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The Origin of ‘Multisite-like’ Ethylene Polymerization Behavior of Single-Site Non-Symmetrical Bis(imino)pyridine Iron(II) Complex in the Presence of MMAO Nina V. Semikolenova, Wen-Hua Sun, Igor E. Soshnikov, Mikhail A Matsko, Olga V Kolesova, Vladimir A Zakharov, and Konstantin P. Bryliakov ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00486 • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 16, 2017

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ACS Catalysis

The Origin of ‘Multisite-like’ Ethylene Polymerization Behavior of Single-Site Non-Symmetrical Bis(imino)pyridine Iron(II) Complex in the Presence of MMAO Nina V. Semikolenova,a Wen-Hua Sun,b Igor E. Soshnikov,a,c Mikhail A. Matsko,a Olga V. Kolesova,a,c Vladimir A. Zakharov,a,c and Konstantin P. Bryliakov a,c* a

Boreskov Institute of Catalysis, Pr. Lavrentieva 5, Novosibirsk 630090, Russian Federation

b

Key Laboratory of Engineering Plastics, Beijing National Laboratory for Molecular Sciences,

Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China c

Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russian Federation

E-mail: [email protected]

Abstract A detailed study of the effect of reaction temperature, time, and co-catalyst composition on the ethylene polymerization performance of 2-[1-(2,6-dibenzhydryl-4-chlorophenylimino)ethyl]-6-(1mesityliminoethyl)pyridyliron dichloride (1) is reported. In the presence of modified methylaluminoxane (MMAO), 1 behaves like a highly active, ‘multisite-like’ ethylene polymerization catalyst, with the resulting polyethylenes having time-dependent bimodal-like molecular-weight distributions, and featuring saturated (n-propyl and i-butyl terminated) chainends. To readily distinguish between bimodal and bimodal-like molecular-weight distributions, it has been proposed to use the dNf/dLogM − LogM representation further to the mainstream dWf/dLogM − LogM one. The consensus mechanism of chain transfer and chain-end formation in the presence of MMAO has been proposed, which explains the composition and amount of terminal alkyl groups in the polymer, and the apparent ‘multisite-like’ nature of the iron catalyst. A comparison between the catalytic behaviors of the ‘multisite-like’ catalyst system 1/MMAO and the truly multisite catalyst system based on the Brookhart’s symmetrical bis(imino)pyridine iron catalyst 2 is given.

1

ACS Paragon Plus Environment

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Keywords Bis(imino)pyridine;

chain

transfer;

ethylene

polymerization;

MMAO;

molecular-weight

distribution; multi-site catalysts

1. Introduction The discovery of the highly active single-site ethylene polymerization catalysts on the basis of iron(II) and cobalt(II) complexes bearing 2,6-bis(imino)pyridyl ligands, as reported by groups of Brookhart1 and of Gibson,2-4 had been one of the major milestones of the post-metallocene era of olefin polymerization. Subsequently, single-site ethylene polymerization catalysts based on bis(imino)pyridine complexes have developed greatly; various groups worldwide have invested significant efforts in modification of the ligand frameworks, in order to delineate the directions for the fine tuning of the reactivity of the catalysts. The progress of bis(imino)pyridine transition metal catalysts, affording products ranging from linear oligomers to strictly linear, high-molecular weight polyethylenes, has been surveyed from both catalytic and synthetic perspectives.5-7 Tuning the ligand structures and the activator composition provides the key to controlling the molecular-weight distribution (MWD) of the resulting polymeric products, from typical single-site PEs (Mw/Mn ≤ 2.5) to broad polymodal PEs (Mw/Mn > 20).8-11 A serious drawback of the bis(imino)pyridine iron(II) catalysts, limiting their industrial application, had been the generally low thermal stability, leading to fast deactivation at temperatures above ca. 50 ºC (while the industrial processes prefer polymerization temperatures between 60 °C and 90 °C). 12 The way to overcome this limitation runs through the design of ligand structures, tolerating higher temperatures, with retention of high catalytic activity. Recently, the introduction of bulky substituents - bis(4-fluorophenyl)methyl and dibenzhydryl12-16 into the bis(imino)pyridine derivatives has been reported to afford bis(imino)pyridine iron(II) complexes with improved thermal stability: with either methylaluminoxane (MAO) or modified methylaluminoxane

(MMAO),

the

resulting

catalysts

showed

polymerization activities at temperatures up to 80 °C. 2


high

apparent

ethylene

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Previously, the catalytic behavior of bis(imino)pyridine iron(II) based complexes was found to be rather complicated. Kinetic studies by Barabanov and co-workers, making use of 14CO inhibition, showed that two types of active sites are present in the systems 2/MAO and 2/Al(iBu)3: the more active (and unstable) centers operated at the early stages of polymerization, gradually transforming into the less active sites; the former sites afforded the low-MW polyethylene fraction, while the latter sites were responsible for the higher-MW PE fraction.17,18 The data on ethylene polymerization kinetics by Kissin with co-workers supported those conclusions.19 Subsequent spectroscopic study suggested that formation of the less active catalytic sites may be associated with the reductive transformations of the initially formed heterobinuclear iron(II) active species.11 Recently,

non-symmetrical

2-[1-(2,6-dibenzhydryl-4-chlorophenylimino)ethyl]-6-[1-

mesityliminoethyl] pyridyliron dichloride 1 (Figure 1) with enhanced thermal stability has been reported; in the presence of MMAO, its catalytic performance and the MWD of the resulting PE strongly depended on the reaction conditions.12 The present work was initiated aiming at getting insight into the nature of the factors controlling the MWD of polyethylenes. Surprisingly, although 1 afforded PEs with bimodal-like MWD, the time-dependent apparent “bimodality” appeared to be different from that of the truly multisite catalyst system based on symmetrical bis(imino)pyridine Fe complex 2 (Figure 1),17-19 and was found to be due to the chain transfer - to aluminum alkyls present in the co-catalyst. The molecular weight distributions of the bimodal-like PE are unimodal in the dNf/dLogM – LogM coordinates (dealing with the number fraction of macromolecules having molecular weight M). The consensus reaction mechanism, explaining the mechanisms of chain transfer and of chain-ends formation has been proposed.

N N

N

Fe Cl

Cl

N

Cl

N Cl

1

Cl

2

3

N

Fe


ACS Catalysis

Figure 1. Non-symmetrical bis(imino)pyridyliron precatalyst 1 and symmetrical complex 2.

