Powerful Ring-Closure Method for Preparing Varied Cyclic Polymers

Jun 3, 2014 - Journal of the American Chemical Society 2017 139 (44), 15812-15820. Abstract | Full .... Bin Mu , Qian Li , Xiao Li , Shi Pan , Yang Zh...
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Powerful Ring-Closure Method for Preparing Varied Cyclic Polymers Qingquan Tang, Ying Wu, Peng Sun, Yongming Chen, and Ke Zhang* State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: A powerful ring-closure method was developed for the formation of cyclic polymers by combining reversible addition−fragmentation chain transfer polymerization (RAFT) and light-induced Diels−Alder click reaction. The outstanding features of this novel method were demonstrated from the following four aspects. This convenient and efficient technique could produce cyclic polymers in air at room temperature without any other catalyst or stimulus requirements other than a mild UV irradiation. The universality of this method was demonstrated by successfully producing five different types of cyclic homopolymers including polystyrene, poly(methyl methacrylate), poly(tert-butyl acrylate), poly(N,N-dimethylacrylamide), and poly(2-vinylpyridine) and two kinds of block copolymers of poly(methyl methacrylate)-block-polystyrene and poly(methyl methacrylate)-block-poly(tert-butyl acrylate). This is the first time to report a one-pot ring-closure method for the formation of cyclic polymers, in which the crude monomer polymerization solution was directly diluted as precursors and no more requirements were needed to purify and postfunctionalize the linear polymer intermediates. When combing with a batchwise operation, this novel light-induced ring-closure method could significantly improve the notorious disadvantage of low cyclic polymer yield accompanied by the ring-closure strategy.

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poly(methyl methacrylate) (PMMA),13 polystyrene (PS)28 and poly(N-isopropylacrylamide),25,26 but also a library of cyclic topologies,32−43 such as eight and tadpole shapes. In addition, ATRP has been combined with atom transfer radical coupling14,44 and ring-closing metathesis coupling15,24 reactions to produce cyclic polymers successfully. However, all of the aforementioned cyclization strategies do require large amount of metal catalysts for a high cyclization efficiency, which may cause the contamination of the resultant cyclic polymers. The metal-free cyclization methods have been developed on the combination of ATRP and Diels−Alder reaction,17,21 in which the cyclic PS, PMMA, poly(tert-butyl acrylate) (PtBA), and their block copolymers have been successfully prepared from the Diels−Alder reaction of maleimide/anthracene21 and maleimide/cyclopentadiene.17 Unfortunately, these two methods all require high temperature (refluxing in toluene) to carried out the cyclization reaction and suffer from multistep end group modifications of the linear polymer precursors. Reversible addition−fragmentation chain transfer polymerization (RAFT) plays a critical role to prepare varied cyclic polymers in the absence of the metal catalysts,10−12,45 since the highly reactive mercapto groups could be conveniently produced at the end of polymer chains by aminolyzing or reducing the thiocarbonylthio end groups. The thiol-related click chemistry including thiol−ene11,12 or thio−bromo45

INTRODUCTION Cyclic polymers have gained more and more interest in the recent developments of topological polymers.1−9 Compared to their linear counterparts, the endless molecular topology endows the cyclic polymers with significantly different characteristics, such as a smaller radius of gyration and hydrodynamic volume, lower melt viscosity, and higher thermostability.1−9 To date, the known synthesis methods for cyclic polymers can be generalized into two categories: the ringexpansion and ring-closure strategies. Although the ringexpansion techniques could produce cyclic polymers with high purity and large molecular weight at concentrated solution, it is hardly to control the molecular weight and polydispersity of the resultant cyclic polymers. Comparatively, the ring-closure methods hold the advantage to prepare well-defined cyclic polymers with a medium and small molecular weight. In addition to the simple monocyclic topology, the ring-closure strategy takes advantage to produce the cyclic polymer derivatives with complex architectures, such as theta and eight shapes. The combination of controlled/living radical polymerization (CRP) and highly efficient coupling chemistry has been demonstrated as a convenient and powerful ring-closure strategy for preparing cyclic polymers.10−31 The most popular method was established on the combination of atom transfer radical polymerization (ATRP) and copper-catalyzed azide− alkyne cycloaddition reaction (CuAAC),13,16,20,22,25,26,28 first reported by Grayson’s group.28 To date, this method has been used to prepare not only a variety of cyclic polymers including © 2014 American Chemical Society

Received: April 16, 2014 Revised: May 25, 2014 Published: June 3, 2014 3775

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Figure 1. Preparation of linear polymers by RAFT polymerization and the corresponding cyclic polymers by light-induced Diels−Alder click reaction.

