Living Anionic Polymerization in Continuous Flow: Facilitated

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Living Anionic Polymerization in Continuous Flow: Facilitated Synthesis of High Molecular Weight Poly(2-vinyl pyridine) and Polystyrene Adrian Natalello, Jan Morsbach, Andreas Friedel, Arda Alkan, Christoph Tonhauser, Axel Mueller, and Holger Frey Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/op500149t • Publication Date (Web): 21 Jul 2014 Downloaded from http://pubs.acs.org on August 1, 2014

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Organic Process Research & Development

Living Anionic Polymerization in Continuous Flow: Facilitated Synthesis of High Molecular Weight Poly(2-vinyl pyridine) and Polystyrene Adrian Natalello,†,‡ Jan Morsbach,† Andreas Friedel,† Arda Alkan,†,§ Christoph Tonhauser,† Axel H. E. Müller*,† and Holger Frey*,† †

Institute of Organic Chemistry, Johannes Gutenberg-University (JGU), Duesbergweg 10-14, 55099 Mainz, Germany Graduate School Materials Science in Mainz, Staudinger Weg 9, D-55128 Mainz, Germany § Max Planck Institute for Polymer Research (MPI-P), Ackermannweg 10, 55128 Mainz, Germany ‡

KEYWORDS: Continuous flow, microstructured reactor, multilamination mixing, jet mixing, polystyrene, poly(2-vinyl pyridine) ABSTRACT: We describe the living anionic polymerization of 2-vinyl pyridine (2VP) and styrene (S) in continuous flow, comparing two micromixing devices with different mixing principle. The use of a continuous flow setup reduces the experimental effort for living anionic polymerizations significantly, compared to a conventional batch system. By adjusting the ratio of the flow rates of the monomer and initiator solutions a variety of different molecular weights can be rapidly synthesized within several minutes, using one setup. Additionally, a comparison of the influence of the two different mixing devices - an interdigital micromixer (SIMM-V2) leading to laminar mixing and a tangential 4-way jet mixing device leading to a turbulent mixing pattern - has been achieved. Both setups allow living anionic polymerization in polar solvents at room temperature with full monomer conversion within seconds and yield polymers with narrowly distributed molecular weights. A maximum Mn of approximately 149,000 g mol-1 (PS-9, PDI = 1.04) for PS and 96,000 g mol-1 for P2VP (P2VP-15, PDI = 1.05) was obtained. Clearly, the turbulent 4-way jet mixing device led to lower polydispersity than the laminar mixing device. All polymers were characterized by 1H NMR spectroscopy and size exclusion chromatography (SEC).

INTRODUCTION In recent decades important progress has been made in the field of controlled polymerization techniques.1–3 Despite these advances, living anionic polymerization still plays the key role with respect to industrial application.4,5 This method avoids the usual limitations of radical polymerization processes. For example, the theoretical limit for the polydispersity index (PDI = Mw/Mn) is decreased for living anionic polymerization due to the absence of chain transfer and termination reactions.6 Furthermore, the living character of the anionic polymerization leads to outstanding control over the molecular weight. However, a drawback of the method is its high sensitivity towards impurities, necessitating careful drying of all reaction vessels employed.7 The high productivity of a microstructured flow reactor, improved heat transfer and reduced reaction times have motivated several groups in recent years to transfer polymerizations from batch systems to microfluidic devices.8– 15 Both theoretical and synthetic works have dealt with the influence of temperature, mixing device geometry, overall flow rates and retention times.16–18 In order to study the polymerization kinetics of anionic polymerizations, microfluidic approaches have been used in a few works.19–21 In 2008 Wilms et al. utilized a continuous flow setup to reduce the experimental effort to synthesize welldefined polystyrene (PS) in a microfluidic device.22 A library of PS samples with varying molecular weights were accessible with one reaction setup, because the monomer/initiator ratio can be adjusted during the polymerization process by changing

