Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
From Stereochemically Tunable Homopolymers to Stereomultiblock Copolymers: Lewis Base Regulates Stereochemistry in the Coordination Polymerization of 2‑Vinylpyridine Chao Yan, Tie-Qi Xu,* and Xiao-Bing Lu State Key Laboratory of Fine Chemicals, College of Chemistry, Dalian University of Technology, Dalian 116024, P. R. China S Supporting Information *
ABSTRACT: Excellent isoselectivity (mmmm > 99%) and high activity (TOF > 4000 h−1) were achieved for the first time in the polymerization of the polar 2-vinylpyridine by using the simple lutetium-based catalysts, Lu(CH2SiMe3)3(THF)x(Py)2−x (x = 0, 1, 2). The isoselectivity (mm) of the polymer could be varied between 37% and 99% through simply altering the quantity of Lewis base (tetrahydrofuran, 1,4-diazabicyclo[2.2.2]octane, 3-bromopyridine, and pyridine) added. Moreover, a novel method for preparing isotactic−atactic stereomultiblock poly(2vinylpyridine)s was developed by the addition and removal of THF during polymerization. The polymerization process including stereoselective control mechanism was deduced by DFT calculations.
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attention, though the first attempt appeared in 1960 by Natta et al. using magnesium amide-catalyzed anionic polymerization. Although an isoselectivity (mm) of 80% was obtained, atactic polymer impurities were also detected.31,32 In 2011, Mashima reported the preparation of end-functional low molecular weight isotactic P2VP (Mn ∼ 3000 g/mol) through the C−H bond activation of N-heterocycles using an yttrium diamide catalyst.33 Furthermore, Reiger and co-workers obtained an isorich polymer with an isotacticity (mm) of 61% using an yttrium catalyst and successfully increased the isotacticity of the polymer to 87% by the introduction of sterically hindered substituent ligands.34,35 Recently, our group achieved the highly isoselective polymerization of 2-VP using a tetradentate phenol ether yttrium catalyst, and an mm (mmmm) value of 96% (88%) was attained.36 Although the optimized isoselectivity of such yttrium catalysts is close to 90%, there are a number of issues with such processes, including the occurrence of nonliving polymerization reactions, low catalyst initiation efficiencies, poor control of the polymer isotacticity, and the inability to prepare stereoblock polymers. Also, in order to achieve an acceptable degree of isotacticity with these catalysts, the use of ancillary ligands is necessary. As a consequence, the development of a ligand-free, living, and stereoselective 2-VP polymerization system remains a challenging task. Herein, we report a highly active and stereoselective catalyst based on simple lutetium alkyl complex for the living polymerization of 2-VP, affording P2VP with mmmm > 99% isoselectivity. Added Lewis bases can precisely control the
INTRODUCTION Since the discovery of the isoselective polymerization of propene by Natta’s research group in the 1950s,1 the stereoselective polymerization of vinyl monomers has received widespread attention. Indeed, the stereoselective polymerization of vinyl monomers yields a series of polymers with rich stereochemistry such as isotactic, syndiotactic, and isotactic− atactic block polymers. The stereochemistry of polymers strongly influences their properties. For example, isotactic polypropylene, consisting of a regular arrangement of stereocenters, is a crystalline thermoplastic with a melting point of ∼165 °C, whereas atactic (stereorandom) polypropylene is an amorphous gum elastomer.2 Polypropylene consisting of blocks of atactic and isotactic stereosequences is a rubbery material with properties of a thermoplastic elastomer.2 Various catalysts have been employed in the stereoselective polymerization of nonpolar olefin monomers to obtain polymers exhibiting highly tactic structures, and several elegant approaches including ligand oscillation,2 chain shuttling,3−7 and degenerative methyl and transfer8−11 have been developed to create new classes of stereoblock and multiblock copolymers. Recently, research focus has shifted gradually toward the stereoselective polymerization of heteroatom-containing polar vinyl monomers.12−19 2-Vinylpyridine (2-VP) is a polar vinyl monomer containing a nitrogen heteroatom. Its homopolymers and copolymer have wide applications in many fields, including photochemistry, ion exchange resins, and self-assembling materials.20−29 Poly(2vinylpyridine) (P2VP) can be obtained through various methods, including free radical polymerization,30 anionic polymerization,31,32 coordination polymerization,33−39 and Lewis pair polymerization.40 Unfortunately, the synthesis of P2VP through stereoselective polymerization has received little © XXXX American Chemical Society
Received: January 18, 2018 Revised: February 22, 2018
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DOI: 10.1021/acs.macromol.8b00125 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules isoselectivity of polymers. Also, isotactic−atactic stereomultiblock poly(2-vinylpyridine)s were successfully prepared via the addition and removal of THF during polymerization.
