Sequence and Regularity Controlled Coordination Copolymerization

Jan 18, 2017 - Sequence and Regularity Controlled Coordination Copolymerization of Butadiene and Styrene: Strategy and Mechanism. Fei Lin†‡, Meiya...
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Sequence and Regularity Controlled Coordination Copolymerization of Butadiene and Styrene: Strategy and Mechanism Fei Lin,†,‡ Meiyan Wang,§ Yupeng Pan,†,‡ Tao Tang,† Dongmei Cui,*,† and Bo Liu*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ University of Chinese Academy of Sciences, Changchun Branch, Changchun 130022, China § Institute of Theoretical Chemistry, Jilin University, Changchun 130022, China S Supporting Information *

ABSTRACT: The stereoselective and sequence controlled coordination copolymerizations of butadiene (BD) and styrene (St) with diblock, tapered, gradient, and random sequence distributions were achieved for the first time through varying the central metals of the rare-earth metal bis(alkyl) catalyst precursors (Flu-CH2-Py)Ln(CH2SiMe3)2(THF)x (Flu = fluorenyl, Py = pyridyl, x = 1: Ln = Nd(1), Y(2), Tm(3); x = 0, Ln = Sc(4)). The thermal behavior, morphology, and the mechanical property of these copolymers were analyzed, and their relationships were established for the first time. The mechanism of central metal size tuning the sequence distribution was discussed based on DFT calculations.

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radical polymerization (ATRP)14 or concurrent tandem living radical polymerization. 1 5 Stereo-multi-block poly(methacrylate)s were synthesized by dually active and diastereoselective chiral cation−anion ion pairs.16 Macrocyclic ring-opening metathesis polymerization was used to produce polymers with arbitrary functionality.17,18 These achievements mainly relate to acrylates and cyclic esters, etc., polar monomers, and particularly the radical and ionic polymerization processes of weak stereocontrol. Coordination polymerization as a milestone in polymer science has obtained great successes in stereoselective polymerization of olefin and conjugated diene, contributing significantly to modern plastic and rubber industries. However, a coordination polymerization rarely performs in living fashion owing to the chain transfer reaction, and particularly, the reactivity ratios of the olefinic monomers are inflexible, which obstruct to control the sequence distribution of the resultant copolymers. Waymouth and Coates developed oscillating catalysts that isomerize between achiral and chiral coordination geometries in the polymerization process to produce multi isotactic−atactic polypropylene.19 Sita established a living coordinative chain transfer polymerization (LCCTP) to obtain a new family of stereochemical grades of multiblock polyolefin via controlled stereo error incorporation.20 More elegantly, they employed the LCCTP combined with the fast and reversible chain transfer between tight and loose ion pairs of the active species to tune the comonomer relative reactivities and prepared new polyolefin materials with a single precursor.21

INTRODUCTION The sequence controlled polymerization where monomer units of different chemical nature are arranged in an ordered fashion, such as alternating, di- and multiblock, tapered, gradient, and random, as well as topological microstructures, has been the chased target of polymer chemists, anticipated to create new materials with unique bulk and surface properties (Figure 1).1

Figure 1. Schematics of diblock (a), tapered (b), gradient (c), and random (d) microstructures of linear copolymers. Black and white ellipsoids represent different monomeric units.

The modern polymer synthetic methods of living polymerization−ionic polymerizations,2 controlled radical polymerizations,3 and ring-opening metathesis polymerization,4 in combination with some organic reactions such as the “click” reactions5 and multicomponent reaction6 as well as Diels− Alder addition reactions,7 allow to synthesize such copolymers in ways that were impossible years ago.8 By means of crosspropagation being favored over homopropagation, precise AB or multiple alternating sequence can be obtained through choosing properly electron-donor/acceptor monomers,9 different enantiomeric monomers,10 metal template monomers or initiators containing a built-in recognition site,11,12 and the degenerative transfer,13 while the programmed gradient microstructures were achieved through kinetic control atom transfer © XXXX American Chemical Society

