Sequence Features of Sequence-Controlled Polymers Synthesized by

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Cite This: Macromolecules XXXX, XXX, XXX−XXX

Sequence Features of Sequence-Controlled Polymers Synthesized by 1,1-Diphenylethylene Derivatives with Similar Reactivity during Living Anionic Polymerization Lincan Yang, Hongwei Ma,* Li Han, Pibo Liu, Heyu Shen, Chao Li, and Yang Li State Key Laboratory of Fine Chemicals, Department of Polymer Science and Engineering, Liaoning Key Laboratory of Polymer Science and Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China

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S Supporting Information *

ABSTRACT: Living anionic polymerization (LAP) is the primary representative method of living polymerization and exhibits the ability to synthesize sequence-regulated functionalized polymers. However, there has always been a lack of special features for LAP to guide these cutting-edge syntheses in sequence-controlled polymers. In this study, the copolymerizations of a series of alkyl-substituted 1,1-diphenylethylene derivatives (DPE-alkyls), which display similar reactivities but different structures, and St were performed. In addition, the processes of whole chain propagation were monitored by the in situ 1H NMR method. The results show that changing the alkyl type of DPE-alkyls has little influence on their sequence distributions during copolymerization. Therefore, we could deduce a reasonable principle called “sequence equivalence” in LAP: the copolymers of different DPE derivatives and St would have identical sequence structures when DPE derivatives exhibit exactly the same reactivity ratio (rSt) during the copolymerization under the same conditions. Furthermore, the combination of in situ 1H NMR experiments and kinetic Monte Carlo model (KMC) simulations was conducted synchronously. The results of simulations show that the KMC model not only can simulate detailed information for LAP of DPE derivative and St but also could quantify the precision of the corresponding sequence distributions. Additionally, the KMC model that we constructed for the simulation of sequence control in LAP can give us a new insight into the possibility of sequence tailoring. Next, the relationship among the glass transition temperatures (Tgs), types of alkyl substituent, and sequence structures of DPE derivative units in the chains was investigated. Through DSC analysis, this study’s results indicated that polymers with similar sequences but different alkyl substituents present contrasting differences in thermal property. These results could provide a broader understanding of the thermal properties of polymers.

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INTRODUCTION Peculiar enchantment of sequence-controlled polymers has been gradually displayed in polymer chemistry in the past 10 years because relatively precise and variable monomer sequences in polymer chains could bring large potential advantages for material properties.1−5 Naturally, polymer chemists have explored corresponding synthetic methods to prepare sequence-controlled polymers, such as solid-phase chemistry,6−8 controlled monomer stepwise addition,9−11 and living chain-growth polymerization (LCGP).12−16 Compared to the first two methods, LCGP can prepare well-defined polymers with controlled molecular weights and architectures on the premise that the statistical precision for sequence control can be endured for advanced applications. As a result, the LCGP method has played a critical role in synthesizing sequence-controlled polymers. Thus, controlled radical polymerization (CRP) and living anionic polymerization (LAP), as primary strategies of the LCGP, have been widely applied for achieving omnidirectional control in the degree of functionalization,17−21 topology,22−24 and, especially, monomer sequence.25−29 © XXXX American Chemical Society

Additionally, it should be pointed out that the synthesis of sequence-controlled polymers with CRP or LAP methods complies with kinetic control.30 For instance, the monomer pair of styrene (St) and N-substituted maleimides (MI) has been extensively applied to synthesize sequence-controlled polymers with the CRP method; the corresponding kinetics were clearly characterized and confirmed the method’s effectiveness.26,31−33 The reactivity ratio product of St (rSt) and MI (rMI) is approximatively equal to 0;31 alternating chain propagation thus could be rapidly generated when MI was added into the homopolymerization of St, and the insertion sites of MI in chains could be easily and precisely controlled.31 Correspondingly, during LAP, there are special kinds of monomers, 1,1-diphenylethylene (DPE) derivatives, that exhibited the feature that only the copolymerization existed (i.e., the rDPE was direct equal to 0), and a widely potential possibility exists for the synthesis of sequence-controlled polymers with the LAP method. Received: July 12, 2018

