The authors acknowledge funding from Fonds de Recherche du Qu&#233;bec - Nature et Technologies, from Conseil Recherches en Sciences Naturelles et G&#233;nie (program INNOV) and PrimaQu&#233;bec.

1. Preparation of Poly(5-norbornene-2-carboxylic acid), PNBE(CO2H)
<ol>
	<li>Preparation of the monomer NBE(CO2H)
	<ol>
		<li>Weigh out acrylic acid (AA) (327 g, 4.5 mol, 2 eq.) and hydroquinone (4.9 g, 4.5 x 10-2 mol, 0.02 eq.) and add them to a 2 L round-bottom flask equipped with a condenser and a magnetic stir bar. Heat the flask at 150 &#176;C using a silicone oil bath.</li>
		<li>Once reflux is settled, add DCPD (300 g, 2.3 mol, 1 eq.) in a single portion, and then increase the temperature to 170 &#176;C.</li>
		<li>Leave the reaction at this temperature for 16 h. Observe the color change from clear to yellow-brown.</li>
		<li>Take a sample by extraction with a Pasteur pipet, and analyze it by 1H NMR (using CDCl3 as the solvent)15. Observe the apparition of NBE (double bond signals between 6.0 and 6.5 ppm, Figure 2 top).16</li>
	</ol>
	</li>
	<li>Purification of NBE(CO2H)
	<ol>
		<li>Replace the condenser with a simple distillation setup (one plateau) connected to a condenser in which cold water is circulated.</li>
		<li>Put the reaction setup under a vacuum set to approx. 1 mmHg. Heat the mixture at 100 &#176;C, and collect a clear liquid (ca. 40 mL) that can be discarded.</li>
		<li>Replace the collection flask with a 500 mL round bottom flask. Heat the oil bath to 155 &#176;C, and observe the dropwise distillation of NBE(CO2H) (317 g, 2.3 mol. Yield = 98%). The distillation takes over 7 h.</li>
		<li>Analyze the colorless liquid by 1H NMR to assess purity as well as endo:exo proportions (Figure 2, bottom).15 The endo:exo ratio changes with the time used for distillation as well as with the heating time used for the preparation of the crude NBE(CO2H). Typically, endo:exo ratios between 50:50 and 80:20 are obtained (60:40 in this case).</li>
	</ol>
	</li>
	<li>Polymerization of NBE(CO2H)
	<ol>
		<li>Place 300 g (2.3 mol, 5,000 eq.) of NBE(CO2H) in a 500 mL round bottom flask equipped with a magnetic stir bar. Degas the liquid by bubbling nitrogen for 30 min.</li>
		<li>Weigh allylpalladium(II) chloride dimer, [PdCl(C3H5)]2 (76 mg, 4.2 x 10-1 mmol, 1 eq. of Pd) and add it to the solution. Add silver antimonate AgSbF6 (180 mg, 5.2 x 10-1 mmol, 1.2 eq.).</li>
		<li>Under stirring, dissolve the Pd salt by heating at 70 &#176;C, and maintain the temperature at 70 &#176;C under a slight nitrogen flux. After 7 to 8 h, the stirring stops due to a viscosity increase.</li>
		<li>Stop the reaction after 36 h.</li>
		<li>Cool the round bottom flask with liquid nitrogen. With a spatula, break the polymer into small pieces.</li>
		<li>In a 2 L beaker equipped with a magnetic stir bar, add 750 mL of ethyl acetate. Add the polymer chunks to the ethyl acetate under vigorous stirring. Continue stirring for 2 h.</li>
		<li>Filter the solution over a 15 cm diameter Buchner funnel equipped with a paper filter (grade 413, 15 cm diameter).</li>
		<li>Wash the polymer with ethyl acetate three times (500 mL each washing). Dry the polymer (123 g, 9.4 x 10-1 mol, yield = 41%) in a vacuum oven set at 50 &#176;C for 12 h.</li>
	</ol>
	</li>
</ol>
2. Preparation of PNBE(vinyl)
<ol>
	<li>Polymerization of NBE(vinyl)
	<ol>
		<li>Degas toluene (ca. 200 mL) and NBE(vinyl) (ca. 200 mL) by bubbling with N2 for 30 min and place them in a glove box.</li>
		<li>Within the glove box, load toluene (100 g) in a 250 mL round bottom flask.</li>
		<li>Add Pd2(dba)3 (76 mg, 1.6 x 10-1 mmol, 1 eq. of Pd), AgSbF6 (68 mg, 2.0 x 10-1 mmol, 1.2 eq.) and triphenylphosphine, PPh3 (43 mg, 1.6 x 10-1 mmol, 1 eq.) successively to the toluene solution.</li>
		<li>Heat the mixture to 70 &#176;C until complete dissolution occurs. It occurs within 10 min.</li>
		<li>Add 100 g (8.0 x 10-1 mol, 5,000 eq.) of NBE(vinyl) to this purple solution.</li>
		<li>Stir at 70 &#176;C for 72 h.</li>
		<li>Remove the solution from the glovebox, and transfer the viscous black solution to a 1 L glass bottle containing a magnetic stir bar.</li>
		<li>Add toluene (200 mL) and stir.</li>
		<li>Add silica powder (silica gel 40-63 &#181;m, 10 g). Stir at room temperature for 16 h.</li>
		<li>Stop stirring and let the powder settle for at least 2 h in order for the silica particles to sedimentate.</li>
