An Efficient and General Strategy toward the Synthesis of

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

An Efficient and General Strategy toward the Synthesis of Polyethylene-Based Cyclic Polymers Yu Jiang, Zhen Zhang, De Wang, and Nikos Hadjichristidis* Physical Sciences and Engineering Division, KAUST Catalysis Center, Polymer Synthesis Laboratory, King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia S Supporting Information *

ABSTRACT: A novel strategy toward well-defined polyethylene-based cyclic homo/copolymers is presented. Tris(3(anthracen-9-ylmethoxy)propyl)borane, prepared by hydroboration of 9-((allyloxy)methyl)anthracene with BH3, was used to initiate the polyhomologation of dimethylsulfoxonium methylide to afford well-defined anthracene-teminated linear polyethylene (PE). The azido and alkynyl groups at α and ω positions of the PE chain were introduced via the anthracene/ maleimide Diels−Alder (D−A) reaction and esterification, respectively. Subsequent intramolecular “click” cyclization of the α,ωheterofunctionalized linear PE gave cyclic PE. Combining this efficient strategy with ring-opening polymerization (ROP), more complex PE-based cyclic block copolymer architectures have been designed and synthesized, such as diblock cyclic and triblock tadpole copolymers. All intermediates and final products were characterized by high-temperature gel permeation chromatography, proton nuclear magnetic resonance spectroscopy, and differential scanning calorimetry. Initial studies on the thermal behavior of the cyclic homo- and block copolymers revealed the big influence of the cyclic structure on the melting temperature and crystallinity as compared to their corresponding precursors.

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Polyhomologation, recently developed by Shea et al.38−40 and developed by others,41−56 is a powerful tool for the synthesis of well-defined linear polymethylene (equivalent to polyethylene, PE). It involves three steps: (a) formation of a complex between an ylide (monomer) and an organoborane (initiator); (b) migration/insertion of −CH2− moiety from the ylide into the three alkyl branches of borane with simultaneous release of dimethyl sulfoxide and regeneration of the alkylborane species ready for the next cycle, affording a borane-link 3-arm star PE; (c) oxidation/hydrolysis of the 3arm star PE to give hydroxyl-terminated linear PE (PE-OH). This efficient strategy, in combination with other polymerizations, was used to construct different PE-based architectures. For example, the terminal −OH group of PE-OH was directly used for ring-opening polymerization (ROP),42 and after modification to appropriate initiating sites for atom transfer radical (ATRP)43−46 or reversible addition−fragmentation chain-transfer polymerization (RAFT),47 to afford PE-based block polymers. PE-OH can also react with appropriate functionalized monomers to give macromonomers, which by (co)polymerization produced graft(comb) (co)polymers or molecular brushes.48 Furthermore, the sensitive to the air 3-arm star PE, with boron junction point, can be transformed to stable carbon-linked 3-arm star PE by using “stitching” reactions and thus to afford more complex PE-based architectures.49,50

INTRODUCTION

Cyclic homo- and copolymers have attracted considerable attention in past decades due to their non-chain-ends unique structure and different physical properties1−4 compared to the linear analogues. Recently, in addition to the single cyclic polymers, 5−9 a few cyclic-based structures have been synthesized, including tadpole-shaped,10,11 sun-shaped,12−15 and eight-shaped.16−18 Two main strategies were used to synthesize cyclic polymers: the ring closure (RC) and the ring expansion (RE).19,20 In the case of RC strategy, linear α,ωfunctionalized precursors are synthesized first, followed by intra- or intermolecular cyclization to afford the cyclic structure. RC reactions are usually performed in high dilution conditions. A few cyclization methods were developed for this strategy, e.g., “click”,21,22 olefin metathesis,23 Glaser coupling,24 Diels−Alder (D−A),25 thiol−ene,26 and esterification.27 RE strategy is induced by the insertion of monomer into an activated cyclic initiator/nucleophile cyclic catalyst. Cyclic tin oxide initiators,28−30 Grubbs metathesis,31,32 and nucleophilic ring-opening polymerization (ROP) catalysts33−35 are the most widely used for this strategy. The polymers involved in the above methods are mainly polystyrene, polydienes, poly(2-vinylpyridine), polylactides, and poly(ethylene oxide). With respect to polyethylene, only two examples have come to our attention: the one by Grubbs and collaborators36 who prepared cyclic PE by hydrogenation of cyclic polybutadiene-1,4 and the other by Shea and collaborators37 by polyhomologation of dimethylsulfoxonium methylide followed by Brown “stitching” reaction. © XXXX American Chemical Society

Received: February 13, 2018 Revised: April 8, 2018

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DOI: 10.1021/acs.macromol.8b00333 Macromolecules XXXX, XXX, XXX−XXX

