Article pubs.acs.org/Macromolecules
Ethylene Polymerization and Ethylene/1-Octene Copolymerization with rac-Dimethylsilylbis(indenyl)hafnium Dimethyl Using Trioctyl Aluminum and Borate: A Polymerization Kinetics Investigation Saeid Mehdiabadi and Joaõ B. P. Soares* Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada
Daniel Bilbao and Jeffrey Brinen Baytown Technology and Engineering Complex, ExxonMobil Chemical Company, Baytown, Texas 77522, United States ABSTRACT: The polymerization of ethylene and ethylene/1octene in a semibatch solution reactor using rac-dimethylsilylbis(indenyl)hafnium dimethyl and tetrakis(pentafluorophenyl) borate dimethylanilinium salt ([B(C6F5)4]−[Me2NHPh]+) as the catalytic system and trioctylaluminum (TOA) as the scavenger was investigated. Ethylene and 1-octene concentrations, polymerization temperature, borate/catalyst ratio, and TOA concentration were changed to study the ethylene polymerization kinetics with this system. The mode of addition for catalyst, borate, and TOA was also studied. When TOA and borate were added sequentially to the reactor followed by the catalyst, the polymerization activity was low and the molecular weight distribution (MWD) bimodal. Contrarily, when TOA was added first to the reactor and a mixture of catalyst and borate were added to the reactor, the polymerization rate was much higher and the MWD unimodal. An augmented 2 × 2 central composite design was used to investigate this phenomenon. Borate helped stabilize the active sites and reduce potential deactivation reactions with excess TOA.
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INTRODUCTION A quantitative model for the polymerization kinetics of αolefins with coordination catalysts is essential for the design and optimization of polymerization reactors. 1 While many publications have proposed fundamental and empirical models for α-olefin polymerization kinetics with coordination catalysts, several aspects of these systems are still poorly quantified.2−10 Among these still controversial subjects are the hydrogen effect on the polymerization kinetics of propylene11−14 and ethylene15−17 and the rate enhancement of ethylene polymerization by α-olefins.18−21 The type of cocatalyst used during olefin polymerization with metallocene catalysts is another very important (and poorly understood from a quantitative point of view) factor affecting polymerization kinetics, polymer activity, and polymer properties.22,23 Methylalumoxane24 (MAO) and bulky, noncoordinating anions from molecules such as organoborate salts are commonly used with these systems.25,26 Both cocatalyst families generate active sites for olefin polymerization, but the properties of the polymer produced and the polymerization kinetics can differ significantly among them.27−29 The use of organoborate salts in solution polymerization is considered by many as advantageous over the use of MAO because (1) the chemical structure of MAO is still poorly understood and (2) high MAO/catalyst ratios (often in excess © 2013 American Chemical Society
of 1000) are required to attain adequate catalyst activities, increasing polymerization costs and ash content in the polymer. On the other hand, borates can be used near 1:1 stoichiometric ratios. As a consequence, several noncoordinating anionic species have been developed and optimized to interact with catalyst precursors in specific ways to produce polymers having particular properties.27,30 However, organoborate compounds are not adequate impurity scavengers and therefore require that a small amount of MAO, other aluminoxanes or alkylaluminum compounds be added to the reactor to fit this role. The presence of this third component in the reactor is bound to affect the way the catalyst/borate complex functions as the polymerization active site, and it is also likely to influence polyolefin microstructural properties. These complex interactions are poorly understood and seldom reported in the literature. In our previous publication, we compared the effect of activating rac-dimethylsilylbis(indenyl)hafnium dimethyl (Hf) with MAO or tetrakis(pentafluorophenyl) borate dimethylanilinium salt (B).31 Effects on catalyst activities and molecular weights of the polymers produced by each type of catalyst/ Received: December 6, 2012 Revised: January 21, 2013 Published: February 5, 2013 1312
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upon catalyst injection for some runs, the temperature was kept at 120 °C ± 0.2 °C throughout the polymerization. After 15 min, the polymerization was stopped by closing the monomer valve and immediately blowing out the reactor contents into a 2-L beaker filled with 400 mL of ethanol. The polymer suspension was kept overnight, filtered, washed with ethanol, dried in air, and further dried under vacuum. Polymer Characterization. Molecular weight distributions (MWD) were determined with a Polymer Char high-temperature gel permeation chromatographer (GPC), run at 145 °C under a flow rate of 1,2,4-trichlorobenzene (TCB) of 1 mL/min. The GPC was equipped with three detectors in series (infrared, light scattering and differential viscometer) and calibrated with polystyrene narrow MWD standards. CRYSTAF analysis was performed using a Polymer Char CRYSTAF model 200. Polymer samples were dissolved in 47 mL of TCB at a concentration of 0.6 mg/mL. The polymer solution was heated to 160 °C and held for 2 h to ensure complete dissolution, at which point the temperature was lowered to 105 °C and allowed to stabilize for 55 min. A constant cooling rate of 0.1 °C/min was applied during analysis until the final temperature reached 30 °C. The polymer concentration in solution was monitored using an in-line infrared detector.
