Synchronous Regulation of Periodicity and Monomer Sequence

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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Synchronous Regulation of Periodicity and Monomer Sequence during Living Anionic Copolymerization of Styrene and Dimethyl-[4(1-phenylvinyl)phenyl]silane (DPE-SiH) Wei Huang, Hongwei Ma,* Li Han, Pibo Liu, Lincan Yang, Heyu Shen, Xinyu Hao, 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, Liaoning 116024, China S Supporting Information *

ABSTRACT: To investigate the specific strategy for sequence regulation with living anionic polymerization, three sequencedefined polymers with similar number of SiH functional groups per chain but totally different periodicities which distributed as gradient, tandem, and symmetrical structures were synthesized. Through different feeding methods and coupling reaction after polymerization during the living anionic copolymerization of styrene (St) and dimethyl-[4-(1-phenylvinyl)phenyl]silane (DPESiH), the synchronous regulation of monomer sequence and functionalized block periodicity in polymer chains was successfully achieved. The monomer sequence distributions in these three structures were confirmed by in situ 1H NMR. As the monomer sequence and block periodicity are synchronously regulated, this effort could promote the further development of the sequence regulation and the design of novel functionalized polymers with living anionic polymerization method, and the strategy we investigated also can improve the novel polymer designs. Then, these gradient, tandem, and symmetrical polymers were applied as backbones for the synthesis of corresponding bottlebrush polymers. The polymeric branches (the chain-end alkynylfunctionalized polystyrenes, PS-yne) were conveniently and efficiently grafted onto the backbones via hydrosilylation, with all conversions above 97%. The basic solution and thermal properties of the bottlebrush polymers and their corresponding backbones were investigated, and the results indicated that the different sequence structures and block periodicities of the polymers display remarkable influences.

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polymer chemistry, which also has been regarded as the “holy grail”.11 Thus far, many sequence-controlled polymerization strategies to effectively control the monomer sequence distributions in polymer chains during the polymerization process have been actualized or are under development. Among the recent strategies for realizing sequence control in recent reports, step-growth polymerization and chain-growth polymerization have been commonly applied for the synthesis of sequence-controlled polymers due to their wide suitability.1,18−20 Additionally, for step-growth polymerization, poor control of the molecular weight, molecular weight distribution, and

INTRODUCTION Sequence-controlled macromolecules in which the monomer units are dispersed in an orderly manner along the chain play a vital role in nature, such as biomacromolecules.1−4 Because of their specific features, imitating biomacromolecules, polymer scientists have developed novel applications in information encoding materials with the synthetically sequence-controlled polymers; all the progress also explored advanced perspectives for sequence-controlled polymers.5−10 However, the precise synthesis of these highly sequence-defined natural polymers is an unreached goal for polymer chemistry, but the concept that structures and properties of synthetic polymers depend on not only monomer compositions but also their sequence distributions can be clearly convinced.1,2,11−17 The control of monomer sequence distributions in synthetic polymers has become one of the most emerging and challenging focus in © XXXX American Chemical Society

Received: March 29, 2018 Revised: April 30, 2018

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DOI: 10.1021/acs.macromol.8b00666 Macromolecules XXXX, XXX, XXX−XXX

