This research forms part of the research program of the Dutch Polymer Institute, project #749.

1. Rubber Modification
<ol>
	<li>Prepare the maleated EPM (EPM-g-MA, 49 wt% ethylene, 2.1 wt% MA, Mn = 50 kg/mol, PDI = 2.0) rubber and furfurylamine (FFA) before starting the experiment as indicated in steps 1.1.1-1.1.4.5
	<ol>
		<li>Dry the EPM-g-MA rubber in a vacuum oven for one hour at 175 &#176;C to convert present di-acid into anhydride.11</li>
		<li>Compression mold a 0.1 mm thick rubber film in a hot press for 10 min at 150 &#176;C and 100 bar.</li>
		<li>Record a transmission infrared spectrum of the resulting film after placing it in a KBr tablet holder. 
		NOTE: The conversion of the hydrolyzed di-acid into anhydride is complete if the typical carboxylic acid peak (<img alt="Equation" src="/files/ftp_upload/54496/54496eq_v-sCO.jpg" /> = 1,710 cm-1) is absent and the characteristic cyclic anhydride peak (<img alt="Equation" src="/files/ftp_upload/54496/54496eq_v-asCO.jpg" /> = 1,856 cm-1) is present.5</li>
		<li>Using standard distillation glassware, distill 2.8 g FFA (boiling point = 145 &#176;C, 28.9 mmol; 3.0 eq. based on MA content in EPM-g-MA) under atmospheric pressure.</li>
	</ol>
	</li>
	<li>Prepare a 10 wt% rubber solution by weighing 45.0 g of EPM-g-MA rubber (9.6 mmol MA) and dissolving it in 500 ml tetrahydrofuran (THF) at 23 &#176;C in a closed beaker under vigorous stirring.</li>
	<li>Add the 2.8 g freshly distilled FFA to the 10 wt% rubber solution.</li>
	<li>Stir the reaction mixture in a closed system at 23 &#176;C for at least 1 hr.</li>
	<li>Precipitate the reaction mixture by pouring it slowly into a tenfold (5 L) of acetone under mechanical stirring, yielding the polymer product as white threads that are easily fished out of the beaker using large tweezers.</li>
	<li>Dry the collected product (EPM-g-furan) to constant weight in a vacuum oven at 35 &#176;C (this takes approximately 1 day).</li>
	<li>Compression mold the resulting, slightly yellowish product in a mold between two metal plates in a hot press for 10 min at 175 &#176;C and 100 bar to convert the intermediate maleamic acid product into the imide product.</li>
	<li>Cut the resulting plaque of rubber in small pieces (0.05 g) with scissors and wash them thoroughly by immersing them in acetone to remove any unreacted FFA.</li>
	<li>Record a transmission infrared spectrum of the product in a KBr tablet holder after compression molding it into a 0.1 mm thick film.5 
	NOTE: The absence of any remaining amide-acid can be deduced from the absence of a peak at 1,530 cm-1.39, 40 The most illustrative indication for the successful modification are the nearly complete disappearance of <img alt="Equation" src="/files/ftp_upload/54496/54496eq_v-asCO.jpg" /> at 1,856 cm-1 of the anhydride and the appearance of <img alt="Equation" src="/files/ftp_upload/54496/54496eq_v-sCO.jpg" /> at 1,710 cm-1 and the C-N stretching vibration (<img alt="Equation" src="/files/ftp_upload/54496/54496eq_v-sCN.jpg" /> = 1,378 cm-1) of the maleimide.</li>
	<li>Determine the reaction conversion of EPM-g-MA to EPM-g-furan from the decrease in absorbance of the C=O symmetrical stretch vibration of the anhydride groups (<img alt="Equation" src="/files/ftp_upload/54496/54496eq_v-asCO.jpg" />= 1,856 cm-1) by integrating the areas under the individual infrared (FT-IR) peaks after deconvolution (R2 &#62; 0.95).5 
	NOTE: The methyl rocking vibration (<img alt="Equation" src="/files/ftp_upload/54496/54496eq_rho_ip_CH3.jpg" /> = 723 cm-1), originating from the EPM backbone, remains unchanged upon modification and can be used as an internal reference.</li>
	<li>Determine the modification conversions by performing Elemental Analysis (EA) for N, C and H on the washed and dried rubber samples.5 
	NOTE: The molar contents can be derived from the measured mass percentages. The molar nitrogen content in EPM-g-furan is equal to that of the grafted furan groups. The conversion can be calculated by comparing the molar ratio of MA-grafted monomer to the non-grafted EPM monomers in the EPM-g-MA precursor (7.69 x 10-3) with the molar ratio N:EPM of the EPM-g-furan sample.</li>
	<li>Measure the Shore A Hardness of the compression molded samples at least 10 times pressing a durometer onto a rubber sample, covering the entire cylindrical surface of the Durometer.5 
	NOTE: Samples with a thickness of 2 &#177; 0.1 mm should be used for these tests.</li>
	<li>Measure the elongation (at break) and the ultimate tensile strength by performing tensile tests on samples of approximately 1 mm thick and 5 mm wide using a clamp length of 15 mm and a strain rate of 500 &#177; 50 mm/min. Determine the Young&#39;s modulus from the initial slope of the resulting stress-strain curves. 
	NOTE: For each measurement, test 10 samples and exclude two outliers with the highest and the lowest values.</li>