		<li>Filter the solution over a 15 cm diameter Buchner funnel equipped with a paper filter (grade 413, 15 cm diameter). Avoid pouring sedimented silica particles in the Buchner funnel.</li>
		<li>Rinse the silica particles with toluene (50 mL) and filter it through the Buchner funnel.</li>
		<li>Add methanol (1.2 L) to a 4 L beaker equipped with a magnetic stir bar.</li>
		<li>Add all of the toluene solution containing the polymer to the methanol gradually under vigorous stirring, and continue stirring for 30 min.</li>
		<li>Filter the polymer over a 15 cm diameter Buchner funnel equipped with a paper filter (grade 413, 15 cm diameter). Wash the polymer with three aliquots of methanol (400 mL each). Change the filter paper between each washing.</li>
		<li>Assess the polymer purity by 1H NMR in CDCl3, in order to see if the residual monomer is present (double bond signals between 6.0 and 6.3 ppm).15,16 If this is the case, continue washing with methanol.</li>
		<li>Dry the polymer (75 g, 6.3 x 10-1 mol, yield = 78%) under vacuum at room temperature overnight.</li>
	</ol>
	</li>
</ol>
3. Preparation of PNBE(epoxy)
<ol>
	<li>Epoxidation of PNBE(vinyl)
	<ol>
		<li>Add 150 g of dichloromethane to a 500 mL round bottom flask equipped with a magnetic stirrer and a condenser.</li>
		<li>Add PNBE(vinyl) (15 g, 1.3 x 10-1 mol, 1 eq.) with stirring until complete dissolution.</li>
		<li>Place the flask in an ice-bath and let it cool for 15 min.</li>
		<li>In a separate container, mix together formic acid (30 g, 6.5 x 10-1 mol, 5 eq.) and acetic acid (5 g, 8.3 x 10-2 mol, 0.6 eq.). Add the combined acids to the polymer solution.</li>
		<li>Let it cool for 15 min.</li>
		<li>Add hydrogen peroxide aqueous solution (30%) (75 g, 6.5 x 10-1 mol, 5 eq.) to the polymer solution.</li>
		<li>Stir for 18 h. The ice bath does not need to be removed, as the temperature will gradually increase to ambient temperature.</li>
		<li>Take a small sample, precipitate the polymer with acetone, and analyze it by 1H NMR in CDCl3.15 If the signal for the double bond (&#948; = 4.5-6.0 ppm) is sufficiently decreased (Figure 3), pass to the next step. Typically, the ratio of the integral of the double bonds to the other protons is less than 1:20 (1:83 in Figure 3). Otherwise, continue the reaction.</li>
		<li>Add acetone (750 mL) to a 4 L beaker equipped with a magnetic stir bar.</li>
		<li>Add the polymer solution to the acetone gradually under vigorous stirring for 15 min.</li>
		<li>Filter the polymer over a 15 cm diameter Buchner funnel equipped with a paper filter (grade 413, 15 cm diameter).</li>
		<li>Wash the polymer four times with acetone (200 mL each time).</li>
		<li>Change the filter paper between each washing.</li>
		<li>Dry the polymer (7.5 g) under vacuum at room temperature overnight.</li>
	</ol>
	</li>
</ol>
<img alt="Figure 2" src="/files/ftp_upload/54552/54552fig2.jpg" /> 
Figure 2: 1H NMR spectra of crude (top) and purified (bottom) NBE(CO2H). The purified product is obtained by simple distillation. Note the peaks which are used to assess the endo:exo ratio. <a href="https://cloudfront-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54552/54552fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/54552/54552fig3.jpg" /> 
Figure 3: 1H NMR spectra of PNBE(vinyl) (blue) and PNBE(epoxy) (red). Note the ratio 1:3 between the integrals of the vinyl group (&#948; = 4.5-6.0 ppm) and the other protons in the PNBE(vinyl) spectrum. After reaction with H2O2, the ratio decreases to 1:83, thus confirming that epoxidation of vinyl group has occurred. <a href="https://cloudfront-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54552/54552fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/54552/54552fig4.jpg" /> 
Figure 4: FTIR spectra of PNBE(vinyl) (black) and PNBE(epoxy) acquired in attenuated total reflectance mode. Insert shows a zoom of the characteristic bands of PNBE(vinyl) and PNBE(epoxy). The 902 cm-1 and 992 cm-1 bands correspond to the C=C-H out-of-plane bending, whereas the 875 cm-1 band corresponds to the epoxide ring deformation. <a href="https://cloudfront-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54552/54552fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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The authors have nothing to disclose.

