Living Polymer Chains with Predictable Molecular Weight and

Article pubs.acs.org/Macromolecules

Living Polymer Chains with Predictable Molecular Weight and Dispersity via Carbanionic Polymerization in Continuous Flow: Mixing Rate as a Key Parameter Jan Morsbach, Axel H. E. Müller, Elena Berger-Nicoletti, and Holger Frey* Institute of Organic Chemistry, Johannes Gutenberg-University (JGU), Duesbergweg 10-14, D-55099 Mainz, Germany S Supporting Information *

ABSTRACT: Aiming at systematic variation of the parameter dispersity, Đ (or “polydispersity”), living polymers with predictable dispersity (Đ = 1.15−2.20) and controlled molecular weights (Mn = 3200−18 500 g mol−1) were prepared via carbanionic polymerization. The approach relies on a continuous flow reactor equipped with a tangential fourway jet micromixing device. By varying the total flow rate, the mixing efficiency of the initiator (sec-BuLi) and the corresponding vinyl monomers is controlled, resulting in polymers with predefined dispersity, while the number-average molecular weight, Mn, is kept constant. In this manner living polystyrene (PS), poly(p-methylstyrene) (PpMeS), and poly(2-vinylpyridine) (P2VP) samples with systematically varied Đ (studied by SEC) were prepared. All polymerizations were carried out at room temperature in a 50:50 solvent mixture of THF with either hexane for PS and PpMeS or with benzene for the polymerization of 2VP. To prove the living character of all polymer chains of the distributions obtained, all carbanionic chains were labeled, i.e., end functionalized via addition of an epoxide (benzyl glycidyl ether, BGE) as a termination reagent, when full conversion of the monomer was reached. Subsequent MALDI-ToF characterization confirmed the living character of all chains of the distributions. This is key for the further generation of complex polymer architectures with tailored polydispersity. Using the living carbanions with different dispersity, in an exploratory study, PS-b-PI block copolymers with controlled dispersity of the styrene block have been prepared via direct addition of isoprene as a second monomer.

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INTRODUCTION Since the discovery of the living carbanionic polymerization by Michael Szwarc in 1956,1 this technique has been known to give excellent control over a variety of key parameters.2−4 Because of the absence of termination and transfer reactions, the ratio of monomer and initiator concentration allows precise control of molecular weights.5 As shown by Flory already in 1940, living anionic polymerization results in a Poisson distribution with very low dispersity, Đ = Mw/Mn.6 The living character of this polymerization method has often been employed to introduce a large variety of functional end groups at the chain termini.7−11 Another unique feature of living polymerizations is the synthesis of block copolymers and more complex polymer architectures like, e.g., multiblock-, star-, comb-, or H-shaped topologies.12−15 However, systematic variation of the dispersity aiming at the question how the molecular weight distribution (MWD) influences polymer properties is rarely studied.16,17 To date, only few methods to systematically control the resulting MWD and thus dispersity of a polymerization, retaining control over molecular weights, are available.18,19 Circumventing this challenge, several works capitalized on mixing different polymer samples to generate the desired molecular weight distributions artificially.20,21 Although this approach permits to obtain information regarding © XXXX American Chemical Society

the effect of the molecular weight distribution on various polymer properties, it remains an intriguing challenge to vary the dispersity in a polymerization systematically. In 2007, Lynd and Hillmyer presented the consequences of an artificially prolonged initiation step, relying on the continuous addition of initiator to the monomer solution. This interesting approach resulted in polystyrenes with varied dispersities,22 albeit the setup does not allow to mimic Gaussian MWD profiles. Very recently, Fors and co-workers used a modular strategy that enables deterministic control over molecular weight distributions via temporal regulation of initiation in the nitroxide-mediated radical polymerization of styrene.23 Yet, to the best of our knowledge, no systematic approach for polydispersity control in the living carbanionic polymerization of vinyl monomers has been reported to date. Polymerization in microstructured, continuous flow reactors is receiving increasing attention.24−31 On the one hand, continuous flow strategies permit to shorten reaction times, considerably reducing experimental effort, and on the other hand, they enable the anionic synthesis of libraries of polymer Received: May 11, 2016 Revised: July 3, 2016

