Cooperative Catalysis by Multiple Active Centers of a Half-Titanocene

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Cooperative Catalysis by Multiple Active Centers of HalfTitanocene Catalyst Integrated in Polymer Random Coil Ashutosh Thakur, Ryuki Baba, Toru Wada, Patchanee Chammingkwan, and Toshiaki Taniike ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00214 • Publication Date (Web): 14 Mar 2019 Downloaded from http://pubs.acs.org on March 14, 2019

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ACS Catalysis

Cooperative Catalysis by Multiple Active Centers of Half-Titanocene Catalyst Integrated in Polymer Random Coil

Ashutosh Thakur, Ryuki Baba, Toru Wada, Patchanee Chammingkwan, and Toshiaki Taniike*

Graduate School of Advanced Science and Technology, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan

ACS Paragon Plus Environment

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ABSTRACT:

To study cooperative catalysis in ethylene polymerization, a series of polynorbornene (PNB)-supported half-titanocene catalysts were synthesized by grafting a half-titanocene complex to monodisperse PNB chains through aryloxide ligands. Their performance in ethylene polymerization was extensively investigated to uncover the mechanism of cooperative catalysis among multiple Ti centers those are confined in a nano-sized random coil of PNB chains. It was found that under full activation, the activity of the PNBsupported catalysts was consistently higher than that of the unsupported molecular catalyst. At a lower polymerization temperature, the cooperation effect prevailed and the activity tended to improve as the number of Ti centers per chain was increased. At a higher temperature, a smaller number of Ti centers per chain led to better activity due to the active-site isolation effect. Based on a kinetic examination, the cooperation at a lower temperature was attributed to the enhancement of the propagation rate constant, where confined active centers as monomer trapping sites plausibly suppressed the entropic loss in the π-complexation.


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ACS Catalysis

KEYWORDS: cooperative catalysis, polymer-supported catalyst, half-titanocene catalyst, ethylene polymerization, active centers per random coil


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INTRODUCTION In catalysis, “cooperative effect” is a term which describes the improvement in catalytic performance by synergistic combination of two or more proximate active centers during a chemical transformation.1–3 It is an intrinsic feature of enzymes, where chemically and conformationally favorable cooperation among multiple active centers within a confined space synergistically activates a specific reactant, leading to superior activity and selectivity.4–6 In the field of abiotic homogeneous catalysis, synthesis of nature inspired multicenter complexes to explore cooperative catalysis has received great interest. For example, cooperation of Lewis base and acid centers within a single catalyst molecule was embodied for a variety of organic transformations, which included Corey-Chaykovsky epoxidations, asymmetric Strecker reactions, Diels-Alder reactions, metallosalencatalyzed asymmetric ring opening of epoxides, and enantioselective cycloaddition reactions between ketene enolates and various electrophiles.7–11 Besides, hybrid heterogeneous catalysts that co-immobilize two or more molecular catalytic components on a single support have also been investigated in recent years to explore cooperative catalysis in organic transformations.12–14


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In the field of catalytic olefin polymerization, it has been assumed that the co-presence of proximate metal centers would improve selectivity for a distinctive enchainment pathway to access a unique polymer microstructure, which directly impacts the polymer properties and their end-use applications. Thus, in the past two decades, great efforts have been devoted to design multinuclear complexes for cooperative olefin polymerization.15

Representative

examples

are

early

transition

metal-based

binuclear/trinuclear metallocene and half-metallocene catalysts,16,17 late transition metalbased binuclear/tetranuclear phenoxyiminato and α-diimine catalysts,18–21 and so on. These catalysts generally display improved activity and produce polymers with higher molecular weight as compared to their mononuclear analogues. It has been suggested that the local high concentration of metal centers in multinuclear catalysts enhances monomer trapping and accounts for said positive consequences.15 Progress in this area was augmented by the elegant research of Marks et al., in which constrained geometry complexes of homo and hetero binuclear types in combination with a binuclear borate complex were employed in olefin polymerization.15,22–24 Synergistic catalyst-cocatalyst nuclearity helps to bring two active sites to an optimum distance for cooperative catalysis,


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which results in the marked alteration of polymer microstructures in terms of molecular weight, branching as compared to their mononuclear analogues. In addition, using rigid ligands, groups of Osakada, Agapie and Ma, independently demonstrated the importance of metal-metal distance for effective cooperation between the active sites of binuclear nickel phenoxyiminato catalysts.25–27 These binuclear catalysts offered higher activity, polymers with higher molecular weight, and improved tolerance toward polar monomers in ethylene (co)polymerization. Surely, the advantage of the multinuclear (mostly binuclear) complexes is attributed to the design precession at molecular level, while the integration of a multiple number of nuclei in a desired ligand framework is restricted by synthetic limitations. On the other hand, such limitations could be offset to some extent by co-immobilizing multiple molecular catalysts on solid supports.28–30 However, cooperation on a solid surface has been scarcely established owing to the surface heterogeneity, diffusion limitation in the presence of pores, and unfavorable interactions with surfaces of the commonly employed supports. From this perspective, we have recently developed a new catalyst system for the integration of functions in a well-defined way by confining multiple active centers in a


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ACS Catalysis

random coil of a single polynorbornene (PNB) chain.31 Monodisperse PNB chains bearing a predefined number of ancillary donor ligands at the side chain were synthesized by Grubbs ring-opening metathesis polymerization. They were used as soluble supports for half-titanocene complexes to yield PNB-supported catalysts with a controlled number of active centers per random coil. When activated with a sterically less frustrated borate system, the PNB-supported catalysts exhibited higher activity than that of the corresponding molecular analogues, plausibly due to cooperation among multiple active centers in a nano-sized random coil of the PNB chains. These previous findings have motivated us to systematically investigate the mechanism of cooperation in ethylene polymerization. In this study, a series of PNB-supported aryloxide-containing half-titanocene catalysts with different numbers of Ti centers per random coil were synthesized. The origin of cooperation among confined active centers and other consequences of supporting a molecular catalyst were investigated. The cooperation among multiple functional groups in a single polymer chain has been known as the “cluster effect”, for example, in the protein recognition by glycopolymers and in the electrical conductivity of radical


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polymers.32,33 This study corresponds to one of few works focused on the cluster effect in the field of catalysis.

