This work was supported by start-up funding from Virginia Polytechnic Institute and State University. Q.F. acknowledges support from National Natural Science Foundation of China (21504047), Natural Science Foundation of Jiangsu Province (BK20150834), Nanjing University of Posts and Telecommunications Scientific Foundation NUPTSF (NY214179).

Caution: Please consult all relevant materials safety data sheets (MSDS) before use. Many chemicals used in the synthesis are acutely toxic and carcinogenic. Please use all appropriate safety practices when performing reaction including the use of engineering controls (fume hood and glovebox) and personal protective equipment (safety glasses, gloves, lab coat, full-length pants, closed-toe shoes, blue-light blocking safety goggles). Following procedures involve standard air-free handling techniques in a glove box. All so...

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Rev. 47, 113-131 (2001).</li><li>Nagahata, R., Sano, D., Suzuki, H., Takeuchi, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microwave-assisted+single-step+synthesis+of+poly+(lactic+acid)+by+direct+polycondensation+of+lactic+acid.">Microwave-assisted single-step synthesis of poly (lactic acid) by direct polycondensation of lactic acid.</a> Macromol. Rapid Commun. 28, 437-442 (2007).</li><li>Albertsson, A. -. C., Varma, I. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Recent+developments+in+ring+opening+polymerization+of+lactones+for+biomedical+applications.">Recent developments in ring opening polymerization of lactones for biomedical applications.</a> Biomacromolecules. 4, 1466-1486 (2003).</li><li>Vert, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Aliphatic+polyesters:+great+degradable+polymers+that+cannot+do+everything.">Aliphatic polyesters: great degradable polymers that cannot do everything.</a> Biomacromolecules. 6, 538-546 (2005).</li><li>Thillayedu Boullay, O., Marchal, E., Martin-Vaca, B., Coss&#237;o, F. 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Res. 48, 1777-1787 (2015).</li><li>Xia, H., 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=N-heterocyclic+carbenes+as+organocatalysts+in+controlled/living+ring-opening+polymerization+of+O-carboxyanhydrides+derived+from+l-lactic+acid+and+l-mandelic+acid.">N-heterocyclic carbenes as organocatalysts in controlled/living ring-opening polymerization of O-carboxyanhydrides derived from l-lactic acid and l-mandelic acid.</a> J. Polym. Sci., Part A: Polym. 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Sci. 29, 197-202 (2010).</li><li>He, Z., Jiang, L., Chuan, Y., Li, H., Yuan, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ring-opening+polymerization+of+L-lactic+acid+O-carboxyanhydrides+initiated+by+alkoxy+rare+earth+compounds.">Ring-opening polymerization of L-lactic acid O-carboxyanhydrides initiated by alkoxy rare earth compounds.</a> Molecules. 18, 12768-12776 (2013).</li><li>Jia, F., 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=One-pot+atom-efficient+synthesis+of+bio-renewable+polyesters+and+cyclic+carbonates+through+tandem+catalysis.">One-pot atom-efficient synthesis of bio-renewable polyesters and cyclic carbonates through tandem catalysis.</a> Chem. 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The authors declare no competing financial interests. A provisional patent (U.S. Patent Application No: 62/414,016) has been filed pertaining to the results presented in this paper.

The critical step within the protocol is maintaining the reaction temperature at -15 &#177; 5 &#176;C. All catalysts solutions and OCA monomers have to be stored in a glove box freezer at -35 &#176;C before the polymerization. The reaction vials have to be pre-cooled in the cold trap. During the reaction, because the LED light dissipates heat, it is necessary to monitor the reaction every 15 - 20 minutes. Once the temperature is raised up to -10 &#176;C, liquid nitrogen should be added into the dewar to cool the trap. Th...

