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Kinetics, Catalysis, and Reaction Engineering

Achieving Low-cost and Accelerated Living Cationic Polymerization of Isobutyl Vinyl Ether in Microflow System DAN XIE, and Yangcheng Lu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01256 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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Industrial & Engineering Chemistry Research

Achieving Low-cost and Accelerated Living Cationic Polymerization of Isobutyl Vinyl Ether in Microflow System

Dan Xie† and Yangcheng Lu*,† †

State Key Laboratory of Chemical Engineering, Department of Chemical Engineering,

Tsinghua University, Beijing 100084, China

ABSTRACT In this work, we proposed and realized a strategy for fast and low-cost living cationic polymerization of IBVE based on temporary stability of propagating chain. It combines the decrease of added base addition to increase the polymerization rate and the use of microflow system to enhance process regulation. To assure the living characteristic, the concentration of added base DO was allowed to be as low as 0.1 M for IBVE-HCl/SnCl4/DO initiation system and 0.5 M for IBVE-HCl/FeCl3/DO initiation system, respectively; the time to achieve 80% conversion could decrease to around 5 s. We also discussed the effect of reaction mode on the polymerization performances, and recognized the mechanism and requirement to achieve living cationic polymerization at decreased added base addition. These recognitions may broaden the horizon of living cationic polymerization, and improve its commercial value with respect to productivity and cost.

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

KEYWORDS Living cationic polymerization; Low base addition; Microflow system; Mixing effect; Kinetic mechanism

INTRODUCTION Living cationic polymerization was achieved first with the HI/I2 system of vinyl ethers (VEs) in 1984.1 It enables the control of molecular weight distribution (MWD) and sequence of constitutional repeating units or segments of polymers, and has become a powerful tool to manufacture functional polymers with well-defined architecture.2-7 Conventionally, the propagating carbocations in the normal cationic polymerization process are usually unstable and tend to undergo irreversible termination and chain transfer reactions via proton elimination.2,8 So, the key to realize living cationic polymerization is to find suitable conditions to stabilize propagating carbocations. So far, three methods were developed. The first is using suitable nucleophilic counteranions (derived from a weak Lewis acid: I2, ZnX2, BCl3, TiX4 etc.).9 The second is using added base.10-13 The third is using added salt (nBu4N+Y-, nBu4P+Y-, Y = Cl-, Br-, I-).14-16 Accordingly, various initiation systems were developed for living cationic polymerization. For examples, in conjunction with VE-HCl adduct, VE-CH3COOH or methanol as cationogen, various metal halides combining suitable base can realize living cationic polymerization of VEs, such as FeCl3/cyclic ether (1,4-dioxane/1,3-dioxolane/THF),17 SnCl4/cyclic ether (1,4-dioxane) or ethyl acetate,18,19 EtxAlCl3-x/ ethyl acetate,20,21 2


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FeBr3/THF, InCl3/1,4-dioxane(DO) or ethyl acetate(EA), ZnCl2/EA or DO, HfCl4/EA, ZrCl4/EA, BiCl3/EA, GaCl3/THF, GeCl4/DO, TiCl4/EA.22 Efforts also have been channeled into other new type of initiation system, such as iron oxides23 (α-Fe2O3, γ-Fe2O3, Fe3O4) and mental-free initiation system24 (HCl·Et2O). Until now, there are still many challenges in living cationic polymerization, such as achieving productivity capable for commercial production and controlling the stereoselectivity of products.25 In origin, the rate of living cationic polymerization is usually quite slow. For example, in the presence of ethyl acetate, the polymerization of IBVE using TiCl4 required 120 hours for 93% conversion, and conversions reached about 97% in 22 h by using EtAlCl2.20-22 In recent years, some researchers have been devoted to developing new rapid initiation systems for the fast living cationic polymerization of VEs.17-19,22 An example is that living cationic polymerization of IBVE was complete in 15 s with IBVE-HCl/FeCl3 in toluene in the presence of 1,4-dioxane at 0 oC.17 However, to enhance the stability of the propagating carbocations and decrease the polymerization rate to a controllable level in batch, these initiation systems always introduced large amount of base, even over 200 times of initiator and activator. It suffers from the expenses of high costs in the usage and recovery of additives. Therefore, the possibility of achieving living cationic polymerization under decreased addition of base is worth exploring. For this purpose, a strictly controlled reaction system is demanded to avoid undesired chain reactions due to non-uniform reaction environment. Recently, microflow systems have drawn more and more attentions due to their 3



