Ethylene Polymerizations Using Parallel Pressure Reactors and a Kinetic Analysis of Chain Transfer Polymerization

Summary

A protocol for the high-throughput analysis of polymerization catalyst, chain transfer polymerizations, polyethylene characterization, and reaction kinetic analysis is presented.

Abstract

We demonstrate a method for high-throughput catalyst screening using a parallel pressure reactor starting from the initial synthesis of a nickel α-diimine ethylene polymerization catalyst. Initial polymerizations with the catalyst lead to optimized reaction conditions, including catalyst concentration, ethylene pressure and reaction time. Using gas-uptake data for these reactions, a procedure to calculate the initial rate of propagation (k_p) is presented. Using the optimized conditions, the ability of the nickel α-diimine polymerization catalyst to undergo chain transfer with diethylzinc (ZnEt₂) during ethylene polymerization was investigated. A procedure to assess the ability of the catalyst to undergo chain transfer (from molecular weight and ¹³C NMR data), calculate the degree of chain transfer, and calculate chain transfer rates (k_e) is presented.

Introduction

Polyolefins are an important class of industrial polymers with uses in thermoplastics and elastomers. Significant advances in the design of single-site catalysts for the production of polyolefins has led to the ability to tune molecular weight, polydispersity, and polymer microstructure, which leads to a wide range of potential applications.^1-3 More recently, chain transfer and chain shuttling polymerizations have been developed to give an additional route to modify the properties of the polymer without having to modify the catalyst.^4-6 This system employs a single-site transition metal catalyst and a chain transfer reagent (CTR), which is typically a main group metal alkyl. During this polymerization, the growing polymer chain is able to transfer from the catalyst to the CTR, where the polymer chain remains dormant until it is transferred back to the catalyst. Meanwhile, the alkyl group that was transferred to the catalyst can initiate another polymer chain. In a chain transfer polymerization, one catalyst can initiate a greater number of chains compared to a standard catalytic polymerization. The polymer chains are terminated with the chain transfer metal; therefore further end-group functionalization is possible. This system can be used to change the molecular weight and molecular weight distribution of polyolefins,⁷ to catalyze Aufbau-like alkyl chain growth on main group metals,⁸ and for the synthesis of specialty polymers involving multicatalyst systems, such as block copolymers.^9,10

Chain transfer polymerizations have been observed most commonly with early transition metals (Hf, Zr) and alkylzinc or alkylaluminum reagents, although examples exist across the transition metal series.^5,7,8,11-16 In typical early transition metal catalyst systems, chain transfer is fast, efficient and reversible leading to narrow molecular weight distributions. Chain transfer/shuttling has been observed in mid-to-late transition metals (e.g. Cr, Fe, Co and Ni) with group 2 and 12 metal alkyls, although the rates of transfer are highly variable compared to early metals.^4,7,17-19 Two main factors are apparently necessary for efficient chain transfer: a good match of metal-carbon bond dissociation energies for the polymerization catalyst and chain transfer reagent, and an appropriate steric environment to promote bimolecular formation/breakage of alkyl-bridged bimetallic intermediates.²⁰ In the case of late transition metals, if the catalyst does not contain enough steric bulk, beta-hydride (β-H) elimination will be the dominant termination pathway and will generally out-compete chain transfer.

Herein we report on a study of bimetallic chain transfer from nickel to zinc in a bis(2,6-dimethylphenyl)-2,3-butanediimine-based catalyst system with diethylzinc (ZnEt₂) through small-scale high-throughput reactions. Chain transfer will be identified by examining changes in the molecular weight (M_w) and dispersity index of the resulting polyethylene through gel-permeation chromatography analysis. Chain transfer will also be identified through ¹³C NMR analysis of the ratio of vinyl to saturated chain ends as a function of chain transfer agent concentration. An in-depth kinetic analysis of the rates of propagation and chain transfer will also be presented.

Protocol

Caution: Please consult all relevant material safety data sheets (MSDS) before use. Several of the chemicals used in these syntheses are acutely toxic and carcinogenic, while several are pyrophoric and ignite in air. Please use all appropriate safety practices when performing these reactions including the use of engineering controls (fume hood, glovebox) and personal protective equipment (safety glasses, gloves, lab coat, full length pants, closed-toe shoes). Portions of the following procedures involve standard air-free handling techniques.