Poly(&#945;-hydroxy acid) (PAHA) is an important class of biodegradable and biocompatible polymers with applications ranging from biomedical devices to packaging materials.1,2 Although PAHAs can be prepared directly by polycondensation of &#945;-hydroxy acids, the molecular weights (MWs) of the resulting PAHAs are generally low.3 Ring-opening polymerization (ROP) of lactones (e.g., lactide and glycolide) is an alternative synthetic approach that provides the better control on MWs and molecular weight distribution (&#272;) than polycondensation. However, the lack of side-chain functionality in PAHAs and in lactones limit the diversity of physical and chemical properties and their applications.4,5 Since 2006, 1,3-dioxolane-2,4-diones, so-called O-carboxyanhydrides (OCAs), which can be prepared with a rich variety of side-chain functionalities,6,7,8,9,10,11,12,13 have emerged as an alternative class of highly active monomers for polyester polymerization.14,15
Catalytic systems for the ROP of OCAs can be categorized into organocatalysts,8,12,16,17 organometallic catalysts12,18,19,20,21 and biocatalysts.22 Generally, the ROP of OCAs promoted by organocatalyst proceeds in a more or less uncontrolled manner, such as epimerization (i.e., lack of stereoregularity) for OCAs bearing electron-withdrawing groups,8,17 unpredictable MWs, or slow polymerization kinetics.13 To address these problems, an active Zn-alkoxide complex was developed for the ROP of OCAs.12 Well-controlled ROPs were achieved at a low degree of polymerization (DP) without epimerization. However, this Zn-alkoxide catalyst cannot efficiently produce polymers with a high degree of polymerization (DP &#8805; 300).13
We have recently reported a promising approach that greatly improves customizability and efficiency of PAHA synthesis (Figure 1).13 We merge photoredox Ni/Ir catalysts that promote OCA decarboxylation with zinc alkoxide to mediate ring-opening polymerization of OCAs. The use of low temperature (-15 &#176;C) and photoredox Ni/Ir catalysis synergistically accelerates ring-opening and decarboxylation of OCA for chain propagation while avoiding undesired side reactions, e.g., the formation of Ni-carbonyl.23,24 Upon transmetalation with Ni complex the active Zn-alkoxide is located at chain terminus for chain propagation.13
In this protocol, we add fresh prepared (bpy)Ni(COD) (bpy = 2,2&#39;-bipyridyl, COD = 1,5-cyclooctadiene), Zn(HMDS)2 (HMDS = hexamethyldisilazane),25 benzyl alcohol (BnOH) and Ir[dF(CF3)ppy]2(dtbbpy)PF6 (Ir-1, dF(CF3)ppy = 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine, dtbbpy = 4,4&#39;-di-tert-butyl-2,2&#39;-bipyridine) into the monomer l-1 solution26 in a glove box with a cold trap, in the presence of a blue LED light (400-500 nm) and a fan to maintain temperature (Figure 1). The temperature is kept at -15 &#176;C &#177; 5 &#176;C during the polymerization. The conversion of OCA is monitored by Fourier-transform infrared spectroscopy. The resulting polymer MWs and &#272;s is characterized by a gel permeation chromatography (GPC). The homodecoupling 1H NMR study determines whether the obtained polymer is isotactic or not. As most chemicals are highly sensitive to moisture, the detailed video protocol is intended to help new practitioners avoid pitfalls associated with the photoredox ROP of OCAs.

<table>
	<tbody>
		<tr>
			<td>Ni(COD)2</td>
			<td>Strem</td>
			<td>28-0010</td>
			<td>Stored in the glove box freezer.</td>
		</tr>
		<tr>
			<td>2,2&#8242;-bipyridine</td>
			<td>Strem</td>
			<td>07-0290</td>
			<td>Stored in the glove box freezer.</td>
		</tr>
		<tr>
			<td>Zn(HMDS)2</td>
			<td>N/A</td>
			<td>N/A</td>
			<td>Synthesized following reported procedures.25 Stored in the glove box freezer.</td>
		</tr>
		<tr>
			<td>Benzyl alcohol</td>
			<td>Sigma-Aldrich</td>
			<td>402834</td>
			<td>Stored with 4&#197; molecular sieve</td>
		</tr>
		<tr>
			<td>Ir[dF(CF3)ppy]2(dtbbpy)PF6</td>
			<td>Strem</td>
			<td>77-0425</td>
			<td>Stored in the glove box freezer.</td>
		</tr>
		<tr>
			<td>THF</td>
			<td>Sigma-Aldrich</td>
			<td>34865</td>
			<td>Dried by alumina columns and stored with 4&#197; molecular sieve in the dark bottle in the glove box.</td>
		</tr>
		<tr>
			<td>Ethanol</td>
			<td>Sigma-Aldrich</td>
			<td>793175</td>
			<td></td>
		</tr>
		<tr>
			<td>GPC with an isocratic pump</td>
			<td>Agilent</td>
			<td>Agilent 1260 series</td>
			<td></td>
		</tr>
		<tr>
			<td>Dawn Heleos II Light Scatterer</td>
			<td>Wyatt</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Optilab rEX differential refractive index detector</td>
			<td>Wyatt</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Size exclusion columns</td>
			<td>Phenomenex</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Glass Scintillation Vials - 7 ml</td>
			<td>VWR</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>FTIR spectrometer</td>
			<td>Agilent</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Stir bars</td>
			<td>VWR</td>
			<td>58948-091</td>
			<td></td>
		</tr>
		<tr>
			<td>Balance</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Glove box</td>
			<td>Mbraun</td>
			<td>Labstar Pro</td>
			<td></td>
		</tr>
	</tbody>
</table>

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.

