Quantifying the Kinetics of the Gilch Polymerization toward Alkoxy

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Quantifying the Kinetics of the Gilch Polymerization toward AlkoxySubstituted Poly(p‑phenylene vinylene) Ann-Kathrin Schönbein, Manfred Wagner, Paul W. M. Blom, and Jasper J. Michels* Max Planck Institute for Polymer Research Ackermannweg 10, 55128 Mainz, Germany S Supporting Information *

ABSTRACT: The Gilch polymerization is one of the most popular routes toward high molecular weight alkoxy-substituted poly(p-phenylenevinylenes) (PPV) applied in, for instance, organic electronics and bioimaging. As the interplay between optoelectronic performance and (synthesis-related) defects represents an active area of research, control over the polymerization is of utmost importance. In this work we quantify for the first time the rate constants of the reaction steps of the Gilch polymerization. We obtain these values by fitting concentration transients of various key intermediates, measured by in situ low-temperature (−68 °C) 1H NMR spectroscopy, to kinetic models based on sets of coupled rate equations. The modeling not only accounts for the usual processes of initiation, propagation, transfer, and radical recombination but also involves the side reaction cascade associated with the presence of residual water in the reaction mixture. The results demonstrate that chain growth initiation by active monomer dimerization is slow and rate determining. We show that a low temperature suppresses the occurrence of bisbenzyl and bisbromobenzyl coupling defects. The initiation rate is reduced by orders of magnitude compared to the propagation rate. Hence, fast chain growth occurs at a relative low concentration of radical intermediates, which suppresses defect formation due to both active monomer dimerization and radical−radical recombination.

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INTRODUCTION Poly(p-phenylene vinylene)s (PPVs) form an important class of semiconducting polymers as they are widely applied as active material in organic light-emitting diodes (OLEDs),1−3 organic field-effect transistors (OFETs),4 and organic photovoltaics (OPVs).5,6 Although they have been replaced in state-of-the-art OLED devices by materials giving higher performance efficiencies, PPVs remain workhorse polymers in fundamental research. Despite three decades of OLED research, important aspects concerning the charge carrier dynamics in the active layer are still not fully understood.7−9 Especially phenomena related to trapping10,11 and degradation12 form the focus of active research. In order to study these phenomena in detail, PPVs are highly attractive since they belong to the best studied materials in organic electronics.13−15 A vast amount of available physical data on this class of polymers has accumulated. Additionally, PPVs have recently shown great potential as bioimaging agents owing to their low toxicity, biocompatibility, and high fluorescence quantum yield.16,17 Besides the well-known step-growth polymerizations18 such as Wittig,19 Horner,20 or McMurry21 polycondensations, the so-called precursor routes22−28 have shown to be the most efficient pathways toward high molecular weight and wellprocessable PPVs. The precursor routes are chain growth polymerizations and usually give high yields and molecular weights, do not require a catalyst, and are much faster than cross-coupling reactions.22 Of all precursor routes used to synthesize PPVs, the Gilch23 route stands out because it allows © XXXX American Chemical Society

for a fast, one-pot synthesis of high molecular weight material with excellent yields based on a starting material, obtained in a two-step synthesis.29,30 Nevertheless, polymers obtained via this route often lack reproducibility in terms of molecular weight and solubility,31 which can have a significant impact on device performance. Much research in the past years has mainly been focused on the general polymerization mechanism of the Gilch reaction, i.e., radical32,33 or anionic.34,35 It has been shown that the mechanisms for all precursor routes can depend strongly on the reaction conditions, in particular the used solvent.32,36,37 For the Gilch route performed in THF with potassium tertbutoxide (KOtBu) as base, it is now generally accepted that the polymerization proceeds via a (free) radical mechanism (see Scheme 1).38,39 Somewhat surprisingly, and in contrast to other precursor routes, such as the sulfinyl40−43 and Wessling44,45 routes, for the Gilch route to date no detailed quantitative studies of the kinetics of the complete polymerization exist in the current literature. A reason for this may be the fact that the Gilch route comprises a complex one-pot reaction cascade, which besides the chain initiation, growth, and termination steps includes halide elimination toward the final conjugated polymer. UV/vis spectroscopy has shown to be useful in monitoring the conversion of the premonomer PM into to the p-quinodiReceived: April 3, 2017 Revised: June 12, 2017

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DOI: 10.1021/acs.macromol.7b00697 Macromolecules XXXX, XXX, XXX−XXX

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a

From premonomer PM the active monomer p-quinodimethane Q is generated via a base-induced 1,6-elimination. Through thermally activated dimerization of two p-quinodimethanes Q the biradical D is formed, which either starts the radical chain growth towards prepolymer Pre or undergoes cyclization, if the concentration of Q is low. Elimination of HBr from Pre mediated by KOtBu leads to the conjugated poly(pphenylenevinylene) PPV.

methane Q.41,42 However, as this technique usually requires very low concentrations, formation of the initiating dimer D and subsequent polymerization to prepolymer Pre is strongly suppressed. Instead, cyclization, leading to the cyclophanes 6 and 7, takes place.38,46 Alternatively, microflow reactors have been used to study, for instance, the sulfinyl route toward PPV.40 For the Gilch reaction this approach is not suitable due to the formation of gel particles31 capable of blocking the reactor. In this work we apply in situ 1H NMR spectroscopy to study the kinetics associated with the synthesis of poly(2,5-bis(2′ethylhexyloxy)-1,4-phenylene vinylene) (BEH-PPV) via the Gilch route. Advantageously, 1H NMR analysis is compatible with a concentration range relevant to batch reactions (10−30 mmol L−1). At room temperature, though, NMR data acquisition is slow compared to the rates of change in the concentrations of the Gilch reaction intermediates. Even by using rapid injection devices47,48 standard NMR analysis would not provide sufficient temporal resolution. For this reason, we perform our measurements at −68 °C. A notable second advantage is the fact that side reactions are strongly suppressed at low temperature.49,50 We obtain rate constants for the various reaction steps by fitting the concentration transients to numerical models based on the rates of change of reaction intermediates. We demonstrate that the NMR data by itself can be accurately fitted using a model which treats polymerization in an effective manner using a reduced number of rate laws. Besides, we provide a more extended analysis using a second model that covers the full cascade including the changes of molecular weight distribution by polymer chain growth. We provide for the first time quantitative information on the kinetics of the Gilch polymerization, not only for the reaction steps of the polymerization itself, including the elimination step toward PPV, but also for side reactions associated with the presence of moisture in the reaction mixture.