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topological architecture restricts its applications in polymeric constructions. Controlled chain-growth polymerizations, which involved reversible-deactivation radical polymerization (RDRP) and living anionic polymerization (LAP), allow the synthesis of polymers with well-defined molecular weights and topological structures and endow polymer chains with good control of monomer sequence. RDRP methods have been widely employed to synthesize well-defined functionalized polymers, and many significant efforts have been explored to achieve deep control of monomer sequence in polymer chains; Lutz2,4 and Satoh21 et al. used RDRP methods to develop a versatile strategy for sequence control and synthesized various sequencecontrolled polymers. In addition to traditional one-pot polymerization to control monomer sequence, the strategies were also developed for the synthesis of controlled periodic structures22−26 through sequential monomer feeding and RDRP methods.16,27,28 The question of how the control of the synchronous structures of polymers (such as monomer sequence, period of repeating units, and so on) impacts the polymer properties is an exciting issue for polymer synthesis and applications. LAP has been displayed outstanding control in polymeric architectures; how to regulate the monomer sequence becomes a significant tendency for LAP. Only the full-scale control for polymers synthesized with LAP method is achieved; it can open up whole new perspectives for LAP, while the sequence control is the missing link.29−31 LAP has been recently utilized to synthesize a variety of sequence-controlled polymers since its special living characteristics which can construct well-defined polymeric architectures.32,33 Because of their steric hindrance, 1,1-diphenylethylene (DPE) and its derivatives cannot be homopolymerized in LAP,34 and this distinct feature makes it possible to control the sequence distribution of functional groups in polymer chains. Based on this point, a variety of functional copolymer with controlled sequence distribution have been prepared.35−40 Additionally, LAP can not only control monomer sequence in the same manner as RDRP methods but also easily combine coupling reaction after polymerization to design and synthesize more specific polymer architectures, such as cyclic, star, and grafted structures.31,41−44 Therefore, LAP has extensive applications in the field of sequence-controlled copolymers and complex topological polymers for achieving synchronous control of both monomer sequence and periodicity. In this work, we synthesize three sequence-defined SiH functional polymers with different periodic structures through one-pot living anionic copolymerization of styrene and dimethyl(4-(1-phenylvinyl)phenyl)silane (DPE-SiH), two-step equivalent feeding in polymerization, and coupling reaction after polymerization. Thus, a kind of periodic-controlled sequence-defined polymer can be facilely obtained. Herein, the detailed kinetic characteristics of copolymerization and the specific monomer sequence distributions in the copolymer chains were investigated by in situ 1H NMR. Additionally, the incorporation of SiH functional groups in the three polymer chains endows these copolymers with the possibility of further postfunctionalization. Then, chain-end alkynyl-functionalized polystyrenes (PS-yne) were efficiently grafted onto these three polymers through hydrosilylation, and sequence-defined bottlebrush polymers with diverse periodic structures were synthesized.

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EXPERIMENTAL SECTION

Synthesis of Monomers. Dimethyl-[4-(1-phenylvinyl)phenyl] silane (DPE-SiH) and trimethyl((4-(1-phenylvinyl)phenyl)ethynyl) silane (DPE-yne) were prepared as in the previous work.39,45 The synthetic routes of the monomers are shown in Scheme S1. Synthesis of Periodic-Controlled Sequence-Defined Si−H Functionalized Polymers. In this work, three sequence-defined polymers (G-M, T-M, and S-M) with similar number of Si−H functional groups per chain but totally with different periodicities were synthesized; the synthetic routes are as follows. Living anionic copolymerizations were carried out in a glovebox under high vacuum conditions. The initial molar feed ratio of styrene/DPE-SiH was set as 3, and the specific synthetic routes were as follows. G-M was synthesized via one-pot polymerization. First, DPE-SiH (1.60 × 10−3 mol, 0.382 g) was dissolved in dry benzene and added to polyplant. sec-BuLi (0.36 mol/L, 0.445 mL) as initiator was then injected into the solution of DPE-SiH. After 30 min, the polymeric mixture showed the characteristic red color, which indicated that all 1,1-diphenylalkyllithium was successfully initiated. Then, styrene (4.80 × 10−3 mol, 0.552 mL) was added quickly and stirred at 25 °C for 20 h. The concentration of comonomers in benzene was 10 wt %. The synthetic route of T-M involves two-step copolymerization of styrene and DPESiH through two-step equivalent feeding, and the details of the polymerization were the same as those for G-M. sec-BuLi (0.360 mol/ L, 0.540 mL) and DPE-SiH (9.56 × 10−4 mol, 0.228 g) were distilled into device. After 30 min, styrene (2.88 × 10−3 mol, 0.331 mL) was added and stirred at 25 °C for 12 h. Afterward, DPE-SiH (9.56 × 10−4 mol, 0.228 g) and styrene (2.88 × 10−3 mol, 0.331 mL) were added sequentially into the polymeric solutions and mixed thoroughly, and the copolymerization was allowed to continue for 12 h at 25 °C. The synthetic route of S-M was as follows: First, sec-BuLi (0.360 mol/L, 0.890 mL) and DPE-SiH (1.60 × 10−3 mol, 0.382 g) were added to the device and reacted for 30 min. Styrene (4.80 × 10−3 mol, 0.552 mL) was added and stirred at 25 °C for 12 h; dichlorodimethylsilane (0.554 g, 3.17% in benzene) was distilled into polymeric solutions and stirred sequentially for 48 h. Subsequently, all copolymerization was terminated with degassed isopropanol. The polymers were precipitated into excess methanol. Residual monomers of DPE-SiH in polymers were removed by flash column chromatography (n-hexane), and the polymers were washed by ethyl acetate. The products were precipitated in excess methanol and dried in a vacuum oven to constant weight at room temperature. Synthesis of PS-yne. The process for the copolymerization of PSyne is shown in Scheme S2 and described as follows: Dry benzene (60 mL) was used as the solvent, sec-BuLi (0.36 mol/L 5.60 mL) and styrene (5.76 × 10−2 mol, 6.62 mL) were added into benzene, and the color changed to red; the system was stirred at 25 °C for 12 h, then DPE-yne (3.0 × 10−3 mol, 0.829 g) was added to the polymeric solution, and the color changed into dark red immediately. The endcapping reaction was allowed to react for another 12 h at 25 °C. It was finally terminated with degassed isopropanol. Then, the product was poured into a large amount of methanol to precipitate. Residual monomer of DPE-yne in the polymer was removed by Soxhlet extraction using methanol. Deprotection of the polymer was implemented by KOH/CH3OH in dry THF under an argon atmosphere. The solution was stirred at 25 °C for 2 h. After filtration to remove the residual solid, the solution was concentrated by rotary evaporator, the final product was dissolved in moderate ethyl acetate and then precipitated in excess methanol, and PS-yne was dried in a vacuum oven to constant weight. Synthesis of Periodic-Controlled Sequence-Defined Bottlebrush Polymers. Different dosages of branches were added to investigate the effect on efficiency of hydrosilylation, and the molar ratios of alkynyl/SiH were used as 1.1 for G-M, 1.5 for T-M, and 1.2 for S-M. The synthetic routes for the periodic-controlled sequencedefined bottlebrush polymers were as follows. The polymer backbone (G-M: 0.156 g; T-M: 0.208 g; S-M: 0.164 g) and branch (PS-yne: 1.23, 2.30, and 1.42 g for these corresponding main chains) dissolved in toluene were added into flask and then stirred immediately. Karstedt’s catalyst (SiH/Pt = 40/1) was added sequentially. The reaction was B

