Regioselective Chain Shuttling Polymerization of Isoprene: An

Aug 18, 2016 - Chain shuttling polymerization (CSP) has exhibited unique privilege to combine monomer sequences of different properties into one ...
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Regioselective Chain Shuttling Polymerization of Isoprene: An Approach To Access New Materials from Single Monomer Bo Liu and Dongmei Cui* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, No. 5625, Renmin Street, Changchun 130022, China S Supporting Information *

ABSTRACT: Chain shuttling polymerization (CSP) has exhibited unique privilege to combine monomer sequences of different properties into one macromolecular chain, which, however, is difficult to achieve because of low chain transfer efficiency and thus lead to poor architecture control over the resulting polymers. Herein, we reported that the pyridyl−methylene fluorenyl scandium complex 1 in combination with [Ph3C][B(C6F5)4] and AliBu3 showed a high transfer efficiency (93.8%) in the presence of 10 equiv of AliBu3 toward the chaintransfer polymerization (CTP) of isoprene (IP) in high 1,4-selectivity (83%). Meanwhile, under the same conditions, the analogous lutetium precursor 3 based system was 3,4-regioselective and exhibited almost perfect chain transfer efficiency (96.5− 100%) in a wide range of AliBu3-to-Lu ratios from 10:1 to 100:1, indicating that each Lu generated apparently 100 polyisoprene (PIP) macromolecules. Both CTPs performed fluently without compromising the selectivity and the activity and had comparable chain transfer rate constants. Based on this, 1,4- and 3,4-regioselective CSPs were realized by mixing 1 and 3 in various ratios to give a series of PIPs bearing different distribution of 1,4- and 3,4-PIP sequences and Tg values. This work provides a new strategy to access stereoregular and architecture controlled polymers from a single monomer.

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INTRODUCTION

nBu2Mg albeit with dramatic decrease of trans-1,4-selectivity from 98% to 61%.9 On the other hand, the coordination polymerizations of the conjugated diene monomers provide important rubbers applied in tires industry to replace the natural rubber10 or plastic elastomers as an important component of green tires11 or high performance resins,12 depending on the cis-1,4, 1,2 (or 3,4) and trans-1,4 selectivities of the employed catalysts. Therefore, incorporation of polydiene sequences with different regularities into one macromolecular chain in a controlled manner, anticipated to provide new type of high performance materials,13 is an obvious promising but challenging project. Herein we reported the successful and highly efficient 1,4and 3,4-selective CTPs of IP were established by using the pyridyl−methylene functionalized fluorenyl scandium and lutetium precursors, respectively. The kinetics of propagation and chain transfer reaction were investigated to construct the total rate equation, and the bond strengths of the active species were calculated, based on which the key factors of influencing the efficiency and specific selectivity of CTP were revealed. Moreover, the unprecedented CSP between 1,4- and 3,4selectivities was realized by mixing the two systems under different ratios. The microstructures of the resultant polymers were identified and the glass transition temperatures were measured, which directed to novel materials bearing various

Chain shuttling polymerization (CSP) has exhibited unique privilege to combine different types of sequences into one macromolecular chain via cross-coordination chain transfer polymerization (CCTP).1 The challenge is to construct the catalytic systems because except for a few cases, the transfer efficiency of CCTP scarcely achieves 100%, that is not every chain-transfer-agent (CTA) molecule involving in the transfer reaction with the active species, leading to monomer distribution varying from chain to chain; meanwhile, the newly generated active species probably do not show similar activity as the virgin catalyst due to the decrease of stereospecific selectivity and the activity of catalytic systems, arousing poor architecture control over the resulting polymers.2 To date, most of the successful chain-shuttling polymerizations mainly relate to the copolymerization of ethylene and α-olefins catalyzed by two group 4 metal catalysts of different monomer selectivities in the presence of CTA, ZnEt2, by merit of the efficient CCTP of olefins.3 In contrast, the CSP of the conjugated dienes has still remained less explored.4 There are some reports relating to the CCTP of cis-1,4-regioselective polymerization of butadiene or IP catalyzed by rare-earth metal complex in the presence of AlEtCl2,5 AlHiBu2,6 AliBu3,7 or ZnEt28 as the chain transfer agent, in which both polymerization rate and cis-1,4 selectivity drop obviously with increasing CTA loading, and the transfer efficiency is lower than 37%.6b The trans-1,4 selectivity CCTP of IP with moderate monomer conversion is reached recently by using the half-sandwich lanthanum borohydrides in the presence of 10 equiv of CTA © XXXX American Chemical Society

Received: April 29, 2016 Revised: June 30, 2016

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DOI: 10.1021/acs.macromol.6b00904 Macromolecules XXXX, XXX, XXX−XXX

