Octahedral Group IV Bis(phenolate) Catalysts for 1-Hexene

Octahedral group IV bis(phenolate) catalysts are highly active catalysts for the isospecific polymerization of 1-hexene and the copolymerization of et...
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Octahedral Group IV Bis(phenolate) Catalysts for 1‑Hexene Homopolymerization and Ethylene/1-Hexene Copolymerization Elizabeth T. Kiesewetter and Robert M. Waymouth* Department of Chemistry, Stanford University, Stanford, California 94305-5080, United States S Supporting Information *

ABSTRACT: Octahedral group IV bis(phenolate) catalysts are highly active catalysts for the isospecific polymerization of 1-hexene and the copolymerization of ethylene with 1-hexene. These catalysts are active for the production of high molecular weight copolymers even at 130 °C. The copolymerization parameters for these complexes were determined; all of the bis(phenolate) complexes tested incorporate 1-hexene with high efficiency to give random copolymers. The complexes prepared from the more sterically demanding ligands showed higher molecular weights but similar comonomer incorporations to those prepared from the less sterically demanding ligands.

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INTRODUCTION Ethylene copolymers are an important class of industrial materials.1−3 The properties and processing characteristics of ethylene/α-olefin copolymers are strongly dependent on the amount and distribution of the comonomers.3 Advances in catalyst research with homogeneous metallocene and coordination catalysts have led to new classes of ethylene copolymers, as these catalysts can afford uniform comonomer compositions, uniform2,4 or programmed2,5−12 comonomer distributions, and controlled molecular weight distributions.1,13 Studies of metallocene catalysts have provided useful insights into the influence of the nature of the transition metal and ligand geometry on the copolymerization behavior;8,12−26 nevertheless, a limitation of many of the metallocene catalysts is the increased rates of chain termination at elevated temperatures, resulting in low molecular weight polymers.27 Considerable efforts have focused on nonmetallocene coordination catalysts for olefin polymerization2,28,29 and copolymerization.1,4,18 In light of the vast array of different ligand/metal combinations for these catalysts,1 our understanding of the influences of the metal and ligands on copolymerization behavior4,18,30−33 is still evolving. Herein, we report the 1-hexene homopolymerization and ethylene/1-hexene copolymerization behavior of a family of octahedral group IV metal bis(phenolate) catalysts34−38 and the effect of both ligand and metal on the copolymerization behavior and polymer microstructure.

a maximum deviation in bond length of only 0.062 Å for the slightly smaller Hf complex.36 1-Hexene Homopolymerization. The high activity and stereospecificity of these complexes for propylene polymerization34−36,38 motivated us to investigate their polymerization behavior with 1-hexene (Table 1). When the Hf complex 3b is activated with MAO in 1-hexene at 50 °C, more than 7 g of polyhexene is produced, giving a productivity of 4.9 kg PH (mmol Hf)−1 h−1,4,39 comparable to that (2.5 kg PH (mmol Zr)−1 h−1) reported by Ishii for [OSSO]-type phenolate complexes.40 As observed for propylene polymerization,36,38 the Zr complex 3a gives a lower productivity of 1.8 kg PH (mmol Zr)−1 h−1, yielding ∼1.5 g of polymer. The Zr and Hf complexes 3a, 3b both produce polyhexenes with high molecular weights (Mn = 94 500 g/mol and Mn = 81 600 g/ mol, respectively), even at 50 °C. Highly isotactic polyhexene was obtained with the Hf complex 3b ([mmmm] = 0.89, see Figure S1).40,41 As observed for propylene, the Hf complex 3b produces a more highly isotactic polyhexene than the Zr complex 3a ([mmmm] = 0.89 and 0.66, respectively). Under comparable conditions, complexes 3a and 3b afford higher tacticities for polyhexene than the corresponding polypropylenes,36,38 as observed previously for both metallocene42 and non-metallocene complexes.43 Hydrogen is an effective chain-transfer agent for catalysts derived from complexes 3a and 3b. Addition of 15 mmol of H2 leads to a reduction in the molecular weight of the polyhexene by a factor of 5 (Mn = 81 600 vs 15 800 g/mol Table 1, entries 1 and 5).

