Financial support was provided by the University of Minnesota (start up funds) and the ACS Petroleum Research Fund (54225-DNI3). Equipment purchases for the Chemistry Department NMR facility were supported through a grant from the NIH (S10OD011952) with matching funds from the University of Minnesota. We acknowledge the Minnesota NMR Center for high-temperature NMR. Funding for NMR instrumentation was provided by the Office of the Vice President for Research, the Medical School, the College of Biological Science, NIH, NSF, and the Minnesota Medical Foundation. We thank John Walzer (ExxonMobil) for a gift of PEEK high-throughput stirring paddles.

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
<ol>
	<li>Preparation of bis(2,6-dimethylphenyl)-2,3-butanediimine: (&#945;-diimine)
	<ol>
		<li>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.</li>
		<li>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.</li>
		<li>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): &#948; 7.06 (d, J = 7.6 Hz, 4H), 6.92 (t, J = 7.5 Hz, 2H), 2.00-1.99 (m, 19H).</li>
	</ol>
	</li>
	<li>Preparation of (1,2-dimethoxyethane)NiBr2: DME-NiBr2
	<ol>
		<li>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.</li>
		<li>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.</li>
		<li>After allowing the mixture to cool, concentrate the reaction mixture using the vacuum pump on the Schlenk line to form a dark green gel.</li>
		<li>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 &#176;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.</li>
		<li>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&#176; causing the orange reaction mixture to enter the Schlenk frit and filter the reaction mixture.</li>
		<li>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.</li>
	</ol>
	</li>
	<li>Preparation of [bis(2,6-dimethylphenyl)-2,3-butanediimine]NiBr2: ([&#945;-diimine]NiBr2)
	<ol>
		<li>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.</li>
		<li>Add 20 ml of dichloromethane and stir overnight at room temperature.</li>
		<li>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.</li>
	</ol>
	</li>
</ol>
2. Preparation of Catalytic Stock Solutions
<ol>
	<li>Preparation of [[&#945;-diimine]Ni(CH3)]+ [MAO]- stock solution
	<ol>
		<li>Prepare a 1.0 x 10-3 M catalyst stock solution by adding 0.0041 g of [&#945;-diimine]NiBr2 (8.0 x 10-6 mol) into a vial with 7.5 ml of toluene.</li>
		<li>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 &#176;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.</li>
	</ol>
	</li>
	<li>Preparation of ZnEt2 stock solution
	<ol>
		<li>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.</li>
	</ol>
	</li>
</ol>
3. Catalytic Polymerizations Using a Parallel Pressure Reactor
<ol>
	<li>Pressure Scope
	<ol>
		<li>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.</li>
		<li>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.</li>
		<li>Fill the reaction vials according to Table 1:
		<table border="0" cellpadding="0" cellspacing="0">
			<tbody>
				<tr>
					<td>Reaction Vessel</td>
					<td>Pressure (psi)</td>
					<td>Catalyst Vol. (ml)</td>
					<td>ZnEt2 Vol. (ml)</td>
					<td>Toluene Vol. (ml)</td>
				</tr>
				<tr>
					<td>1</td>
					<td>15</td>
					<td>0</td>
					<td>0</td>
					<td>3</td>
				</tr>
				<tr>
					<td>2</td>
					<td>15</td>
					<td>0.1</td>
					<td>0</td>
					<td>2.9</td>
				</tr>
				<tr>
					<td>3</td>
					<td>30</td>
					<td>0</td>
					<td>0</td>
					<td>3</td>
				</tr>
				<tr>
					<td>4</td>
					<td>30</td>
					<td>0.1</td>
					<td>0</td>
					<td>2.9</td>
				</tr>
				<tr>
					<td>5</td>
					<td>60</td>
					<td>0</td>
					<td>0</td>
					<td>3</td>
				</tr>
				<tr>
					<td>6</td>
					<td>60</td>
					<td>0.1</td>
					<td>0</td>
					<td>2.9</td>
				</tr>
				<tr>
					<td>7</td>
					<td>150</td>
					<td>0</td>
					<td>0</td>
					<td>3</td>
				</tr>
				<tr>
					<td>8</td>
					<td>150</td>
					<td>0.1</td>
					<td>0</td>
					<td>2.9</td>
				</tr>
			</tbody>
		</table>
		Table 1. Reaction conditions for the pressure scope reactions.</li>
