Isomeric Dicyclic Polymers via Atom Transfer Radical Polymerization

Article pubs.acs.org/Macromolecules

Isomeric Dicyclic Polymers via Atom Transfer Radical Polymerization and Atom Transfer Radical Coupling Cyclization Shuangshuang Wang,† Ke Zhang,† Yongming Chen,*,†,‡ and Fu Xi† †

Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100190, China Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, No. 135, Xingang Xi Road, Guangzhou 510275, China

‡

ABSTRACT: Intramolecular terminal−terminal coupling of the star polymers with a defined number of arms would form multiple cyclic polymers. If the core structure is asymmetric in its composition, the statistically occurred coupling reactions may generate isomerism in the multicyclic products. Herein, this phenomenon was confirmed by applying well-defined star polystyrenes, (S)4, with four arms and a compositional asymmetrical core. The star polymers were prepared by ATRP of styrene using a designed initiator that had a −S−S− center group and four ester groups. Then, intramolecular atom transfer radical coupling (ATRC) reactions among four Br terminals of the star polymers were conducted under a high dilute condition. Depending on how the coupling occurred statistically, the obtained dicyclic polymers contained two topological isomers of which two cyclic polymeric units were linked by different manners. The presence of two isomers was confirmed by cleavage of the disulfide bonds, and the content of each isomer was evaluated. A complete cyclization of the (S)4 and structure of topological isomers were characterized by size exclusion chromatography, 1H NMR and matrix-assisted laser desorption ionization-time-of-flight mass spectrometry, and different cleaving strategies.

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radical coupling (ATRC) reaction,25,26 and Diels−Alder cyclic reaction27 have been used as cyclization chemistry for α,ωdifunctional polymers to synthesize various cyclic polymers. In addition to these chemistries, electrostatic self-assembly and subsequently covalent fixation28,29 and zwitterionic polymerization30 have also been used as efficient cyclization strategies to construct cyclic topology polymers. The second strategy is ringexpansion polymerization techniques. In this case, the monomers insert into a cyclic initiator to form larger rings.31,32 The ringexpansion metathesis polymerization (REMP) technique is based on the use of a cyclic Ru catalyst that was proposed by Grubbs et al., and some cyclic topology polymers have been formed by this strategy.33−38 Except for these traditional techniques, dynamic covalent bonds were also induced to form cyclic polymers to discuss the topology conversion between cyclic polymers and linear polymers.39−41 Cyclic polymers with one ring have been mostly reported, and the monocyclic polymers were applied as building blocks to fabricate more complex topological polymers. Various polymers with cyclic units have been reported, such as 8-shaped,42 Q-shaped,43 P-shaped,43 α-shaped,44 jellyfish-shaped,45 tadpoleshaped,46 quatre-foil-shaped,47 flower-shaped,48 sun-shaped,49 dendric cyclic polymers, and other more sophisticated multicyclic

INTRODUCTION Synthesis of polymers with a defined chain topology has always been a fantastic topic for polymer sciences since diverse unique properties and functions rely on the macromolecular architectures. Formation of cyclic polymers that have no terminals is challenging the synthetic chemistry. This is because that the properties of cyclic polymers at solution and bulk differ their linear counterparts.1−3 Studies have revealed that cyclic polymers demonstrated unique functions. For example, cyclic polymers showed an obviously different circulation time compared to their linear counterparts, and it may be useful for creating novel drug carriers with improved delivery property.4 It was also proposed that cyclic polymers can be served as additives to linear polymers for tuning rheological properties.5,6 In addition, cyclic polymers with pendant azobenzene mesogenic side groups were synthesized, and the cyclic topology would have an effect on liquid crystalline phase transitions of the mesogenic side groups.7 Though still challenging, a great progress has been made in synthesizing cyclic topological polymers. Two main approaches have been applied for formation of cyclic polymers. The first one is the ring-closure technique. In this case, homodifunctional or heterodifunctional polymers were used as precursors, and then cyclization coupling α,ω-terminals was conducted. Recently, copper-catalyzed azide−alkyne cycloaddition (CuAAC) reaction,8−19 thiol−ene click reaction,20 ring-closing metathesis (RCM) reaction,21,22 Glaser coupling reaction,23,24 atom transfer © 2014 American Chemical Society