EXPERIMENTAL SECTION Materials Commercially available chemicals of a reagent grade were used without further purification unless specified. The Grubbs 1st generation catalyst and ethyl vinyl ether were

received

from

Sigma-Aldrich.

pentamethylcyclopentadienyltitanium

(IV)

Bicyclo[2.2.1]hept-2-ene trichloride

(Cp*TiCl3)

(norbornene), and

trityl

tetrakis(pentafluorophenyl)borate (Ph3CB(C6F5)4) were received from Tokyo Chemical Industry. 4-Bicyclo[2.2.1]hept-5-en-2-yl-2,6-dimethylphenol (2-aryloxonorbornene) and the molecular catalyst (Cp*Ti(O-2,6-Me2C6H3)Cl2) were synthesized according to previous reports.31,34 Ethylene of a polymerization grade and tri-iso-butylaluminum (TIBA) were

donated

by

Sumitomo

Chemical

and

Tosoh

Finechem,

respectively.

Dichloromethane (CH2Cl2) and toluene of an anhydrous grade were obtained from Sigma-


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Aldrich and Wako Pure Chemical Industries. They were additionally dried over molecular sieves 3A and 4A with N2 bubbling. Diethyl ether of an anhydrous grade was purchased from Tokyo Chemical Industry and degassed prior to use. Deuterated chloroform (CDCl3, Kanto Chemical) was stored over molecular sieves 3A and degassed before use.

Measurements 1H

and

13C

{1H} NMR were recorded on a 400 MHz Bruker Advance FT-NMR

spectrometer in CDCl3 at 25 °C. The chemical shifts were referenced to the peak of CHCl3 at 7.26 ppm and the peak of CDCl3 at 77.16 ppm in the 1H and

13C

NMR, respectively.

Gel permeation chromatography (GPC, Shimadzu SCL-10A) equipped with a RID-10A detector was used to determine the molecular weight distribution of the PNB supports against polystyrene standards at 35 °C, using tetrahydrofuran as the eluent. UV-Vis spectroscopy (JASCO V670) was utilized to acquire the absorption spectrum of the molecular and PNB-supported catalysts in a toluene solution at the concentration of 0.25– 0.4 μmol-Ti/mL. The molecular weight of polyethylene (PE) was acquired against polystyrene standards by high-temperature GPC (Waters 150C) equipped with


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polystyrene gel columns (Showa Denko AD806M/S) and an IR detector (MIRAN 1A) at 140 C, using 1,2,4-trichlorobenzene as the eluent. The melting temperature (Tm) and crystallinity (Xc) of PE were measured on a differential scanning calorimeter (DSC, Mettler Toledo DSC-822) based on the first heating cycle at the rate of 10 °C/min up to 180 °C, and under N2 flow of 200 mL/min.

Synthesis of PNB supports To investigate the effect of numbers of active centers per chain on cooperative catalysis, four PNB supports with different aryloxide ligand contents were synthesized (Table 1). For this purpose, 0.10–1.6 mmol of 2-aryloxonorbornene was mixed with a fixed amount of norbornene (12.8 mmol) and the resultant mixture was dissolved in CH2Cl2 (12 mL) in a Schlenk tube under N2. The polymerization was initiated by adding a CH2Cl2 solution of the Grubbs 1st generation catalyst (8.0 mmol/L) at room temperature. After 3 h, polymerization was terminated by adding an excess of ethyl vinyl ether. The resultant polymer solution was poured into methanol, collected by filtration after repetitive washing and dried under vacuum. The polymer was additionally purified by reprecipitation from


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ACS Catalysis

CH2Cl2/diethyl ether and finally collected as a white solid after vacuum drying for several hours. The peaks in the 1H NMR spectra (Figure S1) were assigned according to literature.35,36

Synthesis of PNB-supported catalysts (1a–d) Four PNB-supported catalysts with different numbers of active centers per chain were synthesized using the PNB supports (Table 1). A flame-dried Schlenk tube was exchanged with N2 and charged with a solution of Cp*TiCl3 (1.5–2.7 mmol) in CH2Cl2 (20– 25 mL). To this, a solution of a PNB support (1/26–1/6 equivalent of the aryloxide ligand with respect to Cp*TiCl3) in CH2Cl2 (20–30 mL) was added dropwise under N2 with continuous stirring at room temperature. The solution was then slowly warmed to the boiling temperature of the solvent and then refluxed for 36–42 h. It should be noted that the Ti concentration was maintained over 10 mg/mL to avoid catalyst deactivation by trace water. Thereafter, the product was precipitated out by adding diethyl ether, and the unreacted portion of Cp*TiCl3 was washed out. The catalysts were obtained in 75–80%


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yield as pale to intense red solids after repetitive washing with diethyl ether and vacuum drying for several hours at room temperature.

Ethylene polymerization In the previous study,31 it was found that the activation of a PNB-supported catalyst was sterically hindered when bulky activator such as modified methyl aluminoxane was used. Therefore, a less bulky TIBA/Ph3CB(C6F5)4 activator system was used throughout this study. Ethylene polymerization was conducted in a 1 L stainless steel reactor equipped with a mechanical stirrer rotating at 350 rpm. In a typical experiment, e.g. run 1 in Table 2, the reactor was charged with toluene (300 mL) and TIBA (Al = 2.5 mmol) under N2. After replacing the N2, the solution was saturated with ethylene (0.6 MPa, absolute pressure) at 0 °C. Thereafter, a toluene solution (0.5 mL) of the catalyst (Ti = 2.5 µmol) was injected into the reactor, followed by the injection of a toluene solution (0.5 mL) of Ph3CB(C6F5)4 (2.5 µmol) to initiate the polymerization. The polymerization was performed for 10 min with a continuous supply of ethylene at 0.6 MPa and finally quenched by adding


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ACS Catalysis

an excess of ethanol. The resultant polymer was collected by filtration and dried under vacuum for several hours at 60 °C.