		<li>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.</li>
		<li>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.</li>
		<li>Calculate the activity of the catalyst, which is the mass of polymer formed per mole of catalyst, per hour of polymerization.</li>
		<li>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 &#176;C. Run the GPC according to manufacturer&#39;s protocol. 
		Note: An Agilent PL-GPC 220 High Temperature GPC/SEC system was used at 135 &#176;C and data was fit using polystyrene standards to analyze the molecular weight of the polymer.</li>
		<li>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 &#176;C. Run the samples on a 600 MHz NMR at 130 &#176;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 &#176;C was used for 13C NMR analysis.</li>
	</ol>
	</li>
	<li>Chain Transfer Polymerization
	<ol>
		<li>Following the same procedures and analysis as Section 3.1, fill the reactor and program the software according to the parameters in Table 2:
		<table border="0" cellpadding="0" cellspacing="0">
			<tbody>
				<tr>
					<td>Reaction Vessel</td>
					<td>Pressure (psi)</td>
					<td>Catalyst Vol. (ml)</td>
					<td>ZnEt2 Vol. (ml)</td>
					<td>Toluene Vol. (ml)</td>
				</tr>
				<tr>
					<td>1</td>
					<td>60</td>
					<td>0.1</td>
					<td>0</td>
					<td>2.9</td>
				</tr>
				<tr>
					<td>2</td>
					<td>60</td>
					<td>0.1</td>
					<td>0.005</td>
					<td>2.9</td>
				</tr>
				<tr>
					<td>3</td>
					<td>60</td>
					<td>0.1</td>
					<td>0.01</td>
					<td>2.89</td>
				</tr>
				<tr>
					<td>4</td>
					<td>60</td>
					<td>0.1</td>
					<td>0.015</td>
					<td>2.89</td>
				</tr>
				<tr>
					<td>5</td>
					<td>60</td>
					<td>0.1</td>
					<td>0.025</td>
					<td>2.88</td>
				</tr>
				<tr>
					<td>6</td>
					<td>60</td>
					<td>0.1</td>
					<td>0.042</td>
					<td>2.86</td>
				</tr>
				<tr>
					<td>7</td>
					<td>60</td>
					<td>0.1</td>
					<td>0.06</td>
					<td>2.84</td>
				</tr>
				<tr>
					<td>8</td>
					<td>60</td>
					<td>0.1</td>
					<td>0.085</td>
					<td>2.82</td>
				</tr>
			</tbody>
		</table>
		Table 2. Reaction conditions for the chain transfer polymerization reactions.</li>
		<li>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 
		<img alt="Equation 1" src="/files/ftp_upload/53212/53212eq1-2.jpg" width="250" /> 
		(M-R)Extended - moles of chains extended due to the presence of ZnEt2 (mol) 
		YieldPolymer - Mass of polymer formed (g) 
		Mn - Number average molecular weight of the polymer from GPC (g/mol) 
		Chainsinitiated - Number of chains initiated per mole of catalyst 
		Molescatalyst - Moles of catalyst used in the polymerization</li>
		<li>Make a plot of the number chains initiated versus the concentration of ZnEt2.</li>
	</ol>
	</li>
</ol>
4. Kinetic Analysis of Polymerizations: Rates of Chain Transfer and Propagation
<ol>
	<li>Rate of Propagation (kp)
	<ol>
		<li>For each run where only [&#945;-diimine]NiBr2 is present, make a plot of ethylene consumption versus time.</li>
		<li>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.</li>
		<li>Use the average of the slopes to obtain the rate of propagation (kp) for the particular catalyst at the desired pressure.</li>
	</ol>
	</li>
	<li>Rate of chain transfer (kp)
	<ol>
		<li>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.</li>
		<li>Make a Mayo plot of 1/Mn (from the molecular weight data obtained from the GPC) versus [ZnRx]/[C2=] (from the concentration of ZnEt2 placed in the reactor).7, 29</li>
		<li>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 (ke/kp). 
		<img alt="Equation 3" src="/files/ftp_upload/53212/53212eq3.jpg" width="230" /> 
		MN - Number average molecular weight of the polymer from GPC with ZnEt2 
		MNo - Number average molecular weight of the polymer from GPC without ZnEt2 
		ke - Rate of chain transfer 
		kp - Rate of propagation 
		[ZnRx] - Concentration of ZnEt2 in the reaction 
		[C2=] - Concentration of ethylene in the reaction 
		28 - The molecular weight of ethylene</li>
		<li>Multiply ke/kp by kp obtained in the previous section to get the rate of chain transfer ke.</li>
	</ol>
	</li>
</ol>

