Effect of the Substituent Position on the Anionic Copolymerization of

Article Cite This: Macromolecules 2019, 52, 4545−4554

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Effect of the Substituent Position on the Anionic Copolymerization of Styrene Derivatives: Experimental Results and Density Functional Theory Calculations Tobias Johann,†,‡ Daniel Leibig,†,§ Eduard Grune,†,§ Axel H.E. Müller,*,† and Holger Frey*,†,§ †

Institute of Organic Chemistry, Johannes Gutenberg University Mainz, Duesbergweg 10-14, D-55128 Mainz, Germany Max Planck Graduate Center, Forum Universitatis 2, D-55122 Mainz, Germany § Graduate School Materials Science in Mainz, Staudingerweg 9, D-55128 Mainz, Germany Downloaded via GUILFORD COLG on July 30, 2019 at 12:30:20 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: In a combined synthetic, kinetic and theoretical study, the living anionic copolymerization of styrene and its ring-methylated derivatives ortho-, meta-, and para-methylstyrene (MS) was examined by real-time 1H NMR spectroscopy in the nonpolar solvents toluene-d8 and cyclohexane-d12 as well as by density functional theory calculations. Based on the NMR kinetics data, reactivity ratios for each comonomer pair were determined by the Kelen−Tüdő s method and numerical integration of the copolymerization equation (Contour software). The reaction pathway was modeled and followed by density functional theory (DFT) calculations to validate and predict the experimentally derived reactivity ratios. Unexpectedly, two of the three styrene derivatives showed completely different reactivities in copolymerization, governed by the position of the methyl group. While para-MS is considerably less reactive than styrene, resulting in gradient copolymers (rS = 2.62; rpMS = 0.37), ortho-MS showed the opposite behavior and is more reactive than styrene (rS = 0.44; roMS = 2.47), leading to a reversal of the copolymers’ gradient. The substitution in the meta-position had nearly no influence on monomer reactivity, and copolymers with close to random comonomer distribution were formed (rS = 0.81; rmMS = 1.21). In all cases, the theoretical calculations showed good to excellent agreement with the experimental data. Monomer reactivities correlate with the chemical shifts of the β-carbon signals in 13C NMR spectra that are predictive for the gradient structure. The results demonstrate the possibility of tailoring and validating the polymer structures of functional polystyrene copolymers by the choice of the substitution pattern of styrene derivatives, using both experimental and theoretical approaches.

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or butadiene,7 as well as copolymerization of several methacrylates.8,9 The copolymerization of styrene and isoprene strongly depends on solvent polarity. In nonpolar solvents such as cyclohexane or benzene, isoprene is incorporated preferably, while in polar solvents such as tetrahydrofuran (THF), the monomer order is reversed, and styrene is incorporated first. Nevertheless, both polymerizations result in a strong gradient, affording so-called “tapered” block copolymers.10,11 The polymerization kinetics of styrene and its ring-methylated derivatives have hardly been studied, and advanced techniques were not available at the time the few studies were conducted.12−16 In recent years, real-time monitoring of copolymerization kinetics has become an established technique to directly follow monomer consumption during copolymerization. Because of the absence of termination or chain transfer reactions, for the living anionic copolymerization, real-time kinetics experiments permit to determine the monomer conversion that represents

INTRODUCTION The living nature of the anionic polymerization was established by Michael Szwarc more than 60 years ago.1,2 From the living character, he deduced the possibility to generate block copolymers by sequential monomer addition. In the following decades, anionic polymerization evolved to the most precise method to synthesize complex polymer architectures, ranging from simple block copolymers to intricate star and multiblock structures.3,4 In this context, the crossover reaction from the living chain end constituted by one monomer to the second monomer is essential for the formation of block copolymers. It is well known that not every living chain end can propagate with certain monomers.5,6 For instance, it is impossible to add styrene or isoprene monomers to a carbanionic poly(vinyl pyridine) or poly(methyl methacrylate) chain end. Key features for block copolymer formation are the nucleophilicity of the living chain end, the electrophilicity of the propagating monomer, and sterics. Carbanionic polymerization enables a living statistical copolymerization; however, there is only a limited number of studies in this field, and the published reports deal almost exclusively with the copolymerization of styrene with isoprene © 2019 American Chemical Society