Received: November 12, 2013 Revised: February 21, 2014 Published: March 7, 2014 1993

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Scheme 1. Formation of Cyclic Topology Isomers, c(S)4(I) and c(S)4(II), via Intramolecular ATRC of (S)4a

a

Dashed circle: core of star polymer; dashed arrows: coupling reaction.

topology polymers like polymer catenanes.50−56 In addition, cyclic polymers as constructed units have been used to fabricate polymer gels with more available network and properties.57 Although topological polymers composed of several cyclic structures have been reported, smart design of the macromolecular structures is still needed in order to understand complexity of cyclic polymers. It is well-known that the isomerism, organic molecules with the same molecular weights but different molecular structure, is a common phenomenon. The same is true for polymer architectures. The polymer chains with the similar average molar masses but different topologies, like linear, starlike, grafted, brushlike, and cyclic structures may be also considered as the isomers. These polymer isomers may demonstrate varied properties in terms of the number of terminals, chain entanglements, and hydrodynamic size. Actually cyclic polymers may also demonstrate various isomeric structures. Though it seems to be a tendency to synthesize more complicated cyclic polymers, the polymers containing two cyclic units are the simplest example of multicyclic polymers and their chain structures may have different isomers. To our knowledge, the synthesis of isomeric cyclic polymers has only been studied by Tezuka et al.58−62 In their report, a pair of topological isomers of polytetrahydrofurans formed by self-assembly and covalent fixation (ESA-CF) process using complicated bimolecular reactions.58 However, a more universal and convenient method that can be applied to construct topological isomers is still needed. In order to produce the isomers, the structure of dicyclic polymers should be properly designed. Herein, we proposed a method to build dicyclic polymers containing two types of isomers whose structures and even contents could be evaluated by smart molecular design. The star polymers with defined arm numbers and arm length may be applied to prepare the topological polymers composed of several cyclic units by terminal−terminal coupling intramolecularly. For example, a quatre-foil-shaped cyclic polymer could be formed by intramolecular coupling of a star polymer with eight arms.47 Intramolecular coupling of four-arm star polymers with four terminals may produce 8-shaped dicyclic polymers. As shown in Scheme 1, well-defined four-arm star polystyrene, (S)4, was first obtained from an initiator with asymmetric composition, Br4,

whose four Br groups were connected nonequivalently. The isomerism would be generated depending upon how the terminal−terminal cyclization would occur statistically. If the cyclization had occurred between terminals a−b and c−d, dicyclic polymer c(S)4(I) with a bridge should have been obtained. If the reaction between a−c and b−d, dicyclic polymer c(S)4(II) with a shared segment should have been obtained. The same polymer could be obtained for the crosscoupling of a−d and b−c. Since each type of coupling should be occurred statistically, the molar ratio of c(S)4(II) should have doubled that of c(S)4(I). Two isomers have a very similar structure, and thus it is very difficult to distinguish them. Therefore, we applied a multifunctional initiator Br4 to prepare the star polymers through ATRP of styrene and then to conduct intrastar Br terminal-Br terminal radical coupling. As shown in Scheme 1, the Br4 had one −S−S− group and four ester groups that were previously applied by us to prepare stepwise cleavable star polymers.63 The terminal coupling by ATRC would generate the cyclic units by forming stable −C−C− bonds. For the isomer c(S)4(I), cleaving −S−S− center should form two smaller monocyclic polymers with half of the molar mass (Scheme 2). For the isomer c(S)4(II), the cleaved product should become monocyclic structure, but the Scheme 2. Transformation of Cyclic Topology by Cleaving −S−S− Bonds

1994

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molar mass remains unchanged. Therefore, if there were isomers in the cyclization products, the cleaving center disulfide should have generated two peaks in SEC traces. The area ratio of two peaks would allow evaluating the contents of two isomers.

