Aqueous RAFT Polymerization of Acrylonitrile - Macromolecules (ACS

Aug 8, 2016 - Controlled radical polymerization of acrylonitrile (AN) in concentrated aqueous solutions of sodium thiocyanate (NaSCN, 50 wt %) and zin...
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Aqueous RAFT Polymerization of Acrylonitrile Maciej Kopeć, Pawel Krys, Rui Yuan, and Krzysztof Matyjaszewski* Center for Macromolecular Engineering, Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States S Supporting Information *

ABSTRACT: Controlled radical polymerization of acrylonitrile (AN) in concentrated aqueous solutions of sodium thiocyanate (NaSCN, 50 wt %) and zinc chloride (ZnCl2, 60 wt %) is reported. Reversible addition−fragmentation chain transfer (RAFT) polymerization was successfully accomplished with 4cyano-4-(phenylcarbonothioylthio)pentanoic acid (CPAD) as a chain transfer agent (CTA) and AIBN as a radical initiator at 65 °C. First-order linear kinetic plots were observed in aqueous NaSCN with molecular weights (MW) between 3000 and 60 000 and relatively narrow molecular weight distributions (Mw/Mn = 1.2−1.4). Chain extension of the synthesized PAN macroinitiator with AN in aqueous NaSCN showed a clear shift in MWs which indicated high retention of chain end functionality. Furthermore, RAFT polymerization of AN in aqueous ZnCl2 (60 wt %) was conducted with CPAD and 2,2′-azobis(4-methoxy-2,4dimethylvaleronitrile) (V-70) as radical initiator at 30 °C. This system also exhibited linear kinetics and produced PAN with Mn = 13 300 but slightly higher dispersity (Mw/Mn = 1.38).

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INTRODUCTION Polyacrylonitrile (PAN) is one of the most important industrial polymers due to its fiber-forming properties and facile carbonization into graphitic structures.1,2 However, the low solubility of PAN, resulting from strong dipole−dipole interactions between the highly polar nitrile groups, generates significant processing challenges.3 Indeed, PAN is not soluble in its own monomer, rendering bulk polymerization of acrylonitrile (AN) unfeasible due to precipitation of low molecular weight (MW) polymers. Hence, free radical polymerization (FRP) in solution, suspension, or emulsion is usually employed. PAN is soluble in polar organic solvents such as dimethylformamide (DMF), dimethylacetamide (DMAA), dimethyl sulfoxide (DMSO), and ethylene carbonate (EC).4 These solvents are toxic, expensive and exhibit high boiling points, which makes polymer purification difficult.5,6 Alternatively, concentrated aqueous solutions of particular inorganic salts, such as sodium thiocyanate (NaSCN, 50 wt %) or zinc chloride (ZnCl2, 60 wt %), have been used in fiber technology to dissolve high-MW PAN.4,7−9 The fiber spinning process is followed by precipitation in water bath, which simultaneously removes the excess salt. Although FRP in aqueous NaSCN solution is industrially used to manufacture spin-ready PAN dopes,2,7 only a few literature reports have investigated this system, including homopolymerization,10 copolymerization with methyl acrylate (MA)11 or vinyl acetate (VAc),12 and grafting from casein fibers.13 The development of reversible deactivation radical polymerization (RDRP) techniques, such as atom transfer radical polymerization (ATRP) and reversible addition−fragmentation chain transfer polymerization (RAFT), has opened a simple © XXXX American Chemical Society

route for the preparation of an expanded range of polymers with predefined MW and narrow molecular weight distribution (MWD).14−20 In RAFT polymerization, control over polymer structure is achieved through rapid establishment of an equilibrium between active propagating radicals (Pn• and Pm•) and dormant thiocarbonylthio compounds (Scheme 1). This Scheme 1. Addition−Fragmentation Equilibrium in RAFT Polymerization

provides an equal probability for all chains to propagate, leading to polymers with narrow MWD. As in any radical polymerization, termination events in RAFT are unavoidable.21 Nevertheless, chains originating from thiocarbonylthio compound are prevalent over those originating from free radical initiator. Thus, the majority of chains remain “living” after polymerization and can be reused as (macro)chain transfer agents to form block copolymers.15,16 Both ATRP,22−29 including recently reported metal-free ATRP,30 and RAFT31−36 were used to synthesize well-defined PAN and AN copolymers with controlled compositions and Received: June 21, 2016 Revised: July 30, 2016

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Macromolecules architectures.37 This resulted in applications of PAN-based (co)polymers as precursors to various nanostructured carbon materials, e.g., films with ordered morphology,38,39 nanoparticles26,35 or mesoporous, and nitrogen-doped carbons.40−42 RDRP techniques such as ATRP43−46 and RAFT47 were also successfully applied to polymerize a broad range of monomers in aqueous media. Specifically, aqueous RAFT is efficient due to high tolerance to different functional groups, relatively good hydrolytic stability of chain transfer agents at acidic pH, and ability to conduct reaction under mild conditions. However, to date, only DMF, DMSO, or EC were used as the reaction media for RDRP of AN. Herein, we report a new RAFT system that can be employed to polymerize AN in an aqueous solution of NaSCN or ZnCl2.