1. Preparation of [bis(2,6-dimethylphenyl)-2,3-butanediimine]NiBr₂,^21-25

1. Preparation of bis(2,6-dimethylphenyl)-2,3-butanediimine: (α-diimine)
   1. Dissolve 2,3-butanedione (1.0 ml, 11 mmol) and 2,6-dimethylaniline (2.8 ml, 23 mmol) in 20 ml of methanol in a 100 ml round bottom flask.
   2. Add formic acid (0.4 ml, 11 mmol) and stir the reaction at room temperature until the diimine precipitates (typically 1-2 hr, but can be left overnight). If a precipitate does not form, concentrate the reaction mixture using a rotary evaporator and cool in an ice bath.
   3. Filter the reaction mixture using glass frit and filter flask and wash the yellow solid with 20 ml of cold methanol and dry in vacuo.
      Note: ¹H NMR (500 MHz, CD₂Cl₂): δ 7.06 (d, J = 7.6 Hz, 4H), 6.92 (t, J = 7.5 Hz, 2H), 2.00-1.99 (m, 19H).
2. Preparation of (1,2-dimethoxyethane)NiBr₂: DME-NiBr₂
   1. Under a nitrogen atmosphere, combine anhydrous nickel bromide (NiBr₂ , 15 g, 0.069 mol), triethylorthoformate (30 ml, 0.185 mol) and methanol (100 ml) in a 2-neck round-bottom flask fitted with a reflux condenser and rubber septum on a Schlenk line under a nitrogen atmosphere.
   2. Reflux the brown reaction mixture under a nitrogen atmosphere until a green solution forms. This typically takes 2-3 hr, however this reaction can be left refluxing overnight if necessary.
   3. After allowing the mixture to cool, concentrate the reaction mixture using the vacuum pump on the Schlenk line to form a dark green gel.
   4. Cannula transfer an excess 1,2-dimethoxyethane (DME, 100 ml) onto the green gel, which instantly forms orange solid. Once all the DME is added, heat the reaction at 85 °C for an additional 2 hr until a light orange reaction mixture forms. Cool the reaction to room temperature, at which point a light-orange solid precipitates.
   5. Remove the reflux condenser from the reaction flask and replace with a Schlenk frit connected to a second Schlenk receiving flask under a nitrogen atmosphere. Rotate the entire apparatus by 180° causing the orange reaction mixture to enter the Schlenk frit and filter the reaction mixture.
   6. Remove the original reaction flask and place a septum on the Schlenk frit. Cannula transfer DME (100 ml) and then pentane (100 ml) to wash the DME-NiBr₂. Dry the orange solid in vacuo and bring into the glovebox.
      Note: DME-NiBr₂ can be stored indefinitely under a nitrogen atmosphere at room temperature. When left open to the atmosphere, it forms a green, hydrated gel.
3. Preparation of [bis(2,6-dimethylphenyl)-2,3-butanediimine]NiBr₂: ([α-diimine]NiBr₂)
   1. In the glovebox, combine DME-NiBr₂ (1.0 g, 3.2 mmol) and bis(2,6-dimethylphenyl)-2,3-butanediimine (1.1 g, 3.8 mmol) in a 50 ml round bottom flask.
   2. Add 20 ml of dichloromethane and stir overnight at room temperature.
   3. Filter the reaction mixture with a glass frit and filter flask. Wash the brown solid with 75 ml of dichloromethane and dry in vacuo.
      Note: This nickel catalyst can be stored indefinitely in an inert-atmosphere glovebox at room temperature. It is also insoluble in most solvents, so standard characterizations were not performed.

2. Preparation of Catalytic Stock Solutions

1. Preparation of [[α-diimine]Ni(CH₃)]⁺ [MAO]^- stock solution
   1. Prepare a 1.0 x 10^-3 M catalyst stock solution by adding 0.0041 g of [α-diimine]NiBr₂ (8.0 x 10^-6 mol) into a vial with 7.5 ml of toluene.
   2. To the stirred nickel suspension, add 0.50 ml of 30% methylaluminoxane (MAO) in toluene and stir for 1 min. The color will change from a brown suspension to a blue-green solution. This solution can be stored at -30 °C for up to 6 hr and used in subsequent polymerizations.
      Note: Methylaluminoxane is pyrophoric and will smoke in air. It should only be used in an inert-atmosphere glovebox and care should be taken when removing contaminated syringes/glassware from the glovebox. Typically, the syringes and glassware are washed with toluene in the glovebox and placed in a metal can, separate from Kimwipes or any other flammable substances. The contaminated toluene is capped in a vial and removed from the glovebox with the metal can. These items are transferred to the fume hood where they open to air and allowed to slowly quench behind the protective glass.
2. Preparation of ZnEt₂ stock solution
   1. Prepare a 1.2 M solution by dissolving 0.25 ml of ZnEt₂ in 1.75 ml toluene.
      Note: Diethylzinc is pyrophoric and will ignite in air. It should only be used in an inert atmosphere glovebox and care should be taken when removing contaminated syringes/glassware from the glovebox. Typically, the syringes and glassware are washed with toluene in the glovebox and placed in a metal can, separate from Kimwipes or any other flammable substances. The contaminated toluene is capped in a vial and removed from the glovebox with the metal can. These items are transferred to the fumehood where they open to air and allowed to slowly quench behind the protective glass.