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reactions could then be performed for the formation of cyclic polymers in situ. However, the resultant cyclic polymers from above methods still suffer from the purification process to remove the introduced aminolysis or reduction agents and the side products from the cleavage of thiocarbonylthio end group, especially considering the notorious low yield of cyclic polymer from ring-closure strategy. Light-induced Diels−Alder click reactions have earned rapid developments in recent years, since they achieve high reaction efficiency under mild conditions without byproduct production and catalyst requirement.46−52 This facilitates them to become ideal techniques for preparing topological polymers. The pioneered works have been done by Barner−Kowollik and co-workers in this field.46,48,49,51,52 They have successfully employed the Diels−Alder reaction between light-induced photoenols and activated alkenes (e.g., maleimide and acrylate moieties) to prepare block copolymers51,52 and cyclic polyesters46 most recently. In addition, the photoenols have been demonstrated to behave high cycloaddition efficiency with the dithioester moieties of RAFT polymers for the formation of block copolymers.48 Inspired by this, if a functional RAFT agent was designed with an orthoquinodimethane end group (the precursor of photoenol), a “universal” ring-closure method for preparing varied cyclic polymers could be developed based on the combination of RAFT and light-induced Diels−Alder click reaction, considering that RAFT is one of the most powerful CRP and allows for the preparation of a great variety of polymers. Herein, the functional RAFT agent 1 (Figure 1) has been developed to demonstrate the above idea. Taking advantaging of the standard RAFT polymerization procedure, five different types of linear homopolymers and two kinds of block copolymers were synthesized including PS, PMMA, PtBA, poly(N,N-dimethylacrylamide) (PDMA), poly(2-vinylpyridine) (P2VP), poly(methyl methacrylate)-block-polystyrene (PMMAb-PS), and poly(methyl methacrylate)-block-poly(tert-butyl acrylate) (PMMA-b-PtBA) (Figure 1, first step). By irradiating the diluted solution of these linear precursors under UV light, a library of pure cyclic polymers were obtained conveniently by evaporating the solvents without any more requirement for further purification (Figure 1, second step).

EXPERIMENTAL SECTION

All experimental information was detailed in the Supporting Information.

RESULTS AND DISCUSSION Preparation of RAFT Agent 1 and Linear Polymers. Functional RAFT agent 1 was synthesized by a DCC coupling reaction between 4-cyano-4-((thiobenzoyl)sulfanyl)pentanol and 2-(3-hydroxypropoxy)-6-methylbenzaldehyde. The detailed synthesis procedures were shown in Supporting Information. Figure S1A shows the 1H NMR and corresponding peak assignments. Figure 2 (black curve) shows the UV−vis

Figure 2. UV−vis spectra of RAFT agent 1 (black), the resultant linear PS (red) and corresponding cyclic PS (blue) in THF.

spectrum, in which a typical strong adsorption peak was observed at 305 nm ascribed to the characteristic π−π* transition of the thiocarbonyl moiety of RAFT agent. The agent 1 was then used to polymerize varied types of monomers including styrene (St), methyl methacrylate (MMA), tert-butyl acrylate (tBA), N,N-dimethylacrylamide (DMA), and 2-vinylpyridine (2VP) by a standard RAFT process. Table 1 summarizes the syntheses and characterizations of the resultant telechelic polymers bearing the orthoquinodimethane and dithioester end groups, from which the molecular weight distributions were well-controlled below 1.1 for all cases. On the basis of the well-defined linear 3776

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Table 1. Synthesis and Characterization of Linear PS, PMMA, PtBA, PDMA, P2VP, PMMA-b-PS, and PMMA-b-PtBA runa

polymer

feed ratiob

time (h)

convn (%)c

Mnd

Mw/Mnd

1 2 3 4 5 6 7 8 9

PS PMMA PtBA PDMA P2VP PMMA PMMA-b-PS PMMA PMMA-b-PtBA

300:1:0 300:1:0.2 200:1:0.2 300:1:0.1 300:1:0.2 100:1:0.2 300:1:0 100:1:0.2 200:1:0.2

3.5 2 4 10.5 3.5 3 2 3 2

8.2 28.2 15.2 22.7 22.1 30.1 8.1 32.7 20.4

3330 10 080 3470 5440 28 180 3290 6290 4190 8630

1.04 1.10 1.09 1.06 1.06 1.10 1.10 1.08 1.08

RAFT polymerizations were all performed at 60 °C except for runs 1 and 7, in which the polymerization temperature was used as 110 °C. bInitial molar ratio of monomer/RAFT agent/AIBN. cCalculated from the 1H NMR spectrum. dCalculated from GPC, in which THF or DMF (only for run 5) were used as the eluent and polystyrene standards were used for the calibration. a