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RESULTS AND DISCUSSION The ligand precursors 1 and 2 (Scheme 1) were prepared as previously described.34−36 Catalysts 3a,b and 4a,b were prepared by reaction of ligand precursors 1 and 2 in toluene with ZrBn4 or HfBn4. Catalysts 3a and 3b were previously shown by X-ray crystallography to be closely isostructural, with © XXXX American Chemical Society

Received: January 17, 2013 Revised: March 13, 2013

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Scheme 1. Synthesis of Group 4 Bisphenolate Ether Complexes

Table 1. 1-Hexene Homopolymerization with Catalysts 3a,b catalyst

runa

M

T (°C)

yield (g)

activityb

Mnc

Mw/Mnc

[mmmm]d

3b 3b 3a 3a 3b(H2)

1e 2e 3f 4f 5e,g

Hf Hf Zr Zr Hf

50 25 50 25 25

7.29 2.69 1.55 0.37 11.3

4.9 1.1 1.8 0.25 7.7

81 600 89 500 94 500 84 600 15 800

2.04 2.25 1.92 1.75 1.93

0.89 0.87 0.66 0.65 n.d.

a Conditions: activated with 254 mg of MAO; 55 mL of 1-hexene; total solution volume = 60 mL; [cat.] = 7.3 × 10−5 M; [Al]:[M] = 1000; time = 20 min. bIn kg PH/(mmol h). cDetermined by gel permeation chromatography (GPC). dDetermined by 13C NMR. eMetal complex generated in situ by mixing 1 equiv of 1 with HfBn4. fMetal complex generated in situ by mixing 1 equiv of 1 with 1 equiv of ZrBn4. gPolymerization was run in the presence of 15 mmol of H2.

Table 2. Ethylene/1-Hexene Copolymerization with Bis(phenolate)/MAO System at 25 °C catalyst e

3a

3be

4ae

4be

5

runa

[M] (μM)

Xe/Xh

polymer yield (g)

prodb

% Ec

Mnd (g/mol)

Mw/Mnd

1 2 3 4 5 6 7 8 9 10 11 12 13

0.4 0.4 0.8 0.8 0.8 0.8 1.6 1.6 1.6 1.6 2.4 0.8 112

0.224 0.122 0.0883 0.224 0.122 0.0883 0.224 0.122 0.0883 0.224 0.122 0.0883 0.0713

1.49 0.50 0.67 0.27 0.74 0.41 0.48 1.14 0.28 0.77 2.99 0.46 0.32

224 75.0 50.3 20.0 55.6 30.8 18.0 42.8 10.5 28.9 74.8 34.4 0.171

79 68 58 83 65 53 76 64 53 83 60 55 26

334 900 459 800 368 700 15 300 27 000 56 800 808 900 384 200 748 100 1 375 600 462 300 1 215 700 29 600

2.10 1.99 2.20 2.30 2.49 4.43 2.35 2.11 2.17 1.68 2.47 1.69 1.96

a

Conditions: activated with 110 mg of MAO; solvent = add desired amount of 1-hexene and dilute to 50 mL total volume; ethylene pressure = 22 psig; time = 20 min; temp = 25 °C. bProductivity = (kg polymer)/(mmol h). cPercent ethylene in the polymer; determined by 13C NMR. d Determined by gel permeation chromatography (GPC). eMetal complex generated in situ by mixing 1 equiv of ligand with MBn4.

Ethylene/1-Hexene Copolymerization. Ethylene/1-hexene copolymerizations were carried out in a 300 mL stainless steel reactor in either neat 1-hexene or a toluene/1-hexene mixture with an overpressure of ethylene. Catalysts 3a,b and 4a,b were generated in situ in toluene, and the resulting solution was injected into the reactor filled with a preequilibrated solution of MAO. Low conversion of 1-hexene was maintained to ensure a constant monomer feed throughout the polymerization. Polymer yields were kept low to avoid limitations due to mass transport.44 Copolymerizations were performed at a variety of feed ratios to yield polymers with a

range ethylene contents for the determination of reactivity ratios. Complexes 3a,b and 4a,b are highly active catalyst precursors for the copolymerization of ethylene and 1-hexene35,45,46 (Table 2). When activated with MAO at 25 °C with Xe/Xh = 0.224, the most active catalyst, 3a (0.4 μM), yields 1.4 g of high molecular weight (Mn = 334 900 g/mol) polymer in 20 min, which corresponds to a productivity of 224 kg polymer (mmol Zr)−1 h−1. Under slightly different conditions ([Hf] = 0.8 μM vs [Zr] = 0.4 μM; Xe/Xh = 0.0544), 3b yields 2.2 g of polymer giving a productivity of 166 kg polymer (mmol Hf)−1 h−1. B