cocatalyst system were quantified and statistical methods were applied to estimate propagation and chain transfer constants. The present investigation extends this work by providing additional details on the activation and polymerization kinetics for the Hf/B system. The effect of temperature on ethylene polymerization will also be discussed, allowing for calculation of the kinetic propagation and deactivation activation energies according to the fundamental polymerization model previously developed for this system.31 In addition, the effect of borate and TOA concentrations and modes of addition on the polymerization rate of ethylene and copolymerization rate of ethylene and 1-octene with Hf was systematically studied using an augmented 2 × 2 central composite design. It will be shown that varying the mode of addition (sequential versus simultaneous Hf/B modes) and the concentrations of borate and TOA have a large influence on polymerization kinetics, catalysts activity, and polymer properties. These experiments were used to quantify catalyst deactivation and propagation rates as a function of borate and TOA concentrations in the reactor. To the best of our knowledge, this is the first time such a systematic study is performed for a catalyst system involving a catalyst precursor, a borate cocatalyst, and an alkylaluminum scavenger.
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RESULTS AND DISCUSSION
In our previous publication,31 eight polymerizations were run at different ethylene pressures (40−200 psi) to investigate the effect of ethylene concentration on polymerization kinetics and polymer molecular weight. We also developed a mathematical model for polymerization kinetics and used it to estimate the apparent propagation (kp) and deactivation (kd) rate constants for this catalyst system, assuming that all catalyst molecules added to the reactor were active at the beginning of the polymerization. Even though this hypothesis was not tested in the present investigation, it is required for the estimation of these constants, since polymerization kinetic studies alone cannot be used to determine the fraction of catalyst molecules that become active sites. Several methods to measure the concentration of active sites have been proposed in the literature,32−36 but there is no consensus on the best method to obtain these values. In the present investigation, no attempt has been made to measure the concentration of actives sites and, therefore, the reported kinetic constants must be considered apparent values only. A first order propagation rate with respect to ethylene concentration and first order catalyst deactivation kinetics were observed. This mathematical modeling treatment will not be repeated herein, but will be used to demonstrate that the model was still applicable under the different conditions used in the present investigation. Table 1 summarizes the polymerization steps considered in the proposed mechanism. Effect of Polymerization Temperature. Five polymerization runs were conducted to study the effect of temperature on the polymerization kinetics. The simultaneous procedure was used for adding the catalyst and borate solutions. Figure 1 shows the ethylene uptake curves for the five runs and Figure 2 plots ln(FM,in/VR) versus time for these same runs (FM,in = ethylene molar flow rate to the reactor, VR = reactor volume). The linear plots shown in Figure 2 confirm that the first order decay model developed in our previous publication was valid over the temperature range 110−130 °C.31 Estimates for kd and kp[M] (where [M] is ethylene concentration in the reactor) reported in Table 2 were calculated using the slope and intercept of the lines shown Figure 2, as proposed in our previous publication.31
EXPERIMENTAL SECTION