Figure 3. (A) GPC curves of linear PS (black) and the resultant cyclic PS (red); (B) GPC curves of PS with different exposure time on UV light; (C) GPC curves of cyclic PS prepared with purified (black) and unpurifed (red) linear precursors; (D) GPC curves of cyclic PS prepared in one batch (black) and five batch (red) methods. THF was used as the eluent, and polystyrene standards were used for the calibration.

peak was observed with Mn =3330 and Mw/Mn =1.04. Figure S1B (Supporting Information) shows the 1H NMR spectrum of purified PS, where the peaks at 10.66 (a), 7.86 (l), 4.25 (f), 4.10 (h), and 2.58 (b) ppm of end groups were inherited from the RAFT agent 1. Figure 2 (red curve) shows the corresponding UV−vis spectrum, in which the characteristic π−π* adsorption peak of the thiocarbonyl moiety were remained at 305 nm. This indicates the success of RAFT polymerization and the orthoquinodimethane and dithioester moieties was kept at the end of PS chains. The cyclic PS was subsequently prepared by a light-induced Diels−Alder click reaction to ring-close the linear precursor in a highly diluted solution. When dissolving 3 mg linear PS in 600 mL mixed solvents of acetonitrile/dichloromethane (v/v = 2/ 1), the UV irradiation was employed to produce photoenols from orthoquinodimethane end moieties. The Diels−Alder cycloaddition reaction was then performed between the in situ formed photoenols and dithioester moieties at the other end of linear PS at room temperature in air. After 9 h irradiation, the resultant cyclic PS was simply collected by evaporation of the solvent without any other additional purification process.

precursors, the regular low pressure mercury lamp (120 W) was selected as a UV light source to induce the Diels−Alder reaction between the end moieties for the formation of corresponding cyclic polymers. This novel light-induced ringclosure technique is indeed powerful in that a library of cyclic polymers could be quickly produced at room temperature in air without any more catalyst or stimulus requirements other than a mild UV irradiation. In addition, this technique allows the formation of cyclic polymer directly using the diluted monomer polymerization solution as precursors, avoiding the purification and postfunctionalization of linear polymer intermediates. To concisely and clearly present the novel ring-closure method in this paper, the formation of cyclic PS was selected as a representative to be demonstrated in detail as follows. Preparation of Cyclic PS. As shown in Table 1 (run 1), the polymerization of St was carried out in bulk at 110 °C and the St/RAFT agent ratio was used as 300/1. After 3.5 h thermal polymerization, a monomer conversion of 8.2% was obtained from 1H NMR characterization of crude polymerization solution. The GPC curve is shown in Figure 3A (black curve), in which a well-defined, monomodal, and symmetric 3777

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Figure 4. MALDI−TOF mass spectra for linear PS precursor and the corresponding cyclic PS.

The UV−vis, 1H NMR, GPC, and MALDI−TOF MS were used to verify the successful Diels−Alder reaction and the formation of cyclic polymers. As shown in the UV−vis spectra (Figure 2), the π−π* adsorption peak of the thiocarbonyl moiety at 305 nm were completely disappeared from the curve of the resultant cyclic PS (blue), compared to that of linear precursor (red curve). The 1H NMR spectrum of cyclic PS (Figure S1C) showed the complete disappearance of the peaks at 10.66 and 2.58 ppm (peaks a and b in Figure S1B) belonging to orthoquinodimethane end group of the linear precursor. In addition, a new peak was observed at 5.27 ppm, assigned to a′ proton of the new formed isothiochroman group48 (Figure S1C). These strongly indicated that a high efficiency was achieved for the light-induced Diels−Alder reaction under the used condition. The direct evidence of the cyclic PS formation was demonstrated by GPC and MALDI−TOF MS analyses. From Figure 3A, the GPC curve (red) of cyclic PS preserved the welldefined monomodal and symmetric peak shape but the whole peak position shifted to the lower molecular weight direction completely, compared to that (black curve) of the linear precursor. The integration of GPC curves shows a similar Mw/ Mn (1.05 for cyclic PS vs 1.04 for linear PS) and a much smaller Mn (2660 for cyclic PS vs 3330 for linear PS) between cyclic and linear PS. This is resulted from the characteristic smaller hydrodynamic volume of cyclic polymers than that of linear