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Polymerization of Acrylonitrile in an Aqueous Solution of ZnCl2. 17 mg of CPAD (0.061 mmol, 1 equiv), 5 mL of an aqueous solution of ZnCl2 (60 wt %), and 0.2 mL of DMF (NMR standard) were added to a 10 mL Schlenk flask and degassed for 30 min. 1.63 g (31 mmol, 500 equiv) of degassed AN was then added to the flask. The polymerization was started by adding 7.6 mg of V-70 (0.025 mmol, 0.4 equiv) and immersing the flask in an oil bath at 30 °C for 24 h. The final polymer was isolated by adding the reaction mixture to a mixture of methanol/water (1:1, v/v), and the precipitate was dried under vacuum at room temperature overnight. Characterization. 1H and 13C NMR measurements were performed on a Bruker Avance 300 or 500 MHz spectrometer in DMSO-d6. Spectra were used to determine the conversion of monomer, the resulting molecular weights (Mn,NMR), and tacticity of the PAN homopolymer. The apparent molecular weights (Mn,GPC) and molecular weight distributions (Mw/Mn) were determined by gel permeation chromatography (GPC). The samples were dried under a stream of air to remove water and then redissolved in DMF. The GPC system used consisted of a Waters 515 HPLC pump and a Waters 2414 refractive index detector using Waters columns (Styrogel 102, 103, and 105 Å) with DMF containing 10 mM LiBr as the eluent at a flow rate of 1 mL/min at 50 °C. Linear PEO (for PAN) and PMMA (for PAN-b-PBA) standards were used for calibration. UV−vis measurements were conducted on an Agilent 8453 UV−vis spectrophotometer.

EXPERIMENTAL SECTION

Materials. Acrylonitrile (AN, Sigma-Aldrich, >99%) and n-butyl acrylate (BA, Sigma-Aldrich, >99%) were purified by passing through a column of basic alumina to remove the inhibitor. Azobis(isobutyronitrile) (AIBN, Sigma-Aldrich, 98%) was recrystallized from methanol. 4-Cyano-4-(phenylcarbonothioylthio)pentanoic acid (CPAD, >97%, Sigma-Aldrich), 2-cyano-2-propyl benzodithioate (CPBDT, >97%, Sigma-Aldrich), 2,2′-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (VA-044, Wako), 2,2′-azobis(4-methoxy2,4-dimethylvaleronitrile) (V-70, Wako), sodium thiocyanate (NaSCN, >98%, Sigma-Aldrich), zinc chloride (>98%, Sigma-Aldrich), dimethylformamide (DMF, Fisher, 99.9%), and methanol (Fisher, 99.9%) were used as received. Milli-Q water (Millipore) was used in all experiments. Polymerization of Acrylonitrile in an Aqueous Solution of NaSCN. In a typical procedure, 4 mg of AIBN (0.025 mmol, 0.4 equiv), 17 mg of CPAD (0.061 mmol, 1 equiv), 3 mL of an aqueous solution of NaSCN (50 wt %), and 0.2 mL of DMF (NMR standard) were added to a 10 mL Schlenk flask followed by 30 min of degassing by bubbling nitrogen. 1.63 g (31 mmol, 500 equiv) of degassed AN was then added to the flask. The polymerization was started by immersing the flask in an oil bath at 65 °C for 24 h. The final polymer was isolated by addition of the polymerization solution to a mixture of methanol/water (1:1, v/v). The precipitate was filtered, collected, and dried under vacuum overnight at room temperature. Polymerization of Acrylonitrile in DMSO. 24 mg of AIBN (0.15 mmol, 0.4 equiv), 81 mg of CPBDT (0.37 mmol, 1 equiv), 9 mL of DMSO, and 0.7 mL of DMF were added to a 25 mL Schlenk flask followed by 30 min of degassing by bubbling nitrogen. 4.89 g (92 mmol, 250 equiv) of degassed AN was then added to the flask. The polymerization was started by immersing the flask in an oil bath at 65 °C for 8 h. The final polymer was isolated by adding the polymerization solution to a mixture of methanol/water (1:1, v/v). The precipitate was filtered, collected, and dried under vacuum overnight at room temperature. Chain Extension of a PAN-CTA with AN. 153 mg (0.03 mmol, 1 equiv) of dithiobenzoate-terminated PAN (Mn,GPC = 7200, Mn,NMR = 5300, Mw/Mn = 1.24) was dissolved in 2 mL of aqueous NaSCN (50 wt %), placed in a 10 mL Schlenk flask, and degassed for 30 min. 1 mL (15 mmol, 500 equiv) of degassed AN and 2 mg (0.012 mmol, 0.4 equiv) of AIBN were then added to the flask. The polymerization was started by immersing the flask in an oil bath at 65 °C for 20 h. The same conditions were used for chain extension experiments conducted in DMSO. Chain Extension of a PAN-CTA with BA. 175 mg (0.035 mmol, 1 equiv) of dithiobenzoate-terminated PAN (Mn,GPC = 7200, Mn,NMR = 5300, Mw/Mn = 1.24) synthesized in aqueous NaSCN was dissolved in 2 mL of DMF, placed in a 10 mL Schlenk flask, and degassed for 30 min. 1 mL (7 mmol, 200 equiv) of degassed BA and 2.3 mg (0.014 mmol, 0.4 equiv) of AIBN were then added to the flask. The polymerization was started by immersing the flask in an oil bath at 65 °C for 22 h. The same conditions were used for a macro-CTA synthesized in DMSO.