Received: April 12, 2019 Revised: May 23, 2019 Published: June 12, 2019 4545

DOI: 10.1021/acs.macromol.9b00747 Macromolecules 2019, 52, 4545−4554

Article

Macromolecules the exact mean incorporation into the polymer chain. However, because of the high sensitivity of carbanionic polymerization to impurities, it is challenging to establish methods to analyze the exact copolymerization, avoiding termination reactions. In a key publication, Long and coworkers presented a real-time kinetics study of isoprene and styrene copolymerization via near-infrared spectroscopy,17,18 both at low temperatures in THF as well as in nonpolar solvents at higher temperatures. The method was extended by the use of a UV−vis spectrometer that also identified the nature of the active chain end.19 Combining mid-infrared and UV−vis spectra, Fontanille and Gnanou et al. determined the reaction rate constants for homopolymerization and crossover reactions and calculated the reactivity ratios for isoprene/ styrene copolymerization in nonpolar media. Online smallangle neutron scattering has also been used, but a deuterated monomer is required to assess differences in the copolymerization behavior.20 Real-time 1H NMR kinetics studies were used to determine homopolymerization kinetics of carbanionic polymerization of various monomers.21−23 However, this method has recently become an important tool to monitor the anionic ring opening copolymerization of epoxides.24−28 In 2013 Natalello et al. introduced this method to follow the living carbanionic copolymerization of vinyl monomers.29 The decrease of the integrals of the double-bond signals in standard 1H NMR spectra can be followed, which directly translates to monomer incorporation in the resulting copolymers. The shift of the double-bond signals in 1H NMR is highly sensitive; thus, rather similar monomers can be compared with respect to their incorporation.29−31 Several very recent works by our group and by others have capitalized on this method.30,31 On a molecular level, the anionic polymerization of styrene in apolar media is rather complex due to the formation of polystyryl−lithium dimers (PS−Li)2. The widely accepted mechanistic hypothesis suggests that only nonassociated, that is, unimeric species of PSLi are capable of propagation.32,33 As a result, the copolymerization follows an order of 0.5 with respect to the living chain end concentration. Morita and van Beylen recently reported a theoretical approach relying on quantum chemical methods to provide insights into the mechanism of styrene homopolymerization.34 Except for this fundamental work, only theoretical studies of the mechanism of copolymerization with dienes, such as isoprene or butadiene, have been reported to date.35−39 Recent developments in the field of density functional theory (DFT) enable the calculation of larger molecules with overall high precision. By using the RIJCOSX approximation,40 optimized basis sets such as the Ahlrichs def2 family,41,42 and dispersion correction,43,44 the computation of reaction pathways can be performed even on a personal home computer within reasonable time spans. This facilitates the efficient calculation of various sets of comonomer pairs. Especially for similar reaction pathways such as the polymerization of styrene and its ring-methylated derivatives, the calculated energies can be quantitatively examined. In this work, we focus on a fundamental question, that is, the influence of the position of substituents in styrene derivatives as well as the effect of different nonpolar solvents on the copolymerization behavior of methylstyrene (MS) isomers, both from an experimental and theoretical approach (Scheme 1). We believe that the implications of this work are of general interest with respect to the introduction of functional groups in a polymer backbone via carbanionic copolymerization.