3. Catalytic Polymerizations Using a Parallel Pressure Reactor

1. Pressure Scope
   1. Set up all polymerization reactions in a parallel pressure reactor with overhead stirring housed in an N₂ atmosphere glovebox. Program the polymerization in the software: indicate total reaction volume (3.0 ml), the purge gas (N₂), the desired reaction gas (ethylene), the desired pressure (15-150 psi), and the desired reaction time (1 hr).
      Note: A Biotage Endeavor Parallel Pressure Reactor with a separate computer capable of monitoring the gas uptake was used for polymerizations. All polymerizations were done at least three times to ensure accuracy and reproducibility. Using this type of reactor several variables can be adjusted within a single experiment or over the course of multiple experiments including: the reaction gases, pressure, temperature, time, solvent, volume, catalyst, or chain transfer reagent. By doing eight reactions at a time with real-time gas uptake, catalytic reactions are efficiently tested. In this case, the reaction vials have a maximum volume of 5 ml, therefore a volume of 3 ml was chosen to account for the formation of polymer. Also, standard concentrations, pressures and times were based of literature conditions.²¹ These polymerizations can all be done in individual pressure reactors or even on a Schlenk line, however it is more difficult to achieve higher pressures or gas uptake for the reactions.
   2. Insert glass liner reaction vials into the eight wells. Use the depth tool to ensure the glass liners are at the appropriate height. Insert the blade impellers into the overhead assembly.
   3. Fill the reaction vials according to Table 1:

      Reaction Vessel	Pressure (psi)	Catalyst Vol. (ml)	ZnEt2 Vol. (ml)	Toluene Vol. (ml)
      1	15	0	0	3
      2	15	0.1	0	2.9
      3	30	0	0	3
      4	30	0.1	0	2.9
      5	60	0	0	3
      6	60	0.1	0	2.9
      7	150	0	0	3
      8	150	0.1	0	2.9

      Table 1. Reaction conditions for the pressure scope reactions.
   4. Ensure the O-rings are properly seated in the metal grooves and carefully place the overhead stirring assembly on the base and screw down in an alternating fashion. Ensure all screws are tight and press start in the software. Monitor the reaction via gas uptake measurements.
   5. After 1 hr of polymerization, remove the reaction vials from the glove box and precipitate polyethylene with the addition of 5% hydrochloric acid in methanol, remove solvent and dry in vacuo. Mass the polymer to obtain the yield and compare to the ethylene consumption during the reaction.
   6. Calculate the activity of the catalyst, which is the mass of polymer formed per mole of catalyst, per hour of polymerization.
   7. Analyze the molecular weight and dispersity index of the dried polyethylene using high temperature Gel Permeation Chromatography (GPC). Dissolve 0.002 g of the polymer in 2 ml 1,2,4-trichlorobenzene at 135 °C. Run the GPC according to manufacturer's protocol.
      Note: An Agilent PL-GPC 220 High Temperature GPC/SEC system was used at 135 °C and data was fit using polystyrene standards to analyze the molecular weight of the polymer.
   8. Analyze the degree of branching, the branch type and the end group type of the dried polyethylene using high-temperature ¹³C NMR. Dissolve 0.050-0.080 g of the polymer in 0.5 ml of tetrachloroethane-d₂ (C₂D₂Cl₄) at 130 °C. Run the samples on a 600 MHz NMR at 130 °C for at least 2,000 scans. Assign polymer branching according to the literature.^26-28
      Note: An Agilent/Varian 600 MHz spectrometer at 130 °C was used for ¹³C NMR analysis.
2. Chain Transfer Polymerization
   1. Following the same procedures and analysis as Section 3.1, fill the reactor and program the software according to the parameters in Table 2:

      Reaction Vessel	Pressure (psi)	Catalyst Vol. (ml)	ZnEt2 Vol. (ml)	Toluene Vol. (ml)
      1	60	0.1	0	2.9
      2	60	0.1	0.005	2.9
      3	60	0.1	0.01	2.89
      4	60	0.1	0.015	2.89
      5	60	0.1	0.025	2.88
      6	60	0.1	0.042	2.86
      7	60	0.1	0.06	2.84
      8	60	0.1	0.085	2.82