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alkyls are not efficient CTAs as compared with ZnEt2 and MgR2.9b To the best of our knowledge, this is the first example that within CTP of conjugated dienes, the chain transfer efficiency reaches 100% at such a high loading of chain transfer agent. Under the same conditions, switching to the larger rareearth metals based precursors, complexes 4−9,15 the chain transfer efficiency followed the inverse trend of the metal radii. The larger the metal size is the lower the transfer efficiency is (Table 1, entries 4−9). According to DFT calculation, the metal carbon bond strength in complexes 3−9 becomes weaker with the increase of metal size,16 suggesting that the compatibility of the metal−carbon bond energy of metal alkyls between the active species and the CTA was crucial for the successful transmetalation, in consistence with the previous reports.17 The catalytic systems [Flu-CH2-Py]Ln(CH2SiMe3)2(THF)x/ [Ph3C][B(C6F5)4]/AliBu3 based on the Sc and Lu metals were subjected to investigate the CTP of IP at a 2000 equiv monomer loading under various Al-to-Ln ratios. Almost complete monomer conversions (>99%) were reached within the screened range for both systems, but the chain transfer behavior was strongly depending on the amount of AliBu3 (Table 2). For the scandium complex 1, when the Al-to-Sc ratio was 1:1, 2:1, and 5:1, respectively 3, 5, and 8 PIP macromolecular chains were generated from one active scandium center, suggesting that more than one up to three alkyl moieties attaching to aluminum participated the chain transfer reaction (Table 2, entries 2−4). Increasing the AliBu3 loading to 10 equiv, each scandium active species grew 9 PIP macromolecular chains to reach an as high as 93.8% transfer efficiency, indicating that almost all aluminum involved in the transmetalation, each for once (one Al−alkyl moiety of the aluminum tris(alkyl)s involved into the chain-transfer reaction) (Table 2, entry 5).18 Further increasing the Al-to-Sc ratio from 20:1 to 80:1, the transfer efficiency dropped dramatically from 66.3% to 26.9% (Table 2, entries 6−9), indicating that only quarter of the loaded AliBu3 participated the transmetalation, whereas the number of macromolecular chains grown from each active scandium center kept increasing and 1,4-selectivity decreased slightly. In striking contrary, using lutetium complex 3 as the catalytic precursor, increasing Al-to-Lu ratios from 0:1 to 60:1 led to the regular decrease of the molecular weight of the resultant PIP from 12.74 × 104 to 0.24 × 104 without influencing the 3,4-selectivity and the polydispersity index (Table 2, entries 10−16). The molar mass values determined by GPC (Mn,exp) were in quite good agreement with the calculated ones (Mn,cal), which varied linearly with the [IP]0/ [AliBu3]0 ratios (Figure S1), indicating that all the added AliBu3 contributed to the coordinative chain transfer polymerization.18 This system could polymerize up to 10000 equiv of IP in the presence of 100 equiv of AliBu3 without detrimental affecting the catalytic activity. Therefore, the chain transfer reaction between the active metal centers and the CTA molecules was fast and reversible, generating up to 100 macromolecules per lutetium active species (Table 2, entry 17). As further documented hereafter, these observations suggested that the polymer appeared to grow from every aluminum, a special case of CTP, the so-called catalyzed-chain-growth (CCG) polymerization.19 Hence, there is no doubt the afforded PIP was capped by isobutyl group arising from the CTA, AliBu3. Indeed, such end-group fidelity was corroborated by the MALDI-TOF MS that macromolecule corresponding to (CH 3 ) 2 CHCH 2 (C5H8)n-H was the only detected product (Figure 1).

distributions of 1,4- and 3,4-PIP sequences, unable to access through any other methods to date.

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RESULTS AND DISCUSSION Our recent work demonstrated that upon activation of [Ph3C][B(C6F5)4] and 10 equiv of AliBu3, the pyridyl− methylene functionalized fluorenyl ligated yttrium complex 2 (Chart 1) could catalyze IP polymerization to afford 3,4Chart 1. Structures of Pyridyl−Methylene Functionalized Fluorenyl Ligated Rare-Earth Metal Complexes 1−9

enriched (84%) PIP, and the transmetalation between the active yttrium alkyl species and the aluminum alkyl moiety was observed with a transfer efficiency around 47%.14 In this work we found, however, increasing the amount of AliBu3 to 40 equiv, the transfer efficiency decreased to 22.1%, meaning that each yttrium active center apparently grew nine PIP chains (Table 1, entry 1). In contrast, the scandium complex 1 Table 1. Polymerization Data of IP Polymerization Catalyzed by Complexes 1−9/[Ph3C]/[B(C6F5)4]/AliBu3a selectivityb/%

entry

Ln

1,4

3,4

Mn,cal × 10−4 c

1 2 3 4 5 6 7 8 9

Y Sc Lu Tm Er Ho Dy Tb Gd

18 79 16 12 10 14 17 20 38

82 21 84 88 90 86 83 80 62

0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35

Mn,exp × 10−4 d

PDI

1.58 0.89 0.35 0.41 0.89 1.44 1.13 1.90 2.86

2.01 1.70 1.72 2.34 2.61 2.22 2.85 2.22 1.80

d

transfer efficiencye/% (Nf) 22.1(9) 40.0(15) 100(40) 85.4(34) 39.3(15) 24.3(10) 31.0(12) 18.4(7) 12.2(5)