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F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chain+Growth+Polymerization+of+Isoprene+and+Stereoselective+Isoprene-Styrene+Copolymerization+Promoted+by+an+ansa-Bis(indenyl)allyl-Yttrium+Complex.">Chain Growth Polymerization of Isoprene and Stereoselective Isoprene-Styrene Copolymerization Promoted by an ansa-Bis(indenyl)allyl-Yttrium Complex.</a> Macromolecules. 44 (18), 7158-7166 (2011).</li><li>Kretschmer, W. 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Ed. 41 (3), 489-491 (2002).</li><li>Xiao, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+Novel+Linear-Hyperbranched+Multiblock+Polyethylene+Produced+from+Ethylene+Monomer+Alone+via+Chain+Walking+and+Chain+Shuttling+Polymerization.">A Novel Linear-Hyperbranched Multiblock Polyethylene Produced from Ethylene Monomer Alone via Chain Walking and Chain Shuttling Polymerization.</a> Macromolecules. 42 (6), 1834-1837 (2009).</li><li>Simon, L. C., Mauler, R. S., De Souza, R. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+the+alkylaluminum+cocatalyst+on+ethylene+polymerization+by+a+nickel-diimine+complex.">Effect of the alkylaluminum cocatalyst on ethylene polymerization by a nickel-diimine complex.</a> J. Polym. Sci. A Polym Chem. 37 (24), 4656-4663 (1999).</li><li>Hue, R. J., Cibuzar, M. P., Tonks, I. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Analysis+of+Polymeryl+Chain+Transfer+Between+Group+10+Metals+and+Main+Group+Alkyls+during+Ethylene+Polymerization.">Analysis of Polymeryl Chain Transfer Between Group 10 Metals and Main Group Alkyls during Ethylene Polymerization.</a> ACS Catalysis. 4 (11), 4223-4231 (2014).</li><li>Johnson, L. K., Killian, C. M., Brookhart, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=New+Pd(II)-+and+Ni(II)-Based+Catalysts+for+Polymerization+of+Ethylene+and+.alpha.-Olefins.">New Pd(II)- and Ni(II)-Based Catalysts for Polymerization of Ethylene and .alpha.-Olefins.</a> J. Am. Chem. Soc. 117 (23), 6414-6415 (1995).</li><li>Ittel, S. D., Johnson, L. K., Brookhart, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Late-Metal+Catalysts+for+Ethylene+Homo-+and+Copolymerization.">Late-Metal Catalysts for Ethylene Homo- and Copolymerization.</a> Chem. Rev. 100 (4), 1169-1204 (2000).</li><li>Bautista, R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Synthesis+and+Diels-Alder+Cycloadditions+of+exo-Imidazolidin-2-one+Dienes.">Synthesis and Diels-Alder Cycloadditions of exo-Imidazolidin-2-one Dienes.</a> J. Org. Chem. 76 (19), 7901-7911 (2011).</li><li>Rulke, R. E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=NMR+study+on+the+coordination+behavior+of+dissymmetric+terdentate+trinitrogen+ligands+on+methylpalladium(II)+compounds.">NMR study on the coordination behavior of dissymmetric terdentate trinitrogen ligands on methylpalladium(II) compounds.</a> Inorg. Chem. 32 (25), 5769-5778 (1993).</li><li>Ward, L. G. L., Pipal, J. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Anhydrous+Nickel+(II)+Halides+and+their+Tetrakis+(Ethanol)+and+1,2-Dimethoxyethane+Complexes.">Anhydrous Nickel (II) Halides and their Tetrakis (Ethanol) and 1,2-Dimethoxyethane Complexes.</a> Inorg. Syntheses. 13, 154-164 (1972).</li><li>Galland, G. B., de Souza, R. F., Mauler, R. S., Nunes, F. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=13C+NMR+Determination+of+the+Composition+of+Linear+Low-Density+Polyethylene+Obtained+with+[&#951;3-Methallyl-nickel-diimine]PF6+Complex.">13C NMR Determination of the Composition of Linear Low-Density Polyethylene Obtained with [&#951;3-Methallyl-nickel-diimine]PF6 Complex.</a> Macromolecules. 32 (5), 1620-1625 (1999).</li><li>Cotts, P. M., Guan, Z., McCord, E., McLain, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Novel+Branching+Topology+in+Polyethylenes+As+Revealed+by+Light+Scattering+and+13C+NMR.">Novel Branching Topology in Polyethylenes As Revealed by Light Scattering and 13C NMR.</a> Macromolecules. 33 (19), 6945-6952 (2000).</li><li>Wiedemann, T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Monofunctional+hyperbranched+ethylene+oligomers.">Monofunctional hyperbranched ethylene oligomers.</a> J. Am. Chem. Soc. 136 (5), 2078-2085 (2014).</li><li>Mayo, F. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chain+Transfer+in+the+Polymerization+of+Styrene:+The+Reaction+of+Solvents+with+Free+Radicals1.">Chain Transfer in the Polymerization of Styrene: The Reaction of Solvents with Free Radicals1.</a> J. Am. Chem. Soc. 65 (12), 2324-2329 (1943).</li></ol>

The authors declare no competing financial interest.