Scheme 1. Copolymerization/Monomer Systems Employed in This Work

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RESULTS AND DISCUSSION Real-Time 1H NMR Kinetics and Polymer Synthesis. Styrene is one of the most widely used monomers for anionic polymerization. In the case of styrene derivatives, the nature and position of functional groups at the phenyl ring can significantly alter reactivity and copolymerization behavior. This is due to (i) variation of the electron density, which determines the reactivity of the styrene double bond as well as of the anionic chain end and (ii) steric impact in the case of bulky substituents at the phenyl ring. This work focusses on the effect of simple substituents on the monomer reactivity of substituted styrene. The three methyl-substituted styrenes, ortho-, meta-, and para-MS (oMS, mMS, pMS), were chosen due to the fact that the small substituent is expected to have only a minimal steric effect. Experimental details regarding monomer synthesis, the kinetics experiments, and theoretical details regarding the calculations of the reaction pathway are given in the Supporting Information. Table 1 shows the synthesized polymers with numberaverage molecular weights ranging between 2300 and 32 300 g mol−1. The variation of molecular weights arises from the very difficult initiator dosing. Nevertheless, this fluctuation can be neglected because the initiator concentration merely influences the reaction rate but not the copolymerization behavior. All polymers show narrow dispersity monomodal distributions with dispersity Đ around 1.10 (see SEC traces in Figures S59− S62, Supporting Information). The broader distributions in the case of mMS in toluene-d8 are a consequence of the very viscous solution due to the high molecular-weight polymer formed. Modeling of the Reaction Pathway Using DFT. The reaction pathway of the copolymerization was modeled starting from a model of the dimeric chain end, (HS−Li)2, followed by the theoretical dissociation into unimeric structures, HS−Li (eq 1.1). After coordination of the monomer, a precursor complex leads to the propagation transition state, which then forms the polymerization product (see Figure 2 for exemplary 3D illustration of the reaction pathway) (eq 1.2), followed by association to dimers. To speed up the calculations, the polymer chain was assumed to have only one proton in the calculations, for example, “HS−Li”. This simplification is necessary to reduce the overall calculation effort. Also, the penultimate effect was neglected due to only limited influence on the copolymerization behavior. 4546


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Macromolecules Table 1. Overview of the Polymers Synthesized via Carbanionic Copolymerization in the NMR Tubea toluene-d8 pMS

mMS

oMS

cyclohexane-d12

[MS]/[S]

Mn/g mol−1

Đ

[MS]/[S]

Mn/g mol−1

Đ

2.14 1.06 0.77 2.21 1.13 0.57 1.71 0.87 0.41

15 500 10 300 32 300 8550 13 400 22 700 6170 27 000 4730

1.09 1.08 1.08 1.11 1.16 1.25 1.05 1.07 1.05

2.21 1.12 0.59 2.01 1.00 0.55 1.86 0.85 0.39

12 700 7780 7610 6160 2290 6070 5530 7540 2850

1.10 1.08 1.08 1.06 1.07 1.07 1.05 1.06 1.09

a

Comonomer ratios were calculated by comparison of the respective double-bond 1H NMR signals. Number-average molecular weights, Mn, and dispersity, Đ, were determined by SEC in THF with polystyrene standards.

(HS−Li)2 V 2HS−Li

(1.1)

HS−Li + S V [HS−Li−St]‡ → HSS−Li

(1.2)

The reactivity ratio is defined as homopolymerization rate constant k11 versus crossover rate constant k12. To estimate the reactivity ratios, the rate constants can be derived using the Arrhenius equation from the activation energy of the polymerization (eqs 1.4 and 1.5). This activation energy is the difference between the transition state and the reagents, in this case, the 1/2 dimer and the reacting monomer (eq 1.3). EA = E(TS) −

1 E(dimer) − E(monomer) 2

(1.3)

k = Ae−EA / RT

(1.4)

k A e−EA,11/ RT r1 = 11 = = e(EA,12 − EA,11)/ RT k12 A e−EA,12 / RT

(1.5)

Figure 1. Plots of the single monomer conversion ([MS]/[S] = 1.12) vs time for the copolymerization of styrene (red) with pMS (blue) in cyclohexane-d12. The single monomer concentrations of the monomers were normalized.