Received: November 7, 2016 Revised: December 15, 2016

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DOI: 10.1021/acs.macromol.6b02413 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scientists at Dow Chemical Company invented the “chainshuttling” polymerization by using ZnEt2 as an effective “chainshuttling” agent to mediate polymeryl groups transferring between two different transition-metal active sites with different monomer selectivities for ethylene and 1-octene copolymerization to obtain a series of “blocky” poly(ethylene-co-1-octene) copolymers.22 This method was employed by Hou and Zinck to prepare isoprene−styrene copolymers based on rare-earth complexes.23,24 Although undeniably elegant, these strategies based on two carefully chosen or in situ generated active species usually afford multiblock structure and are effective to limited monomers. Therefore, the sequence controlled coordination polymerizations of broad monomers for developing a new generation of polymers, in particular, by a using single catalytic system, has still been the challenging project of polymer science. Butadiene (BD) and styrene (St) are commonly used monomers, which are easily copolymerized via radical or anionic mechanism to give copolymers without regularity.25−27 With respect to the coordination copolymerization of dienes and styrene, only few titanium28−33 and rare-earth metal based systems34−37 exhibited activity, although many transition and rare-earth metal catalytic systems were highly active and regioand stereospecific selective for the coordination homopolymerization of conjugated dienes38−40 or styrene,41−46 respectively. And for the limited reports, diblock products or mixtures of homopolymers were obtained owing to the rather different reactivity ratios of the two monomers. Thus, the sequence controlled and highly stereoselective copolymerization in efficient manner is still an important research area for developing a new generation of synthetic macromolecules. Recently, we reported the selective polymerization of diene,47,48 styrene,44,46,49 and its derivative.50,51 Herein we report the unprecedented syndio-selective copolymerization of styrene and butadiene with diblock, tapered, gradient, and random sequence distributions by changing the relativity ratios of the monomers through varying the central metals of the catalyst precursors (Py-CH 2 -Flu)Ln(CH 2 SiMe 3 ) 2 (THF) n (Chart 1). The relationships among catalysts, sequence

calculated ones and monomodal molecular weight distributions (Mw/Mn = 1.14−2.10),52 indicating the single-sited nature of these catalytic systems and the formation of copolymers instead of a mixture of homopolymers.53 Moreover, the polybutadiene (PBD) sequences possessed high 1,4-regularity in the copolymers isolated from the large central metal Nd(1) and Y(2) based systems, while the 1,4-regularity drops in those obtained from the smaller Tm(3) and Sc(4) systems. The polystyrene (PS) sequences in all isolated copolymers were almost perfect syndiotactic. Interestingly, the monomer sequence distributions of these copolymers varied significantly with the central metal type. This was confirmed by the kinetics investigation of the copolymerization under a more reasonable BD-to-St molar ratio of 2000/2000. When the Nd(1) precursor was employed, St started to incorporate into chains after BD consumed almost completely (Figure 2A) because of the rather different reactivity ratios (rBD ≫ rSt).37 No signal arising from the BD-St joints was observed in the 13C NMR spectrum of the resultant copolymer because of their extremely low content (Table S1 and Figure S9d). The copolymer has two Tgs (−88 and 93 °C, Figure 3A) close to those of PBD and sPS, suggesting the diblock microstructure. The Y(2) catalytic system also showed overwhelming preference of BD incorporation than St by generating a nearly pure PBD segment before the 21.0% conversion, and then St gradually inserted into the PBD chain, leading to the tapered structure (Figure 2B). This was further demonstrated by the big gap between rBD = 27.18 vs rSt = 0.51 (Table 2, entry 1). The limited joint signals in the 13C NMR spectrum (Table S2 and Figure S9c) and the two Tgs (−77 and 84 °C, Figure 3B) of the copolymer distinguished it from the diblock and gradient copolymers (vide inf ra). With respect to the copolymerization catalyzed by precursor Tm(3), BD content showed gradient decline accompanied by St content increasing gradually (Figure 2C), corresponding to the larger rBD = 15.65 than rSt = 0.86 (Table 2, entry 2).54,55 This copolymer gives a Tg at −58 °C from BD enriched sequences, and a Tg at 80 °C and a Tm at 238 °C from St enriched sequences (Figure 3C). Both Tgs are extremely broad and quite different from those of the corresponding homopolymers, in accordance with a gradient microstructure, since the presence of comonomer units with repulsive interactions affects Tg significantly.56 For the copolymerization catalyzed by the Sc precursor (4), the BD content reached to 65.4% at the initial stage and then decreased to 50% finally, while the St content continuously increased until achieving to the same level of BD (Figure 2D), in agreement with the comparable rBD = 2.46 vs rSt = 1.69 (Table 2, entry 3) as compared to the other systems. Strikingly, under a broad BD-to-St ratios ranging from 250/ 2000 to 2000/250, only single Tg is observed for all the resultant copolymers (Table 1, entries 16−20, and Figure 3D), which decreases with the increase of BD content, a typical feature of a random copolymer.57 For the gradient and random copolymers, the 13C NMR spectra give similar topologies albeit with different relative integration intensities. The resonances at δ 27.91 and 30.27−30.66 ppm arise from the methylene carbons of the cis-1,4 BD units within the BD−St triads, while those at 37.49, 41.50, 43.25, 43.98, and 45.11 ppm should be attributed to the methylene carbons of St units connecting to cis-1,4 BD units (Tables S3 and S4, Figure S9).31,33,58,59 Morphology and Mechanical Property. The crystalline sPS and the amorphous PBD segments are incompatible; therefore, copolymers with different sequence distributions