potentials in enhancing the controllability on polymerization process and products.26,27 For examples, fast cationic polymerization of isobutyl vinyl ether (IBVE) can be completed within the residence time of 0.37-1.5 s using trifluoromethane-sulfonic acid (TfOH) as initiator and the molecular weight distribution was narrower (Mw/Mn = 1.19) than in the macroscale batch system (Mw/Mn = 2.73-4.71);28 Yoshida et.al reported that the controlled/living anionic polymerization of styrene using sec-BuLi as an initiator could be achieved under easily accessible conditions, such as at 0 oC (Mw/Mn = 1.08) and 24 oC (Mw/Mn = 1.10);29,30 The living anionic polymerization of tert-butyl acrylate was also achieved in a flow microreactor system under easily accessible conditions, for example at -20 oC.31 These results are commonly attributed to the obvious advantages of microflow system, including fast mixing stemming from short diffusion time, fast heat transfer by virtue of high surface-to-volume ratio, precisely controlled residence time, and so on.32 In general, we envision that for a living cationic polymerization system, the stability loss of propagating chains with the decrease of the added base addition may be offset in a microflow system due to the intensified process regulation, and a fast and low-cost living cationic polymerization may be achievable with this strategy. Herein, the polymerization experiments at low added base addition were carried out in a batch reactor and a microflow system comparatively. Through determining the time and conversion dependences of molecular weight distribution, we explored the feasibility of the strategy, discussed the effect of reaction mode on the polymerization performances, and 4


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recognized the mechanism and requirement to achieve living cationic polymerization at decreased added base addition.

EXPERIMENTAL SECTION Materials. IBVE (TCI; > 99.0%) were refluxed for several hours and distilled over calcium hydride before use. Toluene, 1,4-dioxane and diethyl ether (A.R. Beijing Lanyi chemical products) were refluxed for several hours and distilled over calcium hydride and were stored in 4A molecular sieve. Commercially available FeCl3 (Anhydrous, Acros, 98%) and SnCl4 (Anhydrous, Acros, 99%) were used without further purification. The adduct of IBVE with HCl (IBVE-HCl) was prepared from the addition reaction of IBVE with HCl.33 All chemicals were stored in glove box under dry argon.

Characterization. The number-average molecular weight (Mn), weight-average molecular weight (Mw), and molecular weight distribution (MWD, Mw/Mn) of poly isobutyl vinyl ether (PIBVE) were measured by gel permeation chromatography (GPC). GPC using a Waters 1515 isocratic HPLC pump connected to four Waters Styragel HT2, HT3, and HT4 columns and a Waters 2414 Refractive Index Detector at 30 oC [Waters; styragel HT2 (THF), styragel HT3 (THF) and styragel HT4 (THF); molecular weight range = 100-10 k, 500-30 k, 5-600 k; particle size = 10 µm; column size = 7.8 mm (internal diameter) × 300 mm; flow rate = 1.0 mL/min].

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Polymerization in a batch reactor. The polymerization was carried out at 0 oC under dry nitrogen in a glass reaction tube equipped with a side branch. A monomer solution (9 mL) of IBVE, DO, and IBVE-HCl was added into reaction tube (50 mL) with dry syringes first, and then stirring for 5 min at 0 oC. Immediately after mixing, the polymerization was started with the addition of 1 mL prechilled FeCl3 solution in a mixture of diethyl ether and toluene (1:3, v/v) or SnCl4 solution in toluene. As the preset time interval reached, the reaction was terminated with prechilled methanol containing 0.1 vol% aqueous ammonia solution.