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Scheme 1. Synthetic Routes for Three Periodic-Controlled Sequence-Defined Gradient, Tandem, and Symmetrical Copolymers with Same Equivalents of St and DPE-SiH

allowed to react for 2 h at 25 °C. Then, the solvent was removed using a rotary evaporator, and Karstedt’s catalyst was removed by flash column chromatography (ethyl acetate). The bottlebrush polymers were isolated by fractionation using a combination of toluene and methanol. The fractionated bottlebrush polymers were purified by precipitation in methanol three times and dried in a vacuum oven to a constant weight.

distribution of DPE-SiH units in the chains were carefully investigated with varying monomer feed ratios. While the feed ratio of styrene/DPE-SiH was set as 3, the sequence distribution was determined as gradient structure (composition of DPE-SiH gradually decreased with chain growth) via the timing-sample method.38 As is well-known, LAP exhibits special features due to its living characteristics.32,33 Through sequential feeding or coupling reaction, many architectures that are totally different from those generated in traditional one-pot propagation procedures can be easily formed. These methods have been widely applied over the past decades to prepare triblocks (such as SBS), topological architectures (such as 4-arm stars and DendriMacs), and so on.47,51−54 However, the sequence features of polymer chains have never been considered in these efforts. The combination of monomer feeding manners and sequence control can definitely inspire some novel structures, especially the application of coupling reaction after sequence-controlled synthesis. Thus, in this research, sequential feeding and coupling methods were introduced to sequencecontrolled synthesis during the living anionic copolymerization of St and DPE-SiH. The monomer sequence and functionalized block periodicity are synchronously regulated in order to investigate the unique characteristics for synthesis of sequencedefined polymers with LAP method. Furthermore, even simple coupling or sequential feeding during LAP can obtain a variety of novel polymers, and these structures can take us a new insight into studies on novel application or polymer properties. As shown in Scheme 1, varying the monomer feeding sequence during the living anionic copolymerization of DPESiH and styrene, three sequence-defined copolymers (G-M, TM, and S-M) with similar number of Si−H functional groups per chain but totally different periodicities were synthesized. All