	<li>Determine the compression set at 23 &#176;C by compressing cylindrical samples with a thickness of 6 &#177; 0.1 mm (t0) and a diameter of 13 &#177; 0.1 mm between to metal plates to 3/4th of their initial thickness (tn) for 70 hr, let them relax at 50 &#176;C for &#189; hr and measure the thickness (ti). 
	NOTE: The compression set value can be determined from (t0-ti)/(t0-tn).</li>
</ol>
2. Diels-Alder Cross-linking and Reprocessing
<ol>
	<li>Before the experiment, synthesize the aliphatic bismaleimide (BM) from didodecylamine and maleic anhydride (MA) according to a reported procedure.41</li>
	<li>Weigh 40.0 g of EPM-g-furan rubber (8.6 mmol furan content) and 0.04 g phenolic anti-oxidant (octadecyl-1-[3,5-di-tert-butyl-4-hydroxyphenyl]propionate) and dissolve them in a large, closed beaker with 500 ml THF at 23 &#176;C.</li>
	<li>Add 1.48 g of the aliphatic bismaleimide (4.3 mmol, 0.5 eq. based on furan content in EPM-g-furan) to beaker with the 10 wt% solution.</li>
	<li>Stir the reaction mixture for at least 1 hr at 50 &#176;C in the closed beaker, then remove the cap to open the system to evaporate the solvent. Evaporation of the THF can also be performed using a rotary evaporator.</li>
	<li>Dry the collected product to constant weight in a vacuum oven at 35 &#176;C.</li>
	<li>Compression mold the product for 30 min at 175 &#176;C and 100 bar.</li>
	<li>Anneal the resulting product by storing it in an oven at 50 &#176;C for at least three days.</li>
	<li>Cut the resulting plaque of rubber in small pieces (0.05 g) using scissors and wash them thoroughly by immersing them in acetone to remove any unreacted components and compression mold it into a 0.1 mm thick film.</li>
	<li>Record an transmission infrared spectrum of the resulting film in a KBr tablet holder, using the same set-up as described for 1.9.1.5</li>
	<li>Determine the cross-linking conversion from relative decrease in the C-O-C symmetric stretch vibration of the furan rings (<img alt="Equation" src="/files/ftp_upload/54496/54496eq_v-sCOC.jpg" /> = 1,013 cm-1) as described in 1.10.5</li>
	<li>Determine the cross-linking conversion by performing EA for N, C and H on the washed and dried rubber samples.5</li>
	<li>Determine the Shore A hardness, Young&#39;s modulus, the elongation at break, the ultimate tensile strength and compression set at 23 &#176;C in the same way as described in 1.12-14.</li>
	<li>Reprocess the samples after testing by cutting them into small pieces using scissors (&#177; 50 mm3) and compression molding these under the same conditions into new, homogeneous samples with the same dimensions.</li>
</ol>
3. Peroxide and Sulfur Curing of ENB-EPDM
<ol>
	<li>Heat an internal batch mixer to 70 &#176;C and let it rotate at 50 rpm. 
	NOTE: An initial conditioning step by flushing the chamber with nitrogen gives better control of the cross-linking processes.</li>
	<li>Feed 18.1 g of ENB-EPDM (48 wt% ethylene, 5.5 wt% ENB) to the internal batch mixer to reach a 70% fill factor and mix for 2 min to yield a homogeneous melt.</li>
	<li>Feed 1.25 phr of peroxide di(tert-butylperoxy-isopropyl) benzene) or 1.88 phr of a 80% pure, standard, semi-efficient sulfur vulcanization system and mix it with the rubber for 3 min at 70 &#176;C.</li>
	<li>Compression mold the resulting compound in a hot press for 30 min at 175 &#176;C and 100 bar to cure it.</li>
	<li>Determine the Shore A hardness, Young&#39;s modulus, the elongation at break and the ultimate tensile strength compression set in the same way as described in 1.12-14.</li>
</ol>
4. Cross-link Density Determination
<ol>
	<li>Cut a piece of compression molded, cross-linked rubber of approximately 50 mg using scissors.
	<ol>
		<li>Determine the initial weight of the rubber sample precisely by weighing it in a 20 ml glass vial (W0).</li>
		<li>Immerse the weighed rubber in 15 ml of decalin.</li>
	</ol>
	</li>
	<li>Let the rubber swell in the decalin until its weight does not increase anymore and equilibrium swelling is reached (approximately 3 days).</li>
	<li>Carefully take the swollen sample out of the vial and carefully dab the surface with a paper tissue to remove any solvent from the surface without squeezing it.</li>
	<li>Weigh the swollen rubber sample in a new sample vial (W1).</li>
	<li>Dry the swollen sample in a vacuum oven at 80 &#176;C until a constant weight is reached and determine the sample&#39;s dry weight (W2).</li>
	<li>Obtain the gel content from W2/W0 x 100%</li>
	<li>Determine the cross-link density ([XLD] in mol/cm3) using the Flory-Rehner equation 42, 43, [XLD]= (ln(1-VR)+VR +&#967;VR2)/(2VS(0.5VR-VR1/3) ) with VS the molar volume of the solvent (decalin: 154 ml/mol at 23 &#176;C), &#967; the interaction parameter (decalin-EPDM: 0.121 + 0.278VR44) and VR the volume fraction of rubber in swollen sample that can be determined from W2/(W2+(W1-W2)&#8729;&#961;EPM-g-furan/&#961;decalin) with the densities (&#961;) being 860 kg/m3 for EPM-g-furan and 896 kg/m3 for decalin, respectively.</li>
</ol>