counterparts, strongly indicating the successful formation of cyclic PS. Figure 4 shows the MALDI−TOF mass spectra of linear (A) and cyclic PS (B). From the full spectra (left), the absolute molecular weights were similar for both cases expanding from 2500 to 5500 and centring at 4000. Comparing to the much smaller apparent Mn of cyclic PS than that of linear precursor from GPC (Figure 3A), the similar absolute molecular weight indicated a more compact molecular structure for cyclic PS, again confirming the successful formation of cyclic PS. In addition, the detailed descriptions of the MALDI−TOF mass spectra are shown in Figure 4 (right) and Tables S1 and S2. In the expanded spectrum of linear PS (A), two main peak distributions were accurately assigned to linear PS having different chain ends. They completely disappeared from the expanded spectrum of cyclic PS (B), in which two new formed peak distributions were precisely ascribed to the cyclic PS with a cleavage of sulfur ionized with K+/Ag+. Furthermore, a regular m/z interval of ca. 104 was observed between neighboring peaks in each distribution for both cases, which corresponds to the molar mass of the St monomer unit. By virtue of the cyclic PS formation, the outstanding features of this novel ring-closure method were further explored from the following addtional aspects: cyclization rate and one-pot and batchwise operations. The cyclization rate was traced by GPC characterization of the resultant cyclic PS. Although the pure cyclic PS analyzed above was prepared with a 9 h UV 3778

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Figure 5. GPC curves of (A) linear PMMA (black) and the resultant cyclic PMMA (red), (B) linear PtBA (black) and the resultant cyclic PtBA (red), (C) linear PDMA (black) and the resultant cyclic PDMA (red), (D) linear P2VP (black) and the resultant cyclic P2VP (red), (E) linear PMMA-b-PS (black) and the resultant cyclic PMMA-b-PS (red), and (F) linear PMMA-b-PtBA (black) and the resultant cyclic PMMA-b-PtBA (red). THF or DMF (only for P2VP) was used as the eluent, and polystyrene standards were used for the calibration.

completely overlapped GPC curves with that from the regular one batch operation. As a resultant, by combining with the batchwise operation, the novel ring-closure method could produce pure cyclic polymers with a significantly increased yield. Preparation of Varied Cyclic Polymers. Inspired by the successful formation of cyclic PS, the universality of this novel ring-closure method has been further extended to prepare a variety of cyclic polymers. Four more types of homopolymers including PMMA, PtBA, PDMA, P2VP, and two kinds of block copolymers of PMMA-b-PS and PMMA-b-PtBA have been successfully prepared from agent 1 with a standard RAFT polymerization procedure. Table 1, Figure S2, and Figure 5 (black curves) summarized the respective polymerization condition and the corresponding GPC characterization, where the molecular weights were manipulated by monomer conversion and the polydispersities were well-controlled below 1.1 for all cases. On the basis of these well-defined linear precursors with varied chemical structure and property, a standard cyclization reaction condition was applied to produce varied cyclic polymers derived from the above formation of cyclic PS. In short, the cyclization reactions were performed by 9 h UV, irradiating the diluted linear precursor solutions in acetonitrile (3 mg/600 mL) at room temperature in air. The cyclic products were simply collected by evaporation of the solvent without any requirement of additional purification.

irradiation, the linear precursor could be nearly ring-closed in only 30 min under the same reaction conditions. This is evidenced by the almost overlapped GPC curves (Figure 3B) of cyclic PS prepared under 30 min (dark cyan) and 9 h (magenta) UV irradiation. In addition, the formation of cyclic PS was investigated by using the original St polymerization solution as precursor instead of the purified linear PS. When applying 9 h UV irradiation on the precursor solution directly diluted from the St polymerization solution without any purification, the resultant cyclic PS showed a completely overlapped GPC curve (Figure 3C) with that from the purified linear PS precursor having the same diluted concentration. Resultantly, this method could be used as a convenient one-pot technique for the formation of cyclic polymers directly starting from the monomer polymerization step. No more requirements were needed to purify or postfunctionalize the linear polymer precursor intermediates for the final cyclic polymer formation. At last, to improve the notorious low yield disadvantage of ringclosure strategy, a batchwise operation was introduced to combine with the novel ring-closure technique. As an example, a 5 times more linear PS (15 mg) was evenly added in 600 mL solvents in 5 batches by alternating every 30 min, by which the starting polymer concentration was kept as same as that of the regular one batch operation (3 mg/600 mL). After the addition, the cyclization reaction was allowed to perform for 5 h more. As shown in Figure 3D, the resultant cyclic PS showed 3779