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spectrometer with 500.134 MHz proton frequency, equipped with a 5 mm PABBO (19F, 1H/D) probe with z-gradients. NMR samples were prepared in 5 mm amberized tubes (Wilmad LabGlass, 527-PP7AMB). THF-d8 (99.5 atom % D) was purchased from Deutero in 10 mL bottles. Measurements were performed in THF-d8 at 205 K, using a nitrogen evaporator, and without spinning the sample. For 1D proton spectra FIDs with 64K data points were sampled within 2.185 s over a sweep width of 30 ppm. Prior to Fourier transformation FIDs were zero filled to 128K data points and apodized with an exponential function (LB = 1 Hz). Resulting spectra were automatically phase corrected using TopSpin and automatically baseline corrected using MestReNova (Version 11.0.2), applying a polynomial fit polynom 5. The first 500 1H experiments were executed as single scan experiments; afterward, eight scans were accumulated per experiment, using a relaxation delay of 20 s for all experiments. Spectra were deconvoluted using a multi-Lorentzian fitting procedure. For the determination of concentrations the average of the normalized integrals of the different species was used. Concentrations were calculated based on the concentration of the added internal standard tetraethylsilane (−CH2−, 0.44−0.61 ppm; 31.46 mmol/L). Sample Preparation. A stock solution of 59.15 mg (0.1137 mmol) of 1,4-bis(bromomethyl)-2,5-bis((2-ethylhexyl)oxy)benzene (PM, 1) and 15 μL (7.48 × 10−2 mmol) of the internal standard tetraethylsilane was prepared in 500 μL of degassed THF-d8. 100 μL of the PM stock solution was transferred into an NMR tube, which was flushed with argon, and tightly closed with a screw cap. A second stock solution of 79.11 mg (0.705 mmol) of potassium tert-butoxide (KOtBu) with 5 μL (2.43 × 10−2 mmol) of tetraethylsilane was prepared in 3 mL of degassed THF-d8 and kept under argon. A reference sample containing only PM stock solution (100 μL) diluted with THF-d8 (600 μL) was prepared and inserted into the spectrometer and cooled to predetermine the lock and shim parameters. The actual polymerization sample was prepared as follows. The NMR tube containing 100 μL of PM stock solution and chilled at ca. −90 °C in an ethanol/N2 bath was opened. A cannula connected to an argon supply was inserted to very slowly add 600 μL of the base stock solution by letting it flow along the wall of the tube to guarantee that both solutions are cold upon mixing. The resulting concentrations are corrected for a volume shrinkage of ∼12% resulting in a total sample volume of 616 μL, leading to the following concentrations: 35.83 mmol/L of premonomer, 228.53 mmol/L of KOtBu, and 31.46 mmol/L of tetraethylsilane. To guarantee a fully mixed solution, the cannula was dipped into the solution for 5 s and the tube was closed and subsequently transferred into the precooled spectrometer to start data acquisition.

EXPERIMENTAL SECTION

Starting materials and solvents were purchased from Sigma-Aldrich and Fisher Scientific and used as received. The synthesis of premonomer (PM, 1) is described in the Supporting Information. 1 H NMR measurements were performed using a Bruker Avance III B


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Figure 1. 1H NMR spectra of the Gilch reaction mixture at t = 0 h (bottom), t = 0.4 h (middle), and t = 11.1 h (top). The assignment of the various signals has been implemented using the molecular structures of premonomer PM, p-quinodimethane Q, prepolymer Pre, and PPV, depicted on the left.

Figure 2. Concentration of Gilch reaction intermediates recorded using in situ 1H NMR spectroscopy plotted as a function of reaction time (symbols). Panels a−d show the concentration of premonomer PM, p-quinodimethane Q, prepolymerized monomer Pre, and repeat units in PPV, respectively. The inset in panel d shows an enlarged section of the PPV concentration vs time profile. The solid lines represent fits to the kinetic model I (see main text).

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RESULTS AND DISCUSSION We mix a solution of premonomer PM in THF with a separately prepared solution of KOtBu at −90 °C in an NMR tube and transfer the sample into a precooled spectrometer to record the concentrations of discernible reaction intermediates and products as a function of time until a conversion of ∼85% is reached (approximately after 11 h). For a detailed description of the experimental procedure we refer to the Experimental Section. In Figure 1, we show a set of representative 1H NMR spectra. From bottom to top, the spectra are shown of

premonomer PM, p-quinodimethane Q, and the complete reaction mixture after 11 h. The latter contains besides residual starting material a substantial amount of prepolymer Pre and PPV, evidenced by the broad signals in the low field region. A detailed assignment of all peaks can be found in the Supporting Information. The absence of signals correlating to cyclophanes 6 and 7 (see Supporting Information) shows that the concentration of Q is sufficiently high for polymerization to predominate over cyclization. C