Materials. Ethylene and nitrogen, supplied by Praxair, were purified by passing through beds packed with molecular sieves (a mixture of 4-Å and 13X sieves) and copper(II) oxide. HPLC grade toluene from Kaledon was purified by refluxing over metallic sodium for 40 h and then distilled under nitrogen atmosphere. Triisobuthylaluminum (TIBA, used during the reactor preparation stage) and tryoctylaluminum (TOA, used as impurity scavenger during the polymerizations), were purchased from Aldrich. The catalyst species, rac-dimethylsilylbis(indenyl)hafnium dimethyl, was donated by ExxonMobil in solid form and dissolved in distilled toluene for polymerization. All air-sensitive compounds were handled under inert atmosphere in a glovebox. Polymer Synthesis. All polymerizations were performed in a 500 mL Parr autoclave reactor operated in semibatch mode. The polymerization temperature was controlled using an electrical band heater and internal cooling coils. The reaction medium was mixed using a pitched-blade impeller connected to a magneto-driver stirrer, rotating at 2000 rpm. Prior to use, the reactor was heated to 125 °C, evacuated and refilled with nitrogen six times to reduce the oxygen level in the reactor, then charged with 250 mL of toluene and 0.5 g of TIBA as a reactor scavenger. The temperature was then increased to 120 °C and kept constant for 20 min. Finally, the reactor contents were blown out under nitrogen pressure. This procedure ensured excellent removal of impurities from the reactor walls. In a typical polymerization run, 200 mL of toluene was charged into the reactor, followed by an appropriate amount of TOA, introduced via a 10 mL vial at room temperature. The temperature was then increased to 120 °C and ethylene was supplied to saturate the toluene to the desired pressure. Two procedures were applied for adding catalyst and borate solution to the reactor. In the first procedure (simultaneous) the hafnium catalyst solution was mixed with the borate cocatalyst solution and injected via a 5 mL tube and a 20 mL sampling cylinder connected in series under an ethylene pressure differential of 40 psig. A specified volume of toluene was placed in the sampling cylinder before injection to wash the tube walls from any remaining catalyst solution. In the second method (sequential), the borate solution was injected first using a 5 mL vial; 5 min later, the catalyst solution was injected using a 5 mL tube and a 20 mL sampling cylinder connected in series, similarly to the injection of the catalyst and borate in the simultaneous procedure. Ethylene was supplied on demand to maintain a constant reactor pressure and monitored with a mass flow meter. With the exception of a 1−3 °C fluctuation in temperature 1313
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Table 1. Proposed Polymerization Mechanisma mechanism steps initiation propagation deactivation
Table 2. Estimated Polymerization Rate Constants run
T (°C)
slope
intercept
kp[M] (s−1)
kd (s−1)
yield (g)
1 2 3 4 5
120 110 130 120 120
−0.00346 −0.00176 −0.0053 −0.00317 −0.00329
−6.491 −6.589 −6.598 −6.57212 −6.6484
603.4 547.1 542 556.4 515.5
0.00346 0.00176 0.0052 0.00317 0.00329
3.41 4.49 2.22 3.33 3.12
chemical equation kp
C + M → P0 kp
Pr + M → Pr + 1 kd
Pr → Dr + Cd kd
C → Cd transfer reactions β-hydride elimination transfer to monomer transfer to hydrogen transfer to cocatalyst
Arrhenius plots for kd and kp are shown in Figure 3. The point estimate for the activation energy for kd was calculated
kβ
Pr → Dr + P0 kM
Pr + M → Dr + P1 kH
Pr + H 2 → Dr + P0 kAl
Pr + Al → Dr + P0
a
C = active site, M = monomer, H2 = hydrogen, Al = cocatalysts (excluding borates), Pr = living chains of length r, Dr = dead chains of length r, Cd = deactivated site, kp = propagation rate constant, kd = deactivation rate constant, kβ = β-hydride elimination rate constant, kM = transfer to monomer rate constant, kH = transfer to hydrogen rate constant, and kAl = transfer to cocatalyst rate constant.
Figure 3. Arrhenius plots for kd and kp..
from the slope of the line in Figure 3 (Ed = 70 kJ/mol). The same approach was used to estimate the point estimate for the activation energy for kp (Ep = 8.6 kJ/mol). As expected, the activation energy for catalyst deactivation is higher than that for propagation, which implies that the average polymer yield for a certain polymerization time will pass through a maximum as the polymerization temperature increases. Figure 4 shows how the molecular weight averages and polydispersity index (PDI) depend on the polymerization temperature. The molecular weight averages decrease with increasing temperature, as expected due to higher β-hydride
Figure 1. Ethylene uptake curves for polymerizations at different temperatures (experimental conditions: ethylene pressure =120 psig, solvent = toluene, solvent volume = 222.8 mL, catalyst concentration = 2.51× 10−6 mol/L, Al/Hf = 487, B/Hf = 1.81, and polymerization time = 11 min).
