAAC) click processes for the synthesis of complex macromolecular structures such as sidechain functional polymers, graft copolymers, and organogels is described. In the approach, first a set of copolymers possessing side-chain alkyne or halide functionalities, namely, poly(cyclohexene oxide-co-glycidyl propargyl ether) (P(CHO-co-GPE)), poly(cyclohexene oxide-co-epichlorohydrin) (P(CHO-coECH)), and poly(tetrahydrofuran-co-epichlorohydrin) (P(THF-co-ECH)), were synthesized by photoinitiated free-radical-promoted cationic copolymerization of the corresponding monomers using phenylbis(2,4,6trimethylbenzoyl)phosphine oxide (BAPO) and diphenyl iodonium hexafluorophosphate (Ph2I+PF6−) as free radical photoinitiator and oxidant, respectively. While P(CHO-co-GPE) readily contained clickable alkyne side-chains, the halide groups of P(THF-co-ECH) were converted to azide groups by conventional azidation procedure using NaN3 in DMF. Model side-chain functionalization, grafting onto, and organogel formation were demonstrated by using P(CHO-co-GPE) and azidated P(THF-co-ECH) via photoinduced CuAAC reactions. The intermediate polymers formed in various stages and final polymers were characterized by spectral analysis and gel permeation chromatography.

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generally include onium salts,6 such as diphenyliodonium,7 triphenylsulfonium,8 or alkoxypyridinium salts;9 however, such initiating systems do not absorb at wavelengths above 300 nm, and various strategies have been developed to carry out cationic polymerization at the near-UV−vis region of the electromagnetic spectra. These strategies include the use of free radical photoinitiators for radical promoted cationic polymerization,10 utilization of the charge transfer complexes,11 and use of singlet and triplet photosensitizers.12 In the free-radical-promoted cationic polymerization, the radicals generated by the photolysis of the initiator are oxidized by the onium salts to yield the corresponding cationic species. Thus, the irradiation wavelength for polymerization can be tuned by the choice of the free radical photoinitiator. Although a wide range of photoinitiators have been successfully used in this redox process,13 acyl phosphine oxides appeared to be one of the most suitable oxidants due to their long wavelength absorption characteristics and redox properties. Moreover, depending on the type of the monomers used, these promoters may generate additional cationic species. The photochemically generated phosphonyl radicals are highly reactive toward vinyl monomers due to their tetrahedral structure, and the formed adduct

INTRODUCTION Photopolymerization and its advantages are well-documented and make it an attractive process for the production of polymeric and macromolecular structures.1 These advantages include temporal and spatial control and high rates of polymerization with often no need for use of volatile organic solvents giving rise to environmental and health benefits.2 Economy of energy can be obtained with the use of photo conditions as opposed to thermal, which can be desirable, especially in an industrial setting. These advantages underline the importance of photopolymerization as an example of a “green” process, which is such an important concept in the 21st century as we move toward a more sustainable and less wasteful world.2c,3 One of the primary advantages of cationic polymerization is its activity despite presence of oxygen unlike radical polymerization processes. It also does not suffer from typical radical termination processes because the polymerization is propagated by ionic species. Cationic polymerization is widely applied in the polymerization of epoxide and vinyl-ether-type monomers, finding particular applications commercially in the production of coatings, inks, and adhesives.4 As regards cationic photopolymerization, lifetimes of the propagating ionic species with respect to their radical counterparts can be of benefit as propagation can continue in the dark once initiated by light.5 Photoinitiating systems for cationic photopolymerizations © XXXX American Chemical Society

Received: August 25, 2015 Revised: September 21, 2015

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Table 1. Photoinduced Cationic Copolymerization and the Molecular Weight Characteristics of the Polymers Obtaineda entry

t (min)

monomers (A/B)

feed ratio A/B (eq )