RESULTS AND DISCUSSION Synthesis and characterization The PNB supports were prepared by ring opening metathesis polymerization between norbornene and 2-aryloxonorbornene (Scheme 1) using the Grubbs 1st generation catalyst. Random copolymerization was conducted at a fixed norbornene/catalyst ratio, while varying the amount of 2-aryloxonorbornene in the feed (Table 1). Briefly, more than 90% of the yield and nearly complete comonomer incorporation were observed for all the feed compositions. The obtained PNB supports contained a mixture of trans and cis olefinic double bonds, where the trans double bonds dominated over 82%, which was consistent with the previous results.37 The PNB supports exhibited Mw/Mn values close to one, validating the well-defined nature of the synthesized PNB supports. The narrow molecular weight distribution allowed to reasonably estimate the number of pendant groups per chain for each PNB support from the corresponding Mn value and comonomer


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content, which was found to vary in the range of 3‒73. It should be noted that compared with this variation, the deviation in the Mn values of the resultant PNB supports was not so significant, suggesting that any difference in the performance of the PNB-supported catalysts should dominantly arise from difference in the number of active centers per random coil rather than its size. To synthesize PNB-supported catalysts with different numbers of active centers per chain, Cp*TiCl3 was grafted to the PNB supports bearing different contents of aryloxide groups (Table 1). In 1H NMR (Figure S2), the peak shift of the methyl protons of the Cp ring from 2.39 ppm in Cp*TiCl3 to 2.20 ppm validated successful grafting. In

13C

NMR

(Figure S3), the peak of the carbon (Cd) that is bonded to the oxygen of the aryloxo group was found to be shifted from 150.2 ppm in the PNB support to 160.5 ppm. The other peaks of the aryloxide and Cp* ligands were also shifted to prove the successful grafting of Cp*TiCl3 to the PNB supports,31 irrespective of the aryloxide content. In addition, the similar chemical shifts for the peaks in the 1H and

13C

NMR spectra (Figure S4) of the

molecular catalyst (2) also validated successful synthesis of the PNB-supported catalysts.


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ACS Catalysis

For all the four PNB-supported catalysts (1a–d), the graft yield was derived from 1H NMR based on

𝐺𝑟𝑎𝑓𝑡 𝑦𝑖𝑒𝑙𝑑 (%) =

1 15 𝐼𝐶𝑝 ― 𝐶𝐻3 1 2 𝐼 𝐻𝑏

× 100

(1).

It was found that the graft yield became constant, which was approximately 88% (Table 1). The number of Ti centers per PNB chain of a PNB-supported catalyst was estimated based on the graft yield and the number of pendant groups per chain of the corresponding PNB support, in the range of 2.6–65 (Table 1).

Scheme 1. Synthesis of PNB-supported catalysts.


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Table 1. Summary of PNB-supported catalyst synthesis PNB supportsa Catalyst

Graft yieldf (%)

Ti centers/chaing

3

88.0

2.6

1.20

11

88.5

9.7

7.0

1.18

34

88.3

30

7.7

1.16

73

88.6

65

Comonomer in Comonomer

Mnd ×

feedb (mol%)

contentc (mol%)

10−4

1a

0.7

0.5

5.5

1.17

1b

2.2

2.0

5.3

1c

5.0

4.8

1d

11

10

aPolymerization

Mw/Mnd

Pendant groups/chaine

conditions: Norbornene = 12.8 mmol, catalyst = 64.0 µmol, CH2Cl2 = 20 mL, T = 25 °C, t = 3.0 h. b[2-

Aryloxonorbornene/(Norbornene + 2-Aryloxonorbornene)] × 100. cDerived from 1H NMR. dDetermined by GPC. eCalculated based on the Mn value and the comonomer content. fCalculated based on Eq. 1. gCalculated from the graft yield and the number of pendant groups per chain of the corresponding PNB support.


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ACS Catalysis

Ethylene polymerization In general, aryloxide-containing half-titanocene molecular catalysts exhibit superior performance at a lower temperature, as is evident from higher catalytic activity as well as narrower molecular weight distribution of the product polymers.34,38 In order to separately comprehend the cooperative effect from the other effects of supporting, the polymerization was respectively conducted at 0 °C and 30 °C. The results are summarized in Table 2, in which the polymerization results using the molecular analogue (2) are also included for comparison. It was found that 2 exhibited a high activity, comparable to previously reported activity of aryloxide-containing half-titanocene catalysts.34,38,39 The activity of the PNB-supported catalysts (1a–d) at 0 °C was consistently higher than that of 2, while the activity tended to increase with the increase of the number of Ti centers per chain of the catalysts as depicted in Figure 1a. The highest activity was observed for 1d bearing 65 Ti centers per chain, which was roughly double of the activity of 2 (run 9 vs. 1). The activity of the PNB-supported catalysts was also higher than that of 2 at 30 °C. However, contrary to the trend at 0 °C, the activity tended to decrease with the increase of the number of Ti centers per chain (Figure 1b), in which


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the activity of 1a was nearly double of the activity of 2 (run 4 vs. 2). All the catalysts in Table 2 produced PE with unimodal and narrow molecular weight distribution, typical for aryloxide-containing half-titanocene catalysts.34,38,39 The molecular weight distribution tended to be narrower at a lower temperature, suggesting the improved single site nature of the catalysts. The catalysts bearing a smaller number of Ti centers per chain (1a,b) produced PE with higher molecular weight, probably due to the suppression of chain transfer reactions under low activity conditions (i.e. less heat evolved). The measured Tm and Xc values of the nascent PE samples were typical for high density PE and similar with each other. These results suggest that the observed difference in activity among the catalysts was not likely attributed to diffusion limitation of reagents driven by difference in crystallinity of the formed polymers.


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ACS Catalysis

Table 2. Ethylene polymerization resultsa

Run

Catalyst

1

Ti centers T

Activity

(kg-PE/mol-Ti.h)

Mwb ×

Mw/Mn

b

Tmc

Xcc

(°C)

(%)

/chain

(°C)

2

–

0

16400 ± 390

5.7

2.5

137.0

78

2

2

–

30

15900 ± 490

2.3

3.6

134.0

74

3

1a

2.6

0

22100 ± 50

6.5

2.9

138.0

80

4

1a

2.6

30

29800 ± 150

4.6

3.7

137.0

74

5

1b

9.7

0

22800 ± 100

6.2

2.9

137.5

78

6

1b

9.7

30

20000

–

–

136.0

76

7

1c

30

0

28600 ± 110

4.9

2.3

137.2

77

8

1c

30

30

17300 ± 650

4.0

3.7

137.5

76

9

1d

65

0

29800 ± 200

5.8

2.6

137.5

81

aPolymerization

10−5

conditions: Ethylene pressure = 0.6 MPa, toluene = 300 mL, catalyst = 2.5 µmol, TIBA = 2.5 mmol,

Ti/Ph3CB(C6F5)4 = 1 mol/mol, t =10 min. bDetermined by high-temperature GPC. cMeasured by DSC.