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RESULTS AND DISCUSSION Stereoselective and Living Polymerization of 2-VP. Simple metal alkyl complexes were initially employed to examine the polymerization of 2-VP (Scheme 1). In the Scheme 1. Structures of Complexes 1−5 Figure 1. Plot of P2VP Mn versus [2-VP]0/[complex 3]0 ratio.
this reaction. As indicated, the molecular weight increased linearly with the [2-VP]/[3] ratio from 100 to 600, while the polydispersity index (PDI) was maintained within an extremely narrow range (1.22−1.30) (entries 3−6 in Table 1 and Figure 1). In addition, at a fixed [2-VP]/[3] ratio of 400/1, a straight line was obtained (R2 = 0.999) upon plotting the molecular weight of the polymer against the monomer conversion. In this case, the polymer also exhibited a narrow PDI (1.19−1.29) (Figure 2). Moreover, the successive chain extension experi-
presence of the yttrium complex Y(CH2SiMe3)3(THF)2 (1) as a catalyst at 25 °C using toluene as a solvent, polymerization occurred rapidly. 200 equiv of the monomer was polymerized within 1 min, and the resultant polymer has an isotacticity (mm) of 93%. However, the molecular weight of the polymer was 18 900 g/mol, which was slightly lower than the theoretical molecular weight of 21 000 g/mol (entry 1 in Table 1). In this reaction, an initiation efficiency of 111% was achieved, and chain transfer reactions were also found. Upon the use of a scandium catalyst Sc(CH2SiMe3)3(THF)2 (2), the activity decreased significantly, with only 28% of the 200 monomer equivalents being polymerized after 1 h and a low isotacticity (mm) of 73% being achieved (entry 2 in Table 1). Owing to the lower Lewis acidity of scandium, the weaker polarization of the coordinating monomer in the propagation mechanism may be decisive for the higher activity. In contrast, the use of Lu(CH2SiMe3)3(THF)2 (3) resulted in complete polymerization within 1 min, in addition to an isotacticity (mm) of 95%, a polymer molecular weight of 21 000 g/mol, and an initiation efficiency of ∼100% (entry 3 in Table 1). Furthermore, a narrow molecular weight distribution was obtained, thereby suggesting a controllable polymerization mode and the singlesite nature of the active species. Thus, the three curves shown in Figures 1−3 confirm that the living polymerization nature of
Figure 2. Linear growth of the molecular weight (Mn) as a function of 2-VP conversion. Inset: GPC traces as detected by retention time.
ment catalyzed by complex 3 produced a precise high molecular weight polymer (Figure 3). When polymerization was conducted with a [2-VP]/[3] ratio of 100/1, complete
Table 1. Results of 2-Vinylpyridine (2-VP) Polymerization Using Complexes 1−5a entry
cat.