In this particular case of similar reaction pathways, preexponential factor A can be estimated to be invariant for all transition states. By this approach, absolute errors are eliminated and therefore only the subtle energy differences between the transition states are considered for the calculation of the reactivity ratio ensuring overall high accuracy. Because of the low dielectric strength of the experimentally used solvents cyclohexane and toluene, only negligible solvent effects for the different reaction pathways were calculated in preliminary evaluations. Therefore, all calculations have been performed in the gas state without any solvent effects. para-MS. We will first discuss the copolymerization of para-MS (pMS) and styrene. Selected NMR spectra for the three comonomer ratios studied are shown in Figures S1−S6 (Supporting Information) for toluene-d8 and in Figures S8− S13 for cyclohexane-d12. In toluene-d8, the vinyl signals at 5.62 ppm (pMS) and 5.61 (S) were monitored. Because of the very small chemical shift difference of only 0.01 ppm, the signals overlap which impedes the analysis of the exact integrals and only one-half of the double bond signal can be integrated for the ensuing calculations. Therefore, the measured data show a certain scatter in the plots (Figure S7). The NMR spectra in cyclohexane-d12 show a significant shift difference for the monomer double bond signals at 5.09 ppm (pMS) and 5.14 ppm (S). For this monomer combination, the methylsubstituted styrene derivative always shows considerably lower reactivity than styrene. Figure 1 demonstrates the significantly faster consumption of styrene as compared to that

of pMS. The same results can be observed for all comonomer ratios in the plots of monomer concentration versus total conversion and versus reaction time (Figures S7 and S14). The reactivity ratios for the various monomer combinations were calculated by Kelen−Tü d ő s45 and by numerical integration of the copolymerization equation (Contour software46). The corresponding plots are shown in Figures S50, S51, and S58. The results are given in Table 2. From the data, it is obvious that reactivity ratios do not differ significantly between both nonpolar solvents. The hypothetical gradient structure of a polymer chain with a 50/50 ratio of both monomers is shown in Figure 11a. In the neighborhood of the initiator, the unsubstituted styrene is enriched, while closer to the chain end, the fraction of pMS is significantly higher. To further elaborate the difference in reactivity, the overall reaction pathways for all four possible propagation steps of the copolymerization of pMS and styrene were investigated by DFT calculations. As shown in Figure 2 and Figure 3, the existence of free unimers is highly unlikely due to the high energy difference between the unimeric and dimeric state of approximately 110 kJ/mol (see Tables S1−S3 for detailed values). Considering the transition-state energy levels, the crossover reaction from a poly(pMS)−Li chain end toward styrene exhibits the lowest energy barrier, followed by the homopolymerization of styrene. The highest energy barrier is the crossover reaction from a PS−Li chain end to pMS. The 4547


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Table 2. Experimentally Determined Reactivity Ratios for Styrene (rS) and MSs (rMS) Estimated by Kelen−Tüdős Formalism45 and Nonlinear Least Squares Fitting (Contour46), as Well as Calculated Reactivity Ratios Using the DFT Approach toluene-d8 Kelen−Tüdő s pMS mMS oMS

rS

rMS

2.18 0.85 0.47

0.47 1.17 2.19

cyclohexane-d12 NLLS (Contour)a rS

Kelen−Tüdő s

rMS

rS

rMS

2.62 0.81 0.44

0.37 1.21 2.47

2.58

0.33

a

a

0.40

2.10

DFT

NLLS (Contour)a rS

rMS

rS

rMS

3.28 0.68 0.10

0.32 1.51 5.29

2.74

0.28

a

a

0.38

2.47

a

The determination of reactivity ratios by NLLS was not possible for the copolymerization of styrene and meta-MS because of the insignificant change in mole fractions during copolymerization reaction.

Figure 2. Reaction pathways of all four propagation steps of the copolymerization of styrene and pMS. The 3D illustrations show the corresponding molecules of the styrene homopolymerization. The orbitals shown in the transition state correspond to the vinyl bond (green/yellow) and the carbanion (blue/purple).

incorporation of styrene is both favored by the crossover reaction toward styrene and the energetically favored homopolymerization. The predicted reactivity ratios show very good agreement with the experimentally determined values (Table 2). On the molecular level, the different reactivities can be explained by the positive inductive effect of the methyl group. Styrene exhibits a lower charge at the β-carbon (−0.3645e) compared to pMS (−0.3707e) (Figure 4, see Table S4 for further values); therefore, the nucleophilic attack during the propagation step is more likely to occur. Supporting this argument, the 13C NMR β-carbon shift of pMS (112.34 ppm) is shifted upfield compared to styrene (113.36 ppm). In contrast, the poly(pMS)−Li chain ends are expected to be a little more reactive due to the higher electron density at the αcarbon (−0.3272e vs −0.3203e; Δqα = −0.0069e). meta-MS. The NMR kinetics studies of the copolymerization of styrene with meta-MS (mMS) showed a completely different behavior. The double-bond signals of the two monomers overlay in both solvents; therefore, a similar electronic structure can be expected. The observed protons

Figure 3. Detailed view of the transition-state energy levels of pMS/S copolymerization.