Cat.: 10 μmol, IP:Ln:[Ph3C][B(C6F5)4]:AliBu3 = 2000:1:1:40; toluene: 8 g; temperature: 25 °C; time: 8 h; monomer conversion determined by weight: >99%. bDetermined by 1H NMR spectrum in CDCl3 at 25 °C. cCalculated from Mn,cal = 2000 ÷ ([Al]0/[Ln]0) × MIP + MIB.18 dDetermined by GPC in THF at 40 °C against a polystyrene standard and corrected for PIP. eTransfer efficiency calculated from = Mn,cal ÷ Mn,exp. fThe number of growing chains per rare-earth metal was calculated from N = 2000 × MIP ÷ (Mn,exp− MIB). a

catalyzed CTP of IP to afford 1,4 regulated (79%) PIP with a medium transfer efficiency (40.0%) (Table 1, entry 2). To our delight, when employing the corresponding alkyl complex of the smaller sized lutetium as the precursor, a 100% transfer efficiency was achieved without changing the activity and 3,4selectivity (84%) of the system (Table 1, entry 3), suggesting that all aluminum participated the transmetalation to generate 40 macromolecular chains per Lu center, although aluminum B

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Macromolecules Table 2. IP Polymerization Catalyzed by Complexes 1 and 3 at Different AliBu3-to-Ln Ratiosa selectivityb/% entry

Cat.

[Al]0/[Cat.]0

1,4

3,4

Mn,cal × 10−4 c

Mn,exp × 10−4 d

PDId

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

1 1 1 1 1 1 1 1 1 3 3 3 3 3 3 3 3

0 1 2 5 10 20 40 60 80 0 2 5 10 20 40 60 100

83 82 83 84 83 81 79 77 76 16 17 18 16 16 16 17 14

17 18 17 16 17 19 21 23 24 84 83 82 84 84 84 83 86

13.61 13.61 6.81 2.73 1.37 0.69 0.35 0.24 0.18 13.61 6.81 2.73 1.37 0.69 0.35 0.24 0.69

14.62 4.50 2.80 1.78 1.46 1.04 0.89 0.82 0.67 12.74 7.98 3.00 1.40 0.72 0.35 0.24 0.69

1.28 1.72 1.81 1.82 1.65 1.90 1.70 1.73 1.66 1.17 1.53 1.82 1.93 1.86 1.66 1.65 1.68

transfer efficiencye/%(Nf) 302.4(3) 243.2(5) 153.4(8) 93.8(9) 66.3(13) 40.0(15) 29.3(17) 26.9(20) 85.3(2) 91.0(5) 96.5(10) 95.8(19) 100(40) 100(60) 100(100h)

Cat.: 10 μmol; [IP]0:[Cat.]0 = 2000:1; toluene: 8 g; temperature: 25 °C; monomer conversion determined by weight: >99%; time: 8 h. Determined by the 1H NMR spectrum of the polymer in CDCl3. cCalculated from Mn,cal = 2000 ÷ ([Al]0/[Cat.]0) × MIP + MIB.18 dDetermined by GPC in THF at 40 °C against a polystyrene standard and corrected for PIP. eCalculated from transfer efficiency = Mn,cal ÷ Mn,exp. fThe number of growing chains per rare-earth metal was calculated from N = 2000 × MIP ÷ (Mn,exp − MIB). g3: 10 μmol; IP:3 = 10000:1; toluene: 20 g. hN = 10000 × MIP ÷ (Mn,exp − MIB). a b

Scheme 1. Four Kinds of End Groups Generated during Initiation and Termination Steps of CTP Catalyzed by 3

Figure 1. MALDI-TOF MS spectrum (major population: Ag+) of a PIP sample (Mn,exp = 2400 g mol−1, PDI = 1.65) prepared with the ternary catalytic system 3/[Ph3C][B(C6F5)4]/AliBu3 (Mn,cal = 68.13 × DP + 57.13 + 1.01 + 107.87, where DP is the degree of polymerization, MIP = 68.13 g mol−1, MIB = 57.13 g mol−1, MH = 1.01 g mol−1, and MAg = 107.87 g mol−1).