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DOI: 10.1021/acs.macromol.6b00975 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Scheme 1. Synthetic Strategy To Obtain PS, PpMeS, and P2VP with Different End Groups in a Continuous Flow Reactor with a Microstructured Mixing Device (BGE = Benzyl Glycidyl Ether)

samples in the same setup in one run by merely changing the relative flow rates of monomer and initiator.32 In early works, G. V. Schulz and co-workers studied the influence of the flow situation inside flow tubes in continuous flow reactors on the dispersity of anionic polymerizations both theoretically and experimentally. For turbulent flow they showed that the MWD is not affected by the flow rate as soon as the monomer conversion is nearly quantitative, but in laminar flow regimes an increase of the dispersity is observable.33 Additionally, ultrafast mixing of monomer and initiator is mandatory to neglect the influence of the mixing device on the MWD.34,35 In this context, one of the most important structural elements of every continuous flow setup is the mixing device.36 Different types of micromixers are used to ensure constant flow profiles: (i) multilamination mixers based on fast diffusion between alternating solution layers or (ii) mixing devices like the traditional T-junctions or (iii) improved liquid-jet based multijunction devices.37,38 Especially in the latter case the nature of the mixing process is directly influenced by the flow rates of the incoming solutions. The mixing efficiency drops dramatically at low flow rates. Navarro et al. showed that the flow rate applied in a continuous flow reactor with a tangential four-way jet mixing device affects the Đ of poly(methyl methacrylate)s prepared by anionic polymerization, when micromixing (characterized by the mixing time, tm) is slower than the polymerization kinetics.39 We limited this study to modest molecular weights (maximum value of Mn: 19 100 g mol−1) due to increasing viscosity of the reaction solution, which is problematic in the microstructured reaction setup. However, the molecular weight range can be extended by further adjustment of the reaction conditions (solvent, concentration, mixing device, etc.), as demonstrated in recent work by our group.38 In the current work we aim at utilizing a continuous flow setup to establish a method to control the parameter dispersity. We rely on living carbanionic polymerization in a continuous tubular reactor, permitting to generate living polymer chains with predefined dispersity (Đ) by merely altering the pump rates and consequently the mixing efficiency of a four-way jet mixing device (Scheme 1 and Figure 1). Established monomers, i.e., styrene, p-methylstyrene (pMeS) and 2-vinylpyridine (2VP), were polymerized at room temperature in a 50:50 solvent mixture of THF with hexane (S, pMeS) or benzene (2VP). We varied the flow rates of both monomer and initiator

Figure 1. Scheme of the four-way jet mixing device employed with monomer and initiator inlets.

solutions. In addition, we applied terminal functionalization with a reactive epoxide derivative (benzyl glycidyl ether, BGE) and subsequent MALDI-ToF characterization to demonstrate the living nature of all chains present. Using the living polymer distributions, we have also prepared a series of block copolymers with systematic variation of the dispersity of one block.

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EXPERIMENTAL SECTION

Reagents. All solvents and reagents were purchased from SigmaAldrich or Acros Organics and used as received, unless otherwise declared. Chloroform-d1 was purchased from Deutero GmbH. The solvent tetrahydrofuran (THF) used for the carbanionic polymerization in continuous flow was condensed from sodium/benzophenone under reduced pressure into a liquid nitrogen-cooled reaction vessel (cryo-transfer). n-hexane stored over molecular sieve was used for the initiator solution. The monomers styrene, p-methylstyrene, and 2-vinylpyridine were dried over calcium hydride (CaH2) and cryotransferred. sec-Butyllithium (sec-BuLi, 1.3 M, Acros) was used as received. Benzyl glycidyl ether was dried over CaH2 and subsequently cryo-transferred before usage as termination reagent. B