A

DOI: 10.1021/acs.macromol.8b01491 Macromolecules XXXX, XXX, XXX−XXX

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It is important to note that the sequential feeding at specific times has been demonstrated to be a powerful strategy to achieve the precise insertion of MI units into chains during CRP. However, this effective strategy, traditionally applied in CRP, is not adaptable for LAP to achieve precise insertion, although certain functional DPE derivatives exhibited high reactivity. First, the DPE and St in LAP cannot form the acceptor and donor pair such as the situation of MI and St in CRP. Thus, when the DPE derivatives are added by sequential addition into the homopolymerization of St, the small amount feeding of DPE units cannot ensure rapid insertion into chains (RD = kSD × [S−]1/2 × [D] ≪ Rs = kSS × [S−]1/2 × [S], because of the low concentration of DPE derivatives). The sequence distribution of the incorporated DPE units in the chains absolutely depended on the reactivity of DPE derivatives (i.e., rSt). Second, the crossover reaction (i.e., kDS ≪ kSS and kSD) existed and remarkably affected the copolymerization, which can lead to a relatively broad molecular weight distribution, especially for nonalternating copolymerization; thus, the precise control of the number of interval St units at low values (DPS-betw-D lower than 10) is almost impossible to achieve with the staged end-capping strategy. On the basis of these two notable disadvantages, we attempted a different approach to determine the sequence distribution of DPE units in polymer chains during one-pot copolymerization and regulate the sequence structure with varying the rSt of DPE derivatives,34 rather than the known sequential addition strategydif ferent methods and dif ferent paths, but the same purpose! The special features exhibited by LAP impel totally different methods to synthesize the sequence-controlled polymers. What we should do is to explore ample methods to precisely and conveniently regulate the sequence structure for LAP.16,34,35 The regulation of rSt is a very important path for the sequence regulation during living anionic copolymerization of St and DPE derivatives. Different rSt resulted in different sequence distributions of functional DPE units and multivariate properties for postfunctionalized polymers.36 However, another natural issue is raised: what happens to the monomer sequence structure when the rSt of DPE derivatives is controlled to be the same under the same conditions? The underlying principle in the synthesis of sequencecontrolled polymers with the LAP method can give us a new insight into the sequence analysis for these functionalized polymers or monomers that is difficult to directly obtain. Thus, in this paper, we conducted a series of copolymerizations of St and various alkyl-substituted DPE derivatives that exhibited similar reactivity under similar conditions via LAP. Next, a principle for sequence control was revealed for living anionic copolymerization with DPE derivatives: “sequence equivalence”. In the copolymerization of different DPE derivatives that exhibited similar reactivity with St under similar reaction conditions, the corresponding copolymers would display similar sequence distributions. We believe this is a significant criterion for sequence control with the LAP method. Then, the kinetic Monte Carlo model was developed and applied to analyze the sequence precision based on the “sequence equivalence” principle, and the differences of sequence distribution were quantitatively studied with the simulation method. The effects of sequence structure and substituent on chain glass transition temperatures were also investigated.

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EXPERIMENTS

Materials. 4-Bromobenzophenone (Energy Chemical, 98.0%), methyltriphenylphosphonium bromide (Energy Chemical, 98.0%), and potassium tert-butoxide (Energy Chemical, 98.0%) were user for Wittig reaction as purchased. Magnesium turnings (Energy Chemical, 99.5%), iodine particles (Energy Chemical, 99.8%), and [1,3bis(diphenylphosphino)propane]nickel(II) chloride (Ni(dppp)Cl2, Energy Chemical, 98.0%) were used as purchased. Bromopropane (Energy Chemical, 98.0%), 5-bromo-1-pentene (Energy Chemical, 98.0%), 1-bromo-3-methylbutane (Energy Chemical, 98.0%), 1bromopentane (Aladdin, 99.0%), 1-bromoheptane (Macklin, 98.0%), 1-bromodecane (Macklin, 98.0%), and 1-bromododecane (Macklin, 98.0%) were used for the Grignard reaction as purchased. n-Butyllithium (n-BuLi, J&K Chemical, 1.6 M solution in hexanes) was used as purchased. Tetrahydrofuran (THF) was dried by reflux over a Na− benzophenone and then distilled. Benzene (Certified ACS, EM Science), benzene-d6 (Aladdin, 99.5%) and styrene (Aldrich, 99%) were purified as previously described.34 sec-Butyllithium (sec-BuLi) was prepared using 2-chlorobutane and lithium metal in benzene under high-vacuum conditions; the concentration was double-titrated and determined to be 0.338 mol L−1. sec-BuLi dissolved in benzene-d6 was prepared, and the concentration was determined to be 0.371 mol L−1. Tetramethylsilane (TMS) dissolved in benzene-d6 was purified as similarly described in this paper.37 Measurements. Size exclusion chromatographic (SEC) analyses of the copolymers were performed on a Waters HPLC component system (2414 refractive index detector) at a flow rate of 1.0 mL min−1 in THF at 30 °C after calibration using polystyrene standard polymers. 1H NMR spectra were recorded on a Bruker Avance II 400 M NMR spectrometer at ambient temperature using CDCl 3 (tetramethylsilane, TMS) as the solvent. MALDI-ToF MS analysis was carried out on a Waters MALDI micro MX mass spectrometer. 2[(2E)-3-(4-tert-Butylphenyl)-2-methyprop-2-enylidene]malonitrile (DCTB) and sodium trifluoroacetate were used as dopants; details of the sample preparation are provided in a previous study.38 The glass transition temperatures (Tg) of the polymers were measured under a nitrogen atmosphere using a TA Instruments Universal Analysis 2000 differential scanning calorimeter (DSC) from 0 to 200 °C at a heating rate of 10.0 °C min−1. Synthesis of Various Alkyl-Substituted DPE Derivatives (DPE-Alkyls). The experimental operations of 1-bromo-4-(1phenylvinyl)benzene (DPE-Br) have been reported in other studies via the Wittig reaction.39 Various alkyl-substituted DPE derivatives, including 1-propyl-4-(1-phenylvinyl)benzene (DPE-propyl), 1-pentyl4-(1-phenylvinyl)benzene (DPE-pentyl), 1-isoamyl-4-(1phenylvinyl)benzene (DPE-isoamyl), 1-(pentenyl)-4-(1-phenylvinyl)benzene (DPE-pentenyl), 1-heptyl-4-(1-phenylvinyl)benzene (DPEheptyl), 1-decyl-4-(1-phenylvinyl)benzene (DPE-decyl), and 1dodecyl-4-(1-phenylvinyl)benzene (DPE-dodecyl) have similar synthesis procedures. Therefore, the synthesis of DPE-heptyl is provided as an example for illustration. After drying under vacuum and flushing with Ar, 3.72 g (155 mmol) of magnesium turnings and a small iodine particle were placed inside a three-necked round-bottom flask. In a separate flask, 6.94 g (38.77 mmol) of 1-bromoheptane was dissolved in 50 mL of freshly dried THF. The solution was added dropwise at room temperature to the three-necked round-bottom flask to initiate the Grignard reaction. After 4 h of stirring with the Grignard reagent, the supernatant was drained into a dropping funnel and later added dropwise to a two-necked flask, dried, and flushed with Ar, where a solution of Ni(dppp)Cl2 (0.3 g) and 5 g (19.38 mmol) of DPE-Br in 50 mL of dry THF was prepared. The reaction mixture was left stirring for 24 h at room temperature for completion. Column chromatography was used to obtain the crude product (eluent: nhexane; Rf = 0.42). Further purification involved stirring and degassing over n-BuLi, keeping the typical red wine color of DPE derivative anions for at least 4 h, and finally high vacuum distillation to obtain pure monomers. All the characterizations of these DPEB