2. Results Effect of temperature on polymerization kinetics and polyethylene MWD. The effect of polymerization temperature on the performance of catalyst 1 was tested by conducting the polymerization in the presence of MMAO at three different temperatures: 40, 65 and 80 °C (Table 1). In Figure 2 one can see the effect of temperature on the rate of polymerization. At 40 °C, the latter decreased smoothly, ca. twofold within 20 min. With the increase in polymerization temperature from 40 to 65 °C, the initial catalytic activity (during 1-3 min) was much higher and then dropped sharply. At 80 °C, the reaction rate decreased more slowly within the first 15 min of polymerization to ca. the same value as for the experiment at 65 °C (Figure 2). 65 °C kg PE/ mol Fe bar min

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800

40 °С 400 80 °С

0 5

10

15 t, min

20

25

Figure 2. Ethylene polymerization rate vs. time for the 1/MMAO/C2H4 system at different polymerization temperatures.

The polymerization temperature also affected the molecular weight (MW) of the resulting PE, as well as its molecular weight distribution (MWD) (Table 1 and Figure 3). In contrast to ultrahigh-molecular weight polyethylene obtained at 40 °C (viscosity-average molecular weight Mv =1.2·106 g/mol, Table 1), the polymer formed at 65 °C had Mw of 2.9·105 g/mol and broad MWD (Mw/Mn = 9.1) (Figure 3, blue curve). The polymer obtained at 80 °C exhibited a broad, bimodallike molecular weight distribution (Figure 3, red curve).

4


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The theoretical splittings of the experimental GPC curve for the obtained PEs are provided with the SI (Figure S1). Two main fraction can be distinguished in the GPC trace of the polymer, obtained at 65 °C: the low-molecular weight (low-MW) fraction (24 %, Mw = 30·103 g/mol) and the high-MW fraction (76 %, Mw = 370·103 g/mol). Both fractions showed rather narrow MWD (Mw/Mn = 2.0 and 3.5, respectively). 65 °С

0.6

d Wf / d log M

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

80 °С

0.4

0.2

0.0 2

4

6

log M

8

Figure 3. GPC curves for PEs, obtained at different polymerization temperatures within 30 min.

Table 1. Effect of polymerization temperature on ethylene polymerization by catalyst system 1/MMAO. a No

Т / °С

initial activity b

activity c

Mn×10-3

Mw×10-3

Mw/Mn

1

40

450

290

−

1200 d

−

2

65

850

220

32

290

9.1

3

80

480

190

18

270

15

a

Polymerization conditions: [Fe] = 0.5 µmol, activator: ММАО (1 mmol Al), Р(С2Н4) = 5 bar, in

200 ml of heptane, polymerization time 30 min.

b

Initial activity Kg PE·(mol Fe)-1·bar-1·min-1,

calculated from the PE yield obtained within the first 5 min of the reaction. c Average activity, Kg PE·(mol Fe)-1· bar-1·min-1. d Viscosity-average molecular weight Mv.

The polymer obtained at 80 °C, had a rather similar share of the low-MW fraction (28 %). As compared with the sample obtained at 65 °C, the low-MW fraction showed lower Mw (Mw = 12·103 g/mol) and a similar MWD (Mw/Mn = 2). The high-MW fraction with Mw= 330·103 g/mol, i.e. slightly lower than that of the PE obtained at 65 °C, had a broader MWD (Mw/Mn = 5.5,

5


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Table S1). The question on what is the origin of this bimodality is not trivial; essentially, two hypotheses may be considered: (1) the existence of two types of active centers, and (2) of two different chain-terminations pathways in the system 1/MMAO. Regarding the first hypothesis, the emergence of (at least) two types of catalytically active sites in the system 1/MMAO/ethylene should be assumed. The first type of active sites might be associated

with

the

formation

of

the

low-MW

PE

fraction

(with

MWD

centered

at Mw 12·103…30·103), the second type of active sites being ascribed to the high-MW PE fraction (with MWD centered at Mw = 330·103 to 370·103). Such picture would agree with the data obtained for symmetrical bis(imino)pyridine iron(II) catalysts.11,17-19 However, further detailed study, witnessing the time-dependence of the polyethylene GPC traces, reveal the primary role of chain transfer reactions with regard to the molecular-weight distribution of the resulting PE. The details are provided below.

Effect of polymerization time on the polyethylene MWD. The polymerization rates vs. time for the samples 2, 3, 4 of Table 2 are presented in Figure 4. Within the first 3 min, the rate was nearly constant, followed by an abrupt drop of the activity at ca. 5 min; subsequently, polymerization rate nearly stabilized at a very low level. Data on the effect of polymerization time on the molecular weights and MWD of polymers obtained with catalyst 1 in the presence of MMAO at 65 °C and 80 °C are collected in Table 2. Figure 5 shows the corresponding GPC curves. For all polymers (obtained in 0.75, 3, 15, and 30 min), the MWDs were polymodal-like (Figure 5, S2, Table S1),20 but their GPC curves dramatically differed from each other. For the 0.75 and the 3-min PE (65 °C), the major part of the PE had low MW (Table 2, entries 1, 2, Figure 5A; for theoretical splitting details see SI, Figure S2 and Table S1). The extension of polymerization time to 15 and then to 30 min (Figure 5A) increased the high-MW fraction content (SI, Figure S2 and Table S1). A qualitatively similar picture was observed in polymerization runs at 80 °C (Table 2, Figure 6B, and Figure S3 and Table S2). In this case, however, the share of the low-MW component was in all 6


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ACS Catalysis

cases somewhat higher, and the low-MW and the high-MW components had lower molecular weights than those of the 65 °C PE (SI, cf. Tables S1 and S2). We also note that with the extension of polymerization time, the positions of the low-MW and the high-MW peaks were not constant and gradually shifted in the high-MW direction (see Figure S2 and S3 for the details of theoretical splitting of the GPC curves of Figure 5A and 5B). Such picture is drastically different from that reported for symmetrical bis(imino)pyridine iron(II) complexes: for the latter, only the proportion of the low-MW and the high-MW components in the MWD curves changed with changing polymerization time, while the positions of the components remained constant (in the presence of MAO or Al(iBu)3 as co-catalysts).17

Table 2. Effect of polymerization time on ethylene polymerization by catalyst system 1/MMAO.a No

Т / °С

PE yield b

t / min

activity c

GPC

NMR

Mn×10-3 Mw×10-3 Mw/Mn

Mn×10-3 nPr…iBu chains

a

d

nPr...nPr chains e

1

65

0.75

880

1173

1.8

31

17

2.3

77

23

2

65

3

2260

753

5.5

140

25

7.8

66

34

3

65

15

5040

336

18

145

8.1

20.1

69

31

4

65

30

6640

221

32

290

9.1

−f

−

5

80

3

2400

800

7.7

120

16

10.1

67

33

6

80

15

4600

306

10

90

9.0

11.0

65

35

7

80

30

5560

190

18

270

15

−f

−

f

f

−

−

f

f

Polymerization conditions: [Fe] = 0.5 µmol, activator: ММАО (1 mmol Al), Р(С2Н4) = 5 bar, in

200 ml of heptane.

b

Kg PE·(mol Fe)-1·bar-1.

c

Average activity, Kg PE·(mol Fe)-1·bar-1·min-1 .

d

Content of PE chains with one isobutyl and one n-propyl end (mol %). e Content of PE chains with two n-propyl ends (mol %).

f

Polymers chains were too long for reliable end-group analysis by

natural-abundance 13C NMR.