the flow rates of initiator and monomer solutions. Additionally, the synthesis was carried out in a polar solvent mixture at room temperature in the continuous flow reaction setup. The analogous synthesis in batch is usually performed at -78 °C, due to the limited heat transfer and the highly exothermic nature of the polymerization.23 Also, the reaction of carbanions with THF at room temperature plays a role at extended reaction times. Besides the well-known polymerization of styrene, anionic homo and copolymerization of various alkyl methacrylates has been studied in great detail, capitalizing on microfluidic systems.24–28 Although the utilization of microreactor systems possesses several advantages, the accessible molecular weights are often limited due to an increase in viscosity of the reaction mixture with increasing chain length, which is often accompanied by a decrease of the solubility of the high molecular weight polymer structures.29 Consequently, the mixing devices are susceptible to obstruction. Therefore a variety of mixing devices have been designed and studied in recent years in theoretical and experimental works.30–33 Although poly(2-vinyl pyridine) (P2VP) gained considerable attention because of its remarkable characteristics that are related to the pH-responsive properties of the aromatic pyridine system,34–37 to the best of our knowledge no continuous polymerization technique for 2VP has been reported to date that is based on a microfluidic setup. Although anionic polymerization is a useful method to obtain P2VP, an undesired side reaction between the carbanionic chain ends and the electron-poor pyridine ring has to be taken into account.38,39 The established strategy to overcome this

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deficit in a batch system is the addition of inorganic salts, such as lithium chloride.40 We expected that the short reaction times in a microfluidic system combined with direct, subsequent termination of the reaction would enable us to circumvent the chain coupling side reaction, which is considerably slower than the polymerization. Consequently, a microfluidic process would render the addition of inorganic salts unnecessary. In this work we have employed two different mixing devices aiming at high molecular weight samples for two different monomers, 2VP) and S, respectively. We compare the influence of an interdigital micromixer (SIMM-V2, Fraunhofer Institute for Chemical Technology, Mainz Branch (IMM), Figure S1) based on ultrafast diffusion between alternatingly combined solution layers of the initiator and monomer solution (Figure 1, left) to a tangential 4-way jet mixing device (Figure 1, right), an improvement of the common T-junction that leads to a turbulent mixing pattern. Both mixing systems have been utilized for the carbanionic polymerization of the two vinyl monomers S and 2VP in a polar solvent at room temperature, and the results have been compared with respect to molecular weights achievable and molecular weight distribution.

EXPERIMENTAL SECTION Reagents. All solvents and reagents were purchased from Sigma Aldrich or Acros Organics and were used as received unless otherwise declared. Chloroform-d1 and dimethyl sulfoxide-d6 was purchased from Deutero GmbH. The solvents (tetrahydrofuran (THF) and benzene) used for the living carbanionic polymerization in continuous flow were distilled from sodium/benzophenone under reduced pressure into a liquid nitrogen-cooled reaction vessel (cryo-transfer). The monomers styrene and 2-vinyl pyridine were dried over calcium hydride (CaH2) and cryo-transferred prior to use. sec-Butyllithium (sec-BuLi, 1,3 M in cyclohexane/hexane, Acros) was used as received. Instrumentation. NMR spectra were recorded at 300 MHz or 400 MHz on a Bruker AC300 or Bruker AMX400 spectrometer, respectively, and were referenced internally to the residual proton signals of the deuterated solvent. For size exclusion chromatography (SEC) measurements in DMF (containing 0.25 g/L of lithium bromide as an additive) an Agilent 1100 Series was used as an integrated instrument, including a PSS HEMA column (300/100/40 g mol-1), a UV detector (operating at 275 nm) and a RI detector. SEC measurements in THF were carried out with an instrument containing a Waters 717 plus auto sampler, a TSP Spectra Series P 100 pump, a set of three PSS SDV columns (104/500/50 Å) and RI and UV detectors. All SEC diagrams show the RI detector signal, and the molecular weights were referenced to linear polystyrene (PS) standards provided by Polymer Standards Service (PSS). All polymers were synthesized by living anionic polymerization in continuous flow with purified reagents. Reactor setup. A schematic overview of the continuous flow reaction setup is presented in Scheme 1. Both the monomer and initiator solution were prepared using high vacuum techniques established for living carbanionic polymerization conditions and connected via stainless steel capillary tubes (diameter (ø) = 700 µm) to the pump system (HPLC pumps, Knauer WellChrom K-501, inert 10 mL pump heads with