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Macromolecules Instrumentation. NMR spectra were recorded at 300 or 400 MHz on a Bruker AC300 or Bruker AMX400 spectrometer, respectively, and are referenced internally to residual proton signals of the deuterated solvent. For size exclusion chromatography (SEC) measurements in DMF (containing 0.25 g L−1 of lithium bromide as an additive) an Agilent 1100 Series was used as an integrated instrument, including a PSS HEMA column (cutoff molecular weight: 5 × 105/105/4 × 104 g mol−1), a UV detector (275 nm), and a RI detector. SEC measurements were carried out in THF on an instrument containing a Waters 717 plus autosampler, a TSP Spectra Series P 100 pump, a set of three PSS SDV columns (104/500/50 Å), and RI and UV detectors. If not indicated differently, all SEC curves show the RI detector signal, and the molecular weight refers to linear polystyrene (PS) standards provided by Polymer Standards Service (PSS). For the matrix-assisted laser desorption/ionization time-offlight (MALDI-ToF) mass spectroscopy measurements a Shimadzu Axima CFR mass spectrometer equipped with a nitrogen laser with a pulse rate of t = 3 ns at a wavelength of λ = 337 nm was used. A dithranol matrix, silver trifluoracetate as an ion source, and a polymer concentration of c = 1−2 mg mL−1 were used for sample preparation. Continuous Flow Reactor. A schematic overview of the continuous flow reactor setup is presented in Scheme 1. No heating or cooling was applied to the reactor at any stage of the reaction. Both monomer and initiator solutions were prepared under high-vacuum techniques suitable for living anionic polymerization conditions and connected via stainless steel capillary tubes (diameter = 1.0 mm) to the pump system (HPLC pumps, Knauer WellChrom K-501, inert 10 mL pump heads with ceramic inlays). Both HPLC pumps were connected to common T-junctions to separate the monomer and initiator solution in two streams, before they were connected to the four-way jet micromixing device via steel capillaries with a diameter of d = 1 mm. Depending on the different monomers employed, the length of the residence tube that the reaction mixture passes when leaving the micromixer was varied between l = 3.0 (S, 2VP) and 6.0 m (pMeS) with a constant diameter of d = 1.0 mm. Subsequently, the reaction solution was combined with either a degassed solution of benzyl glycidyl ether (BGE) in THF for the BGE functionalized samples or with a degassed solution of methanol in THF for the Hterminated samples via a subsequent T-junction. In the following, a second retention tube (flow tube) with a length of 1 m and a diameter of 1 mm was attached to ensure quantitative end group functionalization. Polymerization in the Flow Tube Reactor. Prior to polymerization, the setup was purged with dry THF via both HPLC pumps (total flow rate = 1 mL min−1) for approximately 10 min to remove potential impurities. For the polymerization of S and pMeS 1.0 M solutions in dry and degassed THF were prepared as well as an initiator solution of dry and degassed n-hexane with an appropriate amount of sec-BuLi (cI = 0.008−0.033 M). In the case of 2VP a 0.75 M monomer solution of dry and degassed THF was used, and the initiator was dissolved in dry and degassed benzene. After connecting the reaction flasks to the pump system, the pump connected to the initiator solution was activated (flow rate = 1 mL min−1) for approximately 3 min to flush the continuous flow setup with the secBuLi solution in order to eliminate potential trace impurities. To start the polymerization the pumps connected to the monomer solution and to the termination reagent were switched on. To achieve the targeted molecular weight for every polymer sample during the polymerization, the flow ratio of monomer and initiator was kept constant. The total flow rate was adjusted by decreasing/increasing the flow rate of the monomer and initiator pump simultaneously, keeping the ratio constant. The resulting P2VP samples were precipitated in petroleum ether directly after leaving the retention tube, whereas PS and PpMeS samples were precipitated in methanol (MeOH). To guarantee full conversion, the length or retention tube between the four-way jet micromixer and the T-junction was adjusted resulting in retention times (tr) between tr = 1 and 15 min. Synthesis of Poly(styrene)-block-poly(isoprene) Block Copolymers with Varied Molecular Weight Distribution in the PS Block. A schematic overview of a continuous flow reaction setup is

