Isoprene Regioblock Copolymerization: Switching the Regioselectivity

Subscriber access provided by Eastern Michigan University | Bruce T. Halle Library

Isoprene Regioblock Copolymerization: Switching the Regioselectivity by the in situ Ancillary Ligand Transmetalation of Active Yttrium Species Xiaying Yu, Qing You, Xigeng Zhou, and Lixin Zhang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00600 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Isoprene Regioblock Copolymerization: Switching the Regioselectivity by the in situ Ancillary Ligand Transmetalation of Active Yttrium Species Xiaying Yu,† Qing You,† Xigeng Zhou*, †, ‡ and Lixin Zhang*,† †. Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University Shanghai 200433, China. ‡. State Key Laboratory of Organometallic Chemistry, Shanghai 200032, China.

ABSTRACT: Regioblock copolymers of single alkene hold great promise for modifying the properties of polymer materials, but remain scarce due to the lack of viable synthetic methodologies. Here we describe a method for switching the regioselectivity of the cationic yttrium-catalyzed polymerization of conjugated dienes during chain growth, which leads to the formation of a series of di- and multi-regioblock homo-/mixed-copolymers with different properties from isoprene and myrcene. Mechanistic data demonstrate that the amidinate yttrium active species [LbYPIP3,4]+ (Lb = [PhC(NC6H4iPr2-2,6)2]-) changes to the tetramethylaluminate yttrium active center {LsYPIP3,4}+ (Ls = [AlMe4]-) in situ by the amidinate ligand transfer in the presence of AlMe3. The transformation of active species switches the regioselectivity from 3,4to cis-1,4 polymerization while the polymer chain keeps propagating. AliBu3 not only functions as a chain transfer agent but also plays a key role in preventing the chain termination during the amidinate transmetalation. These results highlight the versatility and potential utility of a strategy for the design and precision control of polymer structure and physical properties.

ACS Paragon Plus Environment

1

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 26

KEYWORDS: isoprene polymerization; myrcene polymerization; regio-block copolymer; rareearth metal catalyst; coordination chain transfer polymerization; amidinate ligand

INTRODUCTION The development of synthetic methods that can produce polymer with special structures has been of great importance in polymer chemistry, because adjusting the physical properties of polymeric materials depends on the ability to control their microstructures, molecular weights and molecular weight distritution.1 Much effort has been devoted to the development of synthetic techniques that effect the formation of block copolymers in one-pot transformations and the synthesis of copolymers with special structures or properties. Despite significant progress in this area, the known block copolymers are limited primarily to those comprised of different monomeric segments or different stereospecific segments.2–6 Noticeably, controllable regioblock copolymerization of single alkene has met with very limited success to date. This may be due to the fact that each of the two different sequential regioselective polymerizations generally requires very specific catalyst system to achieve high regioselectivity and optimal control of block length. Generally, switching the regioselectivity by in situ modification of the active species after the first regioselective polymerization through extraneous reagents or by variation of the reaction conditions readily leads to the chain-termination or the occurrence of different catalytic centers,7 thus producing a polymer blend rather than a regioblock copolymer. Over the past decades, a great deal of effort has been devoted to isoprene (IP) polymerization due to the potential to access a wide variety of polymers and copolymers as well as its involvement in natural rubber.8–14 There are now numerous examples of regioselective cis-1,4 and trans-1,4 coordination polymerization of IP using transition metal9 and rare earth metal10–11


2

Page 3 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

catalysts. Moreover, methods for synthesizing highly 3,4-regioselective PIP have also been developed.12 Given the fact that there is a crucial difference in physical properties between 3,4PIP and 1,4-PIP,15 we became interested in the development of IP regioblock copolymers with controllable block number and length. We also hoped that an incorporation of perfect 3,4-PIP and 1,4-PIP blocks into the well-organized regioblock copolymer might lead to the distinct physical properties of the materials for final application.

Scheme 1. Alkylaluminum-assisted alkene block copolymerization.