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Scheme 2. List of All Reactions Taken into Account by Model I, Including Side Reactions Originating from Water Uptake by THF and Elimination of Hydrogen Bromide

argue that the effect becomes manifest by the observation that apparently the summed concentrations of monomer in PM, Q, Pre, and PPV remain somewhat lower than the initial premonomer concentration. As chemical decomposition is unlikely we ascribe the deviation to signal suppression by paramagnetic relaxation. Since this mechanism is known to be effective within a range of approximately 3.5 nm,51 especially signals from protons in the peripheral regions of the growing prepolymer Pre intermediates are expected to be suppressed. For this reason two different experimental data sets are shown in Figure 2c, the diamonds referring to the observed signal intensity and the triangles representing the same data after correction to enforce conservation. We assume that all other species diffuse in and out the 3.5 nm range too fast to become significantly affected by paramagnetic relaxation, and, if so at all, to equal extent. In order to quantitatively analyze the NMR data and for the first time obtain (effective) rate constants for the various reaction steps comprised by the Gilch route, we fit the experimental curves depicted in Figure 2 to two different kinetic models. The first model (“model I”) takes the dehydrohalogenation of PM and the dimerization of Q explicitly into account but treats further chain growth in an effective manner by letting dimer D react with p-quinodimethane Q to give a concentration of monomers embedded in the “prepolymer”. The second model (“model II”) covers the full reaction cascade of the Gilch route, including actual propagation, chain transfer, and radical−radical recombination. We follow this dual approach to (i) demonstrate that fitting the 1 H NMR data is to some extent ambiguous, with satisfactory fits already obtained with a reduced number of rate laws, and (ii) provide a reasonable picture of the mechanistic course of events during polymerization. As an extra feature, and specific to the Gilch route, both models include halide elimination, either via prior deprotonation by base or via self-elimination (see below). In Scheme 2 we list all (sub)reactions and associated rate constants included in model I. As in the excess of base the dehydrohalogenation is expected to fully deplete the premonomer, we assume the occurrence of a side reaction to account for the low premonomer consumption rate at later stages.

Addition of tetraethylsilane at a known concentration as a reference allows us to retrieve the actual concentrations of the Gilch intermediates after phase and baseline correction. In order to minimize error due to signal broadening and overlap, we deconvolute all spectra using a multi-Lorenzian fitting procedure. For a detailed description of the spectral corrections and deconvolution we refer to the Supporting Information. In Figure 2 we plot as a function of time (symbols) the concentrations of premonomer PM and p-quinodimethane Q as well as the total concentration of monomer present in the prepolymer Pre and PPV. The error bars have been determined using the propagation of uncertainty principle. It includes errors originating from integration, weighing, and mixing (see the Supporting Information). The concentration of the premonomer PM (Figure 2a) initially decays fast but then seems to cross over to a regime characterized by a much lower decay rate. Hence, even after 11 h full conversion of premonomer PM is not reached. Interestingly, this result contradicts the observation by Schwalm et al., who claim full conversion of BEH-PPV premonomer into the p-quinodimethane at −80 °C within a time span of several minutes.30 The concentration profile of the p-quinodimethane Q (Figure 2b) has a shape characteristic for an intermediate. In the first 30 min the concentration increases steeply to reach a maximum after 47 min. Subsequently, the concentration decreases toward a plateau-like value due to dimerization and consecutive polymerization. This demonstrates that at the late stages the rates of dimerization and polymerization drop substantially. The concentration of monomer in the prepolymer Pre (Figure 2c) rises pronouncedly after a lag time of about 1 h and seems to reach its maximum value just within the time scale of the experiment. Eventually, the value is expected to decrease due to conversion of the prepolymer Pre into the PPV (Figure 2d). However, due to the fact that the elimination step is apparently slow, the decrease in prepolymer content is not observed within the 11 h time span. The fact that the initiating dimer D is not observed may well be a consequence of very fast propagation, i.e., reaction of a radical with p-quinodimethane D. We should, on the other hand, not rule out the suppression of 1H signals due to paramagnetic relaxation induced by radical end groups. We D


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according to the forward Euler time stepping method.55 We note that the formation of “prepolymer” via reaction between the biradical dimer D and p-quinodimethane Q merely represents polymerization in an effective manner. Our primary aim here is to show that a model with a reduced number of free parameters is already capable of fitting the NMR data. Whether the obtained fit values are reasonable will be assessed below in conjunction to the fitting to the more extended model II. The solid lines in Figure 2 represent the best fit of model I to the NMR data, using the rate constants as free fitting parameters and keeping the experimental values for the initial concentrations of PM (35.83 mmol L−1) and KOtBu (228.89 mmol L−1) fixed. The initial concentration of water is unknown and treated as free parameter as well. By minimizing the leastsquares error between the measured and calculated concentrations [PM], [Q], [Pre], and [PPV], the values for the rate constants as listed in Table 1 are obtained. Figure S5 in the Supporting Information plots the concentrations of all species calculated by model I as a function of time.