Figure 4. 3D visualization of pMS and HpMS−Li with the corresponding partial charges at the α- and β-carbon calculated by the natural bond orbital (NBO) method.47,48 The values in parenthesis show the difference to the corresponding partial charge of styrene and HS−Li, respectively. 4548


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The lower impact of the positive methyl group in the metaposition can also be verified by the molecular electron density distribution (Figure 7). The partial charge of mMS at both αand β-carbons shows less difference to styrene compared to pMS. The increased partial charge at the β-carbon suggests that mMS is slightly less reactive than styrene, while poly(mMS)−Li chain ends are expected to be more reactive due to a higher charge at α-carbon. The 13C NMR β-carbon shift (113.06 ppm) also correlates with the higher negative partial charge and thus upfield shift of the carbon compared to styrene (113.36 ppm). Thus, the subtle difference in reactivity ratios in the case of this copolymerization system of mMS (rMS = 1.21) of styrene (rS = 0.81) cannot be solely explained by partial charges or 13C NMR β-carbon shifts. Thus, one has to consider the propagation transition states, as shown above. ortho-MS. The last monomer combination examined was ortho-MS (oMS) and styrene. For this combination, a significant difference in chemical shifts for the double-bond signals was observed. The monitored signals are at 5.16 ppm (oMS) and 5.09 ppm (styrene) in toluene-d8 and at 5.20 ppm (oMS) and 5.14 ppm (styrene) in cyclohexane-d12. For all comonomer ratios in both solvents, oMS was consumed faster than styrene (Figures S36−S41 and S43−S49, Supporting Information). A strong difference in the monomer reactivity can be observed in the total conversion versus reaction time plot in Figure 8. Surprisingly, the methyl group in the orthoposition leads to an increase of monomer reactivity, in contrast to the para isomer. The reactivity ratios calculated with Kelen−Tüdő s formalism are summarized in Table 2 and show nearly the inverted reactivity ratios of styrene and pMS. The visualized polymer chain with a comonomer ratio of 1:1 shows a higher oMS content in proximity of the initiator, while styrene is enriched at the chain end (Figure 11c). The comonomer reactivities are confirmed by the calculated reaction pathways shown in Figure 9. As discussed before, the different energy levels of the transition states directly correlate with the estimated reactivity ratios of the comonomer pair. Even though in this example the predicted values show higher deviations (roMS,DFT = 5.29, roMS,exp = 2.47) from the experimentally determined reactivity ratios, the overall estimated gradient structure is still valid. For this system, the crossover reaction from a PS−Li chain end to the oMS terminus exhibits the lowest energy barrier. This transition state is surprisingly low, which further explains the preferential incorporation of oMS. Surprisingly, the homopolymerization of styrene shows a slightly lower energy barrier compared to the

are the signals at 5.64 ppm (mMS) and 5.61 ppm (styrene) in toluene-d8 and at 5.12 ppm (mMS) and 5.14 ppm (S) in cyclohexane-d12. Because of the signal overlap, only the separated half of the double-bond signals can be used for calculations. In both solvents, the double-bond signals of the two monomers decrease at nearly the same rate (Figures S22− S27, S29−S34, Supporting Information). Closer inspection of the plot of the monomer concentration versus reaction time in Figure 5 shows a slightly faster incorporation of the substituted

Figure 5. Plots of the single monomer conversion ([MS]/[S] = 2.01) vs time for the copolymerization of styrene (red) with meta-MS (yellow) in cyclohexane-d12. The single monomer concentrations were normalized.

styrene into the polymer chain. The resulting reactivity ratios in both solvents are close to unity (Table 2) with a slight gradient from mMS toward styrene in the polymer chain. The nearly random distribution of the methyl-substituted styrene derivatives does not depend on the monomer ratio used in the copolymerization. The reactivity ratios predicted by DFT fit very well (Table 2) with the experimental values. The homopolymerization of mMS shows the lowest energy barrier, while the homopolymerization of styrene is least kinetically favored (Figure 6). Even though the overall energies only differ within a range of 1.63 kJ/mol, the DFT calculations explain precisely the subtle gradient in the resulting polymer chain. Both the homopolymerization of mMS and the crossover reaction toward mMS are slightly energetically favored, leading to the preferential incorporation of mMS.