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Scheme 1. Various Alkyl-Substituted DPE Derivatives (DPE-Alkyls) and Their Theoretical Reactivity Ratio (rth St) Calculated by Hammett Equations

substituents is at or beyond the third carbon atoms,40 and alkyl-substituted groups are relatively stable in carbanion environments compared to other substituent groups with heteroatoms.41 Therefore, we designed and synthesized a series of alkyl-substituted DPE derivatives (DPE-alkyls: including propyl, pentyl, heptyl, decyl, dodecyl, and isoamyl; see Scheme 1) for the copolymerization with St. Based on the electronic effects mentioned above, the reactivity ratios (rSt) for these DPE derivatives were expected to be approximately identical. According to the Hammett equations applied in LAP42 and the corresponding σ value of propyl (σprop= −0.13),43 their rth St can be calculated as 0.77. Therefore, the theoretical propagation of DPE-alkyls would be slightly faster than that of St because the rth‑St is lower than 1. Furthermore, in order to precisely monitor the monomer incorporation in real time during chain propagation, the in situ 1H NMR method was employed to track the entire process of living anionic copolymerization of St and six kinds of DPE-alkyls. Next, the sequence distribution of DPE units in the resulting polymer chains can be calculated by monitoring the continuous changes of characteristic peaks. An experiment implemented using [DPE-heptyl]0/[St]0/ [sec-BuLi]0 = 10:40:1 (see Scheme 2) is selected as an example

Alkyls have been listed in Supporting Information, see Figures S1−S13. In Situ 1H NMR Kinetics Studies of the Copolymerization of St/DPE-Alkyl. The in situ 1H NMR kinetics studies of the St/DPEheptyl copolymerization in a 4:1 feed ratio is provided as an example for illustration. All preparatory work before carrying out the in situ 1H NMR study was performed inside a glovebox. For the 1H NMR kinetics study of the St/DPE-heptyl copolymerization, benzene-d6 (1.6 mL), several drops of TMS dissolved in benzene-d6, and DPEheptyl (0.067 g, 0.241 mmol) were added to a 20 mL sealed vial, and sec-BuLi dissolved in benzene-d6 (65 μL, 0.0241 mmol) was injected. After 20 min, St (0.102 g, 0.962 mmol) was added to the vial. Next, approximately 2 mL of the dark red solution was rapidly transferred to a sealed NMR tube using a glass syringe. The first spectrum was used for confirming the specific amounts of all components. To monitor the reaction by in situ 1H NMR, all spectra were recorded at 400 MHz with four scans and a time interval between each measurement of 1 min. The total test time was approximately 3 h. Synthesis of the Copolymers of Styrene and Various DPEAlkyls at Different Feed Ratios. Synthesis of the copolymers of styrene (St) and various DPE-alkyls at different feed ratios have similar synthesis procedures. Therefore, the synthesis of copolymer of DPE-heptyl and St at [DPE-heptyl]0/[St]0 = 1:4 condition is provided as an example for illustration. The copolymerization of styrene and DPE-heptyl in benzene was performed using sec-BuLi as the initiator under glovebox conditions. Benzene (10% w/v) and DPE-heptyl (0.067 g, 0.241 mmol) were added to a 20 mL sealed vial, and sec-BuLi (70 μL, 0.0237 mmol) was injected to initiate the DPEheptyl. The typical red wine color of the DPE-heptyl anion appeared immediately following the injection. To achieve sufficient initiation, the solution was stirred for 20 min. Next, St (0.10 g, 0.962 mmol) was added to the bottle rapidly, and the solution was stirred for 12 h at room temperature. Next, the copolymerization was terminated with isopropanol. The product was precipitated with sufficient methanol and subsequently dissolved in an appropriate amount of toluene; this process was repeated twice to ensure that the residual monomers were completely removed.

Scheme 2. Polymerization Route of DPE-Heptyl and St Initiated by sec-BuLi in Benzene-d6 Solvent (10% w/w) at 25 °C

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RESULTS AND DISCUSSION Living Anionic Copolymerization of Styrene and the DPE Derivatives with Similar Reactivity. To achieve the goal mentioned in the Introduction, DPE derivatives with similar reactivities but different structures should be first designed. There are several basic criteria that should be met. (1) Substituent groups cannot introduce any side reactions during the processes of living anionic polymerization (LAP). (2) Substituent constants (i.e., the σ values) for these DPE derivatives should be as similar as possible according to Hammett equations. Based on these considerations, the alkylsubstituted groups with different carbon chain structures are used to design DPE derivatives that exhibit similar reactivity. That finding is observed because it has been demonstrated that the electronic effects of alkyl substituents on the benzene ring would become less significant when the length of alkyl

for universal discussion. Figure 1a shows its stacked overlay of all the in situ 1H NMR spectra during the copolymerization. The local enlarged drawing (Figure 1b) displays the continuous change of the characteristic peaks of the two monomers in detail. The corresponding molecular weight and polydispersity were characterized with SEC (the results can be seen in entry I-3 of Table 1 and Figure S42 in the Supporting Information). During the real-time monitoring, purified tetramethylsilane (TMS) was applied as an internal standard to calculate the accurate monomer molar quantities in each 1H NMR spectrum every 60 s. The continuous changes in the integral ratios of three characteristic peaks (see Figure 2a) can be used to calculate the conversion of the two monomers (see C


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Figure 1. (a) In situ 1H NMR spectra stacked overlay of [DPE-heptyl]0/[St]0/[sec-BuLi]0 = 10:40:1 polymerization. (b) Local enlarged drawing of the characteristic peaks overlay of DPE- heptyl (left) and styrene (right).