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EXPERIMENTAL SECTION

Materials. Styrene (St; Beijing Chemical Factory, China), tetrahydrofuran (THF; Beijing Chemical Factory, China), and anisole (Sinopharm Chemical Reagent, China) were all analytical grade; they were dried over CaH2 and distilled prior to use. CuBr (95.5%, Sinopharm Chemical Reagent, China) was purified by stirring with glacial acetic acid followed by filtering and washing the resulting solid with ethanol. N,N,N′,N″,N″-Pentamethyldiethylenetriamine (PMDETA; 99%, Aldrich), dithiothreitol (DTT) (99%, Sinopharm Chemical Reagent, China), and nanosized Cu powder (99.9%; 10−30 nm; Aladdin Chemistry, China) were used as received. Other reagents were commercialized chemicals and used directly unless otherwise noted. Tris-[2-(dimethylamino)ethyl]amine (Me6TREN) was synthesized according to the report.64 Initiator Br4 (Scheme 1) was synthesized according to a previous report.63 Characterization. Average molecular weights and polydispersity index of all the polymer samples were measured with a SEC system equipped with a Postnava PN 1011 pump, three Waters Styragel columns (HT2, HT3, and HT4), and a Precision PD 2100 detector system (RI detector was used), with THF as the eluent at a flow rate of 1 mL/min at 30 °C. Narrow linear polystyrene standards were used to calibrate the SEC system. 1 H NMR spectra were recorded with a Bruker 400 MHz spectrometer with CDCl3 as the solvent and tetramethylsilane as the internal standard. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) analysis was carried out on a Bruker Autoflex III time-of-flight spectrometry equipped with a 355 nm nitrogen laser, anthratriol was used as the matrix, and CF3COOAg as the cationic source. All spectra were recorded in linear mode. Synthesis of (S)4 by ATRP of St. A mixture of St (22.5 g, 0.216 mol), anisole (11.3 mL), PMDETA (62.4 mg, 0.36 mmol), and Br4 (0.35 g, 0.36 mmol) was charged into a Schlenk flask and degassed by three freeze−pump−thaw cycles. The flask was then filled with nitrogen, and CuBr (51.7 mg, 0.36 mmol) was quickly added to the frozen mixture. After sealing the flask, it was evacuated and backfilled with nitrogen three times. The flask was then immersed in an oil bath and heated to 80 °C. After a predetermined time, the final reaction mixture was terminated by exposing to air and diluted with THF. The solution was filtered through a column filled with neutral alumina, and the polymer was precipitated in a large amount of methanol. The precipitation procedure was repeated three times, and the final crude product was dried under vacuum for 48 h. Intramolecular ATRC of the (S)4. A 500 mL of Schlenk flask containing 200 mL of THF solution of Me6Tren (2 g, 8.68 mmol) was purged with N2 for 3 h. Then the solution mixture was charged with CuBr (1.25 g, 8.71 mmol) and Cu (1.10 g, 17.3 mmol) under N2 flow and sealed from the Schlenk line. The flask was then placed in an oil bath and stirred at 70 °C for 20 min. In a different Schlenk flask, (S)4 (10 mg) was dissolved in THF (25 mL), and the mixture was degassed by three freeze−pump−thaw cycles. After that, the degassed solution was dropped through a needle piercing the rubber septum into the THF solution of catalyst over approximately 24 h. Then the reaction mixture was stirred for an additional 4 h. Then the reaction was terminated by exposing to air. The THF was removed under reduced pressure, and dichloromethane was charged into the flask. Most of the copper salt was first removed by extract with water. Then the residual copper salt was removed by passing a short natural aluminum column. Lastly, the content of the flask was concentrated under reduced pressure, and the coupled product was precipitated in methanol and the obtained product was denoted as c(S)4. Cleavage of the c(S)4. Breaking Ester Linkages. The c(S)4 (4 mg) was dissolved in 5 mL of THF/CH3OH (v/v, 2/1), and then excess of KOH was charged into the solution. The mixture was

Figure 1. SEC curves of the (S)4 (solid line) and its intramolecular coupling product c(S)4 (dashed line) by ATRC.