presented in the Supporting Information (Scheme S1). The monomer solutions were prepared by cryo-transfer of freshly dried monomer (60 min of stirring over CaH2) and subsequent distillation of dry solvent (stirring over Na) under high vacuum. The initiator solutions were prepared via addition of sec-BuLi with a gastight glass syringe to nhexane (stored over molecular sieves) under an argon atmosphere via a rubber septum. Both flasks were connected via stainless steel capillary tubes (diameter = 1.0 mm) to the pump system (HPLC pumps, Knauer WellChrom K-501, inert 10 mL pump heads with ceramic inlays) and consecutively to the four-way jet micromixer. Subsequently, the reaction mixture passed a flow tube with a length of 3.0 m (ϕ = 1.0 mm) before the solution of living PS chains was collected in a pretreated flask. The outlet of the reactor was linked to the flask via a rubber septum. Variation of the molecular weight distribution was achieved via an adjustment of the total flow rate of the monomer and initiator solution. To avoid chain termination via impurities, the glass flask was heated with a Bunsen burner under high vacuum three times to remove low boiling impurities. Afterward, 1.5 mL of 1.3 M sec-BuLi solution was added to remove remaining trace impurities. The remaining sec-BuLi was decomposed by the addition of 5 mL of THF prior to the addition of isoprene via a third HPLC pump. In the following, isoprene was added with a HPLC pump and through a rubber septum to the reaction flask. During the polymerization of isoprene the reaction mixture was cooled to 0 °C. After 12 h the reaction was terminated by the addition of 2.5 mL of degassed methanol via a gastight glass syringe. Pure PS-b-PI block copolymer was obtained in high yields over 90% after three cycles of precipitation in cold methanol and drying under high vacuum. 1 H NMR (400 MHz, CDCl3, δ in ppm): 7.50−6.25 (br, PS aromatic), 5.90−4.50 (br, PI backbone), 2.60−0.80 (br, PS backbone −CH2−CH−, PI methyl groups −CH3, initiator).

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RESULTS AND DISCUSSION Mixing Device and Flow Situation. In a first set of experiments, we studied the influence of the flow rate in the continuous flow reactor equipped with the four-way jet mixing device on the polymerization of styrene. The number-average degree of polymerization (DPn) at full conversion is determined by the ratio of the initial monomer and initiator concentrations (DPn = [M]0/[I]0). In a continuous device, this translates to the ratio of the flow rates of monomer and initiator solutions. For all samples, the flow rates used for addition of the monomer and the initiator solution were adjusted in a way that the ratio of them was constant. This was achieved by precise calibration of both pumps at different flow rates prior to the polymerizations. Both solutions were combined in the four-way jet mixing device shown in Figure 1, resulting in a final monomer concentration of [M] = 0.5 M and initiator concentrations of [I] = 0.006−0.033 M after mixing of both solutions. The inlets (diameter = 0.5 mm) of the mixing device led tangentially into a mixing chamber and from there into the flow tube with a diameter of d = 1 mm. For the herein used mixing device, Marcarian and co-workers determined the mixing time, tm, in dependence of the total flow rate.37 At high total flow rates (>60 mL min−1) the mixing times (tm < 2 ms) are suitable for fast initiation and polymerization of styrene in THF with sec-BuLi as an initiator. However, with lower flow rates the mixing time increases rapidly, since the static turbulent mixing principle of the microstructured device strongly depends on the flow rates of the incoming solutions. After passing the mixing device the living polymer solution passed a flow tube of varied length (l = 3.0−6.0 m), attached to run all reactions to full conversion, which ensures constant number-average molecular weight, Mn, for all samples. Within the steel capillary, mixing is influenced by the total flow rate similar to the flow profile in the mixing device. Calculation of the Reynolds C


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Macromolecules number (eq 1) allows to gain further insight into the flow behavior inside the flow tube. Re =

ρd ν η

(1)