composition (A/B)b

conv. (%)c

Mn (g mol−1)d

Mw/Mnd

1 2 3 4 5 6 7 8

30 60 100 100 100 60 60 60

CHO/GPE CHO/GPE CHO/ECH CHO/ECH CHO/ECH CHO/ECH THF/ECH THF/ECH

5/1 5/1 4/1 2/1 1/1 1/1 3/1 2/1

19.0/1.0 19.8/1.0 9.8/1.0 8.1/1.0 3.7/1.0 4.1/1.0 5.1/1.0 4.4/1.0

23.0 27.7 37.1 19.2 18.1 17.3 14.4 8.9

3820 4000 4100 4000 3800 4800 20 600 12 200

1.7 1.8 2.1 1.7 2.1 1.7 1.4 1.4

Entries 1−2: CH2Cl2/CHO/BAPO/Ph2I+PF6− = 1600:1600:1:1, [BAPO] = 0.002 M; Entries 3−6: CH2Cl2/CHO/BAPO/Ph2I+PF6− = 1600:1600:1:1, [BAPO] = 0.002 M; Entries 7−8: no solvent, THF/BAPO/Ph2I+PF6− = 1000:1:1, [BAPO] = 0.006 M, all reactions were carried out under irradiation at 400−500 nm with a light intensity of 45 mW cm−2 at room temperature. bAs determined by 1H NMR. cDetermined gravimetrically. dDetermined by GPC according to PS standards. a

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radicals can efficiently be oxidized to the corresponding carbocations. With cyclic monomers, however, they undergo a hydrogen abstraction from solvent or monomer present, giving rise to carbon centered radicals that do interact with the onium salt, thereby forming cationic initiating species.13a Such photoreduction reactions are also applied for the in situ generation of inorganic seminoble or noble metals. The most important applications of this type of processes are nanocomposite fabrication and the copper-catalyzed azide alkyne cycloaddition (CuAAC) reactions. CuAAC has been perhaps the most important topic for the synthetic polymer community in the past decade and is considered a member of the “click reactions” family, which exhibits distinct characteristics such as conservation of atom economy, rapidity, and ultraefficiency without any side products. The reaction requires Cu(I) compounds as catalysts, which are sensitive to oxygen and need sophisticated experimentation. Photochemical protocols, as well as other approaches, were developed to generate Cu(I) in the reaction media starting from Cu(II) compounds.14 For example, the use of radical photoinitiators was shown to reduce Cu(II) to Cu(I) under light irradiation, which, in turn, catalyzed the “click” reaction.15 This strategy was used to obtain telechelic polymers and block copolymers as previously described.15,16 The same strategy can also be achieved by electrochemical means.17 In this connection, it should be pointed out that the in situ generation of Cu(I) species by photochemical means has been extensively employed in atom transfer radical polymerization.18 Graft polymers are of keen interest due to their applicability for use in biosensors,19 super soft elastomers,20 photonic,21 polyelectrolytes,22 and drug delivery systems23 among others. One of the routes to obtain graft polymers has been termed the “grafting onto” process, whereby polymer chains are attached in bulk to a polymer backbone.24 The “grafting onto” approach is ideal for the use of click chemistry25 due to its nature of being a high-yielding and highly modular process.26 In this work, we describe the photoinduced free-radical-promoted cationic polymerization yielding clickable copolymers, for the synthesis of graft polymers by the “grafting onto” method, via click chemistry. In our group, we have specialized for some time in the research and publication of work on clickable polymers and hydrogels.27 To the best of our knowledge, it is a unique approach to use cationic photopolymerization for the synthesis of clickable polymers and macromolecular structures.

