Solubility of Monomers for Chain Polymerization in Ionic Liquids

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Solubility of monomers for chain polymerization in ionic liquids predicted by conductor-like screening model for real solvents Xiaoqian Zhang, Wenli Guo, Yibo Wu, Wei Li, Shuxin Li, Yuwei Shang, and Jinghan Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04235 • Publication Date (Web): 15 Nov 2017 Downloaded from http://pubs.acs.org on November 16, 2017

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Industrial & Engineering Chemistry Research

Solubility of monomers for chain polymerization in ionic liquids predicted by conductor-like screening model for real solvents

Xiaoqian Zhang1,2,3, Wenli Guo1,3, Yibo Wu1, Wei Li1, Shuxin Li1, Yuwei Shang1, Jinghan Zhang1,3

1. Department of Materials Science and Engineering, Beijing Key Lab of Special Elastomer Composite Materials, Beijing Institute of Petrochemical Technology, Beijing 102617, China 2. Department of Environmental Engineering, North China Institute of Science and Technology, Yanjiao, Beijing 101601, P. R. China 3. College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China

ABSTRACT

Ionic liquids have been extensively investigated as clean solvents for chain polymerization reactions, such as radical polymerization, anionic polymerization, and cationic polymerization. However, the low solubility of the monomers in ionic liquids often plagues the efficiency of these reactions. In this study, solubility of two typical vinyl monomers of p-methylstyrene (p-MeSt) and isobutyl vinyl ether (IBVE) were studied in 1750 possible ionic liquids (50 cations and 35 anions) by conductor-like screening model for real solvents. The influences of cation structures, anion structures, and ion chain length on the solubility were systemically studied. The interaction energies and σ-profiles were also employed to explain major factors on solubility. The results revealed that a larger size of the nonpolar region of the cation or anion results in the higher solubility for p-MeSt and IBVE, as well as the longer alkyl chain in cation or anion. In this study, a theoretical method for selecting ionic liquids for chain polymerization was established.



Corresponding author: [email protected]; [email protected] 1

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1. INTRODUCTION

2

Vinyl monomers are well-known and commercially available materials that can undergo

3

chain-polymerization reactions, such as radical polymerization, anionic polymerization, and

4

cationic polymerization.1-5 The polymerization products of vinyl monomers are widely used in

5

rubber, plastic, and coating manufature.6-9 However, traditional organic solvents for these chain

6

polymerizations often pollute the environment through evaporation or leakage. The use of ionic

7

liquids (ILs) as green solvents are extensively studied because of their attractive properties, such as

8

their negligible vapor pressure, low melting point, good thermal stability, tunability, and chemical

9

inertia.10-12 ILs are regarded as polar but noncoordinating solvents with a high charge density

10

because of their ionic nature, so they do not behave as simple solvents for polymerization. This

11

property of ILs evoked the initial interest of previous researchers.11

12

ILs cause different polymerization mechanisms and characteristics in chain polymerizations. In

13

radical polymerization, ILs is shown to affect the kinetic reactions because of its polarity and high

14

viscosity. An observable increase in the propagation rate constant, kp, and a decrease of the

15

termination rate constant, kt, result to the strong increase in the overall polymerization rate.13-15 In

16

anionic polymerization, imidazolium ionic liquids are not likely to be suitable solvents as they are

17

not entirely stable under basic conditions because of an acidic proton at the 2-position of

18

imidazolium ring.16, 17 For cationic polymerization, ILs that are seen as (weak) Lewis bases can

19

stabilize the propagating carbocationic species and enhance their stability to undergo cationic

20

polymerization, which has been revealed in some papers.11, 18 The number of the ILs that were

21

studied in those polymerizations is very limited compared with the very vast number of ILs (the

22

number of potentials ILs is ~1018).19 One of the important issues is the solubility of the

23

monomer-in-IL, which reduces the efficiency of the polymerizations. Finding ILs with high

24

dissolving capacity for polymerization monomer is necessary.