Figure 2. Methyl region of 13C NMR spectrum (CDCl3, 298 K) of oligomeric PIP (Mn,exp = 1000, PDI = 1.65) (asterisk represents the resonances arising from methyl carbons of 3,4-units (18.26 ppm) and cis-1,4 units separated by 3,4 sequences (25.7 ppm); vide infra).

The 1H and 13C NMR spectra of oligomeric PIP (Mn = 1000) are very complicated as there are four kinds of end groups generated from the initiation (A: isobutyl attached to a 3,4-unit via 4,3-insertion; B: isobutyl attached to a cis-1,4 unit via 4,1-insertion) and termination (C and D) procedures of CTP catalyzed by 3 (Scheme 1). The methyl protons H1 in A show a strong resonance around 0.85 ppm, which is similar to that reported in the literature (Figure 2).20 The corresponding carbons C1 exhibit multisets in the range of 22.55−23.01 ppm. In comparison, the resonances of isobutyl methyl protons H3 (0.87 ppm) and carbons C3 (27.15 ppm) in B shift downfield owing to the electron-withdrawing property of the nearby

unsaturated CC double bond. The chemical shifts of protons H2 and H4 appear at 0.73 and 0.63 ppm, respectively, which are assigned by means of 1H−13C HMQC spectrum analysis (Figure S4). The signals of C2 and C4 shift upfield to 15.14 and 19.33 ppm as compared with the corresponding those from the continuous 3,4-units (18.26 ppm) and cis-1,4 units (23.25 ppm), which was attributed to the electron donating isobutyl substituent at γ position.21 The methyl carbons C5 and C6 of C

DOI: 10.1021/acs.macromol.6b00904 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules isobutenyl arising from the end group C give resonances at 21.11 and 24.18 ppm. The allyl methyl carbon C7 in end group C is embedded in the strong resonance at 18.26 ppm, while the corresponding protons H5, H6, and H7 should give resonances in the range of 1.48−1.74 ppm, overlapping with those attributed to methyl protons of long PIP sequences (Figure S2). The kinetics under a broad Al-to-3 ratios ranging from 5:1 to 40:1 was explored. In all cases, the first-order dependence on IP consumption was observed. Strikingly, the apparent rate constants remained almost unchanged (kapp = 4.61 × 10−2− 4.66 × 10−2 min−1) (Figure 3), which was further confirmed by

Pn =

∫0

=

∫0

t

Pn(t ) t

1 (e ln[IP]0 − e ln[IP]0 − k p[M]0 t ) k tr[M]0 [Al]0

(3)

Deduced via least-squares method on the basis of the molecular weight afforded at different time (Table S1), the chain transfer rate constants of 1 and 3 catalytic systems are 3.59 × 105 mL mol−1 min−1 (1) and 9.80 × 105 mL mol−1 min−1 (3), which are much faster than their corresponding propagation rate constant, 0.13 × 104 mL mol−1 min−1 (1) and 5.26 × 104 mL mol−1 min−1 (3).23 Regioselective Chain Shuttling Polymerization of IP. As shown above, both catalytic systems 1/[Ph3C][B(C6F5)4]/ AliBu3 and 3/[Ph3C][B(C6F5)4]/AliBu3 could catalyze highly efficient CTP of IP and had comparable chain-transfer-rate constants, allowing us to establish a chain shuttling system by mixing them together. The polymerization was performed under the molar ratios of IP:Ln = 1000:1 (Ln = Sc + Lu, Sc(1):Lu(3) = (9:1)−(1:9)) and Al:B:Ln = 10:1:1 to reach full conversion (>99%). When adding a small amount of 3 to 1 (1-to-3 = 9:1), the resultant CSP system gave a PIP with 53% 1,4- regularity, much lower than that obtained by using system 1 alone (83%); when the fraction of 3 was increased to 30% (1-to-3 = 7:3), the 3,4regularity enriched (68%) PIP was obtained (Table 3, entries

Figure 3. Plots of ln{[IP]0/[IP]t} vs time for IP polymerization catalyzed by 3/[Ph3C][B(C6F5)4]/AliBu3 at various ratios of AliBu3 to 3 (Al/Lu = 40, 30, 20, 10, and 5) (3: 20 μmol; IP:Ln = 2000:1; toluene: 16 g; temperature: 25 °C).