EXPERIMENTAL SECTION

Materials. Bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (BAPO, Ciba specialty chemicals), diphenyliodonium hexafluorophosphate (Ph2I+PF6−, 98%, Alfa Aesar), sodium azide (NaN3, 99%, Aldrich), copper(II) chloride (CuCl2, 98%, Aldrich) ethyl acetate (Carlo Erba, HPLC grade), and dimethylformamide (EMPARTA, HPLC grade) were used as received. (±)-Epichlorohydrin (ECH, 99%, Alfa Aesar) and glycidyl propargyl ether (GPE, 90%, Aldrich) were vacuum-distilled prior to use. N,N,N′,N′,N″-Pentamethyldiethylenetriamine (PMDETA; 99%, Aldrich) was distilled before use. Poly(ethylene glycol) methyl ether (Me-PEG, Mn: 2000 g mol−1, Aldrich) was used as received. Tetrahydrofuran was distilled over sodium and benzophenone. Cyclohexene oxide (CHO) was distilled and dried over MgSO4. CH2Cl2 was distilled over CaCl2. Characterization. 1H NMR and 13C NMR of the intermediates and final polymers were recorded in CDCl3 at room temperature at 500 MHz on an Agilent VNMRS 500 spectrometer. FT-IR spectra were recorded on a PerkinElmer FT-IR Spectrum One spectrometer with an ATR Accessory (ZnSe, PikeMiracle Accessory) and an admium telluride detector. The resolution was 4 cm−1, and 24 scans were performed with a 0.2 cm s−1 scan speed. Molecular weights and polydispersities of the polymers and the block copolymer were measured using gel permeation chromatography (GPC) employing an Agilent 1100 instrument equipped with a differential refractometer using THF as the eluent at a flow rate of 0.3 mL min−1 at 30 °C. Molecular weights were determined using polystyrene standards. General Procedure for the Synthesis of Poly(cyclohexene oxide-co-glycidyl propargyl ether) (P(CHO-co-GPE)). Ph2I+PF6− was added to a flame-dried glass tube under nitrogen followed by BAPO, CH2Cl2, and CHO (CH2Cl2/CHO/BAPO/Ph2I+PF6− = 1600:1600:1:1, [BAPO] = 0.002 M). The reaction flask was irradiated by a Ker-Vis blue photoreactor equipped with a circle of 6 lamps (Philips TL-D 18W) emitting light nominally at 400−500 nm with a light intensity of 45 mW cm−2 at room temperature before GPE was added on top of the reaction. It was then allowed to stir again for a given amount of time with irradiation at the same frequency before stirring was stopped. The reaction mixture was then precipitated into excess cold methanol. The precipitate was filtered off and dried in vacuo overnight to yield the polymer as a white solid. (Yields are depicted in Table 1.) 1H NMR (CDCl3) (δ): 4.27 (d, J = 2.4 Hz, HCC−CH2−), 4.23 (dd, J = 5.7, 2.4 Hz, HCC−CH2−), 4.19 (br m,, HCC−CH2−), 3.84 (dd, J = 11.3, 3.0 Hz, −OCH(CH2)CH2−), 3.80−3.60 (br d, −OCH2CH−, −OCH2CH−, −CH2OCH2−), 3.59− 3.26 (br m, −OCH(CH)CH2−), 2.05−1.15 (br m, −CH2CH2CH2−). 13 C NMR (CDCl3) (δ): 80.0−75.2 (−CHCH−, CHO), 70.3−67.9 (−OCH2CH(O)CH2O−, GPE), 58.5 (HCC−CH2−), 30.4−28.0 (−CH2CH2CH2−, CHO), 23.5−21.2 (−CHCH2CH2−, CHO). IR (νmax): 2935 (br s), 2855 (s), 2118 (small s), 1447 (s), 1080 (br s) cm−1. General Procedure for the Synthesis of Poly(cyclohexene oxide-co-epichlorohydrin) P(CHO-co-ECH) and Poly(tetrahydrofuran-co-epichlorohydrin) P(THF-co-ECH). B

DOI: 10.1021/acs.macromol.5b01857 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of Clickable Copolymers Using Cationic Photopolymerization

