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Synthesis and Characterization of Novel Copolymers with Different Topological Structures and TEMPO Radical Distributions Jiaxing Zhang,† Hongying Shen,† Wenguang Song,† and Guowei Wang*,†,‡ †

State Key Laboratory of Molecular Engineering of Polymers, Collaborative Innovation Center of Polymers and Polymer Composite Materials, Department of Macromolecular Science, Fudan University, Shanghai 200433, China ‡ Joint Laboratory for Adsorption and Separation Materials of Zhejiang University-Zhejiang Tobacco Industry Co. Ltd., Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: A series of novel linear and hyperbranched copolymers with different topological structures and 2,2,6,6tetramethylpiperidine-1-oxyl (TEMPO) radical distributions were synthesized, and their paramagnetic property was systematically investigated. Based on the ring-opening polymerization (ROP) mechanism of GTEMPO, glycidol (Gly) or ethylene oxide (EO) monomers, the linear Poly(EO-co-GTEMPO) and hyperbranched Poly(Gly-co-GTEMPO) copolymers were obtained from a bifunctional 2,2-dimethyl-1,3-propanediol initiator and a tetrafunctional pentaerythritol initiator, respectively. Alternatively, from the multifunctional macroinitiator of hyperbranched polyglycerol (HPG), the hyperbranched HPG-g-Poly(Gly-co-GTEMPO) and HPG-g-PGTEMPO copolymers were also targeted. The copolymers were characterized by GPC, DSC, and UV−vis analysis. The paramagnetic property of the copolymers was studied and compared by EPR analysis at different concentrations of copolymers and temperatures. The results displayed that the concentrations of copolymers majorly manipulated the signal intensity of EPR spectra, and the temperature majorly modulated the shape of EPR spectra. Essentially, the TEMPO radical distributions in copolymers played an important role: the higher regional density tended to give the EPR spectra with intense, broad peaks, while the lower one led to the regular, well-pronounced EPR spectra. The difference was rationalized to different intramolecular spin−spin exchange and dipole−dipole interaction modulated by the topological structures and the corresponding TEMPO radical distributions.

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INTRODUCTION Because of the existence of a unique electron, the stable nitroxide radicals, such as 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO),1−3 2,2,5,5-tetramethyl-1-pyrrolidineoxyl,4 galvinoxyl,5−7 and nitroxylphenyls8 or their derivatives, have been found versatile applications in material science, organic chemistry, biochemistry, and polymer chemistry.9−12 Among all the nitroxide radicals, the TEMPO radical is the most prominent example with its inexpensive price and variety of derivatives. For example, the TEMPO radical and its derivatives have been widely used as catalysts for the oxidation of alcohols to the corresponding aldehydes, ketones, and acids in organic synthesis.13−27 Also, the TEMPO radical and its derivatives have been paid considerable attention because of its rapid electron exchange reactions and proposed as the electroactive © XXXX American Chemical Society

materials for electronic devices such as organic rechargeable batteries,28−33 electrochemical diodes,34 and electrochromic displays.19,35−38 Additionally, the TEMPO radical and its derivatives can be used as reactants in nitroxide radical coupling (NRC) reaction39−44 and mediators in nitroxide-mediated radical polymerization (NMRP) mechanism in polymer chemistry.45−50 Alternatively, due to their special stability and paramagnetic property, the TEMPO radical based spin-labeling technique has found potential application in biological probe and medical imaging field.51−54 With the TEMPO radical attached onto a Received: January 22, 2017 Revised: March 16, 2017

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DOI: 10.1021/acs.macromol.7b00159 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. Illustration of Copolymers with Different Topological Structures and TEMPO Radical Distributions

glycidol (Gly) or ethylene oxide (EO) monomers, a series of novel copolymers with different topological structures and TEMPO radical distributions were designed and synthesized (Scheme 1). The linear Poly(EO-co-GTEMPO) and hyperbranched Poly(Gly-co-GTEMPO) copolymers were prepared from a bifunctional 2,2-dimethyl-1,3-propanediol initiator and a tetrafunctional pentaerythritol initiator, respectively. The HPGg-Poly(Gly-co-GTEMPO) and HPG-g-PGTEMPO copolymers were realized from the multifunctional macroinitiator of HPG. The EPR spectra of the copolymers were monitored at different concentrations and temperatures, and their paramagnetic properties were analyzed and compared.