Table 3. Chain Shuttling Polymerization of IP Using 1 and 3 in Combination with [Ph3C][B(C6F5)4] and AliBu3a selectivity/ %b

the linear relationship between ln(kapp) and ln{[AliBu3]0} with the gradient of 0.008, a sign of zero-order dependence on the AliBu3 concentration, indicating the rapid and reversible transmetalation between Lu active species and AliBu3 (Figure S7). Meanwhile, the first-order dependence on the catalyst concentration was deduced from the slope 1.08 of ln(kapp) vs ln([3]0) where kapp ranging 2.69 × 10−2 to 36.03 × 10−2 min−1 (Figure S8) were extracted from the plots of ln{[IP]0/[IP]t} vs time under various initial concentrations of 3 (Figure S9). Finally, the total rate equation was −d[IP]/dt = kp[3]1.0[IP]1.0[Al]0, meaning that the polymerization active intermediate related to lutetium monometallic complex without involving aluminum species. On the basis of the empirical rate equation, the instant degree of polymerization (Pn(t)) can be described by eq 1:22 Pn(t ) =

Rp R tr

=

k p[M]0 [IP] k tr[M]0 [Al]0

=

[Al]0/ [1+3]0

1,4

3,4

Mn,cal × 10−4 c

Mn,exp × 10−4 d

PDId

Tge/°C

1 2 3 4 5 6 7 8 9

10:0 9:1 7:3 5:5 5:5 5:5 3:7 1:9 0:10

10 10 10 10 5 40 10 10 10

83 53 32 21 24 23 19 18 16

17 47 68 79 76 77 81 82 84

0.69 0.69 0.69 0.69 1.37 0.18 0.69 0.69 0.69

0.67 0.77 0.58 0.64 1.23 0.22 0.66 0.68 0.72

1.65 2.13 1.92 1.75 1.83 1.69 1.70 1.75 1.78

−52.6 −38.0 −14.8 −0.5 5.9 −13.0 0.3 2.0 14.9

1+3: 20 μmol; IP:Ln = 1000:1; toluene: 8 g; time: 8 h; monomer conversion: >99%; temperature: 25 °C. bDetermined by 1H NMR spectrum of the polymer in CDCl3. cCalculated from Mn,cal = 1000 ÷ ([Al]0/[1+3]0) × MIP + MIB.18 dDetermined by GPC in THF at 40 °C against polystyrene standard and corrected for PIP. eDetermined by DSC.

(1)

kp, [M]0, [IP], ktr, and [Al]0 are respectively rate constant of propagation, feeding concentration of complex 3, instant monomer concentration, rate constant of chain transfer to AliBu3, and the feeding AliBu3 concentration, wherein the instant monomer concentration [IP] is derived from the empirical rate equation (2). [IP] = e ln[IP]0 − k p[M]0 t

[1]0: [3]0

a

k p[IP] k tr[Al]0

entry

1−3). Mixing equivalent 1 and 3 together, the resultant systems showed 3,4-selectivities in the range of 76−79% (Table 3, entries 4−6). Further increasing 3 loading (1-to-3 = 3:7−1:9), the 3,4-selectivity (Table 3, entries 7, 8) became close to that using system 3 alone (84%) (Table 3, entry 9). The strong dependence of the selectivity on the ratio of the catalytic precursors suggested the CSP was successfully established. Moreover, the CSP was highly efficient. Keeping the 1-to-3 ratio constant (1:1), increasing the aluminum fed amount, the molecular weight of the afforded PIP decreased accordingly in good agreement with those calculated based on ([IP]0/[1+3]0/ [Al]0) × MIP + MIB (Table 3, entries 4−6).

(2)

[IP]0 and t represent IP fed concentration and polymerization time, respectively. Hence, the function relation between polymerization degree (Pn) and polymerization time (t) can be depicted by eq 3: D

DOI: 10.1021/acs.macromol.6b00904 Macromolecules XXXX, XXX, XXX−XXX

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selective and showed a high efficiency (93.8%) under a AliBu3to-Sc ratio of 10:1. In contrast, under the same conditions, the other rare-earth-metal based systems were 3,4-regioselective. Among of them, the lutetium precursor with the highest metal−carbon bond energy exhibited almost perfect chain transfer efficiency (96.5−100%) in a wide range of AliBu3-to-Lu ratios up to 100:1, suggesting that the compatibility of the metal−carbon bond strength within the rare-earth metal alkyl precursors and CTA is crucial for the transmetalation. Moreover, the regio- or stereoselectivity and the catalytic activity were not nearly affected because the chain transfer agent AliBu3 was not involved in the active species of the IP polymerization according to the empirical rate equation: −d[IP]/dt = kp[Lu]1.0[IP]1.0[Al]0. The high transfer efficiencies and matchable transfer rate constants of scandium and lutetium systems allowed us to establish a series of efficient catalytic systems for IP chain shuttling polymerization between 1,4- and 3,4-selectivities by mixing the above two systems under different ratios. The microstructures of the resulting PIPs possessed various distributions of 1,4- and 3,4-sequences, which was more sensitive to the lutetium system having a relatively higher chain transfer rate constant. This work provides a new strategy to achieve novel polymeric materials with controlled stereoregularity and architecture.