Ph2I+PF6−, BAPO, monomer (CHO or THF), and halogen functional monomer (ECH) were added to a flame-dried glass tube under nitrogen with CH2Cl2 as solvent where indicated (For P(CHO-coECH): CH2Cl2/CHO/BAPO/Ph2I+PF6− = 1600:1600:1:1, [BAPO] = 0.002 M (Entries 3−6) and for P(THF-co-ECH): no solvent, THF/ BAPO/Ph2I+PF6− = 1000:1:1, [BAPO] = 0.006 M (Entries 7 and 8)). The reaction vessel was irradiated by a Ker-Vis blue photoreactor equipped with a circle of six lamps (Philips TL-D 18W) emitting light nominally at 400−500 nm with a light intensity of 45 mW cm−2 at room temperature. Upon completion of the allotted time the reaction mixture was diluted with CH2Cl2 and precipitated in excess cold methanol. The precipitates were filtered off and dried in vacuo overnight to yield the polymer as a white solid. (Yields are depicted in Table 1.) Poly(cyclohexene oxide-co-epichlorohydrin) (P(CHO-co-ECH)). 1 H (CDCl3) (δ): 3.78 (br m, −OCH(CH2)CH2−), 3.67(br m, ClCH2−, −OCH2CH−), 3.58−3.26 (br m, ClCH2−, −OCHCHO−, −OCH2CH−), 2.05−1.15 (br m, −CHCH2CH2−, −CH2CH2CH2−). 13 C NMR (500 MHz, CDCl3, δ): 78.9−75.0 (−CHCH−, CHO, −OCH(CH2)CH2, ECH), 69.0 (−OCH2CH−, ECH), 45.4−44.4 (ClCH 2 −), 30.8−28.7 (−CH 2 CH 2 CH 2 , CHO), 24.7−21.1 (−CHCH2CH2− CHO). IR (νmax):2931 (br s), 2853, 1496−1438 (m), 1373(s), 1107 (br s) cm−1. Poly(tetrahydrofuran-co-epichlorohydrin) (P(THF-co-ECH)). 1H NMR (CDCl3) (δ): 4.08 (br s, −CHOH), 3.97 (br s, −CH−), 3.71 (d, J = 5.3 Hz, −CH2Cl), 3.69−3.48 (m, −CH2CHO−, −CH2Cl), 3.42 (br s, −OCH2−), 1.63 (br s, −CH2CH2CH2−). 13C NMR (CDCl 3 ) (δ): 78.2 (−CH 2 CH(CH 2 )O−, ECH), 71.4−71.2 (−OCH2CH−, ECH) 70.5 (−OCH2CH2−, THF), 46.0−43.9 (−CH2Cl, ECH), 26.5 (−OCH2CH2−, THF). IR (νmax): 2944 (br s), 2860 (br s), 2795 (small s), 1484−1438 (m), 1366 (s), 1101 (br s), 990 cm−1. Poly(tetrahydrofuran-co-(2-epiazidohydrine)) (P(THF-co-EAH)). P(THF-co-ECH) (0.6 g, 1.040 mmol) and NaN3 (0.088 g, 1.350 mmol) were stirred in DMF (5 mL) at 95 °C under nitrogen in a flame-dried glass tube. Stirring with heating was continued for 12.5 h before heating was stopped and the vessel was allowed to cool. It was then diluted with excess THF and filtered of salts though filter paper. The solvent of the filtrate was reduced in vacuo before being precipitated in cold methanol. The next day it was isolated by gravity filtration, and it was allowed to fully drain at −20 °C for several hours before it was added to the oven to be dried in vacuo overnight. The polymer was obtained as a white solid (0.19 g, 32% yield, Mn = 28990, PDI = 1.4). 1H NMR (CDCl3) (δ): 3.96 (m, −CH2CH(OH)CH2−), 3.70−3.47 (m, −OCH(CH2)CH2−, −OCH2CH(CH2)O−), 3.42 (br s, −OCH2CH2−), 2.72 (br s, N3CH2−), 2.63 (br s, N3CH2−), 1.62 (br s, −OCH2CH2CH2−). 13C NMR (CDCl3) (δ): 78.24 (N3CH2CH(O−)CH2−), 71.43 (N3CH2CH(O−)CH2−), 70.55