The author Martin van Duin is an employee of LANXESS Elastomers B.V. that produces EPM-g-MA rubbers used in this Article.

A commercial EPM-g-MA rubber was thermo-reversibly cross-linked in a simple two-step approach. The maleated rubber was first modified with FFA to graft furan groups onto the rubber backbone. The resulting pending furans show reactivity as Diels-Alder dienes. An aliphatic BM was used as cross-linking agent, resulting in a thermo-reversible bridge between two furan moieties. Both reactions were successful with good conversions (&#62; 80%) according to infrared spectroscopy, elemental analysis. Cross-linking was shown by swelling and solubility tests, yielding a gel content of 100%.
For the successful execution of the protocol described, it is critical that the individual components are carefully prepared. Unfortunately, the preparation of reworkable rubbers by making use of the method described here does not allow for easy upscaling to larger quantities. Although no other methods of preparing this reworkable rubber product have been described in the open literature, an analogy with the well-known free-radical grafting of maleic anhydride onto EPM rubber can be drawn in this respect.32 Melt modification of the rubber is a promising alternative that does not require a solvent, intermediate purification or drying steps and can be executed on commercial equipment such as melt mixers or extruders. The production of such a reworkable rubber would be much more time and cost efficient if it were to be executed via melt processing. On the other hand, the method described allows for more control of the reaction and a more well-defined end-product.
On a more general level of comparison between the current approach towards reworkable rubbers and other recycling approaches (e.g., devulcanization), it must be noted that the materials have to be modified prior to cross-linking and that the formed cross-links fail at relatively low temperatures (&#62; 180%). Nevertheless, this approach does allow for the production of a &#34;cradle-to-cradle&#34; recyclable rubber product whereas the recyclability of devulcanized rubbers is severely limited up to the reuse of few percentages in combination with virgin components.54, 55
The cross-link densities of the DA cross-linked EPM-g-furan as determined from FT-IR and EA correspond to those determined from swell tests. The resulting values are characteristic for loosely cross-linked, rubbery networks and correspond to the typical cross-link densities of sulfur and peroxide cured EPDM gums (1-5 x 10-4 mol/cm3) reported in literature.53, 54 The cross-link density of the DA cross-linked samples is comparable to those of sulfur- and peroxide-cured EPDM references, allowing for a comparison of properties.
Finally, the Shore A hardness, Young&#180;s modulus, elongation at break, ultimate tensile strength and compression set all indicate the transformation of a viscous polymer into an elastic rubber network upon the addition of BM. These mechanical properties correspond to those of sulphur- and peroxide-cured references samples and they were retained after reprocessing the thermo-reversibly cross-linked rubber. Reprocessing of the material was performed for up to 5 times without any significant loss of properties.
The presented results provide a new route for the (reversible) cross-linking of (EPM) rubbers via a (retro)Diels-Alder reaction, which complements the toolbox of rubber recycling. They pave the way towards the application of such cross-linking strategies (and the recyclability these might entail) for a variety of rubber products.

Sulfur vulcanization and peroxide curing are currently the main industrial cross-linking techniques in the rubber industry, yielding irreversible chemical cross-links that prevent melt reprocessing.1, 2 A &#39;cradle to cradle&#39; approach to recycle cross-linked rubbers requires a material that behaves similar to permanently cross-linked rubbers at service conditions, while having the processability and complete recyclability of a thermoplastic at high temperatures. An approach to achieve such recyclability uses rubbery networks with reversible cross-links that respond to an external stimulus, such as temperature (most feasible from the viewpoint of future industrial applications).3-5 The formation of these cross-links at relatively low service temperatures is required for good mechanical behavior of the rubber, while their cleavage at high temperatures (similar to processing temperature of original non-cross-linked compound) allows for recycling of the material.
Some specific materials can be reversibly cross-linked by making use of so-called dynamic covalent networks via polycondensation reactions6 or by so-called reversible network topology freezing via transesterification reactions.7-9 The disadvantage of these approaches is the necessity of designing and synthesizing new polymers rather than modifying existing, commercial rubbers that already have the desired properties. Techniques to thermo-reversibly cross-link rubbers involve hydrogen bonding, ionic interactions and covalent bonding such as via thermo-activated disulfide rearrangements.10-13 Recently, thermo-reversible cross-linking via Diels-Alder (DA) chemistry was developed.14-21 DA chemistry can be applied to a broad range of polymers and represents a popular choice, especially since the DA reaction allows for relatively fast kinetics and mild reaction conditions.17, 22-24 Their low coupling and high decoupling temperatures make furan and maleimide excellent candidates for reversible polymer cross-linking.18-20, 25-28
The aim of the present work is to provide a method for the use of DA chemistry as a thermo-reversible cross-linking tool for an industrial rubber product (Figure 1).5 First, the reactivity of saturated hydrocarbon elastomers, such as ethylene/propylene rubbers (EPM), has to be increased. A commercially relevant example that facilitates this is the peroxide-initiated free-radical grafting of maleic anhydride (MA).29-34 Secondly, a furan group can be grafted onto such a maleated EPM rubber by inserting furfurylamine (FFA) into the pendant anhydride to form an imide.35, 36 Finally, the furan moieties that are thus attached to the rubber backbone can then participate in thermo-reversible DA chemistry as an electron-rich diene.25, 37 The electron-poor bis-maleimide (BM) is a suitable dienophile for this cross-linking reaction.19, 26, 38
<img alt="Figure 1" src="/files/ftp_upload/54496/54496fig1.jpg" /> 
Figure 1. Reaction scheme. Furan grafting and bismaleimide cross-linking of EPM-g-MA rubber (reprinted with permission from 5). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54496/54496fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

<table border="0" cellpadding="0" cellspacing="0" width="599">
	<tbody>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">ENB-EPDM</td>
			<td style="width:97px;">LANXESS Elastomers B.V.</td>
			<td style="width:119px;">Keltan 8550C</td>
			<td style="width:167px;"></td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">EPM-g-MA</td>
			<td style="width:97px;">LANXESS Elastomers B.V.</td>
			<td>Keltan DE5005</td>
			<td>Vacuum oven for one hour at 175 &#176;C&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">furfurylamine</td>
			<td style="width:97px;">Sigma-Aldrich</td>
			<td style="width:119px;">F20009</td>
			<td>Freshly distillated before use</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">di-dodecylamine</td>
			<td style="width:97px;">Sigma-Aldrich</td>
			<td style="width:119px;">36784</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">maleic anhydride</td>
			<td style="width:97px;">Sigma-Aldrich</td>
			<td style="width:119px;">M0357</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">octadecyl-1-(3,5-di-tert.-butyl-4-hydroxyphenyl)propionate</td>
			<td style="width:97px;">Sigma-Aldrich</td>
			<td style="width:119px;">367079</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">bis(tert.-butylperoxy-iso-propyl) benzene</td>
			<td style="width:97px;">Sigma-Aldrich</td>
			<td style="width:119px;">531685</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">tetrahydrofuran</td>
			<td style="width:97px;">Sigma-Aldrich</td>
			<td style="width:119px;">401757</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">decalin</td>
			<td style="width:97px;">Sigma-Aldrich</td>
			<td style="width:119px;">294772</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">acetone</td>
			<td style="width:97px;">Sigma-Aldrich</td>
			<td style="width:119px;">320110</td>
			<td></td>
		</tr>
	</tbody>
</table>

the preparation and properties of thermo-reversibly cross-linked rubber via diels-alder chemistry

