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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<li>Britovsek, G. J. P., Cohen, S. A., Gibson, V. C., Maddox, P. J., van Meurs, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Iron-Catalyzed+Polyethylene+Chain+Growth+on+Zinc%3A+Linear+%CE%B1-Olefins+with+a+Poisson+Distribution%5BTitle%5D)+AND+%22Angew.+Chem.%2C+Int.+Ed%22%5BJournal%5D)">Iron-Catalyzed Polyethylene Chain Growth on Zinc: Linear α-Olefins with a Poisson Distribution.</a> Angew. Chem., Int. Ed. 41 (3), 489-491 (2002).</li>
<li>Xiao, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((A+Novel+Linear-Hyperbranched+Multiblock+Polyethylene+Produced+from+Ethylene+Monomer+Alone+via+Chain+Walking+and+Chain+Shuttling+Polymerization%5BTitle%5D)+AND+%22Macromolecules%22%5BJournal%5D)">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/pubmed?term=((Effect+of+the+alkylaluminum+cocatalyst+on+ethylene+polymerization+by+a+nickel-diimine+complex%5BTitle%5D)+AND+%22J.+Polym.+Sci.+A+Polym+Chem%22%5BJournal%5D)">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/pubmed?term=((Analysis+of+Polymeryl+Chain+Transfer+Between+Group+10+Metals+and+Main+Group+Alkyls+during+Ethylene+Polymerization%5BTitle%5D)+AND+%22ACS+Catalysis%22%5BJournal%5D)">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/pubmed?term=((New+Pd%28II%29-+and+Ni%28II%29-Based+Catalysts+for+Polymerization+of+Ethylene+and+.alpha.-Olefins%5BTitle%5D)+AND+%22J.+Am.+Chem.+Soc%22%5BJournal%5D)">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/pubmed?term=((Late-Metal+Catalysts+for+Ethylene+Homo-+and+Copolymerization%5BTitle%5D)+AND+%22Chem.+Rev%22%5BJournal%5D)">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/pubmed?term=((Synthesis+and+Diels-Alder+Cycloadditions+of+exo-Imidazolidin-2-one+Dienes%5BTitle%5D)+AND+%22J.+Org.+Chem%22%5BJournal%5D)">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/pubmed?term=((NMR+study+on+the+coordination+behavior+of+dissymmetric+terdentate+trinitrogen+ligands+on+methylpalladium%28II%29+compounds%5BTitle%5D)+AND+%22Inorg.+Chem%22%5BJournal%5D)">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/pubmed?term=((Anhydrous+Nickel+%28II%29+Halides+and+their+Tetrakis+%28Ethanol%29+and+1%2C2-Dimethoxyethane+Complexes%5BTitle%5D)+AND+%22Inorg.+Syntheses%22%5BJournal%5D)">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/pubmed?term=((13C+NMR+Determination+of+the+Composition+of+Linear+Low-Density+Polyethylene+Obtained+with+%5B%CE%B73-Methallyl-nickel-diimine%5DPF6+Complex%5BTitle%5D)+AND+%22Macromolecules%22%5BJournal%5D)">13C NMR Determination of the Composition of Linear Low-Density Polyethylene Obtained with [η3-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/pubmed?term=((Novel+Branching+Topology+in+Polyethylenes+As+Revealed+by+Light+Scattering+and+13C+NMR%5BTitle%5D)+AND+%22Macromolecules%22%5BJournal%5D)">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/pubmed?term=((Monofunctional+hyperbranched+ethylene+oligomers%5BTitle%5D)+AND+%22J.+Am.+Chem.+Soc%22%5BJournal%5D)">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/pubmed?term=((Chain+Transfer+in+the+Polymerization+of+Styrene%3A+The+Reaction+of+Solvents+with+Free+Radicals1%5BTitle%5D)+AND+%22J.+Am.+Chem.+Soc%22%5BJournal%5D)">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><tbody><tr><td>Endeavor Pressure Reactor</td><td>Biotage</td><td>EDV-1N-L</td><td></td></tr><tr><td>Blade Impellers</td><td>Biotage</td><td>900543</td><td></td></tr><tr><td>Glass Liners</td><td>Biotage</td><td>900676</td><td></td></tr><tr><td>2,3-butanedione, 99%</td><td>Alfa Aesar</td><td>A14217</td><td></td></tr><tr><td>2,6-dimethylaniline, 99%</td><td>Sigma Aldrich</td><td>D146005</td><td></td></tr><tr><td>formic acid, 95%</td><td>Sigma Aldrich</td><td>F0507</td><td></td></tr><tr><td>methanol, 99.8%</td><td>Sigma Aldrich</td><td>179337</td><td>ACS Reagent</td></tr><tr><td>nickel (II) bromide, 99%</td><td>Strem</td><td>28-1140</td><td>anhydrous, hygroscopic</td></tr><tr><td>triethylorthoformate, 98%</td><td>Sigma Aldrich</td><td>304050</td><td>dried with K&lt;sub&gt;2&lt;/sub&gt;CO&lt;sub&gt;3&lt;/sub&gt; and distilled</td></tr><tr><td>1,2-dimethoxyethane, 99.5%</td><td>Sigma Aldrich</td><td>259527</td><td>dried with Na/Benzophenone and distilled</td></tr><tr><td>pentane, 99%</td><td>Fisher</td><td>P399</td><td>HPLC Grade *</td></tr><tr><td>dichloromethane, 99.5%</td><td>Fisher</td><td>D37</td><td>ACS Reagent *</td></tr><tr><td>toluene, 99.8%</td><td>Fisher</td><td>T290</td><td>HPLC Grade *</td></tr><tr><td>methylaluminoxane</td><td>Albemarle</td><td>MAO</td><td>pyrophoric, 30% in toluene</td></tr><tr><td>diethylzinc, 95%</td><td>Strem</td><td>93-3030</td><td>pyrophoric</td></tr><tr><td>1,2,4-trichlorobenzene, 99%</td><td>Sigma Aldrich</td><td>296104</td><td></td></tr><tr><td>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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13.2K Views.  University of Minnesota - Twin Cities.  Een protocol voor de high-throughput analyse van polymerisatiekatalysator, ketenoverdracht polymerisaties, polyethyleen karakterisering en reactie kinetische analyse wordt gepresenteerd. 