The method proposed here is simple, and readily amenable to scale-up. All chemicals could be used as received without any purification. Note that performing the reaction at a lower scale (e.g. scales &#8804;1 g) usually yields lower yields due to an unavoidable loss of material during the handling and the collection.
The catalysts are formed in situ upon the reaction of commercial Pd compounds with cationizing agents. In our hands, the yield of the reaction as well as the characteristics of the polymer (e.g. molecular weight distribution) are not changed whether one starts with a well-defined catalyst such as (&#627;3-allyl)PdS2+ SbF6- (S = nitromethane) or with the in situ catalytic system. For the polymerization of NBE(vinyl), only an in situ form of the catalyst is currently known. We did not observe any reactivity difference between Pd2(dba)3 and Pd(dba)2, but the use of the chloroform adduct, Pd2(dba)3. CHCl3 is not recommended (lower yields are obtained). The polymerization reaction is also slightly air sensitive. Compared to a reaction entirely performed in the glove-box (O2 &#60;5 ppm), we obtained similar results in terms of yield and molecular weights when the reaction is performed with roughly degassed reagents (degassed by bubbling with N2, the catalyst still being weighed in the glove-box). The monomers do not need to be dried before use, but the presence of intentionally added water is not recommended, as water coordinates the catalyst and poisons the reaction. In fact, the catalyst (&#627;3-allyl)Pd(OH2)2+ SbF6- was found to be inactive.
The polymerization of NBE(CO2H) stops at 45% yield due to the formation of a solid (vitrification). The residual monomer is extracted with ethyl acetate. Ethyl acetate can be separated from monomer by rotary evaporation and the collected monomer can be engaged in a subsequent polymerization. The endo:exo ratio in the collected monomer is slightly different from the endo:exo ratio of the initial monomer. As we demonstrated in a previous work,13 the catalyst isomerizes the endo isomer to the exo isomer during polymerization but polymerization of the collected monomer proceeds in a similar fashion as the initial monomer.
With the exception of ethyl acetate, no other organic solvent is used for the preparation of PNBE(CO2H), starting from DCPD and acrylic acid. It is possible to replace the ethyl acetate extraction step by a Soxhlet extraction with water. This procedure also allows the separation of the unreacted monomer, leading to a reaction totally free of solvent, but it is tedious (the Soxhlet extraction lasted for two days in our hands).
The polymerization of NBE(vinyl) proceeds with 80% yield with a catalyst loading of 0.02 mol%. With a catalyst loading of 0.01 mol%, the yield decreases to 45%. The reaction stops due to the decomposition of the catalyst, and therefore, the yield is not improved by using longer reaction times. During the purification of the polymer, silica is added in order to remove residual metals. With this procedure, the polymer is virtually free of Ag, Sb and contains less than 15 ppm Pd, as measured by ICP.
The epoxidation of the polymer is performed by peracetic and performic acids formed in situ with H2O2 and the corresponding carboxylic acids. The filtration steps (3.1.11 and 3.1.12) should be performed without any interruption, and as rapidly as possible. Indeed, at this stage, the solid PNE(epoxy) is in contact with a large amount of acid and water, which leads to the ring opening of the epoxy functionality. To assess for the presence of ring-opened functionalities in the polymer, one can simply assess the solubility of the polymer in dimethylformamide (DMF). The ring-opened polymer is not soluble in DMF whereas PNBE(epoxy) is soluble in DMF.
We believe this method is significant because of the important role played by functional PNBEs in the fabrication of photoresists which are, for example, suitable for use in 157 nm photolithography.19 Functional PNBEs can also be employed for the construction of diblock copolymers.20 Due to their saturated hydrocarbon backbone, such polymers are interesting as low dielectric materials for microelectronics.21Recently, we also demonstrated that these materials lead to very high Tg thermosets14 as well as ionomers with possible applications for fuel-cell membranes.18 In view of the wealth of possible applications, the method presented here is of interest for a broad community.