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immediately. 1H NMR (CDCl3, 500 MHz, TMS): δ 1.44 (6H, s), 2.96 (2H, s), 3.80 (2H, m), 4.35 (2H, m), 5.27 (2H, s), 6.51 (2H, s). Synthesis of ant-PE-(OH)2 (Scheme 3). In a typical procedure for the synthesis of ant-PE61-(OH)2, a solution of ant-PE61-OH (800 mg, 0.40 mmol), 2,2,5-trimethyl-1,3-dioxane-5-carboxylic acid62 (348 mg, 2.0 mmol), N,N′-dicyclohexylcarbodiimide (DCC, 824 mg, 4.0 mmol), and N,N-dimethylpyridin-4-amine (DMAP, 49 mg, 0.40 mmol) in toluene (15 mL) was stirred at 90 °C overnight. The solution was cooled down, and the mixture was concentrated by rotor evaporator and precipitated in methanol. The resulted white solid was filtered, dried under vacuum, and characterized by 1H NMR and GPC (810 mg). A solution of the white solid (810 mg, 0.37 mmol) and amberlyst15 (wet) ion-exchange resin (150 mg) in toluene (20 mL) was stirred at 90 °C overnight. The solution was cooled down, and the mixture was concentrated by rotor evaporator and precipitated in methanol. The white solid was filtered, dried under vacuum, and characterized by 1 H NMR and GPC (ant-PE61-(OH)2, 700 mg, Mn,NMR = 2.1 × 103, PDI = 1.13). Synthesis of Anthracene-Terminated Linear and 3-Miktoarm Star Copolymers. In a typical procedure for the synthesis of ant-PE100-b-PCL38-OH, tBu-P2 (0.45 mL, 2 M, 0.9 mmol) was added to the solution of ant-PE100-OH (0.60 g, 0.194 mmol) in toluene (6.0 mL), and then caprolactone (0.98 g, 8.6 mmol) was added to the mixture. The resulted solution was stirred in a preheated oil bath at 80 °C for 14 h. Then the reaction was quenched by adding CH3COOH/MeOH (10 vol %) and cooled to room temperature. The mixture was concentrated by a rotor evaporator and precipitated in methanol. The white solid was filtered, dried under vacuum, and characterized by 1H NMR and GPC (ant-PE100-b-PCL38-OH, 950 mg, Mn,NMR = 7.4 × 103, PDI = 1.29). The synthetic procedures of ant-PE-b-(PCL-OH)2 3-miktoarm star copolymers were similar to that of ant-PE-b-PCL-OH. Synthesis of α,ω-Functionalized Linear Homo/Copolymers and Functionalized 3-Miktoarm Star Copolymers. In a typical procedure for the synthesis of N3-PE30-≡ , a toluene solution of antPE30-OH (880 mg, 0.80 mmol) and freshly prepared MI-N3 (644 mg, 2.0 mmol) was stirred under reflux overnight. The solution was cooled down, concentrated by rotor evaporator, and precipitated in methanol. The white solid was filtered, dried under vacuum, and characterized by 1 H NMR and GPC (N3-PE30-OH, 800 mg, Mn,NMR = 1.4 × 103, PDI = 1.19). A solution of N3-PE30-OH (800 mg, 0.57 mmol), 4-pentynoic acid (280 mg, 2.85 mmol), N,N′-dicyclohexylcarbodiimide (DCC, 1,17 g, 5.7 mmol), and N,N-dimethylpyridin-4-amine (DMAP, 70 mg, 0.57 mmol) in toluene (15 mL) was stirred at 90 °C overnight. The solution was cooled down, concentrated by a rotor evaporator, and precipitated in methanol. The white solid was filtered, dried under vacuum, and characterized by 1H NMR and GPC (N3-PE30-≡ , 770 mg, Mn,NMR = 1.5 × 103, PDI = 1.19). The synthetic procedures of N3-PE-b-PCL-≡ and N3-PE-b-(PCL-≡ )2 were similar to that of N3-PE-≡. Synthesis of Single Cyclic Homo/Copolymers and TadpoleShaped Copolymers. In a typical procedure for the synthesis of cPE 30 , copper(I) bromide (CuBr, 72 mg, 0.50 mmol) and pentamethyldiethylenetriamine (PMDETA, 87 mg, 0.50 mmol) were dissolved in toluene (250 mL). The mixture was degassed by three freeze−pump−thaw (FTP) cycles and then put in a preheated oil bath at 90 °C, then N3−PE30-≡ (150 mg, 0.10 mmol) in toluene (150 mL) was added to the CuBr/PMDETA solution via a peristaltic pump at a rate of 1.4 mL/h. After completion of the addition, the reaction solution was concentrated, purified by passing through a neutral alumina column using toluene as eluent to remove the copper and precipitated in methanol three times. The white solid was filtered, dried under vacuum and characterized by 1H NMR and GPC (c-PE30, 89 mg, Mn,NMR = 1.5 × 103, PDI = 1.19). The synthetic procedures of c-PE-b-PCL and (c-PE-b-PCL)-b-PCL were similar to that of c-PE.

Recently, our group has developed new borane initiators to enrich the family of PE-based architectures.51−55 Lately, we reported a novel strategy, based on the combination of D−A reaction with polyhomologation, for the synthesis of linear di/triblock co/terpolymers.57,58 In the present work, we expand the range of this strategy to welldefined PE-based cyclic homo/copolymers with complex macromolecular architecture.

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EXPERIMENTAL SECTION

Materials. 9-Anthracenemethanol (97%), allyl bromide (97%), copper(I) bromide (CuBr, 99.999%), borane−dimethyl sulfide complex solution (1.0 M in THF), tBu-P2 (2.0 M in THF), furan (99%), maleic anhydride (≥99%), sodium azide (NaN3, 99.5%), αbromoisobutyryl bromide (98%), ethanolamine (≥99%), amberlyst-15 (wet) ion-exchange resin, 4-(dimethylamino)pyridine (DMAP, ≥99%), trimethylamine N-oxide dihydrate (TAO, ≥99%), and N,N′dicyclohexylcarbodiimide (DCC, 99.8%) were purchased from Aldrich and used as received. N,N,N′,N′′,N′′-Pentamethyldiethylenetriamine (PMDETA, 99%, Aldrich) and caprolactone (CL, ≥99%) were distilled over calcium hydride (CaH2) under reduced pressure before use. Tetrahydrofuran (THF) and toluene were refluxed over sodium/ benzophenone and distilled under a nitrogen atmosphere just before use. Dimethylsulfoxonium methylide was prepared according to Corey’s method followed by switching the solvent from THF to toluene.59,60 α-Anthracene-ω-hydroxyl polyethylene (ant-PE-OH) was prepared by adding borane−dimethyl sulfide complex solution (BH3· Me2S, 1.0 M in Et2O) to 9-((allyloxy)methyl)anthracene solution in toluene.57 Measurements. The high-temperature gel permeation chromatography (HT-GPC) measurements were carried out with the Agilent PLGPC 220 instrument equipped with one Plgel 10 μm MIXED-B column and a differential refractive index (DRI) detector. 1,2,4Trichlorobenzene (TCB) was used as eluent at a flow rate of 1.0 mL/ min at 150 °C. The system was calibrated by PS standards. The 1H and 13C NMR spectra were recorded with a Bruker AVANCE III-400, -500, or -600 spectrometer and Fourier transform infrared (FT-IR) spectra with a NICOLET iS10 FT-IR instrument. Differential scanning calorimetry (DSC) measurements were performed using a Mettler Toledo DSC1/TC100 system under an inert atmosphere (nitrogen). The sample was heated from room temperature to 150 °C, cooled to −40 °C, and finally heated again to 150 °C with a heating/cooling rate of 10 °C/min. The second heating curve was used to determine the melting temperature (Tm) and degree of crystallinity. Synthesis of α-Anthracene-ω-hydroxylpolyethylenes, antPE-OH (Scheme S1). In a typical procedure for the synthesis of antPE30-OH, 1.5 mL (1.5 mmol) of a THF solution of BH3·Me2S (1.0 M) was added over 5 min to a toluene solution (3.5 mL, 1.5 g, 6.04 mmol) of 9-anthracenemethyl allyl ether at 0 °C. The reaction was allowed to reach room temperature over 2 h. The final concentration of the initiator solution in toluene was 0.3 M. 2.0 mL of the initiator tris(3-(anthracen-9-ylmethoxy)propyl)borane (0.3 M, 0.60 mmol) was added to a dimethylsulfoxonium methylide solution (150 mL, 0.80 M, 120 mmol) at 65 °C. After consumption of methylide, 0.29 g of TAO was added to the solution. Then, the solution was stirred for 2 h and precipitated in methanol. The white solid was filtered, dried under vacuum, and characterized by 1 H NMR and GPC (1.35 g, Mn,NMR = 1.1 × 103, PDI = 1.18). Synthesis of 2-(1,3-Dioxo-1,3,3a,4,7,7a-hexahydro-2H-4,7epoxyisoindol-2-yl)ethyl 2-azido-2-methylpropanoate (MI-N3, Scheme S2). A solution of 2-bromo-2-methylpropionic acid 2-(3,5dioxo-10-oxa-4-azatricyclo[5.2.1.02,6]dec-8-en-4-yl)ethyl ester61 (2.50 g, 7.0 mmol) and NaN3 (0.65 g, 10 mmol) in DMF (20 mL) was stirred at room temperature for 2 h. The reaction was quenched by adding water (20 mL), and the product was extracted with toluene. The organic phase was washed with brine, dried over Na2SO4, and concentrated under vacuum to give the product as a colorless oil (1.30 g, 58% yield). This compound is not stable and should be used B