Chart 1. Precursors Used in This Study

distributions, thermal behaviors, morphologies, and mechanical properties of the resultant copolymers are established. The possible mechanism of this sequence controlled copolymerization is proposed based on DFT calculation.

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RESULTS AND DISCUSSION Specific Selective and Sequence Controlled St and BD Copolymerization. The copolymerization under BD-to-St ratios from 250/2000 to 2000/250 performed fluently to reach completeness (conversion >99%) albeit at different polymerization times (Table 1). The resultant copolymers had molecular weights (Mn up to 31.2 × 104) close to the B

DOI: 10.1021/acs.macromol.6b02413 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Styrene and Butadiene Copolymerization Using 1−4 as Precursorsa microstructuresb (%) entry

catalyst

BD/St

t (h)

St content (mol %)

cis-1,4

trans-1,4

1,2

Mnc × 10−4

Mw/Mnc

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Nd(1) Nd(1) Nd(1) Nd(1) Nd(1) Y(2) Y(2) Y(2) Y(2) Y(2) Tm(3) Tm(3) Tm(3) Tm(3) Tm(3) Sc(4) Sc(4) Sc(4) Sc(4) Sc(4)

250/2000 1000/2000 2000/2000 2000/1000 2000/250 250/2000 1000/2000 2000/2000 2000/1000 2000/250 250/2000 1000/2000 2000/2000 2000/1000 2000/250 250/2000 1000/2000 2000/2000 2000/1000 2000/250

18 18 36 36 36 2 3 3 2 2 72 72 72 72 24 24 24 48 24 12

88.7 66.6 49.8 33.3 11.1 88.9 66.6 50.0 33.3 11.1 88.8 66.5 49.9 33.3 11.0 88.7 66.8 50.1 33.4 11.0

72.6 72.9 73.7 74.0 74.1 60.7 61.2 61.6 61.4 61.6 51.4 51.2 51.7 51.7 51.8 51.1 51.2 51.3 51.8 51.7

12.3 12.7 12.2 12.2 12.0 11.4 11.3 11.2 11.4 11.5 10.9 11.1 10.8 11.1 11.0 10.1 10.3 10.5 10.2 10.4

15.1 14.4 14.1 13.8 13.9 27.9 27.5 27.2 27.2 26.9 37.7 37.7 37.5 37.2 37.2 38.8 38.5 38.2 38.0 37.9