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

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 ...

Combinatorial Synthesis of and High-throughput Protein Release from Polymer Film and Nanoparticle Libraries

Polyanhydrides are a class of biomaterials with excellent biocompatibility and drug delivery capabilities. While they have been studied extensively with conventional one-sample-at-a-time synthesis techniques, a more recent high-throughput approach has been developed enabling the synthesis and testing of large libraries of polyanhydrides1. This will facilitate more efficient optimization and design process of these biomaterials for drug and vaccine delivery applications. The method in this work describes the combinatorial synthesis of biodegradable polyanhydride film and nanoparticle libraries and the high-throughput detection of protein release from these libraries. In this robotically operated method (Figure 1), linear actuators and syringe pumps are controlled by LabVIEW, which enables a hands-free automated protocol, eliminating user error. Furthermore, this method enables the rapid fabrication of micro-scale polymer libraries, reducing the batch size while resulting in the creation of multivariant polymer systems. This combinatorial approach to polymer synthesis facilitates the synthesis of up to 15 different polymers in an equivalent amount of time it would take to synthesize one polymer conventionally. In addition, the combinatorial polymer library can be fabricated into blank or protein-loaded geometries including films or nanoparticles upon dissolution of the polymer library in a solvent and precipitation into a non-solvent (for nanoparticles) or by vacuum drying (for films). Upon loading a fluorochrome-conjugated protein into the polymer libraries, protein release kinetics can be assessed at high-throughput using a fluorescence-based detection method (Figures 2 and 3) as described previously1. This combinatorial platform has been validated with conventional methods2 and the polyanhydride film and nanoparticle libraries have been characterized with 1H NMR and FTIR. The libraries have been screened for protein release kinetics, stability and antigenicity; in vitro cellular toxicity, cytokine production, surface marker expression, adhesion, proliferation and differentiation; and in vivo biodistribution and mucoadhesion1-11. The combinatorial method developed herein enables high-throughput polymer synthesis and fabrication of protein-loaded nanoparticle and film libraries, which can, in turn, be screened in vitro and in vivo for optimization of biomaterial performance.