25

ILs are defined as designer solvents whose physicochemical properties can be fine-tuned by an

26

appropriate choice of cation and anion for specific tasks.20, 21 Further designing and selecting ILs

27

with high solubility and better physiochemical properties is still necessary. The experimental 2


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examination is difficult to operate and will take a lot of time and cost because of the enormous

29

number of possible ILs. Therefore, an initial prescreening by computational method is necessary for

30

an efficient selection process. Conductor-like screening model for real solvents (COSMO-RS) based

31

on a quantum chemical approach is an effective method for solvent designing and screening.22

32

COSMO-RS is based solely on information concerning the molecular structure; aside from its full

33

predictability, the main advantage lies in its rapid working speed. COSMO-RS integrates dominant

34

interactions among the system, including misfit, H-bonding (HB), and Van der Waals (VdW)

35

interactions. Thus, COSMO-RS can be used for molecular force field analysis to correlate

36

interaction parameters with structural characteristics qualitatively and quantitatively, which is an

37

instruction valuable in designing and selecting ILs for specific tasks.23 COSMO-RS has been widely

38

used to predict the thermodynamic properties of fluids for various systems, especially ILs, such as

39

solubility,24, 25 phase equilibria,26, 27 and activity coefficients.28

40

In this paper, we study the solubility of vinyl monomers of chain polymerization in ILs using

41

COSMO-RS calculation. Two typical vinyl monomers of p-methylstyrene (p-MeSt) and isobutyl

42

vinyl ether (IBVE) in 1750 possible ILs, involving 50 cations and 35 anions (their full name and

43

short names with chemical structure are shown in Table S1), are calculated employing COSMO-RS.

44

The influences of ILs structure on the solubility are studied systematically, including cation family,

45

anion identity, cation alkyl chain length, and anion alkyl chain length; thus, the guiding principle of

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the selection for suitable ILs with high dissolving capacity as chain polymerization solvents is

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obtained. The interaction energies and σ-profiles are employed to analyze the influences of ILs

48

structure on the solubility. This work motivated us in the ILs selection as polymerization solvent.

49

This study will also provide a broad space for future studies on chain polymerization in ILs.

50

2. DETAILS OF COMPUTATION AND EXPERIMENT

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2.1 Computational Details of COSMO-RS

52

COSMO-RS based on a quantum chemical approach was developed by Klamt and his

53

coworkers.29,30 COSMO-RS combines the electrostatic advantages and the computational efficiency 3



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of the quantum chemical dielectric continum solvation model, COSMO, with a statistical

55

thermodynamic approach for local interaction of surfaces, where the local deviations from dielectric

56

behavior, as well as HB, is considered.31

57

The standard procedure of COSMO-RS calculations consists essentially of two major stages:

58

quantum chemical COSMO calculations for the molecular species involved and COSMO-RS

59

statistical thermodynamic calculations performed using the COSMOtherm program.32, 33 Results of

60

the COSMO calculation are stored in the so-called COSMO files, which are collected in a database.

61

In our study, the COSMO input files for most cations and anions were taken from the COSMO

62

database. The other input files of ions, p-MeSt, and IBVE, which were not stored in this database

63

were obtained via the TURBOMOLE program package,34 using the BP functional35 with TZVP

64

basis set.36 In the COSMO calculation, the effect of different conformations was studied beforehand

65

to ensure efficient computation. In our study, all the stable conformations were considered and

66

weighted according to the Boltzmann distribution function. On the other hand, COSMO-RS

67

calculations were performed using the COSMOtherm (Version C30_1501, COSMOlogic Gmb H

68

&Co. KG), which provided an efficient and flexible implementation of COSMO-RS method.32,33

69

The BP_TZVP_C30_1501.ctd parameterization was used.

70

For a better comparison of solubility of p-MeSt or IBVE in different ILs under same conditions,

71

all calculations (regardless of solubility) were performed with a noniterative mode, which means

72

that the solubility computed is in a zeroth order approximation.11 In the COSMO calculations, the

73

IL was treated as independent ions with equal mole fractions (ncation= nanion= nIL). The calculation

74

was based on a ternary mixture (cation, anion, and solute), which differs from an experimental

75

determination treated as a binary system consisting of IL and solute. To compare with the

76

experimental value, we performed transition calculations for the predicted solubility data. Not all

77

examined cation-anion combinations lead to ILs; some of them do not exist as liquid at room

78

temperature. They are perceived as ILs for the convenience of model processing. Intensive interest

79

in the facts mentioned above is not placed on the discussion of the methodology but on the

80

preliminary prediction monomer-in-IL solubility for solvent designing and screening.