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Catalysts 4a and 4b, prepared with ligand 2, show lower productivities than 3a and 3b but produce high molecular weight copolymers, with Mn as high as 808 900 g/mol for 4a and 1 375 600 g/mol for 4b. Catalyst 3a also generates polymers with high molecular weights (Mn = 459 800 g/mol) that are only slightly lower than those of 4a and 4b. The molecular weights of the polymers produced by catalyst 3b are significantly lower than the other three catalysts, indicating that the high molecular weights are a result of the nature of not only the ligand but also the metal.22 The molecular weight distributions are consistent with single-site behavior (Mw/Mn ∼ 2.0). Similar copolymerization behavior is observed with Zr and Hf pyridyl amine complexes,4 but these complexes can generate broader molecular weight distributions due to catalyst modification by the monomer during polymerization.4,30,47−49 For comparison, copolymerization with the metallocene bis(2(3′,5′-di-t-BuPhInd))HfCl2 (5) was carried out under comparable conditions (Table 2, entry 22). Catalyst 5 is known to be an excellent 1-hexene incorporator;17 however, the activities and molecular weights are much lower than those of the octahedral bis(phenolates) 3a,b and 4a,b. Catalyst 3b was screened for activity at an elevated temperature (130 °C) typical of industrial processes.35,45,46 Catalyst 3b yielded 0.41 g of copolymer (Table 3) in 20 min at

Table 4. Ethylene/1-Hexene Copolymerizations for Comparison at Xe/Xh = 0.88 run catalysta

catalyst

hexene (mL)

% Eb

polymer yield (g)

prodc

Mnd

Mw/Mnd

3be

50

15

48

0.41

30.7

69 100

2.40

% Eb

polymer yield (g)

prodc

Mnd

Mw/Mnd

0.8 0.8 1.6 0.8

58 53 53 55

0.67 1.58 0.28 0.46

50.3 118 10.5 34.4

368 700 425 300 748 100 1 215 700

2.20 2.27 2.17 1.69

a

Conditions: activated with 110 mg of mMAO; solvent = 30 mL of toluene + 15 mL of 1-hexene; ethylene pressure = 22 psig; Xe/Xh = 0.88; time = 20 min; temp = 25 °C. bPercent ethylene in the polymer determined by 13C NMR. cProductivity = (kg polymer)/(mmol h). d Determined by gel permeation chromatography (GPC). eMetal complex generated in situ by mixing 1 equiv of ligand with MBn4

re =

2[EEE] + [EEH] X

(2[EHE] + [HHE]) X e

h

X

rh =

(2[HHH] + [HHE]) X e

h

2[EHE] + [HHE]

The reactivity ratios were also calculated over all triads for all feed ratios simultaneously by optimizing the variation of the reaction probabilities Pij (the probability that a monomer j will add to a polymer chain ending in monomer i) until the best fit between experimental triads and those calculated from the firstorder Markov model was obtained. Reactivity ratios are calculated by this method with the following two equations:53,54

Table 3. Ethylene/1-Hexene Copolymerization with 3b at 130 °C E press. (psig)

3ae 3be 4ae 4be

1 2 3 4

[M] (μM)

⎛ 1 ⎞X re = ⎜ − 1⎟ h ⎝ Peh ⎠ Xe

a Conditions: [M] = 0.8 μM; activated with 110 mg of mMAO; solvent = 30 mL of toluene + 15 mL of 1-hexene; ethylene pressure = 50 psig; time = 20 min; temp = 130 °C. bPercent ethylene in the polymer determined by 13C NMR. cProductivity = (kg polymer)/(mmol h). d Determined by gel permeation chromatography (GPC). eMetal complex generated in situ by mixing 1 equiv of ligand with MBn4.