matrix, the electron paramagnetic resonance (EPR) can be directly detected in a microwave frequency region,55 which could potentially offer a significant sensitivity advantage over the conventional magnetic resonance imaging based technique.56 Typically, the environmental factors of polarity, viscosity, steric hindrance, conformation, and configuration can have an important influence on the EPR spectrum of TEMPO radical. Thus, by analyzing an EPR spectrum, the information on the structure and dynamics of the system containing TEMPO radical can thus be captured. The typically concerned systems can be micelles57 or polymers with various topological structures.58−67 For example, Badetti et al. synthesized a series of generations of phosphorus dendrimers, and the TEMPO radical was anchored at the peripheral region.68 Their results showed that the spin−spin exchange interaction strongly depended on the generations of dendrimers. Xia et al. prepared the bottlebrush polymers by ring-opening metathesis polymerization (ROMP) of macromonomers, and the TEMPO radical was distributed at different position of the bottlebrush polymers.55,69 Accordingly, the distribution of the TEMPO radical was confirmed to exert great influence on the EPR spectrum of bottlebrush polymers. Obviously, among the already investigated system, the TEMPO radical was always deliberately and sterically hindered in a bulky polymer, in which the radical could be selectively restricted into a controlled environment. Aiming to further explain the relationship between paramagnetic property with topological structures, as well as to explore the potential applications of TEMPO radical containing system, more efficient systems are still expected to be developed. For example, the hyperbranched, biodegradable, and biocompatible TEMPO radical containing system is still rarely concerned in the literature. The development of such system might pave the way to their practical applications in the biomedical field. The hyperbranched polyglycerol (HPG) was typically synthesized by ring-opening polymerization (ROP) of glycidol (Gly), which represented a reactive oxirane monomer with latent AB2 structure.70,71 Unlike its analogue of dendrimer with perfect architecture and well-defined compositions, the HPG with defects can be readily prepared by a one-pot strategy, which had largely simplified the operation procedure. Additionally, the HPG has a globular 3D structure and a function as polyether,72−74 such as the widely used poly(ethylene oxide) (PEO). Besides the similar biodegradability and biocompatibility as those for PEG, the HPG was also featured with multiple hydroxyl groups, which can be further selectively modified for certain applications. Thus, combining the advantages of the versatile functions, biodegradability and biocompatibility, and easily synthetic procedure, the HPG has becoming a favorite candidate in the biomedical field. In this contribution, based on the controlled ROP of 4glycidoloxy-2,2,6,6-tetramethylpiperidine-1-oxyl (GTEMPO),

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EXPERIMENTAL SECTION

Materials. Glycidol (Gly, >99.0%, Aldrich), dimethyl sulfoxide (DMSO, >99.0%, Sinopharm Chemical Reagent Co., Ltd. (SCR)), and dioxane (>98.0%, SCR) were purified by distillation from CaH2 under reduced pressure. Potassium methoxide (CH3O−K+) (>95%), 2,2dimethyl-1,3-propanediol (>96%), and pentaerythritol (>98%) were all purchased from Aladdin and used as received. 4-Glycidoloxy2,2,6,6-tetramethylpiperidine-1-oxyl (GTEMPO) was synthesized according to the literature from epichlorohydrin (>98.0%, SCR) and 4-hydroxyl-2,2,6,6-tetramethylpiperidine-1-oxyl (HO-TEMPO, J&K, 98.0%) using tetrabutylammonium hydrogen sulfate (TBAHS, 99.0%, SCR) as catalyst,50,75−78 and the purity was estimated as 99.9% by gas chromatograph mass spectrometer (GC-MS). Ethylene oxide (EO, 98%, SCR) was dried by CaH2 for 48 h and then distilled under N2 before use. Diphenylmethylpotassium (DPMK) solution was freshly prepared according to the literature,79 and the concentration was calibrated as 0.70 mol/L. All other reagents were purchased from SCR and used as received except for additional declaration. Characterization. The apparent molecular weight (MW) and molecular weight distribution (Mw/Mn) of polymer were analyzed by gel permeation chromatographic (GPC) measurement, which was performed in LiBr-added DMF ([LiBr] = 14 mM) at 55 °C with an elution rate of 1.0 mL/min on an Agilent 1260 equipped with a G1310B pump, a G1362A refractive index detector, and a G1314F variable wavelength detector. Two 5 μm LP gel columns (500 Å, molecular range 500−1.2 × 105 Da and 200−1.0 × 106 Da) were calibrated using poly(methyl methacrylate) (PMMA) standards. The absolute molecular weight (MW) of polymer was determined by GPC equipped with a G1310B pump, a multiangle (13°−165°) laser light scattering (MALLS) detector (Wyatt Technology, DAWN HELLOS, with the He−Ne light wavelength at 632.8 nm), and a refractive index detector (Wyatt Technology, Optilab T-rEX). The measurement was carried out at 55 °C using LiBr-added DMF ([LiBr] = 14 mM) as the eluent with a flow rate of 1.0 mL/min. The refractive index dn/dc was measured off-line by a refractive index detector at 55 °C. Differential scanning calorimetry (DSC) analysis was carried out on a TA Q2000 thermal analysis system. The sample was first heated from −80 to 100 °C at a heating rate of 10 °C/min under a nitrogen atmosphere, followed by cooling to −80 °C at −10 °C/min after stopping at 100 °C for 2 min, and finally heated to 100 °C at 10 °C/min after stopping at −80 °C for 2 min. The ultraviolet−visible spectroscopy (UV−vis) spectrum was traced on a Shimadzu UV-3150 spectrophotometer. The 1 H NMR spectrum was recorded on a Bruker (400 MHz) B