Polymerization in a microflow system. The polymerization was carried out in a microflow system as shown in Figure 1, which consists of two T-shaped micromixers (M1 for the mixing of monomer solution and Lewis acid solution, and M2 for the injection of termination agent methanol containing a small amount of aqueous ammonia solution (0.1 vol%)), three precooling coiled tubes (C1 and C2, inner diameter 600 µm; C3, inner diameter 1000 µm), and a microtube reactor (R1, inner diameter 600 µm). The reaction time of IBVE polymerization could be adjusted by changing the flow rate or the length of R1. Three syringe pumps were used to deliver monomer, Lewis acid, and MeOH solution, and the flow rates are 8 mL·min-1, 8 mL·min-1, 9.6 mL·min-1 respectively. The yields of polymers are 1.2576 g·min-1 theoretically at a ca.100% conversion. The quenched mixture was washed with dilute hydrochloric acid, an aqueous NaOH solution, and then water to remove the initiator residues. The volatiles were then removed 6


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under reduced pressure at 50 oC and the residue was vacuum-dried for more than six hours at 70 oC to yield a colorless, gummy polymer. Monomer conversions were determined gravimetrically.

Figure 1. Schematic diagram of flow synthesis setup. M1 and M2 are tees as micromixers; C1, C2 and C3 are curved tubes for achieving the pre-set temperature; R1 is the microtube as reactor.

RESULTS AND DISCUSSION 1. IBVE-HCl/SnCl4/DO initiation system (a) Living characteristic at various DO addition In this study, IBVE-HCl, SnCl4, 1,4-dioxane (DO) was used as cationogen, co initiator, and added base, respectively. As previously reported,17-24 the concentration of DO for the living cationic polymerization of IBVE is normally 1.0 M. To examine the possibility of accelerating the polymerization rate and keeping living characteristic at low 7



concentration of DO, a serial of IBVE polymerization experiments were carried out at different concentrations of DO (0, 0.01, 0.1, 0.5 M, respectively). The results were presented in Supporting Information (Table S1), and the GPC profiles were shown in Figure 2. In a batch reactor, the rate of polymerization progressively increased with the decreasing of DO concentration, and the polymerization with base free was even completed within 3 s. At the same time, the MWD of polymers became broader with the decreasing of DO concentration. Only at [DO] = 0.5 M, the MWD of the polymer was narrow enough (Mw/Mn ≤ 1.10) to approach a monodisperse distribution. The concentration of polymer chains, [PIBVE], can be calculated from the molecular weight and conversion. Except for the entry of [DO] = 0.5 M, all the entries using batch reactor produced [PIBVE] over 4.6 mmol·L-1. It is larger than [IBVE-HCl] of 4 mmol·L-1 clearly, indicating chain transfer evidently. The polymerization of IBVE in a microflow system showed similar rules with that in batch reactor. A difference is that the microflow system can always produce polymers with much narrower MWD, and the MWD of polymers is less than 1.10 at [DO] = 0.1 M or more (entry 6, entry 8 in Table S1, Supporting Information). Correspondingly, the concentration of PIBVE is in the range of 3.8 to 4.2 mmol·L-1. Taking into account the experimental error, we may consider that there is no chain transfer during polymerization. In order to verify the living characteristic at low concentration of DO in a microflow system, we further conducted the cationic polymerization of IBVE at different residence 8


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times. As summarized in Supporting Information (Table S2), the SnCl4 system produced polymers with narrow MWDs (Mn/Mw ≈ 1.05-1.10) under all conditions. And Figure 3-4 demonstrated that the Mn value of polymer was directly proportional to monomer conversion and in good agreement with the calculated value (assuming that there exists a one-to-one match between cationogen IBVE-HCl and polymer chain). These results indicated that the polymerization of IBVE proceeded in a living fashion. Moreover, compared to a ca. 93% at 70 s when using DO at the normal concentration (1.0 M),22 the monomer was consumed almost quantitatively within 9.42 s at [DO] = 0.1 M. So far, an accelerated living cationic polymerization of IBVE has been achieved at low concentration of DO with the assist of the microflow platform. In other words, the microflow system enables more controllable cationic polymerization of IBVE. It may broaden the operating window of living polymerization.