16.8 23.8 31.2 20.0 16.1 18.1 21.8 23.5 18.2 12.3 18.9 21.0 25.0 19.8 13.0 18.4 20.9 25.4 19.3 13.6

1.34 1.29 1.24 1.14 1.15 1.27 1.22 1.15 1.28 1.51 1.97 1.82 1.91 1.69 1.46 2.10 1.89 1.48 1.78 1.51

b

Tmd (°C)

ΔH (J/g)

90 93 90

254 254 253 250

15.0 11.8 9.0 7.4

89 84 80

258 256 256 256

14.8 12.5 9.6 6.8

250 241 238 251

14.5 12.1 7.5 5.2

238

18.6

Tgd (°C) 91 −86, −88, −89, −92 94 −77, −77, −79, −89 81 77 −58, −61 −67 80 56 50 28 21

80

Polymerization conditions: Ln 10 μmol, [Ln]/[B]/[AliBu3] = 1:1:10 (mol/mol) (B = [Ph3C][B(C6F5)4]), Tp = 15 °C, toluene 30 mL (entries 1− 5) and 15 mL (entries 6−20), conversion >99%. bMeasured by 1H NMR or 13C NMR in 1,2-dichlorobenzene-d4 at 120 °C. cDetermined by GPC in 1,2,4-trichlorobenzene at 150 °C against a polystyrene standard. dDetermined by DSC. a

Figure 2. Plots of the contents of St (red point) and BD units (black point) in the copolymers versus the conversions. Ln = 40 μmol, [Ln]/[AliBu3]/ [B]/[St]/[BD] = 1/10/1/2000/2000 (mol/mol) (B = [Ph3C][B(C6F5)4]), total volume = 100 mL, Tp = 15 °C. Panel A, B, C, or D represents the polymerization catalyzed by Nd(1), Y(2), Tm(3), or Sc(4) precursor, respectively.

separation from the raisin bread to the salami-like morphologies (Figures 4B,C). This well-fused nanostructures of “soft” and “hard” segments endow the stress−strain behavior of hard elastic materials. At the initial stage of the stress−strain tests, the stress ascends sharply with the increase of strain to reach 12 MPa (for C) and 16 MPa (for B); then, the stress raises slowly until fracture around 200% extensibility (Figures 5B,C). The BD−St joint sequences, although minor in content, play the significant role of compatibilizer to reduce the interphase repulsion as compared to the diblock copolymer by endowing

should exhibit various nanostructures and mechanical properties.60 In the AFM (atomic force microscope) image of sample A, the snowflake-like islands are assigned to the self-assembly sPS block in the soft PBD matrix (Figure 4A).24,61 The image with larger visual field (Figure S11) also presents distinct microphase separation arising from the inter- and intrachain repulsion in this diblock copolymers. This nanostructure contributes to moderate tensile strength and elongation at break (Figure 5A). In samples B and C, both monomer units become more penetration, leading to faint microphase C

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Figure 5. Stress−strain traces of diblock (A), tapered (B), gradient (C), and random (D) copolymers. Figure 3. DSC traces of BD−St copolymers: curve A (Table 1, entry 3, diblock); curve B (Table 1, entry 8, tapered); curve C (Table 1, entry 13, gradient); and curve D (Table 1, entry 18, random).