The alkylaluminum-mediated chain exchange between two distinct catalysts with different or uniuqe monomer selectivities is an effective method for synthesis of block copolymers (Scheme 1, A).16 Besides, pioneering work reported in 2005 by Coates et al. showed that changing the polymerization temperature led to the formation of regioblock copolymers from propylene.17


3

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 26

Shiono and coworkers found that an reversible coordination of Lewis base with CoCl2-based catalyst could affect the catalytic transformation of 1,3-butadiene into regioblock copolymers, but the number and length of blocks is difficult to be controlled.18 Herein, we describe a strategy for regioblock copolymerization of conjugated alkenes, which provides a versatile route to di- and multi-regioblock homo-/mixed- copolymers with different properties from isoprene or/and myrcene. In our design, the stereo- and regio-selectivity of sequential polymerization is switched by the ancillary ligand exchange of metal active species. Replacement of the bulky ligand (Lb) with a small ancillary ligand (Ls) leads to a change of active species to favor 1,4-coordination over 3,4-coordination during the chain propagation (Scheme 1, B).

RESULTS AND DISCUSSION Recently, Kempe and Anwander reported that excess alkylaluminium could abstract mono anionic N-coordinating ancillary ligands from organolanthanide complexes.19 In addition, Hou and

coworkers7

found

that

a

mono(amidinate)

bis(aminobenzyl)

yttrium

complex/[Ph3C][B(C6F5)4] binary catalytic system could initiate 3,4-isospecific isoprene polymerization while yttrium complex/[Ph3C][B(C6F5)4]/(3AlMe3) ternary system gave cis-1,4product. We wonder if these two active species resulting from one precursor can work together in one pot system to achieve the 3,4- and cis-1,4 regioblock polymer by using alkylaluminium reagents. Firstly, treatment of IP (375 equiv. based on Y) with [LbY(CH2C6H4NMe2-o)2] (1), [Ph3C][B(C6F5)4] and AliBu3 in 1:1:5 molar ratio at 25 °C followed by reacting with 7 equiv. of AlMe3 and sequential another 375 equiv. of IP, afforded the desired diblock polyisoprene composed of 3,4-block (PIP3,4) and rubbery cis-1,4-block (cis-PIP1,4) sequence (Table 1, entry 6).


4

Page 5 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Table 1. Synthesis of diblock regio-copolymers of isoprene under various conditions.a

t1

Microstructuresb

t2

Mnc×10-4

PDIc

Tgd (oC)

7.3

1.23

21

0.1

4.7

1.30

21

4.9

0.1

3.6

1.38

21

95.0

4.9

0.1

3.3

1.35

21

45

63.0

35.6

1.4

6.5

1.42

-54/22

10

45

51.9

46.3

1.8

6.1

1.42

-55/20

10

10

45

51.3

46.7

2.0

5.9

1.43

-60/21

0

7

10

45

51.1

47.9

1.0

bimodal

n.d.

n.d.

375

5

7

10

45

52.8

45.5

0.7

bimodal

n.d.

n.d.

375

750

5

7

10

90

38.5

60.1

1.4

8.7

1.51

-57/18

11

375

1125

5

7

10

135

30.1

68.6

1.3

10.9

1.72

-57/18

12

750

375

5

7

20

45

66.8

32.0

1.2

9.1

1.49

-55/19

13

1125

375

5

7

30

45

74.3

24.9

0.8

12.5

1.70

-55/20

14g

375

0

5

0

60

0

99.0

1.0

0

5.7

1.46

25

15g

0

375

5

7

0

150

2.0

97.3

0.7

5.3

1.47

-60

16g

1250

250

5

7

180

120

82.8

17.1

0.1

17.6

1.46

-58/25

17g

750

250

5

7

100

120

74.8

25.1

0.1

13.3

1.42

-58/24

18g

375

375

5

7

60

150

50.5

49.2

0.3

9.6

1.54

-59/25

19g

250

750

5

7

50

360

24.8

74.8

0.4

12.7

1.60

-62/21

20g

250

1250

5

7

50

600

18.3

81.2

0.5

16.1

1.54

-62/21

Entry

m

n

x

y

1

375

0

0

2

375

0

3

375

4

(min) (min)