This method describes the combinatorial synthesis of biodegradable polyanhydride film and nanoparticle libraries and the high-throughput detection of protein release from these libraries.

Polyanhydrides are a class of biomaterials with excellent biocompatibility and drug delivery capabilities. While they have been studied extensively with ...

Bioengineering

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 ...

Anionic Polymerization of an Amphiphilic Copolymer for Preparation of Block Copolymer Micelles Stabilized by &#960;-&#960; Stacking Interactions

In this study, an amphiphilic copolymer that includes a core-forming block with phenyl groups was synthesized by living anionic polymerization of phenyl glycidyl ether (PheGE) on methoxy-polyethylene glycol (mPEG-b-PPheGE). Characterization of the copolymer revealed a narrow molecular distribution (PDI &#60; 1.03) and confirmed the degree of polymerization of mPEG122-b-(PheGE)15. The critical micelle concentration of the copolymer was evaluated using an established fluorescence method with the aggregation behavior evaluated by dynamic light scattering and transmission electronic microscopy. The potential of the copolymer for use in drug delivery applications was evaluated in a preliminary manner including in vitro biocompatibility, loading and release of the hydrophobic anti-cancer drug doxorubicin (DOX). A stable micelle formulation of DOX was prepared with drug loading levels up to 14% (wt%), drug loading efficiencies &#62; 60% (w/w) and sustained release of drug over 4 days under physiologically relevant conditions (acidic and neutral pH, presence of albumin). The high drug loading level and sustained release is attributed to stabilizing &#960;-&#960; interactions between DOX and the core-forming block of the micelles.

The key steps of living anionic polymerization of phenyl glycidyl ether (PheGE) on methoxy-polyethylene glycol (mPEG-b-PPheGE) are described. The resulting block copolymer micelles (BCMs) were loaded with doxorubicin 14% (wt%) and sustained release of drug over 4 days under physiologically relevant conditions was obtained.

In this study, an amphiphilic copolymer that includes a core-forming block with phenyl groups was synthesized by living anionic polymerization of phenyl ...

Controlled Photoredox Ring-Opening Polymerization of O-Carboxyanhydrides Mediated by Ni/Zn Complexes

Here, we describe an effective protocol that combines photoredox Ni/Ir catalysis with the use of a Zn-alkoxide for efficient ring-opening polymerization, allowing for the synthesis of isotactic poly(&#945;-hydroxy acids) with expected molecular weights (&#62;140 kDa) and narrow molecular weight distributions (Mw/Mn &#60; 1.1). This ring-opening polymerization is mediated by Ni and Zn complexes in the presence of an alcohol initiator and a photoredox Ir catalyst, irradiated by a blue LED (400 - 500 nm). The polymerization is performed at a low temperature (-15 &#176;C) to avoid undesired side reactions. The complete monomer consumption can be achieved within 4 - 8 hours, providing a polymer close to the expected molecular weight with narrow molecular weight distribution. The resulted number-average molecular weight shows a linear correlation with the degree of polymerization up to 1000. The homodecoupling 1H NMR study confirms that the obtained polymer is isotactic without epimerization. This polymerization reported herein offers a strategy for achieving rapid, controlled O-carboxyanhydrides polymerization to prepare stereoregular poly(&#945;-hydroxy acids) and its copolymers bearing various functional side-chain groups.

Controlled Photoredox Ring-Opening Polymerization of O-Carboxyanhydrides Mediated by Ni/Zn Complexes

A protocol for the controlled photoredox ring-opening polymerization of O-carboxyanhydrides mediated by Ni/Zn complexes is presented.

Here, we describe an effective protocol that combines photoredox Ni/Ir catalysis with the use of a Zn-alkoxide for efficient ring-opening polymerization, ...