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Macromolecules

Scheme 2. Synthetic Procedure for Linear Poly(EO-co-GTEMPO) and Hyperbranched Poly(Gly-co-GTEMPO) Copolymers

into the flask. After the system was stirred at 60 °C for 4 days, the crude product was extracted from dichloromethane (CH2Cl2) and water for three times, and the CH2Cl2 phase was further dried by anhydrous magnesium sulfate. After the CH2Cl2 solvent was evaporated under reduced pressure, the concentrated solution was precipitated into petroleum ether (60−90 °C) for three times. Finally, the purified product was dried under vacuum at 45 °C for 24 h. Linear Poly(EO-co-GTEMPO)-1: Mn,GPC = 6.05 × 103, Mw/Mn = 1.24. By changing the fed molar ratio of EO to GTEMPO monomers, the copolymer with a different content of GTEMPO unit was obtained. Linear Poly(EO-co-GTEMPO)-2: Mn,GPC = 4.05 × 103, Mw/Mn = 1.15. Synthesis of Hyperbranched Poly(Gly-co-GTEMPO) Copolymer (Scheme 2). The hyperbranched Poly(Gly-co-GTEMPO) copolymer was synthesized by ROP of Gly and GTEMPO monomers using tetrafunctional pentaerythritol as initiator. Typically, into a 150 mL round-bottomed flask, 0.0220 g (0.162 mmol) of pentaerythritol was deprotonated by 0.0407 g (0.581 mmol) of CH3O−K+, and the formed methanol (CH3OH) was removed by an azeotropic distillation with dry toluene. Subsequently, 5.0 g (21.9 mmol) of GTEMPO dissolved in 10 mL (151 mmol) of Gly was dropwisely added by a peristaltic pump over 18 h at 95 °C. After the solution was stirred at 95 °C for another 5 days, the crude product was purified by dialysis in methanol. Finally, the remaining CH3OH was evaporated, and the purified product was further dried under vacuum at 45 °C for 24 h. Poly(Gly-co-GTEMPO)-1: Mn,GPC = 1.94 × 104, Mw/Mn = 1.38, Mw,MALLS = 3.74 × 104. By changing the fed molar ratio of Gly to GTEMPO monomers, the Poly(Gly-co-GTEMPO) copolymer with a different content of GTEMPO unit was obtained. Poly(Gly-co-

spectrometer in CD3OD at 298 K. Electron paramagnetic resonance (EPR) spectrum was performed on a Bruker A300 spectrometer. The parameters with modulation frequency of 100 kHz, modulation amplitude of 1 G, time constant of 20 ms, conversion time of 30 ms, sweep time of 30 s, center field of 3362 G, sweep width of 200 G, and microwave power of 16.1 mW were adopted. Magnetic measurement was carried out with a Quantum Design MPMS (SQUID) VSM magnetometer from 300 K down to 5 K under an applied magnetics field of 0.2 T. Determination of the Reactivity Ratios of Monomers. The copolymerization of GTEMPO and EO or Gly monomers with different feed ratios ([GTEMPO]:[Gly] = 1:9, 3:7, 5:5, 7:3, and 9:1; [GTEMPO]:[EO] = 1:9, 3:7, 5:5, 7:3, and 9:1) was carried out in DMSO solvent. The copolymerization was traced and stopped at monomer conversion less than 10%. After the DMSO solvent was removed by reduced distillation, the products were washed with ethyl ether for three times to remove the remained Gly and GTEMPO monomers. The obtained copolymers were further dried and weighed, and the content of GTEMPO unit was analyzed by UV−vis measurement. The reactivity ratios were calculated by the YBR method.80 Synthesis of Linear Poly(EO-co-GTEMPO) Copolymer (Scheme 2). The linear Poly(EO-co-GTEMPO) copolymer was synthesized by ROP of EO and GTEMPO monomers using difunctional 2,2-dimethyl-1,3-propanediol as initiator. Typically, into a 150 mL round-bottomed flask, 0.1090 g (1.048 mmol) of 2,2dimethyl-1,3-propanediol and 2.80 mL (1.96 mmol) of DPMK were first added. Then, 7.0 g (30.7 mmol) of GTEMPO dissolved in 60 mL of DMSO and 3.0 mL (59.4 mmol) of EO were sequentially charged C


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Macromolecules Scheme 3. Synthetic Procedure for HPG-g-Poly(Gly-co-GTEMPO) and HPG-g-PGTEMPO Copolymers

GTEMPO)-2: Mn,GPC = 1.69 × 104, Mw/Mn = 1.40, Mw,MALLS = 1.76 × 104. Synthesis of Hyperbrached Polyglycerol (HPG) (Scheme 3). The HPG was synthesized by ROP of Gly monomer using tetrafunctional pentaerythritol as initiator. Typically, into a 150 mL three-necked flask equipped with a mechanical stirrer, 0.0230 g (0.169 mmol) of pentaerythritol was deprotonated by 0.0465 g (0.664 mmol) of CH3O−K+, and the formed CH3OH was removed by an azeotropic distillation with dry toluene. Subsequently, 60 mL of dioxane was introduced, and the system was immersed into an oil bath at 95 °C; 60 mL (906 mmol) of Gly monomer was dropwisely added by a peristaltic pump over 20 h. After the reaction was continued for another 48 h, the crude product was dissolved in CH3OH and purified by dialysis. After the remaining CH3OH was evaporated under vacuum, the purified product was further dried under vacuum at 45 °C for 24 h to a constant weight. Mn,GPC = 3.69 × 104, Mw/Mn = 1.56, Mw,MALLS = 5.98 × 105. Synthesis of Hyperbranched HPG-g-(Gly-co-GTEMPO) Copolymer (Scheme 3). The hyperbranched HPG-g-Poly(Gly-coGTEMPO) was synthesized by ROP of Gly and GTEMPO monomers using the presynthesized HPG as multifunctional macroinitiator. Similar to the synthetic procedure for Poly(Gly-co-GTEMPO) copolymer, into a 150 mL round-bottomed flask 4.1 g (55.4 mmol