⎛ 1 ⎞X rh = ⎜ − 1⎟ e ⎝ Phe ⎠ Xh

The optimized reactivity ratios are reported in Table 6. The reactivity ratios were also calculated over all triads for all feed ratios simultaneously by optimizing the variation of the reaction probabilities from the second-order Markov model.10,12,53 The second-order Markov model results in a slightly less satisfactory fit than that obtained using the first-order model; at triad resolution it is not clear that analysis by the second-order Markov model is justified.20 Reactivity ratios were also calculated using the linear methods of Fineman−Ross55 and Kelen−Tudos56,57 and found to follow the same trends as those calculated by optimizing the reaction probabilities, indicating that the first-order Markov model provides an adequate description of the triad data.20,53 Ligand and Metal Effects on Polymer Microstructure. All four complexes, 3a,b and 4a,b, are efficient at incorporating 1-hexene, with re’s ranging from 12.9 to 21.6 and rh’s from 0.090 to 0.15. Although these re values are higher and these rh values lower than those obtained with some of the best 1hexene incorporating catalysts such as 5 (re = 3.2, rh = 0.25)17 and constrained geometry catalyst, Me2Si(Me4Cp)(N-tertbutyl)TiMe2 (re = 4, rh = 0.4),17 they are comparable to those obtained with the metallocenes rac-ethylenebis(indenyl)ZrCl2 (re = 14, rh = 0.027).19 The octahedral bis(phenolates) exhibit higher activities and comonomer incorporations45,46 than the structurally related C2 symmetric amine bis(phenolates)41,58 (re = 98), but the comonomer incorporation is comparable to that of the Cs

130 °C, corresponding to a productivity of 30.7 kg polymer (mmol Hf)−1 h−1. The catalyst incorporates 1-hexene well at this temperature, yielding a polymer with 52% 1-hexene incorporation. Similar results were recently reported by scientists at Dow.45,46 At 130 °C the molecular weights of the copolymers are not significantly different from those generated at 25 °C, indicating that there is not a strong dependence of the molecular weight on the polymerization temperature.34,35 Comonomer feed ratios at 25 °C were determined using an empirical equation developed by Spitz and co-workers50 in combination with experimental data for the solubility of ethylene in 1-hexene.51 Copolymers prepared by all four catalysts at the same feed compositions are shown in Table 4. Incorporation of 1-hexene is similar for all four catalysts, with Zr complex 3a incorporating slightly less 1-hexene than Zr catalyst, 4a as well as both Hf catalysts, 3b and 4b. Ethylene/1-hexene copolymer compositions and sequence distributions calculated from 13C NMR using the method of Cheng are reported in Table 5.52 Reactivity ratios for each run were calculated using a first-order Markov model from these experimental triad distributions according to the following two equations:53 C

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Table 5. 13C NMR Characterization of E/H Copolymers Prepared with Bis(phenolate)/MAO catalyst 3a

d

3bd

4ad

4bd

5

runa

Xe/Xh

% Eb

EEEc

HEEc

HEHc

EHEc

HHEc

HHHc

re

rh

1 4 5 7 9 10 13 15 16 18 19 21 23

0.224 0.0713 0.0611 0.224 0.0883 0.0544 0.224 0.0883 0.0611 0.224 0.0883 0.0713 0.0713

79 48 44 83 53 34 76 53 37 83 55 50 26

0.533 0.169 0.131 0.559 0.178 0.061 0.401 0.153 0.061 0.559 0.222 0.159 0.006

0.234 0.204 0.217 0.237 0.269 0.161 0.282 0.259 0.171 0.237 0.252 0.241 0.093

0.038 0.095 0.094 0.035 0.095 0.116 0.069 0.129 0.134 0.035 0.057 0.107 0.168

0.113 0.082 0.057 0.110 0.109 0.065 0.162 0.127 0.096 0.110 0.142 0.107 0.046

0.069 0.213 0.236 0.059 0.187 0.250 0.084 0.207 0.271 0.059 0.214 0.226 0.295

0.03 0.225 0.264 0 0.169 0.344 0 0.135 0.263 0 0.095 0.165 0.388

19.7 20.1 22.4 19.8 17.5 13.8 11.9 13.8 12.1 19.8 15.9 17.8 3.8

0.098 0.13 0.13 0.034 0.12 0.14 0.046 0.091 0.087 0.034 0.072 0.09 0.20

a

Conditions: activated with 110 mg of MAO; solvent = add desired amount of 1-hexene and dilute to 50 mL total volume; ethylene pressure = 22 psig; time = 20 min; temp = 25 °C. bPercent ethylene in the polymer; determined by 13C NMR. cTriads determined by 13C NMR; triads do not sum to 1.00 for all runs. This reflects the experimental error in the calculation of triads from 13C NMR. dMetal complex generated in situ by mixing 1 equiv of ligand with MBn4.