Norbornene, NBE, the Diels-Alder adduct of ethylene and cyclopentadiene (obtained by &#34;cracking&#34; of dicyclopentadiene (DCPD)), is readily polymerized using either free-radical polymerization,1 cationic polymerization,2 ring-opening metathesis polymerization3 and catalytic insertion polymerization.4,5,6,7 Unlike the other mechanisms, the catalytic insertion polymerization leads to the formation of a very high glass-transition temperature (Tg) polymer whereby the bicyclic backbone of NBE is conserved. A variety of catalysts such as metallocene catalysts and late transition metal catalysts can be used to promote the polymerization of NBE.4,5,6,7 However, due to its low solubility and due to difficulties associated with the processing of a very high Tg polymer, the PNBE homopolymer has, to our knowledge, never found any use.
Functional polynorbornenes (PNBEs) have been the object of considerable scrutiny for the last 20 years, because they combine the high Tg imparted by the bicyclic rigid repeat unit as well as desirable properties endowed by their functionalities.8,9,10 NBE monomers are obtained from rather simple and inexpensive feedstocks, using a one-step Diels-Alder reaction between cyclopentadiene and a functionalized dienophile. However, the Diels-Alder reaction leads to two stereoisomers, endo and exo, which have very different reactivities.11,12 In fact, the endo stereoisomer is less reactive than exo form and deactivates the catalyst.11,12 Thus, in the past, the preparation of functional polynorbornenes usually required the separation of the endo and exo stereoisomers, and only the exo stereoisomer was used. Such a separation procedure was time-consuming, and led to the accumulation of unreacted endo stereoisomers as undesirable waste.
Recently we have shown that the polymerization of functionalized NBEs containing both stereoisomers is in fact feasible.13 We have thus been able to prepare a variety of substituted PNBEs, containing functional groups such as esters, anhydrides, aldehydes, imides, alcohols and double bonds. Due to their high Tg and functionality, these polymers show desirable properties. We describe here two methods to prepare functional polymers. The first one leads to the synthesis of the water soluble polymer poly(5-norbornene-2-carboxylic acid), PNBE(CO2H), using a cationic Pd catalyst (Figure 1).13,14 The same polymerization method can be used to prepare functional PNBEs with various pendant functionalities, such as esters, alcohols, imides, bromo-alkyls, aldehydes and anhydrides. In our hands, this cationic Pd catalyst cannot be used for NBEs containing pendant double bonds such as 5-vinyl-2-norbornene. In this case, a partial insertion of the pendant double bond during the polymerization leads to the formation of a cross-linked material. Therefore, we present here a second method dedicated to the formation of poly(5-vinyl-2-norbornene), PNBE(vinyl), using Pd2(dba)3:AgSbF6:PPh3 as an in situ catalyst.14 The pendant vinyl groups of the polymer are then further epoxidized, to lead to the formation of PNBE(epoxy) (Figure 1). Both PNBE(CO2H) and PNBE(epoxy) have been found to lead to the formation of thermoset resins with a Tg as high as 350 &#176;C.14 Thus, the simple method described here allows one to efficiently prepare polymers with a very high Tg and having a variety of functional groups, which can be used for numerous applications.
<img alt="Figure 1" src="/files/ftp_upload/54552/54552fig1.jpg" /> 
Figure 1: Functional PNBEs prepared by Pd catalyzed polymerization. (A) preparation of PNBE(CO2H), (B) preparation of PNBE(vinyl) and PNBE(epoxy). The dashed bond indicates a mixture of endo and exo isomers. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54552/54552fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