A method for using Diels Alder thermo-reversible chemistry as cross-linking tool for rubber products is demonstrated. In this work, a commercial ethylene-propylene rubber, grafted with maleic anhydride, is thermo-reversibly cross-linked in two steps. The pending anhydride moieties are first modified with furfurylamine to graft furan groups to the rubber backbone. These pendant furan groups are then cross-linked with a bis-maleimide via a Diels-Alder coupling reaction. Both reactions can be performed under a broad range of experimental conditions and can easily be applied on a large scale. The material properties of the resulting Diels-Alder cross-linked rubbers are similar to a peroxide-cured ethylene/propylene/diene rubber (EPDM) reference. The cross-links break at elevated temperatures (&#62; 150 &#176;C) via the retro-Diels-Alder reaction and can be reformed by thermal annealing at lower temperatures (50-70 &#176;C). Reversibility of the system was proven with infrared spectroscopy, solubility tests and mechanical properties. Recyclability of the material was also shown in a practical way, i.e., by cutting a cross-linked sample into small parts and compression molding them into new samples displaying comparable mechanical properties, which is not possible for conventionally cross-linked rubbers.

The successful modification of EPM-g-MA into EPM-g-furan and the cross-linking with the bismaleimide is shown by Fourier transform infrared spectrometry (FTIR) (Figure 2). The presence of furan groups in the EPM-g-furan product can be deduced from the splitting of the CC aliphatic stretching peak (<img alt="Equation" src="/files/ftp_upload/54496/54496eq_v-sCC.jpg" /> = 1,050 cm-1) into two furan peaks (<img alt="Equation" src="/files/ftp_upload/54496/54496eq_v-asCOC.jpg" /> = 1,073 cm-1 and <img alt="Equation" src="/files/ftp_upload/54496/54496eq_v-sCOC.jpg" /> = 1,013 cm-1), the appearance of the C=C furan stretching vibration (<img alt="Equation" src="/files/ftp_upload/54496/54496eq_v-sCC.jpg" /> = 1,504 cm-1) and of the deformation vibration at 599 cm-1.15, 37, 37 The furan ring stretching peaks at 1,436 and 1,345 cm-1 cannot not be observed as they are hidden by the large overlapping CH2-vibrations of the rubber backbone at 1,450 cm-1 and 1,350 cm-1.15 The incorporation of the BM can be deduced from the appearance of <img alt="Equation" src="/files/ftp_upload/54496/54496eq_delta_ip_DAring.jpg" /> at 1,190 cm-1 and of the characteristic succinimide bands (<img alt="Equation" src="/files/ftp_upload/54496/54496eq_v-sCNC.jpg" /> = 1,385 cm-1, <img alt="Equation" src="/files/ftp_upload/54496/54496eq_delta_ip_HNC.jpg" /> = 1,311 cm-1 and <img alt="Equation" src="/files/ftp_upload/54496/54496eq_delta_asoop_OCN.jpg" /> = 620 cm-1).45, 46 The furan-related absorption (<img alt="Equation" src="/files/ftp_upload/54496/54496eq_v-sCOC.jpg" />) decreases upon cross-linking and the <img alt="Equation" src="/files/ftp_upload/54496/54496eq_v-sCO.jpg" /> carbonyl band increases as a result of a second absorption around 1,770 cm-1 that is attributed to the succinimide ring resulting from the cycloaddition.25
FTIR and elemental analysis were used to determine the modification and cross-linking conversions (Table 1). The FT-IR and EA conversions were used to determine the cross-link density of the DA cross-linked EPM-g-furan, which was found to be 1.8 x 10-4 &#177; 3 x 10-5 mol/cm3. According to swell tests (protocol 4), the gel content of all cross-linked samples was approximately 100% and the cross-link density of the DA cross-linked EPM-g-furan was 2.1 x 10-4 &#177; 2 x 10-5 mol/cm3. The cross-link densities of the sulfur- and peroxide-cured EPDM reference systems were found to be respectively 1.7 x 10-4 &#177; 6 x 10-6 and 1.8 x 10-4 &#177; 8 x 10-6 mol/cm3, respectively.
<img alt="Figure 2" src="/files/ftp_upload/54496/54496fig2.jpg" /> 
Figure 2. FT-IR absorption spectra. Spectra of left: EPM-g-MA (red) and modified EPM-g-furan (black) and right: of the non-cross-linked EPM-g-furan (black) and the Diels-Alder cross-linked EPM-g-furan (blue) (reprinted with permission from 5). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54496/54496fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td></td>
			<td>wt% N</td>
			<td>wt% C</td>
			<td>wt% H</td>
			<td>FT-IR conversion (%)</td>
			<td>EA conversion (%)&#160;</td>
		</tr>
		<tr>
			<td>EPM-g-MA</td>
			<td>&#60; 0.01</td>
			<td>84.7</td>
			<td>14.3</td>
			<td></td>
			<td>-</td>
		</tr>
		<tr>
			<td>EPM-g-furan</td>
			<td>0.3</td>
			<td>84.8</td>
			<td>14.2</td>
			<td>96</td>
			<td>93</td>
		</tr>
		<tr>
			<td>DA cross-linked EPM-g-furan</td>
			<td>0.4</td>
			<td>84.2</td>
			<td>14</td>
			<td>72</td>
			<td>80</td>
		</tr>
	</tbody>
</table>
Table 1. Reaction conversions. Results from Elemental Analysis and FT-IR (adapted with permission from 5).