Figure 2. 1H NMR spectra of the (S)4 (top) and c(S)4 (bottom).

Scheme 3. Hydrolysis of (S)4 and c(S)4 by Cleaving Ester Bonds

stirred overnight under reflux. The polymer was precipitated with an excess of CH3OH and analyzed by SEC. The hydrolysis product was subjected to hydrochloric acid treatment before analyzed by MALDI-TOF MS. Cleaving Disulfide Linkages. The c(S)4 (2 mg) was dissolved in 2 mL of de-oxygenated toluene, and then excessive DTT was charged into the solution. The mixture was stirred under nitrogen for 48 h at 60 °C. The solvent was removed under reduced pressure and used for SEC test directly. 1995

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RESULTS AND DISCUSSION

Then the dicyclic polymers, c(S)4, were obtained via intramolecular ATRC reactions between four terminals of one (S)4 macromolecule under an optimized dilute condition (ii, Scheme 1) which was given in the Experimental Section. It is well-known that cyclic polymer has a smaller hydrodynamic volume compared to its linear precursor, so SEC can prove occurrence of the intramolecular cyclization. SEC trace of c(S)4 is given in Figure 1, and it appeared at a longer eluent time after ATRC relative to (S)4, implying the occurrence of cyclization reaction. Moreover, the SEC peak of the product c(S)4 remained unimodal after the cyclization, indicating the absence of intermolecular coupling; otherwise, a shoulder peak in short elution time would be observed. 1H NMR analysis of the products also shows that the coupling of Br terminals occurred in an efficient manner. 1H NMR spectra of the (S)4 and the c(S)4 are shown in Figure 2. A complete loss of resonance signals from the methyne protons (a in Figure 2, 4.31−4.61 ppm) adjacent to the bromine terminals confirmed occurrence of coupling reactions. The efficient Br−Br coupling intramolecularly was further proved by cleavage strategy. The star polymer (S)4 had four ester bonds that were located in center of the star polymer inherited from the initiator Br4. As shown in Scheme 3, by cleavage of ester groups one star polymer (S)4 would produce four linear S32 segments with one COOH terminal. As a contrast, cleaving of esters of one c(S)4 macromolecule should give two linear S64 with two COOH terminals attributed to the stable C−C bonds formed from the ATRC reactions. The molecular weights of the cleaved polystyrene segment from c(S)4 should be 2 times higher than that of (S)4 precursor. If the Br terminals were reacted partially, the SEC trace of the cleaved product should show two peaks: one is attributed to hydrolysate of the coupled arms, and another is attributed to

Synthesis of (S)4 and c(S)4. Star polystyrene (S)4 with four arms, the precursor of c(S)4, was prepared from the designed initiator Br4 (Scheme 1) via ATRP of St in anisole at a molar feed ratio of [St]/[Br4]/[CuBr]/[PMDETA] = 600:1:1:1. The reaction was terminated after 10.5 h. The obtained star polymer was analyzed by SEC, and as shown in Figure 1, the eluent trace showed a symmetric shape with PDI = 1.15, demonstrating that the star polymer with a controlled structure was obtained. The Mn relative to linear polystyrene standards was 12.4 kDa, which should be smaller than the absolute molecular weight due to its compact structure. The actual degree of polymerization (DP) of each arm was determined by the integrated area ratio between the chemical shift of phenyl protons at region of 6.8− 7.1 ppm and methyl protons of the core linked with the arms at 0.9 ppm in the 1H NMR spectrum, as shown in Figure 2. The obtained DP of each arm was 32, and the absolute Mn thus was calculated to be 14.3 kDa.