A methyl-substituted cationic [&#945;-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...

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 (&#946;-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 (Mw) 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.

<table border="0" cellpadding="0" cellspacing="0" style="width:675px;" width="674">
	<tbody>
		<tr height="25">
			<td height="25" style="height:25px;width:209px;">Endeavor Pressure Reactor</td>
			<td style="width:119px;">Biotage</td>
			<td style="width:107px;">EDV-1N-L</td>
			<td style="width:240px;"></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">Blade Impellers</td>
			<td>Biotage</td>
			<td>900543</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">Glass Liners</td>
			<td>Biotage</td>
			<td>900676</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">2,3-butanedione, 99%</td>
			<td>Alfa Aesar</td>
			<td>A14217</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">2,6-dimethylaniline, 99%</td>
			<td>Sigma Aldrich</td>
			<td>D146005</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">formic acid, 95%</td>
			<td>Sigma Aldrich</td>
			<td>F0507</td>
			<td></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">methanol, 99.8%</td>
			<td>Sigma Aldrich</td>
			<td>179337</td>
			<td>ACS Reagent</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">nickel (II) bromide, 99%</td>
			<td>Strem</td>
			<td>28-1140</td>
			<td>anhydrous, hygroscopic</td>
		</tr>
		<tr height="26">
			<td height="26" style="height:26px;">triethylorthoformate, 98%</td>
			<td>Sigma Aldrich</td>
			<td>304050</td>
			<td>dried with K2CO3 and distilled</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">1,2-dimethoxyethane, 99.5%</td>
			<td>Sigma Aldrich</td>
			<td>259527</td>
			<td>dried with Na/Benzophenone and distilled</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">pentane, 99%</td>
			<td>Fisher</td>
			<td>P399</td>
			<td>HPLC Grade *</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">dichloromethane, 99.5%</td>
			<td>Fisher</td>
			<td>D37</td>
			<td>ACS Reagent *</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">toluene, 99.8%</td>
			<td>Fisher</td>
			<td>T290</td>
			<td>HPLC Grade *</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">methylaluminoxane</td>
			<td>Albemarle</td>
			<td>MAO</td>
			<td>pyrophoric, 30% in toluene</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">diethylzinc, 95%</td>
			<td>Strem</td>
			<td>93-3030</td>
			<td>pyrophoric</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;">1,2,4-trichlorobenzene, 99%</td>
			<td>Sigma Aldrich</td>
			<td>296104</td>
			<td></td>
		</tr>
		<tr height="26">
			<td height="26" style="height:26px;">1,1,2,2-tetrachloroethane-D2, 99.6%</td>
			<td>Cambridge Isotopes</td>
			<td>DLM-35</td>
			<td></td>
		</tr>
	</tbody>
</table>

ethylene polymerizations using parallel pressure reactors and a kinetic analysis of chain transfer polymerization

We demonstrate a method for high-throughput catalyst screening using a parallel pressure reactor starting from the initial synthesis of a nickel &#945;-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 (kp) is presented. Using the optimized conditions, the ability of the nickel &#945;-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 (ke) is presented.

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 (kp). Figure 2B shows gel permeation chromatography (GPC) traces for chain transfer polymerizations with 0-1,000 equivalents of diet...

Watch this Scientific Journal Video about Ethylene Polymerizations Using Parallel Pressure Reactors and a Kinetic Analysis of Chain Transfer Polymerization at JoVE.com

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

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

We demonstrate a method for high-throughput catalyst screening using a parallel pressure reactor starting from the initial synthesis of a nickel ...

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12.7K Views.  University of Minnesota - Twin Cities. A protocol for the high-throughput analysis of polymerization catalyst, chain transfer polymerizations, polyethylene characterization, and reaction kinetic analysis is presented.