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RESULTS AND DISCUSSION 4-Cyano-4-(phenylcarbonothioylthio)pentanoic acid (CPAD) was selected as the water-soluble chain transfer agent (CTA) with AIBN as the free radical source. Although solubility of both CPAD and AIBN in aqueous NaSCN (50 wt %) is limited, they dissolved readily after AN was added to the mixture. Thus, the initial AN/solvent ratio was set at 1:1.5 (v/ v) in order to ensure solubility of AIBN, CTA, and the growing polymer at high monomer conversions. Figure 1 shows firstorder kinetic plots and MW evolution for polymerizations conducted at AN/CPAD ratio 500:1 and different concentrations of AIBN. Linear semilogarithmic kinetic plots were observed in all reactions up to 7 h at 65 °C (Figure 1a) with an increasing rate upon increasing initial concentrations of AIBN.

Figure 1. (a) First-order kinetic plots and (b) MW and MWD evolution in RAFT polymerization of AN in aqueous solution of NaSCN. [AN]:[CPAD]:[AIBN] = 500:1:n, n = 0.2, 0.3, 0.4, or 0.5; AN:50 wt % aqueous NaSCN = 1:1.5 (v/v), T = 65 °C. (c) GPC traces of PAN prepared using molar ratios [AN]:[CPAD]:[AIBN] = 500:1:0.4. B

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Macromolecules Table 1. Results of RAFT Polymerization of AN in Aqueous Solution of Sodium Thiocyanatea entry 1

[AN]0/[CPAD]0/[AIBN]0 500/1/0.2

2

500/1/0.3

3

500/1/0.4

4

500/1/0.5

5

100/1/0.4

6

250/1/0.4

7

1000/1/0.4

8

2000/1/0.4

9

2000/1/0.4d

time (h)

conv (%)

Mn,thb

Mn,GPC

7 24 7 24 7 24 7 24 7 24 7 24 7 24 7 24 6 24

28 44 42 49 38 64 56 62 44 82 43 72 36 66 38 43 17 59

7 650 11 800 11 400 13 150 10 450 17 350 15 200 16 500 2 600 4 360 6 000 9 550 19 450 35 300 40 600 45 900 17 900 62 900

6 500 12 450 10 350 15 500 9 700 17 000 14 000 19 700 2 000c 3 300 6 650 10 200 20 200 30 500 26 000 40 000 25 150 57 250

Mn,NMR 12 850 16 150 17 550 16 900 2 700 8 300 28 200 30 100 60 200

Mw/Mn 1.25 1.23 1.23 1.34 1.23 1.30 1.27 1.26 1.11c 1.32 1.17 1.28 1.24 1.31 1.28 1.53 1.24 1.38

AN:50 wt % aqueous NaSCN = 1:1.5 (v/v); T = 65 °C, 24 h. Calculated according to Mn,th = ([AN]0 × conversion × MAN)/[CPAD]0 + MCPAD. Values from deconvoluted peaks; see Supporting Information (Figure S2) for details. dVA-044 as the radical initiator; AN:50% aqueous NaSCN = 1:2 (v/v); T = 44 °C.

a

b

c

However, after the initial 7 h, all reactions exhibited a decrease in polymerization rate due to depletion of AIBN, with final conversions after 24 h ranging between 44 and 64% (Table 1, entries 1−4). MWs increased linearly with conversion and stayed close to theoretical values even for higher AIBN loadings. Mw/Mn values remained low, between 1.20 and 1.25, for all of the reactions. After 24 h, some tailing could be observed in the GPC traces that resulted in a slightly broader MWD, Mw/Mn = 1.30 at 64% conversion for AIBN/CPAD ratio 0.4 (Figure 1c). The structure and molecular weights of precipitated polymers were confirmed by NMR and were in excellent agreement with Mn,th (Table 1 and Figure S1). PAN with a high MW (Mn > 50 000) is necessary for applications in fiber industry, whereas preparation of templated carbon nanostructures, such as mesoporous carbons, requires low-MW PAN (degree of polymerization (DP) 50−200) in order to maintain nanometer-scale phase-separated morphologies.40,48 Thus, different target DPs were examined to investigate the versatility of the developed conditions to synthesize PAN over a broad range of MWs. High monomer conversions of 82% and 72% were achieved when targeting lower DP = 100 or 250, respectively (Table 1, entries 5 and 6). Linear semilogarithmic kinetic plots were observed without any significant decrease in rate even after 24 h (Figure 2). This is due to larger amounts of AIBN used for reactions with lower target DPs. However, AIBN can also initiate additional polymer chains leading to lower MW and increased fraction of dead chains. This was reflected in Mn,NMR values which were slightly lower than the theoretical ones. Nevertheless, MWDs remained narrow with Mw/Mn = 1.11− 1.32 (Figure 2c). On the other hand, when targeting DP = 2000, a significant increase in viscosity of the reaction mixture was observed which was accompanied by a rapid increase in the reaction rate. This resulted in a limited conversion that reached 38% after 7 h and only 43% after 24 h (Table 1, entry 8) in addition to a broader MWD (Mw/Mn = 1.58). In order to overcome this issue, a low-temperature, water-soluble radical initiator VA-044 was used instead of AIBN (Figure 2). This

Figure 2. (a) First-order kinetic plots and (b) MW and MWD evolution in an aqueous RAFT polymerization of AN with different targeted DPs. [AN]:[CPAD]:[AIBN] = DPtarget:1:0.4, AN:50 wt % aqueous NaSCN = 1:1.5 (v/v); T = 65 °C, 24 h. For targeted DP = 2000, VA-044 was used as the radical initiator at 44 °C. (c) GPC traces of PAN prepared using molar ratios [AN]:[CPAD]:[AIBN] = 250:1:0.4.