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RESULTS AND DISCUSSION A series of well-defined α-anthracene-ω-hydroxyl polyethylenes (ant-PE-OH, Scheme S1) with different molecular weight were Table 1. Molecular Characteristics of α-Anthracene-ωhydroxyl Polyethylenes (ant-PE-OH) entry

sample

Mn,NMRa

Mn,theorb

DPa

Đc

1 2 3 4 5 6 7

ant-PE30-OH ant-PE61-OH ant-PE100-OH ant-PE118-OH ant-PE175-OH ant-PE190-OH ant-PE595-OH

1100 2000 3100 3600 5200 5600 17000

1400 1900 3400 2900 5900 6800 22600

30 61 100 118 175 190 595

1.18 1.12 1.06 1.05 1.11 1.05 1.13

a

Mn,NMR and DP were calculated from 1H NMR spectra (600 MHz, C2D2Cl4, 90 °C) using the area ratio of protons of terminal CH2OH at δ = ∼ 3.7 ppm to the ones of the backbone. bCalculated from gmon/ molinitiator. cĐ (Mw/Mn) and Mn,GPC, determined by HT-GPC (1,2,4trichlorobenzene, 150 °C, PS standards).

Figure 1. GPC traces of linear precursor N3-PE190-≡ (a), c-PE190 using a feeding rate of 3.0 mL/h (b), and c-PE190 with a feeding rate of 1.4 mL/h (c) (TCB at 150 °C).

Table 2. Molecular Characteristics of Linear Polyethylenes (N3-PE-≡) and Their Cyclic Products (c-PE)

synthesized (Table 1) and used as precursors for the cyclicbased structures. The molecular weights were determined by 1 H NMR end-group analysis and the polydispersity index by high-temperature gel permeation chromatography (HT-GPC) with polystyrene standards (polydispersity index: Đ (Mw/Mn) = 1.05−1.18). All GPC traces were monomodal with narrow molecular weight distributions (Figure S1). Synthesis and Characterization of Single Cyclic Homopolymers, c-PE. The single cyclic polyethylenes were synthesized in three general steps from α-anthracene-ωhydroxyl polyethylenes (Scheme 1): (a) Diels−Alder reaction of anthracene group with maleimide derivative (MI-N3, Scheme S2) to introduce azido group on one chain end; (b) esterification of the OH group with 4-pentynoic acid, in the presence of N,N′-dicyclohexylcarbodiimide (DCC) and N,Ndimethylpyridin-4-amine (DMAP), to introduce alkynyl group on the other chain end and afford the linear precursor N3-PE-≡ ; (c) intramolecular “click” reaction, in the presence of Cu(I)Br

entry

sample

Mn,NMRa

Mp,GPCb

Đb

Rc

1

N3-PE30-≡ c-PE30 N3-PE190-≡ c-PE190

1500 1500 6000 6000

1520 990 10000 8010

1.19 1.19 1.07 1.08

0.65

2

0.80

a

Mn,NMR were calculated from 1H NMR spectra (600 MHz, C2D2Cl4) at 90 °C. bĐ (Mw/Mn) and Mp,GPC, determined by HT-GPC (1,2,4trichlorobenzene, 150 °C, PS standards). cR is the ratio of the Mp,GPC of cyclic polymers to their linear precursors.

and N,N,N′,N′′,N′′-Pentamethyldiethylenetriamine (PMDETA) to produce the final product, cyclic PE, c-PE. The GPC traces and molecular characteristics of the precursors and the final products are shown in Figure 1, Figure S4a, and Table 2. Mn,NMR were determined by 1H NMR endgroup analysis; Mn,GPC and Đ (Mw/Mn) were determined by HT-GPC with polystyrene standards (Table 2). The hydrodynamic volume of cyclic polymer is noticeably smaller than

Scheme 1. Synthesis of c-PE from α-Anthracene-ω-hydroxyl Polyethylenes, N3-PE-≡

C


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Figure 2. 1H NMR (600 MHz) spectra of the (a) ant-PE30-OH, (b) N3-PE30-OH, (c) N3-PE30-≡, and (d) c-PE30 in 1,1,2,2-tetrachloroethane-d2 at 90 °C.