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

Steady-state, Pre-steady-state, and Single-turnover Kinetic Measurement for DNA Glycosylase Activity

Human 8-oxoguanine DNA glycosylase (OGG1) excises the mutagenic oxidative DNA lesion 8-oxo-7,8-dihydroguanine (8-oxoG) from DNA. Kinetic characterization of OGG1 is undertaken to measure the rates of 8-oxoG excision and product release. When the OGG1 concentration is lower than substrate DNA, time courses of product formation are biphasic; a rapid exponential phase (i.e. burst) of product formation is followed by a linear steady-state phase. The initial burst of product formation corresponds to the concentration of enzyme properly engaged on the substrate, and the burst amplitude depends on the concentration of enzyme. The first-order rate constant of the burst corresponds to the intrinsic rate of 8-oxoG excision and the slower steady-state rate measures the rate of product release (product DNA dissociation rate constant, koff). Here, we describe steady-state, pre-steady-state, and single-turnover approaches to isolate and measure specific steps during OGG1 catalytic cycling. A fluorescent labeled lesion-containing oligonucleotide and purified OGG1 are used to facilitate precise kinetic measurements. Since low enzyme concentrations are used to make steady-state measurements, manual mixing of reagents and quenching of the reaction can be performed to ascertain the steady-state rate (koff). Additionally, extrapolation of the steady-state rate to a point on the ordinate at zero time indicates that a burst of product formation occurred during the first turnover (i.e. y-intercept is positive). The first-order rate constant of the exponential burst phase can be measured using a rapid mixing and quenching technique that examines the amount of product formed at short time intervals (&lt;1 sec) before the steady-state phase and corresponds to the rate of 8-oxoG excision (i.e. chemistry). The chemical step can also be measured using a single-turnover approach where catalytic cycling is prevented by saturating substrate DNA with enzyme (E&gt;S). These approaches can measure elementary rate constants that influence the efficiency of removal of a DNA lesion.

Time courses for the glycosylase activity of 8-oxoguanine DNA glycosylase are biphasic exhibiting a burst of product formation and a linear steady-state phase. Utilizing quench-flow techniques, the burst and the steady-state rates can be measured, which correspond to excision of 8-oxoguanine and release of the glycosylase from the product DNA, respectively.

Human 8-oxoguanine DNA glycosylase (OGG1) excises the mutagenic oxidative DNA lesion 8-oxo-7,8-dihydroguanine (8-oxoG) from DNA. Kinetic characterization ...

Microwave-assisted Functionalization of Poly(ethylene glycol) and On-resin Peptides for Use in Chain Polymerizations and Hydrogel Formation

One of the main benefits to using poly(ethylene glycol) (PEG) macromers in hydrogel formation is synthetic versatility. The ability to draw from a large variety of PEG molecular weights and configurations (arm number, arm length, and branching pattern) affords researchers tight control over resulting hydrogel structures and properties, including Young&#8217;s modulus and mesh size. This video will illustrate a rapid, efficient, solvent-free, microwave-assisted method to methacrylate PEG precursors into poly(ethylene glycol) dimethacrylate (PEGDM). This synthetic method provides much-needed starting materials for applications in drug delivery and regenerative medicine. The demonstrated method is superior to traditional methacrylation methods as it is significantly faster and simpler, as well as more economical and environmentally friendly, using smaller amounts of reagents and solvents. We will also demonstrate an adaptation of this technique for on-resin methacrylamide functionalization of peptides. This on-resin method allows the N-terminus of peptides to be functionalized with methacrylamide groups prior to deprotection and cleavage from resin. This allows for selective addition of methacrylamide groups to the N-termini of the peptides while amino acids with reactive side groups (e.g.&#160;primary amine of lysine, primary alcohol of serine, secondary alcohols of threonine, and phenol of tyrosine) remain protected, preventing functionalization at multiple sites. This article will detail common analytical methods (proton Nuclear Magnetic Resonance spectroscopy (;H-NMR) and Matrix Assisted Laser Desorption Ionization Time of Flight mass spectrometry (MALDI-ToF)) to assess the efficiency of the functionalizations. Common pitfalls and suggested troubleshooting methods will be addressed, as will modifications of the technique which can be used to further tune macromer functionality and resulting hydrogel physical and chemical properties. Use of synthesized products for the formation of hydrogels for drug delivery and cell-material interaction studies will be demonstrated, with particular attention paid to modifying hydrogel composition to affect mesh size, controlling hydrogel stiffness and drug release.