7


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3 min

1000

500 30 min 15 min 0 10

20

t, min

30

Figure 4. Ethylene polymerization rate vs. time for the 1/MMAO/C2H4 system (65 °C) at different polymerization times.

1,0 d Wf / d log M

A

0,8

0.75 min

0,6

3 min

15 min 30 min

0,4 0,2 0,0 2

3

4

5

6

7

log M

B

0.8

3 min

0.6 d Wf / d log M

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

kg PE/ mol Fe bar min

ACS Catalysis

15 min

0.4

30 min

0.2 0.0 2

3

4

5

6

7

log M

Figure 5. GPC curves for the PEs obtained at different polymerization times at 65 °C (A) and 80 °C (B).

In contrast, for ethylene polymerization in the presence of non-symmetrical catalyst 1, activated with MMAO, the time dependence of positions of the low-MW and the high-MW fractions of the resulting PE suggests that the time-dependent “bimodality” may be due to the peculiarities of the chain termination processes rather than due to the existence of two types of catalytically active sites. To get insight into the chain termination processes, the nature and the content of the polymers’ end groups were analyzed by

13

C NMR. The NMR-evaluated Mn demonstrated 8


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satisfactory qualitative agreement with those obtained by GPC (Table 2). Analysis of the 13C NMR spectra of the polymers revealed the absence of measurable quantities of unsaturated chain ends, thus ruling out chain termination via β-H elimination. On the contrary, two sorts of saturated ends have been identified, i.e. i-butyl and n-propyl ends (Figure 6A), in non-equal amounts. With MMAO as co-catalyst, the number of n-propyl ends was ca. twice as high as the number of i-butyls; this proportion was found for the PEs obtained at different polymerization times (3 or 15 min) and temperature (65 or 80°C) (Table 2). The only exception was the low-MW PE obtained during 0.75 min, for which the amount of i-butyls was slightly higher (Table 2, entry 1). Most likely, the two types of different terminal groups reflect the existence of two types of PE macromolecules: (1) chains with one n-propyl and one i-butyl end, and (2) chains with two n-propyl ends. Integration of the 13C NMR spectra has revealed that the share of chains of the first sort is typically in the range 65-69 % (Table 2, entries 2, 3 and 5, 6), and 77 % at 0.75 min polymerization time (Table 2, entry 1). When MMAO was replaced with MAO as activator, no i-butyl end groups were found in the PE (Figure 6C, entry 1 of Table 3), providing evidence that isobutyl groups could only originate from the co-catalyst (MMAO, by preparation, contains aluminum isobutyl groups, see also Figure S5, SI).

9


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−(CH2)n−

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III

3

1 2

4

I II

1

69 % iBu…nPr III

I

1

II

2 4

3

A

86 % iBu…nPr III

3

1

II

2 4

I

B II I

III

III III

II

II

0 % iBu…nPr

I

I

C 40

38

36

34

32

30

28

26

24

22

20

18

16

14

13

C, δ

Figure 6.

13

C NMR spectra (1,2-dichlorobenzene, 100 °C) of PE obtained by ethylene

polymerization on catalyst 1 during 15 min at 65 °C in the presence of different activators: MMAO (A); MMAO + TIBA (B); MAO (C) (Table 3). The above findings are very intriguing, rising questions on (1) the mechanism of chain transfer and (2) how the chain transfer is related to the formation of the end groups. These issues will be discussed below.

Effect of co-catalyst on the polyethylene MWD. In attempts to clarify the effect of co-catalyst composition on the polymerization kinetics and the molecular weight of the resulting PE, we have conducted ethylene polymerizations with different co-catalysts (Table 3). The corresponding reaction rate vs. time plots for the polymerizations on catalyst 1 at 65 °C with various activators, as well as the GPC traces of the resulting polyethylenes are presented in Figures 7 and 8, respectively. 10


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The addition of TIBA to MMAO resulted in the increase of the initial activity of the system, which further gradually declined (Fig. 7A). When co-catalyst composed of B(C6F5)3, taken as the cationizing agent, and TIBA as alkylating agent, was used, the initial catalytic activity was lower, and declined more slowly (Fig. 7B).

A kg PE/ mol Fe bar min

1500

MMAO+TIBA MMAO

1000 500 0 0

B kg PE/ mol Fe bar min

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

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5

t, min

10

15

MMAO+TIBA 1500 B(C6F5)3+TIBA

1000

DMAO+TIBA

500 0 0

5 t, min

10

15

Figure 7. Ethylene polymerization rate vs. time for the 1/activator/C2H4 system (65 °C, 15 min polymerization time) with different activators: MMAO and MMAO+TIBA (A); MMAO+TIBA, DMAO21+TIBA, B(C6F5)3+TIBA (B).

The polymers, obtained within 15 min with MMAO and MMAO+TIBA, had broad (Mw/Mn = 8.1-14.5) bimodal-like MWD (Table 3, entries 1-3 and Fig. 8A). The details of theoretical splitting of the corresponding GPC traces are provided with the SI (Figure S4 and Table S3). As compared with the polymer obtained with 1/MMAO, the PE produced by the system 1/MMAO+TIBA featured a higher share of the high-MW fraction of PE (85 % vs. 73 %), the highMW fraction exhibiting lower Mw (87·103 vs. 200·103), and much broader MWD (Mw/Mn = 10 vs. 3.6, Table S3).

11


ACS Catalysis

A d Wf / d log M

0.8 0.6

MMAO

MMAO+TIBA

0.4 0.2 0.0 2

3

4

5

6

7

log M

B

B(C6F5)3+TIBA

1.0 d Wf / d log M

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

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DMAO+TIBA

0.8 0.6

MMAO+TIBA

0.4 0.2 0.0

2

3

4

5

6

7

log M

Figure 8. GPC curves for PEs produced with complex 1 at 65 °C within 15 min using different activators: MMAO and MMAO+TIBA (A); DMAO+TIBA, B(C6F5)3+TIBA (B).