Gly unit) of dry HPG was deprotonated by 0.5830 g (8.33 mmol) of CH3O−K+, and the formed CH3OH was removed by an azeotropic distillation with dry toluene. Subsequently, 2.0 g (8.77 mmol) of GTEMPO and 4 mL (60.4 mmol) of Gly monomers dissolved 90 mL of DMSO was dropwisely added by a peristaltic pump at 95 °C over 18 h. After the solution was stirred at 95 °C for another 5 days, the crude product was purified by dialysis in methanol. The purified product was further concentrated and dry under vacuum at 45 °C for 24 h. HPG-g-(Gly-co-GTEMPO)-1: Mn,GPC = 7.29 × 104, Mw/Mn = 1.25, Mw,MALLS = 6.16 × 105. By changing the fed molar ratio of HPG macroinitiator to GTEMPO and Gly monomer, HPG-g-(Gly-coGTEMPO) with a different content of GTEMPO unit was obtained. HPG-g-(Gly-co-GTEMPO)-2: Mn,GPC = 7.82 × 104, Mw/Mn = 1.32, Mw,MALLS = 7.03 × 105. Synthesis of Hyperbranched HPG-g-TEMPO Copolymer (Scheme 3). The hyperbranched HPG-g-PGTEMPO was synthesized by ROP of GTEMPO monomer using the presynthesized HPG as multifunctional macroinitiator. Similar to the synthetic procedure for Poly(Gly-co-GTEMPO), into a 150 mL round-bottomed flask, after 6.0 g (81.1 mmol Gly unit) of dry HPG was deprotonated by 1.0950 g (15.6 mmol) of CH3O−K+, 3.5 g (15.4 mmol) of GTEMPO dissolved in 90 mL of DMSO was added. The system was continuously stirred at 60 °C for 4 days, and the DMSO was removed under reduced D


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Macromolecules

pentaerythritol was used as initiator and CH3O−K+ was used as deprotonation agent. In order to avoid the interruption of CH3OH on the initiation, the formed CH3OH was completely removed by an azeotropic distillation with toluene. The GPC traces in Figure 1 displayed the monomodal peaks and low Mw/ Mn values of the synthesized Poly(Gly-co-GTEMPO) copolymers. Based on the measured dn/dc values in N,N-dimethylformamide (DMF) at 55 °C, the absolute MWs of Poly(Gly-coGTEMPO) copolymers were determined by GPC equipped with a MALLS detector (GPC-MALLS) (Table 1). However, due to the presence of the low active, monofunctional GTEMPO monomer and the typical side reaction of living species transferring to substituted epoxide monomers in ROP,81−84 the molecular weights of the synthesized linear Poly(EO-co-GTEMPO) and hyperbranched Poly(Gly-coGTEMPO) copolymers were always limited to several kg/mol. Subsequently, the TEMPO radical was introduced onto a higher molecular weight HPG, which was presynthesized by ROP of Gly monomer using tetrafunctional pentaerythritol as initiator and CH3O−K+ as deprotonation agent (Scheme 3). The monomodal peak (Mn,GPC = 3.69 × 104, based on linear PMMA standards) with Mw/Mn value (1.56) for HPG is shown in Figure 2. Accordingly, the dn/dc value was measured as 0.0640 ± 0.0014 mL/g in DMF at 55 °C, and the absolute MW (Mw,MALLS = 5.98 × 105) was determined by GPC-MALLS. Using the presynthesized HPG as multifunctional macroinitiator and CH3O−K+ as deprotonation agent, the ROP of Gly and GTEMPO monomers was carried out for hyperbranched HPG-g-Poly(Gly-co-GTEMPO) copolymers (Scheme 3). Similarly, the hyperbranched HPG-g-PGTEMPO copolymers were also synthesized by ROP of GTEMPO monomers (Scheme 3). Comparing with the GPC trace for HPG, all the GPC traces for HPG-g-Poly(Gly-co-GTEMPO) and HPG-g-PGTEMPO copolymers shift to the higher molecular weight region (Figure 2), which confirmed that the Gly and/or GTEMPO were successfully grafted onto HPG. The dn/dc values were measured and the absolute MWs of copolymers were also determined by GPC-MALLS (Table 1). As another evidence for the successful synthesis of the copolymers with different topological structures and TEMPO radical distributions, the glass transition temperature (Tg) can be traced to analyze the properties of different copolymers. Typically, the as-synthesized HPG macroinitiator was a liquidlike viscous matter, while the introduction of GTEMPO unit onto HPG or the copolymerization of GTEMPO with Gly monomer would result to a red, solid-like matter. Thus, the DSC measurement was performed to measure the Tg and quantify the properties of copolymers. As shown in Figure 3, each DSC curve showed one Tg transition, and the Tg of HPG occurred at −13.1 °C. When the GTEMPO monomer was copolymerized with Gly monomer, the Tg of Poly(Gly-coGTEMPO)-1 was measured as 0.2 °C, and the Tg of Poly(Glyco-GTEMPO)-2 was given as 7.8 °C. When GTEMPO was introduced onto HPG, the T g of HPG-g-Poly(Gly-coGTEMPO)-1 was given as 4.5 °C, and the Tg of HPG-gPoly(Gly-co-GTEMPO)-2 was given as 7.5 °C. Also, the Tg of HPG-g-PGTEMPO-1 shift to 6.0 °C, and the Tg of HPG-gPGTEMPO-2 shift to 13.3 °C. Thus, the DSC results led to a conclusion that the GTEMPO unit tends to increase the Tgs of hyperbranched Poly(Gly-co-GTEMPO), HPG-g-Poly(Gly-coGTEMPO), and HPG-g-GTEMPO copolymers. The DSC results further confirmed that the copolymers were actually synthesized.