Table 6. Reactivity Ratios for Ethylene/1-Hexene Copolymerizations Determined Using a First-Order Markov Model catalysta

Xe/Xhb

% Ec

3a 3b 4a 4b

0.061−0.22 0.049−0.122 0.061−0.22 0.041−0.22

44−79 29−65 37−76 22−77

red,e 21.6 16.5 12.9 15.4

± ± ± ±

rhd,e

1.44 2.91 1.67 2.80

0.12 0.13 0.090 0.15

± ± ± ±

rerhe

0.020 0.027 0.025 0.07

2.55 2.14 1.17 2.42

± ± ± ±

0.45 0.59 0.36 1.90

rh/ree 545 781 701 1026

± ± ± ±

97 59 215 499

a

Five experiments were run with each catalyst to determine the average reactivity ratios (N = 5). bRange of ratios of mole fractions of ethylene and 1-hexene in the monomer feed. cRange of the mol % E in the copolymers as determined by 13C NMR. dCalculated by optimization of reaction probabilities from the triads over all runs simultaneously. eStandard deviation calculated as [(1/(N − 1))∑(re,h(exp) − re,h(opt))2]1/2.

symmetric amine bis(phenolates)59 (re = 12.5−15.1).18 Using the rh/re value as a measure of selectivity for 1-hexene, 3a,b and 4a,b show a strong preference for incorporating ethylene over 1-hexene. All four catalysts show similar rh values. Catalysts 3a,b generated from the phenyl ligand 1 incorporate 1-hexene with comparable or slightly higher selectivities than the more sterically hindered complexes 4a,b, indicating that the increased steric demands of ligand 2 do not have a significant influence on the comonomer incorporation.45,46 With metallocenes, increased steric hindrance around the metal center usually reduces the ability of the catalyst to incorporate 1-hexene.10,60,61 For complexes 3 and 4, this does not appear to be the case, although the origin of this effect is not yet clear. For complexes 3 and 4, there is not a significant influence of the metal on the comonomer incorporation. The comonomer distribution for EH copolymers derived from complex 4a are close to statistical, as determined by the product of the reactivity ratios rerh ∼ 1.54 In contrast, for the polymers prepared with 3a, 3b, and 4b, rerh ∼ 2, suggesting that these polymers are slightly enriched in EEE and HHH homosequences relative to those generated with 4a.54 As shown in Figure 1, the triad distributions (Table 5) evidence a slight increase in the relative amounts of HHH and EEE homosequences for copolymers produced with 3a, 3b, and 4b relative to those produced by 4a. While ethylene copolymers derived from metallocene catalysts typically exhibit slightly alternating sequence distributions (rerc ∼ 0.5),14,62−64 Galimberti has reported that certain classes of stereospecific metallocene catalysts exhibit blocky copolymer sequence distributions with rerc ∼ 4.5.9,10,12,54,65,66 To assess the influence of the sequence on the thermal properties of copolymers produced by catalysts 3a,b and 4a,b,

Figure 1. Triad distributions as determined by 13C NMR for copolymers prepared at Xe/Xh = 0.88 with 3a (57% E), 3b (53% E), 4a (53% E), and 4b (54% E).

polymers with high ethylene contents were prepared so that melting points could be determined (see Table 7). In a random ethylene/1-hexene copolymer, the introduction of branches results in a decrease in the melting point.8,67 For blocky copolymers, the melting points will be higher than random copolymers depending on the length of the crystallizable sequences.5,8,66,68 In the case of 3a,b and 4a,b, the melting points are not markedly different from that of statistical copolymers,67 indicating that the slight enhancement in the length of the ethylene homosequences suggested by the 13C NMR is not sufficient to manifest a significant difference in the melting points of the copolymers. As 1-hexene content of the polymer increases above 20%, ethylene sequences in the polymer become too short to crystallize.