adventitious oxygen, during the synthetic procedure, with the 3PE borane.64 For the linear polyethylene with the higher molecular weight (Table 1, entry 7), the intramolecular cyclization was not successful (Figure S2); a high molecular weight shoulder was observed, even with lower feeding rate, may be due to the increasing distance of the two terminal groups of polymer chain, which favors rather intermolecular than intermolecular reaction. Triple-detection GPC (refractometry, light scattering at λ = 670 nm, and viscometry) was also used to compare the linear to the corresponding cyclic polymers. Unfortunately, due to the weak signals (low molecular weight) of the light scattering and intrinsic viscosity responses (Figure S3), it was impossible to extract conclusions. All purified polymers were characterized by 1H NMR and FT-IR. For example, the 1H NMR spectrum of polymer N3PE30-OH is shown in Figure 2b. Compared with that of antPE30-OH (Figure 2a), the resonance signals (a), (b), (c), and (d) belonging to anthracene group disappeared completely, and new signals between 7.00 and 7.60 ppm appeared ((b′), (c′) and (d′)), indicating the success of the D−A reaction. The

that of their linear precursors, and this feature could be described by using R, the ratio of the peak value of molecular weight (Mp,GPC) of the cyclic polymer to that of the linear precursor.63 For example, by comparing the GPC traces in Figures 1a and 1c, it is obvious that GPC trace of c-PE190 shifted to longer elution time, meaning smaller hydrodynamic volume compared to the linear one. The R values for c-PE30 and c-PE190 are 0.65 and 0.80, respectively. The Đ (Mw/Mn) of the cyclic products are 1.20 (c-PE30) and 1.08 (c-PE190) (Table 2). All the above indicate that the cyclization process was rather successful. In order to avoid the formation of byproduct from intermolecular condensation, the cyclization was carried out under high dilution condition. The dilute solution of linear precursor was added dropwise to the CuBr/PMDETA solution in toluene. Increasing the feeding rate from 1.4 to 3.0 mL/h results in a small shoulder at shorter retention time (Figure 1b), indicating the generation of byproduct from intermolecular “click” reaction. In the GPC traces (Figure 1a,c and inset of Scheme 2), a small peak at approximately twice the molecular weight was occasionally observed. This peak is due to the coupling products of the radicals formed by reaction of D


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Macromolecules Scheme 2. Synthesis of c-PE-b-PCL and the HT-GPC Traces Corresponding to c-PE118-b-PCL8 (Inset)

We also tried the retro-Diels−Alder reaction to cleave the cyclic polymers and get the linear chain again, without success. A characteristic example is given in Scheme S3 and Figure S15.65 Synthesis and Characterization of Single Cyclic Block Copolymers, c-PE-b-PCL. With the successful results on single cyclic PE in hand, we next tried to synthesize other polyethylene-based cyclic copolymers (Scheme 2). The hydroxyl group of ant-PE-OH was utilized to initiate the ROP of ε-caprolactone with the phosphazene superbase tBu-P2 as catalyst in toluene at 80 °C. Then the same reactions were used to introduce the azido and alkynyl groups into the linear diblock copolymer to form N3-PE-b-PCL-≡. The “click” cyclization of the linear diblock copolymer led to the desired cyclic diblock copolymer of polyethylene and polycaprolactone. The molecular characteristics of the precursors and the final products are shown in Table 3. Cyclic diblock copolymers with low polydispersity index (Đ: 1.17 and 1.38), and R values (0.68 and 0.79) were synthesized. The success of ROP and the subsequent cyclization was confirmed by HT-GPC, 1H NMR, and FT-IR results. For example, as shown in Scheme 2, the elution peak of N3-PE118-bPCL8-≡ shifted to the high molecular weight range, while retaining the narrow distribution profile, when compared with the corresponding ant-PE118-OH. After the intramolecular “click” cyclization, the peak of c-PE118-b-PCL8 shifted to low molecular weight range, indicating the smaller hydrodynamic volume of cyclic polymer (see also Figure S4b) . A typical 1H NMR spectrum of cyclic copolymer c-PE118-b-PCL8 is shown in Figure 3. The fingerprints of PE and PCL blocks are evident in the 1H NMR spectrum. Comparing the NMR spectra of cPE118-b-PCL8 and its linear precursor (Figure 3 and Figure S12), the observed new signals at 2.75, 3.10, and 7.52 ppm indicate the generation of triazole group after “click” reaction. In the FT-IR spectra of all the samples (Figure S6), a strong band at 1734 cm−1, attributed to the polycaprolactone, appeared after ROP. The stretch absorption at 2111 cm−1 for azido group was detected after D−A reaction, and this

Table 3. Molecular Characteristics of Linear Polyethylene-bpolycaprolactones (N3-PE-b-PCL-≡) and Their Cyclic Products (c-PE-b-PCL) entry

sample

Mn,NMRa

Mp,GPCb

Đb

Rc

1

N3-PE118-b-PCL8-≡ c-PE118-b-PCL8 N3-PE100-b-PCL38-≡ c-PE100-b-PCL38

4900 4900 7800 7800

8650 5890 13940 10980

1.12 1.17 1.29 1.38

0.68

2

0.79

a

Mn,NMR were calculated from 1H NMR spectra (600 MHz, C2D2Cl4) at 90 °C. bĐ (Mw/Mn) and Mp,GPC, determined by HT-GPC (1,2,4trichlorobenzene, 150 °C, PS standards). cR is the ratio of the Mp,GPC of cyclic polymers to their linear precursors.

successful esterification was also confirmed by 1H NMR. Comparing the 1H NMR spectra of N3-PE30-OH with the corresponding N3-PE30-≡ sample (Figure 2b,c), new signals (k), (i), and (j) appeared at δ = 2.02 and 2.58 ppm, attributed to the terminal proton of the alkyne functional group and to the methylene protons adjacent to the triple bond, respectively. Furthermore, the signal of the methylene protons adjacent to −OH group also shifted from δ = 3.70 ppm (g′) to 4.20 ppm (g″). These results indicate the successful formation of the ester group. The cyclic polyethylenes were synthesized by intramolecular “click” reaction of the linear α,ω-difunctional precursors. Comparing the 1H NMR of N3-PE30-≡ with c-PE30 (Figure 2c,d), the signal (k) belonging to terminal proton of the alkyne disappeared completely, and a new signal at 7.52 ppm (k′) is assigned to proton of triazole ring formed from the alkyne and azido groups. The signal (i + j) at 2.58 ppm disappeared, and new signals (i′) and (j′) at 2.75 and 3.10 ppm appeared, due to the transformation of alkyne to triazole. From the FT-IR spectra (Figure S5), it could be observed that the characteristic absorption of azido group around 2110 cm−1 appeared after D−A reaction and disappeared again after “click” cyclization. From all above, it could be concluded that the single cyclic polyethylenes were synthesized successfully. E


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Figure 3. 1H NMR (600 MHz) spectrum of c-PE118-b-PCL8 in 1,1,2,2-tetrachloroethane-d2 at 90 °C.