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ceramic inlays). Depending on the micro mixer the length and the diameter of the residence tube varied from 1.2 m (ø = 700 µm) for the interdigital micromixer to 3.7 m (ø = 1000 µm) for the 4-way jet mixing device before quenching the reaction with degassed methanol via a subsequent Tjunction. Polymerization with the microfluidic setup. Prior to polymerization, the micro structured setup was purged with THF via all HPLC pumps (flow rate = 1 mL/min) for approximately 10 min to remove potential impurities. In the case of 2VP a 0.5 м monomer solution of dry and degassed THF as well as an initiator solution of dry and degassed benzene with an appropriate amount of sec-BuLi was prepared. Styrene was polymerized using a 1.0 м monomer solution. After connecting the reaction flasks to the pump system, the pump connected to the initiator solution was activated (flow rate = 1 mL/min) for approximately 3 min to flush the setup with the sec-BuLi solution in order to eliminate residual impurities. To start the polymerization, the pumps connected to the monomer solution and to MeOH were switched on. To achieve the desired molecular weights during the polymerization the flow rate ratio of monomer and initiator was adjusted. The resulting P2VP samples were precipitated in petroleum ether, whereas the PS samples were precipitated in MeOH.

Scheme 1. Schematic setup of the microfluidic device for the polymerization of 2VP and S with direct termination by MeOH.

RESULTS AND DISCUSSION The current work focuses on the homopolymerization of 2VP and S by living carbanionic polymerization in continuous flow at room temperature. It has to be emphasized that commonly anionic polymerization in polar solvents has to be performed at -78 °C. In the microfluidic setup the high surface to volume ratio enables improved heat transfer and extremely short reaction times compared to the common batch system. This allows to carry out exothermic reactions at ambient temperature with good control over the transformation, accompanied by a strong decrease in the reaction time. The centerpiece of each continuous polymerization is the mixing device. In this work we compare two different mixing devices, namely an interdigital micromixer and a tangential 4-way jet mixing device, based on different mixing principles (multilamination vs. turbulent mixing, respectively). In 2008 our group reported the living anionic polymerization of S in continuous flow, utilizing the interdigital micromixer for the first time.22 This concept has been transferred to the polymerization of 2VP. However, this protocol had to be adjusted to deal with the different monomer reactivity and solubility properties of P2VP. Compared to PS, P2VP is not


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soluble in THF/hexane mixtures. Consequently, the initiator (1.3 M sec-BuLi) has been added to a certain amount of benzene instead of hexane. Additionally, the prepared monomer solution was not as concentrated (5 wt.-%) as the corresponding styrene solution (10-20 wt.-%) for the PS synthesis. The initiator concentration was adjusted according to the desired molecular weight range. For the chosen experimental setup, flow rates between 0.1 mL∙min-1 and 1.5 mL∙min-1 were established. The higher propagation rate and possible side reactions complicate the polymerization of 2VP in comparison to S. After complete consumption of the monomer, the carbanionic P2VP chain end can attack the electron poor pyridine ring system of another P2VP chain in the reaction mixture, leading to coupling and broadening of the molecular weight distribution. To overcome this problem, salts like LiCl are usually added to the reaction mixture in order to decrease the reactivity of the active chain ends. However, this leads to additional workup steps after polymerization.38 Using the microfluidic devices this problem can be avoided by adjusting the reaction time via variation of the length of the flow tube. After termination with protons from MeOH, the polymer chains can no longer react with each other to create undesired branched polymeric structures. This allows for a highly controlled polymerization of high molecular weight P2VP without the addition of inorganic salts or cooling of the reaction to -78°C. The addition of the nonpolar solvent benzene containing the initiator to the polar monomer solution (THF) clearly increased the control of the reaction.

P2VP-5 P2VP-6

1.5 1.5

0.3 0.1

1.8 1.6

15.3 17.3

6.2 10.4

1.26 1.26

a Flow rates given in mL∙min-1; bResidence times given in s, cdetermined by SEC in DMF (compared to PS standards).

By adjusting the flow rate ratio of the initiator solution and monomer solution a series of polymers with varying numberaverage molecular weights (Mn) were synthesized using the SIMM-V2 mixing device. Table 1 summarizes characterization data of the synthesized P2VP samples, characterized by 1 H NMR and SEC. All polymers show monomodal molecular weight distributions (MWDs) with moderately low polydispersity indices (PDI). Three different SEC traces are exemplarily presented in Figure 2. The small deviations between the theoretical and observed molecular weights can be explained by the slightly imprecise flow rates of the HPLC-pumps.

Figure 2. SEC traces (eluent: DMF) of selected P2VP samples (P2VP-1 blue, P2VP-4 red, P2VP-6 black) prepared in the continuous flow reaction setup, utilizing the interdigital micromixer.