Figure 6. (a) Reaction pathways of all four possible propagation steps of the copolymerization of styrene and meta-MS. (b) Magnification of the corresponding transition-state energies. 4549


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Figure 7. 3D visualization of mMS and HmMS−Li with the corresponding partial charges at α- and β-carbon calculated by the NBO method.47,48 The values in parenthesis show the difference to the corresponding partial charge of styrene and HS−Li, respectively.

implies a negligible inductive effect of the methyl group. In contrast to the partial charge, the 13C NMR β-carbon shift of oMS (114.78 ppm) is shifted low field compared to styrene (113.36 ppm). After the nucleophilic attack of a polymer chain end at the vinyl bond, the planarity in poly(oMS)−Li is restored. On the other hand, the charge at the α-carbon of the poly(oMS)−Li chain ends is higher (−0.3352e vs −0.3203e; Δqα = −0.0149e), even 0.008e higher than at poly(pMS)−Li chain ends. Thus, the high reaction rate can be attributed to the higher reactivity of the poly(oMS)−Li chain ends. This steric impact is difficult to calculate by this DFT approach, leading to disparate reactivity ratios. Table 2 contains a summary of the reactivity ratios for each comonomer ratio of all monomer systems in both solvents, calculated by two different methods. For each comonomer combination, the values in the two solvents are very similar, leading to practically identical gradients. Because toluene (εr = 2.38) exhibits slightly higher polarity than cyclohexane (εr = 2.02), the question was raised regarding in which manner a further increase of the solvent polarity would influence incorporation. For that reason, we added very small amounts of THF to the comonomer solution, so that the ratio of initiator to THF was [THF]/[I] = 1.0−1.5. The signal of the THF traces is visible in the NMR spectra at 3.65 ppm (Figures S15, S17, and S19) and shifts slightly to 3.45 ppm after the initiation process. The addition of this small amount of polar solvent accelerated the polymerization several times, as expected from the literature.49 Therefore, this experiment was only possible with the slowest comonomer system, styrene and pMS. The polymerization was complete after a few minutes for all experiments. However, the same monomer reactivity as in the system without the polar additive was observed (Figure S21). The reactivity ratios calculated by

Figure 8. Plots of the single monomer conversion ([MS]/[S] = 1.71) vs time for the copolymerization of styrene (red) with oMS (green) in cyclohexane-d12. The single monomer concentrations were normalized.

homopolymerization of oMS. Nevertheless, the crossover from a poly(oMS)−Li chain end to styrene is very unlikely due to the energy barrier of more than 12 kJ/mol. This leads to a crossover-controlled polymerization, in which the crossover primarily leads to a poly(oMS)−Li chain end and thus to oMS homopolymerization. This astonishing order of transition-state energy levels can be explained by the molecular structures of oMS and the poly(oMS)−Li chain end. Because of the steric influence of the methyl group, the vinyl bond is twisted out of the aromatic plane by 29.4°, which disrupts the overall π-conjugation (Figure 10). Therefore, the partial charges at the β-carbon (−0.3660e) of oMS and styrene only differ by −0.0014e which

Figure 9. (a) Reaction pathway of all four possible propagation steps of the copolymerization of styrene and ortho-MS. (b) Magnification of the corresponding transition-state energies. 4550


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Figure 10. 3D visualization of oMS and HoMS−Li with the corresponding partial charges at the alpha and β-carbon calculated by the NBO method.47,48 The values in parenthesis show the difference to the corresponding partial charge of styrene and HS−Li.