Microwave-assisted Functionalization of Polyethylene glycol and On-resin Peptides for Use in Chain Polymerizations and Hydrogel Formation

This video will illustrate a rapid, efficient method to methacrylate poly(ethylene glycol), enabling chain polymerizations and hydrogel synthesis. It will demonstrate how to similarly introduce methacrylamide functionalities into peptides, detail common analytical methods to assess functionalization efficiency, provide suggestions for troubleshooting and advanced modifications, and demonstrate typical hydrogel characterization techniques.

One of the main benefits to using poly(ethylene glycol) (PEG) macromers in hydrogel formation is synthetic versatility. The ability to draw from a large ...

High-pressure Sapphire Cell for Phase Equilibria Measurements of CO2/Organic/Water Systems

The high pressure sapphire cell apparatus was constructed to visually determine the composition of multiphase systems without physical sampling. Specifically, the sapphire cell enables visual data collection from multiple loadings to solve a set of material balances to precisely determine phase composition. Ternary phase diagrams can then be established to determine the proportion of each component in each phase at a given condition. In principle, any ternary system can be studied although ternary systems (gas-liquid-liquid) are the specific examples discussed herein. For instance, the ternary THF-Water-CO2 system was studied at 25 and 40&#160;&#176;C and is described herein. Of key importance, this technique does not require sampling. Circumventing the possible disturbance of the system equilibrium upon sampling, inherent measurement errors, and technical difficulties of physically sampling under pressure is a significant benefit of this technique. Perhaps as important, the sapphire cell also enables the direct visual observation of the phase behavior. In fact, as the CO2 pressure is increased, the homogeneous THF-Water solution phase splits at about 2 MPa. With this technique, it was possible to easily and clearly observe the cloud point and determine the composition of the newly formed phases as a function of pressure.
The data acquired with the sapphire cell technique can be used for many applications. In our case, we measured swelling and composition for tunable solvents, like gas-expanded liquids, gas-expanded ionic liquids and Organic Aqueous Tunable Systems (OATS)1-4. For the latest system, OATS, the high-pressure sapphire cell enabled the study of (1) phase behavior as a function of pressure and temperature, (2) composition of each phase (gas-liquid-liquid) as a function of pressure and temperature and (3) catalyst partitioning in the two liquid phases as a function of pressure and composition. Finally, the sapphire cell is an especially effective tool to gather accurate and reproducible measurements in a timely fashion.

High-pressure Sapphire Cell for Phase Equilibria Measurements of CO2/Organic/Water Systems

The high pressure sapphire cell apparatus is a unique tool to study, without sampling, phase behavior under a wide range of pressures. Using a cathetometer, very precise volume measurements can be recorded to measure liquid expansion and phase composition. Thus, this synthetic method enables the study of (1) phase equilibria of multi-component mixtures and (2) the partition behavior of catalyst or model compounds as a function of pressure.

The high pressure sapphire cell apparatus was constructed to visually determine the composition of multiphase systems without physical sampling. ...

Chemical Gardens as Flow-through Reactors Simulating Natural Hydrothermal Systems

Here we report experimental simulations of hydrothermal chimney growth using injection chemical garden methods. The versatility of this type of experiment allows for testing of various proposed ocean / hydrothermal fluid chemistries that could have driven reactions toward the origin of life in environments on the early Earth, early Mars, or even other worlds such as the icy moons of the outer planets. We show experiments that include growth of chemical garden structures under anoxic conditions simulating the early Earth, inclusion of trace components of phosphates / organics in the injection solution to incorporate them into the structure, a switch of the injection solution to introduce a secondary precipitating anion, and the measurement of membrane potentials generated by chemical gardens. Using this method, self-assembling chemical garden structures were formed that mimic the natural chimneys precipitated at submarine hydrothermal springs, and these precipitates can be used successfully as flow-through reactors by feeding through multiple successive &#8220;hydrothermal&#8221; injections.

We describe chemical garden formation via injection experiments that allow for laboratory simulations of natural chemical garden systems that form at submarine hydrothermal vents.

Here we report experimental simulations of hydrothermal chimney growth using injection chemical garden methods. The versatility of this type of experiment ...

Tuning a Parallel Segmented Flow Column and Enabling Multiplexed Detection

Active flow technology (AFT) is new form of column technology that was designed to overcome flow heterogeneity to increase separation performance in terms of efficiency and sensitivity and to enable multiplexed detection. This form of AFT uses a parallel segmented flow (PSF) column. A PSF column outlet end-fitting consists of 2 or 4 ports, which can be multiplexed to connect up to 4 detectors. The PSF column not only allows a platform for multiplexed detection but also the combination of both destructive and non-destructive detectors, without additional dead volume tubing, simultaneously. The amount of flow through each port can also be adjusted through pressure management to suit the requirements of a specific detector(s). To achieve multiplexed detection using a PSF column there are a number of parameters which can be controlled to ensure optimal separation performance and quality of results; that is tube dimensions for each port, choice of port for each type of detector and flow adjustment. This protocol is intended to show how to use and tune a PSF column functioning in a multiplexed mode of detection.