Figure 1. Left: Operating principle of the interdigital micromixer (SIMM-V2, provided by IMM);22 Right: Operating principle of the 4-way jet mixing device.33 Mixing of the initiator and monomer solution in the interdigital micromixer occurs within milliseconds, exploiting the principle of multilamination combined with geometric and hydrodynamic focusing.41 This concept is visualized in Figure 1. Both solutions are expanded in thin liquid layers and combined alternatingly. Due to an extremely large contact area between the solution layers’ interfaces, effective mixing by diffusion is realized. Table 1. Characterization data of synthesized P2VP samples, utilizing the interdigital micromixer SIMM-V2 sample

flow (M)a)

flow (I)a)

total flowa)

residence timeb)

Mnc) x 103

PDI

P2VP-1

1.5

1.5

3.0

9.2

1.5

1.19

P2VP-2 P2VP-3 P2VP-4

1.5 1.5 1.5

1.2 0.7 0.5

2.7 2.2 2.0

10.2 12.5 13.8

1.8 3.2 5.3

1.23 1.22 1.21

c)

P2VP with Mn exceeding 10,000 g mol-1 could not be realized due to the low solubility of high Mn P2VP in the chosen solvent mixture. Accompanying the increase in viscosity, the multi-lamellar mixing is disturbed, leading to undesired broad MWDs and finally clogging of the micromixer. In comparison, using the SIMM-V2 micro reactor setup for the synthesis of PS, Mns up to 70,000 g mol-1 were realized.22 In summary, the presented strategy for the synthesis of P2VP in continuous flow using the interdigital micromixer possesses several advantages compared to the conventional synthesis in batch, avoiding the addition of salts. Furthermore, the greatly simplified handling of the carbanionic polymerization represents a major advantage. No time-consuming and intense predrying of the reaction equipment is necessary, because after drying of the monomer and initiator solutions the reactor is purged with these solutions and thereby rapidly free from protic impurities. Once the reaction is set up, a variety of samples with different Mn can by synthesized by merely varying the flow rates of the monomer and initiator solutions in the same run. No cooling of the polymerization is necessary. However, the accessible molecular weights of P2VP with this setup are limited, which we attribute to the rapidly increasing viscosity. To overcome this limitation, we used a tangential 4-way jet mixing device in the microfluidic setup (Figure 1). In contrast to the multilamination mixing device, this represents an extension of a common T-junction. Monomer solution as well as



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initiator solution are divided into two capillaries and recombined in the mixing chamber (V ≈ 1 µL), as shown in Figure 1. The mixing time was determined to be below 1 ms at sufficiently high flow rates.33 In a first series of experiments the effect of different total flow rates of the initiator and monomer solutions was studied. As shown in Figure 3, the correlation between PDI and total flow rate behaves similarly for the polymerization of both 2VP and S. At low flow rates, less than of 4 mL∙min-1, polymers with broad MWD are obtained (PDI > 1.25), indicating insufficient mixing. However, with increasing total flow rate polymers with narrowly distributed MWDs (PDI < 1.2) are generated. Based on this observation a variety of different Mn of P2VP and PS have been prepared.

PS-8g) PS-9g)

8 9

3 3

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15.9 14.6

104.0 149.0

1.04 1.04

a Flow rates given in mL∙min-1; bResidence times given in s, cdetermined by SEC in DMF for P2VP and in THF for PS (both compared to PS standards). d-gThese polymers were synthesized from the same respective stock solution.

Especially in the case of P2VP the maximum attainable Mn increased up to 96,000 g mol-1, which is approximately an order of magnitude higher than for the multilamination mixing device (Table 1. P2VP-6 Mn = 10,600 g mol-1). In addition, the PDIs are lower (1.19 – 1.05), even for comparable Mn (below 10,000 g mol-1), indicating improved mixing of the monomer and initiator solution. In the case of PS, similar results are obtained (Table 2). Whereas the multilamination mixer yielded Mns up to 70,000 g mol-1, PS samples with Mns up to 149,000 g mol-1 have been synthesized with remarkably narrow MWDs. In summary, the resulting polymers (P2VP and PS) synthesized using the 4 way jet device are comparable to corresponding polymers synthesized in batch. Figure 4 shows several SEC traces of the synthesized P2VP and PS samples. All samples are monodisperse and exhibit a narrow MWD, as presented in Table 2. In agreement with theory the polydispersity decreases with increasing Mn.