1,4 consecutive BD unit 5NdBD−BD through transition state TS45NdBD−BD with an activation energy of 11.7 kcal mol−1, while η2-St coordination to the 3NdBD affords the BD−St joint unit 5NdBD−St via transition state TS45NdBD−St by overcoming an activation energy of 14.1 kcal mol−1. Similarly, for the η2styrenyl active species 3NdSt, propagating a St−BD sequence needs to overcome an activation energy of 9.1 kcal mol−1, lower than 11.2 kcal mol−1 for forming a St−St unit. Therefore, no matter the active species is ended with BD or St unit, formation of continuous BD sequences is favored. In contrast, for the Sc system (Figure 7), the energy difference between the transition states of TS45ScBD−BD and TS45ScBD−St for generating continuous BD−BD units and BD−St units from π-butenyl species 3ScBD is quite small (0.2 kcal mol−1), while the activation energies for generating St−St and St−BD joints by η2-styrenyl active species 3ScSt are also quite close (16.7 kcal mol−1 vs 17.0 kcal mol−1), suggesting that polymerization rates of self- and cross-propagation is contestable, in line with the comparable reactivity ratios of rBD = 2.46 and rSt = 1.69. DFT calculations satisfactorily explain that varying central metal changes the reactivity ratios of both monomers from thermodynamic point of view, whereas the underlying reason is still unclear. The steric hindrance of the metal center has been widely reported to significantly influence the catalytic activity. For constrain-geometry-configuration catalysts (CGC), the bite

Table 2. Reactivity Ratios rBD and rSt of Copolymerizations Catalyzed by Precursors Y(2), Tm(3), and Sc(4) entry

precursor

rBD

rSt

1 2 3

Y(2) Tm(3) Sc(4)

27.18 15.65 2.46

0.51 0.86 1.69

the corresponding copolymers integrated properties of high tensile strength of crystalline sPS and high extensibility of elastomeric PBD. The random copolymer forms an almost continuous microphase (Figure 4D), where PBD sequences hinder the crystallization of sPS segments, thus leading to a high tensile strength (36.7 MPa) (Figure 5D) and a better elongation at failure (17.3%) than the extremely fragile sPS homopolymer (about 3%).62 Investigation of Mechanism. The mechanism of the dependence of sequence distribution on the central metal type was investigated by DFT calculations of the typical Nd(1) and Sc(4) systems affording diblock and random copolymers, respectively.63 As shown in Figure 6, the Nd-π-butenyl species 3NdBD adopts a η4-BD coordination (4NdBD−‑BD) to form a cis-

Figure 4. AFM images of samples A (Table 1, entry 3, diblock), B (Table 1, entry 8, tapered), C (Table 1, entry 13, gradient), and D (Table 1, entry 18, random). D

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angle is an important factor to estimate the coordination environment of the metal centers. The bite angle Flucent−Ln−N (Nd(1) 88.20(8)°; Y(2) 92.01(9)°; Tm(3) 91.71(10)°; Sc(4)· THF 96.29(12)°) increases with the decrease of metal size (ionic radius: Nd3+ 0.983 Å; Y3+ 0.901 Å; Tm3+ 0.88 Å; Sc3+ 0.745 Å),64 indicating the smaller the metal center, the more crowded coordination sphere around it.65 The Ln−N distance (Nd(1) 2.605(4) Å; Y(2) 2.513(4) Å; Tm(3) 2.477(4) Å; Sc(4)·THF 2.280(5) Å) reflects the same dependence of steric hindrance on metal size. In the copolymerization, BD chelating to the metal center in the cis-η4 mode via the two double bonds is more sterically demanding; in contrast, St coordinates to the metal center in the η2-CC fashion of less steric. Therefore, the large Nd ion based active species 3NdBD and 3NdSt are highly unsaturated coordinate and have more opening coordination sphere, which facilitates to accommodate BD coordination−insertion to form the more stable transition states TS45NdBD−BD and TS45NdSt−BD. While 3ScBD and 3ScSt active species bearing more crowded coordination environment are difficult to hold the transition states TS45ScBD−BD and TS45ScSt−BD, namely, having no privilege for the consecutive propagation of BD sequences. Thus, the gap between the selfand cross-propagation rates becomes narrow (rBD = 2.46 and rSt = 1.69), and the catalyst loses monomer selectivity. As a result, the gradual change of steric space with central metal size from Nd, Y, Tm to Sc arouses diblock, tapered, gradient, and random sequence distribution (Figure 8), respectively. Figure 6. Free energy profiles (in kcal/mol) for chain propagation catalyzed by Nd(1) based species.