Hence, both model I and model II include residual water as a species capable of reacting with KOtBu. By running multiple experiments, we have confirmed that ambient humidity indeed influences the fit quality to some degree. Because of THF’s hydrophilicity, as well as the fact that samples were not prepared in a glovebox, but under reversed gas flow conditions, condensation of water into the reaction mixture proved difficult to avoid completely. Besides the main reactions (eqs 1−4, Scheme 2), also the side reactions associated with water ingress and elimination are listed (eqs 5−11). The abbreviations “Pre” and “PPV” correspond to concentrations of embedded repeat units and not of polymer chains. Furthermore, “Pre” corresponds to the prepolymer concentration curve that has been corrected for paramagnetic relaxation (red triangles in Figure 2c). Classically, Gilch polymerizations start with a 1,6-dehydrohalogenation of premonomer by KOtBu according to an E2 elimination (eq 1). Alternatively, this reaction proceeds via an E1CB elimination mechanism, as is generally accepted for e.g. the Wessling precursor route,45,52,53 but density functional theory (DFT) calculations39 show that an E2 mechanism applies to the Gilch reaction. As mentioned above, the pquinodimethane dimerizes to give the diradical dimer D (eq 2). The diradical monomer is not expected due to the high free enthalpy difference between the quinoid (≡ Q) and diradical form (125 kJ/mol at room temperature), as calculated for a dimethoxy-substituted p-quinodimethane using DFT.54 Residual water reacts reversibly with KOtBu under formation of hydroxide ions (eqs 5 and 6). In separate experiments we have confirmed that OH− deprotonates the premonomer, thereby starting the polymerization, though at a much lower rate than observed for KOtBu. Nevertheless, on the basis of this observation we take initiation by OH− into account (eq 7). To be consequent, we then also include OH−-mediated elimination of HBr from the prepolymer (eq 8). Besides, as separate experiments show HBr to slowly self-eliminate (see Supporting Information), formation of HBr (eq 9), and subsequent protonation of any base present is included as well (eqs 10 and 11). As mentioned above, cyclophane formation46 is not observed, so it is not considered in the modeling. On the basis of equations in Scheme 2, we define the following rate laws for the discernible Gilch intermediates: d[PM] = −(k1[t BuO−][PM] + k6[OH−][PM]) dt

Table 1. Rate Constants Obtained from Fitting the NMR Concentration Data to Model I entry

d[PPV] = k4[Pre][t BuO−] + k 7[Pre][OH−] + k 8 dt

k1

2 3

polymerization (effective)

k3

4

k4

5

elimination from prepolymer pre by KOtBu KOtBu consumption by water

k5

6

deprotonation of tbutanol by OH−

k−5

7 8

PM dehydrohalogenation by OH− elimination from prepolymer pre by OH− self-elimination from pre HBr neutralization by KOtBu HBr neutralization by OH−

k6 k7

9 10 11

k2

k8 k9 k10

value 5.3 × 10−3 L mol−1 s−1 2.8 × 10−3 L mol−1 s−1 −2 2.3 × 10 L mol−1 s−1 −5 1.2 × 10 L mol−1 s−1 4.7 × 10−3 L mol−1 s−1 9.2 × 10−9 L mol−1 s−1 0.0 L mol−1 s−1 1.2 × 10−5 L mol−1 s−1 0.0 mol L−1 s−1 0.0 L mol−1 s−1 0.0 L mol−1 s−1

The fitting procedure roughly comprises two stages. In the first step we fit the concentration profile of PM to evaluate k1, k±5, k6, and the total water concentration, whereas in the second step the profiles for Q, Pre, and PPV are fitted. This approach appeared suitable since the rate constants governing dehydrohalogenation of PM and KOtBu consumption by water do not couple strongly with the ones associated with initiation and polymerization. The fit quality is more than reasonable, since all curves fall within the estimated experimental error ranges, apart from the “overshoot” in the p-quinodimethane concentration observed around 1 h reaction time. The initial water content amounts to 20.3 mmol L−1, which corresponds to 2.6 μL in a reaction volume of 0.7 mL. Its value is robust and insensitive to the initial guess. The same holds for the dehydrohalogenation rate constant k1 and the consumption rate of KOtBu by water (k5). Notably, assuming a nonzero value for k6 does not improve the fit of the profile of PM, indicating that hydroxide-mediated dehydrohalogenation is negligible. We next focus on the fitting of the profiles for Q and Pre (Figures 2b,c) and concomitant quantification of k2 and k3. The temporal changes in [Q] and [Pre] are effectively governed by

(12)

(13)

d[Pre] = k 3[Q][D] − (k4[Pre][t BuO−] + k 7[Pre][OH−] dt + k 8)

rate constant

PM dehydrohalogenation by KOtBu dimerization of two Q molecules

1

d[Q] = k1[t BuO−][PM] + k6[OH−][PM] dt − (2k 2[Q]2 + k 3[Q][D])

reaction

(14)

(15)

The rate laws for the other species in the mixture are formulated in an analogous fashion but not shown here explicitly. The full model I is given in the Supporting Information. The concentrations of the reaction intermediates and product(s) are calculated by numerically solving the coupled rate laws using the finite difference approximation E