(−OCH2CH2CH2−), 52.08, 53.47 (N3CH2−), 26.53 (−OCH2CH2CH2−). IR (νmax): 2944 (br s), 2853 (br s), 2801 (small s), 2100 (small br s), 1671, 1496−1438 (m), 1373, 1101 (large s), 990 cm−1. Synthesis of ω-Azide Functional Polyethylene Glycol (PEG-N3). Me-PEG (Mn: 2000 g/mol, 1 equiv) was dissolved in 25 mL of CH2Cl2. Tosyl chloride (3 equiv) was carefully added to the reaction mixture, which was left to stir overnight at room temperature. The reaction mixture was diluted and precipitated in the diethyl ether and filtered of to give the intermediate polymer which was dried in the oven. Next, thus-obtained polymer was dissolved in DMF in a roundbottomed flask and NaN3 (3 equiv) was added to the mixture. The reaction was stirred overnight and water was added. Then, the mixture was three times extracted with CH2Cl2. The combined organic phases were dried by Na2SO4 and filtered and the solvents were completely removed under vacuum (Mn,GPC: 1800, Mw/Mn: 1.07). General Procedure for the Synthesis of Side Chain Functional Polymers by CuAAC. The clickable polymers (with azido or alkyne side chain functionalities, 1 equiv) and phenyl acetylene or 3azido-2-methylprop-1-ene (matching equivalent concentrations) in appropriate amounts were dissolved in THF in a flask. PMDETA and CuCl2 (equal to the concentrations of alkyne or azide functionalities) were added to the reaction media, and the solution was purged of oxygen by fluxing nitrogen through for 10 min. The reaction flask was irradiated by a Ker-Vis blue photoreactor equipped with a circle of six lamps (Philips TL-D 18W) emitting light nominally at 400−500 nm with a light intensity of 45 mW cm−2 at room temperature. After 6 h, the solution was passed through neutral alumina to remove the inorganic salts and then precipitated in cold methanol and allowed to sit at −20 °C for at least 1 h. The precipitate was then filtered off and dried under vacuum. Synthesis of P(CHO-co-GPE)-g-PEG by CuAAC. P(CHO-co-GPE) (1 equiv-6.5 equiv acetylene units) and PEG-N3 (13 equiv) were taken in a flask with a magnetic stirrer and dissolved in THF. PMDETA (6.5 equiv) and CuCl2 (6.5 equiv) were added, and the solution was purged of oxygen by fluxing nitrogen through for 10 min. The reaction flask was irradiated at visible light, as previously described for 6 h. After the completion of the reaction, the solution was passed through neutral alumina to remove the inorganic salts and then precipitated in cold methanol and allowed to sit at −20 °C for at least 1 h. The precipitate was then filtered and dried under vacuum.

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RESULTS AND DISCUSSION Preparation of Poly(cyclohexene oxide-co-glycidypropargyl ether) (P(CHO-co-GPE)) by Photoinduced Cationic Polymerization. It was conceived that to introduce clickable functionality via cationic photopolymerization it would be possible to use glycidyl propargyl ether (GPE) as a

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GPE with respect to CHO into the main chain was deduced to be 19:1 (CHO/GPE) in all cases. Preparation of Poly(cyclohexene oxide-co-epichlorohydrine) (P(CHO-co-ECH)) and Poly(tetrahydrofuranco-epiazidohydrine) (P(THF-co-EAH)). In a similar strategy, CHO was copolymerized with epichlorohydrin (ECH) to give chloro functionality on the side chain of the polymer. The photoinduced cationic copolymerization of CHO and ECH using the BAPO/Ph2I+PF6− initiation system was also found to be successful. The overall process is presented in Scheme 1. Table 1 depicts the molecular weight characteristics of the polymers obtained in this process (Runs 3−6). As can be seen, decreasing the ratio of CHO in the feed results in lower polymer conversions due to the poor propagating rate of ECH. Moreover, decreasing the time yields polymers in comparably lower conversions, as can be seen in Table 1, entry 6. 1 H NMR was used to confirm the structures of the polymers obtained. In the region from 3.88 to 3.62 ppm, the CH and CH2 peaks pertaining to the ECH and the characteristic signals corresponding to the CH groups of CHO were observed, and in the region from 3.61 to 3.21 ppm a large broad multiplet was observed. (See Supporting Information, FS1). To achieve more favorable azidation conditions, the same strategy by replacing CHO with a more polar monomer, tetrahydrofuran (THF), was applied. Thus, ECH was photochemically copolymerized with THF under similar experimental conditions to afford P(THF-coECH) (Scheme 1). Table 1 summarizes the polymerization conditions, composition, and molecular weight characteristics of the copolymers obtained (Runs 7 and 8). The ratio of THF and ECH monomer incorporation into the polymer chain was judged by 1 H NMR by comparing the combined integration of the CH and CH2 of ECH fraction residing between 3.72 and 3.47 ppm with the integration of the OCH2 signals of THF fraction present at 3.42 ppm. Again, the greater incorporation of THF with respect to ECH can be attributed to its relatively greater reactivity (Supporting Information, FS2). P(THF-co-ECH) was sufficiently soluble in most of the organic solvents so that the copolymer could be effectively azidated to obtain poly(tetrahydrofuran-co-epiazidohydrine), P(THF-co-EAH), by stirring the polymer with NaN3 in DMF for 12 h at 95 °C (Scheme 1). Side-Chain Functionalization by Photoinduced CuAAC. To demonstrate the synthetic versatility and modularity of the approach, we have conducted several CuAAC reactions. First, both alkyne and azide functional polymers were attempted to be functionalized by the antagonist model compounds. The model compounds were selected so as to allow spectral characterization of the ultimate click product through the functional groups present in the structure. In the process, we applied Cu(II) in the presence of photoreducing agents, BAPO. Photolysis of BAPO under proper irradiation wavelengths leads to radicals, which reduces the Cu(II) to Cu(I) complexes mediating the click reactions. Photoreduction technique does not require inert atmosphere as higher oxidation state copper is reduced during the irradiation process. Functionalization of P(CHO-co-GPE) by Photoinduced CuAAC. P(CHO-co-GPE) was clicked with 3-azido-2-methylprop-1-ene in the presence of Cu(II)/PMDETA and BAPO under visible-light irradiation (Scheme 3). The success of the reaction was followed by 1H NMR spectroscopy (Figure 1). The acetylenic protons observed