Scheme 3. Synthesis of ant-PE-(OH)2

Scheme 4. Synthesis of (c-PE-b-PCL)-b-PCL and the HT-GPC Traces Corresponding to (c-PE61-b-PCL7)-b-PCL7 (Inset)

F


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Macromolecules Table 4. Molecular Characteristics of Linear Polyethylene-bpolycaprolactones (N3-PE-b-PCL-≡) and Their Cyclic Products (c-PE-b-PCL) entry

sample

Mn,NMRa

Mp,GPCb

Đb

Rc

1

N3-PE61-b-(PCL7-≡)2 (c-PE61-b-PCL7)-b-PCL7 N3-PE175-b-(PCL10-≡)2 (c-PE175-b-PCL10)-b-PCL10

4200 4200 8100 8100

8240 5580 14220 10080

1.19 1.21 1.17 1.22

0.68

2

0.71

a

Mn,NMR were calculated from 1H NMR spectra (600 MHz, C2D2Cl4) at 90 °C. bĐ (Mw/Mn) and Mp,GPC, determined by HT-GPC (1,2,4trichlorobenzene, 150 °C, PS standards). cR was the ratio of the Mp,GPC of cyclic polymers to their linear precursors.

characteristic signal disappeared again after the “click” reaction. These results were consistent with the production of the target cyclic diblock copolymer structure. Synthesis and Characterization of Tadpole-Shaped Copolymers, (c-PE-b-PCL)-b-PCL. To further develop this successful method, we tried to synthesize tadpole-shaped block polymers. Our synthetic strategy for this kind of polymer is based on the intramolecular “click” reaction of a 3-arm star polymer. We first introduced two hydroxyl group into the linear polyethylene by esterification of ant-PE-OH with 2,2,5trimethyl-1,3-dioxane-5-carboxylic acid, followed by deprotection of the hydroxyl groups (Scheme 3). Figure S10 shows the 1 H NMR spectra of these polymers. In Figure S10a, the signal (g) at 3.70 ppm is attributed to the −CH2−OH group. After esterification of the terminal hydroxyl group, this peak (g) shifted to 4.21 ppm (g′), indicating the formation of ester group. There were also two new peaks at 4.21 and 3.70 ppm, attributed to the two methylene group (i) and (j) (Figure S10b). After deprotection, these two peaks shifted to 3.91 and 3.78 ppm (Figure S10c). It was also observed that the characteristic absorption of ester group around 1732 cm−1 appeared after esterification in FT-IR spectra (Figure S7).

Figure 5. DSC traces for polymer: (a) N3-PE190-≡ , (b) c-PE190, (c) N3-PE100-b-PCL38-≡, (d) c-PE100-b-PCL38, (e) N3-PE175-b-(PCL10-≡)2, and (f) (c-PE175-b-PCL10)-b-PCL10.

As shown in Scheme 4, these two terminal hydroxyl groups were used as initiation site for ROP of ε-caprolactone to give the 3-miktoarm star copolymer. The azido and alkynyl groups were introduced by D−A reaction and esterification, respectively. The tadpole-shaped copolymers were obtained after the “click” cyclization of 3-miktoarm star copolymers. The molecular characteristics of the precursors and the final products are shown in Table 4. Two tadpole-shaped copolymers were synthesized with low polydispersity index (Đ) of 1.21 and 1.22 and R values 0.68 and 0.71. From the HT-

Figure 4. 1H NMR (600 MHz) spectrum of (c-PE175-b-PCL10)-b-PCL10 in 1,1,2,2-tetrachloroethane-d2 at 90 °C. G


Macromolecules

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GPC traces (Scheme 4 and Figure S4c), it could be observed that the trace of 3-miktoarm star copolymer clearly shifted to higher molecular weight range, and the trace of tadpole-shaped copolymer shifted to long retention time again, indicating the decrease in the hydrodynamic volume. The 1H NMR spectrum also shows the successful synthesis of the tadpole-shaped copolymers. For example, in the 1H NMR spectrum of (c-PE175-b-PCL10)-b-PCL10 (Figure 4), both signals of PE and PCL blocks are identified. Comparing the 1H NMR spectra of (c-PE175-b-PCL10)-b-PCL10 and its 3-miktoarm star copolymer precursor (Figure 4 and Figure S14), the observed new signals at 2.78, 3.10, and 7.52 ppm ((a), (b), and (c)) indicate the generation of triazole group after “click” reaction. Most importantly, the signals at 2.58 and 2.10 ppm ((d), (e) and (f)) show that one of the alkynyl groups was not reacted. From FT-IR spectra, similar results were observed (Figure S8). There was a strong absorption at 1726 cm−1 detected after ROP, corresponding to the CO group of the PCL block. The stretch absorption at 2110 cm−1 for azido group appeared after D−A reaction and disappeared again after the “click” reaction. All of the results above are in good agreement with the successful synthesis of tadpole-shaped block copolymers. DSC Characterization. The thermal behavior of the cyclic homo/copolymers and their corresponding precursors, studied by differential scanning calorimetry (DSC), is shown in Figure 5. The linear N3-PE190-≡ with Mn = 6000 exhibited a relatively sharp melting temperature (Tm) at 120.8 °C with moderate crystallinity (Xc) of 32.5% (Figure 5a). For the cyclic homopolymer c-PE190, the melting point (Tm) was slightly decreased (117.2 °C) while the Xc decreased considerably to 21.0% (Figure 5b). The same trend was observed in the case of diblock cyclic (Figure 5d) and tadpole block (Figure 5f) copolymers when compared with the corresponding precursors (Figure 5c,e): slightly lower melting temperatures of the two blocks but significant lowering of the crystallinity. The more complex the structure, the higher the influence on Tm and Xc due to the reduced mobility of the chains.

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00333. Figures S1−S15 and Schemes S1−S3 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(N.H.) Tel + 966-(0)12-8080789; e-mail nikolaos. [email protected]. ORCID

Nikos Hadjichristidis: 0000-0003-1442-1714 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Research reported in this publication was supported by King Abdullah University of Science and Technology (KAUST).