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RESULTS AND DISCUSSION Synthesis of Periodic-Controlled Sequence-Defined Si−H-Functionalized Polymers. With the research on sequence-controlled polymers developing, how to precisely control monomer sequence distributions in synthetic polymers has become one of core focuses in polymer chemistry for potential glamour in further applications. Various synthetic strategies for regulating the monomer sequence distributions of polymer chains have been developed and reported; therein the sequence distribution can be generally controlled by sequential monomers feeding, difference in monomer reactivity, and so on, which has been mentioned in the Introduction.2,16,19−21 Among these strategies, sequential monomer feeding can easily regulate the monomer sequence distribution during chain growth. Lutz and co-workers investigated the possibility of controlling the monomer sequence distribution during RDPP through the sequential addition of various functional comonomers.2,16,27 In living anionic polymerization (LAP), 1,1-diphenylethylene (DPE) and its derivatives have been developed as a distinctive field for the synthesis of in-chain or chain-end functionalized polymers because of their unique characteristics during polymerization.30,46−48 In our previous work, we focused on the synthesis of sequence-controlled functionalized polymers with DPE derivatives via LAP.35−38,45,49,50 In particular, the composition and sequence C

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SiH, which clearly demonstrates that the sharp signals of the monomers disappear, and simultaneously broad polymer peaks become more intense as the reaction continues. Especially the zoom-in shows the intensity of the vinyl signals decreasing from both styrene and DPE-SiH monomers, which indicated the consumption of monomers and the formation of the copolymer. Based on the in situ 1H NMR calculation results, the incorporation of both monomers in the polymer chains can be real-time determined during the whole copolymerization process. This is visualized in Figure S2, and the conversions of DPE-SiH and styrene vs reaction time are also plotted. As seen, both monomers were almost totally consumed in the copolymerization. At first, DPE-SiH was incorporated slightly faster than styrene, indicated by the more rapid consumption of monomer concentration in the reaction mixture. But DPE-SiH cannot be homopolymerized in LAP due to steric hindrance, so DPE-SiH and styrene were incorporated into the chains together. Then, DPE-SiH was almost completely consumed, and the further growth of the polymer chains was mainly due to the homopolymerization of styrene, which can also be confirmed by the obvious transformation in kinetic curves of St, shown in Figure 2. The kinetic curves of chain propagation were obtained from ln[M]0/[M] vs reaction time and are shown in Figure 2a. The apparent kinetic constants of KS and KD were calculated from the slopes of the corresponding linear kinetic curves during the initial stage of copolymerization, as shown in Figure 2a. The values of KS and KD can be obtained from the curve fitting, while the accurate reactivity ratio of styrene (rSt) was recalculated and confirmed as 0.19 from iterative computations of the in situ 1H NMR data.39 When DPE-SiH was almost entirely consumed, styrene was individually incorporated into the polymer chains, and the apparent kinetic constant of KS was also calculated from the curve fitting, as shown in Figure 2a. Based on the kinetic results obtained from the in situ 1H NMR monitoring, this copolymerization seems that strong alternating propagation underwent first and then styrene homopolymerized with the depletion of DPE-SiH, and the sequence distribution can be deduced as a tapered (DPE-SiH-alt-St)-bSt block-like structure. Additionally, when the feed ratio of DPE-SiH/St changed, the sequence distribution of DPE-SiH in chain showed a gradient transition from alternating to the tapered structure as described in our previous work.38,49 Thus, this structure has been previously named as “gradient”, and in this work the idiomatic name was also used. Additionally, through the in situ 1H NMR method, an obvious transformation was clearly observed over the entire propagation process and revised our previous data obtained from timing sample method.38 Based on the conclusions of T-M precursor, the prime 10 h and the final phase of the copolymerization for synthesis of GM were also real-time monitored by in situ 1H NMR. The monomer sequence of G-M also can be deduced based on the corresponding linear kinetic curves which are shown in Figure S3. From the initiation to 10 h of copolymerization, DPE-SiH and styrene were successively incorporated into the polymer chains until DPE-SiH was almost fully consumed. Then, styrene begins to self-propagate, and thus, the intersection point of the two linear kinetic curves represents the transition in the whole copolymerization process. Finally, as conclusions of all in situ 1H NMR, the real-time numbers of both monomers incorporated in the copolymer

anionic polymerizations were carried out in a glovebox, and the materials were purified under high vacuum conditions as described in our previous work.38,45 G-M was prepared via onepot feeding, and the synthetic route was similar to the previous work38 and is described in the Experimental Section; T-M and S-M were synthesized through two-step equivalent feeding (for T-M) and coupling reaction after half feeding (for S-M). Therefore, through simple control of the feeding sequence or coupling, different periodic blocks with a gradient sequence distribution can be easily prepared. For S-M in particular, two similarly gradient blocks were linked together with a “tail-totail” manner, while a “tail-to-head” linkage was observed for TM, so the periodicities of these copolymers were named as “symmetrical” and “tandem” structure in this work, respectively. The number of functional Si−H groups per chain for these three copolymers was controlled approximately 10, and their structures were carefully characterized by 1H NMR and SEC. In the 1H NMR spectra, all characteristic peaks can be attributed to protons in the copolymer, and the coupling efficiency of S-M is 96.4%, which can be calculated from the SEC curves (Figure S1). The characteristics of each copolymer and corresponding precursors are listed in Table 1. Table 1. Results of Three Periodically Sequence-Defined Polymersa sample