Table 1. Characterization Data for Polymerizations of St and Various DPE-Alkyls by in Situ 1H NMR Monitoring entrya

DPE-alkylsb

feedc [D]/[St]

Mnd/kg mol−1

PDId

NDe

NSe

sequencee

KDf/×10−2 min−1

KStf/×10−2 min−1

rStg

I-1 I-2 I-3 I-4 I-5 I-6

D-propyl D-pentyl D-heptyl D-decyl D-dodecyl D-isoamyl

1:4

6.3 6.6 6.3 7.5 7.4 5.5

1.13 1.14 1.14 1.10 1.11 1.15

10.0 9.8 9.7 9.8 10.1 9.8

39.8 39.0 38.8 38.3 40.5 42.3

statistical SSSSD

2.43 2.38 2.19 2.24 2.41 2.91

2.09 1.97 1.87 1.88 2.06 2.50

0.69 0.65 0.65 0.65 0.64 0.66

a

The polymerizations were initiated by sec-BuLi dissolved in C6D6 at rt, and all the progress was monitored by in situ 1H NMR. bThe type of DPE derivative at current polymerization. cPolymerization feed ratio ([D]/[S]/[Li] = 10:40:1) where same feed ratio was set for keeping reaction conditions almost the same. dDetermined by SEC. eDetermined by each in situ 1H NMR progress. fObtained from semilogarithmic curves. g Calculated by the least-squares analysis.

Figure 2. (a) 1H NMR signals in [DPE-heptyl]0/[St]0/[sec-BuLi]0 = 10:40:1 polymerization system. (b) Conversion vs time curves for DPE-heptyl and St.

definitely reveals that the statistic sequence mentioned above exhibited the quasi-tetra St units splitting adjacent DPE units (i.e., statistical SSSSD sequence) under the situation of [DPEheptyl]0/[St]0 equal to 1/4. This also implies that the kinetic propagation trend is that after each additional DPE-heptyl unit four St units were subsequently incorporated, and the corresponding sequence was generated as shown in Figure 3g. Furthermore, the apparent kinetic constants for DPEheptyl and St (KD‑heptyl and KSt) were determined using a semilogarithmic curve with data from the in situ 1H NMR characterization (see Figure 4c). After linear fitting, KD‑heptyl and KSt were calculated to be 2.19 × 10−2 min−1 and 1.87 × 10−2 min−1 (see Table 1, entry I-3). Meanwhile, the corresponding reactivity ratio (rSt) was also calculated to be 0.65 with least-squares analysis.35 Herein, the values of KD‑heptyl and KSt are close but not equal, which indicates in the sequence deduced via the in situ 1H NMR method is not a

Figure 2b). As observed, the conversions of DPE-heptyl and St are close to 100%. Furthermore, dividing the moles of continuous consumption of two monomers by the amount of initiator can deduce the statistic sequence structure during the chain propagation (see Figure S20). To visualize the composition changes occurring over the whole chain length and further illustrate the characteristics of this statistic sequence, Figure 3 plots the cumulative copolymer composition (Fcum) vs normalized chain length. In this study, normalized chain length represents the chain propagation progress, and cumulative copolymer composition represents the percent content of each monomer at a certain chain length. As we can see from Figure 3c, Fcum(St) remains at approximately 80%, and that of DPE-heptyl is approximately 20% during the whole chain propagation. Fcum of each monomer is essentially a near constant value with little fluctuation at all stages in the propagation progress. This result D


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Figure 3. (a−f) Cumulative copolymer composition vs normalized chain length curves of polymerizations of DPE-alkyls and St. (g) Sketch of the statistically tetra-St interval sequence structure.

Figure 4. Semilogarithmic curves of polymerizations of DPE-alkyls and St.

cumulative copolymer composition (Fcum) vs normalized chain length (see Figure 3) showed similar sequence structures during the six copolymerizations. All of these monomer sequences synchronously exhibited a statistically tetra-St interval in each repeating unit when the feed ratio was controlled at [DPE-alkyl]0/[St]0/[sec-BuLi]0 = 10:40:1. Furthermore, the kinetic parameters (K and rSt) for each copolymerization were determined using a semilogarithmic curve (see Figure 4) and least-squares analysis, respectively. As we can see in Table 1, the values of K and rSt for these copolymerizations with different DPE-alkyls are almost the same. By this token, a clear feature can be seen that changing the alkyl type of DPE derivatives has little influence on the sequence structures of copolymerization. In essence, LAP

strictly tetra-St-interval sequence (i.e., the repeating of DSSSS), and a slight gradient feature may be exhibited (the specific explanation is presented later). Sequence Equivalence Principle in Living Anionic Copolymerization with DPE Derivatives. In order to explore features of the monomer sequence structures when the rSt are controlled to be the same, we tracked the entire process of living anionic copolymerization of St and six kinds of DPEalkyls with in situ 1H NMR methods under constant copolymerization conditions. These DPE-alkyls included DPE-propyl, DPE-pentyl, DPE-heptyl, DPE-decyl, DPEdodecyl, and DPE-isoamyl. The specific processes monitored by in situ 1H NMR are as mentioned above and omitted in this section for brevity. All experiments results can be found in Table 1 and Figures S14−S29. Most importantly, the curves of E