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polymer products at each time step, from which (instantaneous) molecular weight distributions and averages are calculated.a A full description of model II, including all subreactions and corresponding rate equations, is given in the Supporting Information. It is noted that we do not preassume a quasi-steady state for any reactive intermediate, which perhaps increases computation time but renders flexibility to the algorithm. Typically, our calculation procedure involves variation of the time step in multiple runs to observe where the calculated concentration profiles become time step invariant (in practice: δt ∼ 0.2 s). Before detailing the fitting procedure of the NMR data with model II, we briefly address the consequence of the fact that radical intermediates that lead to the prepolymer bear two growing chain ends (see Scheme 1). In other words, recombination leads to a new biradical species and not to termination. As the molecular weight of the product is never “infinite”, Junkers et al.40 suggested a termination mechanism that proceeds via radical transfer to the solvent (denoted “X”), followed by chain reinitiation (see entries 2 and 3 in Table 2). Monoradicals that result from the transfer reaction can then terminate via radical−radical recombination or a second radical transfer step. Despite the fact that this termination mechanism is yet to be experimentally confirmed, we here adopt the same approach. In other words, the concentration of THF in the reaction mixture, i.e. 12.3 M, is taken as the initial concentration of “chain transfer agent”. Since we have shown above that the NMR data can already be fitted satisfactorily by a model with a limited number of free variables, ambiguity is expected when using a significantly more extended one. For this reason we subject the fitting against model II to an extra constraint that the calculated weightaverage molecular weight (M̅ w) and polydispersity index (PDI) must compare favorably with the experimentally obtained values of M̅ w ∼ 45 kg/mol and PDI ∼ 2 (Figure S6). These values were obtained from GPC analysis of BEH-PPV samples synthesized via low-temperature Gilch polymerization in a separate lab-scale experiment (it proved impractical to perform GPC analysis on the mixture used for the in situ NMR experiment). The fitting to model II proceeds as follows: we copy the model I values for k1, k4, k±5, k6, k7, k8, k9, and k10 (“SET 1”) into model II and consider k2, kp = kri, ktr, and kre (“SET 2”) as fitting parameters. The dimerization rate constant k2 is assigned a value, upon which the remaining parameters of SET 2 are varied to fit the experimental concentration and molecular weight data. Subsequently, the rate constants of SET 1 are allowed to float to further optimize the fit. Irrespective of the values for SET 2, the variation in the SET 1 parameters remains modest and consistent with model I. In other words, the coupling between SET 1 and SET 2 is weak. The focus of the following discussion is therefore on SET 2. As expected, the fitting to model II proved ambiguous. Based on the information at hand, a satisfactory fit can be achieved with an infinite number of parameter values for SET 2, despite the fact that the rate constants in SET 2 mutually couple very strongly. As an example: a very high dimerization rate k2 has to be compensated for by a low propagation rate to avoid overestimation of p-quinodimethane consumption. To at the same time maintain a sufficiently high molecular weight, the radical transfer rate is suppressed. The opposite reasoning applies for a set based on a low k2, though giving comparably fit quality.

only these two rate constants, since the coupling between premonomer consumption and polymerization is negligible and the conversion of Pre into PPV is comparatively slow. As a result, the fitting ambiguity for k2 and k3 is expected to be low. Indeed, the value reported in Table 1 for k2 is robust and insensitive to its initial guess. The value for k3 is more ambiguous and may be considered a lower limit, as choosing a higher value does not affect the fit. Hence, the dimerization of Q seems to be the rate-determining step. Finally, the concentration profile for PPV (Figure 2d) could only be fitted if HBr elimination is assumed exclusively basemediated and self-elimination is disregarded. If the selfelimination rate k8 is chosen nonzero, the observed time lag before the occurrence of the PPV is not appropriately described. Consequently, neutralization of base by HBr is not considered either, i.e., k9 = k10 = 0 L mol−1 s−1. The values for the deprotonation rate constants k4 and k7 are interchangeable and may be varied within 1 order of magnitude without affecting the fit. Since the pKb’s of HO− and tBuO− are comparable, it seems reasonable to assume k4 to be equal to k7. On the other hand, one could also argue that k7 is somewhat larger than k4, as deprotonation by the smaller hydroxide ion is less sterically hindered than for tBuO−. To obtain insight in the actual polymerization kinetics of the Gilch reaction, we now fit the concentration data in Figure 2 to the more extended model II. As mentioned above, this model takes, besides all the (side) reactions of model I, the rates for propagation, i.e., chain growth via reaction between radical intermediates with Q, as well as radical transfer and recombination explicitly into account (see Table 2). Table 2. Reactions and Corresponding Rate Constants Included in Model II To Describe the Gilch Polymerizationa entry

reaction

rate constant

1 2 3 4 5

propagation radical transfer chain reinitiation recombination

kp ktr kri kre

a

equation [···Ri•] [···Ri•] [X•] + [···Ri•] [···Ri•]

+ [Q] → [···Ri+1•] + [X] → [···Ri−H] + [X•] [Q] → [X−R1•] + [···Rj•] → [···Ri−Rj···] + [X•] → [···Ri−X]

···Ri• denotes one radical chain end with i monomers specified.

Mathematically, for model II the same approach is taken as for model I: also, model II comprises a set of coupled differential equations for the rates of change of the concentrations of reaction intermediates. The equations are numerically solved following the same explicit Euler time integration method. We are aware of the fact that models for radical (chain growth) polymerization have been developed, of which some are commercially available.56 Such models typically calculate molar mass distributions as a function of reaction time using Monte Carlo algorithms41,56 or differential solvers.57,58 Within the context of the present study we deemed it instructive to write our own code (model II). Nevertheless, model II has been formulated analogously to the Predici-based57,58 model used by Zaquen et al.40 to study the kinetics of the sulfinyl route toward the prepolymer of poly[2-methoxy-5-(3′,7′dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV). The set of subreactions of model II is hence directly comparable to the one reported by Zaquen et al. The model stores the concentrations of all (radical) intermediates and F


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Table 3. Exemplary Fit Parameter Values for “SET 2” That Each Give a Satisfactory Fit of Model II with the Experimental Concentration and Molecular Weight Data SET 2a (M−1 s−1)

SET 2b (M−1 s−1)

−3

k2 = 2.8 × 10 kp = 1.0 × 10−2 ktr = 5.0 × 10−8 kri = 1.0 × 10−2 kre = 2.6 × 10−2 SET 2d (M−1 s−1) k2 = 5.0 × 10−4 kp = 0.15 ktr = 3.0 × 10−6 kri = 0.15 kre = 0.25 a

SET 2c (M−1 s−1)

−3

M̅ w = 43 kg/mol PDI = 2.2 ϕbb = 17% ϕbbb = 34%

a

M̅ w = 46 kg/mol PDI = 2.3 ϕbb = 3.0% ϕbbb = 5.6%

k2 = 1.9 × 10 kp = 2.1 × 10−2 ktr = 3.0 × 10−7 kri = 2.1 × 10−2 kre = 5.0 × 10−2 SET 2e (M−1 s−1)