<table><tbody><tr><td>acrylic acid</td><td>Sigma-Aldrich</td><td>147230</td><td></td></tr><tr><td>hydroquinone</td><td>Sigma-Aldrich</td><td>H9003</td><td></td></tr><tr><td>dicyclopendadiene</td><td>Sigma-Aldrich</td><td>454338</td><td></td></tr><tr><td>palladium allyl dichloride dimer</td><td>Sigma-Aldrich</td><td>222380</td><td></td></tr><tr><td>silver hexfluoro antimonate</td><td>Sigma-Aldrich</td><td>227730</td><td></td></tr><tr><td>liquid nitrogen</td><td>Local Facility</td><td>NA</td><td></td></tr><tr><td>ethyl acetate</td><td>Fischer Scientific</td><td>E14520</td><td></td></tr><tr><td>5-vinyl-2-norbornene</td><td>Sigma-Aldrich</td><td>148679</td><td></td></tr><tr><td>toluene</td><td>Fischer Scientific</td><td>T290-4</td><td></td></tr><tr><td>palladium dba</td><td>Sigma-Aldrich</td><td>227994</td><td></td></tr><tr><td>triphenyl phosphine</td><td>Sigma-Aldrich</td><td>93090</td><td></td></tr><tr><td>silica gel 40-63 microns</td><td>Silicycle</td><td>Siliaflash</td><td></td></tr><tr><td>methanol</td><td>Fischer Scientific</td><td>BPA412-20</td><td></td></tr><tr><td>dichloromethane</td><td>EMD Millipore</td><td>DX08311</td><td></td></tr><tr><td>formic acid</td><td>Sigma-Aldrich</td><td>F0507</td><td></td></tr><tr><td>acetic acid</td><td>Sigma-Aldrich</td><td>320099</td><td></td></tr><tr><td>hydrogen peroxide solution</td><td>Sigma-Aldrich</td><td>216763</td><td></td></tr><tr><td>acetone</td><td>Fischer Scientific</td><td>A18-200</td><td></td></tr></tbody></table>

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.

The NBE monomers are prepared by simple Diels-Alder reaction of DCPD and a suitable dienophile, for example acrylic acid (AA). Normally, DCPD is cracked to yield cyclopentadiene (CPD) before reaction.17 Freshly cracked CPD is then engaged in the Diels-Alder reaction. However, in this protocol, both cracking and Diels-Alder steps are performed concomitantly, in a one-pot reaction. Thus, as soon as CPD is formed, it reacts with AA to yield 5-norbornene-2-carboxylic acid, NBE(CO2H) (Figure 1A). After reaction, the crude NBE(CO2H) is purified by simple distillation (Figure 2). This procedure has been repeated with a variety of dienophiles (maleic anhydride13, allyl bromide18, allyl alcohol18, 1-octene18) in a consistent manner (high yields, purification by distillation (Figure 2). The reaction with butadiene as dienophile was not attempted, due to the inherent difficulties associated with the manipulation of butadiene. Interestingly, this reaction is solvent-less.
The polymers are obtained by catalyzed polymerization (Figure 1A and 1B) and collected under the form of dry powders. They are analyzed by 1H NMR, FTIR spectroscopy, and gel permeation chromatography (GPC). The 1H NMR analysis of the polymer (Figure 3) is used to diagnose the presence of leftover solvent and monomer (which appear as sharp peaks, in contrast to the broad resonance of the polymer). The 1H NMR analysis of PNBE(vinyl) (Figure 3 blue) and PNBE(epoxy) (Figure 3 red) permit the confirmation of the epoxidation of PNBE(vinyl). Indeed, the decreasing ratio between the integrals of the vinyl group and the other protons in the PNBE(epoxy) spectrum (1:3 to 1:83) proves the excellent conversion of the reaction (97% in this case). The FTIR spectroscopy can also be used to confirm the outcome of the epoxidation reaction, as shown by the apparition of a characteristic peak at 875 cm-1 (Figure 4) and the disappearance of the 904 and 992 cm-1 which are observed in the PNBE(vinyl) polymer. The GPC is used to confirm the molecular weight distribution of the polymers. Typical Mn for PNBE(CO2H) and PNBE(epoxy) are 100,000 g/mol and 20,000 g/mol, with a PDI comprised between 1.5 and 2.0.