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Figure 1. Effect of the number of Ti centers per chain on the ethylene polymerization activity of the PNB-supported catalysts: a) At 0 °C and b) at 30 °C.

From Table 2, it is clear that the PNB support improved the activity of the aryloxidecontaining half-titanocene catalyst under all conditions. It should be noted that at least


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ACS Catalysis

two mechanisms must exist to explain the opposite activity trends at the two different temperatures. Regarding the activity enhancement along the number of Ti centers per chain at 0 °C, potential cooperation among multiple active centers in a nano-sized random coil needs to be assumed, whose origin will be discussed later. On the other hand, the decreasing tendency at 30 °C can be mainly explained by considering the effect of activesite isolation during polymerization. The occurrence of deactivation at 30 °C was suggested from the fact that the activity of 1c and 2 was lowered and the Mw/Mn value was enlarged by increasing the temperature from 0 to 30 °C. It is known that molecular catalysts are prone to deactivate by a bimolecular pathway i.e. reduction and subsequent dimerization to form inactive species. Supporting of a molecular catalyst prevents the said deactivation by active-site isolation.40,41 Out of all the three PNB-supported catalysts, 1a could facilitate the maximum active-site isolation owing to the least Ti centers per chain, leading to its highest activity among all the catalysts at the polymerization temperature of 30 °C. This explanation is consistent with the decreasing activity of the three PNBsupported catalysts in the order of 1a > 1b > 1c. Moreover, in the case of 1a, the observed higher activity at 30 °C as compared to 0 °C was based on the Arrhenius behaviour, i.e.


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activity is supposed to increase exponentially with the temperature in the absence of catalyst deactivation. These results established that the activity of the PNB-supported catalysts depended on the interplay between cooperation among multiple active centers and stabilization of a catalyst by active-site isolation. At a lower temperature, there was negligible deactivation of the PNB-supported catalysts and therefore the effect of cooperation among multiple active centers dominated to increase the activity with the increase of the number of Ti centers per chain. However, the cooperative activity enhancement in polymerization tended to converge, plausibly due to increase in bulkiness as the number of Ti centers per chain was increased. Contrary to the PNB-supported catalysts, the absence of both the cooperation and active-site isolation effects lets the molecular catalyst (2) exhibit lower activity than the supported catalysts. Activity enhancement by supporting a molecular catalyst has been rarely reported for solid surfaces.28,42,43 In general, a support mitigates bimolecular deactivation of the molecular catalyst by active-site isolation and stabilizes the active sites by dissipating the heat of polymerization. However, deactivation by uncleaved hydroxyl groups, mass transfer limitation (i.e. slower diffusion with respect to catalysis) of reagents by the pores


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of a support, and/or steric hindrance from the support surfaces interfere the catalysis and these interferences overcome the positive effects of the supporting. In contrast to the solid catalysts, the mass transfer limitation of reagents was negligible for the PNB-supported catalysts due to their homogeneous nature. Also, the flexibility of a PNB chain should sufficiently reduce the steric hindrance as compared to the solid supports, especially when a less bulky TIBA/Ph3CB(C6F5)4 activator system was used.31 The performance of an olefin polymerization catalyst is determined by three factors: i) Lifetime of the active sites, ii) the intrinsic activity of the active sites, represented by the propagation rate constant (kp), and iii) the number of active sites, represented by the working active-site concentration ([C*]). At 0 °C, the deactivation of the active sites was believed to be much less than that at 30 °C, at least for the PNB-supported catalysts. Neglecting the contribution from the lifetime of the active sites, the activity of an olefin polymerization catalyst can be approximated by44,45 (2),

𝑅𝑝 = 𝑘𝑝 [𝐶 ∗ ] [𝑀]

which corresponds to the low coverage regime of the Langmuir-Hinshelwood mechanism.46 Here, [M] represents the monomer concentration in the polymerization


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medium. Therefore, the origin of cooperative catalysis, which enhanced the activity of the PNB-supported catalysts along the number of Ti centers at 0 °C, could be attributed to a higher value of both/either kp and/or [C*]. In order to comprehend the origin of cooperative catalysis, a kinetic aspect of the PNB-supported (1a,d) and molecular catalysts (2) was investigated. Ethylene polymerization was conducted at the Al/Ti ratio of 140–1000 and at 0 °C. In our previous study,31 it was found that the alkylation of the PNB-supported catalysts was much more sterically hindered than the ionization. Therefore, the activation/deactivation behavior of the catalysts was studied by changing the Al/Ti ratio through the TIBA amount. The results are summarized in Table 3 and Figure 2. It was found that the activity of 2 became the highest at the Al/Ti ratio of 140, while it decreased with the increase of the Al/Ti ratio. This suggested that the molecular catalyst was most accessible to reagents and therefore it was fully activated even at the lowest Al/Ti ratio. However, owing to the absence of active-site isolation, the molecular catalyst had the highest propensity for deactivation and hence its activity decreased with the increase of the Al/Ti ratio. In contrast, the activity of the PNB-supported catalysts increased with the increase of the


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Al/Ti ratio. This is consistent with our previous results that alkylation of the PNB-supported catalysts was sterically hindered. The activity converged in a similar manner and reached a plateau at the highest Al/Ti ratio, which validated our assumption that deactivation of the active sites of the PNB-supported catalysts was negligible at 0 °C. Hence, [C*] did not increase along the number of Ti centers per chain of the PNB-supported catalysts. The above discussion suggests that the origin of cooperative catalysis is not likely to be attributed to the difference of [C*].