[M]/[Ln]
time (min)
convb (%)
TOF (h−1)
Mn,theoc (kg/mol)
Mn,expd (kg/mol)
PDI (Mw/Mn)c
Ie (%)
mmf (%)
1 2 3 4 5 6 7g 8h 9 10i 11 12j
1 2 3 3 3 3 3 3 4 4 5 5
200 200 200 100 400 600 200 200 200 200 200 200
1 60 (720) 1 1 2 3 30 3360 2 2 2 3
100 28 (100) 100 100 100 100 93.4 96.2 100 100 100 100
12000
21.0 21.0 21.0 10.5 42.1 63.1 19.6 20.2 21.0 21.0 21.0 21.0
18.9 121.1; 9.2 21.0 11.1 44.0 67.9 148.0 12.9 25.0 46.6 27.1 67.9
1.34 1.37; 1.22 1.29 1.25 1.22 1.30 1.65 2.34 1.39 1.64 1.57 1.69
112
93 73 95 95 94 94 36 34 94 98 91 >99
12000 6000 12000 12000 374 3 6000 6000 6000 4000
100 95 96 93 13 157 84 45 77 31
Reaction conditions: 25 °C, toluene, [2-VP] = 1.0 M, and 10 μmol of rare-earth complex. b2-VP conversions were analyzed by 1H NMR spectroscopy. cMn,theo = M(2VP)·(([2-VP]/[cat.])·conversion). dThe molecular weight (Mn) was obtained by gel permeation chromatography (GPC) in DMF relative to P2VP. eI* = Mn,theo/Mn,exp. fTacticity measured by 13C NMR in CD3OD. gTHF as solvent. hPyridine as solvent. i1 equiv of B(C6F5)3 was added. j2 equiv of B(C6F5)3 was added. a
B
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Figure 3. Overlay of GPC traces of the P2VP samples produced by the sequential addition of 2-VP. (A) Mn = 32 600 g/mol, PDI = 1.26; (B) Mn = 20 700 g/mol, PDI = 1.27; (C) Mn = 10 400 g/mol, PDI = 1.29. Figure 4. Overlay of 13C NMR (CD3OD, 125 MHz) spectra of P(2VP) produced by 5/B(C6F5)3 (top; see entry 12, Table 1) and 3 (bottom; see entry 3, Table 1), in the aromatic quaternary carbon region.
polymerization was observed within 5 min (Mn = 10 400 g/mol, PDI = 1.29). Following the subsequent addition of 100 equiv of 2-VP to the aforementioned system, complete polymerization was observed after 5 min (Mn = 20 700 g/mol, PDI = 1.27). Similar observations were made upon the addition of yet another 100 equiv of 2-VP to the system (Mn = 32 600 g/mol, PDI = 1.26). The molecular weight of the final polymer was ∼3 times that of the polymer obtained following the first stage and was consistent with the molecular weight of the polymer obtained using a [2-VP]/[3] ratio of 300/1. Regulation of Stereochemistry of P2VP by Lewis Base. Upon the use of THF as the polymerization solvent, the reaction rate was slightly reduced, with 93.4% of 200 monomer equivalents polymerizing within 30 min. In addition, the obtained polymer was atactic, and the isotacticity (mm) was only 36% (entry 7 in Table 1). The use of pyridine further reduced the polymerization activity, with 96.2% polymerization being achieved after 56 h to yield an atactic polymer with an isotacticity (mm) of 34% (entry 8 in Table 1). These results indicate that Lewis base molecules have a significant impact on the polymerization rate and stereoselectivity. To further investigate the impact of Lewis bases on the polymerization activity and polymer tacticity, pyridine-coordinated complexes 4 and 5 were synthesized. Compared to complex 3, these two complexes exhibited slightly lower polymerization activities in addition to significantly lower initiation efficiencies (i.e., 84% and 77%, respectively) (entries 9 and 11 in Table 1). This further confirms that Lewis bases have a significant impact on the polymerization activity and the rate of chain initiation. The high PDI values (1.64 and 1.57) of obtained polymers also suggest that there is a competitive coordination between THF (Py) and 2-VP. In addition, the polymer tacticity was slightly reduced, affording the polymers with isotacticities (mm) of 94% and 91%, respectively. Interestingly, when pyridine-coordinated complexes 4 or 5 were mixed with B(C6F5)3 prior to the polymerization of 2-VP, the tacticities of the obtained polymers were significantly improved (entries 10 and 12 in Table 1). More specifically, the mixing of complex 3 with 2 equiv of B(C6F5)3 prior to the polymerization of 2-VP produced a highly isotactic polymer with an mmmm value of >99% (Figure 4). This was likely due to the Lewis acid B(C6F5)3 displacing pyridine from the central metal atom to form Py·B(C6F5)3, as supported by NMR spectroscopic measurements.41 To the best of our knowledge, it is the first time to prepare P2VP with mmmm > 99% isotacticity reported to date. In addition, analysis by differential scanning calorimetry (DSC) indicated a polymer melting point of 217 °C (Figure S35 in the Supporting
Information), which was significantly higher than that of the previously reported isotactic P2VP (mmmm = 88%, Tm = 205 °C).36 As described above, Lewis bases have a significant impact on the stereoselectivity. An interesting fundamental and practical question is whether this Lewis bases could control the stereochemistry in the polymerization reaction. To address this question, we examined relationship between the polymer tacticity and the quantity of Lewis base (Figures 5 and 6).