The Reynolds number and consequently the nature of the flow are determined by the density (ρTHF/hexane = 736.8 kg m−3; ρTHF/benzene = 875.1 kg m−3), the diameter (d = 1 mm), the viscosity of the fluid (ηTHF/hexane = 0.334 mPa s; ηTHF/benzene = 0.504 mPa s), and the linear flow rate, ν (in m s−1). The change in viscosity during the polymerization can be assumed to be small because the monomer concentration is in the range of [M]0 = 0.38−0.5 M after the mixing step with the initiator solution and the molecular weights of the polymers (Table S1) are in a range, in which the influence on viscosity is low. The only variable parameter left in the equation is the flow rate of the solution. As a result, a linear dependence between the Reynolds number and the flow rate is obtained. As shown in the Supporting Information (Figure S1), in both solvent mixtures the Reynolds number is between 20 and 300, far below the threshold value of 3000. Thus, the flow is in the laminar regime at the herein used flow rates between (14 > νt > 0.8 mL min−1). In laminar flow, a parabolic flow profile influences the outcome of the polymerization inside a continuous flow reactor because the living polymer chains in the flow streams close to the tube walls show increased retention times. Therefore, these chains can add more monomer than those in the center of the tube. As demonstrated by Löhr et al. long ago, this effect can influence the molecular weight distribution.33 Effect of Flow Rate on Dispersity of Polystyrene. Usually the living carbanionic polymerization in polar solvents is performed at −78 °C to avoid side reactions with the solvent. The microstructured setup with its high surface to volume ratio leads to improved heat transfer compared to a common batch system and thereby permits to carry out exothermic reactions at ambient temperature without losing control over the reaction. As described in the Experimental Section, the flow rates of the HPLC pumps was the only parameter that was changed during the reaction. The initiator concentration was adjusted depending on the desired number-average molecular weight, Mn, of the sample. Within the same polymerization run the flow rates of both pumps were decreased simultaneously, and a sample terminated with methanol (designated as PS-H-X-Y; X: set of samples, Y: sample) was taken for every total flow rate. In Table S1 (Supporting Information) the results obtained in two continuous experiments in the same reaction setup but with different initiator concentration ([I]0 = 6−33 mM) are summarized. SEC traces of selected samples are shown in Figure 2. For additional SEC traces, see Figure S3. The dispersity of all samples is higher than the typical values (Đ = 1.01−1.1) obtained by living anionic polymerization, which can be explained by a slight pulsation of the HPLC pumps leading to a minor periodical variation of monomer to initiator ratio and the laminar flow profile in the flow tube. A comparison of the SEC traces (Figure 2) confirms that the number-average molecular weights of all samples remained constant, regardless of the applied total flow rate. This was expected due to the constant ratio of monomer and initiator concentrations and sufficiently long residence times for full monomer conversion. Thus, the setup permits molecular weight control, while systematically varying the dispersity. Figure 3 shows dispersity as a function of flow rate. The

Figure 2. SEC traces, total flow rate, and dispersity for four PS-H-01 samples (Mn = 3400 g mol−1; THF, RI signal, PS standards).

Figure 3. Dispersity vs total flow rate for PS, P2VP, and PpMeS.

dispersity significantly and systematically increases with decreasing total flow rate, most dramatically for flow rates below νt < 3 mL min−1. At flow rates below νt < 0.3 mL min−1 multimodal size distributions were obtained, and the micromixer tended to clog. The same dependence of the dispersity on flow rates was consistently observed for four sets of PS samples (PS-H-01, PS-H-02, PS-H-03, PS-H-04) with Mn = 3400, 4800, 5600, and 18 500 g mol−1 (Table S1, series PS-H02, PS-H-03, PS-H-04, see Figures S4−S6). In all cases a slight increase of dispersity was observed when lowering the total flow rate to νt < 4.0 mL min−1. At even lower flow rates, a rapid increase of Đ occurs, as it was the case for the samples discussed earlier (see Figure S7). As shown in Figure 4C, the SEC measurements are supported by MALDI-ToF mass spectrometry. The same trend of increasing dispersity with decreasing flow rate was observed for a series of samples. The shift of the distributions to slightly lower molecular weights for the samples with higher dispersity can be explained by the wellknown mass discrimination effect in MALDI-ToF, leading to an overestimation of the lower mass oligomers in the spectrum. We interpret the results in the following way: Two separate effects have an impact on the dispersity of the PS samples. First, the mixing times of both solutions inside the four-way jet mixing device strongly depend on the flow rate. When decreasing the total flow rate to values at which the mixing times are too high for the ultrafast initiation and polymerization D