      Table 2. Reaction conditions for the chain transfer polymerization reactions.
   2. In addition, calculate the moles of chains extended due to the presence of ZnEt₂ and the number of chains initiated per mole of catalyst based on the following equations.⁷
      
      (M-R)_Extended - moles of chains extended due to the presence of ZnEt₂ (mol)
      Yield_Polymer - Mass of polymer formed (g)
      M_n - Number average molecular weight of the polymer from GPC (g/mol)
      Chains_initiated - Number of chains initiated per mole of catalyst
      Moles_catalyst - Moles of catalyst used in the polymerization
   3. Make a plot of the number chains initiated versus the concentration of ZnEt₂.

4. Kinetic Analysis of Polymerizations: Rates of Chain Transfer and Propagation

1. Rate of Propagation (k_p)
   1. For each run where only [α-diimine]NiBr₂ is present, make a plot of ethylene consumption versus time.
   2. Fit the initial ethylene gas uptake in the linear region to obtain the initial rate. In the case of Figure 2a, the traces were fit from 500 to 2,000 sec.
   3. Use the average of the slopes to obtain the rate of propagation (k_p) for the particular catalyst at the desired pressure.
2. Rate of chain transfer (k_p)
   1. Estimate the concentration of ethylene, [C₂⁼], in 3.0 ml of toluene from the average gas uptake (solubility) by finding the number of moles of gas consumed in the reactors that do not contain catalyst.
   2. Make a Mayo plot of 1/M_n (from the molecular weight data obtained from the GPC) versus [ZnR_x]/[C₂⁼] (from the concentration of ZnEt₂ placed in the reactor).^{7, 29}
   3. Fit the data to a linear line based on the Mayo equation. Multiply the slope by 28, the molecular weight of ethylene, to obtain the ratio of the rate of chain transfer to the rate of propagation (k_e/k_p).
      
      MN - Number average molecular weight of the polymer from GPC with ZnEt₂
      M_N^o - Number average molecular weight of the polymer from GPC without ZnEt₂
      k_e - Rate of chain transfer
      k_p - Rate of propagation
      [ZnR_x] - Concentration of ZnEt₂ in the reaction
      [C₂⁼] - Concentration of ethylene in the reaction
      28 - The molecular weight of ethylene
   4. Multiply k_e/k_p by k_p obtained in the previous section to get the rate of chain transfer k_e.

Results

The ethylene gas consumption versus time is presented in Figure 1 for the different ethylene pressures tested. This data is used to determine optimized reaction conditions. The ethylene gas consumption versus time is presented in Figure 2A for the catalyst alone samples, which is used to calculate the rate of propagation (k_p). Figure 2B shows gel permeation chromatography (GPC) traces for chain transfer polymerizations with 0-1,000 equivalents of diet...

Discussion

A methyl-substituted cationic [α-diimine]NiBr₂ ethylene polymerization catalyst activated with MAO was examined for its competency for ethylene chain transfer polymerizations. The reactions were monitored via gas uptake measurements to determine the rate and extent of polymerization and catalyst lifetime, and the molecular weight of the resultant polymers were determined via gel permeation chromatography (GPC). Initially, the nickel catalyst was tested over a range of et...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Endeavor Pressure Reactor	Biotage	EDV-1N-L
Blade Impellers	Biotage	900543
Glass Liners	Biotage	900676
2,3-butanedione, 99%	Alfa Aesar	A14217
2,6-dimethylaniline, 99%	Sigma Aldrich	D146005
formic acid, 95%	Sigma Aldrich	F0507
methanol, 99.8%	Sigma Aldrich	179337	ACS Reagent
nickel (II) bromide, 99%	Strem	28-1140	anhydrous, hygroscopic
triethylorthoformate, 98%	Sigma Aldrich	304050	dried with K2CO3 and distilled
1,2-dimethoxyethane, 99.5%	Sigma Aldrich	259527	dried with Na/Benzophenone and distilled
pentane, 99%	Fisher	P399	HPLC Grade *
dichloromethane, 99.5%	Fisher	D37	ACS Reagent *
toluene, 99.8%	Fisher	T290	HPLC Grade *
methylaluminoxane	Albemarle	MAO	pyrophoric, 30% in toluene
diethylzinc, 95%	Strem	93-3030	pyrophoric
1,2,4-trichlorobenzene, 99%	Sigma Aldrich	296104
1,1,2,2-tetrachloroethane-D2, 99.6%	Cambridge Isotopes	DLM-35