4


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2.2 Experimental section

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2.2.1 Materials

83

1-Ethyl-3-methylimidazolium trifluoroacetate ([Emim][TA]), 1-ethyl-3-methylimidazolium

84

bis(trifluoromethanesulfonyl)amide

([Emim][NTf2]),

1-propyl-3-methylimidazolium

85


([Prmim][NTf2]),

1-butyl-3-methylimidazolium

86


([Bmim][NTf2]),

1-hexyl-3-methylimidazolium

87


([Hmim][NTf2]),

1-octyl-3-methylimidazolium

88


([Omim][NTf2]),

1-butyl-3-methylimidazolium

89

tetrafluoroborate ([Bmim][BF4]), 1-hexy-3-methylimidazolium tetrafluoroborate ([Hmim][BF4],

90

1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]), 1-hexy-3-methylimidazolium

91

hexafluorophosphate

92

([Omim][PF6]), N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)amide ([P14][NTf2]),

93

and 1-butyl-pyridinium bis(trifluoromethanesulfonyl)amide ([Bpy][NTf2]) were purchased from

94

Sigma Aldrich, and the purities were more than 99.0%. The ILs were dried and degassed at 70 °C

95

for a few days on a vacuum line untill the H2O concentration was reduced to less than 30 ppm

96

before use. p-Methylstyrene (p-MeSt, TCI Shanghai) and isobutyl vinyl ether (IBVE, TCI Shanghai)

97

were dried by introduction of CaH2 and freshly distilled under reduced pressure before use.

98

2.2.2 Solubility measurement

([Hmim][PF6]),

1-octyl-3-methylimidazolium

hexafluorophosphate

99

The solubilities were measured by a gravimetric method.37 For each measurement, an excess

100

mass of vinyl monomer (p-MeSt or IBVE) was added to a known mass of IL (m1) in a tightly-closed

101

test tube with a septum cap. The mass of empty test tube was m0. Then the mixture was heated to a

102

constant temperature and at atmospheric pressure with continuous stirring. After at least 2 hour, the

103

stirring was stopped. The solution in the sealed test tube was placed to settle and equilibrate for at

104

least 48 h in an isolated air bath capable of maintaining the temperature within ±0.01 K between

105

0°C and 50°C. The excess mass of vinyl monomer was in the upper portion. The period of the

106

stirring time and equilibrium time proved to be enough to ensure a complete separation of the two

107

phases, as well as their saturation, namely, a complete thermodynamic equilibrium. The upper 5



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portion was withdrawn via a glass syringes maintained dry and kept at the same time temperature of

109

the measurements, and then the total mass of the lower portion and the test tube was weighed (m2).

110

The monomer concentration of the sample solution in mole fraction, x, is determined from Eqs. (1):

x

111

 m2  m1  m0  / M  m2  m1  m0  / M  m1 / M 2 1

(1)

1

112

where M1 and M2 are the mole mass of monomer and IL, respectively.

113

The mass was determined by a balance (CPA324S, Germany) with a precision of ±0.0001 g. The

114

temperature was controlled by an MBraun 150-M glovebox under a dry nitrogen atmosphere ([H2O]

115

< 0.5 ppm; [O2] < 10 ppm). Each measurement, at each temperature, was repeated at least 5 times,

116

and the results are reported as the average solubility value along with the respective standard

117

deviation.

118

3. RESULTS AND DISCUSSION

119

3.1 Validation of COSMO-RS prediction

120 121

Figure 1. Comparison between the experimental and COSMO-RS calculated solubilities of

122

p-MeSt (298.15 K) (a) and IBVE (288.15 K) (b) in different types of ILs

123 124

The feasibility of COSMO-RS method for selecting ILs as the p-MeSt or IBVE chain 6


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polymerization solvent was evaluated by comparing the calculated values with the experimental

126

data (Table S3 and Table S4). Figure 1 shows the calculated solubility of p-MeSt and IBVE in

127

different ILs along with their experimental data. In this work, the experimental solubility was

128

correlated with the calculated values by linear regression (Figure 1); the fitting results were listed in

129

Table S5. The equations for correction were shown in Eq. (2) and Eq. (3). The correlation

130

coefficient of solubilityp-MeSt and solubilityIBVE reached 0.986 and 0.920, respectively, indicating the

131

ability of COSMO-RS method to qualitatively predict the trend of solubilityp-MeSt and solubilityIBVE

132

in a great variety of ILs. For p-MeSt monomer, a systematic deviation between calculated and

133

experimental data for the solubility was observed. The prediction errors are probably attributed to

134

the weak interaction in p-MeSt-IL systems; however, this does not affect the prediction of the trend.