Figure 2. Effect of temperature on ln(FM,in/VR) versus time. (FM,in = ethylene molar flow rate to the reactor, VR = reactor volume.). Figure 4. Effect of polymerization temperature on molecular weight averages and polydispersity index. 1314
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elimination rates, while the PDI remains close to 2.0, confirming the single-site behavior of this catalyst system. Effect of the Method of Catalyst and Cocatalyst Addition. The simultaneous and sequential Hf/B addition methods were compared by running two polymerizations under identical conditions. Ethylene uptake curves are compared in Figure 5 and Table 3 summarizes polymer yield and molecular
Figure 6. Molecular weight distributions for samples made with simultaneous and sequential Hf/B additions.
solution to yellow was observed promptly. Possibly, the reaction between these two species makes a stable complex that TOA may still attack, but only slowly; therefore, the catalyst activity is much higher in the simultaneous method. If this hypothesis is correct, we expect that decreasing TOA concentration in the reactor should lead to lower values for kd. This hypothesis was tested in the following section by polymerizing ethylene under different TOA and cocatalyst concentrations. Effect of TOA and Borate Concentration. A 2 × 2 central composite design with 3 center-point replicates (augmented with some additional runs, as explained below) was used to study the effect of TOA and borate on polymerization kinetics and polymer molecular weight. Replicates at the center point were used to estimate experimental errors associated with polymer yield, molecular weight averages, propagation and deactivation rate constants, and to allow for checking model adequacy. A sketch of the central composite design is shown in Figure 7 and the augmented design is depicted in Figure 8. Catalyst concentration, polymerization temperature, pressure and time were the same for all experiments. Table 4 summarizes the results for this set of runs. Effect of TOA and Cocatalyst Concentration on Polymer Molecular Weight. The set of polymerization runs from A to E constitutes a 2 × 2 factorial design with three replicates at the center point (E). TOA and borate concentrations are the variables of interest; catalyst activity, Mn, and Mw are the responses. The linear model given by eq 1, including an interaction term, was selected to fit the data,
Figure 5. Ethylene uptake profiles for simultaneous and sequential Hf/ B additions (experimental conditions: ethylene pressure = 120 psig, temperature = 120 °C, solvent = toluene, solvent volume = 222.8 mL, catalyst concentration = 2.51 × 10−6 mol/L, Al/Hf = 243, B/Hf = 1, and polymerization time = 11 min).
weight measurements for these two runs. The polymerization rate is high upon injection of the catalyst for the simultaneous addition case, causing the flow meter to saturate for a few seconds, while for the sequential run the polymerization rate is much lower. The overall activity for the sequential procedure is 1 order of magnitude lower than that for the simultaneous procedure. Interestingly, Figure 6 shows that the MWD for the polymer made in the sequential procedure was bimodal, indicating that two catalyst site types were active in this mode of addition. A possible explanation for this phenomenon is that only a fraction of TOA acts as an impurity scavenger, while some of the TOA molecules will react with the borate to form a new chemical species. This new species is responsible for creating a different catalyst site type during catalyst activation, likely the one that makes the polymer population with lower molecular weights. Another catalyst site type would also be formed by the direct complexation of catalyst and borate, as in the simultaneous addition method. These sites would be responsible for the production of the high molecular weight chains, similar to the ones made during the simultaneous addition method. The lower polymerization rate for the sequential addition method may be linked to a decrease in the effective borate concentration, resulting from reaction between TOA and the borate. For the simultaneous addition method, when the borate is added to the catalyst solution, a change in color from clear
y = β0 + β1 × [TOA] + β2 × [B] + β12 × [TOA] × [B] (1)
+ε
Table 3. Polymer Yield and Molecular Weight Averages for Polymer Made with Sequential and Simultaneous Hf/B Addition
a
run
yield (g)
Mn
Mw
PDI
Mwa
activityb
simultaneous sequential
3.55 0.55
144 000 66 900
333 000 238 000
2.31 3.56
350 000 237 000
54 133 5365
Determined by light scattering detector at 15° (absolute measurement). bActivity in kg PE/(mol catalyst × h). 1315
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Figure 7. Two-factor central composite design.