Figure 5. Scopes of LBs investigated in this study.
Figure 6. Relationships between polymer tacticity and the amount of Lewis base (THF, Py, BPy, and DABCO) added during 2-VP polymerizations catalyzed by complex 3.
When THF was used as the Lewis base, the polymer tacticity decreased upon increasing the [THF]/[3] ratio, with a ratio of 800 giving an mm value of 49%. Likewise, other LBs including 1,4-diazabicyclo[2.2.2]octane (DABCO), 3-bromopyridine (BPy), and pyridine (Py) also can control polymer isotacticity. When DABCO was employed, a lower polymer isotacticity (mm) of 45% was obtained with a [DABCO]/[3] ratio of 800. There were more prominently fall in polymer isotacticity with pyridines complexes than with THF or DABCO. A [BPy]:[3] ratio of 400 can produce a polymer with mm of 40%. Similarly, C
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Figure 7. Preparation of stereomultiblock P2VP through the addition and removal of THF using complex 3: (left) variations of block polymer tacticity with the amount of monomers and THF added; (right) GPC curves for the formation of stereotetrablock copolymers: (A) isotactic (Mn = 11 000 g/mol, PDI = 1.25), (B) isotactic−atactic diblock (Mn = 22 400 g/mol, PDI = 1.20), (C) isotactic−atactic−isotactic triblock (Mn = 30 900 g/ mol, PDI = 1.29), and (D) isotactic−atactic−isotactic−atactic tetrablock (Mn = 44 500 g/mol, PDI = 1.29).
when the [Py]:[3] ratio is 100, a polymer mm of 37% was obtained. These results indicate that the stereochemical control of the polymerization is strongly affected by the relative nucleophilicity of the LBs employed herein. Isotactic−Atactic Stereomultiblock P2VP. On the basis of these results, we examined whether the addition and removal of a Lewis base could result in stereoblock polymer formation. As shown in Figure 7, 100 equiv of 2-VP was completely polymerized after 5 min in the presence of toluene and complex 3. More specifically, a rapid, living polymerization reaction was achieved, and the polymer exhibited a high isotacticity (mm = 95%). Following the subsequent addition of a THF solution containing 100 equiv of 2-VP (1 mol/L), complete polymerization was observed within 5 min, and the polymer tacticity was significantly reduced (mm = 70%). This indicates that the newly added 2-VP monomers formed an atactic chain segment (mm = 45%). THF was then removed from the system under vacuum, and a toluene solution containing 100 equiv of 2-VP was added. After 5 min, polymerization was complete and the polymer tacticity was increased, giving an isotacticity value (mm) of 79%. This indicates that in this case the newly added 2-VP monomers formed an isotactic chain segment (mm = 97%). Subsequently, a THF solution containing 100 equiv of 2VP was added once again, and after 5 min a reduction in polymer tacticity was observed for the newly formed polymer (mm = 71%) (Figure S36). In addition, the molecular weight of the polymer gradually increased upon the introduction of additional monomers, giving a final molecular weight of 44 500 g/mol, in addition to a narrow molecular weight distribution. These findings indicate that the stereoblock polymerization of 2-VP was successful through the addition and removal of THF. Initiation and Propagation in 2-VP Polymerization. To elucidate the potential initiation process taking place in our system, a low molecular weight oligomer was synthesized with a [3]:[2-VP] ratio of 1:10, using toluene as the solvent. Characterization of the synthesized oligomer was then performed using matrix-assisted laser desorption ionization− time-of-flight spectroscopy (MALDI-TOF). The 1H NMR spectrum of the oligomer showed a signal corresponding to SiMe3, which indicates that the initiation step of the polymerization process involves the insertion of 2-VP into