Kinetics of Addition Polymerization to Polydimethylsiloxane

Source: <a href="https://www.jove.com/author/Kerry+M._Dooley">Kerry M. Dooley</a> and Michael G. Benton, Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA
Polymers are molecules consisting of many repeating monomer units that are chemically bonded into long chains. They exhibit a broad range of physical properties, which are affected by their chemical structure, molecular weight and degree of polymerization. The polymer industry manufactures thousands of raw materials used in a broad variety of commercial products.1,2
The goal of this video is to perform an addition polymerization reaction and then evaluate the resulting product to understand how viscosity can be used to determine polymer molecular weight. Additionally, this experiment will investigate how molecular weight can be related to monomer conversion.

Synthesis of Polydimethylsiloxane via Addition Polymerization

Source: Kerry M. Dooley and Michael G. Benton, Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA
Polymers are molecules ...

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Free-Radical Chain Reaction and Polymerization of Alkenes

The conversion of alkenes to macromolecules called polymers is a reaction of high commercial importance. The structure of the polymer is defined by a repeating unit, while the terminal groups are considered insignificant. The average degree of polymerization represents the number of repeating units in the polymer molecule and is denoted by the subscript n.
<img alt="Polymer chain structure diagram; hydrocarbon backbone with side groups, chemistry, material science." src="/files/ftp_upload/11773/11773_PT_Figure_1.svg" style="text-align: center; height: 60px;" data-altupdatedat="1747908711000" data-alt="Figure1">
Alkenes undergo polymerization via a free-radical mechanism involving three steps: initiation, propagation, and termination. 
Radicals are generated in the initiation step by heating a small quantity of free-radical initiators such as benzoyl peroxide. The benzoyloxy radical loses CO2 and forms a phenyl radical (Ph·), which adds to the double bond of the alkene to initiate the polymerization process. One of the electrons of the alkene π bond pairs with one electron from the phenyl radical to form a new C–C bond.
<img alt="Radical reaction mechanism diagram, benzoyl peroxide decomposition into radicals, chemical process." src="/files/ftp_upload/11773/11773_PT_Figure_2.svg" style="text-align: center; height: 60px;" data-altupdatedat="1747908711000" data-alt="Figure2">
<img alt="Chemical reaction diagram of phenyl cation formation via decarboxylation process with equations." src="/files/ftp_upload/11773/11773_PT_Figure_3.svg" style="text-align: center; height: 60px;" data-altupdatedat="1747908711000" data-alt="Figure3">
<img alt="Chemical reaction diagram showing radical polymerization steps with benzene and propylene." src="/files/ftp_upload/11773/11773_PT_Figure_4.svg" style="text-align: center; height: 80px;" data-altupdatedat="1747908711000" data-alt="Figure4">
In the propagation step, the carbon radical generated in the initiation step adds to another molecule of alkene to generate a new radical. The continuous addition of alkene monomers at the radical site yields the polymer.
<img alt="Allylic polymerization process, chemical reaction equation, polymer chain formation." src="/files/ftp_upload/11773/11773_PT_Figure_5.svg" style="text-align: center; height: 80px;" data-altupdatedat="1747908711000" data-alt="Figure5">
The termination step of the free-radical mechanism occurs via recombination or disproportionation. In a recombination step, two growing chains form a bond at their radical sites.
<img alt="Polymerization process diagram with structural formulas and reaction arrows." src="/files/ftp_upload/11773/11773_PT_Figure_6.svg" style="text-align: center; height: 60px;" data-altupdatedat="1747908711000" data-alt="Figure6">
In termination by disproportionation, a β-hydrogen atom is transferred from one radical to another radical center, resulting in two non-radical products.
<img alt="Free radical reaction diagram; benzyl radical formation; organic chemistry mechanism." src="/files/ftp_upload/11773/11773_PT_Figure_7.svg" style="text-align: center; height: 300px;" data-altupdatedat="1747908711000" data-alt="Figure7">
The free-radical polymerization of ethylene yields low-density polyethylene (LDPE), where the low density is a consequence of significant branching in the polymer chains. Articles made from polymers surround us in various forms, such as food packaging materials, plastic bags, bottles, stationery, and automotive parts.