Figure 8. Augmented experimental design.
alternative hypothesis H1 = β12 ≠ 0
In eq 1, y is the observed data for the response, [TOA] and [B] are the molar concentrations of TOA and borate, respectively, β0, β1, β2, β12 are the model coefficients, and ε is the random error component. A single degree of freedom sum of squares for pure quadratic curvature was calculated using the equation SSpure quadratic =
Table 5 shows the analysis of variance for Mn. The estimated pooled variance for Mn was calculated using replicates at points A and E. Since the observed F values for the pure quadratic term, borate concentration, and the interaction term are less than F0.05,1,2 = 18.51, we conclude that that those terms are not significant. Consequently, the only term affecting Mn seems to be TOA concentration. A similar analysis shows that Mw is also only affected by TOA concentration in the reactor. Fitting all Mn data of the augmented design (covering a wider data range) using a second order model leads to the same conclusion that Mn is only influenced by TOA concentration at a given polymerization pressure and temperature.
nf nc(yf̅ − yc ̅ )2 nf + nc
(2)
where nf is the number of factorial design points (4 in this case) and nc is the number of replicates at the center point. The test for curvature examines the hypotheses, null hypothesis Ho = β12 = 0 1316
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Table 4. Polymerization Results for Designs Shown in Figures 7 and 8a polymerization conditions run
results
[B] (mol/L × 106)
[TOA] (mol/L × 104)
B/Hf
2.514 2.514 2.514 2.514 4.19 4.19 2.51
6.12 6.12 6.12 6.12 6.12 18.36 18.36
1.00 1.00 1.00 1.00 1.67 1.67 1.00
3.35 3.352 3.35 3.35 4.54 3.352 2.17 5.03 3.35 2.51
12.24 12.24 12.24 20.9 12.24 3.59 12.24 6.12 6.12 12.24
2.51 2.51 2.51 2.51 3.91
3.06 3.06 1.53 0 6.12
A A1 A2 A3 A4 B C D E E1 E2 E3 F G H I J K L M M1 M2 N O P
activity kg PE/(mol catalyst × h)
Mw
Mn
34 367 23 573 24 261 31 200 48 680 26 069 10 564
330 000 319 000 287 000 294 000 280 000 174 000 190 000
139 000 143 000 139 000 137 000 128 000 74 400 83 300
2.4 2.2 2.1 2.2 2.2 2.3 2.3
893 553 661 795 769 803 375
0.0060 0.0048 0.0056 0.0057 0.0029 0.0058 0.0099
1.33 1.33 1.33 1.33 1.81 1.33 0.86 2.00 1.33 1.00
27 163 27 675 23 116 20 380 38 813 59 154 18 209 81 890 33 886 20 839
236 000 220 000 231 000 184 000 226 000 319 000 228 000 336 000 283 000 236 000
91 600 103 000 101 000 85 800 91 800 142 000 106 000 132 000 136 000 102 000
2.6 2.2 2.3 2.2 2.5 2.3 2.2 2.6 2.1 2.3
703 712 635 653 779 1057 433 1115 681 577
0.0068 0.0052 0.0058 0.007 0.0036 0.0031 0.0059 0.0023 0.0041 0.0069
1.00 1.00 1.00 1.00 1.56
71 472 74 783 76 827 negligible 58 326
316 000 341 000 355 000 NA 308 000
144 000 150 000 155 000 NA 115 000
2.2 2.3 2.3 NA 2.7
768 988 1201 NA 735
0.0032 0.0027 0.0033 NA 0.0043
PDI
kp[M] (1/s)
kd (1/s)
Experimental conditions: ethylene pressure = 120 psig, temperature = 120 °C, solvent = toluene, solvent volume = 222.8 mL, catalyst concentration = 2.51× 10−6 mol/L, polymerization time = 11 min.
a
Table 5. ANOVA Table for Mn source of variation TOA cocatalyst TOA × cocatalyst pure quadratic error total
sum of squares 2.958 1.020 1.528 1.024 6.977 3.234
× × × × × ×
degrees of freedom
109 108 106 108 107 109
F0
P-value
84.80 2.90 0.044 2.94