Noteworthy was that despite bearing similar 3,4-unit contents, PIPs isolated from systems with various 1-to-3 ratios exhibit different Tg values ranging from −0.5 to 14.9 °C, which should be attributed to different distributions of 3,4- and 1,4PIP sequences confirmed by 13C NMR spectrum analysis. The PIP sample (3,4- 83%) obtained by using system 3 alone shows a resonance at 25.7 ppm assigned to methyl carbon (C1) of cis1,4 unit shifting downfield compared to those in continuous cis1,4-sequences (23.25 ppm), suggesting the cis-1,4 units are discrete and separated by 3,4 units (Figure 4, line 1).24 In

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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.6b00904. General methods and material, typical polymerization procedures, NMR analyses of oligomeric PIP and kinetic study curves of CTP using complex 3 as catalytic precursor (PDF)

Figure 4. 13C NMR spectra of PIP bearing similar 3,4-units afforded via regio-chain shuttling polymerization at different ratios of 1-to-3 (from top to bottom, 1:3 = 0:10 (line 1), 1:3 = 1:9 (line 2), 1:3 = 3:7 (line 3), and 1:3 = 5:5 (line 4)).

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corresponding to this microstructure, the isolated PIP shows a Tg at 14.9 °C (Table 3, entry 9). When mixing a small amount of 1 with complex 3 (1-to-3 = 1:9), the obtained PIP gives additional signals at 16.0 and 16.2 ppm (Figure 4, line 2), which belong to methyl carbon (C3) arising from the continuous trans-1,4 sequence generated by complex 1 and methyl carbon (C4) of trans-1,4 unit connected with 3,4 unit derived from cross-chain growth between scandium species and lutetium species,9a respectively, indicating that chain shuttling polymerization truly occurred. The Tg of afforded PIP decreased to 2.0 °C (Table 3, entry 8). With increasing the fraction of 1 to 30%, the intensity of C3 becomes a little stronger (Figure 4, line 3), and the Tg dropped to 0.3 °C (Table 3, entry 7). Under the presence of equivalent of 1 and 3, the amount of 1,4 sequences did not change much (Figure 4, line 4) because propagation and chain-transfer reaction in CTP of IP catalyzed by 3 were much rapid than those by precursor 1 (vide supra), leading to the fact that the 3,4-polyisoprenyl attached lutetium active species was easily generated and performed transmetalation with aluminum alkyls (or polyisoprenyls).

AUTHOR INFORMATION

Corresponding Author

*(D.C.) E-mail [email protected]; Fax (+86) 431 85262774; Tel +86 431 85262773. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by MST for “973” project No. 2015CB654702 and NSFC for project No. 21634007, 51321062, 51673184, 21274143 and 21374112.

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REFERENCES

(1) (a) Matyjaszewski, K.; Möller, M. Chain Shuttling Catalysis and Olefin Block Copolymers; Hustad, P. D., Kuhlman, R. L., Li, C., Shan, P., Eds.; Elsevier: Amsterdam, 2012; Vol. 3, p 699. (b) Guan, Z. Metal Catalysts in Olefin Polymerization. Topics in Organometallic Chemistry; Wenzel, T. T., Arriola, D. J., Carnahan, E. M., Hustad, P. D., Kuhlman, R. L., Eds.; Springer-Verlag: Berlin, 2009; Vol. 26, p 65. (2) Valente, A.; Mortreux, A.; Visseaux, M.; Zinck, P. Coordinative Chain Transfer Polymerization. Chem. Rev. 2013, 113, 3836−3857. (3) (a) 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. (b) Zintl, M.; Rieger, B. Novel Olefin Block Copolymers through Chain-Shuttling Polymerization. Angew. Chem., Int. Ed. 2007, 46, 333− 335. (c) Alfano, F.; Boone, H. W.; Busico, V.; Cipullo, R.; Stevens, J. C. Polypropylene “Chain Shuttling” at Enantiomorphous and

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CONCLUSION We have demonstrated the catalytic systems for highly efficient 3,4- and 1,4 chain transfer polymerizations (CTP) of isoprene (IP) have been established by combining the pyridyl− met hylene fluorenyl p recursors [Flu-CH 2 -Py]Ln(CH2SiMe3)2(THF) with the activator [Ph3C][B(C6F5)4] and the chain transfer agent AliBu3. The scandium precursor was 1,4 E

DOI: 10.1021/acs.macromol.6b00904 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