comonomer with cyclohexene oxide (CHO). To initiate the photoinduced cationic copolymerization of cyclohexene oxide (CHO) and GPE, we used diphenyliodonium hexafluorophosphate (Ph2I+PF6−) and bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (BAPO) photoinitiating system (Scheme 1). During the irradiation, GPE was slowly added to the polymerization mixture and allowed to stir because GPE is a less reactive monomer in comparison with CHO. Experiments conducted by the addition of the two monomers at once failed to produce any precipitable polymer. It is well known26 that the photopolymerization of alkyl glicydyl ethers, initiated by photochemically formed Bronsted acids, displays a prominent induction period at room temperature, as the result of the formation of long-lived, relatively stable secondary oxonium ions (Scheme 2). Scheme 2. Formation of Secondary Oxonium Ions

It was reported that the input of only a small amount of thermal activation energy was required to induce the further reaction of these species. In our case, the independently generated oxonium ions formed during CHO polymerization enable the polymerization to proceed. As confirmed by 1H NMR analysis, the obtained polymer contained GPE segments in the structure. Table 1 depicts the polymerization conditions and the molecular weight characteristics of the polymers obtained (Runs 1 and 2). Clearly, increasing the time gave rise to higher conversions without very much affecting the molecular composition of the structures, which were verified by 1H NMR analyses (vide infra). 1 H NMR confirmed incorporation of the propargyl ether groups into the copolymer chain as pendant functions (Figure 1A). The peaks appearing in the 4.26 to 4.16 ppm region could

Figure 1. 1H NMR spectra of P(CHO-co-GPE) (A) and its click product (B).

be attributed to the propargyl CH2 groups, and the signals between 3.85 and 3.61 ppm corresponded to CH and CH2 protons coming from the GPE fraction. The large broad peaks between 3.61 and 3.27 ppm are the characteristic signals of the PCHO moiety. By comparing the alkyne adjacent CH2 signal with the characteristic signals of CHO, the incorporation of D

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Macromolecules Scheme 3. Functionalization of Copolymers by Photoinduced CuAAC

functionalities were subjected to CuAAC with azide terminal PEG in the presence of CuCl2/BAPO catalyst system. The process is demonstrated in Scheme 3. The success of the click reaction was confirmed by 1H NMR spectroscopy. The NMR spectrum of P(CHO-co-GPE)-g-PEG showed the characteristic signals of both copolymer and PEG components, as can be seen in Figure 3. The peaks at 3.1 and