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Figure 2. (A) MWD curves for poly (IBVE) obtained in a batch reactor; (B) MWD curves for poly (IBVE) obtained in a microflow system. (Polymerization conditions: [IBVE]0 = 0.76 M, [IBVE-HCl]0 = 4.0 mM, [SnCl4]0 = 5.0 mM, [DO] = 0/0.01/0.1/0.5 M, in toluene at 0 oC

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Figure 3. (A) Time-conversion curves, (B) Mn and Mw/Mn for the polymerization of IBVE, (C) MWD curves for poly(IBVE). (Polymerization conditions: [IBVE]0 = 0.76 M, [IBVE-HCl]0 = 4.0 mM, [SnCl4]0 = 5.0 mM, [DO] = 0.5 M, in toluene at 0 oC).

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Figure 4. (A) Time-conversion curves, (B) Mn and Mw/Mn for the polymerization of IBVE, (C) MWD curves for poly(IBVE) (Polymerization conditions: [IBVE]0 = 0.76 M, [IBVE-HCl]0 = 4.0 mM, [SnCl4]0 = 5.0 mM, [DO] = 0.1 M, in toluene at 0 oC).

(b) Mixing effect on living characteristic The remarkable difference between the microflow system and the batch reactor in polymerization performance may be attributed to intensive mixing, which is an important merit of microflow system. Compared with batch methods, mixing time (tm) in a microflow system is much shorter. Masoud Rahimi et.al used the incorporation model to determine tm for three shapes of micro channels (T, Y, and oriented Y mixers), which is in 12


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the range of 0.001-0.1 s.34 Madhvanand Kashid et.al also proposed tm is in the range of 0.01-0.1 s for most of flow rates in microflow system.35 L. Falk et al. used the Villermaux-Dushman test reaction to estimate the tm of T-shaped mixers, which was nearly 0.1 s.36 For batch reactor, F. Al-Qaessi used MASM and TRM method to predict the tm for 340 and 100 s, respectively.37 Herein, to confirm the mixing effect on living characteristic, the cationic polymerization of IBVE using SnCl4 coinitiation system at various flow rates in a microflow system was investigated.

Table 1. Effects of mixing on the cationic polymerization of IBVE initiated by IBVE-HCl/SnCl4/DO in microflow system Fmonomer

FLewis acid

FMeOH

t

Conv.a

Mn,calb

Mnb

/mL·min-1

/mL·min-1

/mL·min-1

/s

/%

/g·mol-1

/g·mol-1

1

2

2

2.4

7.54

77

15095

14486

1.21

4.04

2

4

4

4.8

7.54

85

16650

16626

1.10

3.89

3

5

5

6

7.54

93

18282

17260

1.08

4.10

4

6

6

7.2

7.54

82

16078

17313

1.09

3.60

5

8

8

9.6

9.42

91

17842

17447

1.09

3.97

Entry

Mw/Mn

[PIBVE]c

b

/mmol·L-1

All experiments: [IBVE]0 = 0.76 M, [IBVE-HCl]0 = 4 mM, [SnCl4]0 = 5 mM, [DO]0 = 0.1 M, in toluene at 0 oC.

a

Gravimetric conversion.

b

Mn: number-average molecular

weight, Mw/Mn: molecular weight distribution. c [PIBVE] = [IBVE] × 100.16 × conv./Mn.

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Figure 5. MWD curves for poly(IBVE) obtained with SnCl4 in the presence of 1,4-dioxane at various flow rates.