Here, we present a protocol for the operation and tuning of parallel segmented flow chromatography columns to enable multiplexed detection.

Active flow technology (AFT) is new form of column technology that was designed to overcome flow heterogeneity to increase separation performance in terms ...

Synthesis of Programmable Main-chain Liquid-crystalline Elastomers Using a Two-stage Thiol-acrylate Reaction

This study presents a novel two-stage thiol-acrylate Michael addition-photopolymerization (TAMAP) reaction to prepare main-chain liquid-crystalline elastomers (LCEs) with facile control over network structure and programming of an aligned monodomain. Tailored LCE networks were synthesized using routine mixing of commercially available starting materials and pouring monomer solutions into molds to cure. An initial polydomain LCE network is formed via a self-limiting thiol-acrylate Michael-addition reaction. Strain-to-failure and glass transition behavior were investigated as a function of crosslinking monomer, pentaerythritol tetrakis(3-mercaptopropionate) (PETMP). An example non-stoichiometric system of 15 mol% PETMP thiol groups and an excess of 15 mol% acrylate groups was used to demonstrate the robust nature of the material. The LCE formed an aligned and transparent monodomain when stretched, with a maximum failure strain over 600%. Stretched LCE samples were able to demonstrate both stress-driven thermal actuation when held under a constant bias stress or the shape-memory effect when stretched and unloaded. A permanently programmed monodomain was achieved via a second-stage photopolymerization reaction of the excess acrylate groups when the sample was in the stretched state. LCE samples were photo-cured and programmed at 100%, 200%, 300%, and 400% strain, with all samples demonstrating over 90% shape fixity when unloaded. The magnitude of total stress-free actuation increased from 35% to 115% with increased programming strain. Overall, the two-stage TAMAP methodology is presented as a powerful tool to prepare main-chain LCE systems and explore structure-property-performance relationships in these fascinating stimuli-sensitive materials.

A novel methodology is presented to synthesize and program main-chain liquid-crystalline elastomers using commercially available starting monomers. A wide range of thermomechanical properties was tailored by adjusting the amount of crosslinker, while the actuation performance was dependent on the amount of applied strain during programming.

This study presents a novel two-stage thiol-acrylate Michael addition-photopolymerization (TAMAP) reaction to prepare main-chain liquid-crystalline ...

Atom Transfer Radical Polymerization of Functionalized Vinyl Monomers Using Perylene as a Visible Light Photocatalyst

A standardized technique for atom transfer radical polymerization of vinyl monomers using perylene as a visible-light photocatalyst is presented. The procedure is performed under an inert atmosphere using air- and water-exclusion techniques. The outcome of the polymerization is affected by the ratios of monomer, initiator, and catalyst used as well as the reaction concentration, solvent, and nature of the light source. Temporal control over the polymerization can be exercised by turning the visible light source off and on. Low dispersities of the resultant polymers as well as the ability to chain-extend to form block copolymers suggest control over the polymerization, while chain end-group analysis provides evidence supporting an atom-transfer radical polymerization mechanism.

A method for the atom transfer radical polymerization of functionalized vinyl monomers using perylene as a visible-light photocatalyst is described.

A standardized technique for atom transfer radical polymerization of vinyl monomers using perylene as a visible-light photocatalyst is presented. The ...

Facile Synthesis of Worm-like Micelles by Visible Light Mediated Dispersion Polymerization Using Photoredox Catalyst