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synthesized from DPE derivatives, such as the copolymers of dienes and DPE. On the other hand, the “sequence equivalence principle” could solve the difficulties existing in analyzing the sequences of complex monomers with the in situ 1 H NMR method. For example, DPE-pentenyl (no. 5 in Scheme 1) is versatile due to its pendant double bonds. However, it is tough to determine DPE-pentenyl’s propagation progress in polymerization using the in situ 1H NMR method because the characteristic peaks of pendant double bonds are seriously overlapped with those of polymeric double bonds. In this situation, we could seek a substitute with a similar reactivity to replace DPE-pentenyl for the analysis of the propagation using this principle. In brief, “sequence equivalence” does possess important significance for conducting syntheses of advanced polymers by LAP. Evaluation of the Sequence Structure with Kinetic Monte Carlo Simulation. In recent decades, progressive discussions and plentiful efforts in sequence control have been undertaken. Particularly, the statistical sequence was obtained when living chain growth polymerization methods were selected to synthesize sequence-controlled polymers.12−16 Thus, each functional unit in the chain is distributed in a specific domain, not a specific site. Especially in the sequencecontrolled polymers synthesized by LAP with DPE derivatives, the number of interval units (such as St) sandwiched in between adjacent DPE units was regulated and calculated as average values. The precision of the statistical sequence structure in the same DPE derivative with different sequences, or the same sequence but different DPE derivatives, has become a critical problem for us when we explored plentiful sequence-controlled polymers with abundant DPE derivatives.34,35,37,38,44,45 The simulation of copolymerization with the kinetic Monte Carlo model (KMC), which is based on the exact stochastic simulation algorithm developed by Gillespie,46,47 has been widely used to interpret detailed chain propagation, and proximate results have been reported.48,49 Additionally, the “sequence equivalence” principle, as raised above, gives the possibility for us to carry out the simulation of kinetic propagation with the KMC model. Thus, in order to reveal the explicit sequence structure in copolymerization of DPE-heptyl and St, and to further estimate whether the KMC model is universal for LAP systems of DPE derivatives and St, we developed the corresponding KMC program based on the elementary reactions of the living anionic polymerization of DPE derivatives and St (Scheme 3). For specific implementation processes, see Figure S44. Sequence Structure Regulated by Feed Ratios and Calculated via the in Situ 1H NMR Method. First, we conducted enough copolymerization experiments for comparisons of calculations and simulations to estimate whether the

belongs to kinetic control to synthesize sequence-controlled polymers. Thus, the kinetic parameters are almost changeless with variation of the type of alkyl substituents could perfectly verify this feature. Here, different alkyl substituents we introduced exhibited similar electronic effects, which caused the approximately equal reactivity (rSt and KD) in copolymerization. Next, the amounts of monomers inserted into the chain during the same period are almost the same (see Scheme 3), which also means the sequence structures obtained are almost equivalent. Therefore, this experimental result can be summarized as a principle: Scheme 3. Chain Propagation Elementary Reaction and Propagation Equation of Living Anionic Polymerization

During living anionic copolymerization with DPE derivatives as f unctional monomers, when the DPE derivatives exhibit similar reactivity ratios in different polymerizations under the same reaction conditions, the polymers prepared could display similar sequence structures. Axioms in mathematics and postulates in physics have one thing in common: that both are self-evident statements to serve as a premise for further reasoning and arguments. We have a great mind to synthesize two DPE derivatives with exactly the same rSt to study the sequence structures under exactly identical copolymerization conditions, but it is clear that this cannot be achieved. However, according to the above experimental results obtained under nearly identical copolymerization conditions, we could deduce a reasonable “axiom” applied to living anionic polymerization: in living anionic polymerization, the copolymers of different DPE derivatives and St would have exactly the same sequences when DPE derivatives have exactly the same rSt under same copolymerization conditions. Herein, we call this principle “sequence equivalence in living anionic polymerization”. This principle is instructive for monomer sequence control in LAP. On the one hand, compared with other methods, LAP is the typical and representative method of true living polymerization. LAP has the ability and responsibility to obtain sequence-defined and controlled synthetic polymers. However, there has always been a lack of understanding of the special features for LAP to guide these cutting-edge syntheses. Now, this condition is improved. The “sequence equivalence” could give us a new insight into analyze the monomer sequences of these functionalized polymers which were

Table 2. Characterization Data for Polymerizations of St and DPE-Heptyl in Different Feed Ratios by in Situ 1H NMR Monitoring entrya

feedb [D]/[St]

Mnc/kg mol−1

PDIc

NDd

NSd

sequenced

KD‑heptyle/×10−2 min−1

KSte/×10−2 min−1

H-1 H-2 H-3 H-4

1:6 1:4 1:2 1.5:1

5.5 6.3 7.3 4.4

1.14 1.14 1.13 1.11

6.5 9.7 13.3 13.7

40.7 38.8 26.5 13.4

sta-SSSSSSD sta-SSSSD sta alt-SSD alt-SD

4.15 2.19 0.66 0.37

3.57 1.87 0.64 0.39

a

The polymerizations were initiated by sec-BuLi dissolved in C6D6 at rt, and all the progress was monitored by in situ 1H NMR. bPolymerization feed ratio, [St]0/[Li]0 = 40:1 for all entries. cDetermined by SEC. dDetermined by each in situ 1H NMR progress. eObtained from semilogarithmic curves. F


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Figure 5. MALDI ToF of polymerization of St and DPE-heptyl in [DPE-heptyl]0/[St]0 = 1.5:1 by in situ 1H NMR monitoring.