The de-cross-linking of the DA cross-linked EPM-g-furan product was followed by transmission FT-IR (Figure 3). Some characteristic furan peaks such as and at respectively 1,504 and 1,013 cm-1 decrease upon thermal annealing at 50 &#176;C and increases after compression molding at 175 &#176;C. This indicates that cross-linking and de-cross-linking takes place via a reversible DA reaction between the grafted furan groups and the added BM cross-linking agents.47 Solubility tests are a more practical method to observe the effects of cross-linking and de-cross-linking. EPM-g-furan is soluble in decalin (5 wt%) at 23 &#176;C. The same material cross-linked with BM is clearly insoluble under the same conditions. Thermo-reversibility of the cross-linking reaction was shown by dissolving the product during 1 hour of heating to 175 &#176;C. Finally, a practical way of testing the material&#39;s reworkability is by grinding or cutting the cross-linked rubber and compression molding it at 160 &#176;C and 100 bar for 30 minutes. The resulting reprocessed material was found to have a cross-link density of 2.0 x 10-4 &#177; 2 x 10-5 mol/cm3. When an EPDM rubber is irreversibly cross-linked with peroxide, remolding the cut pieces under the same conditions does not yield a coherent sample (Figure 4).
<img alt="Figure 3" src="/files/ftp_upload/54496/54496fig3.jpg" /> 
Figure 3. FT-IR absorption spectra. Spectra of EPM-g-furan and DA (de/re-)cross-linked EPM-g-furan (reprinted with permission from 5). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54496/54496fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/54496/54496fig4.jpg" /> 
Figure 4. Reprocessing rubber samples. Sample bars of thermoreversibly cross-linked and sulphur- and peroxide-cured rubbers that are all reprocessed under the same conditions. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54496/54496fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
The hardness increases and the compression set decreases when going from the non-cross-linked EPM-g-MA and EPM-g-furan precursors to the DA cross-linked EPM-g-furan (Figure 5). This clearly indicates the conversion of the viscous polymer into an elastic network that occurs upon cross-linking. The hardness and compression set of reprocessed samples of respectively 44 Shore A and 5% are comparable to those of the original DA cross-linked samples. The hardness and compression set of the sulfur and peroxide cross-linked EPDM rubbers were 60 and 61 Shore A and 5% and 8% respectively. Although these irreversibly cross-linked samples have higher hardness values, their compression set is slightly inferior to the DA cross-linked samples.
The elongation at break decreases when going from the non-cross-linked EPM-g-MA to the modified EPM-g-furan (Figure 6). This difference could be explained by synergetic effects of the pending, conjugated furan groups.48 Their increased rigidity, &#960;-stacking stabilization and a very low degree of radical cross-linking between the furans could be enough to decrease the rubber&#39;s elasticity to a certain extent. The cross-linked EPM-g-furan samples show significantly higher tensile moduli and lower elongation at break values compared to their non-cross-linked precursors. These high tensile moduli and low elongation are indicative for cross-linked rubbers.49, 50 It also appears that the recycled cross-linked rubbers retain these characteristic properties of cross-linked rubbers, indicating that these cross-linked rubbers can be reshaped or reworked, regardless of their high modulus and low elongation. The median stress-strain curves also show that the thermo-reversibly, DA cross-linked rubber, before and after reprocessing, yields at higher stresses and lower strains than their non-cross-linked EPM-g-MA and EPM-g-furan precursors. This distinction is illustrative for the different behavior of cross-linked and non-cross-linked rubbers as is illustrated by the peroxide and sulfur cured EPDM samples.51, 52 These peroxide and sulfur cured reference samples appear to have slightly higher Young&#39;s moduli than the DA cross-linked rubbers even though they were measured to have similar cross-link densities. The tensile strength and elongation at break of the DA cross-linked samples however, are at least as good as those of the peroxide and sulfur cured samples.
<img alt="Figure 5" src="/files/ftp_upload/54496/54496fig5.jpg" /> 
Figure 5. Hardness and compression set. Results for non-cross-linked EPM-g-MA and EPM-g-furan, DA cross-linked EPM-g-furan and irreversibly sulfur- and peroxide-cured EPDM rubbers. Error bars represent the standard deviations (reprinted with permission from 5). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54496/54496fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" src="/files/ftp_upload/54496/54496fig6.jpg" /> 
Figure 6. Tensile test. Results for EPM-g-MA (1), EPM-g-furan (2), DA cross-linked EPM-g-furan (3) and reprocessed, DA cross-linked EPM-g-furan (4) together with sulfur- (5) and peroxide- (6) cured EPDM. Median stress-strain graphs (left) and the corresponding Young&#39;s modulus, tensile strength and elongation at break (right). Error bars represent the standard deviations. (Reprinted with permission from 5) <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54496/54496fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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The Preparation and Properties of Thermo-reversibly Cross-linked Rubber Via Diels-Alder Chemistry

A simple two-step approach involving rubber modification and cross-linking yields fully reworkable, elastic rubber products.

A method for using Diels Alder thermo-reversible chemistry as cross-linking tool for rubber products is demonstrated. In this work, a commercial ...

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Dienes, Conjugated Pi Systems, and Pericyclic Reactions

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13.5K Views.  University of Groningen. A simple two-step approach involving rubber modification and cross-linking yields fully reworkable, elastic rubber products. 