Watch this Scientific Journal Video about A Simple and Efficient Protocol for the Catalytic Insertion Polymerization of Functional Norbornenes at JoVE.com

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

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 ...

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10.3K Views.  Université de Sherbrooke. 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. 

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

Particles without a Box: Brush-first Synthesis of Photodegradable PEG Star Polymers under Ambient Conditions

Convenient methods for the rapid, parallel synthesis of diversely functionalized nanoparticles will enable discovery of novel formulations for drug delivery, biological imaging, and supported catalysis. In this report, we demonstrate parallel synthesis of brush-arm star polymer (BASP) nanoparticles by the &#34;brush-first&#34; method. In this method, a norbornene-terminated poly(ethylene glycol) (PEG) macromonomer (PEG-MM) is first polymerized via ring-opening metathesis polymerization (ROMP) to generate a living brush macroinitiator. Aliquots of this initiator stock solution are added to vials that contain varied amounts of a photodegradable bis-norbornene crosslinker. Exposure to crosslinker initiates a series of kinetically-controlled brush+brush and star+star coupling reactions that ultimately yields BASPs with cores comprised of the crosslinker and coronas comprised of PEG. The final BASP size depends on the amount of crosslinker added. We carry out the synthesis of three BASPs on the benchtop with no special precautions to remove air and moisture. The samples are characterized by gel permeation chromatography (GPC); results agreed closely with our previous report that utilized inert (glovebox) conditions. Key practical features, advantages, and potential disadvantages of the brush-first method are discussed.

Poly(ethylene glycol) (PEG) brush-arm star polymers (BASPs) with narrow mass distributions and tunable nanoscopic sizes are synthesized in via ring opening metathesis polymerization (ROMP) of a PEG-norbornene macromonomer followed by transfer of portions of the resulting living brush initiator to vials containing varied amounts of a rigid, photo-cleavable bis-norbornene crosslinker.

Convenient methods for the rapid, parallel synthesis of diversely functionalized nanoparticles will enable discovery of novel formulations for drug ...

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 ...

Mizoroki-Heck Cross-coupling Reactions Catalyzed by Dichloro{bis[1,1',1''-(phosphinetriyl)tripiperidine]}palladium Under Mild Reaction Conditions

Dichloro-bis(aminophosphine) complexes of palladium with the general formula of [(P{(NC5H10)3-n(C6H11)n})2Pd(Cl)2] (where n = 0-2), belong to a new family of easy accessible, very cheap, and air stable, but highly active and universally applicable C-C cross-coupling catalysts with an excellent functional group tolerance. Dichloro{bis[1,1',1''-(phosphinetriyl)tripiperidine]}palladium [(P(NC5H10)3)2Pd(Cl)2] (1), the least stable complex within this series towards protons; e.g. in the form of water, allows an eased nanoparticle formation and hence, proved to be the most active Heck catalyst within this series at 100 &#176;C and is a very rare example of an effective and versatile catalyst system that efficiently operates under mild reaction conditions. Rapid and complete catalyst degradation under work-up conditions into phosphonates, piperidinium salts and other, palladium-containing decomposition products assure an easy separation of the coupling products from catalyst and ligands. The facile, cheap, and rapid synthesis of 1,1',1"-(phosphinetriyl)tripiperidine and 1 respectively, the simple and convenient use as well as its excellent catalytic performance in the Heck reaction at 100 &#176;C make 1 to one of the most attractive and greenest Heck catalysts available.
We provide here the visualized protocols for the ligand and catalyst syntheses as well as the reaction protocol for Heck reactions performed at 10 mmol scale at 100 &#176;C and show that this catalyst is suitable for its use in organic syntheses.