Mnb × 10−3 (g/mol)

Đb

NS/NDc

NDc

NSc

G-M T-M samplingd T-M S-M samplingd S-M

6.3 3.0 5.8 2.8 5.4

1.28 1.24 1.25 1.26 1.32

3.30 3.59 3.05 3.56 3.28

10.8 4.9 10.4 4.6 9.3

35.7 17.6 31.7 16.4 30.5

The reaction was carried out in benzene at 25 °C under high vacuum conditions; the monomer molar feed ratio [Ms]0/[MD]0 = 3:1. b Determined by SEC with tetra-detectors. cThe average numbers of styrene, DPE-SiH, and NS/ND in each chain were calculated from the 1 H NMR spectra and SEC curves. dT-M sampling and S-M sampling were the precursors of T-M and S-M. The coupling efficiency of S-M is 96.4%, calculated by Mn,S‑M/(2Mn,S‑M sampling). a

Although we determined the sequence distribution during the copolymerization of St and DPE-SiH by the timing-sample method ([St]0/[DPE-SiH]0 = 3), in pursuit of investigating more detailed sequence, the precursors for T-M and S-M were real-time monitored by the in situ 1H NMR method, which has been widely used to study the kinetics of chain growth.55−57 The reaction mixture was prepared inside an argon-filled glovebox, and the anionic copolymerization was carried out in a stopcock-sealed NMR tube. In order to avoid any unexpected derivations, the reaction mixture was quickly and thoroughly mixed together in a glass bottle in glovebox, and then it was injected into a stopcock-sealed NMR tube. Prior to the initiation of the copolymerization, a 1H NMR spectrum of deuterated benzene (C6D6) was recorded to calibrate the NMR equipment. Subsequently, the reaction was initiated by the addition of sec-BuLi and the copolymerization was monitored by in situ 1H NMR spectroscopy over 10 h. All spectra were recorded at 400 MHz. From the synthetic routes for T-M and S-M, in order to investigate the monomer sequence distributions of them, so we also carried out in situ 1H NMR monitoring of the T-M precursor. Figure 1 shows a diagram of in situ 1H NMR spectra measured during the copolymerization of styrene and DPED

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Figure 1. In situ 1H NMR copolymerization kinetics. Bottom: overlay of spectra of in situ 1H NMR kinetics study. Top: zoom-in showing the consumption of vinyl signals of monomers.

block periods under control were successfully synthesized, while a similar number of Si−H functional groups in chain distributed as gradient, tandem, and symmetrical structure. Synthesis of Periodic-Controlled Sequence-Defined Bottlebrush Polymers. Synchronous control over the polymer sequence and repeating period is significant to both structure−property relationships and functional materials design. In order to achieve these goals, we developed a synthetic route that enables the facile manipulation of the distribution of grafts in polymers via LAP and hydrosilylation, so that densely grafted bottlebrush polymers with gradient, tandem, and symmetrical sequence structure can be successfully synthesized. Hydrosilylation was utilized to accomplish the coupling reaction of the backbones and branches due to its high efficiency and mild reaction conditions. Unlike in our previous works,38,45 alkynyl and hydrosilyl groups instead of vinyl groups were used for hydrosilylation in this work because alkynyl

chains were calculated, as shown in Figure 2b and Figure S3b, which illustrates the definite monomer sequence distributions of the T-M precursor and G-M. In terms of the characteristics of the three periodically sequence-defined copolymers and the corresponding precursors, as shown in Table 1, T-M and S-M had almost twice the molecular weight of their corresponding precursors. Furthermore, T-M and S-M were synthesized via two-step feeding and coupling reaction, respectively; two-step feeding strategy can ensure the propagation of living chains in the second copolymerization was the same as the T-M precursor, and the sequence structure of S-M can be deduced through coupling reaction and its efficiency, which was 96.4%. Therefore, on the basis of the above results and conclusions, we can confirm that the monomer sequence distributions of the three periodically sequence-defined copolymers are consistent with the experimental design, as shown in Scheme 1. As discussed above, three in-chain Si−H-functionalized copolymers with both monomer sequence distributions and E

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Figure 2. Kinetic curves of copolymerization of DPE-SiH and styrene for T-M precursor: (a) the kinetics of propagation for DPE-SiH and styrene; (b) the monomer sequence distributions in the polymer chains.