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CONCLUSIONS A powerful ring-closure technique was established for the formation of cyclic polymers based on the combination of RAFT and light-induced Diels−Alder click reaction. A variety of well-defined telechelic polymers including PS, PMMA, PtBA, PDMA, P2VP, PMMA-b-PS, and PMMA-b-PtBA were synthesized by a standard RAFT polymerization procedure to have the orthoquinodimethane and dithioester end groups. By virtue of the light-induced Diels−Alder reaction between these end moieties, a library of cyclic polymers were then quickly produced by ring-closing the corresponding linear precursors in air at room temperature with a mild UV irradiation. The pure cyclic polymers were conveniently collected by evaporating the solvents without any requirement of additional purification. As one more striking advantage, this method could produce cyclic polymers by a one-pot operation directly from the diluted monomer polymerization solution precursors, avoiding the purification and postfunctionalization of linear polymer intermediates. In addition, when combining with a batchwise operation, this technique could produce cyclic polymers with a significantly improved yield. Resultantly, we expect that this novel light-induced ring-closure technique could become one of the most powerful tools for producing various well-defined cyclic polymers. Additionally, the preparation of varied cyclic polymer derivatives, such as tadpole and eight shape topological polymers, are exploring in our group based on the same chemistry.

Figure 5 shows the GPC curves of the linear precursors and the corresponding cyclic polymers for all cases. Compared to those (black curves) of linear counterparts, the GPC curves (red) of resultant cyclic polymers preserved the well-defined monomodal and symmetric peak shape but the whole peak position shifted to the lower molecular weight direction completely. This convincingly supported the successful formation of various cyclic polymers by this novel ring-closure method. The more evidence is demonstrated by UV−vis, 1H NMR, and MALDI−TOF MS characterizations. From the UV−vis spectra shown in Figure S3, the π−π* adsorption peak of the thiocarbonyl moiety were completely disappeared at 305 nm from the curves (red) of the resultant cyclic polymers, compared to those (black curves) of linear precursors. In addition, the 1H NMR spectrum of cyclic polymers (curves B in Figures S4−S9) showed the complete disappearance of the peaks at 10.66 ppm (peak a of curves A in Figures S4−S9) belonging to orthoquinodimethane end group of the linear precursors. The simultaneous disappearance of dithioester and orthoquinodimethane moieties strongly indicated the high cyclization efficiency from the light-induced Diels−Alder cycloaddition reaction. Furthermore, MALDI−TOF MS characterizations were performed for all of the linear precursors and the resultant cyclic counterparts, which were detailed in Figures S10−S13. Compared to the much smaller apparent Mn of cyclic polymers than that of linear precursors from GPC (Figure 5), the absolute molecular weights from MALDI−TOF MS were distributed in the same range for each type of cyclic polymer with its corresponding linear precursor. This again confirmed the successful formation of varied cyclic polymers. The one-pot and batchwise operations were demonstrated for all cases. As shown in Figure S14, the GPC curves (red) of cyclic polymers directly using original monomer polymerization solutions as precursors were completely overlapped with those (black) of cyclic polymers from purified linear polymer precursors. This suggested that the one-pot operation could be employed to prepare all kinds of cyclic polymers regardless of the monomer types, thanking to the low cyclization reaction concentration. From Figure S15, the cyclic polymers from batchwise operation using 5 times more linear precursors produced completely overlapped GPC curves with those from the regular one batch operation. This indicated that the batchwise operation could be employed to increase the cyclic polymer yield from the novel ring-closure method regardless of the polymer kinds. Furthermore, the effect of initial polymer concentration on the formation of cyclic polymer from this technique was demonstrated by ring-closing linear PtBA precursor (run 3, Table 1) at two higher concentrations, 3 mg/150 mL and 3 mg/50 mL. As shown in Figure S16, the GPC curve (blue) of the resultant cyclic PtBA from 3 mg/150 mL nearly overlapped with that (red) of cyclic PtBA from 3 mg/600 mL. However, when further increasing the initial linear PtBA concentration to 3 mg/50 mL, a shoulder peak was clearly observed from the GPC curve (dark cyan) at the higher molecular weight direction, compared to those of cyclic PtBA at 3 mg/600 mL and 3 mg/150 mL. This indicated that the inter-chain coupling reaction maybe happened when the initial polymer concentration used above 3 mg/150 mL for the given PtBA system.

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ASSOCIATED CONTENT

* Supporting Information S

Experimental section, Table S1 and S2 (MALDI−TOF data), and Figures S1−S16 (NMR spectra, GPC curves, UV−vis spectra, and MALDI−TOF spectra). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(K.Z.) Fax: +86-010-62559373. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Generous support was primarily provided by National Science Foundation of China (21090353) Ministry of Science and Technology of China (2012CB933200 and 2014CB932200) and National Science Foundation of China (21374122). K.Z. thanks the Bairen project from The Chinese Academy of Sciences for support.

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