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CONCLUSIONS Octahedral bis(phenolate) catalysts 3a,b and 4a,b are highly active catalysts for the isospecific polymerization of 1-hexene D

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Table 7. Melting Points of Polymers with High Ethylene Contents Produced by Bis(phenolate) Catalysts entrya c

1 2e 3c 4e 5c 6e 7c 8e

catalyst d

3a 3ad 3bd 3bd 4ad 4ad 4bd 4bd

[M] (μM)

Xe/Xh

polymer yield (g)

proda

% Eb

Tm (°C)

ΔH (J/g)

0.4 0.4 0.8 0.8 0.8 1.6 2.4 1.6

1.04 0.22 1.04 0.22 1.04 0.22 1.04 0.22

1.09 1.49 0.28 0.16 0.85 0.48 0.93 2.8

164 224 20.6 20.0 21.3 18.0 69.8 105

93 79 95 81 97 76 90 77

86 45 100 45 76 none 94 none

44 0.84 72 3.7 16 none 54 none

a

Productivity = (kg polymer)/(mmol h). bPercent ethylene in the polymer; determined by 13C NMR. cConditions: activated with 110 mg of MAO; solvent = 49 mL of toluene + 1 mL of 1-hexene; ethylene pressure = 22 psig; time = 20 min; temp = 25 °C. dMetal complex generated in situ by mixing 1 equiv of ligand with MBn4. eConditions: activated with 110 mg of MAO; solvent = 45 mL of toluene + 5 mL of 1-hexene; ethylene pressure = 22 psig; time = 20 min; temp = 25 °C. and was quenched by addition of methanol (10 mL). The polymer was precipitated from acidified methanol. It was then filtered, washed with methanol, and dried under vacuum at 60 °C. 1-Hexene Polymerization with H2(g). The 300 mL stainless steel Parr reactor was evacuated for at least 1 h on a vacuum line and then refilled and flushed three to four times with Ar(g). In the reactor, a solution of 254 mg of MAO in 55 mL of 1-hexene was equilibrated for 30 min at the polymerization temperature under 30 psig of Ar(g). A 150 mL, single-ended injection tube was pressurized with 35 psig of H2(g). Ar pressure was removed, and H2 was injected into the reactor. The ligand 1 or 2 was combined with MBn4 (M = Zr, Hf) in toluene (5 mL) in the drybox and loaded into a double-ended injection tube and introduced into the reactor without venting, under 50 psig of Ar. Ar pressure was removed, and the polymerization was run for 20 min and was quenched by addition of methanol (10 mL). The polymer was precipitated from acidified methanol. It was then filtered, washed with methanol, and dried under vacuum at 60 °C. Polymer Analysis. 13C NMR spectra for polymer analysis were run at 75.4 MHz on a Varian UI-300 NMR spectrometer using a 10 mm broad-band probe operating at 95 °C with an inverse gated decoupled pulse sequence with an acquisition time of 1.8 s.69 Copolymer samples were prepared as solutions of ca. 50 mg of polymer in 2.5 mL of 90:10 (v/v) 1,2-dichlorobenzene/benzene-d6 containing ca. 5 mg of chromium(III) acetylacetonate as a spin relaxation agent. Polyhexene samples were prepared as solutions of ca. 50 mg of polymer in 0.8 mL of CDCl3 with ca. 2 mg of chromium(III) acetylacetonate as a spin relaxation agent. Melting points and heats of fusion were determined by differential scanning calorimetry (DSC) using a TA Instruments Q100 DSC. All DSC samples were heated to 180 °C, held at that temperature for 10 min, cooled to 20 °C at a rate of 10 °C/min, and then were aged at room temperature for 48 h. DSC scans were obtained by heating the samples to 180 °C at a rate of 20 °C/min. High-temperature gel permeation chromatography of the ethylene/ 1-hexene copolymers (HT GPC) was performed on a PL 220 series HT GPC equipped with a built-in RI detector and 4 PLgel 20 μm Mixed-A columns (Polymer Laboratories Inc.). The mobile phase and sample preparation solvent are nitrogen-purged 1,2,4-trichlorobenzene (TCB) with ∼200 ppm antioxidant 2,6-di-tert-butyl-4-methylphenol (BHT). The flow rate was 1.0 mL/min. The column temperature was 150 °C while the autosampler temperatures were 130 °C (warm zone) and 135 °C (hot zone). Samples were prepared by dissolving sample in preheated TCB (150 °C) for 2.5 h with gentle shaking. The concentration of solution sample was 2 mg/mL, and the injection volume was 200 μL. Molecular weights were calibrated against polystyrene standards. Gel permeation chromatography (GPC) of the polyhexenes was performed in tetrahydrofuran (THF) at a flow rate of 1.0 mL/min on a Waters chromatograph equipped with four 5 μm Waters columns (300 mm × 7.7 mm) connected in series. The column temperature was 35 °C. A Viscotek S3580 refractive index detector and Viscotek