Interestingly, the activation of 1 with DMAO21+TIBA and B(C6F5)3+TIBA, resulted in the formation of polymers with visibly unimodal and relatively narrow (Mw/Mn = 3.7-4, Table 3) MWD (Fig. 8B), illustrating the crucial effect of co-catalyst composition on the ‘multisite-like’ catalytic performance of 1. In the latter case, the polymer demonstrated much higher molecular weight (Mn = 100·103), indicating that the chain transfer is substantially suppressed in the system B(C6F5)3+TIBA, eventually leading to longer PE macromolecules. FTIR data witnessed the presence of 0.12 vinyl end groups per 1000 C. If all PE molecules had one vinyl end, this would correspond to PE with Mn = 117·103, which is rather close to the GPC evaluated Mn = 100·103 (Table 3, entry 5). One can conclude that in the system presence 1/B(C6F5)3/TIBA, the chain transfer is substantially suppressed, and β-H transfer is the major chain termination pathway. Variation of the co-catalyst composition also affected the proportion of the ‘nPr…iBu’ versus ‘nPr…nPr’ polymer chains. The 13C NMR spectra of PEs from entries 2 and 4 of Table 3 are shown in Figures 6A and 6B, respectively, together with the spectrum of polyethylene, obtained

12


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ACS Catalysis

using commercial MAO (Table 3, entry 1; Figure 6C). When MMAO (Table 3, entry 2) was replaced with DMAO+TIBA (Table 3, entry 3) and further with MMAO+TIBA (Table 3, entry 4), the content of the ‘nPr…iBu’ polymer chains increased from 69 % to 81 % and further to 86 %, respectively, indicating that an increase of the Al(iBu)3 content in the co-catalyst results in the increase in the proportion of the ‘nPr…iBu’ PE macromolecules.

Table 3. Effect of various co-catalysts on ethylene polymerization with complex 1.a No

a

co-catalyst

PE yield b activity c

NMR

Mn×10-3 Mw×10-3 Mw/Mn

Mn×10-3 nPr…iBu chains d

nPr...nPr chains e

1

MAO f

2920

195

11

160

14.5

21.0

0

100

2

MMAO

5040

336

18

145

8.1

20.1

69

31

3

DMAO+TIBA g

5040

336

12

44

3.7

14.6

81

19

4

MMAO+TIBA h

6400

430

7.7

75

9.7

10.0

86

14

5

B(C6F5)3/TIBA i

4120

275

100

400

4

−j

−k

−k

Polymerization conditions: [Fe] = 0.5 µmol, T = 65 °C, Р(С2Н4) = 5 bar, in 200 ml of heptane,

polymerization time 15 min. 1

GPC

·min-1.

d

b

Kg PE·(mol Fe)-1·bar-1.

c

Average activity, Kg PE·(mol Fe)-1·bar-

Content of PE chains with one isobutyl and one n-propyl end (mol %). e Content of PE

chains with two n-propyl ends (mol %). f Activator: МАО (1 mmol Al, Al/Fe=2000; polymerization in 200 ml of toluene).

g

Activator: DMAO (1 mmol Al) + TIBA (0.5 mmol Al), AlΣ/Fe=3000.

h

Activator: MMAO (1 mmol) + TIBA (0.55 mmol), AlΣ/Fe=3100. i Activator: B(C6F5)3 (5 µmol) + TIBA (0.5 mmol), Al/Fe=1000. j Polymers chains were too long for reliable end-group analysis by natural-abundance 13C NMR.

k

FTIR data witness the presence of vinyl end groups (0.12 per 1000

C), identified by characteristic band at 909 cm-1. At the same time, replacing MMAO as co-catalyst with DMAO+TIBA or with MMAO+TIBA reduced the average molecular weight of the resulting PE (cf. entries 2, 3, 4 of Table 3). The inspection of Table 3 and Figure 8 reveals a correlation between the co-catalyst composition and the average molecular weight Mn of the polymers. In particular, the use of B(C6F5)3+TIBA affords the most high-MW PE, reflecting that TIBA, which is the only available aluminum alkyl in the catalyst system 1/B(C6F5)3+TIBA, is a very poor chain-transfer agent,22 β-H 13


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transfer accounting for the major chain termination pathway (see above). All other systems of the series, in addition to Al(iBu)3, contain Al-Me groups. Commercial MMAO contains aluminum alkyls with Al-Me groups (existing mostly as mixed dimers, e.g. (iBu)2Al(µ-Me)2Al(iBu)2, Figure S5, SI),23 and those Al-Me groups are also present in the MMAO+TIBA mixture. In the DMAO+TIBA co-catalyst, some Al-Me groups, introduced with the DMAO, could have undergone exchange with TIBA, to afford the (iBu)2Al(µ-Me)2Al(iBu)2 aluminum alkyls. Apparently, for the systems containing Al-Me groups, the chain transfer is substantially facilitated, leading to PE with shorter chains.

3. Discussion Proposed mechanism of iBu end group formation. As reported above, the systems containing both Al-Me and Al-iBu groups, in most cases afford PE with bimodal-like MWD (Figure 8A). This fact has prompted us to conclude that this apparent bimodality is most likely due to the peculiarities of the chain-termination pathways. In particular, while in the catalyst system 1/B(C6F5)3+TIBA, mostly β-H transfer operates, chain transfer to Al(iBu)3, if any, being a minor termination pathway, in the systems 1/MMAO, 1/MMAO+TIBA, the chain transfer to aluminum alkyls R1R2AlMe (R1, R2 can be iBu and Me) may occur as well.24 Therefore, at the initial and middle stages of polymerization, chain transfer most likely occurs to (iBu)2AlMe, with formation of low-MW PE (Figure 5). At the late stages, the concentration of (iBu)2AlMe decreases and further vanishes, eventually leaving β-H transfer as the major termination pathway, which apparently results in the increase of the share of the higher-MW fraction, along with increase of the MW of the high-MW component (Figure 5, Tables S1, S2). To establish the correlation between the chain length and the amount of the iBu groups, the PE sample obtained at 65 °C in the 1/MMAO system (entry 2 of Table 3) was separated into narrow-MWD fractions (Figure 9), and the individual fractions were analyzed by GPC and

13

C

NMR (Table S4). The results obtained suggest two important conclusions. First of all, the 14


Page 15 of 31

proportion of the ‘iBu…nPr’ chains of the first fraction was nearly equal to that for the whole polymer (Table S4, entries 0 and 1). Apparently, in the broad-MWD polyethylene, it is only the shortest PE molecules that significantly contribute to the NMR-derived count of the ratio of the iBu / nPr groups, due to their numerical prevalence. This also applies to all studied polymers (Tables 2 and 3), thus providing plausible explanation for the observed weak dependence of the share of ‘iBu…nPr’ chains for the PE, obtained in catalyst system 1/MMAO, on the polymerization time (cf. entry 2 vs. 3 and 5 vs. 6). The second finding is that the share of the ‘iBu…nPr’ chains monotonously decreases when going from low-MW fractions to high-MW fractions (from 63 % for F1 to 33 % for F4, Table S4). This result is not trivial and clearly indicates some relationship between the mechanisms of chain termination and of formation of terminal iBu groups.