M̅ w = 49 kg/mol PDI = 2.3 ϕbb = 12% ϕbbb = 24%

k2 = 1.0 × 10−4 kp = 1.2 ktr = 2.5 × 10−5 kri = 1.2 kre = 3

M̅ w = 48 kg/mol PDI = 2.5 ϕbb = 0.59% ϕbbb = 1.1%

k2 = 1.0 × 10−3 kp = 5.0 × 10−2 ktr = 1.0 × 10−6 kri = 5.0 × 10−2 kre = 0.1 SET 2f (M−1 s−1) k2 = 5.0 × 10−5 kp = 2.0 ktr = 4.5 × 10−5 kri = 2.0 kre = 4

M̅ w = 45 kg/mol PDI = 2.3 ϕbb = 6.8% ϕbbb = 13%

M̅ w = 45 kg/mol PDI = 2.5 ϕbb < 0.4% ϕbbb = 0.56%

All values calculated for M̅ w, PDI, and ϕ correspond to 11 h reaction time.

Figure 3. (a) Representative best fit of the experimental concentrations of premonomer PM, p-quinodimethane Q, prepolymerized monomer Pre, and polymerized monomer PPV to model II. (b) Calculated percentages of bisbenzyl (circles) and bisbromobenzyl (squares) defects plotted against dimerization rate constant k2. The solid lines in (b) represent empirical second-order polynomial fits.

Table 3 lists six exemplary variations for SET 2 that all yield satisfactory fits (Figure 3a) and give similar values for M̅ w and PDI (after 11 h reaction time), consistent with the GPC data (see Supporting Information). The sets have been arranged according to decreasing dimerization rate constant, chosen in the range 2.8 × 10−3 ⩾ k2 ⩾ 5 × 10−5 L mol−1 s−1. For the upper limit of this range we take the value produced by model I, as it is expected that model I overestimates k2 in order to maintain a sufficiently high consumption rate of the pquinodimethane in the absence of multiple radical intermediates. The lower limit of the above range for k2 has been chosen arbitrarily. Obviously, to reduce the ambiguity in the fit values of SET 2, an additional discriminator is required to determine which subset seems the most reasonable. The fitting ambiguity is decisively reduced if we additionally take into account the percentage of monomer−monomer coupling defects which form due to (i) dimerization of pquinodimethane (“bisbenzyl defects”) and (ii) radical−radical recombination (“bisbromobenzyl defects”). The motivation for involving these defect percentages is the fact that they can in principle be experimentally quantified by 1H NMR. In Table 3 the simulated bisbenzyl (ϕbb) and bisbromobenzyl (ϕbbb) percentages after 11 h reaction time are listed for each variation of SET 2 and plotted as an empirical function of the value for k2 in Figure 3b. Note that ϕbbb ≈ 2 × ϕbb, which likely expresses the relative geometric recombination probabilities. In Figure 4 (inset) we indicate the 1H NMR chemical shift (δ ≈ 2.6 ppm59) corresponding to the bisbenzyl defect. The bisbromobenzyl signal is expected around 5.9 ppm but remains

Figure 4. 1H NMR spectrum of the low-T Gilch reaction mixture, recorded after 11 h time. The arrow in the inset points toward the ppm value where the signal corresponding to the protons associated with a bisbenzyl defect is expected.

elusive as it overlaps with the broad peak corresponding to the “conventional” monobromobenzylic protons. Clearly, even after 11 h reaction time, the bisbenzyl signal remains very small and hence difficult to integrate. We can now nevertheless safely state that a set of fit values giving ϕbb > ∼1% is not properly representing polymerization. We therefore conclude that of the six parameter sets shown in Table 3 SET 2e and 2f represent the most realistic values and that the dimerization rate is indeed much lower than suggested by model I. Our interpretation of this result is as follows: a low k2 is accounted for by a high propagation rate kp, which suppresses the formation of bisbenzyl defects. Hence, chains grow fast but their number density remains low, which in turn suppresses the G


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with Zaquen, as transfer (and reinitiation) is not considered by Steenberge. Again, in principle the agreement between our work and Zaquen (ktr(300 K) = 0.05 M−1 s−1) is very good upon extrapolation of our ktr ∼ 104 M−1 s−1 assuming the “ruleof-thumb” activation barrier of Ea ∼ 36 kJ/mol. A little more discussion is dedicated to the comparison of the radical recombination (“termination”) rates. On this matter Steenberge and Zaquen strongly disagree. The former concludes that recombination is not playing a significant role and reports calculations using a low value of kre = 0−5 M−1 s−1. In contrast, Zaquen argues that the activation energy associated with radical−radical recombination should be low and that termination must therefore be fast with kre becoming diffusionlimited under dilute conditions. Indeed, a satisfactory fit is obtained with kre = 107 M−1 s−1, exceeding kp by 3 orders of magnitude. Despite the fact that Zaquen’s argument is intuitively general, we obtain a good fit with a relatively low kre (Table 3). In order to identify possible ambiguity, we refitted the experimental data using SET 2f, but with significantly increased values for kre of 100 and 1000 M−1 s−1. Indeed, acceptable fits are obtained (Figure S6), but only if the propagation rate is raised as well, respectively to kp = kri = 10 and 30 M−1 s−1. Coupling defect densities remain comparable to those obtained for SET 2f. The molecular weight increase is accounted for by increasing the radical transfer rate constant to ktr = ∼5 × 10−4 M−1 s−1, without significantly influencing the fit. Coupling defect densities are comparable to those obtained for SET 2f (Table 3). Further increase in kre (and kp) significantly compromises the fit quality. In view of the above, we keep the discussion regarding the magnitude of the recombination rate open and do not exclude the possibility of kre being suppressed in the Gilch polymerization. Radical recombination is known to be retarded by a rise in viscosity,63,64 during polymerization, which is not uncommon for the Gilch polymerization but usually absent for the sulfinyl prepolymerization. We speculate that besides the low temperature, chain stiffening upon halide elimination retards molecular motion. Since these factors are absent in the sulfinyl prepolymerization, we conclude that arguments can be given in favor of kre being lower for Gilch compared to sulfinyl.