allowed the reaction to be conducted at 44 °C and decrease the polymerization rate. Linear kinetic plots were observed with VA-044 as opposed to AIBN. The reaction reached 59% conversion in 24 h and produced PAN with Mn,NMR ≈ 60 000, which was in excellent agreement with the theoretical value. The polymer exhibited relatively narrow MWD, Mw/Mn = 1.38 (Table 1, entry 9). In all polymerizations, a progressive color change from pink to orange was observed (Figure S3). Such a behavior is often associated with degradation of dithiobenzoate end groups due to hydrolysis or aminolysis under basic conditions, resulting in the loss of chain-end functionality.49−51 Notably, a freshly C

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Macromolecules

Figure 3. (a) Chain extension of a PAN macro-CTA (Mn,GPC = 6900, Mn,NMR = 5300, Mw/Mn = 1.24) with AN in aqueous NaSCN. Conditions for macro-CTA: [AN]0:[CPAD]0:[AIBN]0 = 250:1:0.4, AN:50 wt % aqueous NaSCN = 1:1.5 (v/v); T = 65 °C, 8 h. Conditions for chain extension: [AN]:[PAN-CTA]:[AIBN] = 500:1:0.4, AN:50 wt % aqueous NaSCN = 1:2 (v/v); T = 65 °C, 20 h. (b) Chain extension of a PAN macro-CTA (Mn,GPC = 7600, Mn,NMR = 5600, Mw/Mn = 1.25) synthesized in DMSO. Conditions for macro-CTA: [AN]0:[CPBDT]0:[AIBN]0 = 250:1:0.4, AN:DMSO = 1:1.5 (v/v); T = 65 °C, 8 h. Conditions for chain extension: [AN]:[PAN-CTA]:[AIBN] = 500:1:0.4, AN:DMSO = 1:2 (v/v); T = 65 °C, 20 h.

Figure 4. Chain extension of dithiobenzoate-terminated PAN synthesized in (a) aqueous NaSCN and (b) DMSO with n-butyl acrylate. [BA]:[PANCTA]:[AIBN] = 200:1:0.4; BA:DMF = 1:2 (v/v); T = 65 °C, t = 22 h. Conversion in (a) and (b): 63%.

suggesting some retardation of polymerization in DMSO, as compared to aqueous NaSCN. Since the primary motivation behind this study was to synthesize block copolymer precursors for nanostructured carbons, chain extension of PAN-CTA with BA in DMF was conducted. However, a pronounced retardation was observed for macro-CTAs synthesized in both aqueous NaSCN and DMSO (Figure 4). Even though the GPC traces clearly shifted toward higher MWs and both reactions reached 63% conversion after 22 h, dispersities increased to 1.5−1.6 in both cases. This suggests slow addition of BA to dithiobenzoate-terminated macroradicals due to low activity of dithiobenzoates in polymerization of acrylates that often leads to retardation.55 The developed strategy was then extended to ZnCl2 which also dissolves PAN in aqueous solution. A 60 wt % aqueous ZnCl2 was used as solvent, and reaction was conducted under similar conditions as in aqueous NaSCN. As the solubility of AN is worse in aqueous ZnCl2 than in aqueous NaSCN, the monomer-to-solvent ratio was set to 1:2.5 (v/v). The pH of the solvent was 1.2, which should prevent hydrolysis of the CTA. However, a similar color change as in the case of aqueous NaSCN was observed shortly after polymerization started. Additionally, when AIBN was used as the radical initiator, uncontrolled polymerization occurred. Indeed, ZnCl2 can form complexes with different vinyl monomers, including acrylonitrile. Such complexation drastically increases the polymerization rate.57,58 Thus, V-70 was used as initiator and the polymerization was conducted at 30 °C in order to reduce the propagation rate. Decreased temperature led to a pronounced, 3 h induction period after which slow polymerization with

prepared aqueous solution of NaSCN (50 wt %) had pH = 7.8, which decreased to 4.8 upon addition of CPAD (containing carboxylic acid functionality) and AN ([AN]:[CPAD] = 500:1). In the literature, some side-reactions of dithiobenzoate moieties were reported.52−55 A structural change of the end group is possible which might affect its spectroscopic properties but retain chain extension capability. In fact, the absorption of the dithiobenzoate moiety may be different when attached to a an initiating alkyl group or to a growing macroradical as well as due to changes in polarity of the system.56 Thus, a macro-CTA could exhibit different spectral characteristics than the original CTA. A detailed UV−vis analysis of this phenomenon in aqueous NaSCN as well as in DMSO and DMF is included in Figures S4−S6. Chain extension experiments were performed in order to determine if there was any detrimental effect on the end group fidelity due to potential degradation of dithiobenzoate moieties. Control experiments were conducted in DMSOthe most common solvent for RDRP of AN. A PAN sample synthesized in aqueous NaSCN within 8 h (Figure S1) was precipitated, dried and redissolved in the same solvent. Chain extension was conducted with AN using AIBN at 65 °C. High conversion (77%) was obtained in 20 h. A clean shift was observed in the GPC traces without any significant tailing (Figure 3a), and MWD remained narrow (Mw/Mn < 1.3) demonstrating high chain end functionality. Interestingly, a different macro-CTA, synthesized using CPBDT in DMSO, was extended in a parallel experiment under the same conditions in DMSO. The reaction reached 46% conversion in 20 h and showed tailing toward low MWs as well as broadening of MWD to 1.48 (Figure 3b), D

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Macromolecules linear first-order kinetics was observed (Figure 5a). Conversion reached only 10% in 8 h but increased to 52% in 24 h. MWDs

presence of NaSCN displayed a slightly reduced fraction of rr triads and an increased fraction of isotactic mm triads, when calculated from the CN peak. However, the ratio of respective triads was generally similar throughout all samples, suggesting that the presence of large quantities of inorganic salts had no significant effect on tacticity and led to mostly atactic polymer.