0,6

PE

0,5 d Wf / d log M

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

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F1 F2 F3 F4 F5

0,4 0,3 0,2 0.1 0.0 2

3

4

5

6

7

log M

Figure 9. GPC curves for PE produced with complex 1 at 65 °C within 15 min using MMAO as activator (black line), and GPC curves for individual fractions F1…F5 (colored lines).

On the other hand, the share of ‘nPr…iBu’ chains increases in parallel with the increase of the amount of Al-iBu groups in the composition of the activator (cf. entries 2, 3, 4 of Table 3). Summarizing these facts, the following reaction scheme can be proposed, assuming the commonly accepted mechanism6,11,25 of activation of bis(imino)pyridine iron(II) based catalysts (Scheme 1). According to this scheme, the ratio of the ‘nPr…nPr’ and ‘nPr…iBu’ chains in the resulting PE, formed in the presence of iBu2AlMe as the main chain transfer agent, is in fact determined by only the concentration of iBu-containing aluminum alkyls in solution.26 Indeed, at higher [Al-iBu] 15


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concentration, the transient species [LFe-Me]+, generated after chain transfer to (iBu)2AlMe, can form heterobimetallic adducts with iBu-containing aluminum alkyls; the adducts, upon the interaction with ethylene, may give rise to species [LFe-Me(C2H4)]+ (responsible for pathway A) or [LFe-iBu(C2H4)]+ (involved in pathway B). At lower [Al-iBu] concentration, it may become insufficient to ensure fast formation of the bimetallic adducts, so that the transient species [LFeMe]+ preferentially react with ethylene via polymerization pathway A, eventually leading to the formation of the ‘nPr…nPr’ chain. Formation of each polymer chain is associated with irreversible consumption of one (iBu)2AlMe molecule (the latter is transferred to one Al-polymeryl molecule); the exchange of the active species [LFe-Me]+ with Al(iBu)3 either restores the (iBu)2AlMe molecule (pathway B), or not (pathway A). At any rate, the concentrations of (iBu)2AlMe and Al(iBu)3 both gradually decrease in the course of polymerization, thus accounting for the gradual increase of the MW of the resulting polymers, with simultaneous decrease of the proportion of the ‘nPr…iBu’ chains when going from low-MW to high-MW fractions. At some point (at long polymerization times), (iBu)2AlMe concentration may run low, thus leaving β-H transfer as the major termination pathway. Model conditions for such situation are provided in entry 5 of Table 3; in effect, the system 1/B(C6F5)3/Al(iBu)3 has afforded a much higher-MW PE. On the other hand, when using smaller concentration of MMAO (Al/Fe = 500 instead of 2000), the contribution of β-H transfer to the chain termination becomes significant, leading to the formation of high-MW fraction of polyethylene (Figure S6 of the SI). An important feature of this model is that the formation of one PE macromolecule takes away one molecule of aluminum alkyls.27 In effect, the total number of the PE molecules formed could be expected not to substantially exceed the number of Al atoms initially introduced into the system (assuming that there is an upper limit to the number of Al-alkyl bonds that can be involved in chain transfer22). Our estimates show that this condition (within the experimental error, Table S5, SI) is fulfilled, thus corroborating the proposed model (Scheme 1).28

16


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Scheme 1. Proposed mechanism of activation of catalyst 1 (upper sequence; R = Me, Cl) and the mechanisms of chain-ends formation. For simplicity, chain-transfer agent R1R2AlMe, is presented as (iBu)2AlMe, and the counteranion is omitted in some structures for brevity.

The origin of the apparent bimodality. Comparison of non-symmetrical and symmetrical bis(imino)pyridine iron(II) catalysts. At first glance, our conclusion that the chain transfer can be the origin of the apparent bimodality might look inconsistent: indeed, one would expect that, upon decreasing the chain-transfer agent concentration, the chain transfer rate is reduced with respect to propagation, thus leading to simply broadened, not bimodal, MWD. Hereinafter, we will demonstrate that this intuitively clear expectation and the catalytic behavior of the system 1/MMAO do not contradict one another. In fact, the crucial point is the commonly used MWD curves ( dWf/dLogM vs. LogM ), dealing with the weight fraction dWf versus LogM,29 rather than with the number of polymer macromolecules versus LogM. The dWf/dLogM − LogM graph provides in fact a distorted, MWweighted distribution, with the high-MW fraction being represented with higher weight (in mathematical sense), which is perceived by our eyes as (sometimes significant) overestimation of

17


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the high-MW fraction. In some cases this can even lead to bimodal-like MWD curves for polyolefins that are not actually bimodal, but simply have a “tail” of the high-MW fractions. In disputable cases, to sort out the bimodality issues carefully, it is a powerful tool to convert the dWf/dLogM – LogM curves into the dNf/dLogM – LogM ones, where dNf stands for the number fraction of macromolecules having molecular weight M: this should eliminate the highMW overestimation and provide the non-weighted distribution, sc. the undistorted picture. To illustrate this approach, we have converted the conventional, bimodal-like dWf/dLogM – LogM MWD curves of the PEs obtained in catalyst systems 1/MMAO at 65 °C within 0.75 min, 3 min, 15 min, and 30 min (Figure 5A), into the dNf/dLogM − LogM curves (Figure 10). One can see that in the latter coordinates, there is no evidence of bimodality. With the increase of polymerization time, the distribution maximum (correlating with the average molecular weight) simply monotonously shifts in the high-MW direction. The widths of the distributions also increase with time, as can be expected, provided that the chain transfer rate gradually decreases in the course of polymerization. A weak point of the “dNf/dLogM - LogM” presentation is the lowest-MW region, for which there may be visible artifacts (as those marked with asterisks for the 0.75 min and 3 min samples): the latter are apparently due to (1) low intensity – low sensitivity problem of the GPC machines equipped with DRI detectors, and (2) relatively high contribution of the initial, nonstationary period of polymerization. 3 min

1,0

0.75 min

15 min

0,8 dNf / d log M

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

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0,6

*

0,4

30 min 0,2

* 0,0 2

3

4

5

6

7

log M

Figure 10. The MWD curves for the PEs obtained on the 1/MMAO catalyst system at 65 °C in the dNf/dLogM − LogM coordinates. The original dWf/dLogM – LogM plot for the same samples is

18


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given in Figure 5A. Asterisks mark artifacts originating mostly from the initial, non-stationary period of polymerization.