pressure. The crude product was dissolved in methanol and purified by dialysis. After the remaining methanol was evaporated under vacuum, the purified product was further dried under vacuum at 45 °C for 24 h to a constant weight. HPG-g-PGTEMPO-1: Mn,GPC = 9.54 × 104, Mw/ Mn = 1.41, Mw,MALLS = 1.17 × 106. By changing the fed molar ratio of HPG macroinitiator to GTEMPO monomer, the HPG-g-PGTEMPO copolymer with a different content of GTEMPO unit was obtained. HPG-g-PGTEMPO-2: Mn,GPC = 9.33 × 104, Mw/Mn = 1.38, Mw,MALLS = 1.34 × 106.

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RESULTS AND DISCUSSION Synthesis and Characterization of Copolymers with Different Topological Structures and TEMPO Radical Distributions. In order to systematically investigate the effect of topological structures and TEMPO radical distributions on the paramagnetic property of copolymers, a series of novel copolymers were designed and synthesized. Specifically, the linear Poly(EO-co-GTEMPO) and hyperbranched Poly(Gly-coGTEMPO), HPG-g-Poly(Gly-co-GTEMPO) and HPG-gPGTEMPO copolymers were included (Schemes 2 and 3). The reactivity ratio of the monomer pair of EO and GTEMPO was determined as r1(EO) = 5.42 and r2(GTEMPO), and that of the monomer pair of Gly and GTEMPO was determined as the r1(Gly) = 4.33, and r2(GTEMPO) = 0.63. Obviously, the reactivities of EO and Gly were higher than that of the GTEMPO monomer. In order to reduce the influence of reactivity ratio on the distribution of GTEMPO unit, a dropwise addition procedure assisted by a peristaltic pump was adopted for the hyperbranched Poly(Gly-co-GTEMPO) and HPG-g-(Gly-co-GTEMPO) copolymers. Thus, the distribution of the TEMPO radical in copolymers would be predominantly manipulated by the addition rate of Gly and GTEMPO monomers. First, using the difunctional 2,2-dimethyl-1,3-propanediol as initiator and DPMK as deprotonation agent, the linear Poly(EO-co-GTEMPO) copolymers were synthesized by ROP of EO and GTEMPO monomers in DMSO (Scheme 2). By changing the fed molar ratio of EO to GTEMPO monomers, the copolymers with different content of GTEMPO unit were obtained. The monomodal peaks and low Mw/Mn values shown in Figure 1 gave solid evidence that the copolymers have been successfully synthesized. Alternatively, when the GTEMPO monomer was copolymerized with Gly monomer, a hyperbranched Poly(Gly-co-GTEMPO) copolymer can be formed. In this case, the tetrafunctional

Figure 1. GPC traces for linear Poly(EO-co-GTEMPO) and hyperbranched Poly(Gly-co-GTEMPO) copolymers by using DMF as eluent. E


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Macromolecules Table 1. Data for Copolymers samples linear poly(EO-coGTEMPO)-1 linear poly(EO-coGTEMPO)-2 hyperbranched poly(Gly-coGTEMPO)-1 hyperbranched poly(Gly-coGTEMPO)-2 HPG-g-Poly(Glyco-GTEMPO)-1 HPG-g-Poly(Glyco-GTEMPO)-2 HPG-gPGTEMPO-1 HPG-gPGTEMPO-2

yield (%)

Mn,GPCa (×104)

Mw/Mna

67

0.61

1.24

dn/dcb (mL/g)

theor content of GTEMPO unitc (g %)

content of GTEMPO unit (g %) by UV−visd

content of GTEMPO unit (g %) by EPRe

0.464h

27

22

20

h

73

69

64

Mw,MALLS (×104)b

0.750

regional density of GTEMPO unitf (g %)

Tgg (°C)