135

COSMO-RS is a suitable tool for ranking ILs qualitatively regarding the solubility of vinyl

136

monomers in ILs.38

137 138

So lubility p MeSt, Experimental  1.083  So lubility p MeSt,COSMO  5.470

(2)

139

So lubility IBVE , Experimental  0.972  So lubility IBVE ,COSMO  0.835

(3)

140

141

3.2 Solubility of p-MeSt and IBVE in ILs

142

The solubility of p-MeSt and IBVE in 1750 types ILs (50 cations  35 anions) was predicted

143

by COSMO-RS, and the contour plot of those 1750 ILs is shown in Figure 2. The short names and

144

chemical structures of all the IL cations and anions investigated in our study are shown in the

145

supporting information. In Figure 2, the vertical axis corresponds to the cationic label order in Table

146

S1, while the horizontal axis corresponds to the anionic label order in Table S2. The solubility of

147

monomers is mainly influenced by IL molecular structure, that is, cation nature, anion nature, or the

148

cation-anion combination as shown in Figure 2. The cations and anions all control the solubility of

149

the p-MeSt and IBVE in ILs. In the following parts, we will discuss the influences of IL molecular

150

structure variations on the monomer solubility in detail. The reaction temperature of styrene and its

151

derivatives is between −25 °C to 60 °C,39, 40 while the reaction temperature of IBVE is 0 °C;41-45 in 7



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our COSMO-RS calculation, the predicted temperature for p-MeSt is 25 °C and for IBVE is 0 °C.

153 154

155 156

157 158

Figure 2. Predicted solubilities of p-MeSt (A), and IBVE (B) in 1750 types (50 cations  35 anions)

159

of ILs.

160

Predicted temperature: (A) p-MeSt at 25 °C; (B) IBVE at 0 °C

8


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3.3 Influence of the cation family

162

Most widely used cations contain imidazolium, piperidinium, pyrrolidinium, and pyridinium,

163

including aromatic or aliphatic rings. In this study, we analyzed the p-MeSt and IBVE solubility in

164

the following ILs: [C4mim]+, [C4mpip]+, [C4mpyr]+, and [C4mpy]+, which have the same alkyl

165

substituent group but different cation cores (Figure 3). p-MeSt-in-IL solubility roughly increased

166

accordingly to [C4mpip]+≥[C4mpy]+≈[C4mpyr]+>[C4mim]+>[BuMe3N]+>[Ch]+, as shown in Figure

167

3A. The influences of the cation family on p-MeSt-in-IL solubility can be explained based on their

168

σ-profiles (Figure 4) and multiple interaction energies from COSMO-RS computation (Table 1).46-48

169

The peak of the p-MeSt is located in the nonpolar region from σ=−0.0082 e/Å2 to σ=+0.0082 e/Å2,

170

as shown in Figure 4; the peaks of [C4mim]+, [C4mpy]+, [C4mpyr]+, and [C4mpip]+are also located

171

in

172

[C4mpip]+>[C4mpyr]+>[C4mpy]+>[C4mim]+; the peaks of [BuMe3N]+ and [Ch]+ are located in

173

nonpolar region (σ=−0.004 e/Å2 and σ=−0.0003 e/Å2) and also in polar region (σ=−0.009 e/Å2 and

174

σ=−0.009 e/Å2), and the peak area of [Ch]+ is larger than that of [BuMe3N]+. Thus, the size of the

175

nonpolar

176

[C4mpip]+>[C4mpyr]+>[C4mpy]+>[C4mim]+>[BuMe3N]+>[Ch]+, which is roughly in the same order

177

as the p-MeSt-in-IL solubility and implies that larger size of the nonpolar region for cations results

178

in higher solubility. The multiple interaction energies can be computed from COSMO-RS, including

179

misfit energy, HB energy, and, VdW energy. The p-MeSt-in-IL solubility can be theoretically