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CONCLUSIONS

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ASSOCIATED CONTENT

In summary, the first example of a controlled radical polymerization of acrylonitrile in aqueous media was demonstrated using RAFT in aqueous solution of NaSCN or ZnCl2. Polymers in a broad predetermined range of MWs up to 60 000 and relatively narrow MWDs were obtained. Although a commercially available 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid was successfully employed as a chain transfer agent, the polymerization of AN in aqueous media may require a more suitable CTA. A color change during polymerization was observed, indicating that a side reaction of the dithiobenzoate group might have occurred. Although the origin of this phenomenon is not clear, high chain end fidelity of synthesized PAN was preserved as confirmed by excellent chain extension efficiency with AN in aqueous NaSCN. Nevertheless, the use of water-soluble trithiocarbonates or xanthates could be considered to enable synthesis of well-defined block copolymers. Possibility to directly synthesize well-defined, high MW PAN in the media industrially used for preparation of PAN fibers may lead to novel fiber precursors.

Figure 5. (a) First-order kinetic plots. (b) MW and MWD evolution. (c) GPC traces in RAFT polymerization of AN in aqueous solution of ZnCl2. [AN]:[CPAD]:[V-70] = 500:1:0.4, AN:60 wt % aqueous ZnCl2 = 1:2.5 (v/v), T = 30 °C.

remained narrow, with Mw/Mn between 1.25 and 1.40 with no significant tailing visible in GPC traces (Figure 5c). MW of the obtained polymer was confirmed by NMR to be in excellent agreement with theory (Mn,th = 14 100; Mn,NMR = 13 300). Finally, precipitated homopolymers were analyzed via 13C NMR in order to determine their tacticity (Figure S7). Radical polymerization of AN typically yields an atactic polymer. One approach to synthesize isotactic PAN was γ-ray initiated polymerization in urea canals.59 However, some salts, Lewis acids in particular, can influence the tacticity of polymers synthesized by radical polymerization.60−62 The presence of a large excess of NaSCN or ZnCl2 in the polymerization media could affect PAN stereoregularity. Thus, the tacticity of PAN samples prepared under these conditions was evaluated by calculating relative amounts of meso−meso (mm), meso− racemo (mr), and racemo−racemo (rr) triads. The results were compared with polymers prepared by conventional FRP in aqueous NaSCN, commercially available PAN, as well as polymers synthesized by ATRP or RAFT in DMSO. The results are presented in Table 2. Samples synthesized in the

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01336. 1

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H and 13C NMR spectra of PAN; deconvolution of GPC peaks; discussion of color change of the reaction with appropriate UV−vis spectra (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.M.). Notes

The authors declare no competing financial interest.

Table 2. Fraction of Triads in PAN Samples Synthesized under Different Conditions CN fraction of triads

C−H fraction of triads

solvent

description

mm

mr

rr

mm

mr

rr

aq NaSCN aq NaSCN aq NaSCN aq NaSCN aq ZnCl2 DMSO DMSO

RAFT (Mn = 2700)a RAFT (Mn = 5300)b RAFT (Mn = 17 000)c conventional RPd RAFT (Mn = 13 300)e RAFT (Mn = 3700)f ATRP (Mn = 5700)g Sigma-Aldrich

0.29 0.29 0.26 0.28 0.28 0.24 0.26 0.26

0.52 0.51 0.54 0.54 0.53 0.50 0.51 0.55

0.19 0.21 0.19 0.18 0.19 0.26 0.23 0.20

0.30 0.28 0.28 0.26 0.30 0.26 0.26 0.28

0.49 0.51 0.50 0.51 0.50 0.51 0.51 0.50

0.21 0.22 0.22 0.23 0.20 0.24 0.23 0.22

Table 1, entry 5 (24 h). b[AN]0:[CPAD]0:[AIBN]0 = 250:1:0.4, AN:50 wt % aq NaSCN = 1:1.5 (v/v); T = 65 °C. cTable 1, entry 3 (24 h). d[AN]: [AIBN] = 200:1, AN:50 wt % aq NaSCN = 1:1.5 (v/v); T = 65 °C. e[AN]0:[CPAD]0:[AIBN]0 = 500:1:0.4, AN:60 wt % aq ZnCl2 = 1:2.5 (v/v); T = 65 °C. fSynthesized according to published procedures.25 gSynthesized according to published procedures.29 a

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DOI: 10.1021/acs.macromol.6b01336 Macromolecules XXXX, XXX, XXX−XXX