the Lu−CH2SiMe3 bond and that the polymerization process is achieved through 2,1-insertion reactions (Figure S37). This result is similar to the previously reported yttriumcatalyzed33−39 2-VP polymerization process. MALDI-TOF analysis indicated that the molecular weight of the oligomer was nM2‑VP + 89 or nM2‑VP + 106, where the addition fractions correspond to the chain-initiating CH2SiMe3, H+(NH4+), and H originating from methanol during reaction termination (Figure S38). Attempts to synthesize active intermediates through reactions at room temperature or low temperatures using a [3]:[2-VP] ratio of 1:3 were unsuccessful, instead in the synthesis of 2-VP oligomers. Upon increasing the [3]:[2-VP] ratio to 1:10, no active intermediate was obtained, although the Lu-CH2SiMe3 signal was absent from the NMR spectrum, indicating that all Lu−CH2SiMe3 bonds reacted with 2-VP during the polymerization process (Figure S39). Based on previous literature regarding the yttrium-catalyzed polymerization of 2-VP,33−39 the following initiation process was deduced: (1) an active intermediate is formed by the 2,1insertion of three 2-VP molecules into three Lu−CH2SiMe3 bonds; (2) coordination of a further 2-VP molecule and insertion of the newly formed Lu−CH(Py)(CH2)2SiMe3 bond initiate polymerization to give an 8-membered cyclic intermediate; and (3) chain propagation occurs via repeated coordination of 2-VP to produce the desired polymer (Figure 8). Density functional theory (DFT) calculations were then performed using the B3LYP method to further validate the possibility of an active intermediate and determine the rationale behind the influence of Lewis bases on the polymerization stereoselectivity.42 Initially, the geometry of the active intermediate, 5AS, was optimized. Subsequently, intermediates INT1-Re and INT1-Si were formed through coordination of the first 2-VP molecule from the Re-face or the Si-face, respectively, relative to the Lu−C bond. Four-membered cyclic transition states TS1-Re and TS1-Si were then formed by reaction between the olefinic double bond and the Lu−C bond, and this was followed by formation of the 8-membered cyclic intermediates INT2-Re and INT2-Si to complete the chain initiation process (Figure 9). As the energy of TS1-Si is 2.9 kJ/ mol lower than that of TS1-Re, the formation of INT2-Si D
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through coordination of 2-VP from the Si-face relative to the Lu−C bond is more favorable. The chain propagation process (Figure 10), which determines the polymerization stereoselectivity, was then examined in further detail. The stereoselectivity was investigated through the successive insertion of two 2-VP molecules into the lutetium center of INT2-Si. In contrast to the coordination of the first 2-VP molecule, coordination of the second 2-VP molecule from the Re-face relative to the Lu−C bond in INT2-Si is more favorable, as the energy of the corresponding transition state (i.e., TS2-Re) is 2.2 kJ/mol, lower than that of TS2-Si. This polymerization process was also confirmed through the coordination of a third 2-VP molecule to INT4-Re. To form an isotactic polymer, an energy barrier of 13.5 kJ/mol must be overcome, while in the case of a syndiotactic polymer, this energy barrier is 17.3 kJ/mol. Thus, based on the above results, the energy of 2-VP coordination and insertion from the Re-face into the Lu−C bond of the catalyst intermediate is lower than the coordination from the Siface, thereby rendering isotactic polymer formation more favorable. Upon in the introduction of pyridine (Py) as the Lewis base, insertion of the third 2-VP molecule initially produced intermediate INT4-Re-Py from INT4-Re. The energy of this intermediate was calculated to be −22.5 kJ/mol, which is significantly lower than that of INT4-Re (−18.4 kJ/mol). This indicates that Py has a stabilizing effect on INT4-Re, which results in variation in the transition state energy during the subsequent 2-VP coordination processes. Furthermore, upon the addition of Py, the energy difference between TS3-Si and TS3-Re was reduced to 1.1 kJ/mol (cf. 3.8 kJ/mol in the absence of Py), which in turn reduced the stereoselectivity of the polymerization process.