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REFERENCES

(1) Orrah, D. J.; Semlyen, J. A.; Ross-Murphy, S. B. Studies of Cyclic and Linear Poly(dimethylsiloxanes): 28. Viscosities and Densities of Ring and Chain Poly(dimethylsiloxane) Blends. Polymer 1988, 29, 1455−1458. (2) Qiu, X.-P.; Tanaka, F.; Winnik, F. M. Temperature-Induced Phase Transition of Well-Defined Cyclic Poly(Nisopropylacrylamide)s in Aqueous Solution. Macromolecules 2007, 40, 7069−7071. (3) Shin, E. J.; Jeong, W.; Brown, H. A.; Koo, B. J.; Hedrick, J. L.; Waymouth, R. M. Crystallization of Cyclic Polymers: Synthesis and Crystallization Behavior of High Molecular Weight Cyclic Poly(εcaprolactone)s. Macromolecules 2011, 44, 2773−2779. (4) Nasongkla, N.; Chen, B.; Macaraeg, N.; Fox, M. E.; Fréchet, J. M. J.; Szoka, F. C. Dependence of Pharmacokinetics and Biodistribution on Polymer Architecture: Effect of Cyclic versus Linear Polymers. J. Am. Chem. Soc. 2009, 131, 3842−3843. (5) Touris, A.; Hadjichristidis, N. Cyclic and Multiblock Polystyreneblock-polyisoprene Copolymers by Combining Anionic Polymerization and Azide/Alkyne “Click” Chemistry. Macromolecules 2011, 44, 1969− 1976. (6) Goldmann, A. S.; Quémener, D.; Millard, P. E.; Davis, T. P.; Stenzel, M. H.; Barner-Kowollik, C.; Müller, A. H. E. Access to Cyclic Polystyrenes via a Combination of Reversible Addition Fragmentation Chain Transfer (RAFT) Polymerization and Click Chemistry. Polymer 2008, 49, 2274−2281. (7) Zhang, Y. N.; Wang, G. W.; Huang, J. L. Synthesis of Macrocyclic Poly(ethylene oxide) and Polystyrene via Glaser Coupling Reaction. Macromolecules 2010, 43, 10343−10347. (8) Eugene, D. M.; Grayson, S. M. Efficient Preparation of Cyclic Poly(methyl acrylate)-block-poly(styrene) by Combination of Atom Transfer Radical Polymerization and Click Cyclization. Macromolecules 2008, 41, 5082−5084. (9) Adachi, K.; Honda, S.; Hayashi, S.; Tezuka, Y. ATRP−RCM Synthesis of Cyclic Diblock Copolymers. Macromolecules 2008, 41, 7898−7903. (10) Oike, H.; Washizuka, M.; Tezuka, Y. Designing an “A-Ringwith-Branches” Polymer Topology by Electrostatic Self-Assembly and Covalent Fixation with Interiorly Functionalized Telechelics Having Cyclic Ammonium Groups. Macromol. Rapid Commun. 2001, 22, 1128−1134. (11) Dong, Y. Q.; Tong, Y. Y.; Dong, B. T.; Du, F. S.; Li, Z. C. Preparation of Tadpole-Shaped Amphiphilic Cyclic PS-b-linear PEO via ATRP and Click Chemistry. Macromolecules 2009, 42, 2940−2948.

CONCLUSION

A novel synthetic strategy toward well-defined PE-based cyclic homo/copolymers was developed by combining polyhomologation with D−A and “click” reactions. By using this strategy, a series of PE-based materials with well-defined complex architectures and low polydispersity were synthesized. Tris(3(anthracen-9-ylmethoxy)propyl)borane was used as initiator for the polyhomologation of dimethylsulfoxonium methylide to give anthracene-terminated linear polyethylene; subsequent D− A reaction, esterification, and intramolecular “click” cyclization led to the single cyclic polymers. Combining this method with ROP, by using the terminal −OH group in linear polyethylene as initiate site, afforded more complex cyclic copolymers. This general strategy opens new horizons toward PE cyclic-based complex architectures when combined with other living and living/controlled polymerization techniques. Initial studies on the thermal behavior of the cyclic homo- and block copolymers revealed the big influence of the cyclic structure on the melting temperature and crystallinity as compared to their corresponding precursors. H