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(20) Matyjaszewski, K.; Tsarevsky, N. V. Macromolecular engineering by atom transfer radical polymerization. J. Am. Chem. Soc. 2014, 136, 6513−33. (21) Zhong, M.; Matyjaszewski, K. How Fast Can a CRP Be Conducted with Preserved Chain End Functionality? Macromolecules 2011, 44, 2668−2677. (22) Matyjaszewski, K.; Jo, S. M.; Paik, H.-j.; Gaynor, S. G. Synthesis of Well-Defined Polyacrylonitrile by Atom Transfer Radical Polymerization. Macromolecules 1997, 30, 6398−6400. (23) Matyjaszewski, K.; Jo, S. M.; Paik, H.-j.; Shipp, D. A. An Investigation into the CuX/2,2′-Bipyridine (X) Br or Cl) Mediated Atom Transfer Radical Polymerization of Acrylonitrile. Macromolecules 1999, 32, 6431−6438. (24) Dong, H.; Tang, W.; Matyjaszewski, K. Well-Defined HighMolecular-Weight Polyacrylonitrile via Activators Regenerated by Electron Transfer ATRP. Macromolecules 2007, 40, 2974−2977. (25) Tang, C.; Kowalewski, T.; Matyjaszewski, K. Preparation of Polyacrylonitrile-block-poly(n-butyl acrylate) Copolymers Using Atom Transfer Radical Polymerization and Nitroxide Mediated Polymerization Processes. Macromolecules 2003, 36, 1465−1473. (26) Tang, C.; Qi, K.; Wooley, K. L.; Matyjaszewski, K.; Kowalewski, T. Well-Defined Carbon Nanoparticles Prepared from Water-Soluble Shell Cross-linked Micelles that Contain Polyacrylonitrile Cores. Angew. Chem. 2004, 116, 2843−2847. (27) Tang, C.; Dufour, B.; Kowalewski, T.; Matyjaszewski, K. Synthesis and Morphology of Molecular Brushes with Polyacrylonitrile Block Copolymer Side Chains and Their Conversion into Nanostructured Carbons. Macromolecules 2007, 40, 6199−6205. (28) Dufour, B.; Tang, C.; Koynov, K.; Zhang, Y.; Pakula, T.; Matyjaszewski, K. Polar Three-Arm Star Block Copolymer Thermoplastic Elastomers Based on Polyacrylonitrile. Macromolecules 2008, 41, 2451−2458. (29) Lamson, M.; Kopeć, M.; Ding, H.; Zhong, M.; Matyjaszewski, K. Synthesis of well-defined polyacrylonitrile by ICAR ATRP with low concentrations of catalyst. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 1961−1968. (30) Pan, X.; Lamson, M.; Yan, J.; Matyjaszewski, K. Photoinduced Metal-Free Atom Transfer Radical Polymerization of Acrylonitrile. ACS Macro Lett. 2015, 4, 192−196. (31) Tang, C.; Kowalewski, T.; Matyjaszewski, K. RAFT Polymerization of Acrylonitrile and Preparation of Block Copolymers Using 2Cyanoethyl Dithiobenzoate as the Transfer Agent. Macromolecules 2003, 36, 8587−8589. (32) An, Q.; Qian, J.; Yu, L.; Luo, Y.; Liu, X. Study on kinetics of controlled/living radical polymerization of acrylonitrile by RAFT technique. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 1973−1977. (33) Spörl, J. M.; Ota, A.; Beyer, R.; Lehr, T.; Müller, A.; Hermanutz, F.; Buchmeiser, M. R. Carbon fibers prepared from tailored reversibleaddition-fragmentation transfer copolymerization-derived poly(acrylonitrile)-co-poly(methylmethacrylate). J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 1322−1333. (34) Moskowitz, J. D.; Abel, B. A.; McCormick, C. L.; Wiggins, J. S. High molecular weight and low dispersity polyacrylonitrile by low temperature RAFT polymerization. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 553−562. (35) Aqil, A.; Detrembleur, C.; Gilbert, B.; Jérôme, R.; Jérôme, C. Controlled RAFT Synthesis of Polyacrylonitrile-b-poly(acrylic acid) Diblocks as Precursors of Carbon Nanocapsules with Assistance of Gold Nanoparticles. Chem. Mater. 2007, 19, 2150−2154. (36) Moskowitz, J. D.; Wiggins, J. S. Semibatch RAFT copolymerization of acrylonitrile and N-isopropylacrylamide: Effect of comonomer distribution on cyclization and thermal stability. Polymer 2016, 84, 311−318. (37) Grishin, D. F.; Grishin, I. D. Radical-initiated controlled synthesis of homo- and copolymers based on acrylonitrile. Russ. Chem. Rev. 2015, 84, 712. (38) Kowalewski, T.; Tsarevsky, N. V.; Matyjaszewski, K. Nanostructured Carbon Arrays from Block Copolymers of Polyacrylonitrile. J. Am. Chem. Soc. 2002, 124, 10632−10633.

ACKNOWLEDGMENTS Support from the NSF (DMR 1501324) is acknowledged. M.K. thanks the Fulbright Program and Polish Ministry of Science and Higher Education (“Mobilnosc Plus” grant no. 1055/ MOB/2013/0) for financial support.