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Table 3. Effect of the Al/Ti ratio on the polymerization activitya Run

Catalyst

Al/Ti (mol/mol)

Activity (kg-PE/mol-Ti.h)

10

2

140

22300 ± 360

11

2

260

19900

12

2

500

20300 ± 340

13

2

1000

16400 ± 390

14

1a

140

14900

15

1a

260

17500 ± 200

16

1a

500

19200 ± 240

17

1a

1000

22100 ± 50

18

1d

140

20100 ± 700

19

1d

260

23800 ± 900

20

1d

500

28800

21

1d

1000

29500

a Polymerization

conditions: Ethylene pressure = 0.6 MPa, toluene = 300 mL, catalyst =

2.5 µmol, TIBA = 0.35–2.5 mmol, Ti/Ph3CB(C6F5)4 = 1 mol/mol, T = 0 °C, t =10 min.


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Figure 2. Effect of the Al/Ti ratio on the ethylene polymerization activity of the PNBsupported catalysts (1a,d) and the molecular analogue (2).

Next, polymerization was conducted, varying the ethylene pressure in the range of 0.12–0.6 MPa at 0 °C. The results are summarized in Table 4 and Figure 3. It was found that the activity of the molecular catalyst increased upto the ethylene pressure of 0.4 MPa and then saturated plausibly due to the deactivation by the heat of the polymerization. In contrast, the activity of the two PNB-supported catalysts showed a first-order dependence on the ethylene pressure, which again confirmed our assumption that the deactivation was negligible at 0 °C. While the activity of both 1a and 1d increased linearly with the


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ethylene pressure (Figure 3), the gradient of 1d was greater than that of 1a. In Figure 3, the gradient of a straight line is proportional to the product of kp and [C*] (Rp = kp [C*] [M]). Since, [C*] was assumed to be similar for 1a and 1d, the ratio of kp for the two PNBsupported catalysts could be determined by the ratio of the gradients of their respective straight lines in Figure 3, which was found to be kp,1d/kp,1a ≈ 1.4. Thus, the origin of cooperation among multiple active centers confined in a random coil was plausibly attributed to the kp enhancement.


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Table 4. Effect of the ethylene pressure on the polymerization activitya Run

Catalyst

Ethylene pressure (MPa) Activity (kg-PE/mol-Ti.h)

22

2

0.12

9800

23

2

0.2

11700 ± 100

24

2

0.3

13400

25

2

0.4

15400

26

2

0.6

16400 ± 390

27

1a

0.12

6200

28

1a

0.2

9600

29

1a

0.3

13400 ± 210

30

1a

0.4

16100

31

1a

0.6

22100 ± 50

32

1d

0.12

6700

33

1d

0.2

11000

34

1d

0.3

15100 ± 100

35

1d

0.4

18000

36

1d

0.6

29500

a

Polymerization conditions: Toluene = 300 mL, catalyst = 2.5 µmol, TIBA = 2.5 mmol,

Ti/Ph3CB(C6F5)4 = 1 mol/mol, T = 0 °C, t =10 min.


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Figure 3. Effect of the ethylene pressure on the ethylene polymerization activity of PNBsupported catalysts (1a,d) and the molecular analogue (2).

The kp of polymerization can be expressed by the Eyring equation47, which has the form of

𝑘𝑝 =

𝑘𝐵𝑇 ℎ

‡  𝑆

( )𝑒𝑥𝑝(

𝑒𝑥𝑝

𝑅

― ‡ 𝐻 𝑅𝑇

)

(3),

where kB is the Boltzmann constant, h is the Planck’s constant, and ‡H and ‡S are the enthalpy and entropy of activation, respectively. ‡H is mainly related to the energetic barrier for monomer insertion, whereas ‡S is mainly affected by the monomer-catalysts complexation.44 Thus, Eq. 3 explains that kp increases, when both/either enthalpic penalty


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in insertion and/or entropic loss in complexation are reduced. For any reaction, ‡H can be lowered by stabilizing the transition state and in catalysis, energy of the transition state depends on the active-site nature. The similar 1H and 13C NMR spectra of the employed catalysts (both supported and molecular catalysts) as well as similar molecular weight of produced PE suggested that all the catalysts had an identical active-site nature. However, to obtain a direct picture of the electronic properties of the catalysts, UV-Vis spectroscopy was used. The spectra of Cp*TiCl3 and the molecular catalyst (2) in toluene are represented in Figure 4a. For Cp*TiCl3, an absorption band with its maximum (λmax) located at 452 nm was observed due to the ligand-to-metal charge-transfer transitions (LMCT)48 of Cp* to Ti. By replacing one electron withdrawing chloride ligand with the electron donating aryloxide ligand, the LMCT was blue shifted for 2 with the λmax value observed at 416 nm. Similar hypsochromic shifts were reported upon increase of electron density on the metal center of zirconocene and half-titanocnene catalysts.40,49 It was observed that another absorption feature arose in the spectrum of 2 in the UV range with the λmax value centerd at 333 nm. This second peak was attributed to the π–π* transition of the benzene ring of the 2-aryloxide ligand.50 Figure 4b represents the absorption


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spectra of a PNB support and homo PNB dissolved in toluene at a polymer concentration of 2.0 mg/mL. Both the polymers did not have specific absorptions in the visible region, whereas only the PNB support exhibited a typical absorption band in the UV region due to the π–π* transition of the benzene ring of the 2-aryloxide pendant groups with the λmax value located at 332 nm. Figure 4c represents the absorption spectra of three PNBsupported catalysts (1a,c,d) dissolved in toluene along with the molecular catalyst (2), in which the peak due to the absorption corresponding to the π–π* transition of the benzene ring of the 2-aryloxide ligand was observed in the case of 1c,d. Contrary, the same peak was not observed in the case of 1a plausibly due to the scarce concentration of aryloxide ligands distributed in a large amount of polymer. In Figure 4c, the λmax value for the LMCT transition was identified in the range of 418–422 nm, almost identical to the corresponding λmax value of 2. The similar absorption patterns in the case of the molecular catalyst (2) and the PNB-supported catalysts assured that the electronic environment around the Ti centers was similar with each other. Most importantly, the electronic environment was unaffected by the number of Ti centers per chain of the PNB-supported catalysts.