Article

Macromolecules (12) Fan, X. S.; Wang, G. W.; Huang, J. L. Synthesis of Macrocyclic Molecular Brushes with Amphiphilic Block Copolymers as Side Chains. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 1361−1367. (13) Li, H. Y.; Jérôme, R.; Lecomte, P. Amphiphilic Sun-Shaped Polymers by Grafting Macrocyclic Copolyesters with PEO. Macromolecules 2008, 41, 650−654. (14) Xia, Y.; Boydston, A. J.; Grubbs, R. H. Synthesis and Direct Imaging of Ultrahigh Molecular Weight Cyclic Brush Polymers. Angew. Chem., Int. Ed. 2011, 50, 5882−5885. (15) Boydston, A. J.; Holcombe, T. W.; Unruh, D. A.; Fréchet, J. M. J.; Grubbs, R. H. A Direct Route to Cyclic Organic Nanostructures via Ring-Expansion Metathesis Polymerization of a Dendronized Macromonomer. J. Am. Chem. Soc. 2009, 131, 5388−5389. (16) Lonsdale, D. E.; Monteiro, M. J. Various Polystyrene Topologies Built from Tailored Cyclic Polystyrene via CuAAC Reactions. Chem. Commun. 2010, 46, 7945−7947. (17) Tezuka, Y.; Komiya, R.; Washizuka, M. Designing 8-Shaped Polymer Topology by Metathesis Condensation with Cyclic Poly(THF) Precursors Having Allyl Group. Macromolecules 2003, 36, 12− 17. (18) Fan, X.; Huang, B.; Wang, G.; Huang, J. Synthesis of Amphiphilic Heteroeight-Shaped Polymer Cyclic-[Poly(ethylene oxide)-b-polystyrene]2 via “Click” Chemistry. Macromolecules 2012, 45, 3779−3786. (19) Josse, T.; De Winter, J.; Gerbaux, P.; Coulembier, O. Cyclic Polymers by Ring-Closure Strategies. Angew. Chem., Int. Ed. 2016, 55, 13944−13958. (20) Tu, X.-Y.; Liu, M.-Z.; Wei, H. Recent progress on cyclic polymers: Synthesis, Bioproperties, and Biomedical Applications. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 1447−1458. (21) Xu, J.; Ye, J.; Liu, S. Synthesis of Well-Defined Cyclic Poly(Nisopropylacrylamide) via Click Chemistry and Its Unique Thermal Phase Transition Behavior. Macromolecules 2007, 40, 9103−9110. (22) Han, D.; Tong, X.; Zhao, Y.; Galstian, T.; Zhao, Y. Cyclic Azobenzene-Containing Side-Chain Liquid Crystalline Polymers: Synthesis and Topological Effect on Mesophase Transition, Order, and Photoinduced Birefringence. Macromolecules 2010, 43, 3664− 3671. (23) Quirk, R. P.; Wang, S. F.; Foster, M. D.; Wesdemiotis, C.; Yol, A. M. Synthesis of Cyclic Polystyrenes Using Living Anionic Polymerization and Metathesis Ring-Closure. Macromolecules 2011, 44, 7538−7545. (24) Polymeropoulos, G.; Bilalis, P.; Hadjichristidis, N. Well-Defined Cyclic Triblock Terpolymers: A Missing Piece of the Morphology Puzzle. ACS Macro Lett. 2016, 5, 1242−1246. (25) Durmaz, H.; Dag, A.; Hizal, G.; Tunca, U. Cyclic Homo and Block Copolymers Through Sequential Double Click Reactions. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 5083−5091. (26) Stanford, M. J.; Pflughaupt, R. L.; Dove, A. P. Synthesis of Stereoregular Cyclic Poly(lactide)s via “Thiol−Ene” Click Chemistry. Macromolecules 2010, 43, 6538−6541. (27) Lepoittevin, B.; Perrot, X.; Masure, M.; Hemery, P. New Route to Synthesis of Cyclic Polystyrenes Using Controlled Free Radical Polymerization. Macromolecules 2001, 34, 425−429. (28) Kricheldorf, H. R.; Lee, S. Polylactones. 35. Macrocyclic and Stereoselective Polymerization of.beta.-D,L-Butyrolactone with Cyclic Dibutyltin Initiators. Macromolecules 1995, 28, 6718−6725. (29) Li, H.; Debuigne, A.; Jérome, R.; Lecomte, P. Synthesis of Macrocyclic Poly(ε-caprolactone) by Intramolecular Cross-Linking of Unsaturated End Groups of Chains Precyclic by the Initiation. Angew. Chem., Int. Ed. 2006, 45, 2264−2267. (30) Ryner, M.; Finne, A.; Albertsson, A.-C.; Kricheldorf, H. R. LLactide Macromonomer Synthesis Initiated by New Cyclic Tin Alkoxides Functionalized for Brushlike Structures. Macromolecules 2001, 34, 7281−7287. (31) Bielawski, C. W.; Benitez, D.; Grubbs, R. H. Synthesis of Cyclic Polybutadiene via Ring-Opening Metathesis Polymerization: The Importance of Removing Trace Linear Contaminants. J. Am. Chem. Soc. 2003, 125, 8424−8425.

(32) Boydston, A. J.; Xia, Y.; Kornfield, I.; Gorodetskaya, A.; Grubbs, R. H. Cyclic Ruthenium-Alkylidene Catalysts for Ring-Expansion Metathesis Polymerization. J. Am. Chem. Soc. 2008, 130, 12775− 12782. (33) Culkin, D. A.; Jeong, W.; Csihony, S.; Gomez, E. D.; Balsara, N. P.; Hedrick, J. L.; Waymouth, R. M. Zwitterionic Polymerization of Lactide to Cyclic Poly(lactide) by Using N-Heterocyclic Carbene Organocatalysts. Angew. Chem., Int. Ed. 2007, 46, 2627−2630. (34) Brown, H. A.; Xiong, S.; Medvedev, G. A.; Chang, Y. A.; AbuOmar, M. M.; Caruthers, J. M.; Waymouth, R. M. Zwitterionic RingOpening Polymerization: Models for Kinetics of Cyclic Poly(caprolactone) Synthesis. Macromolecules 2014, 47, 2955−2963. (35) Guo, L.; Lahasky, S. H.; Ghale, K.; Zhang, D. H. N-Heterocyclic Carbene-Mediated Zwitterionic Polymerization of N-Substituted NCarboxyanhydrides toward Poly(α-peptoid)s: Kinetic, Mechanism, and Architectural Control. J. Am. Chem. Soc. 2012, 134, 9163−9171. (36) Bielawski, C. W.; Benitez, D.; Grubbs, R. H. An “Endless” Route to Cyclic Polymers. Science 2002, 297, 2041−2044. (37) Shea, K. J.; Lee, S. Y.; Busch, B. B. A New Strategy for the Synthesis of Macrocycles. The Polyhomologation of Boracyclanes. J. Org. Chem. 1998, 63, 5746−5747. (38) Luo, J.; Shea, K. J. Polyhomologation. A Living C1 Polymerization. Acc. Chem. Res. 2010, 43, 1420−1433. (39) Shea, K. J.; Walker, J. W.; Zhu, H.; Paz, M. M.; Greaves, J. Polyhomologation. A Living Polymethylene Synthesis. J. Am. Chem. Soc. 1997, 119, 9049−9050. (40) Shea, K. J. Polyhomologation: The Living Polymerization of Ylides. Chem. - Eur. J. 2000, 6, 1113−1119. (41) Wang, D.; Zhang, Z.; Hadjichristidis, N. C1 Polymerization: A Unique Tool Towards Polyethylene-based Complex Macromolecular Architectures. Polym. Chem. 2017, 8, 4062−4073. (42) Li, Q.-Z.; Zhang, G.-Y.; Chen, J.-Z.; Zhao, Q.-L.; Lu, H.-C.; Huang, J.; Wei, L.-H.; D’Agosto, F.; Boisson, C.; Ma, Z. Well-defined Polyolefin/poly(ε-caprolactone) Diblock Copolymers: New Synthetic Strategy and Application. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 511−517. (43) Lu, H.-C.; Xue, Y.; Zhao, Q.-L.; Huang, J.; Xu, S.-G.; Cao, S.-K.; Ma, Z. Well-defined Amphiphilic Polymethylene-b-poly(acrylic acid) Diblock Copolymers: New Synthetic Strategy and Their Self-assembly. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 3641−3647. (44) Xue, Y.; Lu, H.-C.; Zhao, Q.-L.; Huang, J.; Xu, S.-G.; Cao, S.-K.; Ma, Z. Polymethylene-b-poly(styrene-co-2,3,4,5,6-pentafluoro styrene) Copolymers: Synthesis and Fabrication of Their Porous Films. Polym. Chem. 2013, 4, 307−312. (45) Chen, J.; Cui, K.; Zhang, S.; Xie, P.; Zhao, Q.; Huang, J.; Shi, L.; Li, G.; Ma, Z. New Strategy Targeting Welldefined Polymethyleneblock-Polystyrene Copolymers: The Combination of Living Polymerization of Ylides and Atom Transfer Radical Polymerization. Macromol. Rapid Commun. 2009, 30, 532−538. (46) Li, J.; Zhao, Q.-L.; Chen, J.-Z.; Li, L.; Huang, J.; Ma, Z.; Zhong, Y.-W. Highly Ordered Microporous Films Containing a Polyolefin Segment Fabricated by the Breath-figure Method Using Well-defined Polymethylene-b-polystyrenecopolymers. Polym. Chem. 2010, 1, 164− 167. (47) Wang, X.; Gao, J.-P.; Zhao, Q.-L.; Huang, J.; Mao, G.-L.; Wu, W.; Ning, Y.-N.; Ma, Z. Polymethylene-block-polystyrene Copolymers: A New Synthetic Approach Using a Combination of Polyhomologation and Reversible Addition-fragmentation Chain-transfer Polymerization and Their Microfibers and Microspheres Fabricated through Electrospinning Process. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2892−2899. (48) Zhang, H.; Gnanou, Y.; Hadjichristidis, N. Well-defined Polyethylene Molecular Brushes by Polyhomologation and Ring Opening Metathesis Polymerization. Polym. Chem. 2014, 5, 6431− 6434. (49) Zapsas, G.; Ntetsikas, K.; Kim, J.; Bilalis, P.; Gnanou, Y.; Hadjichristidis, N. Boron “Stitching” Reaction: A Powerful Tool for the Synthesis of Polyethylene-based Star Architectures. Polym. Chem. 2018, 9, 1061−1065. I