Let us now consider a really multisite catalyst system. In 2005, Barabanov and co-workers showed that catalyst 2 (Figure 1) affords bimodal PE, when activated with MAO or Al(iBu)3.17 The GPC curves for the polymers obtained in the system 2/Al(iBu)3 at different polymerization times are presented in Figure 11. In this case, the low- and the high-MW fractions can be clearly distinguished both in the dWf/dLogM – LogM (Figure 11A) and in the dNf/dLogM − LogM plot (Figure 11B). The increase in polymerization time leads to only redistribution between the shares of the two fractions. Their peak-MWs keep the same positions, reflecting two types of catalytically active sites (the sites affording the low-MW fraction were assumed to decline first, while the sites responsible for the high-MW fraction persist for longer17).

A

7 min

d Wf / d log M

0,8 0,6 2 min

0,4

15 min

0,2 0,0 2

3

4

5

6

7

log M

B

Low-MW fraction 1,2 1,0 dNf / d log M

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

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0,8 0,6

High-MW fraction

0,4 0,2 0,0 2

3

4

5

6

7

log M

Figure 11. GPC curves for the PEs obtained on the 2,6-Me2LFeCl2/Al(iBu)3 catalyst system at 35 °C in the “dWf/dLogM – LogM” coordinates (A) and in the “dNf/dLogM - LogM” coordinates (B).

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To compare the catalytic behaviors of the non-symmetrical catalyst 1 and the symmetrical catalyst 2, we have performed a series of polymerization experiments with the system 2/MMAO under identical conditions (except temperature: the symmetrical catalyst is much less temperature stable, which required lower T). The 30-min experiment was not conducted, because the system 2/MMAO completely lost the activity by the end of the 15-min experiment. One can see that the catalytic behavior of 2, activated by MMAO, is somewhat different: on the one hand, the MWDs (both the dWf/dLogM – LogM and the dNf/dLogM – LogM plots) broaden with increasing polymerization time (Figure 12), like for the non-symmetrical catalyst 1. On the other hand, the peak Mw does not drift in the high-MW direction with increasing time (Figure 12B), yet average Mn grows up at the expense of the growing high-MW tail (Table 4). The most evident reason of this distinction is the more complex chain termination pattern of catalyst 2: for the latter, both chain transfer to aluminum alkyls and β-H transfer operate (Table 4), which hinders the analysis of the overall picture. On the other hand, the MWD curves (dNf/dLogM – LogM) for the PEs obtained over the 2/MMAO system exhibit no evidence of bimodality (just get broader with time, Figure 12B). Such behavior is similar to this of the ‘multisite-like’ 1/MMAO system (Figure 10),30 and different from that of the really multisite 2/Al(iBu)3 system (Figure 11B). The co-existence of two chain termination channels gives rise to four types of PE macromolecules (Table 4 and Figure S7, SI); all of them can be readily quantified by

13

C NMR

(Figure S7, SI). Upon increasing the polymerization time from 0.75 to 15 min, the proportion of vinyl-terminated macromolecules increases from 31 % to 54 % (Table 4), indicating that β-H transfer actually becomes the major chain termination pathway at late stages of polymerization, when significant part of Al-alkyls, initially brought with MMAO, is consumed. At the same time, upon increasing the polymerization time from 0.75 to 15 min, the proportion of iBu-ended PE chains gradually decreases from 48 % to 37 % (Table 4), in line with decreasing concentration of Al-iBu alkyls in the reaction mixture.

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A 1,0 d Wf / d log M

0.75 min 0,8

3 min

0,6 0,4 15 min 0,2 0,0 2

3

4

5

6

log M

B 1,0 0.75 min 0,8 dNf / d log M

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

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3 min 0,6 0,4 0,2

15 min

0,0 2

3

4

5

6

log M

Figure 12. MWD curves for the PEs obtained on the 2/MMAO catalyst system at 40 °C in the dWf/dLogM – LogM coordinates (A) and in the dNf/dLogM – LogM coordinates (B).

Table 4. Effect of polymerization time on ethylene polymerization by catalyst system 2/MMAO.a No

GPC

chains content (%) by 13C NMR

Mn×10-3 Mw×10-3 Mw/Mn

nPr… nPr... vinyl vinyl unsatu- iBu-ended i iBu d nPr e …iBu f …nPr g rated h

t

PE

activity

min

yield b

c

1

0.75

5400

7200

2.1

6.1

2.9

33

36

15

16

31

48

2

3

7200

2400

4.1

20

4.9

26

35

17

22

49

43

3

15

9600

640

5.1

40

7.8

17

29

20

34

54

37

a

Polymerization conditions: 40 °C, [Fe] = 0.5 µmol, activator: ММАО (1 mmol Al), Р(С2Н4) = 5

bar, in 200 ml of heptane. b Kg PE·(mol Fe)-1·bar-1. c Average activity, Kg PE·(mol Fe)-1·bar-1·min1 d

.

Content of PE chains with one isobutyl and one n-propyl end (mol %). e Content of PE chains

with two n-propyl ends (mol %).

f

Content of PE chains with one isobutyl and one vinyl end

(mol %). g Content of PE chains with one n-propyl and one vinyl end (mol %). h Sum of vinyl…nPr and vinyl…iBu chains (mol %). i Sum of vinyl…iBu and nPr…iBu chains (mol %).

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4. Conclusions In summary, upon the activation with MMAO, 2-[1-(2,6-dibenzhydryl-4-chlorophenylimino)ethyl]6-[1-mesityliminoethyl] pyridyliron dichloride 1 behaves like a highly active catalyst, affording linear PE with broad, bimodal-like, molecular-weight distribution. At short reaction time (≤ 3 min), low-MW PE (Log Mw < 4) is formed at 65 and at 80 °C. In the course of polymerization, the share of the high-MW PE fractions increases, in parallel with the increase of the Mw values of both the low-MW and the high-MW fractions. End-group analysis reveals the presence of two types of PE chains: (1) chains with one ibutyl and one n-propyl end (nPr…iBu) and (2) those with both n-propyl ends (nPr…nPr). The ratio of the (nPr…nPr) / (nPr…iBu) chains strongly depends on the co-catalyst composition, from 100:0 (with MAO) to ca. 30:70 (with commercial MMAO) and up to 14:86 (with MMAO + additional Al(iBu)3). Splitting the broad-MWD PE into narrow-MWD fractions, and

13

C NMR analysis of

each fraction, witnesses monotonous decrease of the share of the ‘iBu…nPr’ chains when going from low-MW fractions to high-MW fractions (from > 60 % to ca. 30 %). No vinyl-ended chains for PEs obtained on 1 have been observed by