3,4 (%)

cis-1,4 (%)

trans1,4(%)

0

2

0

91.0

9.0

0

3

0

10

0

93.1

6.8

0

5

0

10

0

95.0

375

0

7

0

10

0

5

375

375

5

5

10

6

375

375

5

7

7

375

375

5

8e

375

375

9f

375

10

a

Polymerization conditions: C6H5Cl (20 mL); 25 oC; 1 (20 µmol); [1]0/borate = 1; borate = [Ph3C][B(C6F5)4]; yield: ~100%. bDetermined by 1H NMR and 13C NMR spectroscopy. cDetermined by GPC with respect to a polystyrene standard. dDetermined by DSC. eA polymer blend of 3,4-PIP and cis-1,4-PIP was obtained, GPC curve is an overlap bimodal, so PDI hasn’t been determined. f Replacement of AliBu3 by ZnEt2, giving a polymer blend of 3,4-PIP and cis-1,4-PIP, GPC curve is an overlap bimodal, so PDI hasn’t been determined. g Polymerization temperature: -10 oC.


5

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 26

However, in the absence of AliBu3 only a polymer blend of PIP3,4 and cis-PIP1,4 was obtained (Table 1, entry 8). To get more insight into the role of AliBu3 in regioblock copolymerization of IP and find out the optimum reaction conditions, a mixture of complex 1, IP (375 equiv.) and 0–7 equiv. of AliBu3 in C6H5Cl was treated with one equiv. of [Ph3C][B(C6F5)4]. With the gradual increase of AliBu3 used, the Mn of the resulting PIP3,4 decreased and the catalyst efficiency increased (catalyst efficiency: 0.35, 0.54, 0.71, 0.77, entries 1–4, Table 1), indicating AliBu3 works as a chain transfer agents (CTA) in the polymerization process.20 Furthermore, we determined the chain-end

groups

of

the

PIP3,4

produced

from

the

45:1:1:5

feed

ratio

of

IP/1/[Ph3C][B(C6F5)4]/AliBu3. The MALDI-TOF mass spectrum (Figure S29 in SI) consisted of two series of molecular ion peaks, which corresponded to the linear PIP with the iBu and the CH2C6H4(NMe2-o) as the chain ends, respectively. These results proved that AliBu3 has a dual role: stabilizing the first active species–PIP3,4 segment and playing a CTA. Noticeably, replacement of AliBu3 by ZnEt2 as a CTA cannot prevent the chain termination of the previously formed segment when AlMe3 was added under the same conditions (Table 1, entry 9). Subsequently, we investigated the cooperative roles of AliBu3 and AlMe3 in the IP regioblock copolymerization process. As shown in Table 1, the ratio of AlMe3/AliBu3 used has a large impact on the regioblock copolymerization. Generally, excess AlMe3 (relative to AliBu3) is required for the regioselectivity switch. Keeping the loading of 5 equiv. of AliBu3 (based on Y), when the amount of AlMe3 was varied from 5 to 7 equivalents, the subsequent polymerization exhibited a higher 1,4-regioselectivity (entries 5–6). The 1,4-regioselectivity did not improve significantly when the loading amount of AlMe3 continued to increase (entry 7). Thus, we