(10) It is well-known that cis-1,4-regulated polyisoprene is the main component of natural rubber (NR), which exhibits outstanding low temperature performance and processability and low hysteresis. (11) (a) Wolpers, J. 3,4-Polyisorene Containing Rubber Blend Mixtures for Tire Treads. US Patent US5104941, 1992. (b) Song, J. S.; Huang, B. C.; Yu, D. S. Progress of Synthesis and Application of trans1,4-Polyisoprene. J. Appl. Polym. Sci. 2001, 82, 81−89. (12) (a) Tsukada, G.; Tokuda, M.; Torii, M. Temperature Triggered Shape Memory Effect of Transpolyisoprene-based Polymer. J. Endod. 2014, 40, 1658−1662. (b) Liu, B.; Liu, D.; Li, S.; Sun, G.; Cui, D. High Trans-1,4 (Co)Polymerization of β-Myrcene and Isoprene with an Iminophosphonamide Lanthanum Catalyst. Chin. J. Polym. Sci. 2016, 34, 104−110. (13) Nordsiek, K. H. The “Integral Rubber” Concept An Approach to An Ideal Tire Tread Rubber. Kaut. Gummi Kunst. 1985, 38, 178− 185. (14) Liu, B.; Wang, X.; Pan, Y.; Lin, F.; Wu, C.; Qu, J.; Luo, Y.; Cui, D. Unprecedented 3,4-Isoprene and cis-1,4-Butadiene Copolymers with Controlled Sequence Distribution by Single Yttrium Cationic Species. Macromolecules 2014, 47, 8524−8530. (15) Lin, F.; Wang, X.; Pan, Y.; Wang, M.; Liu, B.; Luo, Y.; Cui, D. Nature of the Entire Range of Rare Earth Metal-Based Cationic Catalysts for Highly Active and Syndioselective Styrene Polymerization. ACS Catal. 2016, 6, 176−185. (16) The transfer efficiency decreased with the metal carbon bond energy decreasing. The metal−carbon energies are 83.5 kcal/mol (Lu), 80.4 kcal/mol (Tm), 80.3 kcal/mol (Y), 79.9 kcal/mol (Er), 79.2 kcal/ mol (Ho), 78.5 kcal/mol (Dy), 77.8 kcal/mol (Tb), 77.1 kcal/mol (Gd), and 75.3 kcal/mol (Sc) based on the DFT calculation. (17) Britovsek, G. J. P.; Cohen, S. A.; Gibson, V. C.; van Meurs, M. Iron Catalyzed Polyethylene Chain Growth on Zinc: A Study of the Factors Delineating Chain Transfer versus Catalyzed Chain Growth in Zinc and Related Metal Alkyl Systems. J. Am. Chem. Soc. 2004, 126, 10701−10712. (18) After the insertion of the IP unit into Al−C bond within AliBu3, the bond energy of new Al−C bond (73.8 kcal mol−1) is ca. 6.5 kcal mol−1 lower than that of the other Al−CH2CH(CH3)2 bond (80.3 kcal mol−1) according to the DFT calculation. Hence, the transmetalation was facile to occur between the polymer chain attached to the active species and that connected to Al atom, leading to one molecule AliBu3 generating one polymer chain. (19) Ziegler, K. Aluminium-Organische Synthese im Bereich Olefinischer Kohlenwasserstoffe. Angew. Chem. 1952, 64, 323−329. (20) Zinck, P.; Terrier, M.; Mortreux, A.; Visseaux, M. On the Number-Average Molecular Weight of Poly(1,4-trans Isoprene) Determined by Conventional GPC. Polym. Test. 2009, 28, 106−108. (21) Suzuki, S.; Horii, F.; Kurosu, H. Theoretical Investigations of The γ-Gauche Effect on the 13C Chemical Shifts Produced by Oxygen Atoms at The γ Position by Quantum Chemistry Calculations. J. Mol. Struct. 2009, 919, 290−294. (22) Naga, N.; Mizunuma, K. Chain Transfer Reaction by Trialkylaluminum (AlR3) in the Stereospecific Polymerization of Propylene with Metallocene/AlR3 /[Ph3C][B(C6F5)4]. Polymer 1998, 39, 5059−5067. (23) The kp of IP polymerization catalyzed by 1 and 3 was determined under the following conditions: Ln: 20 μmol; IP:Ln:Al = 2000:1:10; toluene: 16 g; temperature: 25 °C. (24) (a) Gronski, W.; Murayama, N.; Cantow, H.-J.; Miyamoto, T. On the Structure of 3,4-/cis-1,4-Polyisoprene by 13C NMR. Polymer 1976, 17, 358−360. (b) Suman, P. T.; Werstler, D. D. 13C NMR of Polybutadienes. J. Polym. Sci., Polym. Chem. Ed. 1975, 13, 1963−1967.