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and the flow situation inside the ensuing flow tube, we studied the polymerization of two other vinyl monomers with different reactivity and polymerization kinetics. Because the flow profile should not be influenced by the nature of the monomer, we expected the dispersity to be similar for monomers with similar polymerization kinetics. Differences are expected due to changes in the propagation rate constant. We utilized the same setup for thecompared to styrene (kp = 550 L mol−1 s−1 in THF)faster polymerization of 2-vinylpyridine (2VP, kp = 3500 L mol−1 s−1) and the slower polymerization of pmethylsytrene (pMeS, kp = 220 L mol−1 s−1).40 The reaction parameters, namely temperature (25 °C), initiator concentration, termination reagent, mixing device, and dimensions of the flow tube, were the same as in the polymerization of styrene. For the polymerization of 2VP we changed the solvents for the initiator and cut the monomer concentration in half, also to prevent clogging of the reactor due to the known low solubility of P2VP. The results obtained are shown in Figure 3 and Table S2. The comparison of the dispersity vs total flow rate dependence leads to two observations: first, at flow rates above 3.0 mL min−1 the increase in dispersity with decreasing νt is similar for the P2VP samples as for the polymerization of PS. One observation is a significant broadening of the MWD at low total flow rates, νt < 3.0 mL min−1, which is more pronounced for the polymerization of 2VP than for styrene. Furthermore, the reaction became uncontrolled already at flow rates below 1.5 mL min−1, resulting in multimodal molecular weight distributions and clogging of the reactor. We explain this different behavior by the faster propagation rate of 2VP because rapid mixing is more important when the polymerization is faster. In this case, the micromixing time is insufficient already at higher flow rates, and the effect of MWD broadening by an artificially prolonged initiation is observed at higher flow rates. Nevertheless, all samples again showed monomodal SEC profiles (Figure S8). This supports the hypothesis that the mixing pattern is the main reason for the high Đ values at low flow rates. The next logical step was to perform a reaction with a monomer with lower propagation rate constant as compared to styrene and 2-vinylpyridine, i.e., p-methylstyrene. In this case, the effect on the molecular weight distribution should be weaker because chain propagation is slower. In fact, for pMeS only a minimal broadening of the molecular weight distribution from Đ = 1.10 at high flow rates to Đ = 1.21 at lower flow rates was observed (Figure 3, black squares). This result is consistent, since the influence of the rate of mixing on the molecular weight distribution is weaker for polymerizations with slower propagation kinetics. Thus, we conclude that the reason for the different behavior of pMeS and S is the ratio between the times of monomer addition and that of mixing in the four-way jet mixing device. Again, all SEC traces showed monomodal molecular weight distributions (see Figure S9). Therefore, the polymerization is controlled even at low flow rates, which is further demonstrated in the following sections. To sum up, all results substantiate the hypothesis that the flow profile created inside the four-way jet mixing device is the main reason for the systematic influence of the mixing rate on the dispersity. The system can be transferred to other monomers with varying degree of effect onto the dispersity, depending on the respective reactivity, i.e., their propagation rate. Living Nature of Polymer Chains. In the context of this work, it is crucial to demonstrate that all chains of the

Figure 4. (A) Exemplary 1H NMR spectrum of PS-BGE-01-02 (CD2Cl2, 400 MHz). (B) MALDI-ToF mass spectrum with zoom in of PS-BGE-01-02 sample. (C) MALDI-ToF spectra of a series of PSBGE samples with Mn = 6600−7000 g mol−1 (SEC).

of styrene in THF by sec-BuLi, slow mixing leads to a prolonged initiation and thereby broadening of the molecular weight distribution. Second, with decreasing flow rate the influence of the laminar flow profile on the dispersity increases. Nevertheless, at this point it is not clear if the main parameter is the variation of the mixing pattern inside the mixing device or the inconstant flow profile in the flow tube. In summary, with this microreactor-based approach several sets of living PS samples with a broad range of dispersities (Đ = 1.15−2.21) and defined average molecular weights (Mn = 3400, 4800, 5600, and 18 500 g mol−1) were generated, as shown in Figure 3. In the following section, we transfer this concept to two further vinyl monomers, showing the universal nature of the approach. Polymerization of p-Methylstyrene and 2-Vinylpyridine. To generalize this strategy and to confirm that broadening of the molecular weight distribution is caused by the flow rate-dependent mixing quality inside the mixing device E