Results presented on Table 1 and Figure 5 indicate that living polymers were obtained at higher flow rates (entry 2-5), and the MWD of polymers kept at no more than 1.10. But at the lowest flow rate (entry 1), the MWD reached at 1.21. It is well known that the mixing efficiency decreases with a decrease in flow rate,38 so low mixing efficiency corresponds to broad MWD evidently. On the other hand, for all the entries in Table 1, [PIBVE] is always around 4 mmol·L-1. It indicates that there is almost no chain 14


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transfer when residence time changes little, independent on the flow rate and mixing efficiency. We inferred that living characteristic was determined by the residence time, and the MWD of product were determined by the mixing efficiency and the residence time simultaneously. To illustrate the mechanism determining the living characteristic and MWD of product in the cationic polymerization of IBVE initiated by IBVE-HCl/SnCl4/DO, we proposed two scenarios under different mixing conditions, as shown in Figure 6. The polymerization system experiences four stages. At t0, it forms effective active species consisting of initiator, added base and Lewis acid, and the chain initiation is also started. At t1, the polymerization just reaches about 100% conversion in the microflow system. At t2, corresponding to the lifetime of living chains, the chain transfer started to take place. At t3, the polymerization reached about 100% conversion in the batch reactor. Under good mixing conditions, all species mixed intensely well including initiator, Lewis acid, added base and monomer. Subsequently, they can form effective active species, leading to simultaneous chain initiation and chain propagation, resulting in a very narrow molecular weight distribution of polymers. On the contrary, under poor mixing conditions, it may exist relatively high concentration gradients,39 affecting the formation of active species and sufficient supply of monomer. Thereafter, uneven initiation can not guarantee that the chain propagation is synchronous, and chain transfer may occur before the monomer was depleted in time. As a result, polymers with broad MWD were obtained, and the concentration of chain was higher than the concentration of 15



cationogen. However, the living characteristic with low concentration added base could exist at relatively short residence time, which can be utilized through prompt monomer supply with assist of the microflow system.

Figure 6. Proposed Mechanisms of cationic polymerization of IBVE under different mixing conditions.

2. IBVE-HCl/FeCl3/DO initiation system The microflow system proved to be a useful tool for realizing accelerated living cationic polymerization of IBVE initiated by IBVE-HCl/SnCl4/DO. This finding encouraged us to examine the universality of this method. Therefore, we studied IBVE-HCl/FeCl3/DO initiation systems under the similar conditions. Polymerization experiments were first conducted in a batch reactor. As shown in Supporting Information (Table S3) and Figure 7, it obtains the concentration of PIBVE much higher than the 16


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concentration of cationogen (entry 19, entry 21 in Table S3, Supporting Information), and very broad MWD (Mw/Mn = 1.24, 2.05), indicating that the chain transfer occurred significantly. Compared with the SnCl4 coinitiation system (entry 5 and entry 7 in Table S1, Supporting Information), the FeCl3 coinitiation system corresponds to higher polymerization rate and more intensive chain transfer. In contrast, the microflow system produced polymers with much lower MWD, which is 1.08 at [DO] = 0.5 M and 1.28 at [DO] = 0.1 M, respectively. In particular, the concentration of PIBVE in entry 22 (Supporting Information, Table S3) was 3.89 mmol·L-1, a little less than the concentration of [IBVE-HCl], reflecting some living characteristic.

Figure 7. (A) MWD curves for poly (IBVE) obtained using a batch reactor; (B) MWD 17



curves for poly (IBVE) obtained using a microflow system. (Polymerization conditions: [IBVE]0 = 0.76 M, [IBVE-HCl]0 = 4.0 mM, [FeCl3]0 = 5.0 mM, [DO] = 0.1/0.5 M, in toluene at 0 oC).

Furthermore, Figure 8 revealed that the Mn value of polymers obtained at [DO] = 0.5 M (entries 23-29 in Table S4, Supporting Information) was directly proportional to monomer conversion and in good agreement with the calculated value, presenting the living characteristic clearly. Meanwhile, a ca.85% conversion was achieved in 6.36 s. Comparatively, at [DO] = 0.1 M, the conversion was almost 100% in 0.27 s, at the same time scale with the tm of T-mixer (about 0.1 s). Figure 9 shows experimental Mn is obviously smaller than what calculated. It indicates that the chain transfer occurred to some extent and the extremely fast polymerization was out of the control of the microflow system.