Free-radical Polymerization Reaction of Alkenes

The conversion of alkenes to macromolecules called polymers is a reaction of high commercial importance. The structure of the polymer is defined by a ...

Organic Chemistry

Unit 1

Reactions of Alkenes

KEY TERMS AND CONCEPTS

Radical Chain-Growth Polymerization: Overview

Chain-growth or addition polymerization is successive addition reactions of monomers with a polymer chain. In radical chain-growth polymerization, the reaction proceeds via a free-radical intermediate. The free radical is formed from radical initiators, which spontaneously generate free radicals by homolytic fission. Organic peroxides (such as dibenzoyl peroxide, as shown in Figure 1) or azo compounds are popular radical initiators. A low concentration ratio of radical initiator to monomer is used to minimize radical coupling.
<img alt="Figure1" src="/files/ftp_upload/12920/12920_PT_Figure_1.svg" style="text-align: center; height: 75px;" />
Figure 1: The homolytic fission reaction of radical initiator dibenzoyl peroxide to generate benzoyloxy radicals and the subsequent decomposition to phenyl radicals
This free radical initiates polymerization by reacting with a monomer molecule. Unsaturated monomers are suitable for radical chain-growth polymerization; a substituent group across the unsaturated bond that stabilizes the free radical increases the reaction feasibility. Ethylene, propylene, vinyl chloride, and styrene are examples of monomers that can be polymerized via free-radical chain-growth polymerization.
The reaction between a monomer and a free radical generates a new free radical. This new radical reacts with another monomer and creates yet another free radical. The polymer chain grows as the reaction between free-radical intermediates and monomers repeats sequentially. Typically, the propagation step repeats one thousand to ten thousand times before the termination of polymerization.
Often, chain transfer reagents, such as thiol, are used to control the molecular weight (Figure 2). A chain transfer agent must be sufficiently reactive to transfer a hydrogen atom to the growing chain to terminate the polymer growth. The resultant radical must add to a double bond in the monomer to initiate polymerization.
<img alt="Figure2" src="/files/ftp_upload/12920/12920_PT_Figure_2_.svg" style="text-align: center; height: 150px;" />
Figure 2: (Top): The termination reaction of chain transfer agent thiol and a growing polymer chain; (Bottom): Subsequent reaction of thiol radical and monomer molecule to initiate polymer growth
Inhibitors are the reagents used to reduce the reactivity of radicals in the growing polymer chain. Figure 3 depicts the reaction of benzoquinone as an inhibitor.
<img alt="Figure3" src="/files/ftp_upload/12920/12920_PT_Figure_3.svg" style="text-align: center; height: 200px;" />
Figure 3: The reaction between chain inhibitor benzoquinone and a growing polymer chain to generate a less reactive free radical

Chain-growth or addition polymerization is successive addition reactions of monomers with a polymer chain. In radical chain-growth polymerization, the ...