Figure 8. Possible reactive species, chain initiation, and chain propagation during the polymerization of 2-VP catalyzed by complex 5.
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CONCLUSIONS In conclusion, we have achieved the synthesis of highly isoselective P2VP with mmmm > 99% for the first time, using a simple lutetium-based catalyst at mild conditions. Notably, the isoselectivity (mm) of the polymer could be adjusted within a large range (37%−99%) by changing the amount of Lewis base
Figure 9. Energy profiles for the initiation process (first 2-VP insertion into the Lu−CHPy(CH2)2SiMe3 bond).
Figure 10. Energy profiles for the propagation process. E
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Lanthanidocene: Straightforward Access to a New Type of Thermoplastic Elastomers. Angew. Chem., Int. Ed. 2014, 53, 4638−4641. (8) Harney, M. B.; Zhang, Y.; Sita, L. R. Discrete, Multiblock Isotactic−Atactic Stereoblock Polypropene Microstructures of Differing Block Architectures through Programmable Stereomodulated Living Ziegler− Natta Polymerization. Angew. Chem. 2006, 118, 2460−2464. (9) Harney, M. B.; Zhang, Y.; Sita, L. R. Bimolecular Control over Polypropene Stereo- chemical Microstructure in a Well-Defined TwoState System and a New Fundamental Form: Stereogradient Polypropene. Angew. Chem., Int. Ed. 2006, 45, 6140−6144. (10) Zhang, Y.; Keaton, R. J.; Sita, L. R. Degenerative Transfer Living Ziegler-Natta Polymerization: Application to the Synthesis of Monomodal Stereoblock Polyolefins of Narrow Polydispersity and Tunable Block Length. J. Am. Chem. Soc. 2003, 125, 9062−9069. (11) Sita, L. R. Ex Uno Plures (“Out of One, Many”): New Paradigms for Expanding the Range of Polyolefins through Reversible Group Transfers. Angew. Chem., Int. Ed. 2009, 48, 2464−2472. (12) Chen, E. Y.-X. Coordination Polymerization of Polar Vinyl Monomers by Single-Site Metal Catalysts. Chem. Rev. 2009, 109, 5157−5214. (13) Vidal, F.; Gowda, R. R.; Chen, E. Y.-X. Chemoselective, Stereospecific, and Living Polymerization of Polar Divinyl Monomers by Chiral Zirconocenium Catalysts. J. Am. Chem. Soc. 2015, 137, 9469−9480. (14) Zhang, Y.; Ning, Y.; Caporaso, L.; Cavallo, L.; Chen, E. Y.−X. Catalyst-Site-Controlled Coordination Polymerization of Polar Vinyl Monomers to Highly Syndiotactic Polymers. J. Am. Chem. Soc. 2010, 132, 2695−2709. (15) Chen, X.; Caporaso, L.; Cavallo, L.; Chen, E. Y.-X. Catalyst-SiteControlled Coordination Polymerization of Polar Vinyl Monomers to Highly Syndiotactic Polymers. J. Am. Chem. Soc. 2012, 134, 7278− 7281. (16) Mariott, W. R.; Chen, E. Y.-X. Stereospecific, Coordination Polymerization of Acrylamides by Chiral ansa-Metallocenium Alkyl and Ester Enolate Cations. Macromolecules 2004, 37, 4741−4743. (17) Liu, D.; Yao, C.; Wang, R.; Wang, M.; Wang, Z.; Wu, C.; Lin, F.; Li, S.; Wan, X.; Cui, D. Highly Isoselective Coordination Polymerization of ortho-Methoxystyrene with β-Diketiminato Rare-EarthMetal Precursors. Angew. Chem., Int. Ed. 2015, 54, 5205−5209. (18) Bolig, A. D.; Chen, E. Y.-X. Isotactic-b-Syndiotactic Stereoblock Poly(methyl methacrylate) by Chiral Metallocene/Lewis Acid Hybrid Catalysts. J. Am. Chem. Soc. 2002, 124, 5612−5613. (19) Chen, E. Y.