6

Page 7 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

selected the 1:5:7 stoichiometric ratio of 1/AliBu3/AlMe3 as the optimized conditions for further investigations. The GPC analyses indicate that the molecular weights of the resulting regioblock copolymers increase in proportion to the amount of IP used and are close to the sum of PIP3,4 and cis-PIP1,4 generated independently under the same conditions (entry 18 vs entries 14 and 15). A good linearity between monomer feed and block length in each step was observed at [IP]/[Y] = 375– 1150 (entries 6, 10–13) and the dispersity kept relatively narrow (1.4–1.7). Furthermore, the polymerization temperature exerted a significant influence both on the regioselectivity and on the reaction activity. When the polymerization was carried out at -10 oC, a higher level of regio- and stereoselectivity was achieved in each step, giving the diblock copolymers composed of almost perfect PIP3,4 (3,4-selectivity up to 99%) and cis-PIP1,4 (cis-1,4selectivity up to 97.3%) with controllable length, albeit with a requirement of a longer reaction time (Table 1, entries 14–20). The glass-transition temperature (Tg) of materials is of practical importance.21 In contrast to only one Tg observed commonly for previous stereo- and regioblock copolymers from single alkene, all the present diblock PIP displayed two Tg values, which are near to the corresponding Tg values of cis-PIP1,4 and PIP3,4, respectively.7,10–12 This highlights the unique physical properties of the diblock PIP. The tensile mechanical properties of different PIP synthesized at -10 oC are shown in Table 2. cis-1,4-PIP (entry 15) is too sticky to give a measurable value. On the contrary, the plastic-like 3,4-PIP is brittle and easy to break upon elongation (ε = 4.5%) (entry 14). Significantly, the resulting diblock PIP exhibited improved performance over isotactic PIP. Within the series of diblock materials tested, Young’s modulus decreases as the soft block content increases. All


7

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 26

samples have a lower Young’s modulus than that of the perfect 3,4-PIP, probably as a consequence of two Tg values. Moreover, the true stress at break increases with the increase of the fraction of 3,4-PIP in diblock samples. These diblock regio-copolymers can impart better miscibility between the hard and soft blocks that could be useful for adjustment elastomeric performance. For example, when the hard/soft block ratio of the diblock copolymers is changed from 3:1 to 1:3, the elastic recovery values were significantly increased from 9.4% to 147%. Table 2. Mechanical properties of different PIP synthesized by the method described in Table 1. Sample a

E /(MPa) σb/(MPa) εc/(%) a

entry 14

entry 16

entry 17

entry 18

entry 19

entry 20

505 16.9 4.5

241 21.3 15.7

58.5 2.5 9.4

5.8 2.0 42.4

3.13 1.43 147.9

0.95 0.22 112.8

Young’s modulus. b Tensile strength. c Elongation at break.

Figure 1. AFM height images of PIP3,4-b-cis-PIP1,4 generated in Table 1, entry 12 (a) and polymer blend (b).

In addition, the difference in surface information and nanostructure between regioblockcopolymer and polymer blend was also confirmed by the AFM analyses. A relatively flat surface


8

Page 9 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

of the regioblock copolymer sample (Table 1, entry 12) gave an evidence of a well-distributed phase (Figure 1, a), whereas a clearly micro phase separation was observed for the corresponding 2:1 polymer blend of PIP3,4 and cis-PIP1,4 (Figure 1, b). Table 3. Synthesis of diblock polymyrcene. a

Entry 1 2 3 4d 5e

m

n

3,4 (%)b

cis-1,4 (%)b

Mn c ×10-4

Mw/Mnc

188 0 188 375 188

0 188 188 188 375

96.7 3.8 50.2 65.5 34.9

3.3 96.2 49.8 34.5 65.1

3.2 3.1 6.0 8.5 8.2

1.70 1.80 1.77 1.72 1.78

a

Conditions: C6H5Cl (20 mL); 25 oC; 1 (20 µmol); [1]0/borate = 1; yield: ~100%. b Determined by 1H and 13C NMR spectroscopy. cDetermined by GPC with respect to a polystyrene standard. dt1 = 1.5 h. et2 = 5 h.

β-Myrcene (7-methyl-3-methylene-octa-1,6-diene) is a kind of bio-renewable monomer which is thought to be an alternative of petroleum-based olefins. Recently, polymyrcene (PM) with high 1,4-22 or 3,4-selectivity23 has been prepared by coordination polymerization. Encouraged by the above results, we further examined the applicability of the present method in the synthesis of polymyrcene analogues. Similarly, when AlMe3 was added to an effective catalytic system combining 1, [Ph3C][B(C6F5)4] and AliBu3, that catalyzes regioselective 3,4polymerization of myrcene (Table 3, entry 1), the subsequent polymerization of the later inputting myrcene changed to 1,4-regiospecificity, giving a diblock polymyrcene PM3,4-b-cisPM1,4 (Table 3, entries 3–5). The activity of myrcene regioblock copolymerization was somewhat lower than that of isoprene under the same conditions, but showed equally good control over the polymerization.