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REFERENCES

(1) Morris, E. A.; Matthew, C. W. Solution Spinning of PAN-Based Polymers for Carbon Fiber Precursors. In Polymer Precursor-Derived Carbon; American Chemical Society: 2014; Vol. 1173, pp 189−213. (2) Frank, E.; Steudle, L. M.; Ingildeev, D.; Spörl, J. M.; Buchmeiser, M. R. Carbon Fibers: Precursor Systems, Processing, Structure, and Properties. Angew. Chem., Int. Ed. 2014, 53, 5262−5298. (3) Eom, Y.; Kim, B. C. Solubility parameter-based analysis of polyacrylonitrile solutions in N,N-dimethyl formamide and dimethyl sulfoxide. Polymer 2014, 55, 2570−2577. (4) Iovleva, M. M.; Smirnova, V. N.; Budnitskii, G. A. The Solubility of Polyacrylonitrile. Fibre Chem. 2001, 33, 262−264. (5) Morosoff, N.; Stannett, V. Some effects of small amounts of residual solvent on polyacrylonitrile film. J. Macromol. Sci., Part B: Phys. 1980, 17, 157−161. (6) Kim, D.; Moreno, N.; Nunes, S. P. Fabrication of polyacrylonitrile hollow fiber membranes from ionic liquid solutions. Polym. Chem. 2016, 7, 113−124. (7) Serkov, A. T.; Budnitskii, G. A. Mechanism of polyacrylonitrile fibre spinning by the thiocyanate method. Fibre Chem. 1994, 25, 335− 341. (8) Nam, C.-W.; Kim, Y. H.; Ko, S. W. Blend fibers of polyacrylonitrile and water-soluble chitosan derivative prepared from sodium thiocyanate solution. J. Appl. Polym. Sci. 2001, 82, 1620−1629. (9) Bajaj, P.; Roopanwal, A. K. Thermal Stabilization of Acrylic Precursors for the Production of Carbon Fibers: An Overview. J. Macromol. Sci., Polym. Rev. 1997, 37, 97−147. (10) Goldfein, M. D.; Kozhevnikov, N. V.; Rafikov, E. A.; Stepukhovich, A. D.; Kosyreva, R. V. Polymerization of acrylonitrile in an aqueous solution of sodium thiocyanate. Polym. Sci. U. S. S. R. 1975, 17, 2630−2637. (11) Gol’dfein, M. D.; Rafikov, E. A.; Kozhevnikov, N. V.; Stepukhovich, A. D.; Rabinovich, I. S. Copolymerization of acrylonitrile and methacrylate in an aqueous solution of sodium thiocyanate. Polym. Sci. U. S. S. R. 1977, 19, 318−324. (12) Huang, W.; Liu, C.; Yang, H.; Xue, X.; Jiang, B.; Zhang, D.; Kong, L.; Zhang, Y.; Komarneni, S. Facile synthesis of highly branched poly(acrylonitrile-co-vinyl acetate)s with low viscosity and high thermal stability via radical aqueous solution polymerization. Polym. Chem. 2014, 5, 3326−3334. (13) Dong, Q.; Gu, L. Synthesis of AN-g-casein copolymer in concentrated aqueous solution of sodium thiocyanate and AN-g-casein fiber’s structure and property. Eur. Polym. J. 2002, 38, 511−519. (14) Moad, G.; Rizzardo, E.; Thang, S. H. Radical addition− fragmentation chemistry in polymer synthesis. Polymer 2008, 49, 1079−1131. (15) Moad, G.; Rizzardo, E.; Thang, S. H. Living Radical Polymerization by the RAFT Process − A Third Update. Aust. J. Chem. 2012, 65, 985−1076. (16) Hill, M. R.; Carmean, R. N.; Sumerlin, B. S. Expanding the Scope of RAFT Polymerization: Recent Advances and New Horizons. Macromolecules 2015, 48, 5459−5469. (17) Matyjaszewski, K.; Xia, J. Atom Transfer Radical Polymerization. Chem. Rev. 2001, 101, 2921−2990. (18) Matyjaszewski, K.; Tsarevsky, N. V. Nanostructured functional materials prepared by atom transfer radical polymerization. Nat. Chem. 2009, 1, 276−288. (19) Matyjaszewski, K. Atom Transfer Radical Polymerization (ATRP): Current Status and Future Perspectives. Macromolecules 2012, 45, 4015−4039. F