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Scheme S1, the living chains can be gathered and stored in a thoroughly predried flask. The pretreatment involved heating of the flasks under high vacuum three times and subsequent addition of a small amount of 1.3 M solution of sec-butyllithium (sec-BuLi) in hexane to remove all further impurities. Because remaining sec-BuLi can react as an initiating species after an addition of the second monomer, an excess of tetrahydrofuran (THF) was added to the flask. By this procedure decomposition of THF via a deprotonation step in the α-position to the oxygen, followed by a reverse [3 + 2] cycloaddition of the resulting anion leads to the formation of ethylene as well as a lithium enolate of acetaldehyde.28 In each reaction setup, several PS macroinitiators were produced with varied dispersities, retaining the same molecular weight by changing the total flow rate. For each sample the same amount was gathered in the pretreated flask. The total flow rate and the time at which the solution was collected were tuned accordingly. Before and after the outlet of the continuous flow reactor was connected to the pretreated flask, a sample of the PS homopolymer was taken and precipitated in a solution of methanol in THF to directly terminate the living chain ends for characterization of the precursors. On the basis of these samples, we are able to compare the pure PS macroinitiators with their respective PS-b-PI block copolymers. SEC traces for all samples from one reaction setup are shown in Figure S11. After the macroinitiators were collected in the reaction flask, the solution was cooled to 0 °C, and afterward the same amount of isoprene was added to all samples to achieve the same molecular weight for all block copolymers. Isoprene was cryo-transferred prior to usage and was added to the reaction chamber via a third HPLC pump (see Scheme S1). The originally orange-colored solution of living PS chains slowly changed to the slightly yellow color of the living isoprene chain ends. The reaction was completed after 12 h and terminated by the addition of methanol. All block copolymer samples were characterized by SEC and NMR spectroscopy. In Figure 5 SEC traces for a typical PS macroinitiator and the respective PS-b-PI block copolymer are shown. The distribution curve of the block copolymer is shifted to higher molecular weight values in the expected range of

distribution are living, and premature termination is absent and not a cause for the dispersity effects observed. On the one hand, it is unlikely that termination is the reason for the abovedescribed dispersity effect because the setup is sealed and selfpurging; therefore, no terminating impurities are present in the system. Additionally, the observed constant number-average molecular weight of all samples would not be achieved with uncontrolled termination reactions occurring. Obviously only living polymer chains are applicable to generate block copolymers or more complex polymer architectures. To support the living character of all chains of the distribution, we used benzyl glycidyl ether (BGE), a reactive epoxide, as a termination reagent, i.e., to trap the living chains. A third HPLC pump was used to add a solution of BGE in THF (cBGE = 0.47 M, 10-fold excess in comparison to initiator) to the reaction medium after the first retention tube via a T-mixer. A second retention tube (l = 1.0 m; d = 1.0 mm) was attached to achieve complete chain end functionalization. The resulting samples were analyzed by SEC, 1H NMR, and MALDI-ToF mass spectrometry. The SEC results are shown in Figure S10. It is important that regardless of the end group the same dispersity was observed as described in the previous sections. Thus, termination of the polymersemployed to evidence their living characterdoes not have any impact on the molecular weight distribution. 1H NMR spectra of all samples confirm quantitative functionalization of the polystyrene chains with BGE. In Figure 4A all signals are assigned, and a comparison of the initiator methyl signals at δ = 0.85−0.51 ppm (6H) with the signals of the termination reagent at δ = 4.55−3.05 ppm (5H) is an indication for the successful termination reaction. Since NMR spectroscopy only gives global information, MALDI-ToF MS measurements were carried out to obtain a better understanding regarding all polymer chains formed. MALDITOF mass spectra shown in Figure 4B are composed of two molecular weight distributions with a molecular weight of 104.14 g mol−1 as the repeating unit. The main distribution is unequivocally assigned to the desired PS-BGE polymers. The molecular weight of the smaller subdistribution derives from a minor amount of chains with two benzyl glycidyl ether end groups. Because of the strong interaction between the oxyanion and the lithium counterion, further propagation of the alkoxide termini is usually inhibited.11 Nevertheless, the large excess of termination reagent and the unusual conditions, i.e., combination of a polar solvent and room temperature, can lead to the addition of a second epoxide to the chain end, which is shown by the lower intensity distribution in the MALDI-ToF spectra. The absence of a signal with a molecular weight of M = 5894.4 g mol−1 (C4H9(C8H8)55HAg) and the respective distribution confirm quantitative functionalization. Another important observation is the monomodal distribution and the numberaverage molecular weight, which both are in good agreement with the results obtained by SEC. In summary, these results demonstrate that all polymer chains formed maintain their living character also at low flow rates and are amenable to further terminal functionalization and direct polymerization of a second monomer, which opens manifold pathways to polymer architectures with controlled dispersity. Synthesis of PS-b-PI Block Copolymers with Predictable Molecular Weight Distribution of the PS Block. As described previously, polymerization in a microstructured continuous flow reactor permits the synthesis of PS with controlled dispersity and with living chain ends. As shown in