Figure 6. Copolymerization simulation using kinetic Monte Carlo model. (a) Simulation results of [DPE-heptyl]0:[St]0 = 1.5:1. (b) Simulation results of [DPE-heptyl]0:[St]0 = 1:2. (c) Simulation results of [DPE-heptyl]0:[St]0 = 1:4. (d) Simulation results of [DPE-heptyl]0:[St]0 = 1:6.

feed ratio is also a slight gradient. Here we give a detailed explanation according to the chain propagation equation of LAP41 (see Scheme 3). Because different K values could cause different insertion amounts at a given time (−d[M]/dt), and then −d[M] could further affect the residual amount [M], the −d[M]/dt would be further changed by K[M]. Finally, the sequence structure would become a slight gradient, and the degree of polymerization (DPS) of St between each two adjacent DPE-heptyls (DPS-betw-D) would deviate from the initial feed ratio. As a comparison, however, the KD‑heptyl and KSt of the 1.5:1 feed are almost equal. According to the chain propagation equation of LAP (Scheme 3), equal K values could cause the same insertion amount at a given time (−d[M]/dt). As a result, the value of [DPE]/[St] would remain static, and the

KMC model is universal for LAP of DPE derivatives and St. Similarly, the in situ 1H NMR method was employed to monitor all the processes of polymerization. According to the “sequence equivalence” principle mentioned above, we just carried out chain propagation monitoring at different feed ratios of DPE-heptyl to St including [DPE-heptyl]0:[St]0 = 1:6, 1:4, 1:2, and 1.5:1. (The in situ 1H NMR of [DPE-heptyl]0: [St]0 = 1:4 has been discussed in the above section, and it is just for convenience of comparison that we put it here.) Table 2 lists all the experimental results, and the details are in the Supporting Information (Figures S30-S42). After linear fitting from semilogarithmic curves, we can see from the K values the relationship between KD‑heptyl and KSt at a 1:6 feed ratio is similar to that of the 1:4 feed ratio: KD‑heptyl is larger than KSt. This result shows that the sequence in the 1:6 G


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Figure 7. DSC curves (10 °C/min) of polymers of DPE-alkyls and St in different feed ratios including [DPE-alkyls]0/[St]0 = (a) 1.5:1, (b) 1:2, (c) 1:4, and (d) 1:6 where alkyls include propyl (black), pentyl (red), heptyl (cyan), decyl (blue), and dodecyl (violet).

To compare with the results obtained from in situ 1H NMR, the weighted averages of DPS-betw-D were calculated, and these values are shown in the histogram in Figure 6. The average DPS-betw-D drifts from 1.43 initially to 1.12 finally for the weighted average of 1.5:1 (D/S), and the value drifts from 2.37 initially to 1.86 finally for the 1:2 (D/S). However, referring to the MALDI results (see Figure 5), we can surely affirm that the sequences of 1.5:1 (D/S) are strictly alternating, rather than a reverse gradient. As a comparison, the average DPS-betw-D drifts from 3.66 to 3.93 for the 1:4 (D/S), and the corresponding value drifts from 5.08 to 6.02 for the 1:6 (D/S). Both of these present slight gradient features. It should be noted that the statistically equal-St interval sequence was still labeled because the trend of the gradient is slight. Therefore, the simulation results match the in situ 1H NMR experiment: two of them (1:4 and 1:6) have slight gradient features, while the other two (1.5:1 and 1:2) exhibit alternating features. These results definitely confirm that the KMC model is appropriate for LAP systems of DPE derivatives and St, especially for analysis of their sequence features. The analysis of explicit sequence structure by quantification of the sequence precision was additionally described. As we can see from the linear chart in Figure 6, for a specific feed ratio, all the curves of weight fraction vs DPS-betw-D are dramatically overlapping. This result indicated that the DPS between every two adjacent DPE-heptyl units remains nearly constant from chain initiation to termination. However, significant differences in the curve slopes can be observed when the feed ratios are changed. For the simulation results of 1.5:1, the curve slopes are strictly steep, and the cutoff DPSbetw-D is below two St units. For the results of 1:2, the curve

ratio of insertion rate of DPE-heptyl to that of St is consistent with the feed ratio. Therefore, the sequence in the 1.5:1 feed ratio is a strictly alternating structure rather than statistical sequence. The corresponding MALDI-TOF-mass spectrum can verify this alternating tendency (see Figure 5). Meanwhile, the K values of DPE-heptyl and St are also almost equal when the feed ratio is 1:2. Analogous to the above analysis, the sequence at 1:2 is still a quasi-alternating structure, and the corresponding repetitive unit “DSS” or “SDS” is dominating. Sequence Structure Simulated by KMC Simulation. Thus, four KMC simulations were conducted to maintain consistency with the copolymerization of DPE-heptyl and St regulated at four feed ratios and monitored with in situ 1H NMR. All of the simulations were set to be consistent with corresponding copolymerizations with different feedings, and each copolymer was defined to contain 1 × 104 polymer chains. The reaction rate constants (k) of this polymerization system (see Scheme 3) could be deduced according to kSt,23 °C,48 rSt (calculated above), and the living species proportion ([S−]/([S−] + [D−]), see Figure S32 Figure S45, not S32). After calculation, kSS is set as 6.4 × 10−3 (L/mol)1/2 s−1, kSD as 9.32 × 10−3 (L/mol)1/2 s−1, and kDS as 9.41 × 10−4 (L/mol)1/2 s−1. The results of the copolymerization simulations are shown in Figure 6. D1 represents the DPE-heptyl initiated by sec-butyl, which obeyed real copolymerization, D2 is the second insertion of DPEheptyl, and so on. Because of the non-homopolymerization of DPE derivatives, we could count the degree of polymerization of St (DPS) between every two adjacent DPE-heptyls (D) (DPS-betw-D). S1−2 represents the DPS between D1 and D2, and the pattern continues. H