62

0.40

1.15

58

1.94

1.38

0.0656 ± 0.0010

3.74

31

15

11

15

0.2

56

1.69

1.40

0.0764 ± 0.0013

1.76

47

26

21

26

7.8

86

7.29

1.25

0.0517 ± 0.0009

61.6

23

12

13

24

4.5

71

7.82

1.32

0.0549 ± 0.0013

70.3

40

15

18

28

7.5

78

9.54

1.41

0.0718 ± 0.0030

117

37

21

22

100

6.0

74

9.33

1.38

0.0714 ± 0.0016

134

63

37

37

100

13.3

a

The Mn,GPC and Mw/Mn of copolymer were estimated by GPC measurement in DMF elution using PMMA as standards. bThe dn/dc value was measured by refractive index detector, and Mw,MALLS was measured by GPC equipped with a MALLS detector (GPC-MALLS). cThe theoretical content of GTEMPO unit was calculated according to the fed EO, GTEMPO, Gly, and/or HPG. dThe content of GTEMPO unit was obtained by UV−vis measurement. eThe content of GTEMPO unit was obtained by EPR measurement. fThe regional density of GTEMPO unit in bulk state was calculated according to the content of GTEMPO unit by UV−vis measurement and the molecular weight of copolymer. gThe Tg was determined in the second heating cycle in DSC measurement. hThe Mw was an apparent molecular weight.

TEMPO radical were always interrupted, and the resonance signals were difficult to be discriminated. For example, in the 1 H NMR spectrum for GTEMPO monomer (Figure S1), except for the signals at 2.0−4.5 ppm for protons (−OCH2CH(CH2)O−) on epoxide ring (which was far away from the nitroxide radical), all the resonance signals at 0.60− 1.50 ppm for protons (−C(CH3)2CH2) close to TEMPO radical cannot be clearly discriminated. Thus, the contents of the GTEMPO unit in copolymers cannot be determined by 1H NMR spectra. Alternatively, the UV−vis measurement was further adopted to give a more accurate and quantitative information on the content of GTEMPO unit. According to the literature,86−90 the UV−vis spectrum for a nitroxide radical typically showed two absorption peak at 242 nm (for π−π* transition, ε = 2 × 103 M−1 cm−1) and 424 nm (for n−π* transition, ε = 10−13 M−1 cm−1), respectively. The higher molar extinction coefficient at 242 nm was expected to give a more accurate information. The UV−vis spectrum of GTEMPO monomer closely resembled that of the copolymer containing TEMPO radical, which was a prerequisite for a reliable analysis using this technique. Actually, all the absorption spectra recorded for GTEMPO monomer, linear Poly(EO-co-GTEMPO) and hyperbranched Poly(Gly-coGTEMPO), HPG-g-Poly(Gly-co-GTEMPO), and HPG-gPGTEMPO have a maximum peak at 242 nm. The interruption of the HPG can also be excluded because the absorption spectrum for HPG was flattened without any signal at this region (Figure 4A). Focused on the intensity at 242 nm, a calibration plot was drawn from a series of methanol solutions of GTEMPO monomer (Figure 4B), and a molar extinction coefficient of 2047.5 M−1 cm−1 was calculated. Based on the calibration plot, the contents of GTEMPO unit in all copolymers were calculated and listed in Table 1. Obviously, the measured content of GTEMPO unit was largely lower than the theoretical one. The reason can be attributed to the lower activity of GTEMPO monomers. Additionally, the UV−vis results were rather consistent with those obtained from DSC

Figure 2. GPC traces for HPG, HPG-g-(Gly-co-GTEMPO), and HPG-g-PGTEMPO copolymers by using DMF as eluent.

Figure 3. DSC curves for HPG (A), Poly(Gly-co-GTEMPO)-1 (B), Poly(Gly-co-GTEMPO)-2 (C), HPG-g-Poly(Gly-co-GTEMPO)-1 (D), HPG-g-Poly(Gly-co-GTEMPO)-2 (E), HPG-g-PGTEMPO-1 (F), and HPG-g-PGTEMPO-2 (G).

However, due to the paramagnetic nature of the TEMPO radical,85 the 1H NMR spectra for copolymers containing a F


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Figure 4. (A) UV−vis absorption spectra for GTEMPO (0.05 mg/mL), HPG-g-PGTEMPO-1 copolymer (0.33 mg/mL), and HPG (0.30 mg/mL) in methanol solvent. (B) Beer−Lambert linear calibration plot obtained for GTEMPO monomer in methanol at 242 nm used to calculate the content of GTEMPO unit.

broadened peaks were accorded to a typical EPR spectrum for a polyradical polymer due to the comprehensive intramolecular spin−spin exchange and dipole−dipole interaction.2,68,92,93 Nevertheless, because of the relatively low molecular weight of linear Poly(EO-co-GTEMPO) copolymer and its completely stretched main chain in good solvent of methanol, the GTEMPO unit can be uniformly distributed in solution, and its EPR spectrum totally resembled that for GTEMPO monomer. Alternatively, when the hyperbranched Poly(Glyco-GTEMPO) was concerned, the globular 3D structure of hyperbranched copolymers seriously limited the arrangement of GTEMPO unit. In fact, in the case for the linear Poly(EO-coGTEMPO), the regional density of TEMPO radical can be regarded as the mean concentration of TEMPO radical in solution. However, in the case for hyperbranched copolymer, the copolymer can only be swollen (rather than dissolved) in methanol solvent because of the restriction of their 3D structure. Thus, the accurate regional density of TEMPO radical was difficult to measure because of the limitation on quantifying of the actual volume of the swollen copolymer in solution. Alternatively, the regional density in bulk state can be approximately referred, which can be calculated according to the content of GTEMPO unit and the molecular weight of copolymers (Table 1). Correspondingly, the hyperbranched Poly(Gly-co-GTEMPO) copolymer would contribute to a higher regional density of TEMPO radical than that for linear Poly(EO-co-GTEMPO) copolymer. The intramolecular spin− spin exchange and dipole−dipole interaction was thus enhanced, and the peaks in EPR spectra were further broadened. However, during the synthesis of hyperbranched Poly(Gly-co-GTEMPO) copolymers by direct copolymerization of monofunctional GTEMPO and difunctional Gly monomers, the copolymers were always limited with low degree of branching and low molecular weights. The hyperbranched Poly(Gly-co-GTEMPO) copolymer still have some solubility in methanol solvent. Thus, the amplification of the regional density of TEMPO radical was still limited. Furthermore, for the hyperbranched HPG-g-Poly(Gly-coGTEMPO) copolymers, the peripheral Poly(Gly-co-GTEMPO) section can be uniformly and densely grafted onto HPG by growing GTEMPO and Gly monomers from a large amount of the hydroxyl groups. The densely grafted Poly(Gly-coGTEMPO) section would actually decrease the solubility of copolymers and increase the regional density of TEMPO radical, which enhanced the intramolecular spin−spin exchange and dipole−dipole interaction. Comparing with the EPR