-X.; Cooney, M. J. Amphicatalytic Polymerization: Synthesis of Stereomultiblock Poly(methyl methacrylate) with Diastereospecific Ion Pairs. J. Am. Chem. Soc. 2003, 125, 7150−7151. (20) Kang, N. G.; Cho, B.; Kang, B. G.; Song, S.; Lee, T.; Lee, J. S. Structural and Electrical Characterization of a Block Copolymer-Based Unipolar Nonvolatile Memory Device. Adv. Mater. 2012, 24, 385−390. (21) Jang, S. G.; Audus, D. J.; Klinger, D.; Krogstad, D. V.; Kim, B. J.; Cameron, A.; Kim, S. W.; Delaney, K. T.; Hur, S. M.; Killops, K. L.; Fredrickson, G. H.; Kramer, E. J.; Hawker, C. J. Striped, Ellipsoidal Particles by Controlled Assembly of Diblock Copolymers. J. Am. Chem. Soc. 2013, 135, 6649−6657. (22) Klinger, D.; Wang, C. X.; Connal, L. A.; Audus, D. J.; Jang, S. G.; Kraemer, S.; Killops, K. L.; Fredrickson, G. H.; Kramer, E. J.; Hawker, C. J. A Facile Synthesis of Dynamic, Shape-Changing Polymer Particles. Angew. Chem., Int. Ed. 2014, 53, 7018−7022. (23) Jang, S. G.; Kramer, E. J.; Hawker, C. J. Controlled Supramolecular Assembly of Micelle-Like Gold Nanoparticles in PSb-P2VP Diblock Copolymers via Hydrogen Bonding. J. Am. Chem. Soc. 2011, 133, 16986−16996. (24) Higashihara, T.; Ito, S.; Fukuta, S.; Koganezawa, T.; Ueda, M.; Ishizone, T.; Hirao, A. Synthesis and Characterization of ABC-Type Asymmetric Star Polymers Composed of Poly(3-hexylthiophene), Polystyrene, and Poly(2-vinylpyridine) Segments. Macromolecules 2015, 48, 245−255. (25) Wen, T.; Shen, H. Y.; Wang, H. F.; Mao, Y. C.; Chuang, W. T.; Tsai, J. C.; Ho, R. M. Controlled Handedness of Twisted Lamellae in
added. Also, isotactic−atactic stereomultiblock poly(2-vinylpyridine) was obtained through the addition and removal of THF during the polymerization process. DFT calculations indicated the insertion of 2-VP into three Lu−C bonds in Lu(CH2SiMe3)3 to form a six-coordinated active intermediate, 5AS, which enables the coordination of 2-VP solely from the Reface to the metal center throughout the chain propagation process, thus reasonably forming a highly isotactic polymer. With the addition of a Lewis base, the energy difference between the transition states formed by the coordination of 2VP from the Re-face and Si-face to the metal center is decreased, which leads to a decrease in polymer tacticity.
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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.8b00125. Full experimental and computational details as well as additional figures and tables (PDF) X-ray crystal structure of complex 5 (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (T.-Q.X.). ORCID
Tie-Qi Xu: 0000-0003-1777-630X Xiao-Bing Lu: 0000-0001-7030-6724 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21574016, 21774017, and 51473027), Program for Liaoning Excellent Talents in University (LJQ2015025), and the Fundamental Research Funds for the Central Universities (DUT17ZD210).
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REFERENCES
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DOI: 10.1021/acs.macromol.8b00125 Macromolecules XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.macromol.8b00125 Macromolecules XXXX, XXX, XXX−XXX