Figure 8. Possible mechanism for the sequence controlled copolymerization.

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CONCLUSION We opened an interesting avenue to control over sequence distribution of copolymers. By varying central metal of the catalytic precursors, the specific and sequence controlled coordination polymerization of BD and St was achieved for the first time to give block, tapered, gradient, and random copolymers efficiently and conveniently. Therefore, the relationships among the catalyst structure, monomer sequence distribution, thermal behavior, morphology, and mechanical property of the resultant products were established unprecedentedly. DFT calculations revealed the possible mechanism that varying the central metal from Nd, Y, Tm to Sc changed the coordination environment of the corresponding active species to tune the reactivity ratios of monomer couple. At the

Figure 7. Free energy profiles (in kcal/mol) for chain propagation catalyzed by Sc(4) based species.

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“click chemistry” as a novel route to graft copolymers. Macromolecules 2006, 39, 5330−5336. (8) Bates, F. S.; Hillmyer, M. A.; Lodge, T. P.; Bates, C. M.; Delaney, K. T.; Fredrickson, G. H. Multiblock Polymers: Panacea or Pandora’s Box? Science 2012, 336, 434−440. (9) Pfeifer, S.; Lutz, J. F. A facile procedure for controlling monomer sequence distribution in radical chain polymerizations. J. Am. Chem. Soc. 2007, 129, 9542−9543. (10) Kramer, J. W.; Treitler, D. S.; Dunn, E. W.; Castro, P. M.; Roisnel, T.; Thomas, C. M.; Coates, G. W. Polymerization of Enantiopure Monomers Using Syndiospecific Catalysts: A New Approach To Sequence Control in Polymer Synthesis. J. Am. Chem. Soc. 2009, 131, 16042−16044. (11) Hibi, Y.; Ouchi, M.; Sawamoto, M. Sequence-Regulated Radical Polymerization with a Metal-Templated Monomer: Repetitive ABA Sequence by Double Cyclopolymerization. Angew. Chem., Int. Ed. 2011, 50, 7434−7437. (12) Ida, S.; Terashima, T.; Ouchi, M.; Sawamoto, M. Selective Radical Addition with a Designed Heterobifunctional Halide: A Primary Study toward Sequence-Controlled Polymerization upon Template Effect. J. Am. Chem. Soc. 2009, 131, 10808−10809. (13) Gody, G.; Maschmeyer, T.; Zetterlund, P. B.; Perrier, S. Rapid and quantitative one-pot synthesis of sequence-controlled polymers by radical polymerization. Nat. Commun. 2013, 4, 2505. (14) Qin, S. H.; Saget, J.; Pyun, J. R.; Jia, S. J.; Kowalewski, T.; Matyjaszewski, K. Synthesis of block, statistical, and gradient copolymers from octadecyl (meth)acrylates using atom transfer radical polymerization. Macromolecules 2003, 36, 8969−8977. (15) Nakatani, K.; Terashima, T.; Sawamoto, M. Concurrent Tandem Living Radical Polymerization: Gradient Copolymers via In Situ Monomer Transformation with Alcohols. J. Am. Chem. Soc. 2009, 131, 13600−13601. (16) 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. (17) Gutekunst, W. R.; Hawker, C. J. A General Approach to Sequence-Controlled Polymers Using Macrocyclic Ring Opening Metathesis Polymerization. J. Am. Chem. Soc. 2015, 137, 8038−8041. (18) Moatsou, D.; Hansell, C. F.; O’Reilly, R. K. Precision polymers: a kinetic approach for functional poly(norbornenes). Chem. Sci. 2014, 5, 2246. (19) Coates, G. W.; Waymouth, R. M. Oscillating Stereocontrol - a Strategy for the Synthesis of Thermoplastic Elastomeric Polypropylene. Science 1995, 267, 217−219. (20) Harney, M. B.; Zhang, Y. H.; Sita, L. R. Discrete, multiblock isotactic-atactic stereoblock polypropene microstructures of differing block architectures through programmable stereomodulated living Ziegler-Natta polymerization. Angew. Chem., Int. Ed. 2006, 45, 2400− 2404. (21) Wei, J.; Zhang, W.; Wickham, R.; Sita, L. R. Programmable Modulation of Co-monomer Relative Reactivities for Living Coordination Polymerization through Reversible Chain Transfer between “Tight” and “Loose” Ion Pairs. Angew. Chem., Int. Ed. 2010, 49, 9140−9144. (22) Arriola, D. J.; Carnahan, E. M.; Hustad, P. D.; Kuhlman, R. L.; Wenzel, T. T. Catalytic production of olefin block copolymers via chain shuttling polymerization. Science 2006, 312, 714−719. (23) Pan, L.; Zhang, K. Y.; Nishiura, M.; Hou, Z. M. Chain-Shuttling Polymerization at Two Different Scandium Sites: Regio- and Stereospecific “One-Pot” Block Copolymerization of Styrene, Isoprene, and Butadiene. Angew. Chem., Int. Ed. 2011, 50, 12012− 12015. (24) Valente, A.; Stoclet, G.; Bonnet, F.; Mortreux, A.; Visseaux, M.; Zinck, P. Isoprene- Styrene Chain Shuttling Copolymerization Mediated by a Lanthanide Half- Sandwich Complex and a Lanthanidocene: Straightforward Access to a New Type of Thermoplastic Elastomers. Angew. Chem., Int. Ed. 2014, 53, 4638−4641.