probability for radical recombination and hence the formation of bisbromobenzyl defects. In contrast, in the case k2 is large in relation to kp the p-quinodimethane is mostly consumed by dimerization, which leaves insufficient material for chain growth by propagation. Instead, chains grow mostly via radical recombination, giving an unphysically high density of both bisbenzyl and bisbromobenzyl defects. As a final remark, the uncertainty in the fit valuesb and the low signal-to-noise ratio of the bisbenzyl signal do not allow for more accurate quantification of the rate constants. It is therefore fair to say that the values for k2 in SET 2e and 2f probably indicate an upper limit. Now that the first rate constants have been obtained for the various stages of the Gilch polymerization, we dedicate the final part of our discussion to a loose “order-of-magnitude” comparison with kinetic data obtained from/used in modeling the sulfinyl route toward PPV prepolymer. Two recent studies serve as the basis for our comparison: (i) the kinetic Monte Carlo study by van Steenberge et al.41 and (ii) the Predici-based approach by Zaquen and Junkers et al.,40 below referred to as “Steenberge” and “Zaquen”. Although a comparison is in place, we are aware that care must be taken concerning differences in premonomer substitution pattern. For the tBuO−-mediated 1,6-dehydrohalogenation, Steenberge and Zaquen respectively report values of k1 = 1.2 M−1 s−1 and 1.4 < k 1 < 14 M −1 s −1 for 1-(chloromethyl)-4[(octylsulfinyl)methyl]benzene and 1-(chloromethyl)-5-((3,7dimethyloctyl)oxy)-2-methoxy-4-[(octylsulfinyl)methyl]benzene at T ∼ 300 K. The upper limit of the latter was retrieved from an earlier study.42 If one would extrapolate our 205 K value of k1 of (5−6) × 10−3 M−1 s−1 to 300 K according to the “rule-of-thumb” of a doubled reaction rate with every 10 degrees temperature rise, we arrive at k1 ∼ 3 M−1 s−1, which agrees surprisingly very well with the earlier studies considering the difference in halide substitution (i.e., −Cl for the sulfinyl reaction and −Br in our case). However, this “rule of thumb” not necessarily holds across such a wide temperature range. If so, the associated activation energy speculatively amounts to Ea ∼ 36 kJ/mol. Comparison between the dimerization (initiation) rates is less straightforward. Steenberge works with a value of k2 = (3− 5) × 10−3 M−1 s−1, reconstructed from a trimolecular rate constant for initiation of polymerization from the vapor phase.60 The value of k2 = 2 M−1 s−1 reported by Zaquen is considerably higher, but that study considers initiation to be a two-step mechanism involving dimerization and subsequent trimer formation, with a separate rate constant defined for the latter process of k = 1 × 104 M−1 s−1. Extrapolating our value of k2 = 5 × 10−5 M−1 s−1 using the calculated38 activation energy of Ea ∼ 80 kJ/mol gives a value that exceeds those of both Steenberge and Zaquen, the former by many orders of magnitude. On the one hand, this indicates that our k2 might indeed represent an upper bound, whereas on the other hand the dimerization in particular is likely influenced by the nature of the substituent functional groups, perhaps prohibiting a fair comparison. For the rate constant of radical-quinoid-based reaction (i.e., propagation) an activation barrier of Ea ∼ 36 kJ/mol has been reported.61 Extrapolating our kp = 1−10 M−1 s−1 at 205 K assuming this activation energy gives good agreement with the values used or retrieved by Steenberge and Zaquen, i.e., kp = 1340 M−1 s−1 and kp = 104 M−1 s−1, respectively.62 As for the radical transfer rate constant we can only make a comparison

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SUMMARY The Gilch polymerization toward BEH-PPV has been studied in detail using low temperature in situ 1H NMR spectroscopy. We have measured the concentrations of the premonomer, pquinodimethane, prepolymer, and the PPV product as a function of reaction time. The experimental concentrations have been fitted to models based on coupled rate laws for the reaction steps of the Gilch polymerization. The rate constants of the initiating steps, i.e., the 1,6-dehydrohalogenation and pquinodimethane dimerization, as well as those associated with the polymerization itself, i.e., the propagation, radical transfer, chain reinitiation, and radical−radical recombination, have been quantified. The same holds for the hydrogen bromide elimination step that converts the prepolymer into the actual PPV. We have shown that the fitting ambiguity is significantly reduced by involving experimental molecular weight information and coupling defect concentrations as additional discriminators. Besides the polymerization itself, the modeling shows that reaction between the KOtBu base and residual water in the reaction mixture can significantly influence the formation rate of the p-quinodimethane. Despite its long tradition, now H


Article

Macromolecules for the first time rate constants have been quantified for the Gilch polymerization, allowing comparison of the present results with rate constants previously determined for the sulfinyl prepolymerization.