9

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 26

Scheme 2. Yttrium-catalyzed mixed-regioblock copolymerization of isoprene and myrcene and the characterization of the resulting copolymers.

In addition, we proceeded to explore the utility of this method for synthesis of mixedregioblock copolymers based on isoprene and myrcene. Scheme 2 depicts the general strategy for the formation of isoprene- and myrcene-based mixed di-, tri- and tetrablock copolymers. DSC studies showed that all of the mixed-regioblock copolymers have two different Tg values. Noticeably, there is a fairly good correlation between the content of hard block PIP3,4 and the Tg of the copolymers, indicating that the Tg values can be finely adjusted by the volume fraction and composition of the blocks, as well as the overall size of the macromolecules. For example, a decrease in Tg value is observed with a decrease in the portion of PIP3,4.


10

Page 11 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

In order to better understand the mechanism of the regioblock copolymerization, some control experiments and model reactions were conducted. Firstly, according to the first step polymerization conditions, reaction of complex 1 with [Ph3C][B(C6F5)4] and AliBu3 in PhCl gave an oily product [LbY(CH2C6H4NMe2-o)][B(C6F5)4] (2), that was highly efficient for catalytic isoprene 3,4-polymerization (Scheme 3, (a), (c)). Furthermore, treatment of a 1:1:5 mixture of 1, [Ph3C][B(C6F5)4] and AliBu3 with 7 equivalents of AlMe3 in PhCl led to the occurrence of a sequential aminobenzyl and amidinate transmetalation, affording Me2Al(CH2C6H4NMe2-o) (3),24 LbAlMe2 (4),25 and [YMe2][B(C6F5)4] (5)10c (Scheme 3, (b)). 5 could also be obtained by the reaction of 2 with AlMe3, and was highly effective for catalytic IP cis-1,4-polymerization (Scheme 3, (d)). In addition, it was found that 5 reacted with AlMe3 to form the adduct [(AlMe4)2Y][B(C6F5)4] (7), which might play a key role in preventing the Y-Me bond insertion.10c Although the low solubility of 2 and 5 in nonpolar solvent and their oiliness precluded the direct acquisition of NMR spectra and solid state structures, 2 and 5 are soluble in THF-d8, enabling 1H, 11B and 19F NMR data of their THF adducts to be obtained (Figure S30 – S36 in SI), which were consistent with the formulas of original [LbY(CH2C6H4NMe2o)][B(C6F5)4]

and

[Me2Y][B(C6F5)4],

respectively.

To

our

delight,

replacement

of

[Ph3C][B(C6F5)4] with [Et3NH][BPh4] to carry out analogous reactions under the same conditions, followed by crystallization in THF, gave the corresponding cationic model complexes

[LbY(CH2C6H4NMe2-o)(THF)2][BPh4]

(8)

and

[YMe2(THF)5][BPh4]10c

(9),

respectively (Scheme 3, (e) and (f)). 8 and 9 were structurally characterized by X-ray diffraction analysis. X-ray structural data of 8 clearly reveal a distorted tetragonal bipyramidal coordination geometry around the yttrium center (Figure 2). The Y1–N1 and Y1–N2 distances of 2.374(3) and 2.321(3) Å are comparable to the corresponding values found in other amidinate yttrium


11

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 26

complexes,26 whereas the Y1–C32 bond length of 2.377(1) is shorter than those in [YMe2(THF)5][BPh4].10c Noticeably, no ligand exchange reaction involving AliBu3 was observed in these controlled experiments, indicating that AliBu3 seems to take part only as a chain transfer and stabilizing chain agent in the polymerization process.

Scheme 3. Mechanistic studies for the regioselectivity difference between two active species in reaction with isoprene.