Article

Macromolecules Table 3. Characterization Data for Polymerizations of St and Various DPE-Alkyls in Different Feed Ratios entrya

subb

feedc

Mnd

PDId

Tge

entry

sub

feed

Mn

PDI

Tg

T-1 T-2 T-3 T-4 T-5 T-6 T-7 T-15 T-16 T-17 T-18 T-19 T-20 T-21

propyl pentyl heptyl decyl dodecyl isoamyl pentenyl propyl pentyl heptyl decyl dodecyl isoamyl pentenyl

1.5:1

9.7 11.9 11.1 13.5 12.4 11.1 10.9 6.9 7.7 7.8 8.4 8.4 7.5 7.4

1.08 1.07 1.07 1.08 1.08 1.08 1.08 1.13 1.12 1.13 1.12 1.12 1.13 1.13

139 109 83 62 42 123 104 111 99 86 68 59 104 97

T-8 T-9 T-10 T-11 T-12 T-13 T-14 T-22 T-23 T-24 T-25 T-26 T-27 T-28

propyl pentyl heptyl decyl dodecyl isoamyl pentenyl propyl pentyl heptyl decyl dodecyl isoamyl pentenyl

1:2

8.2 8.6 9.8 12.2 11.4 9.3 9.0 5.9 6.1 6.3 7.3 7.8 6.2 6.5

1.12 1.13 1.11 1.12 1.11 1.12 1.13 1.20 1.19 1.19 1.19 1.15 1.20 1.19

127 99 83 64 47 114 104 105 93 83 71 63 100 96

1:4

1:6

a

The polymerizations were initiated by sec-BuLi at rt in benzene. bThe type of substituents of DPE-alkyls at current polymerization. Polymerization feed ratio, [D]0/[St]0; the theoretical number of St per chain is set to be 40. dDetermined by SEC and SEC curves in Figure S43. e Glass transition temperature (°C), determined by DSC. c

The glass transition temperature (Tg) values of P[(DPEalkyl)-St]s containing different length alkyl substituent groups with various sequence structures were studied experimentally by DSC under nitrogen. Figure 7 shows their detailed DSC curves, and Table 3 shows detailed experiment results. A 3D histogram was drawn to facilitate understanding of the relationship among sequence, alkyl length, and Tg (see Figure 9a). As we can see from each colorful stair (same sequence structure but different alkyl length), with the increase of the alkyl side-chain length from propyl to dodecyl, Tg shows a significant decline at each type of sequence structure. For example, Tg declines from 139 to 42 °C when the alkyl changes from propyl to dodecyl with an alternating SD structure! We know from the discussion above that the length of alkyl chain has little influence on the polymerization properties, such as sequence or kinetics, because they have an almost identical electronic effect. However, different alkyl side-chain lengths can cause completely different results for physical properties like Tg because their steric effects or flexibility are different. A short alkyl is disadvantageous for the movement of polymer chain segments as it is rigid. On the other hand, a relatively long alkyl is beneficial for reducing the entanglement between the polymer chains to increase the movement of chain segments. Therefore, Tg shows a significant decline with the increase of the alkyl side-chain length from propyl to dodecyl. From the unicolor 3D stairs (same alkyl length but different sequence structure), we can see an attractive phenomenon. For the heptyl substituent group, Tg remains stable, regardless of sequence. This result indicates that (1) the effect of heptylsubstituted DPE derivative on chain segment movement is the same with that of St and (2) there is little influence of polymer molecular weights on Tg, at least when the molecular weight is greater than 6.3K. For the short alkyl substituent groups, like propyl and pentyl, Tg shows a dramatic decrease when the sequence structure changes from SD alternation into the six-St interval. For the long alkyl substituent groups like decyl and dodecyl, an opposite trend is seen in comparison with short alkyl substituent groups: Tg shows an obvious increase when the sequence structure changes from SD alternation into the six-St interval. This is because SD alternating and SSD alternating sequences have crowded alkyl substituents compared with four-St and six-St interval sequences. These