DOI: 10.1021/acs.macromol.6b01336 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (39) Tang, C.; Tracz, A.; Kruk, M.; Zhang, R.; Smilgies, D.-M.; Matyjaszewski, K.; Kowalewski, T. Long-Range Ordered Thin Films of Block Copolymers Prepared by Zone-Casting and Their Thermal Conversion into Ordered Nanostructured Carbon. J. Am. Chem. Soc. 2005, 127, 6918−6919. (40) Zhong, M.; Kim, E. K.; McGann, J. P.; Chun, S. E.; Whitacre, J. F.; Jaroniec, M.; Matyjaszewski, K.; Kowalewski, T. Electrochemically active nitrogen-enriched nanocarbons with well-defined morphology synthesized by pyrolysis of self-assembled block copolymer. J. Am. Chem. Soc. 2012, 134, 14846−57. (41) Lin, Y.; Wang, X.; Qian, G.; Watkins, J. J. Additive-Driven SelfAssembly of Well-Ordered Mesoporous Carbon/Iron Oxide Nanoparticle Composites for Supercapacitors. Chem. Mater. 2014, 26, 2128−2137. (42) Wu, D.; Li, Z.; Zhong, M.; Kowalewski, T.; Matyjaszewski, K. Templated synthesis of nitrogen-enriched nanoporous carbon materials from porogenic organic precursors prepared by ATRP. Angew. Chem., Int. Ed. 2014, 53, 3957−60. (43) Simakova, A.; Averick, S. E.; Konkolewicz, D.; Matyjaszewski, K. Aqueous ARGET ATRP. Macromolecules 2012, 45, 6371−6379. (44) Konkolewicz, D.; Magenau, A. J. D.; Averick, S. E.; Simakova, A.; He, H.; Matyjaszewski, K. ICAR ATRP with ppm Cu Catalyst in Water. Macromolecules 2012, 45, 4461−4468. (45) Chmielarz, P.; Park, S.; Simakova, A.; Matyjaszewski, K. Electrochemically mediated ATRP of acrylamides in water. Polymer 2015, 60, 302−307. (46) Fantin, M.; Isse, A. A.; Venzo, A.; Gennaro, A.; Matyjaszewski, K. Atom Transfer Radical Polymerization of Methacrylic Acid: A Won Challenge. J. Am. Chem. Soc. 2016, 138, 7216−7219. (47) Lowe, A. B.; McCormick, C. L. Reversible addition− fragmentation chain transfer (RAFT) radical polymerization and the synthesis of water-soluble (co)polymers under homogeneous conditions in organic and aqueous media. Prog. Polym. Sci. 2007, 32, 283− 351. (48) Zhong, M.; Tang, C.; Kim, E. K.; Kruk, M.; Celer, E. B.; Jaroniec, M.; Matyjaszewski, K.; Kowalewski, T. Preparation of porous nanocarbons with tunable morphology and pore size from copolymer templated precursors. Mater. Horiz. 2014, 1, 121−124. (49) Thomas, D. B.; Convertine, A. J.; Hester, R. D.; Lowe, A. B.; McCormick, C. L. Hydrolytic Susceptibility of Dithioester Chain Transfer Agents and Implications in Aqueous RAFT Polymerizations. Macromolecules 2004, 37, 1735−1741. (50) Gregory, A.; Stenzel, M. H. Complex polymer architectures via RAFT polymerization: From fundamental process to extending the scope using click chemistry and nature’s building blocks. Prog. Polym. Sci. 2012, 37, 38−105. (51) Albertin, L.; Stenzel, M. H.; Barner-Kowollik, C.; Davis, T. P. Effect of an added base on (4-cyanopentanoic acid)-4-dithiobenzoate mediated RAFT polymerization in water. Polymer 2006, 47, 1011− 1019. (52) David, G.; Asri, Z. E.; Rich, S.; Castignolles, P.; Guillaneuf, Y.; Lacroix-Desmazes, P.; Boutevin, B. Peculiar Behavior of Degenerative Chain Transfer Polymerization of a Phosphonated Methacrylate. Macromol. Chem. Phys. 2009, 210, 631−639. (53) Hosseini Nejad, E.; Castignolles, P.; Gilbert, R. G.; Guillaneuf, Y. Synthesis of methacrylate derivatives oligomers by dithiobenzoateRAFT-mediated polymerization. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 2277−2289. (54) Nguyen, D. H.; Vana, P. On the Mechanism of Radical Polymerization of Methyl Methacrylate with Dithiobenzoic Acid as Mediator. Aust. J. Chem. 2006, 59, 549−559. (55) Moad, G. Mechanism and Kinetics of Dithiobenzoate-Mediated RAFT Polymerization - Status of the Dilemma. Macromol. Chem. Phys. 2014, 215, 9−26. (56) Skrabania, K.; Miasnikova, A.; Bivigou-Koumba, A. M.; Zehm, D.; Laschewsky, A. Examining the UV-vis absorption of RAFT chain transfer agents and their use for polymer analysis. Polym. Chem. 2011, 2, 2074−2083.

(57) Imoto, M.; Otsu, T.; Nakabayashi, M. Vinyl polymerization. LXIX. The polymerization of a complex of acrylonitrile with zinc chloride. Makromol. Chem. 1963, 65, 194−201. (58) Imoto, M.; Otsu, T.; Shimizu, S. Vinyl polymerization. LXVI.. The effect of zinc chloride on the radical polymerization of vinyl monomers. Makromol. Chem. 1963, 65, 174−179. (59) Minagawa, M.; Yamada, H.; Yamaguchi, K.; Yoshii, F. Gammaray irradiation canal polymerization conditions ensuring highly stereoregular (>80%) polyacrylonitrile. Macromolecules 1992, 25, 503−510. (60) Lutz, J.-F.; Neugebauer, D.; Matyjaszewski, K. Stereoblock Copolymers and Tacticity Control in Controlled/Living Radical Polymerization. J. Am. Chem. Soc. 2003, 125, 6986−6993. (61) Lutz, J.-F.; Jakubowski, W.; Matyjaszewski, K. Controlled/Living Radical Polymerization of Methacrylic Monomers in the Presence of Lewis Acids: Influence on Tacticity. Macromol. Rapid Commun. 2004, 25, 486−492. (62) Ray, B.; Isobe, Y.; Morioka, K.; Habaue, S.; Okamoto, Y.; Kamigaito, M.; Sawamoto, M. Synthesis of Isotactic Poly(Nisopropylacrylamide) by RAFT Polymerization in the Presence of Lewis Acid. Macromolecules 2003, 36, 543−545.

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DOI: 10.1021/acs.macromol.6b01336 Macromolecules XXXX, XXX, XXX−XXX