Scheme 2. Synthetic Routes for Periodic-Controlled Sequence-Defined Bottlebrush Polymers

groups have higher reaction activities than vinyl groups. Thus, terminal alkynyl-functionalized polystyrene (PS-yne) was used as the branches, and three sequence-defined copolymers (G-M, T-M, and S-M) bearing similar number of Si−H functional groups per chain but totally different periodicities were used as the backbones to synthesize the corresponding bottlebrush

polymers, in which the branches were grafted onto with gradient, tandem, and symmetrical sequences (the products were Grad-g-(PS)10, Tand-g-(PS)10, and Symm-g-(PS)10, respectively). The synthetic routes for the bottlebrush polymers are shown in Scheme 2. F

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Figure 3. MALDI-TOF MS of polystyrene precursor and PS-yne.

Figure 4. FT-IR spectra and SEC curves recorded during hydrosilylation of T-M with branches.

We monitored the coupling reaction between T-M and PSyne (with alkynyl/SiH = 1.5 equiv) in toluene over 2 h. The conditions, including the polymer and catalyst concentrations, were identical to the previous literature.38,45 The reaction mixtures were sampled during the coupling process, and the samples were characterized by FT-IR and SEC to confirm the efficiency of hydrosilylation between the alkynyl and hydrosilyl groups. The FT-IR spectra of the samples taken during T-M coupling with PS-yne are presented in Figure 4a. A significant decrease in the intensity of the infrared band at ∼2160 cm−1 due to the Si−H bond stretching vibrations (signals highlighted in light green in Figure 4a) in all the final FT-IR spectra with respect to the starting one is the most remarkable characteristic revealed upon spectra examination. The SEC traces indicated the continuing depletion of branches as well as the concomitant growth of the bottlebrush polymers within 5 min, as shown in Figure 4b. From FT-IR and SEC analyses, the fact that all

First, PS-yne was synthesized through end-capping and deprotection reaction after the homopolymerization of styrene, as shown in Scheme S2. The structure of PS-yne was determined through the characteristics of 1H NMR and SEC (Figure S4), and the end-capping efficiency of PS-yne was 97.8%, which can be calculated from the corresponding 1H NMR spectrum (Figure S4) according to eq S1. MALDI-TOF mass spectrometry of the polystyrene precursor and PS-yne further verified the definite structure of PS-yne, and the spectra are shown in Figure 3. As expected, the representative peak of m/z at 4551.16 corresponding to a 41-mer of PS-yne was in good agreement with the calculated mass ([M + Na]+, calcd: 4551.62), and a regular interval 104.06 Da was observed in the MALDI-TOF MS spectra, which can be attributed to the difference in the numbers of styrene units in the chains. All the evidence illustrates the high chain-end functionality and the definite structure of PS-yne. G

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Macromolecules Table 2. Characteristic of Periodic-Controlled Sequence-Defined Bottlebrush Polymersa sample PS-yne G-M Grad-g-(PS)10 T-M Tand-g-(PS)10 S-M Symm-g-(PS)10

NS/NDb 3.30 3.05 3.28

Mnc × 10−3 (g/mol)

Đc

4.2 6.3 50.2 5.8 48.8 5.4 44.1

1.08 1.28 1.30 1.25 1.27 1.30 1.33

Fd

fe

Ef (%)

g′g

10.8 10.5

97.2

0.51

10.2

98.1

0.40

9.2

98.9

0.45

10.4 9.3

Rhc

Tgh (°C)

1.7 2.3 4.6 2.2 4.4 1.8 3.9

118 98 124 89 104 100

a

The reaction was carried out in toluene under high vacuum conditions. bMonomer unit ratio of St/DPE-SiH in copolymer chains, calculated by 1H NMR. cDetermined by SEC with tetra-detectors. dNumber of SiH groups in the backbone, calculated by 1H NMR. eNumber of branches grafted onto backbones which was calculated as f = (Mn,C − Mn,M)/Mn,PS‑yne. fE (%) = ( f/F) × 100%. gBranching factor g′ = [η]C/[η]L; [η]C was investigated by using an Ubbelohde viscometer, and [η]L = K(Mn,C)α (K = 0.012 and α = 0.71 for linear polystyrene analogues at 30 °C in toluene). hThe glass transition temperature, characterized by DSC.