Video: The Preparation and Properties of Thermo-reversibly Cross-linked Rubber Via Diels-Alder Chemistry

Microwave-assisted Intramolecular Dehydrogenative Diels-Alder Reactions for the Synthesis of Functionalized Naphthalenes/Solvatochromic Dyes

Functionalized naphthalenes have applications in a variety of research fields ranging from the synthesis of natural or biologically active molecules to the preparation of new organic dyes. Although numerous strategies have been reported to access naphthalene scaffolds, many procedures still present limitations in terms of incorporating functionality, which in turn narrows the range of available substrates. The development of versatile methods for direct access to substituted naphthalenes is therefore highly desirable.
The Diels-Alder (DA) cycloaddition reaction is a powerful and attractive method for the formation of saturated and unsaturated ring systems from readily available starting materials. A new microwave-assisted intramolecular dehydrogenative DA reaction of styrenyl derivatives described herein generates a variety of functionalized cyclopenta[b]naphthalenes that could not be prepared using existing synthetic methods. When compared to conventional heating, microwave irradiation accelerates reaction rates, enhances yields, and limits the formation of undesired byproducts.
The utility of this protocol is further demonstrated by the conversion of a DA cycloadduct into a novel solvatochromic fluorescent dye via a Buchwald-Hartwig palladium-catalyzed cross-coupling reaction. Fluorescence spectroscopy, as an informative and sensitive analytical technique, plays a key role in research fields including environmental science, medicine, pharmacology, and cellular biology. Access to a variety of new organic fluorophores provided by the microwave-assisted dehydrogenative DA reaction allows for further advancement in these fields.

Microwave-assisted intramolecular dehydrogenative Diels-Alder (DA) reactions provide concise access to functionalized cyclopenta[b]naphthalene building blocks. The utility of this methodology is demonstrated by one-step conversion of the dehydrogenative DA cycloadducts into novel solvatochromic fluorescent dyes via Buchwald-Hartwig palladium-catalyzed cross-coupling reactions.

Functionalized naphthalenes have applications in a variety of research fields ranging from the synthesis of natural or biologically active molecules to ...

Preparation of Hydrophobic Metal-Organic Frameworks via Plasma Enhanced Chemical Vapor Deposition of Perfluoroalkanes for the Removal of Ammonia

Plasma enhanced chemical vapor deposition (PECVD) of perfluoroalkanes has long been studied for tuning the wetting properties of surfaces. For high surface area microporous materials, such as metal-organic frameworks (MOFs), unique challenges present themselves for PECVD treatments. Herein the protocol for development of a MOF that was previously unstable to humid conditions is presented. The protocol describes the synthesis of Cu-BTC (also known as HKUST-1), the treatment of Cu-BTC with PECVD of perfluoroalkanes, the aging of materials under humid conditions, and the subsequent ammonia microbreakthrough experiments on milligram quantities of microporous materials. Cu-BTC has an extremely high surface area (~1,800 m2/g) when compared to most materials or surfaces that have been previously treated by PECVD methods. Parameters such as chamber pressure and treatment time are extremely important to ensure the perfluoroalkane plasma penetrates to and reacts with the inner MOF surfaces. Furthermore, the protocol for ammonia microbreakthrough experiments set forth here can be utilized for a variety of test gases and microporous materials.

Herein the procedures for plasma enhanced chemical vapor deposition of perfluoroalkanes on microporous materials such as metal-organic frameworks to enhance their stability and hydrophobicity are described. Furthermore, breakthrough testing of milligram quantities of samples is described in detail.

Plasma enhanced chemical vapor deposition (PECVD) of perfluoroalkanes has long been studied for tuning the wetting properties of surfaces. For high ...

Preparation and Use of Carbonyl-decorated Carbenes in the Activation of White Phosphorus

Here we present a protocol for the synthesis of two distinct carbonyl-decorated carbenes. Both carbenes can be prepared using nearly identical procedures in multi-gram scale quantities. The goal of this manuscript is to clearly detail how to handle and prepare these unique carbenes such that a synthetic chemist of any skill level can work with them. The two carbenes described are a diamidocarbene (DAC, carbene 1) and a monoamidoaminocarbene (MAAC 2). These carbenes are highly electron-deficient and as such display reactivity profiles that are atypical of more traditional N-heterocyclic carbenes. Additionally, these two carbenes only differ in their electrophilic character and not their steric parameters, making them ideal for studying how carbene electronics influence reactivity. To demonstrate this phenomenon, we are also describing the activation of white phosphorus (P4) using these carbenes. Depending on the carbene used, two very different phosphorus-containing compounds can be isolated. When the DAC 1 is used, a tris(phosphaalkenyl)phosphane can be isolated as the exclusive product. Remarkably however, when MAAC 2 is added to P4 under identical reaction conditions, an unexpected carbene-supported P8&#160;allotrope of phosphorus is isolated exclusively. Mechanistic studies demonstrate that this carbene-supported P8allotrope forms via a [2+2] cycloaddition dimerization of a transient diphosphene which has been trapped by treatment with 2,3-dimethyl-1,3-butadiene.

Here, we present a protocol for the synthesis of two carbonyl-decorated carbenes. The protocol makes these interesting compounds readily available to chemists of all skill levels. In addition to the synthesis of these two carbenes, their use in the activation of white phosphorus is also described.

Here we present a protocol for the synthesis of two distinct carbonyl-decorated carbenes. Both carbenes can be prepared using nearly identical procedures ...