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(8) Kuik, M.; Vandenbergh, J.; Goris, L.; Begemann, E. J.; Lutsen, L.; Vanderzande, D. J. M.; Manca, J. V.; Blom, P. W. M. Optical detection of deep electron traps in poly(p-phenylene vinylene) light-emitting diodes. Appl. Phys. Lett. 2011, 99, 183305. (9) Gassmann, A.; Yampolskii, S. V.; Klein, A.; Albe, K.; Vilbrandt, N.; Pekkola, O.; Genenko, Y. A.; Rehahn, M.; von Seggern, H. Study of electrical fatigue by defect engineering in organic light-emitting diodes. Mater. Sci. Eng., B 2015, 192, 26−51. (10) Abbaszadeh, D.; Kunz, A.; Wetzelaer, G.; Michels, J. J.; Craciun, N. I.; Koynov, K.; Lieberwirth, I.; Blom, P. W. Elimination of charge carrier trapping in diluted semiconductors. Nat. Mater. 2016, 15, 628− 633. (11) Kunz, A.; Blom, P. W.; Michels, J. J. Charge carrier trapping controlled by polymer blend phase dynamics. J. Mater. Chem. C 2017, 5, 3042−3048. (12) Niu, Q.; Wetzelaer, G. J. A.; Blom, P. W.; Crăciun, N. I. Modeling of Electrical Characteristics of Degraded Polymer LightEmitting Diodes. Adv. Electron. Mater. 2016, 2, 1600103. (13) Fink, J. K. High Performance Polymers; William Andrew: 2014. (14) Braun, D.; Heeger, A. J. Visible light emission from semiconducting polymer diodes. Appl. Phys. Lett. 1991, 58, 1982− 1984. (15) Scott, J. C.; Kaufman, J. H.; Brock, P. J.; DiPietro, R.; Salem, J.; Goitia, J. A. Degradation and failure of MEH-PPV light-emitting diodes. J. Appl. Phys. 1996, 79, 2745−2751. (16) Peters, M.; Zaquen, N.; D’Olieslaeger, L.; Bové, H.; Vanderzande, D.; Hellings, N.; Junkers, T.; Ethirajan, A. PPV-Based Conjugated Polymer Nanoparticles as a Versatile Bioimaging Probe: A Closer Look at the Inherent Optical Properties and Nanoparticle−Cell Interactions. Biomacromolecules 2016, 17, 2562−2571. (17) Zaquen, N.; Lu, H.; Chang, T.; Mamdooh, R.; Lutsen, L.; Vanderzande, D.; Stenzel, M.; Junkers, T. Profluorescent PPV-Based Micellar System as a Versatile Probe for Bioimaging and Drug Delivery. Biomacromolecules 2016, 17, 4086−4094. (18) Blayney, A. J.; Perepichka, I. F.; Wudl, F.; Perepichka, D. F. Advances and Challenges in the Synthesis of Poly(p-phenylene vinylene)-Based Polymers. Isr. J. Chem. 2014, 54, 674−688. (19) McDonald, R. N.; Campbell, T. W. The Wittig Reaction as a Polymerization Method1a. J. Am. Chem. Soc. 1960, 82, 4669−4671. (20) Pfeiffer, S.; Hörhold, H.-H. Investigation of poly(arylene vinylene)s, 41. Synthesis of soluble dialkoxy-substituted poly(phenylene alkenylidene)s by applying the Horner-reaction for condensation polymerization. Macromol. Chem. Phys. 1999, 200, 1870−1878. (21) Rehahn, M.; Schlüter, A.-D. Soluble poly(p-phenylenevinylene)s from 2,5-dihexylterephthalaldehyde using the improved McMurry reagent. Makromol. Chem., Rapid Commun. 1990, 11, 375−379. (22) Junkers, T.; Vandenbergh, J.; Adriaensens, P.; Lutsen, L.; Vanderzande, D. Synthesis of poly (p-phenylene vinylene) materials via the precursor routes. Polym. Chem. 2012, 3, 275−285. (23) Gilch, H.; Wheelwright, W. Polymerization of α-halogenated pxylenes with base. J. Polym. Sci., Part A-1: Polym. Chem. 1966, 4, 1337− 1349. (24) Wessling, R. A. The polymerization of xylylene bisdialkyl sulfonium salts. J. Polym. Sci., Polym. Symp. 1985, 72, 55−66. (25) Louwet, F.; Vanderzande, D.; Gelan, J. A general synthetic route to high molecular weight poly(p-xylylene)-derivatives: a new route to poly(p-phenylene vinylene). Synth. Met. 1995, 69, 509−510. (26) Kesters, E.; Gillissen, S.; Motmans, F.; Lutsen, L.; Vanderzande, D. Polymerization Behavior of Xanthate-Containing Monomers toward PPV Precursor Polymers: Study of the Elimination Behavior of Precursor Polymers and Oligomers with in-Situ FT-IR and UV−Vis Analytical Techniques. Macromolecules 2002, 35, 7902−7910. (27) Hsieh, B. R.; Antoniadis, H.; Bland, D. C.; Feld, W. A. Chlorine precursor route (CPR) chemistry to poly(p-phenylene vinylene)-based light emitting diodes. Adv. Mater. 1995, 7, 36−38. (28) Zaquen, N.; Lutsen, L.; Vanderzande, D.; Junkers, T. Controlled/living polymerization towards functional poly(p-phenylene vinylene) materials. Polym. Chem. 2016, 7, 1355−1367.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00697. Figures S1−S7 and Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.J.M.). ORCID

Jasper J. Michels: 0000-0003-1591-4449 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Prof. T. Junkers and Prof. D. J. Vanderzande of Hasselt University are kindly acknowledged for fruitful discussions. ADDITIONAL NOTES To remain consistent with the experimentally recorded GPC elugram (see Figure S6), model II assumes a lower limit molecular weight cutoff of ∼2 kg/mol, i.e., roughly corresponding to an oligomer containing five monomer units. Considering the uncertainty in the experimental data, we refrain from making a distinction between prepolymer and (partially) HBr-eliminated PPV product. b We note that for a given k2 the listed rate constants may vary within a “scanning range” spanning roughly a factor 2−3 and still give a reasonable fit. a

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REFERENCES

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