Mizoroki-Heck Cross-coupling Reactions Catalyzed by Dichloro{bis[1,1',1''-phosphinetriyltripiperidine]}palladium Under Mild Reaction Conditions

Dichloro{bis[1,1',1''-(phosphinetriyl)tripiperidine]}palladium [(P(NC5H10)3)2Pd(Cl)2] (1) is an easy accessible, cheap, and air stable, but highly active Heck catalyst with an excellent functional group tolerance that efficiently operates under mild reaction conditions to give the coupling products in very high yields.

Dichloro-bis(aminophosphine) complexes of palladium with the general formula of [(P{(NC5H10)3-n(C6H11)n})2Pd(Cl)2] (where n = 0-2), belong to a new family ...

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.

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 ...

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, ...

Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization

We report a method to generate the N-heterocyclic carbene (NHC) 1,3-dimesitylimidazol-2-ylidene (IMes) under UV-irradiation at 365 nm to characterize IMes and determine the corresponding photochemical mechanism. Then, we describe a protocol to perform ring-opening metathesis polymerization (ROMP) in solution and in miniemulsion using this NHC-photogenerating system. To photogenerate IMes, a system comprising 2-isopropylthioxanthone (ITX) as the sensitizer and 1,3-dimesitylimidazolium tetraphenylborate (IMesH+BPh4-) as the protected form of NHC is employed. IMesH+BPh4- can be obtained in a single step by anion exchange between 1,3-dimesitylimidazolium chloride and sodium tetraphenylborate. A real-time steady-state photolysis setup is described, which hints that the photochemical reaction proceeds in two consecutive steps: 1) ITX triplet is photo-reduced by the borate anion and 2) subsequent proton transfer takes place from the imidazolium cation to produce the expected NHC IMes. Two separate characterization protocols are implemented. Firstly, CS2 is added to the reaction media to evidence the photogeneration of NHC through formation of the IMes-CS2 adduct. Secondly, the amount of NHC released in situ is quantified using acid-base titration. The use of this NHC photo-generating system for the ROMP of norbornene is also discussed. In solution, a photopolymerization experiment is conducted by mixing ITX, IMesH+BPh4-, [RuCl2(p-cymene)]2 and norbornene in CH2Cl2, then irradiating the solution in a UV reactor. In a dispersed medium, a monomer miniemulsion is first formed then irradiated inside an annular reactor to produce a stable poly(norbornene) latex.

We describe a protocol to photogenerate N-heterocyclic carbenes (NHCs) by UV irradiation of a 2-isopropylthioxanthone/imidazolium tetraphenylborate salt system. Methods to characterize the photoreleased NHC and elucidate the photochemical mechanism are proposed. The protocols for ring-opening metathesis photopolymerization in solution and miniemulsion illustrate the potential of this 2-component NHC photogenerating system.

We report a method to generate the N-heterocyclic carbene (NHC) 1,3-dimesitylimidazol-2-ylidene (IMes) under UV-irradiation at 365 nm to characterize IMes ...

Depolymerizable Olefinic Polymers Based on Fused-Ring Cyclooctene Monomers

The growing consumption of synthetic polymers and the accumulation of polymer waste have led to a pressing need for new routes to sustainable materials. Achieving a closed-loop polymer economy via chemical recycling to monomer (CRM) is one such promising route. Our group recently reported a new CRM system based on polymers prepared by ring-opening metathesis polymerization (ROMP) of trans-cyclobutane fused cyclooctene (tCBCO) monomers. This system offers several key advantages, including the ease of polymerization at ambient temperatures, quantitative depolymerization to monomers under mild conditions, and a broad range of functionalities and thermomechanical properties. Here, we outline detailed protocols for the preparation of tCBCO-based monomers and their corresponding polymers, including the preparation of elastic polymer networks and compression molding of linear thermoplastic polymers. We also outline the preparation of high ring strain E-alkene tCBCO monomers and their living polymerization. Finally, the procedures for the depolymerization of linear polymers and polymer networks are also demonstrated.