13

C NMR, thus indicating that in the presence of

MMAO β-H transfer is virtually suppressed for this sterically demanding catalyst. The plausible consensus reaction scheme, explaining the mechanism of chain termination via transfer to aluminum alkyls (i.e. predominantly to (iBu)2AlMe at the early and middle stages of polymerization, and to Al(iBu)3 at late stages of polymerization), and the origin of the iBu and nPr end groups, has been proposed. In the framework of this mechanism, the (nPr…nPr) / (nPr…iBu) ratio is governed by the concentration of iBu-containing Al alkyls, introduced into the reaction mixture with the co-catalyst. In the course of polymerization, the content of the major chain transfer agent gradually decreases, which suppresses the chain transfer, giving rise to the monotonous increase of the share of the high-MW fraction, and of its molecular weight, which leads to ‘multisite-like’ polymerization behavior of catalyst 1, affording PE with bimodal-like molecularweight distribution. We notice that it is only the MW-weighted dWf/dLogM – LogM GPC traces 22


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that look bimodal-like, while the non-weighted, dNf/dLogM – LogM distributions exhibit no signs of bimodality. In the latter coordinates, the MWD curves monotonously broaden with increasing polymerization time, in parallel with increasing peak-MW of the curves. This ‘multisite-like’ catalytic pattern is different from that exhibited by truly multisite bis(imino)pyridine iron catalysts (for that at least two types of different active sites were found),11,17-19 as well as from that of industrially relevant hafnium pyridyl-amido catalysts (where peculiarities of activation processes have been invoked).31-33 In the catalyst system 1/B(C6F5)3+Al(iBu)3/C2H4, containing no (iBu)2AlMe, Al(iBu)3 is the only available aluminum alkyl in solution. It appears to be a much less efficient chain transfer agent compared with iBu2AlMe, leading to the prevalence of chain termination via β-H transfer, with the formation of high-MW polyethylene (Log Mw ≈ 5.6). In this catalyst system, the resulting PE has visibly unimodal and relatively narrow MWD. The catalytic patterns of the related symmetrical catalyst 2 are drastically different in the presence of different activators: Al(iBu)3 and MMAO. In the first case, the polyethylene MWDs are evidently bimodal (both the dWf/dLogM – LogM and the dNf/dLogM – LogM plots), reflecting two types of catalytically active sites.17-19 With MMAO, the catalytic behavior of 2 resembles the multisite-like behavior of catalyst 1, however, with some peculiarities stemming from the coexistence of two comparably important chain termination pathways: (1) chain transfer to Al-alkyls, and (2) β-hydride transfer.

5. Experimental Section Toluene was dried over molecular sieves (4Å), purified by refluxing over sodium metal and distilled in dry argon. Methylene chloride was dried over P2O5 and distilled in a vacuum. All solvents and prepared solutions were stored and handled in vacuum. All experiments were carried out in sealed high vacuum systems using breakseal techniques.

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Methylaluminoxane (MAO) was obtained from Crompton GmbH (Bergkamen) (toluene solution with total Al content 1.8 M). Polymeric AlMe3-depleted MAO (DMAO) - was prepared by vacuum distillation of commercial MAO at 50 °C according to ref. 34 and used as the toluene solution (total [Al] concentration 0.1 M, [Al] as AlMe3 0.005 M). Methylaluminoxane, modified with Al(iBu)3 (MMAO), was obtained from Akzo Nobel Corporation (as heptane solution with total [Al] concentration 1.7 M). The bis(imino)pyridyl iron (II) complexes 112 and 23 (Scheme 1) were prepared according to previously published procedures. In polymerization experiments, the iron complexes were used as solutions in CH2Cl2 (1 µmol Fe in 1mL). Ethylene polymerization was performed in a 0.5 L steel reactor. A sealed glass ampoule with 0.5 ml of the solution of complex 1 in CH2Cl2 was placed into the reactor. The reactor was heated at 80 ºC under vacuum for 1 h and cooled to 25 ºC, then charged with the solution of the cocatalyst in toluene (MAO, DMAO) or heptane (MMAO) (typically 1 mmol Al, 200 ml of 5·10-3 M solution). After setting up the desired polymerization temperature and ethylene pressure, the reaction was started by breaking the ampoule with the complex. During the reaction, the stirring speed, the temperature and the ethylene pressure were maintained constant through the automatic computer-controlled system for the ethylene feed, recording the ethylene consumption and providing the kinetic curve output both as a table and as a graph. After a prescribed time the reactor was vented, the obtained solid product was separated and dried at ambient conditions to constant weight. The detailed experimental conditions are given below in the footnotes of the tables. GPC measurements were performed using a PL-220 High Temperature Chromatograph equipped with set of PLgel Olexis columns. Run conditions used: 160 °C; flow rate 1 cm3/min; 1,2,4-trichlorobenzene was used as a solvent. Conventional calibration was made using PS standards and PE standards with narrow MWD. For the purposes of this study, the conventional dWf/dLogM – LogM MWD functions were converted into the dNf/dLogM – LogM MWD functions according to dNf(M)/dLogM = (1/M)⋅dWf(M)/dLogM, with subsequent normalization to unity. Viscosity (η) of the polymers was measured in decalin at 135 ºC on an Ubbelohde viscosimeter. The 24


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viscosity-average molecular weight Mv was calculated according to the Mark–Houwink equation: Mv = (η/K)1/α, with the Mark–Houwink coefficients K = 67.7×10-5; α = 0.67.35 For theoretical splitting, the experimental GPC traces were represented as a weighted sum of Gaussian curves. Separation of polymer into fractions with narrow MWD was performed using a PolymerChar fractionation station PREP mc2. 1 g of a sample was dissolved in a defined volume of xylene for 2 hours, then calculated amount of 2-(2-butoxyethoxy)-ethanol was added to the polymer solution to precipitate part of the polymer. Hot solution of the polymer was filtered into collecting flask. The precipitated polymer was dissolved in a new portion of xylene, and partially precipitated with a new portion of 2-(2-butoxyethoxy)-ethanol. The precipitation-dissolution procedure was repeated to obtain 5 fractions with a narrow MWD. The total volume of liquid (xylene and 2-(2butoxyethoxy)-ethanol) was always 180 mL. The last fraction was washed with pure xylene. 1

H and

13

C NMR spectra of polymers were measured on a Bruker Avance 400

spectrometer at 400.13 and 100.613 MHz, respectively, in 10 mm o.d. glass tubes in 1,2dichlorobenzene at 100 °C. IR spectra were measured on a Shimadzu 8400S FTIR spectrophotometer.