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Figure 4. UV-Vis spectra: a) Cp*TiCl3 and molecular catalyst (2), b) homo PNB and PNB support (4.8 mol% of comonomer content), and c) PNB-supported catalysts along with the molecular catalyst (2).

Thus, it was believed that ‡H in ethylene polymerization was similar for all the PNBsupported catalysts and their molecular analogue. This implies that the improvement of

kp by the increase of Ti centers per chain was due to a lower entropic loss during monomer-catalyst complexation. In general, a complex between a supported active site and a substrate is formed as a sequential result of adsorption of the substrate on the surface of a large area and its diffusion to the active site.51,52 In the adsorption, the substrate transfers only the degrees of freedom in the surface-normal direction, and maintains the other degrees of freedom in the surface-parallel direction. During the diffusion over the surface, the substrate gradually loses the remaining degrees of freedom, and finally get entrapped in the reaction sphere of the active site. In the case of molecular catalysts, the substrate must lose all the degrees of freedom at once in the complexation event. Thus, the entropic loss in the complexation is suppressed in the case


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of solid catalysts, which offsets the low collision frequency of the substrate at the active site as compared to molecular catalysts. Similarly, in the case of the PNB-supported catalysts with a higher number of Ti centers per chain, a high local concentration of active sites offers a number of monomer-trapping sites, which is expected to decrease the entropic loss in the π-complexation to improve kp of polymerization, as represented by Scheme 2.

Scheme 2. Proposed mechanism of cooperation and active-site isolation in ethylene polymerization by PNB-supported catalysts.

CONCLUSIONS


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In order to exploit cooperative catalysis in ethylene polymerization, a series of polynorbornene (PNB)-supported half-titanocene catalysts bearing different numbers of Ti centers per PNB chains were synthesized. The bottom-up strategy starting from the synthesis of well-defined PNB supports containing a predefined number of grafted aryloxide ligands per chain enabled to precisely control the number of Ti centers per PNB chain for the systematic investigation of the cooperation among multiple active centers in a random coil. Through extensive investigation on the ethylene polymerization performance of these catalysts, two important mechanisms were uncovered i.e. cooperation among concentrated active sites and active-site isolation. A kinetic study of the ethylene polymerization by the PNB-supported and molecular catalysts postulated that the cooperation was attributed to the enhancement of the propagation rate constant (kp). It was plausibly driven by the suppression of the entropic loss in the π-complexation due to the confinement of multiple active centers as monomer trapping sites in a random coil. Based on the findings of this research, we have successfully established a new direction for the exploitation of cooperative catalysis in the field of catalytic olefin polymerization.


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Most importantly, the precise catalysts design using well-defined polymer supports enabled to study cooperative catalysis for the first time in supported molecular catalysts.

AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]; Tel: +81-761-51-1630; Fax: +81-761-51-1635

Author Contributions

A.T. performed experiments and wrote the manuscript. R.B. performed experiments and co-wrote the manuscript. T.W. and P.C. analyzed data and supported the discussion. T.T. conceived the idea and supervised the entire research. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.


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SUPPORTING INFORMATION

The 1H NMR spectra of PNB supports, 1H and 13C NMR spectra of the PNB-supported and molecular catalysts.

ACKNOWLEDGMENT

The Authors acknowledge Japan Polychem Corporation, Sumitomo Chemical Co., Ltd., Tosoh Finechem Corporation and Prof. Noriyoshi Matsumi (Japan Advanced Institute of Science and Technology), for the donation of reagents and GPC measurements. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

REFERENCES

1) Shibasaki, M.; Yoshikawa, N. Lanthanide Complexes in Multifunctional Asymmetric Catalysis. Chem. Rev. 2002, 102, 2187−2209.


38

Page 39 of 50 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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2) Endo, K.; Ogawa, M.; Shibata, T. Multinuclear Catalyst for Copper-Catalyzed Asymmetric Conjugate Addition of Organozinc Reagents. Angew. Chem. Int. Ed. 2010, 49, 2410−2413. 3) Haak, R. M.; Wezenberg, S. J.; Kleij, A. W. Cooperative Multimetallic Catalysis Using Metallosalens. Chem. Commun. 2010, 46, 2713−2723. 4) Gahan, L. R.; Smith, S. J.; Neves, A.; Schenk, G. Phosphate Ester Hydrolysis: Metal Complexes As Purple Acid Phosphatase and Phosphotriesterase Analogues. Eur. J.

Inorg. Chem. 2009, 2009, 2745−2758. 5) Mitic, N.; Smith, S. J.; Neves, A.; Guddat, L. W.; Gahan, L. R.; Schenk, G. The Catalytic Mechanisms of Binuclear Metallohydrolases. Chem. Rev. 2006, 106, 3338−3363. 6) Collman, J. P.; Boulatov, R.; Sunderland, C. J.; Fu, L. Functional Analogues of Cytochrome c Oxidase, Myoglobin, and Hemoglobin. Chem. Rev. 2004, 104, 561−588.


39

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 40 of 50

7) Sone, T.; Yamaguchi, A.; Matsunaga, S.; Shibasaki, M. Catalytic Asymmetric Synthesis of 2,2-Disubstituted Terminal Epoxides via Dimethyloxosulfonium Methylide Addition to Ketones. J. Am. Chem. Soc. 2008, 130, 10078−10079. 8) Kato, N.; Mita, T.; Kanai, M.; Therrien, B.; Kawano, M.; Yamaguchi, K.; Danjo, H.; Sei, Y.; Sato, A.; Furusho, S.; Shibasaki, M. Assembly State of Catalytic Modules as Chiral Switches in Asymmetric Strecker Amino Acid Synthesis. J. Am. Chem. Soc. 2006, 128, 6768−6769. 9) Yamatsugu, K.; Yin, L.; Kamijo, S.; Kimura, Y.; Kanai, M.; Shibasaki, M. A Synthesis of Tamiflu by Using a Barium-Catalyzed Asymmetric Diels–Alder-Type Reaction.