Facile and Efficient Preparation of Tri-component Fluorescent Glycopolymers via RAFT-controlled Polymerization

Synthetic glycopolymers are instrumental and versatile tools used in various biochemical and biomedical research fields. An example of a facile and efficient synthesis of well-controlled fluorescent statistical glycopolymers using reversible addition-fragmentation chain-transfer (RAFT)-based polymerization is demonstrated. The synthesis starts with the preparation of &#946;-galactose-containing glycomonomer 2-lactobionamidoethyl methacrylamide obtained by reaction of lactobionolactone and N-(2-aminoethyl) methacrylamide (AEMA). 2-Gluconamidoethyl methacrylamide (GAEMA) is used as a structural analog lacking a terminal &#946;-galactoside. The following RAFT-mediated copolymerization reaction involves three different monomers: N-(2-hydroxyethyl) acrylamide as spacer, AEMA as target for further fluorescence labeling, and the glycomonomers. Tolerant of aqueous systems, the RAFT agent used in the reaction is (4-cyanopentanoic acid)-4-dithiobenzoate. Low dispersities (&#8804;1.32), predictable copolymer compositions, and high reproducibility of the polymerizations were observed among the products. Fluorescent polymers are obtained by modifying the glycopolymers with carboxyfluorescein succinimidyl ester targeting the primary amine functional groups on AEMA. Lectin-binding specificities of the resulting glycopolymers are verified by testing with corresponding agarose beads coated with specific glycoepitope recognizing lectins. Because of the ease of the synthesis, the tight control of the product compositions and the good reproducibility of the reaction, this protocol can be translated towards preparation of other RAFT-based glycopolymers with specific structures and compositions, as desired.

An efficient, three-step synthesis of RAFT-based fluorescent glycopolymers, consisting of glycomonomer preparation, copolymerization, and post-modification, is demonstrated. This protocol can be used to prepare RAFT-based statistical glycopolymers with desired structures.

Synthetic glycopolymers are instrumental and versatile tools used in various biochemical and biomedical research fields. An example of a facile and ...

Synthesis of Protein Bioconjugates via Cysteine-maleimide Chemistry

The chemical linking or bioconjugation of proteins to fluorescent dyes, drugs, polymers and other proteins has a broad range of applications, such as the development of antibody drug conjugates (ADCs) and nanomedicine, fluorescent microscopy and systems chemistry. For many of these applications, specificity of the bioconjugation method used is of prime concern. The Michael addition of maleimides with cysteine(s) on the target proteins is highly selective and proceeds rapidly under mild conditions, making it one of the most popular methods for protein bioconjugation.
We demonstrate here the modification of the only surface-accessible cysteine residue on yeast cytochrome c with a ruthenium(II) bisterpyridine maleimide. The protein bioconjugation is verified by gel electrophoresis and purified by aqueous-based fast protein liquid chromatography in 27% yield of isolated protein material. Structural characterization with MALDI-TOF MS and UV-Vis is then used to verify that the bioconjugation is successful. The protocol shown here is easily applicable to other cysteine - maleimide coupling of proteins to other proteins, dyes, drugs or polymers.

Synthesis of Protein Bioconjugates via Cysteine-maleimide Chemistry

This protocol details the important steps required for the bioconjugation of a cysteine containing protein to a maleimide, including reagent purification, reaction conditions, bioconjugate purification and bioconjugate characterization.

The chemical linking or bioconjugation of proteins to fluorescent dyes, drugs, polymers and other proteins has a broad range of applications, such as the ...

Preparation of Chitosan-based Injectable Hydrogels and Its Application in 3D Cell Culture

The protocol presents a facile, efficient, and versatile method to prepare chitosan-based hydrogels using dynamic imine chemistry. The hydrogel is prepared by mixing solutions of glycol chitosan with a synthesized benzaldehyde terminated polymer gelator, and hydrogels are efficiently obtained in several minutes at room temperature. By varying ratios between glycol chitosan, polymer gelator, and water contents, versatile hydrogels with different gelation times and stiffness are obtained. When damaged, the hydrogel can recover its appearances and modulus, due to the reversibility of the dynamic imine bonds as crosslinkages. This self-healable property enables the hydrogel to be injectable since it can be self-healed from squeezed pieces to an integral bulk hydrogel after the injection process. The hydrogel is also multi-responsive to many bio-active stimuli due to different equilibration statuses of the dynamic imine bonds. This hydrogel was confirmed as bio-compatible, and L929 mouse fibroblast cells were embedded following standard procedures and the cell proliferation was easily assessed by a 3D cell cultivation process. The hydrogel can offer an adjustable platform for different research where a physiological mimic of a 3D environment for cells is profited. Along with its multi-responsive, self-healable, and injectable properties, the hydrogels can potentially be applied as multiple carriers for drugs and cells in future bio-medical applications.

Here we describe a facile preparation of chitosan-based injectable hydrogels using dynamic imine chemistry. Methods to adjust the hydrogel&#8217;s mechanical strength and its application in 3D cell culture are presented.

The protocol presents a facile, efficient, and versatile method to prepare chitosan-based hydrogels using dynamic imine chemistry. The hydrogel is ...

Determination of Thermodynamic Properties of Alkaline Earth-liquid Metal Alloys Using the Electromotive Force Technique

A novel electrochemical cell based on a CaF2 solid-state electrolyte has been developed to measure the electromotive force (emf) of binary alkaline earth-liquid metal alloys as functions of both composition and temperature in order to acquire thermodynamic data. The cell consists of a chemically stable solid-state CaF2-AF2 electrolyte (where A is the alkaline-earth element such as Ca, Sr, or Ba), with binary A-B alloy (where B is the liquid metal such as Bi or Sb) working electrodes, and a pure A metal reference electrode. Emf data are collected over a temperature range of 723 K to 1,123 K in 25 K increments for multiple alloy compositions per experiment and the results are analyzed to yield activity values, phase transition temperatures, and partial molar entropies/enthalpies for each composition.

This protocol describes the measurement of the electromotive force of alkaline-earth elements in liquid metal alloys at high temperatures (723-1,123 K) to determine their thermodynamic properties, including activity, partial molar entropy, partial molar enthalpy, and phase transition temperatures, over a wide composition range.

A novel electrochemical cell based on a CaF2 solid-state electrolyte has been developed to measure the electromotive force (emf) of binary alkaline ...