and copolymerization of ethylene and hexane to give high molecular weight 1-hexene homopolymers and ethylene/1hexene copolymers. These catalysts are active for the production of high molecular weight copolymers even at 130 °C. All four complexes, 3a,b and 4a,b, are efficient at incorporating 1-hexene, with re’s ranging from 12.9 to 21.6 and rh’s from 0.090 to 0.15. Complexes 3a,b derived from the less sterically demanding phenyl-substituted ligand 1 are more active than complexes 4a,b prepared with the more sterically demanding bis-tBuphenyl-substituted ligand, 2, but the latter complexes afford higher molecular weight copolymers while maintaining high comonomer incorporation

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EXPERIMENTAL SECTION

General Considerations. All solvents and reagents were purchased from Aldrich and were used without further purification unless otherwise stated. Tetrabenzylhafnium and tetrabenzylzirconium were purchased from Strem and were used without further purification. Ligands 1 and 2 were prepared as previously reported.36 Toluene was distilled from sodium/benzophenone ketyl. 1-Hexene was dried over sodium or triisobutylaluminum. Ethylene was purchased from Matheson TriGas (Research Purity) and passed through a purification column packed with activated alumina and copper catalyst. Modified methylaluminoxane (type IV) was purchased from Akzo Nobel as a solution in toluene; before use, the volatile materials were removed in vacuo to yield a powdery solid. Ethylene/1-Hexene Copolymerizations. The 300 mL stainless steel Parr reactor was evacuated for at least 1 h on a vacuum line and then refilled and flushed three to four times with ethylene. A solution of 110 mg of MAO in 45 mL of solvent was prepared in a drybox and loaded into a 150 mL double-ended injection tube. This MAO solution was injected into the reactor and was equilibrated for 30 min at the polymerization temperature. The ligand 1 or 2 was combined with MBn4 (M = Zr, Hf) in toluene (5 mL) in the drybox and loaded into a double-ended injection tube and introduced into the reactor via a small vent (1−2 psig) followed by injection with ethylene pressure. The polymerization was run for 20 min and was quenched by addition of methanol (10 mL). The polymer was precipitated from acidified methanol. It was then filtered, washed with methanol and dried under vacuum at 60 °C for at least 6 h. Conversion of 1-hexene and polymer yield were both kept to a minimum to ensure constant monomer concentration and to avoid mass transport limitations. 1-Hexene Polymerization. The 300 mL stainless steel Parr reactor was evacuated for at least 1 h on a vacuum line and then refilled and flushed three to four times with Ar(g). In the reactor, a solution of 254 mg of MAO in 55 mL of 1-hexene was equilibrated for 30 min at the polymerization temperature under 50 psig of Ar(g). The ligand 1 or 2 was combined with MBn4 (M = Zr, Hf) in toluene (5 mL) in the drybox and loaded into a double-ended injection tube and introduced into the reactor via a small vent (1−2 psig), followed by injection with Ar(g) pressure. The polymerization was run for 20 min E

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GPCmax autosampler were employed. The system was calibrated using monodisperse polystyrene standards (Polymer Laboratories).

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ASSOCIATED CONTENT

S Supporting Information *

13

C NMR of poly(1-hexene) and ethylene/hexane copolymers, polymerization data, and sequence distributions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the NSF (CHE-0910729) for financial support. We thank Dow Chemical for high temperature GPC analysis.

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