Associated content Supporting information, including the GPC traces with their theoretical splitting, details of PE fractionation, NMR spectra of the MMAO used, additional figures and tables. This material is available free of charge via the Internet at http://pubs.acs.org.

Author information Corresponding author E-mail: [email protected].

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ORCID Konstantin P. Bryliakov: 0000-0002-7009-8950 Wen-Hua Sun: 0000-0002-6614-9284

Notes The authors declare no competing financial interest.

Acknowledgments The NMR and GPC data were obtained within the framework of budget project 0303-2016-0009 for the Boreskov Institute of Catalysis. Catalyst 1 was prepared with the support of the joint grant of the Russian Foundation for Basic Research/National Natural Science Foundation of China (16-5353014 / 21611130026). We thank Dr. A. Barabanov for providing the original GPC data, used for the preparation of Figure 11A, and Mrs. M. Vanina for the FTIR measurements.

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(9)

Wang, Q.; Yang, H.; Fan, Z. Macromol. Rapid Commun. 2002, 23, 639-642.

(10) Semikolenova, N. V.; Zakharov, V. A.; Echevskaya, L. G.; Matsko, M. A.; Bryliakov, K. P.; Talsi, E. P. Catal. Today 2009, 144, 334-340. (11) Bryliakov, K. P.; Talsi, E. P.; Semikolenova, N. V.; Zakharov, V. A. Organometallics 2009, 28, 3225-3232. (12) Cao, X.; He, F.; Zhao, W.; Cai, Z.; Hao, X.; Shiono, T.; Redshaw, C.; Sun, W. H. Polymer 2012, 53, 1870-1880. (13) Yu, J.; Liu, H.; Zhang, W.; Hao, X.; Sun, W. H. Chem. Commun. 2011, 47, 3257-3259. (14) Zhao, W.; Yu, J.; Song, S.; Liu, H.; Hao, X.; Redshaw, C.; Sun, W. H. Polymer 2011, 53, 130-137. (15) Sun, W. H.; Zhao, W.; Yu, J.; Zhang, W.; Hao, X.; Redshaw, C. Macromol. Chem. Phys. 2012, 213, 1266-1273. (16) Zhang, W.; Wang, S.; Du, S.; Guo, C. Y.; Hao, X.; Sun, W. H. Macromol. Chem. Phys. 2014, 215, 1797-1809. (17) Barabanov, A. A.; Zakharov, V. A.; Semikolenova, N. V.; Echevskaja, L. G.; Matsko, M. A. Macromol. Chem. Phys. 2005, 206, 2292-2298. (18) Barabanov, A. A.; Bukatov, G. D.; Zakharov, V. A.; Semikolenova, N. V.; Mikenas, T. B.; Echevskaja, L. G.; Matsko, M. A. Macromol. Chem. Phys. 2006, 207, 1368-1375. (19) Kissin, Y. V. ; Qian, C.; Xie, G.; Chen, Y. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 6159-6170. (20) The only exception was the 3-min PE, for which the contribution of the third, ultrahighmolecular-weight component was observed; this contribution was higher at 65 °C, and much lower 80 °C. At longer polymerization times, the UHMW component virtually vanished. (21) DMAO is AlMe3-depleted MAO, see Experimental Section for details.

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(22) Britovsek, G. J. P.; Cohen, S. A.; Gibson, V. C.; van Meurs, M. J. Am. Chem. Soc. 2004, 126, 10701-10712. (23) Bryliakov, K. P.; Semikolenova, N. V.; Panchenko, V. N.; Zakharov, V. A.; Brintzinger, H. H.; Talsi, E. P. Macromol. Chem. Phys. 2006, 207, 327-335. (24) Apparently, the steric demand of alkyls iBu2AlMe iBuAlMe2 is much lower compared with Al(iBu)3, which makes them efficient chain transfer agents for catalyst 1. (25) Bryliakov, K. P.; Talsi, E. P. Coord. Chem. Rev. 2012, 256, 2994-3007. (26) Previously, formation of i-butyl end groups, origination from the MMAO activator, was described for the polymerization of propene over symmetrical bis(imino)pyridine iron(II) complexes, and was explained by the i-butyl group transfer from MMAO and Al(iBu)3 to the chain-propagating Fe species, see: Small, B. L.; Brookhart, M. Macromolecules 1999, 32, 2120-2130. (27) Once the Al-capped PE chain has formed, it resides in the reaction solution until the reaction is quenched. (28) It has also been assumed that the contribution of β-H transfer is not significant. However, in the catalyst systems 1/B(C6F5)3+TIBA/C2H4, β-H transfer may play the key role; in effect, the rate of chain transfer does not significantly depend on the concentration of Al alkyls, and does not substantially change within the polymerization time, thus resulting in a visibly unimodal PE with relatively narrow MWD (entry 5 of Table 3, and Figure 8B). (29) Lew, R.; Suwanda, D.; Balke, S. T. J. Appl. Polymer Sci. 1988, 35, 1049-1063. (30) The effect of MMAO concentration on the bimodal-like MWD of polyethylene formed on symmetrical bis(imino)pyridine catalyst in the presence of MMAO was reported by Brookhart and co-workers, who pointed out the viable role of chain transfer to aluminium (ref. 1). Similar effects were reported by Wang and co-workers, by using BTEAO as activator: Wang, Q.; Li, L.; Fan, Z. Eur. Polym. J. 2004, 40, 1881-1886.

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(31) Zuccaccia, C.; Macchioni, A.; Busico, V.; Cipullo, R.; Talarico, G.; Alfano, F.; Boone, H. W.; Frazier, K. A.; Hustad, P. D.; Stevens, J. C.; Vosejpka, P. C.; Abboud, K. A. J. Am. Chem. Soc. 2008, 130, 10354–10368. (32) Zuccaccia, C.; Busico, V.; Cipullo, R.; Talarico, G.; Froese, R. D. J.; Vosejpka, P. C.; Hustad, P. D.; Macchioni, A. Organometallics 2009, 28, 5445–5458. (33) Luca Rocchigiani, Vincenzo Busico, Antonello Pastore, Giovanni Talarico, and Alceo Macchioni. Angew. Chem. Int. Ed. 2014, 53, 2157–2161. (34) Talsi, E. P.; Babushkin, D. E.; Semikolenova, N. V.; Zudin, V. N.; Panchenko, N. V.; Zakharov, V. A. Macromol. Chem. Phys. 2001, 202, 2046-2051. (35) Brandrup J.; Immergut, E. H. Polymer Handbook, 4th ed.; Wiley: New York, Sec VII, 1999.

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TOC graphic

LFeCl2 / MMAO + C2H4 3 min

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dNf / d log M

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TOC in TIFF format 44x37mm (300 x 300 DPI)


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