Figure 3. SEC curves of the hydrolysates of (S)4 (dashed line) and c(S)4 (solid line) by cleaving esters groups.

Figure 4. MALDI-TOF mass spectra of the hydrolysates of (S)4 (top) and c(S)4 (bottom). 1996

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that of the uncoupled ones. We applied SEC and MALDI-TOF MS to analyze the cleaved products of (S)4 and c(S)4. Figure 3 shows the SEC traces of the hydrolysates of c(S)4 and (S)4. A unimodal peak was given from the SEC trace of hydrolysate of the c(S)4. The molecular weight of the hydrolysate of the c(S)4 given by SEC, Mn 6.87 kDa and PDI 1.05, was twice as that of the hydrolysate of its precursor (S)4, Mn 3.53 kDa and PDI 1.10. Furthermore, the hydrolyzed product was characterized by MALDI-TOF MS (Figure 4). Both spectra of the cleaved products from two samples showed uniform series of peaks with an interval of 104 mass units, the molecular weight of styrene unit. The m/z of the hydrolysate of c(S)4 was 2 times higher than that of the (S)4. For example, the observed peak of the cleaved (S)4 at m/z = 3628 corresponded to the PS component of DP = 33, MSt × 33 + MC4H6O2 + MAg+ = 3627. For the cleaved c(S)4 with a DP of 66, the observed data, m/z = 7149, matched the calculated one by MSt × 66 + MC8H14O4 + MAg+ = 7149. The above clean results from two characterizations further demonstrated a complete coupling reaction of intrastar polymers. Proof of the Presence of Cyclic Topological Isomers. The cleavage of disulfide by DTT was used as the strategy to prove the existing of topological isomers. As shown in Scheme 2, cleavage of the central disulfide groups of cyclic isomers would

form different products with different molar masses that could be easily confirmed by SEC traces. As shown in Figure 5, the cleaved products revealed two peaks. The molar mass of peak A matched completely to the peak of dicyclic polymers. Thus, the peak A corresponded to the cleaved product of the isomer c(S)4(II). For the shoulder peak (peak B), its peak molar mass (Mp) was estimated to be 7.12 kDa, half of peak A, Mp 12.8 kDa, demonstrating that it was the cleaved product of c(S)4(I). The area ratio of peak A and peak B was given to be 1.9:1 after Gaussian fit processing, which agreed with the expected ratio 2:1. In order to further verify the structure of the reduced cleaved products, the reduced cleaved products of mixed c(S)4(I) and c(S)4(II) from another repeated experiment were fractionated by conducting several SEC measurements and collecting the fraction of lower molecular weights. The collected cleaved c(S)4(I) was used for MALDI-TOF MS test, and the result is shown in Figure 6. For the cleaved c(S)4(I) with a DP of 63, the observed data, m/z = 6995, matched the calculated one by MSt × 63 + MC15H24O6S + MAg+ = 6995. Therefore, the isomeric dicyclic polymers were confirmed by combination of the SEC and MALDI-TOF MS results.

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CONCLUSION We have demonstrated a more universal way to construct topological isomers of dicyclic polymers. The cyclic isomers were formed by completely intramolecular ATRC reaction of (S)4 prepared by ATRP. The topological structure of the cyclic isomers could be clearly understood by simple cleavage of the disulfide bond that located in the focal position of the macromolecular structure. The cyclic isomers can split into either one big loop or two small loops. It is noteworthy that this method would be further applicable for various common polymers that can be prepared by ATRP. Thus, it offers a more convenient strategy for building complicated cyclic topologies and conducting cyclic topology conversion thereof.

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AUTHOR INFORMATION

Corresponding Author

Figure 5. SEC traces of the reduced cleaved products of c(S)4 (solid line) and the original c(S)4 (dashed line).

*E-mail: [email protected] (Y.M.C.).

Figure 6. MALDI-TOF mass spectrum of the cleaved product of c(S)4(I). 1997

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from NSF China (21090350 and 21090353) is gratefully acknowledged. REFERENCES

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