12

Page 13 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figure 2. Molecular structure of the cation of 8. Thermal ellipsoids are set at 30% probability. Hydrogen atoms and BPH4 are omitted for clarity. Selected bond distances (Å) and angles (o): Y1–O1 2.322(3), Y1–O2 2.414(3), Y1–N1 2.374(3), Y1–N2 2.321(3), Y1–N3 2.552(4), Y1–C32 2.377(1), Y1–C1 2.796(4), C1–N1 1.333(5), C1–N2 1.338(5); N1–Y1–N2 56.8(1), N3–Y1–C32 70.1(1).

On the basis of these results, a possible mechanism for the present regioblock copolymerization of isoprene is shown in Scheme 4. The reaction of 1 with 1 equiv. of [Ph3C][B(C6F5)4] gives the cationic Y species A.7 Isoprene coordinates to the Y center preferably in a 3,4-η2 fashion to form species B due to the steric hindrance of the amidinate ligand.6d, 7 Subsequent migratory insertion of the aminobenzyl group to the coordinated isoprene in B, followed by multiple subsequent isoprene 3,4-coordination and insertion steps, affords a yttrium active species C bearing a 3,4-polyisoprene chain. When the chain is long enough to free the chelating coordination amino group, the AliBu3 becomes possible to coordinate the metal center. Thus, C undergoes the ligand exchange with AliBu3 (chain transfer agent) to afford a cationic yttrium isobutyl species D and isotactic 3,4-polyisoprene with an iBu2Al-functionalized end


13

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 26

group E. Continuous isoprene insertion into D gives the yttrium active species bearing a 3,4polyisoprene block F. The fast and reversible chain exchange between the LbY+–PIP3,4 species

Scheme 4. A possible regioblock copolymerization mechanism for the formation of diblock PIP3,4-b-cis-PIP1,4.

and the iBu2Al–PIP3,4 species followed by chain propagation at the cationic LbY+ site would gradually achieve chain unification. Then, the amidinate transfer from cationic yttrium active species H to AlMe3 probably yields a different cationic heterotrinuclear Y/Al species I. The coordination of isoprene to the Y ion bearing the sterically less encumbered ancillary ligation takes place in an η4-fashion by replacement of the iBu2Al–PIP3,4 unit in I to give J. The 1,4addition of a resulting PIP3,4 moiety to the coordinated isoprene gives a yttrium allyl species. Subsequent repeated isoprene coordination and insertion would yield the [Y]–cis-PIP1,4-b-PIP3,4 species L. Analogously, the fast and reversible chain exchange between the [Y]–cis-PIP1,4-bPIP3,4 species and the iBu2Al–cis-PIP1,4-b-PIP3,4 species followed by IP chain propagation at the cationic Y+ site leads to the formation of diregioblock polyisoprene. Obviously, the amidinate


14

Page 15 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

transfer from cationic yttrium active species to AlMe3 switches the stero- and regioselectivity of sequential polymerization, while AliBu3 not only functions as a chain transfer agent but also plays a key role in preventing the chain termination of the previously formed segment during the amidinate transmetalation. The regioselective copolymerization of isoprene and myrcene to give copolymers containing PIP3,4, cis-PIP1,4, PM3,4, and cis-PM1,4 blocks could also take place in an analogous fashion.

CONCLUSIONS In summary, we have developed an efficient and practical method for controllable synthesis of regioblock copolymers via a cationic yttrium-catalyzed sequential 3,4- and 1,4-polymerization of conjugated dienes. This approach leads to the formation of a series of homo- and mixedregioblock copolymers with controllable size and microstructure of each block from isoprene and myrcene in a one-pot process. The mechanistic data reveal a different cooperative role of AliBu3 and AlMe3 additives in switching the polymerization modes of alkenes. This facile method shows significant substrate flexibility and excellent control of the regio- and stereoselectivity of poly-conjugated dienes, giving polymer materials with distinct and tunable physical properties. Further studies on the regioblock copolymerization of other monomers by this synthetic strategy are in progress.

EXPERIMENTAL SECTION General Procedures and Materials. All manipulations were performed with rigorous exclusion of air and water, using Schlenk techniques or an Mbraun glovebox (Unilab Mbraun;

Isoprene Regioblock Copolymerization: Switching the Regioselectivity

Recommend Documents