Preparation of Stable Bicyclic Aziridinium Ions and Their Ring-Opening for the Synthesis of Azaheterocycles

Bicyclic aziridinium ions were generated by the removal of an appropriate leaving group through internal nucleophilic attack by nitrogen atom in the aziridine ring. The utility of bicyclic aziridinium ions, specifically 1-azoniabicyclo[3.1.0]hexane and 1-azoniabicyclo[4.1.0]heptane tosylate highlighted in the aziridine ring openings by the nucleophile with the release of the ring strain to yield the corresponding ring-expanded azaheterocycles such as pyrrolidine, piperidine and azepane with diverse substituents on the ring in regio- and stereospecific manner. Herein, we report a simple and convenient method for the preparation of the stable 1-azabicyclo[4.1.0]heptane tosylate followed by selective ring opening via a nucleophilic attack either at the bridge or at the bridgehead carbon to yield piperidine and azepane rings, respectively. This synthetic strategy allowed us to prepare biologically active natural products containing piperidine and azepane motif including sedamine, allosedamine, fagomine and balanol in highly efficient manner.

Bicyclic aziridinium ions such as 1-azoniabicyclo[4.1.0]heptane tosylate were generated from 2-[4-tolenesulfonyloxybutyl]aziridine, which was utilized for the preparation of substituted piperidines and azepanes via regio- and stereospecific ring-expansion with various nucleophiles. This highly efficient protocol allowed us to prepare diverse azaheterocycles including natural products such as fagomine, febrifugine analogue and balanol.

Bicyclic aziridinium ions were generated by the removal of an appropriate leaving group through internal nucleophilic attack by nitrogen atom in the ...

Disentangling High Strength Copolymer Aramid Fibers to Enable the Determination of Their Mechanical Properties

Traditionally, soft body armor has been made from poly(p-phenylene terephthalamide) (PPTA) and ultra-high molecular weight polyethylene. However, to diversify the fiber choices in the United States body armor market, copolymer fibers based on the combination of 5-amino-2-(p-aminophenyl) benzimidazole (PBIA) and the more conventional PPTA were introduced. Little is known regarding the long-term stability of these fibers, but as condensation polymers, they are expected to have potential sensitivity to moisture and humidity. Therefore, characterizing the strength of the materials and understanding their vulnerability to environmental conditions is important for evaluating their use lifetime in safety applications. Ballistic resistance and other critical structural properties of these fibers are predicated on their strength. To accurately determine the strength of the individual fibers, it is necessary to disentangle them from the yarn without introducing any damage. Three aramid-based copolymer fibers were selected for the study. The fibers were washed with acetone followed by methanol to remove an organic coating that held the individual fibers in each yarn bundle together. This coating makes it difficult to separate single fibers from the yarn bundle for mechanical testing without damaging the fibers and affecting their strength. After washing, fourier transform infrared (FTIR) spectroscopy was performed on both washed and unwashed samples and the results were compared. This experiment has shown that there are no significant variations in the spectra of poly(p-phenylene-benzimidazole-terephthalamide-co-p-phenylene terephthalamide) (PBIA-co-PPTA1) and PBIA-co-PPTA3 after washing, and only a small variation in intensity for PBIA. This indicates that the acetone and methanol rinses are not adversely affecting the fibers and causing chemical degradation. Additionally, single fiber tensile testing was performed on the washed fibers to characterize their initial tensile strength and strain to failure, and compare those to other reported values. Iterative procedural development was necessary to find a successful method for performing tensile testing on these fibers.

The primary goal of the study is to develop a protocol to prepare consistent specimens for accurate mechanical testing of high strength copolymer aramid fibers, by removing a coating and disentangling the individual fiber strands without introducing significant chemical or physical degradation.

Traditionally, soft body armor has been made from poly(p-phenylene terephthalamide) (PPTA) and ultra-high molecular weight polyethylene. However, to ...

Surface Properties of Synthesized Nanoporous Carbon and Silica Matrices

In this work, we report the synthesis and characterization of ordered nanoporous carbon material (also called ordered mesoporous carbon material [OMC]) with a 4.6 nm pore size, and ordered silica porous matrix, SBA-15, with a 5.3 nm pore size. This work describes the surface properties of nanoporous molecular sieves, their wettability, and the melting behavior of D2O confined in the differently ordered porous materials with similar pore sizes. For this purpose, OMC and SBA-15 with highly ordered nanoporous structures are synthesized via impregnation of the silica matrix by applying a carbon precursor and by the sol-gel method, respectively. The porous structure of investigated systems is characterized by an N2 adsorption-desorption analysis at 77 K. To determine the electrochemical character of the surface of synthesized materials, potentiometric titration measurements are conducted; the obtained results for OMC shows a significant pHpzc shift toward the higher values of pH, relative to SBA-15. This suggests that investigated OMC has surface properties related to oxygen-based functional groups. To describe the surface properties of the materials, the contact angles of liquids penetrating the studied porous beds are also determined. The capillary rise method has confirmed the increased wettability of the silica walls relative to the carbon walls and an influence of the pore roughness on the fluid/wall interactions, which is much more pronounced for silica than for carbon mesopores. We have also studied the melting behavior of D2O confined in OMC and SBA-15 by applying the dielectric method. The results show that the depression of the melting temperature of D2O in the pores of OMC is about 15 K higher relative to the depression of the melting temperature in SBA-15 pores with a comparable 5 nm size. This is caused by the influence of adsorbate/adsorbent interactions of the studied matrices.

Here we report the synthesis and characterization of ordered nanoporous carbon (with a 4.6 nm pore size) and SBA-15 (with a 5.3 nm pore size). The work describes the surface and textural properties of nanoporous molecular sieves, their wettability, and the melting behavior of D2O confined in the materials.

In this work, we report the synthesis and characterization of ordered nanoporous carbon material (also called ordered mesoporous carbon material [OMC]) ...