measurement; i.e., the higher content of GTEMPO unit tended to give a higher Tg value. Thus, from the above results analyzed by GPC, DSC, and UV−vis measurements, we can comprehensively conclude that we had successfully synthesized the linear and hyperbranched copolymers with different topological structures and TEMPO radical distributions. EPR Analysis on Copolymers with Different Topological Structures and TEMPO Radical Distributions. The EPR analysis on a series of the synthesized linear Poly(EO-coGTEMPO) and hyperbranched Poly(Gly-co-GTEMPO), HPGg-Poly(Gly-co-GTEMPO), and HPG-g-PGTEMPO copolymers with different topological structures and TEMPO radical distributions was performed under varied concentrations of copolymers and temperatures, and the EPR spectra were systematically compared. First, the effect of the concentrations of copolymers on paramagnetic property was investigated. As a contrast, the EPR spectrum for GTEMPO monomer was also monitored at 190 K (Figure S2). Typically, the EPR spectrum for GTEMPO monomer showed three narrow, gradually weakened, wellpronounced peaks centered at g = 2.0050 for coupling of the free electron to the 14N nucleus, which was rather consistent with the previous report on the derivative of TEMPO with low molecular weight.91 Similarly, the EPR spectra at 190 K for linear Poly(EO-co-GTEMPO) and hyperbranched Poly(Gly-coGTEMPO), HPG-g-Poly(Gly-co-GTEMPO), and HPG-gPGTEMPO copolymers also showed a similar g value close to 2.0050 (Figure 5). For a certain copolymer, the increasing of the copolymer concentration from 0.1 to 25 mg/mL only led to the enhanced signal intensity of EPR spectra and had negligible effect on the shape of EPR spectra. This phenomenon can be rationalized as that the EPR spectra were predominantly influenced by the intramolecular spin−spin exchange and dipole−dipole interaction, rather than an intermolecular one. Additionally, for the copolymers with the same topological structures, less difference can be discriminated with the increasing of content of GTEMPO unit in copolymers. However, for the linear Poly(EO-co-GTEMPO) and hyperbranched Poly(Gly-co-GTEMPO), HPG-g-Poly(Gly-co-GTEMPO), and HPG-g-PGTEMPO copolymers with different topological structures, there were some regular changes can be pronounced in the EPR spectra (Figure 5). Specifically, the EPR spectra for linear Poly(EO-co-GTEMPO) copolymers still showed three gradually weakened peaks, which were somewhat broader than those for GTEMPO monomer (Figure S2). These G


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Figure 5. EPR spectra for linear Poly(EO-co-GTEMPO) (A), hyperbranched Poly(Gly-co-GTEMPO) (B), HPG-g-Poly(Gly-co-GTEMPO) (C), and HPG-g-PGTEMPO (D) from 0.1 to 25 mg/mL in methanol at 190 K.

spectra for the above hyperbranched Poly(Gly-co-GTEMPO) copolymers, the peaks in the EPR spectra for hyperbranched HPG-g-Poly(Gly-co-GTEMPO) copolymers were largely broadened. Continuously, when the PGTEMPO homopolymer was grafted onto HPG for hyperbranched HPG-g-PGTEMPO, the densely grafted PGTEMPO and the uniformly arranged GTEMPO unit provided the highest regional density of TEMPO radical. The EPR spectra for hyperbranched HPG-gPGTEMPO copolymers were further broadened and the typical three peaks in EPR spectra has been difficult to be pronounced. Subsequently, the influence of temperatures (from 190 to 300 K) on EPR spectra was studied by fixing the concentration of copolymers at 10 mg/mL in methanol (Figure 6). For the

linear Poly(EO-co-GTEMPO) copolymers with 22% content of GTEMPO unit, with the increasing of temperature from 190 to 300 K, the EPR spectra were changed from the three gradually weakened peaks to three identical peaks. Differently, in the case with 69% content of GTEMPO unit, the three gradually weakened peaks at 190 K were evolved as intense, broad peaks at 300 K. The broadening of the superfine EPR peaks at higher temperature can be attributed to the enhanced intramolecular spin−spin exchange and diploe−diploe interaction between the TEMPO radicals. Additionally, as discussed in previous section, the EPR spectra for linear Poly(EO-co-GTEMPO) copolymers at lower temperature (190 K) were rather similar to that for a GTEMPO monomer. However, at the higher temperature of H