Here, we describe protocols for the preparation of trans-cyclobutane fused cyclooctenes (tCBCO),&#160;their polymerization to prepare depolymerizable olefinic polymers, and the depolymerization of these polymers under mild conditions. Additionally, protocols for the preparation of depolymerizable networks and compression molding of rigid linear plastics based on this system are described.

The growing consumption of synthetic polymers and the accumulation of polymer waste have led to a pressing need for new routes to sustainable materials. ...

Controlling Particle Fraction in Microporous Annealed Particle Scaffolds for 3D Cell Culture

Microgels are the building blocks of microporous annealed particle (MAP) scaffolds, which serve as a platform for both in vitro cell culture and in vivo tissue repair. In these granular scaffolds, the innate porosity generated by the void space between microgels enables cell infiltration and migration. Controlling the void fraction and particle fraction is critical for MAP scaffold design, as porosity is a bioactive cue for cells. Spherical microgels can be generated on a microfluidic device for controlled size and shape and subsequently freeze-dried using methods that prevent fracturing of the polymer network. Upon rehydration, the lyophilized microgels lead to controlled particle fractions in MAP scaffolds. The implementation of these methods for microgel lyophilization has led to reproducible studies showing the effect of particle fraction on macromolecule diffusion and cell spreading. The following protocol will cover the fabrication, lyophilization, and rehydration of microgels for controlling particle fraction in MAP scaffolds, as well as annealing the microgels through bio-orthogonal crosslinking for 3D cell culture in vitro.

Minimizing the variability in the particle fraction within granular scaffolds facilitates reproducible experimentation. This work describes methods for generating granular scaffolds with controlled particle fractions for in vitro tissue engineering applications.

Microgels are the building blocks of microporous annealed particle (MAP) scaffolds, which serve as a platform for both in vitro cell culture and in vivo ...

Bioengineering

Cationic Chain-Growth Polymerization: Mechanism

The cationic polymerization mechanism consists of three steps: initiation, propagation, and termination. In the initiation step of the polymerization process, the &#960; bond of a monomer gets protonated by the Lewis acid catalyst, which is formed from boron trifluoride and water. The protonation of the &#960; bond generates a carbocation stabilized by the electron&#8208;donating group. In the propagation step, the &#960; bond of the second monomer acts as a nucleophile and attacks the generated carbocation, thereby producing a dimer that acts as a new carbocation. The propagation step repeats itself and builds the polymer chain. In the termination step, the propagating chain gets terminated either by adding a base that deprotonates the carbocation and forms a new &#960; bond or by the attack of a nucleophile on the carbocation, where the nucleophile adds to the cationic end of the chain.

The cationic polymerization mechanism consists of three steps: initiation, propagation, and termination. In the initiation step of the polymerization ...

Organic Chemistry

Unit 1

Synthetic Polymers

KEY TERMS AND CONCEPTS

Ziegler&ndash;Natta Chain-Growth Polymerization: Overview

Ziegler&#8211;Natta polymerization is another form of addition or chain&#8208;growth polymerization used for synthesizing linear polymers over branched polymers. The catalyst used for polymerization is the Ziegler&#8211;Natta catalyst, named after Karl Ziegler and Giulio Natta, who developed it in 1953. This catalyst is an organometallic complex of titanium tetrachloride and triethyl aluminum, with the active form of the catalyst being an alkyl titanium compound. Using the Ziegler&#8211;Natta catalyst, high molecular weight and stereochemically controlled linear polymers can be synthesized at a large scale. For example, high molecular weight and ultrahigh molecular weight polyethylene, used for making underground pipes and bulletproof vests, are synthesized using the Ziegler&#8211;Natta catalyst. This catalyst also facilitates the synthesis of polymers of specific tacticity. For instance, isotactic&#160;polypropylene, with chiral centers on the same side, can be synthesized using a specific Ziegler&#8211;Natta catalyst.

Ziegler&amp;#8211;Natta Chain-Growth Polymerization: Overview

Ziegler&#8211;Natta polymerization is another form of addition or chain&#8208;growth polymerization used for synthesizing linear polymers over branched ...

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

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