Figure 11. Illustration of the monomer distribution of a hypothetical poly(styrene-co-MS) copolymer consisting of 50 mol % styrene and 50 mol % MS for the corresponding reactivity ratios in cyclohexane calculated by Kelen−Tüdő s (Table 2).

discussed before. The higher the β-carbon shift, the higher is the polarization of the double bond and also the monomer reactivity. The crossover reaction from one living chain end to the second monomer is unproblematic, if both monomers show similar chemical shifts in 13C NMR or if the chemical shift of the second monomer is higher. Comparing the βcarbon shifts of the monomers in a statistical copolymerization enables a prediction of the reactivity of the monomers and thus of the microstructure, that is, the gradient direction of the resulting copolymers (Table 3). In this study, the most reactive

Kelen−Tüdő s are rS = 3.36 and rMS = 0.46. These values contain inaccuracies due to the lack of sufficient data points as a consequence of the enhanced polymerization rate. Thus, at least in this case, the addition of THF only accelerates the polymerization reaction and does not alter the microstructure of the synthesized polymer. This is in good agreement with copolymerization behavior described by O’Driscoll and Patsiga (Figure 11).50 The kinetics studies carried out for the three different comonomer systems demonstrate that the methyl group in the ortho-position increases the monomer reactivity due to steric hindrance of the methyl group and vinyl bond, while maintaining chain end reactivity similar to PS−lithium. In the case of pMS, the positive inductive effect decreases monomer reactivity during increasing chain end reactivity, leading to a less preferred incorporation in the copolymerization with styrene. As expected from the low electronic effect, placement of the substituent in the meta-position has only marginal influence on both the monomer and chain end reactivity. These results are in agreement with previous investigations of the copolymerization of protected vinylcatechol monomers.31 While the substitution in ortho- and meta-positions for 3-vinylcatechol acetonide increases the monomer reactivity, monomer 4-vinylcatechol acetonide with substituents in the meta- and para-position is less reactive. Simplified Predictive Tools for Reactivity Ratios. Hirao and co-workers investigated the possibility of block copolymer formation in carbanionic polymerization and demonstrated a reactivity direction for block copolymer synthesis based on different monomer combinations.51,52 It is interesting to note that the monomer reactivity can be correlated to the β-carbon shift in 13C NMR spectra as

Table 3. Chemical Shifts of the 13C NMR β-Carbon Signal of the Used Monomers in Cyclohexane-d12 and the Corresponding Reactivity Ratios Calculated by Kelen− Tü dős, as Well as the Difference in β-Carbon Partial Charge Compared to Styrene monomer

β-carbon δ [ppm]

Δδ [ppm]

rMS

Δq [e]

oMS Styrene mMS pMS

114.78 113.36 113.06 112.34

+1.42 0 −0.30 −1.02

2.47 1 1.21 0.37

−0.0005 0 −0.0014 −0.0062

monomer, oMS, has the highest chemical shift (114.78 ppm), while pMS (112.34 ppm) has the lowest one. Styrene (113.36 ppm) and mMS (113.06 ppm) exhibit similar values (Table 3). In contrast to Hirao’s observation, the δ-value of the metasubstituted styrene is slightly lower than that of styrene; however, it shows slightly higher reactivity. Therefore, the βcarbon shift is a good indicator for the reactivity of vinyl monomers in the carbanionic copolymerization. However, it is important to annotate that this approach only holds for 4551


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reactivity due to the positive inductive effect of the methyl group, whereas surprisingly, methyl substitution in the orthoposition results in considerably higher reactivity than that observed for styrene. The calculated monomer geometry of oMS revealed that this inversed reactivity is induced by the steric hindrance of the methyl group that interferes with the vinyl-aromatic conjugation. An almost negligible effect arises from substitution in the meta-position to the vinyl functionality. In this case, simple methods for correlating the reactivity like partial charges or 13C β-carbon shift failed to predict the determined reactivity ratios. Only the total reaction pathway of mMS/styrene copolymerization represented precisely the gradient of the polymer. In all cases, the different reactivities correlated to the molecular structure of the monomers, both by analyzing the electron densities and the possible reaction pathways of the copolymerization. The use of cyclohexane or toluene as solvents for the polymerization did not produce noticeable differences in the copolymerization characteristics. Remarkably, even the addition of small amounts of THF on the order of the initiator concentration leads to polymers with unchanged microstructure, but a strong increase of the polymerization rate was observed. The data obtained for this simple copolymer system via in situ monitoring confirm the predictive value of the 13C NMR β-carbon shifts for the coarse vinyl monomer reactivity and thus for the gradient structure as well as the direction of the monomer gradient in the copolymers.53 An increase of the chemical shift indicates stronger polarization of the double bond, which translates to higher monomer reactivity. Nevertheless, monomers with only small differences in the β-carbon shift may diverge from this rule. For currently elusive monomers, the Bell−Evans−Polanyi principle can be applied to reduce the calculation effort for estimation of the reactivity ratios. In summary, the results demonstrate the predictive value of DFT calculations for the reactivity of styrene monomers in the carbanionic copolymerization. In addition, these fundamental results reveal a strategy to adjust the gradient of polystyrene copolymers by varying the substitution pattern of styrene derivatives.