Article

Macromolecules (50) Shea, K. J.; Busch, B. B.; Paz, M. M. Polyhomologation: Synthesis of Novel Polymethylene Architectures by a Living Polymerization of Dimethylsulfoxonium Methylide. Angew. Chem., Int. Ed. 1998, 37, 1391−1393. (51) Zhang, Z.; Zhang, H.; Gnanou, Y.; Hadjichristidis, N. Polyhomologation Based on in situ Generated Boron-thexyl-silaboracyclic Initiating Sites: A Novel Strategy Towards the Synthesis of Polyethylene-based Complex Architectures. Chem. Commun. 2015, 51, 9936−9938. (52) Zhang, Z.; Gnanou, Y.; Hadjichristidis, N. Well-defined 4-Arm Stars with Hydroxy-terminated Polyethylene, Polyethylene-b-polycaprolactone and Polyethylene-b-(polymethyl methacrylate)2 Arms. Polym. Chem. 2016, 7, 5507−5511. (53) Zhang, Z.; Altaher, M.; Zhang, H.; Wang, D.; Hadjichristidis, N. Synthesis of Well-Defined Polyethylene-Based 3-Miktoarm Star Copolymers and Terpolymers. Macromolecules 2016, 49, 2630−2638. (54) Zhang, H.; Banerjee, S.; Faust, R.; Hadjichristidis, N. Living Cationic Polymerization and Polyhomologation: an Ideal Combination to Synthesize Functionalized Polyethylene−polyisobutylene Block Copolymers. Polym. Chem. 2016, 7, 1217−1220. (55) Wang, D.; Hadjichristidis, N. Allyl Borates: A Novel Class of Polyhomologation Initiators. Chem. Chem. Commun. 2017, 53, 1196− 1199. (56) Wang, D.; Zhang, Z.; Hadjichristidis, N. Boron-Catalyzed C3Polymerization of ω-2-Methyl Allylarsonium Ylide and Its C3/C1 Copolymers with Dimethylsulfoxonium Methylide. ACS Macro Lett. 2016, 5, 387−390. (57) Alkayal, N.; Hadjichristidis, N. Well-defined Polymethylenebased Block Co/terpolymers by Combining Anthracene/maleimide Diels−Alder Reaction with Polyhomologation. Polym. Chem. 2015, 6, 4921−4926. (58) Alkayal, N.; Zapsas, G.; Bilalis, P.; Hadjichristidis, N. Selfassembly Behavior of Well-defined Polymethylene-block-poly(ethylene glycol) Copolymers in Aqueous Solution. Polymer 2016, 107, 415− 421. (59) Corey, E. J.; Chaykovsky, M. Dimethyloxosulfonium Methylide ((CH3)2SOCH2) and Dimethylsulfonium Methylide ((CH3)2SCH2). Formation and Application to Organic Synthesis. J. Am. Chem. Soc. 1965, 87, 1353−1364. (60) Zhang, H.; Alkayal, N.; Gnanou, Y.; Hadjichristidis, N. Anionic Polymerization and Polyhomologation: an Ideal combination to Synthesize Polyethylene-based Block Copolymers. Chem. Chem. Commun. 2013, 49, 8952−8954. (61) Mantovani, G.; Lecolley, F.; Tao, L.; Haddleton, D. M.; Clerx, J.; Cornelissen, J. J. L. M.; Velonia, K. Design and Synthesis of NMaleimido-Functionalized Hydrophilic Polymers via Copper-Mediated Living Radical Polymerization: A Suitable Alternative to PEGylation Chemistry. J. Am. Chem. Soc. 2005, 127, 2966−2973. (62) Ihre, H.; Hult, A.; Frechet, J. M. J.; Gitsov, I. Double-Stage Convergent Approach for the Synthesis of Functionalized Dendritic Aliphatic Polyesters Based on 2,2-Bis(hydroxymethyl)propionic Acid. Macromolecules 1998, 31, 4061−4068. (63) Roovers, J.; Toporowski, P. M. Synthesis of High Molecular Weight Ring Polystyrenes. Macromolecules 1983, 16, 843−849. (64) Busch, B. B.; Paz, M. M.; Shea, K. J.; Staiger, C. L.; Stoddard, J. M.; Walker, J. R.; Zhou, X.-Z.; Zhu, H. The Boron-Catalyzed Polymerization of Dimethylsulfoxonium Methylide. A Living Polymethylene Synthesis. J. Am. Chem. Soc. 2002, 124, 3636−3646. (65) Bruce, R. Retro-Diels-Alder Reaction. Part I. C-C Dienophiles. Organic Reactions 1998, 52, 1−393.

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An Efficient and General Strategy toward the Synthesis of

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