Figure 3. Influence of the total flow rate (monomer to initiator ratio 1:1) on the PDIs for the polymerization of 2VP (maroon) and S (blue), using the 4 way jet mixing device. Table 2 summarizes the synthesized P2VP and PS samples using the 4-way mixing jet. Compared to the multilamination mixer, considerably higher molecular weights could be realized. Table 2. Characterization data of synthesized P2VP and PS utilizing the 4-way mixing jet flow (M)

flow (I)

residence timeb)

Mnc) x 103

PDI

a)

total flowa)

a)

P2VP-7d)

3

5

8

21.8

1.4

1.17

P2VP-8d) P2VP-9d) P2VP-10d) P2VP-11d) P2VP-12e) P2VP-13e) P2VP-14e) P2VP-15e)

5 5 5 5 5 6 7 7

5 4 3 2 5 5 4 3

10 9 8 7 10 11 11 10

17.5 19.4 21.8 24.9 17.5 15.9 15.9 17.5

1.8 2.3 3.5 6.4 14.3 18.7 49.3 96.0

1.16 1.19 1.17 1.19 1.15 1.17 1.12 1.05

PS-1f) PS-2f) PS-3f) PS-4g) PS-5g) PS-6g) PS-7g)

3 5 5 4 5 6 7

5 4 3 3 3 3 3

8 9 8 7 8 9 10

21.8 19.4 21.8 24.9 21.8 19.4 17.5

2.2 4.7 6.6 32.2 42.5 57.7 74.2

1.09 1.08 1.10 1.08 1.08 1.08 1.05

sample

c)

Figure 4. a) SEC traces (eluent: DMF) of selected P2VP samples (P2VP-8 blue, P2VP-12 red, P2VP-15 black) and b) SEC traces (eluent: THF) of selected PS samples (PS-4 blue, PS-7 red, PS-9 black) prepared in the microfluidic reaction setup utilizing the 4 way jet mixing device.

CONCLUSION We have successfully transferred the protocol for the living carbanionic polymerization of styrene (S) in continuous flow to the synthesis of poly(2-vinyl pyridine) (P2VP). In contrast to PS only limited molecular weights (Mns) up to 10,000 g mol-1 were achieved when using a multilamination mixing device (SIMM-V2), which is attributed to the limited solubility for the used solvent mixture of THF/benzene. For this mixing device, no dependence of the polydispersity on the flow rates was observed. To overcome the limitations of laminar mixing, the SIMM-V2 was replaced by a tangential 4-way jet mixing device resulting in higher Mn limits and considerably lower PDIs. A maximum Mn of approximately 149,000 g mol-1 (PS-9, PDI = 1.04) for PS and 96,000 g mol-1 for P2VP (P2VP-15, PDI = 1.05) was achieved. The low PDIs observed indicate the absence of side reactions. Especially in the case of P2VP the well-known branching side reaction could be suppressed even at room temperature without cooling as it is common in the batch process. The improved heat transfer due to the high surface to volume ratio of the micro reactors allows for transferring this exothermic reaction from batch without additional cooling or additives.


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Additionally, a number of advantages arise simply by the nature of the continuous flow reactor. After starting the microfluidic device a variety of different molecular weights of one polymer can be synthesized in a continuous polymerization by merely adjusting the monomer to initiator ratio. Furthermore, no lengthy drying procedures are required, because all components of the micro structured reactor are flushed with the initiator solution for some time before polymerization. Finally, polymerizing 2VP and S with the microstructured reactor represents a safe process, because the internal volume is very low. In brief, the microfluidic carbanionic polymerization simplifies access to well-defined P2VP and PS in a wide range of molecular weights.

9. 10. 11. 12.

13.

14. 15.

ASSOCIATED CONTENT

16.

Supporting Information. Detailed experimental procedures as well as analytical and spectral characterization data. “This material is available free of charge via the Internet at http://pubs.acs.org

17. 18. 19.

AUTHOR INFORMATION Corresponding Author *E-mail: (A. H. E. M.) [email protected]; (H.F.) [email protected]

20. 21.

Funding Sources A.N. thanks the Graduate School of Excellence MAINZ for financial support and we thank the Fraunhofer Institute for Chemical Technology, Mainz Branch (IMM) for providing the Interdigital micromixer (SIMM-V2).

ACKNOWLEDGMENT We thank Kevin Tritschler for technical assistance.

22. 23. 24. 25. 26.

For Table of Contents only: 27. 28. 29.

30. 31.

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Living Anionic Polymerization in Continuous Flow: Facilitated

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