slopes are still quite steep, and the cutoff DPS-betw-D is approximately five St units. As a comparison, for the results of 1:4 and 1:6, their curve slopes become considerably flattened, and the corresponding cutoff DPS-betw-D becomes very large (about 10 and 15). Therefore, an internal distinction among these sequences is that the sequence precision is totally different when the feed ratios are regulated. The degree of sequence precision becomes rough when increasing the amount of St in feeding. What is more, the standard deviation in the histogram (Y error bars) becomes larger when increasing the amount of St in feeding. A larger standard deviation illustrates that the fluctuation range of DPS-betw-D is larger and further indicates the degree of sequence precision becomes rougher. To evaluate the precision of sequence, the in situ 1H NMR method is powerless. However, we received a glimpse of all explicit sequence structure and deeply analyzed the sequence precision with KMC simulations. These sequences obtained in a high-DPE-heptyl feed possess higher precision compared with those obtained in a high-St feed. Consequently, the sequences in a high-St feed are a statistical structure, and the sequences in a high-DPE-heptyl feed are defined to be an alternating structure. After the above analysis with KMC simulations, it is assured that the KMC model not only is appropriate for LAP of DPE derivatives and St but also could quantify the precision of sequence. We will conduct in-depth research about this in our future progress. Thermal Property (Glass Transition Temperature) of the Sequence-Controlled Polymers. Alkyl substituents with different lengths have been used to adjust the material properties due to their electronic properties, steric effects, and chemical stability. To date, numerous studies have reported many significant impacts of the length of alkyl chains on polymer properties. For conjugated polymers, the alkyl sidechain length mainly affects their electrical properties.50−53 In addition, alkyl substituents with different lengths also affect polymeric molecular ordering or morphology54 and thermal properties.55 The purpose of this section about the relationship among glass transition temperature, length of alkyl substituents, and sequence structure is to provide a broader understanding of the thermal properties of polymers. I


Article

Macromolecules

Figure 8. DSC curves (10 °C/min) of polymers of DPE-alkyls and St in different feed ratios including [DPE-alkyls]0/[St]0 = (a) 1.5:1, (b) 1:2, (c) 1:4, and (d) 1:6 where alkyls include pentyl (black), pentenyl (red), and isoamyl (blue).

Figure 9. 3D histograms of glass transition temperatures. (a) Relationships among sequence structures, alkyl lengths, and glass transition temperatures. (b) Relationships among alkyl types, sequence structures, and glass transition temperatures.

substituents in series increase their influence on chain segment movement. Rigid substituents have higher Tg and flexible substituents have lower Tg. Furthermore, P[(DPE-alkyl)-St]s containing different alkyl substituents (pentyl, isoamyl, and pentenyl) with various sequence structures were investigated by DSC under nitrogen. Figure 8 shows their detailed DSC curves, and Table 3 shows detailed experimental results. Similarly, a 3D histogram was drawn to understand the relationship among sequence, alkyl substituents, and Tg (see Figure 9b). As we can see from each unicolor stair (same alkyl substituent but different sequences), Tg shows a decrease when the sequence structure changes from SD alternation into the six-St interval. This result is consistent with that obtained in short alkyls, like propyl and pentyl. Therefore, isoamyl and pentenyl are disadvantageous for the polymer chain segments of movement. As we can see from each colorful stair (same sequence structure but different alkyl substituents), the Tg of isoamyl is obviously higher than that of pentyl and pentenyl at the same sequence structure. However, the difference between pentyl and pentenyl is not clear. From

the above discussion, it is evident that isoamyl is more rigid for the polymer chain segments, preventing movement, due to its larger bulk.

■

CONCLUSIONS A series of alkyl-substituted DPE derivatives (DPE-alkyls), which possessed similar reactivity but different structures, were designed and successfully synthesized. Next, the copolymerization processes of these DPE-alkyls and St were monitored by the in situ 1H NMR method. The results show that changing the alkyl type of DPE-alkyls has little influence on the sequence distributions of copolymerization. After analysis, the reason for this phenomenon is that the different alkyl substituents we introduced exhibited similar electronic effects, which caused the approximately equal reactivity (rSt) in copolymerization, and the same kinetic behaviors were formed during all these copolymerizations. Therefore, we could deduce a reasonable principle, called “sequence equivalence”: in living anionic polymerization (LAP), the copolymers of different DPE derivatives and St would have exactly the same sequences J


Macromolecules

■

when DPE derivatives exhibit exactly the same rSt under identical copolymerization conditions. Considering that we have explored plentiful sequence-controlled polymers with abundant DPE derivatives, this shows important significance for conducting the subsequent synthesis of advanced polymers by LAP. Furthermore, in order to explore the precision of the statistical sequence structure in the same DPE derivative with different sequences, or the same sequence but different DPE derivatives, the combination of in situ 1H NMR experiments and kinetic Monte Carlo model (KMC) simulations was carried out synchronously. The simulation results about the sequence structures match those obtained from the in situ 1H NMR experiments well. In addition, the KMC model indicated the explicit sequence structure for quantifying the sequence precision, which is hard to determine with the in situ 1H NMR method. The quantization for sequence precision shows that the sequences in high-St feeds are a statistical structure, and the sequences in high-DPE-alkyls feeds are a defined structure. Therefore, it is assured that the KMC model is not only appropriate for LAP of DPE derivatives and St but also could quantify the precision of sequences. Finally, the relationship among glass transition temperature (Tg), length of alkyl substituents, and sequence structure was discussed to a provide broad understanding of the thermal properties of polymers. Through DSC analysis, we obtained an intriguing conclusion that the polymers with almost identical sequence structure but different alkyl substituents present contrasting differences in thermal properties. The reason is that DPE-alkyls have an almost uniform electronic effect for the polymerization properties, such as sequence or kinetics, but their different steric effects or flexibility plays a decisive role in the physical properties, such as Tg.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01491.

■

Article

The original data of all the in situ 1H NMR experiments and corresponding conversion, statistic sequence structure and semilogarithmic curves; SEC curves of copolymerizations monitoring by in situ 1H NMR; SEC curves of P[(DPE-alkyl)-St]s in Table 3; The flowchart of KMC simulation and the computational process of kDS (PDF)

AUTHOR INFORMATION

Corresponding Author

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

Hongwei Ma: 0000-0003-3897-9907 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21674017 and U1508204) and the Fundamental Research Funds for the Central Universities (DUT18LAB12). K


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M


Sequence Features of Sequence-Controlled Polymers Synthesized by

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