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Figure 6. EPR spectra for linear Poly(EO-co-GTEMPO) (A), hyperbranched Poly(Gly-co-GTEMPO) (B), HPG-g-Poly(Gly-co-GTEMPO) (C), and HPG-g-PGTEMPO (D) from 190 to 300 K in methanol at 10 mg/mL.

pronounced. Especially, for the hyperbranched HPG-gPGTEMPO, one can observe that the originally broadened three peaks at 190 K had been gradually evolved as a single broad peak at 300 K, which can be attributed to the higher temperature and densely introduced TEMPO radicals in copolymers. The evolution of the EPR spectra under different temperatures can be explained according to the Boltzmann distribution rule,94,95 in which the contribution of the electron spins on the Zeeman splitting depended on the energy levels. Given a system, when the temperature was lowered, the difference

300 K, the EPR spectra were rather different to that for a GTEMPO monomer (Figure S2). Hence, this result further gave a strong proof that there was actually much difference between the high molecular weight polyradical and small molecular weight GTEMPO monomer. Similar to the evolution of EPR spectra for linear Poly(EOco-GTEMPO) copolymers, the same evolution of EPR spectra from 190 to 300 K can be discriminated for hyperbranched Poly(Gly-co-GTEMPO) and HPG-g-Poly(Gly-co-GTEMPO) copolymers. Also, the obvious derivation of EPR spectra from the lower content of GTEMPO unit to a higher one can be I


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distributions in copolymers mediated by different topological structures exerted a predominant contribution to the EPR spectra. Comparing with the linear Poly(EO-co-GTEMPO) with lower regional density of TEMPO radicals, the hyperbranched Poly(Gly-co-GTEMPO), HPG-g-Poly(Gly-co-GTEMPO), and HPG-g-PGTEMPO with higher regional density of TEMPO radical favored a strong intramolecular spin−spin exchange and dipole−dipole interaction. The progress on these novel TEMPO radical contained copolymers is expected to find potential applications in the biomedical field in the near future.

between high and low energy level increases, and the EPR signal can thus be well pronounced. Additionally, the increasing of the viscosity at lower temperature would also restrict the mobility and collision between TEMPO radicals in copolymers and finally tended to display a well-discriminated EPR spectrum. Based on the EPR spectra, the content of GTEMPO unit in copolymers can be also calculated (Table 1). The obtained values by EPR measurement were rather close to those by UV− vis measurement. Furthermore, as shown in Figure 7, the

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00159. 1 H NMR spectra and EPR spectrum for GTEMPO monomer (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.W.). ORCID Figure 7. Plots of χm−temperature and 1/χm−temeprature for HPG-gPGTEMPO-1 (A) and HPG-g-PGTEMPO-2 (B).

Guowei Wang: 0000-0003-2595-8269 Notes

The authors declare no competing financial interest.

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temperature dependence of the paramagnetic susceptibility (χm) for HPG-g-PGTEMPO-1 and HPG-g-PGTEMPO-2 was measured by SQUID instrument. Obviously, at the temperature above 40 K, the χm reaches an approximate constant value, and the χm−temperature plot conforms to the Curie−Weiss law, which was rather consistent with the literature.96−98 The values of Curie constant (C) and Weiss temperature (Θ) can be drivatived from the 1/χm−temperature plot below 100 K. For HPG-g-PGTEMPO-1, C = 1.006 emu K/g and Θ = −3.56 K. For HPG-g-PGTEMPO-2, C = 2.012 emu K/g and Θ = −6.67 K. Thus, based on the above results from EPR and SQUID measurements, the relationship between the paramagnetic property and topological structures, as well as the TEMPO radical distributions, was systematically compared and extracted. One can concluded that the essential regional density of TEMPO radical always played an important role to modulate the EPR spectra.

ACKNOWLEDGMENTS We appreciate the financial support of this research by the Natural Science Foundation of China (21274024) and Joint Laboratory for Adsorption and Separation Materials of Zhejiang University-Zhejiang Tobacco Industry Co. Ltd.

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REFERENCES

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CONCLUSIONS In summary, a series of novel linear Poly(EO-co-GTEMPO) and hyperbranched Poly(Gly-co-GTEMPO), HPG-g-Poly(Glyco-GTEMPO), and HPG-g-PGTEMPO copolymers with different topological structures and TEMPO radical distribution were constructed by ROP of GTEMPO, Gly, or EO monomers. The successful synthesis of the copolymers was confirmed by GPC measurement with monomodal peaks and narrow Mw/Mn and by DSC with higher Tgs than that for HPG. Also, the UV− vis was used to quantitatively calibrate the contents of GTEMPO unit in copolymers. The paramagnetic property of the copolymers was further studied by EPR analysis at different concentrations of copolymers and temperatures. The concentration of copolymers was confirmed to have an effect on the signal intensity, while the temperature had an important influence on the EPR shape. Essentially, the TEMPO radical J


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