Enantiopure Catalytic Species: Direct and Quantitative Evidence from Polymer Microstructure. Macromolecules 2007, 40, 7736−7738. (d) Xiao, A.; Wang, L.; Liu, Q.; Yu, H.; Wang, J.; Huo, J.; Tan, Q.; Ding, J.; Ding, W.; Amin, A. M. A Novel Linear−Hyperbranched Multiblock Polyethylene Produced from Ethylene Monomer Alone via Chain Walking and Chain Shuttling Polymerization. Macromolecules 2009, 42, 1834−1837. (e) Pan, L.; Zhang, K.; Nishiura, M.; Hou, Z. 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. (f) 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. (g) Hue, R. J.; Cibuzar, M. P.; Tonks, I. A. Analysis of Polymeryl Chain Transfer Between Group 10 Metals and Main Group Alkyls during Ethylene Polymerization. ACS Catal. 2014, 4, 4223−4231. (h) Kempe, R. How to Polymerize Ethylene in a Highly Controlled Fashion? Chem. - Eur. J. 2007, 13, 2764−2773. (4) Wang, Z.; Liu, D.; Cui, D. Highly Selective Polymerization of 1,3Conjugated Dienes by Rare Earth Organometallic Complexes. Acta Polymerica Sinica 2015, 9, 989−1009. (5) Friebe, L.; Nuyken, O.; Windisch, H.; Obrecht, W. Polymerization of 1,3-Butadiene Initiated by Neodymium Versatate/ Diisobutylaluminium Hydride/Ethylaluminium Sesquichloride: Kinetics and Conclusions About the Reaction Mechanism. Macromol. Chem. Phys. 2002, 203, 1055−1064. (6) (a) Friebe, L.; Windisch, H.; Nuyken, O.; Obrecht, W. Polymerization of 1,3-Butadiene Initiated by Neodymium Versatate/ Triisobutylaluminum/Ethylaluminum Sesquichloride: Impact of the Alkylaluminum Cocatalyst Component. J. Macromol. Sci., Part A: Pure Appl.Chem. 2004, 41, 245−256. (b) Fan, C.; Bai, C.; Cai, H.; Dai, Q.; Zhang, X.; Wang, F. Preparation of High cis-1,4 Polyisoprene with Narrow Molecular Weight Distribution via Coordinative Chain Transfer Polymerization. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 4768−4774. (c) Wang, F.; Zhang, C.; Hu, Y.; Jia, X.; Bai, C.; Zhang, X. Reversible Coordinative Chain Transfer Polymerization of Isoprene and Copolymerization with ε-Caprolactone by NeodymiumBased Catalyst. Polymer 2012, 53, 6027−6032. (7) (a) Taniguchi, Y.; Dong, W.; Katsumata, T.; Shiotsuki, M.; Masuda, T. Novel Neodymium-Based Ternary Catalyst, Nd(OiPr)3/ [HNMe2Ph]+[B(C6F5)4]/iBu3Al, for Isoprene Polymerization. Polym. Bull. 2005, 54, 173−178. (b) Kaita, S.; Yamanaka, M.; Horiuchi, A. C.; Wakatsuki, Y. Butadiene Polymerization Catalyzed by Lanthanide Metallocene-Alkylaluminum Complexes with Cocatalysts: MetalDependent Control of 1,4-cis/trans Stereoselectivity and Molecular Weight. Macromolecules 2006, 39, 1359−1363. (c) Visseaux, M.; Mainil, M.; Terrier, M.; Mortreux, A.; Roussel, P.; Mathivet, T.; Destarac, M. Cationic Borohydrido−Neodymium Complex: Synthesis, Characterization and Its Application as An Efficient Precatalyst for Isoprene Polymerisation. Dalton Trans. 2008, 4558−4561. (d) Jian, Z.; Cui, D.; Hou, Z.; Li, X. Living Catalyzed-Chain-Growth Polymerization and Block Copolymerization of Isoprene by Rare-Earth Metal Allyl Precursors Bearing a Constrained-Geometry-Conformation Ligand. Chem. Commun. 2010, 46, 3022−3024. (8) Friebe, L.; Müller, J. M.; Nuyken, O.; Obrecht, W. Molar Mass Control by Diethyl Zinc in the Polymerization of Butadiene Initiated by the Ternary Catalyst System Neodymium Versatate/Diisobutylaluminum Hydride/Ethylaluminum Sesqui-chloride. J. Macromol. Sci., Part A: Pure Appl. Chem. 2006, 43, 11−22. (9) (a) Valente, A.; Zinck, P.; Mortreux, A.; Visseaux, M. Catalytic Chain Transfer (co-)Polymerization: Unprecedented Polyisoprene CCG and a New Concept to Tune the Composition of a Statistical Copolymer. Macromol. Rapid Commun. 2009, 30, 528−531. (b) Valente, A.; Zinck, P.; Vitorino, M. J.; Mortreux, A.; Visseaux, M. Rare Earths/Main Group Metal Alkyls Catalytic Systems for the 1,4-trans Stereoselective Coordinative Chain Transfer Polymerization of Isoprene. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 4640−4647. F

DOI: 10.1021/acs.macromol.6b00904 Macromolecules XXXX, XXX, XXX−XXX