Figure 5. SEC curves of bottlebrush polymers, corresponding backbones, and branch before and after fractionation.

branches due to excess feeding of PS-yne. The 1H NMR spectra and assignment of the corresponding peaks are shown in Figure 6. The peaks of Si−H at δ 4.5−4.3 ppm are observed in the 1H NMR spectra of the sequence-defined backbones, but these peaks disappeared in the spectra of the corresponding bottlebrush polymers. This indicated that all of the Si−H groups effectively reacted with PS-yne in the grafting-onto reactions. The molecular weights and distributions of the bottlebrush polymers were obtained by SEC analyses. Calculated from the molecular weights of the polymers shown in Table 2, the results demonstrated that the number of branches (f in Table 2 and f = (Mn,C − Mn,M)/Mn,PS‑yne) grafted in bottlebrush polymers was similar to the number of Si−H groups in corresponding backbones (F in Table 2). So the conversions of the graftingonto reaction, as calculated by the ratio of the branches grafted onto the backbone and the number of SiH groups in the corresponding backbone, were all above 97%, and the highest efficiency reached 98.9%. These results indicated that the branches were effectively grafted onto three periodically

grafting-onto reactions were essentially completed within 2 h and produced densely grafted polymers with specific sequence structure could be confirmed. Subsequently, and all grafting-onto reactions were accomplished under the same conditions in 2 h except different feed ratios of PS-yne for G-M and S-M (G-M: alkynyl/SiH = 1.1; SM: alkynyl/SiH = 1.2, and the specific experimental procedure is described in the Experimental Section) to investigate how the feed ratios of branches affect the grafting reactions. After fractionation, three bottlebrush polymers were obtained and characterized by 1H NMR and SEC. The characteristics of the three backbones and bottlebrush polymers are presented in Table 2. Meanwhile, the results in Table 2 indicated that the amount of excess branches cannot affect the efficiency of hydrosilylation between polymers. The SEC curves of the backbones, the branch, and the bottlebrush polymers are presented in Figure 5. The graftingonto reaction proceeded efficiently according to the SEC traces of the bottlebrush polymers. The low-molecular-weight peak of the unclassified polymer SEC was attributed to the residual H

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Figure 6. 1H NMR spectra of bottlebrush polymers and their corresponding backbones.

sequence-defined backbone through the hydrosilylation of alkynyl and hydrosilyl groups. The 1H NMR and SEC results illustrated that the hydrosilylation proceeds with high efficiency, which ensured that the branches were efficiently grafted onto the three backbones with the arrangement of SiH groups in the main chains. Meanwhile, the narrow dispersity for the bottlebrush polymers remained because high efficient hydrosilylation ensured the structural homogeneity, and few defective structures existed. Thus, based on the hydrosilylation of hydrosilyl and alkynyl groups, a convenient and efficient “grafting onto” method for the synthesis of sequence-defined bottlebrush polymers was successfully developed, in which the branched points can be easily controlled by regulating the sequence structure of the backbones. Moreover, the bottlebrush polymers exhibited distinct properties, which were uncharacteristic of the linear analogues, due to the polymer backbones bearing densely grafted branches. In order to investigate the relationships of “structure−property”, the hydrodynamic radii (Rh), branching factors (g′), and thermal properties (mainly the glass transition temperatures, Tg) were characterized by SEC with tetradetectors, Ubbelohde viscometer, and DSC, respectively, as presented in Table 2 and Figure 7 (DSC curves). The remarkable changes in Rh between the three periodically sequence-defined bottlebrush polymers and corresponding backbones indicated that the branches were successfully grafted onto the backbone. According to the results of the SEC analyses, the Rh of the bottlebrush polymers were significantly different. Comparing Rh among the bottlebrush polymers with similar brush number and molecular weights, Grad-g-(PS)10 had the highest Rh of 4.6 nm, and Symm-g-(PS)10 had the lowest Rh of 3.9 nm, while the Rh of Tand-g-(PS)10 was in

Figure 7. DSC curves of bottlebrush polymers and their corresponding backbones.