systems without a significant steric effect of the substituents. In the case of oMS, the steric effect is surprisingly well correlated to the 13C NMR β-carbon shift. As another simplified approach to predict reactivity ratios of styrene derivatives, which are not available for 13C measurement because of, for example, elaborate synthesis, the Bell− Evans−Polanyi principle can be applied. For similar molecules such as the shown copolymerization of styrene and MSs, the enthalpy of reaction can be linearly correlated to the activation energy (eq 1.6). By only considering the monomer reactivity and neglecting all chain end influences (i.e., ideal copolymerization with r1 · r2 = 1), the necessary DFT calculation can be reduced to only the monomer and the corresponding H−Li adduct (eq 1.7). These calculations are rather simple geometry optimization problems without the sometimes troublesome need for calculation of a transition state. In this case, a comonomer pair of only five structures (HLi, styrene, the MS derivative, and their corresponding H−Li adducts) have to be calculated. Despite these many simplifications and assumptions which are made, the experimentally determined reactivity ratios agree very well with the calculations (Figure 12).

Figure 12. Linear correlation of the reactivity ratio of styrene to the energy of H−Li addition to a styrene derivative (r2 = 0.9707).

From the linear fit, the scaling factor was calculated as a = 264.6 (Figure 12; 95% confidence interval: 180.6−348.6). With this parameterized equation, the reactivity ratio of similar styrene derivatives can now be estimated by simple DFT calculations of the monomer “MS” and unimeric “HMS−Li” chain end structure. EA = E0 + a ·ΔH ≈ E0 + a ·ΔE El

(1.6)

HLi + S → HSLi

(1.7)

rS =

■

kS = e(E0 + a·ΔEMS − E0 − a·ΔES)/ RT kMS

ln rS = a ·(ΔEMS − ΔES)/RT

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00747.

(1.8)

■

(1.9)

Materials and Methods, 1H NMR kinetics results, Kelen−Tüdő s plots, Contour plots, SEC results, and DFT Calculation data (PDF)

AUTHOR INFORMATION

Corresponding Authors

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

CONCLUSIONS The copolymerization of styrene with its methyl-substituted derivatives was studied in toluene and cyclohexane via in situ 1 H NMR kinetics and DFT calculations in a combined experimental and simulation study. For evaluation of the experimentally determined reactivity ratios, the total reaction pathway of each comonomer system and the partial charges and molecular geometry were calculated. Substitution in the para-position leads to a significant decrease of the monomer

ORCID

Holger Frey: 0000-0002-9916-3103 Author Contributions

T.J. and D.L. contributed equally. Notes

The authors declare no competing financial interest. 4552


Article

Macromolecules

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ACKNOWLEDGMENTS T.J. acknowledges a fellowship through the Max Planck Graduate Center. D.L. and E.G. acknowledge a fellowship through the Excellence Initiative (DFG/GSC 266) in the content of MAINZ “Materials Science in Mainz”. The authors thank Christian Jochum for technical assistance, Monika Schmelzer for SEC measurements, and Dr. Jan Morsbach for valuable discussions. The authors gratefully acknowledge the computing time granted on the super computer Mogon at the Johannes Gutenberg University Mainz (hpc.uni-mainz.de).

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Effect of the Substituent Position on the Anionic Copolymerization of

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