around 3.1 ppm and the CH2 protons of the propargyl groups at 4.2 ppm of the precursor polymer completely disappeared after the click process. The new signal in the 7.5 ppm region corresponds to the triazole protons, whereas the peaks between 5.2 and 5.0 ppm could be attributed to the double-bond protons on the side chain. Functionalization of P(THF-co-EAH) by Photoinduced CuAAC. The process can also be achieved by reverse click components, that is, polymer with azide functionality and model compound with alkyne functionality. Thus, P(THF-coEAH) was reacted with phenyl acetylene by photoinduced click reaction in a very similar procedure previously described (Scheme 3). Figure 2 shows the clear new signals appearing in the aromatic region after the click reaction. The signals around 7.8 ppm correspond to the triazole protons, whereas the peaks between 7.3 and 7.5 ppm can be attributed to the protons of the phenyl group. Grafting of PEG-N3 onto P(CHO-co-GPE) by Photoinduced CuAAC. (P(CHO-co-GPE)) with acetylene pendant

Figure 3. 1H NMR of P(CHO-co-GPE) (black) and P(CHO-coGPE)-g-PEG (blue).

4.1 ppm corresponding to the acetylenic CH and propargyl CH2 protons, respectively, were found to disappear after the grafting process. The new peaks appearing at 4.5 and 7.7 ppm confirmed the success of the CuAAC reaction, as can be seen in Figure 3. Figure 4 demonstrates the GPC traces of the precursor polymers (P(CHO-co-GPE) and PEG-N3 and the resulting graft copolymer (P(CHO-co-GPE)-g-PEG) after CuAAC reaction. The GPC trace of the graft copolymer exhibits a unimodal distribution indicating the absence of any side reactions during the grafting process. Expectedly, the GPC trace of the graft copolymer appeared in the higher molecular weight region, which also confirms the success of grafting.

Figure 2. 1H NMR spectra of P(THF-co-EAH) (black) and its click product (blue). E

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fact, such functionalities may be useful for further functionalization particularly for bioapplications.27c,27d

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CONCLUSIONS In conclusion, we have presented for the first time the utility of the cationic photopolymerization to produce copolymers with clickable functionalities. By employing the BAPO initiator along with diphenyliodonium salt under irradiation at >350 nm CHO and GPE could be copolymerized to achieve P(CHO-co-GPE). Similarly, CHO and THF were copolymerized with ECH to afford P(CHO-co-ECH) and P(THF-co-ECH) having chloro functionalities as pendant groups. By substituting the chloro groups to azido functions, clickable polymers were readily obtained. CuAAC reactions using CuCl2/BAPO photoinitiating system were employed to afford side-chain functional polymers and graft copolymers. The two new clickable copolymers P(THF-co-EAH) and P(CHO-co-GPE) were also clicked together by photo CuAAC to form a cross-linked organogel material. Although these results are preliminary in nature, they clearly demonstrate the simplicity, versatility, and modularity of the approach, which may lead to new pathways for the preparation of functional materials for specific applications.

Figure 4. GPC traces of PEG-N3, P(CHO-co-GPE), and P(CHO-coGPE)-g-PEG.

Organogel Formation. Because of the multifunctional nature of the copolymers, a polyether-type organogel was obtained upon CuAAC (Scheme 4).

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Scheme 4. Photoinduced CuAAC of P(THF-co-EAH) with P(CHO-co-GPE)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01857. 1

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H NMR spectra of P(CHO-co-ECH) and P(THF-coECH). (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. The formed organogel was then swollen in CH2Cl2. Upon soaking in MeOH/H2O with PMDETA, some but not all of the trapped copper salts was leached out (Figure 5a,b). FTIR of the clicked organogel confirmed the presence of azide peaks at 2100 cm−1(Figure 5c). This is expected because exact concentrations of the azido and alkyne functions cannot be estimated. Moreover, they may not be available in the close proximity as the photoinduced cross-linking commences and remains trapped inside the gel structure without reacting. In

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS ̇ AK for the 2236 CoWe acknowledge and thank TÜ BIT Funded Brain Circulation Scheme (Co-Circulation scheme), partially supported by the EC-FP7Marie Curie Actions-Peoplė EB as a source of funding. COFUND and coordinated by BID

Figure 5. Images of organogel prepared by photoinduced CuAAC swollen with CH2Cl2 (a) and its rubbery form after removal of the solvent (b). FTIR of organogel after shrinking (c). F

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DOI: 10.1021/acs.macromol.5b01857 Macromolecules XXXX, XXX, XXX−XXX