between. These results indicated that the diverse sequence arrangement of the branches may encourage the bottlebrush polymers to adopt different conformations in dilute solution. However, Symm-g-(PS)10 had the longest polystyrene block in the backbone; when the branches are closely grafted onto the backbone, the space between the branches may lessen repulsion, and the backbone can be more flexible. Therefore, Symm-g-(PS)10 possibly adopted a smaller conformation in solution. The above results showed that the solution properties of bottlebrush polymers with similar grafts could be affected by regulating the arrangement and branched spacing of the branches. I

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Macromolecules

backbones were also investigated. The results demonstrated that the gradient structure Grad-g-(PS)10 had the highest Rh of 4.6 nm among the three bottlebrushes, indicating that Grad-g(PS)10 exhibited a more extended conformation. Compared with the corresponding linear polystyrene analogues, the glass transition temperatures (Tg) of the backbones in the gradient and tandem structures showed remarkable increases, possibly due to the incorporation of DPE-SiH units and the bottlebrush polymers, except for the symmetrical structure, which showed decreases in the Tg as a result of their densely grafted structure. The above results indicated that the different sequence structures and block periodicities of the polymers display remarkable influences on their basic properties. Further related research will be continued in the future.

As characterized by DSC, the incorporation of DPE-SiH units remarkably increased the glass transition temperature of the three sequence-defined backbones with different periodic structures, especially for G-M and T-M; their Tg increased to 118 and 124 °C, respectively, while the Tg of S-M was 104 °C, which increased slightly when compared to the other two backbones. These three backbones exhibited similar compositions and molecular weights; the only differences between them were their specific sequence and block periodicities. For the coupling method, the S-M exhibited longer polystyrene block in the middle, and most DPE-SiH units distributed in two end sections. The polystyrene block may make the polymer chain more flexible than these chain segments with DPE-SiH units; thus, the Tg of S-M was slightly lower than others. Compared with their corresponding backbones, remarkable decreases in the Tg of the bottlebrush polymers were observed obviously because of their densely grafted structure. But the decreases in the Tg of the three backbones and bottlebrush polymers were not consistent due to the spacing of Si−H functional groups in the corresponding backbones, i.e., the interval number of styrene units between DPE-SiH monomers, and the Tg for bottlebrush polymers displayed an opposite tendency. Because of the long polystyrene block in the backbones and the densely grafted polystyrene branches, Symm-g-(PS)10 was regarded as PS and compared with linear polystyrene analogues to discuss (the Tg of linear PS was about 98 °C, which could be calculated from the equation in the report48). For the gradient and tandem distributions, the Tg of the corresponding bottlebrush polymers decreased to 98 and 89 °C, respectively. Such decrease could be attributed to the abundant chain ends of bottlebrush polymers because polystyrene branches were grafted onto homogeneous backbones. This was confirmed by the characteristic of branching factors g′, which represents the degree of branching (data and calculation method are listed in Table 2), as Tand-g-(PS)10 exhibited the lowest g′ of 0.40 and the corresponding lowest Tg of 89 °C.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00666. Experimental details, related characterization results that were inexistent in the text but were referred to including 1 H NMR, GPC, MALDI-TOF, FT-IR, DSC, and related calculation methods from 1NMR and absorption peak assignment (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

Hongwei Ma: 0000-0003-3897-9907 Notes

The authors declare no competing financial interest.

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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).

CONCLUSIONS In summary, through sequential feeding and coupling reaction after polymerization, we successfully achieved the synchronous regulation of monomer sequence and functionalized block periodicity during the living anionic copolymerization of St and DPE-SiH. The three resultant polymers exhibited similar molecular weights and DPs for St and DPE-SiH in the chains, but totally different monomer sequence and periodicities which distributed as gradient, tandem, and symmetrical structure were obtained. In addition, the monomer sequence distributions in polymer chains were confirmed by in situ 1H NMR. In this work the monomer sequence and functionalized block periodicity are synchronously regulated, and the strategy for sequence regulation with LAP method was successfully investigated. We believe this result can open a new door for polymer chemists to regain a sense of novel polymer design. Then, these gradient, tandem, and symmetrical polymers were applied as backbones for the synthesis of corresponding bottlebrush polymers. The branches were conveniently and efficiently grafted onto the backbones via hydrosilylation of hydrosilys and alkynyls, with all conversions above 97%. The results showed that this method can ensure both effective control of the branch spacing and high efficiency during the synthesis of bottlebrush polymers with gradient, tandem, and symmetrical branch distributions. Basic solution and thermal properties of the three bottlebrush polymers and corresponding

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