Synthetic Polymers

Radical Chain-Growth Polymerization: Mechanism

The radical chain-growth polymerization mechanism consists of three steps: initiation, propagation, and termination of polymerization. The polymerization initiates when a free radical generated from the radical initiator adds to the unsaturated bond in the monomer. The unpaired electron of the free radical and one &#960; electron in the unsaturated bond creates a &#963; bond between the free radical and the monomer. As a result, the other &#960; electron in the unsaturated bond converts this species into the new free-radical. The mechanism of the initiation reaction between a phenyl free radical and a vinyl chloride monomer is shown in Figure 1.
<img alt="Figure1" src="/files/ftp_upload/12921/12921_PT_Figure_1.svg" style="text-align: center; height: 85px;" />
Figure 1: The initiation step of the reaction mechanism of the free-radical polymerization of vinyl chloride using a phenyl free radical.
The propagation step is the addition of this new free radical to the unsaturated bond in another monomer. This propagation step repeats to add more monomers and grow the polymer chain. In each addition, the radical site shifts to the newly added monomer. The number of times the propagation step repeats determines the molecular weight of the polymer chain. The propagation step of vinyl chloride free-radical polymerization is shown in Figure 2.
<img alt="Figure2" src="/files/ftp_upload/12921/12921_PT_Figure_2.svg" style="text-align: center; height: 105px;" />
Figure 2: The propagation step of the reaction mechanism of the free-radical polymerization of vinyl chloride.
The monomers add preferentially such that the unpaired electron is positioned on the more substituted carbon. For example, monosubstituted ethylene monomers such as propylene, vinyl chloride, and styrene add to the growing chain in a head-to-tail manner. Here, the radical site in the growing polymer chain is always on the substituted carbon (tail).
Due to the high reactivity of radical intermediates, the termination of polymer growth is inevitable. Different reaction pathways lead to the termination of polymerization. One is the coupling reaction of a radical site in a growing chain with a radical site in another. Another way of termination is the radical disproportionation reaction, where the radical site of one growing polymer chain abstracts hydrogen from the &#945; carbon of the radical site in the other chain. So, the former chain is terminated with an alkyl group, and the latter is terminated with an alkenyl group. The reaction mechanisms of radical coupling and disproportionation in the termination of the growth of a poly(vinyl chloride) chain are depicted in Figure 3.
<img alt="Figure3" src="/files/ftp_upload/12921/12921_PT_Figure_3.svg" style="text-align: center; height: 225px;" />
Figure 3: The termination of vinyl chloride polymerization via radical coupling (top) and disproportionation reaction (bottom).
Chain transfer reagents and chain inhibitors can also terminate a polymer chain&#39;s growth. However, in this case, the radical site is not eliminated but either removed from the growing chain or made less reactive.

The radical chain-growth polymerization mechanism consists of three steps: initiation, propagation, and termination of polymerization. The polymerization ...

Radical Chain-Growth Polymerization: Chain Branching

The skeletal structure of polymers synthesized via radical polymerization is always branched. For example, the polymerization of ethylene by radical polymerization results in a low-density grade of polyethylene with a heavily branched skeletal structure. Here, the radical site abstracts hydrogen from the growing chain, and the radical site shifts from the end (a primary carbon center) to anywhere within the growing chain (a secondary carbon center). Consequently, the part of the chain from the abstraction site to the end of the chain becomes a branch.
In intermolecular hydrogen abstraction, the radical site abstracts hydrogen from another growing polymer chain and terminates its chain growth. A new branch grows in the other chain where the hydrogen was removed. Figure 1 demonstrates the intermolecular hydrogen abstraction and the consequent chain growth from the middle of the polymer chain.
<img alt="Figure1" src="/files/ftp_upload/12922/12922_PT_Figure_1.svg" style="text-align: center; height: 125px;" />
Figure 1: The intermolecular hydrogen abstraction (top) and the subsequent addition of the newly formed radical site to a monomer (bottom).
The radical site abstracts hydrogen from the same growing chain in intramolecular hydrogen abstraction. The length of the branch depends on the distance between the radical site and the abstraction site. Intramolecular 1,5-hydrogen abstraction frequently occurs due to the relatively stable six-membered transition state, resembling the chair conformation of cyclohexane. The resultant butyl branches are commonly found in branched polymers. Figure 2 shows the intramolecular hydrogen abstraction and the formation of the butyl branch.
<img alt="Figure2" src="/files/ftp_upload/12922/12922_PT_Figure_2v2.svg" style="text-align: center; height: 60px;" />
Figure 2:&#160; The intramolecular hydrogen abstraction (top) and subsequent addition of the newly formed radical site on a monomer (bottom).
In general, intramolecular hydrogen abstraction is more probable than intermolecular hydrogen abstraction. Also, short-chain branching is more common than long-chain branching, as the ends of the chains are more accessible.

The skeletal structure of polymers synthesized via radical polymerization is always branched. For example, the polymerization of ethylene by radical ...

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

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