The conversion of OCA is monitored by Fourier-transform infrared spectroscopy, as shown in Figure 2. The peak at 1805 cm-1 is assigned as the anhydride bond stretch in OCA; the peak at 1760 cm-1 corresponds to the formation of the ester bond in the polymer. Once the monomer&#39;s peak at 1805 cm-1 completely disappears, the polymerization is finished.
The MW and ...

Watch this Scientific Journal Video about Controlled Photoredox Ring-Opening Polymerization of O-Carboxyanhydrides Mediated by Ni/Zn Complexes at JoVE.com

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.

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, ...

The overall goal of this experiment is to synthesize high molecular weight poly(alpha-hydroxy acids)with side chain functional groups in a controlled manner. This method can help answer key questions in the polyester synthesis field about controlled synthesis of polyesters from O-carboxyanhydrides. The main advantage of this technique is that the polymerization is highly efficient and does not affect the serial chemistry of the monomer.Demonstrating the procedure will be Quanyou Feng, a postdoc from my laboratory. To begin the procedure, prepare solutions of bipy-nickel(COD)zinc bis(hmds)benzyl alcohol, and the iridium catalyst, an anhydrous THF. Store the solutions minus 35 degrees Celsius.Then, place 72.2 milligrams of recrystallized L1 in a seven milliliter scintillation vial. Dissolve the L1 in 722 microliters of anhydrous THF. Transfer 200 microliters of the L1 solution to another 7 milliliter scintillation vial with the stir bar.Add 100 microliters of anhydrous THF to this vial, and cap the vial. Place the dilute L1 solution in the cold well, along with a thermometer. Turn off the glove box light before proceeding.Add about 500 milliliters of ethanol to the dewar for the cold well. Then, add liquid nitrogen to the dewar. Place the dewar on a stir plate and jack, and raise the dewar to cool the well.Wait for the cold well to reach minus 50 degrees Celsius. Then, obtain a pair of safety goggles that block blue light, and place them within easy reach. Open the vial of chilled L1 solution.Remove the catalyst solutions from the freezer. Over the course of 30 seconds, add to the vial 16.4 microliters of bipy-nickel(COD)24.4 microliters of zinc bis(hmds)22.5 microliters of benzyl alcohol, and 24.2 microliters of the iridium catalyst solution. Cap the vial when finished.Put on the blue light-blocking safety goggles. Then, turn on a 34 watt blue LED light and a small cooling fan. Direct the light towards the vial.Start the stir motor. Cover the light, cooling fan, and cold well with aluminum foil. Monitor the reaction temperature, and add liquid nitrogen every 15 to 20 minutes, to keep the reaction at about minus 15 degrees Celsius.The critical part of this procedure, is to keep the reaction temperature around minus 15 degrees. If the temperature is higher than minus 10 degrees, liquid nitrogen should be added to the dewar. Monitor the monomer conversion with FDIR spectroscopy.Once the desired degree of polymerization has been achieved, dry and wash the polymer with one milliliter of diethyl ether, containing one percent acetic acid, and one milliliter of methanol. The polymerization of phenyl O-carboxyanhydride was monitored by FDIR. The peak, at about 1805 reciprocal centimeters, corresponded to the anhydride bond stretch of L1.As the reaction progressed, the developing peak at 1760 reciprocal centimeters, indicated the formation of the ester bonds in the polymer.The polymerization was finished when the 1805 peak had completely disappeared. Gel permeation chromatography was used to determine the molecular weights and dispersity of a series of L1 polymers with increasing monomer feed ratios. The number average molecular weight increased linearly with initial monomer catalyst ratios.The dispersity values were all below 1.1. Proton NMR spectroscopy was used to evaluate the polymer stereo chemistry. A homodecoupling study showed a single peak at the alpha methine region, indicating that the polymer was isotactic, and therefore no epimerization had occurred.Visual demonstration of this method is critical, as photoredox polymerization at low temperature is difficult to learn without it. Proper reaction setup and temperature control are both critical for controlled polymerization. When it's mastered, this technique can be done within four to six hours if it's performed properly.Remember to wear safety goggles while attempting this procedure. After watching this video, you should have a good understanding of how to perform photoredox ring-opening polymerization.

controlled-photoredox-ring-opening-polymerization-o-carboxyanhydrides

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JoVE Journal

Chemistry

8.0K 조회수.  Nanjing University of Posts and Telecommunications. The overall goal of this experiment is to synthesize high molecular weight poly(alpha-hydroxy acids)with side chain functional groups in a controlled manner. This method can help answer key questions in the polyester synthesis field about controlled synthesis of polyesters from O-carboxyanhydrides. The main advantage of this technique is that the polymerization is highly efficient and does not affect the serial chemistry of the monomer.Demonstrating the procedure will be Quanyou Feng, ...