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Figure 8. (A) Time-conversion curves of IBVE with FeCl3, (B) Mn and Mw/Mn for the polymerization of IBVE, (C) MWD curves for poly(IBVE) obtained with FeCl3. (Polymerization conditions: [IBVE]0 = 0.76 M, [IBVE-HCl]0 = 4.0 mM, [FeCl3]0 = 5.0 mM, [DO] = 0.5 M, in toluene at 0 oC).

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Figure 9. (A) Time-conversion curves of IBVE with FeCl3, (B) Mn and Mw/Mn for the polymerization of IBVE, (C) MWD curves for poly(IBVE) obtained with FeCl3. (Polymerization conditions: [IBVE]0 = 0.76 M, [IBVE-HCl]0 = 4.0 mM, [FeCl3]0 = 5.0 mM, [DO] = 0.1 M, in toluene at 0 oC).

In summary, to ensure living cationic polymerization of IBVE in microflow system, 0.1 M DO and 0.5 M DO were enough for IBVE-HCl/ SnCl4/DO initiation system and IBVE-HCl/ FeCl3/DO initiation system, respectively. Compared with 1.0 M DO reported elsewhere, the saving of DO usage was quite distinct. For living cationic polymerization, we can suppose a first-order kinetic model and calculate the apparent polymerization rate 20


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(kapp) from the plot of ln([M]0/[M]) vs. reaction time. We summarized all the entries with living characteristic in Figure 10. For IBVE-HCl/ SnCl4/DO initiation system, kapp is 0.277 s-1 at [DO] = 0.1 M and 0.0619 s-1 at [DO] = 0.5 M, respectively. For IBVE-HCl/ FeCl3/DO initiation system, kapp is 0.330 s-1 at [DO] = 0.5 M. These results indicate that the polymerization rate increased with the decrease of DO addition extraordinarily, and the substitute of SnCl4 with FeCl3 could accelerate the polymerization rate much. We noticed at the lower limit of DO addition to achieve living cationic polymerization in experiments, the value of kapp is quite similar (around 0.3 s-1) for two initiation system, corresponding to 2.3 s of t1/2 (time for a ca.50% conversion at constant kapp). As well known, the initiation system for cationic polymerization usually has negatively correlated reactivity and stability. So, long t1/2 corresponds to low reactivity of initiation system as well as high stability. In other words, the living cationic polymerization of IBVE is accessible in our microflow system as an initiation system could provide a t1/2 of 2.3 s or longer, which is approximately one order of magnitude higher than tm. Enhancing mixing performance is key to using high reactivity initiation system with low addition of added base and high productivity.

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Figure 10. (a) ln([M]0/[M]) versus time with SnCl4 system; and (b) ln([M]0/[M]) versus time with FeCl3 system.

Conclusion In this work, we proposed and realized a strategy for fast and low-cost cationic polymerization of IBVE based on temporary stability of propagating chain. It combines the decrease of added-base addition to increase the polymerization rate and the use of microflow system to enhance process regulation. To assure the living characteristic, the concentration of added base DO was allowed to be as low as 0.1 M for IBVE-HCl/SnCl4/DO initiation system and 0.5 M for IBVE-HCl/FeCl3/DO initiation system, respectively; the time to achieve 80 % conversion could decrease to around 5 s. Compared with batch reactor, the advantage of microflow system is enabling the initiation and propagation of active chains to proceed more uniformly. It leads to not only 22


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the decrease of MWD, but also the acceleration of the monomer consumption. The latter could bring the possibility to fulfill polymerization before chain transfer taking place and present living characteristic throughout polymerization process. The fast living cationic polymerization shows first-order kinetics, a sufficient condition for which in our work is that the characteristic reaction time (time to reach 50% conversion) is one order of magnitude longer than the characteristic mixing time. These recognitions may broaden the horizon of living cationic polymerization, and improve its commercial value with respect to productivity and cost.

ASSOCIATED CONTENT *Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:________. Results for molecular weight, MWD and the concentration of polymer chains of PIBVE with different reaction system, various concentration of added base, and different flow rate (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel.: +86 10 62773017. Fax: +86 10 62773017. ORCID 23



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Dan Xie: 0000-0001-6675-6714 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors granted the financial support of the National Natural Science Foundation of China (21422603, U1662120).

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