180

explained through the force field analysis of the COSMO-RS measures. Table 1 shows the predicted

181

solubility of p-MeSt in [BF4]-based ILs and [MeOEtSO4]-based ILs with varying cations and the

182

corresponding solvation interaction energies. Thus, the misfit interactions and VdW interactions

183

between p-MeSt and cations are the determinant for p-MeSt solubility in ILs. Stronger VdW

184

interactions result in higher VdW energies attained; consequently, larger p-MeSt solubility is

185

gained.

the

above

region

nonpolar

of

region,

and

these

their

cations

peak

follows

areas

the

follow

trend

186

The effects of different cation families on the IBVE-in-IL solubility are shown in Figure 3B.

187

The σ-profiles (Figure 4) and multiple interaction energies from COSMO-RS computation (Table

188

S6) also explained the influences of cation families on IBVE-in-IL solubility. The results indicated

189

that

the

IBVE-in-IL

solubility

can

be

ranked

as

[C4mpip]+>[C4mpyr]+≥[C4mpy]+>

9



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190

[C4mim]+≥[BuMe3N]+>[Ch]+, which is consistent with the solubility trend. Among the three

191

dominant interactions in the system, VdW energies are strongly negative for all ILs and the larger

192

VdW energies result in higher solubility. Similar to the effect of VdW energies, the greater misfit

193

energies lead to the larger the solubility. Those results also suggest that more nonpolar cations have

194

larger misfit energies and VdW energies and result in higher IBVE-in-IL solubility. The

195

corresponding COSMO volume of cations [C4mpip]+, [C4mpyr]+, [C4mpy]+, [C4mim]+, [BuMe3N]+,

196

and [Ch]+ are 254.71 Å3, 215.21 Å3, 215.83 Å3, 199.90 Å3, 187.07 Å3, and 154.83 Å3, which affects

197

the monomers-in-IL solubility to a certain extent.

198

199 200

10


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201 202

Figure 3. Predicted solubilities of p-MeSt (A) and IBVE (B) in ILs with different cation family

203 204

205 206

Figure 4. σ-Profiles of p-MeSt, IBVE, and six studied cations

207 208 209 210 211 212 11



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Table 1. The solubility, misfit energy, HB energy and VdW energy for p-MeSt in the [BF4]-based

214

ILs and [MeOEtSO4]-based ILs combinations Solubility

Emisfit

Ehb

EVdW

(xp-MeSt)

(kcal/mol)

(kcal/mol)

(kcal/mol)

[C4mpip][BF4]

0.2527

6.23993

‒0.66753

‒10.48521

[C4mpy][BF4]

0.2228

5.76258

‒1.11608

‒9.48384

[C4mpyr][BF4]

0.2097

6.14482

‒0.62087

‒8.96890

[C4mim][BF4]

0.1177

5.25298

‒1.38884

‒8.99643

[BuMe3N][BF4]

0.0824

5.30388

‒1.07920

‒7.94879

[Ch][BF4]

0.0063

4.05530

‒4.11320

‒6.10058

[C4mpip][MeOEtSO4]

0.4016

6.46280

‒0.99542

‒10.91492

[C4mpy][MeOEtSO4]

0.3793

5.84817

‒1.72792

‒9.90433

[C4mpyr][MeOEtSO4]

0.3697

6.31430

‒0.88646

‒9.39943

[C4mim][MeOEtSO4]

0.2570

5.35885

‒2.15199

‒9.43415

[BuMe3N][MeOEtSO4]

0.2101

5.43311

‒1.54910

‒8.37567

[Ch][MeOEtSO4]

0.0331

3.98130

‒5.27204

‒6.54537

Ionic liquids

215

216

3.4 Influence of the cation alkyl chain length

217

Among all cations of ILs, the imidazolium ions are well-known to have tunable properties,

218

such as viscosity, miscibility, and melting point; thus, tunable properties can be achieved by their

219

structural variations. Figure 5 shows the effects of alkyl chain length of imidazolium cations on

220

p-MeSt-in-IL solubility and IBVE-in-IL solubility. The p-MeSt-in-IL solubility and IBVE-in-IL

221

solubility

222

[C1mim]+

Solubility of Monomers for Chain Polymerization in Ionic Liquids

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