Angew. Chem. Int. Ed. 2009, 48, 1070−1076. 10)Konsler, R. G.; Karl, J.; Jacobsen, E. N. Cooperative Asymmetric Catalysis with Dimeric Salen Complexes. J. Am. Chem. Soc. 1998, 120, 10780−10781. 11)Paull, D. H.; Abraham, C. J.; Scerba, M. T.; Danforth, E. A.; Lectka, T. Bifunctional Asymmetric Catalysis: Cooperative Lewis Acid/Base Systems. Acc. Chem. Res. 2008, 41, 655−663.


40

Page 41 of 50 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

12)Huh, S.; Chen, H. T.; Wiench, J. W.; Pruski, M.; Lin, V. S. Y. Cooperative Catalysis by General Acid and Base Bifunctionalized Mesoporous Silica Nanospheres. Angew.

Chem. Int. Ed. 2005, 117, 1860−1864. 13)Notestein, J. M.; Katz, A. Enhancing Heterogeneous Catalysis through Cooperative Hybrid Organic–Inorganic Interfaces. Chem. Eur. J. 2006, 12, 3954−3965. 14)Chandra, P.; Jonas, A. M.; Fernandes, A. E. Spatial Coordination of Cooperativity in Silica-Supported Cu/TEMPO/Imidazole Catalytic Triad. ACS Catal. 2018, 8, 6006−6011. 15)Delferro, M.; Marks, T. J. Multinuclear Olefin Polymerization Catalysts. Chem. Rev. 2011, 111, 2450−2485. 16)Lee, M. H.; Kim, S. K.; Do, Y. Biphenylene-Bridged Dinuclear Group 4 Metal Complexes: Enhanced Polymerization Properties in Olefin Polymerization.

Organometallics 2005, 24, 3618−3620. 17)Yan, Q.; Tsutsumi, T.; Nomura, K. Synthesis and Structural Analysis of Aryloxomodified Trinuclear Half-Titanocenes, and Their Use As Catalyst Precursors for Ethylene Polymerisation. RSC Adv. 2017, 7, 41345−41358.


41

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 42 of 50

18)Makio, H.; Terao, H.; Iwashita, A.; Fujita, T. FI Catalysts for Olefin Polymerization-A Comprehensive Treatment. Chem. Rev. 2011, 111, 2363−2449. 19)Salata, M. R.; Marks, T. J. Catalyst Nuclearity Effects in Olefin Polymerization. Enhanced

Activity

and

Comonomer

Enchainment

in

Ethylene

+

Olefin

Copolymerizations Mediated by Bimetallic Group 4 Phenoxyiminato Catalysts.

Macromolecules 2009, 42, 1920−1933. 20)Mi, X.; Ma, Z.; Wang, L.; Ke, Y.; Hu, Y. Homo‐ and Copolymerization of Norbornene and Styrene with Pd‐ and Ni‐Based Novel Bridged Dinuclear Diimine Complexes and MAO. Macromol. Chem. Phys. 2003, 204, 868−876. 21)Enus, R. M.; Mapolie, S. F.; Smith, G. S. Norbornene Polymerization Using Multinuclear Nickel Catalysts Based on a Polypropyleneimine Dendrimer Scaffold.

J. Organomet. Chem. 2008, 693, 2279−2286. 22)Li, L.; Metz, M. V.; Li, H.; Chen, M. C.; Marks, T. J.; Sands, L. L.; Rheingold, A. L. Catalyst/Cocatalyst Nuclearity Effects in Single-Site Polymerization. Enhanced Polyethylene Branching and α-Olefin Comonomer Enchainment in Polymerizations


42

Page 43 of 50 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

Mediated by Binuclear Catalysts and Cocatalysts via a New Enchainment Pathway.

J. Am. Chem. Soc. 2002, 124, 12725−12741. 23)Wang, J.; Li, H.; Guo, N.; Li, L.; Stern, C. L.; Marks, T. J. Covalently Linked Heterobimetallic Catalysts for Olefin Polymerization. Organometallics 2004, 23, 5112−5114. 24)McInnis, J. P.; Delferro, M.; Marks, T. J. Multinuclear Group 4 Catalysis: Olefin Polymerization Pathways Modified by Strong Metal−Metal Cooperative Effects. Acc.

Chem. Res. 2014, 47, 2545−2557. 25)Takeuchi, D.; Chiba, Y.; Takano, S.; Osakada, K. Double-Decker-Type Dinuclear Nickel Catalyst for Olefin Polymerization: Efficient Incorporation of Functional Comonomers. Angew. Chem. Int. Ed. 2013, 52, 12536–12540. 26)Radlauer, M. R.; Day, M. W.; Agapie, T. Bimetallic Effects on Ethylene Polymerization in the Presence of Amines: Inhibition of the Deactivation by Lewis Bases. J. Am. Chem. Soc. 2012, 134, 1478−1481.


43

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 44 of 50

27)Chen, Z.; Yao, E.; Wang, J.; Gong, X.; Ma. Y. Ethylene (Co)polymerization by Binuclear

Nickel

Phenoxyiminato

Catalysts

with

Cofacial

Orientation.

Macromolecules 2016, 49, 8848−8854. 28)Severn, J. R.; Chadwick, J. C.; Duchateau, R.; Friederichs, N. “Bound but Not Gagged”-Immobilizing Single-Site α-Olefin Polymerization Catalysts. Chem. Rev. 2005, 105, 4073−4147. 29)Kim, J. D.; Soares, J. B. P. Copolymerization of Ethylene and α-Olefins with Combined Metallocene Catalysts. III. Production of Polyolefins with Controlled Microstructures. J. Polym. Sci. A. 2000, 38, 1417−1426. 30)Hotta S.; Asuka, K.; Kitade, S.; Takahashi, K.; Kouzai, I.; Tayano, T. Polypropylenebased Resin Composition and Foam Sheet. US Patent 20150010747A1 2015, to Japan Polypropylene Corporation. 31)Thakur, A.; Baba, R.; Chammingkwan, P.; Terano, M.; Taniike, T. Synthesis of Aryloxide-Containing Half-Titanocene Catalysts Grafted to Soluble Polynorbornene Chains and Their Application in Ethylene Polymerization: Integration of Multiple Active Centers in a Random Coil. J. Catal. 2018, 357, 69−79.