Figure 5. SEC traces of the PS-H-1-3 macroinitiator (blue) and the respective PS-b-PI-1-3 block copolymer (red) (solvent: THF, PS standard, UV signal). F


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benzyl glycidyl ether, which is crucial for the synthesis of complex polymer architectures (e.g., block copolymers or mictoarm stars) with systematically varied dispersity of one block. In the final part, the synthesis of PS-b-PI block copolymers with varied dispersity of the PS block has been introduced, employing direct polymerization of isoprene with the collected living distribution of polymer chains obtained from the continuous synthesis. In this case the formation of a second polyisoprene block with lower dispersity leads to a reduction of the dispersity of the block structures. Further studies of the block copolymers and their properties and their correlation with dispersity are currently under way. Deliberate control of dispersity is likely to represent a broadly applicable strategy to control processing features and properties of complex polymer materials.

molecular weights. For the block copolymer a small shoulder in the lower molecular weight region is observed. This can be explained by a low amount of macroinitiator chains that are terminated by impurities entering the reaction chamber when the outlet of the third HPLC pump, transporting the isoprene monomer, is connected to the reaction flask. The main distributions of all block copolymer samples are in the same molecular weight range and reflect the trend in molecular weight distribution of their living macroinitiators. All sample data are summarized in Table 1. Table 1. Characteristics of PS Homo- and PS-b-PI Block Copolymers polymer

Mna (g mol−1)

Mnb (g mol−1)

Mw/Mna

PS-H-05-01 PS-H-05-02 PS-H-05-03 PS-b-PI-1-1c PS-b-PI-1-2c PS-b-PI-1-3c

3320 3350 3360 9500 8670 9910

3300 3400 3350 14000 13400 14200

1.24 1.56 1.67 1.13 1.16 1.19

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00975. Calculation of Reynolds numbers, SEC traces for PS-H, PS-BGE, P2VP, and PpMeS samples at different reaction conditions, MALDI-ToF spectra for a set of PS-BGE samples (PDF)

a

Molecular weight and molecular weight distribution determined by SEC in g mol−1 (PS standards, THF, UV signal). bMolecular weight obtained by 1H NMR (CD2Cl2).

The dispersities of the block copolymers are significantly lower than the values of their respective PS macroinitiators, which can be explained by the highly controlled anionic polymerization of isoprene initiated by the living PS chain ends. With respect to the molecular weight fraction (wA, wB) of both blocks and the molecular weight distributions of the macroinitiator and the block copolymer (ĐA, ĐAB), eq 2 allows to calculate the molecular weight distribution of the PI block.29 The dispersities of the PI segments in the range of Đ = 1.20− 1.24 are above the normal values obtained by living polymerizations which can be explained by the marginal amount of inadvertently terminated chains. ĐAB = wA 2(ĐA − 1) + wB 2(ĐB − 1) + 1

ASSOCIATED CONTENT

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors thank Luka Decker and Tobias Kaiser for the help in the laboratory.

(2)

Since all samples show no significant difference in Đ for the PI block, the difference in the dispersity of the block copolymers is only affected by the varied dispersity of the macroinitiators. The results demonstrate the potential of the continuous approach for the synthesis of block copolymers, based on a living PS block with varied dispersity.

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CONCLUSIONS Utilizing a microstructured continuous flow reactor equipped with a micromixer for the living carbanionic polymerization enables to control the often-neglected parameter dispersity, Đ (or “polydispersity”), in a unique manner. By comparison of three monomers with different reaction kinetics, we are able to demonstrate that the ratio between mixing time and propagation rate is the key parameter for the broadening of the molecular weight distribution. A key result of this contribution is that a change of the total flow rate directly influences the mixing quality, and thereby living polymer distributions with a deliberately varied, broad range of dispersities (Đ = 1.15−2.20) are accessible. Furthermore, the living character of the polymerization is retained, enabling (i) control over the molecular weight via the addition rate ratio of monomer and initiator and (ii) quantitative chain end functionalization (i.e., trapping of the living chain ends) with G


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Macromolecules

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Living Polymer Chains with Predictable Molecular Weight and

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