Presented herein is a protocol for the facile synthesis of worm-like micelles by visible light mediated dispersion polymerization. This approach begins with the synthesis of a hydrophilic poly(oligo(ethylene glycol) methyl ether methacrylate) (POEGMA) homopolymer using reversible addition-fragmentation chain-transfer (RAFT) polymerization. Under mild visible light irradiation (&#955; = 460 nm, 0.7 mW/cm2), this macro-chain transfer agent (macro-CTA) in the presence of a ruthenium based photoredox catalyst, Ru(bpy)3Cl2 can be chain extended with a second monomer to form a well-defined block copolymer in a process known as Photoinduced Electron Transfer RAFT (PET-RAFT). When PET-RAFT is used to chain extend POEGMA with benzyl methacrylate (BzMA) in ethanol (EtOH), polymeric nanoparticles with different morphologies are formed in situ according to a polymerization-induced self-assembly (PISA) mechanism. Self-assembly into nanoparticles presenting POEGMA chains at the corona and poly(benzyl methacrylate) (PBzMA) chains in the core occurs in situ due to the growing insolubility of the PBzMA block in ethanol. Interestingly, the formation of highly pure worm-like micelles can be readily monitored by observing the onset of a highly viscous gel in situ due to nanoparticle entanglements occurring during the polymerization. This process thereby allows for a more reproducible synthesis of worm-like micelles simply by monitoring the solution viscosity during the course of the polymerization. In addition, the light stimulus can be intermittently applied in an ON/OFF manner demonstrating temporal control over the nanoparticle morphology.

This article describes a process for producing polymeric self-assembled nanoparticles using visible light mediated dispersion polymerization. Using low energy visible light to control the polymerization allows for the reproducible formation of self-assembled worm-like micelles at high solids content.

Presented herein is a protocol for the facile synthesis of worm-like micelles by visible light mediated dispersion polymerization. This approach begins ...

Spin Saturation Transfer Difference NMR (SSTD NMR): A New Tool to Obtain Kinetic Parameters of Chemical Exchange Processes

This detailed protocol describes the new Spin Saturation Transfer Difference Nuclear Magnetic Resonance protocol (SSTD NMR), recently developed in our group to study processes of mutual-site chemical exchange that are difficult to analyze by traditional methods. As the name suggests, this method combines the Spin Saturation Transfer method used for small molecules, with the Saturation Transfer Difference (STD) NMR method employed for the study of protein-ligand interactions, by measuring transient spin saturation transfer along increasing saturation times (build-up curves) in small organic and organometallic molecules undergoing chemical exchange.
Advantages of this method over existing ones are: there is no need to reach coalescence of the exchanging signals; the method can be applied as long as one signal of the exchanging sites is isolated; there is no need to measure T1 or reach steady state saturation; rate constant values are measured directly, and T1 values are obtained in the same experiment, using only one set of experiments.
To test the method, we have studied the dynamics of the hindered rotation of N,N-dimethylamides, for which much data is available for comparison. The thermodynamic parameters obtained using SSTD are very similar to the reported ones (spin-saturation transfer techniques and line-shape analysis). The method can be applied to more challenging substrates that cannot be studied by previous methods.
We envisage that the simple experimental set up and the wide applicability of the method to a great variety of substrates will make this a common technique amongst organic and organometallic chemists without extensive expertise in NMR.

Spin Saturation Transfer Difference NMR SSTD NMR: A New Tool to Obtain Kinetic Parameters of Chemical Exchange Processes

A detailed protocol describing the SSTD NMR method is presented here to help new users apply this new method to obtain the kinetic parameters of their own systems undergoing chemical exchange.

This detailed protocol describes the new Spin Saturation Transfer Difference Nuclear Magnetic Resonance protocol (SSTD NMR), recently developed in our ...

A Simple and Efficient Protocol for the Catalytic Insertion Polymerization of Functional Norbornenes

Norbornene can be polymerized by a variety of mechanisms, including insertion polymerization whereby the double bond is polymerized and the bicyclic nature of the monomer is conserved. The resulting polymer, polynorbornene, has a very high glass transition temperature, Tg, and interesting optical and electrical properties. However, the polymerization of functional norbornenes by this mechanism is complicated by the fact that the endo substituted norbornene monomer has, in general, a very low reactivity. Furthermore, the separation of the endo substituted monomer from the exo monomer is a tedious task. Here, we present a simple protocol for the polymerization of substituted norbornenes (endo:exo ca. 80:20) bearing either a carboxylic acid or a pendant double bond. The process does not require that both isomers be separated, and proceeds with low catalyst loadings (0.01 to 0.02 mol%). The polymer bearing pendant double bonds can be further transformed in high yield, to afford a polymer bearing pendant epoxy groups. These simple procedures can be applied to prepare polynorbornenes with a variety of functional groups, such as esters, alcohols, imides, double bonds, carboxylic acids, bromo-alkyls, aldehydes and anhydrides.

We describe the catalytic insertion polymerization of 5-norbornene-2-carboxylic acid and 5-vinyl-2-norbornene to form functional polymers with a very high glass transition temperature.

Norbornene can be polymerized by a variety of mechanisms, including insertion polymerization whereby the double bond is polymerized and the bicyclic ...