44

Page 45 of 50 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

32)Miura, Y. Design and Synthesis of Well-Defined Glycopolymers for the Control of Biological Functionalities. Polymer Journal 2012, 44, 679−689. 33)Joo, Y.; Agarkar, V.; Sung, S. H.; Savoie, B. M.; Boudouris, B. W. A Nonconjugated Radical Polymer Glass with High Electrical Conductivity. Science 2018, 359, 1391−1395. 34)Nomura, K.; Naga, N.; Miki, M.; Yanagi, K. Olefin Polymerization by (Cyclopentadienyl)(aryloxy)titanium(IV)

Complexes-Cocatalyst

Systems.

Macromolecules 1998, 31, 7588−7597. 35)Floros, G.; Saragas, N.; Paraskevopoulou, P.; Psaroudakis, N.; Koinis, S.; Pitsikalis, M.; Hadjichristidis, N.; Mertis, K. Ring Opening Metathesis Polymerization of Norbornene and Derivatives by the Triply Bonded Ditungsten Complex Na[W2(µCl)3Cl4(THF)2](THF)3. Polymers 2012, 16, 1657–1673. 36)Düz, B.; Elbistan, C. K.; Ece, A.; Sevin, F. Application of Carbon Arc‐Generated Mo‐ and W‐Based Catalyst Systems to the ROMP of Norbornene. Appl. Organomet.

Chem. 2009, 23, 359−364.


45

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 46 of 50

37)Schwab, P.; Grubbs, R. H.; Ziller, J. W. Synthesis and Applications of RuCl2(=CHR‘)(PR3)2: The Influence of the Alkylidene Moiety on Metathesis Activity.

J. Am. Chem. Soc. 1996, 118, 100−110. 38)Kitiyanan, B.; Nomura, K. Synthesis of Half-Titanocenes Containing Aryloxide Ligands Attached to the ROMP Polymer Chain End: Unique Catalyst Precursors for Ethylene (Co)polymerization. Organometallics 2007, 26, 3461−3465. 39)Nomura, K.; Liu, J.; Padmanabhan, S.; Kitiyanan, B. Nonbridged Half-Metallocenes Containing Anionic Ancillary Donor Ligands: New Promising Candidates as Catalysts for Precise Olefin Polymerization. J. Mol. Catal. A. 2007, 267, 1−29. 40)Goto, K.; Taniike, T.; Terano, M. Dual‐Active‐Site Nature of Magnesium Dichloride‐Supported Cyclopentadienyl Titanium Chloride Catalysts Switched by an Activator in Propylene Polymerization. Macromol. Chem. Phys. 2013, 214, 1011−1018. 41)Müller, C.; Ackerman, L. J.; Reek, J. N. H.; Kamer, P. C. J.; Van Leeuwen, P. W. N. M. Site-Isolation Effects in a Dendritic Nickel Catalyst for the Oligomerization of Ethylene. J. Am. Chem. Soc. 2004, 126, 14960−14963.


46

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42)Jezequel, M.; Dufaud, V.; Garcia, M. J. R; Hermosilla, F. C; Neugebauer, U.; Niccolai, G. P.; Lefebvre, F.; Bayard, F.; Corker, J.; Fiddy, S.; Evans, J.; Broyer, J. P.; Malinge, J.; Basset, J. M. Supported Metallocene Catalysts by Surface Organometallic Chemistry. Synthesis, Characterization, and Reactivity in Ethylene Polymerization of Oxide-Supported Mono- and Biscyclopentadienyl Zirconium Alkyl Complexes: Establishment of Structure/Reactivity Relationships. J. Am. Chem. Soc. 2001, 123, 3520−3540. 43)Amgoune, A.; Krumova, M.; Mecking, S. Nanoparticle-Supported Molecular Polymerization Catalysts. Macromolecules 2008, 41, 8388−8396. 44)Switzer, J. M.; Pletcher, P. D.; Steelman, D. K.; Kim, J.; Medvedev, G. A.; Omar, M. M. A.; Caruthers, J. M.; Delgass W. N. Quantitative Modeling of the Temperature Dependence of the Kinetic Parameters for Zirconium Amine Bis(Phenolate) Catalysts for 1-Hexene Polymerization. ACS Catal. 2018, 8, 10407−10418. 45)Taniike, T.; Nguyen, B. T.; Takahashi, S.; Vu, T. Q.; Ikeya, M.; Terano, M. Kinetic Elucidation

of

Comonomer‐Induced

Chemical

and

Physical

Activation

in


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Heterogeneous Ziegler‐Natta Propylene Polymerization. J. Polym. Sci. A. 2011, 49, 4005−4012. 46)Odian, G. Principle of Polymerization, 4th Ed.; John Wiley and Sons, New York, 2004, pp 661–662. 47)Roduner, E. Understanding Catalysis. Chem. Soc. Rev. 2014, 43, 8226–8239. 48)Jaffe, H. H.; Orchin, M. Theory and Applications of Ultraviolet Spectroscopy; John Wiley and Sons, New York, 1962, pp 508−513. 49)Makela, N. I.; Knuuttila, H. R.; Linnolahti, M.; Pakkanen, T. A.; Leskela, M. A. Activation of Racemic Ethylene-Bridged Bis(indenyl)-Type Siloxy-Substituted Zirconocenes with Methylaluminoxane. A Combined UV/vis Spectroscopic and ab Initio Hartree−Fock Study. Macromolecules 2002, 35, 3395−3401. 50)Sun, M.; Mu, Y.; Liu, Y.; Wu, Q.; Ye, L. Synthesis of the Binuclear Half-Metallocene Chromium(III) Aryloxides [Cp′Cr(OAr)Cl]2 and Their Catalytic Properties for Ethylene Polymerization in the Presence of Alkylaluminum Cocatalyst. Organometallics 2011,

30, 669−675.


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51)Ashby, M. T. Appreciating Formal Similarities in the Kinetics of Homogeneous, Heterogeneous, and Enzyme Catalysis. J. Chem. Educ. 2007, 84, 1515−1519. 52)Bowker, M. The Role of Precursor States in Adsorption, Surface Reactions and Catalysis. Top. Catal. 2016, 59, 663−670.


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Cooperative Catalysis by Multiple Active Centers of a Half-Titanocene

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