presence of both BD and St, the largest Nd based active species possessing more opening space prefers η4-CC coordination of BD and its consecutive insertion and then allows St η2-CC coordination and propagation, affording the diblock product, while the smallest Sc based active species loses its superiority for BD coordination−insertion owing to its crowded coordination sphere and thus shows the similar activation energy for self-propagation and cross-propagation, giving the random copolymer. This work sheds new light on sequence controlled coordination polymerization; it is intensely interesting to systematically vary the ligand environment to provide even a greater range of copolymers and produce different polymeric materials from a common pool of monomers, and extension work is currently in progress to adjust sequence distribution of copolymers from other monomers.

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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.6b02413. Experimental procedures, GPC traces, supplementary experiments, NMR characterization data, AFM images, DFT calculations results and X-ray structure of complex Tm(3) and Sc(4)·THF (PDF) Cartesian coordinates all of the calculated structures (PDF) Crystallographic information on C36H44NOSi2Tm (CIF) Crystallographic information on C31H44NOScSi2 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*Fax +86 431 85262774; Tel +86 431 85262773; e-mail [email protected] (D.C.). *Fax +86 431 85262774; Tel +86 431 85262773; e-mail [email protected] (B.L.). ORCID

Dongmei Cui: 0000-0001-8372-5987 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by “973” project No. 2015CB654702 and the NSFC for Projects Nos. 21643007, 21374112, 21574125, and 51233005. The authors are grateful to Computing Center of Jilin Province for essential support.

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REFERENCES

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DOI: 10.1021/acs.macromol.6b02413 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b02413 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (60) Epps, T. H., III; O’Reilly, R. K. Block copolymers: controlling nanostructure to generate functional materials − synthesis, characterization, and engineering. Chem. Sci. 2016, 7, 1674−1689. (61) AFM line profile analysis is shown in Figure S10. (62) Stress−strain data are shown in Table S8. (63) Free energy profiles for chain initiation are shown in Figures S16 and S17. (64) Cotton, S. A. Scandium, Yttrium & the Lanthanides: Inorganic & Coordination Chemistry. In Encyclopedia of Inorganic Chemistry; John Wiley & Sons, Ltd.: Chichester, UK, 2006; p 2. (65) Selected bond distances (Å) and angles (deg) of complexes 1−4 are shown in Table S9.

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DOI: 10.1021/acs.macromol.6b02413 Macromolecules XXXX, XXX, XXX−XXX