We acknowledge funding support from the University of Akron and the National Science Foundation under grant DMR-2042494.

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A patent application (PCT/US2021/050044) has been filed for this work.

The tCBCO monomers can be prepared from a common precursor: the [2+2] photocycloadduct of maleic anhydride and 1,5-cyclooctadiene, anhydride 1. Since the crude anhydride 1 is difficult to purify but can be hydrolyzed readily, the crude photoreaction mixture is subjected to methanolysis conditions to yield the readily isolable methyl ester-acid 2. The recrystallization of 2 after column chromatography is key to obtaining the pure trans-cyclobutane isomer of 2. 2 can be readily derivatized to prepare several different tCBCO monomers as outlined here, including the diester monomers M1 and M2, imide monomer M3, and ester crosslinker XL. Additionally, the final esterification step in the preparation of M2 and XL can lead to the formation of a side product that, we hypothesize, differs&#160;only in the relative stereochemistry of the ester groups (cis- for M2 and XL vs. trans- for the side products). Being only slightly lower in polarity than the desired products, care must be taken during the purification of M2 and XL so as to ensure efficient separation and minimize the loss of product. Typically, performing column chromatography under gravity (instead of flash chromatography) yields satisfactory results in this case.
The preparation of the highly strained monomer with the trans-cyclooctene, EM1, provides access to depolymerizable polymers with controlled molecular weight distribution. To achieve this, a photochemical isomerization method employing flow chemistry is utilized. This method shows higher yield and functional group tolerance compared to conventional batch-type photoisomerization. In this flow system, silver nitrate is used to immobilize EM1 in a column. The constant removal of EM1 drives the equilibrium in the irradiated reaction mixture toward EM1 and prevents its photodegradation. Active silver nitrate and proper polarity of the solvent mixture are critical for optimal results. Additionally, the pressure buildup can cause leakages; thus, pre-circulation before irradiation is necessary to locate any leakages. Due to the silver nitrate silica gel and Et2O/hexane solvent mixture, this method is limited to compounds with relatively low polarity and sufficiently high solubility in Et2O/hexane. Further, the trans-olefins in these monomers are reactive and prone to dimerization/decomposition in the presence of acidic impurities23. Additionally, if the monomer is not isolable as a solid, it may be stored as a dilute solution or with a small amount of BHT (~3%-5%) added to prevent radical-induced side reactions; these trans-olefin monomers may also be refrigerated to further prevent degradation24.
The tCBCO monomers can be polymerized to high molecular weights at ambient temperatures by ring-opening metathesis polymerization (ROMP) in the presence of G2. A rather high monomer concentration (~2 M) is needed to achieve this, owing to the low ring strain of the tCBCO monomers. If the monomers prove difficult to dissolve in the solvent at such high concentrations, sonication in an ultrasound bath may be helpful. Under these conditions, the polymerization can be performed to conversions &#62;80% and high molecular weights (Mn &#62; 100 kDa), albeit with broad dispersities (&#272; &#62; 1.5)15.
Monomer EM1, on the other hand, can be polymerized to a high conversion in a short time, even at low initial monomer concentrations. We attribute this to the high ring strain in EM1, resulting in a higher driving force for its polymerization. Depolymerization and cross-metathesis are suppressed by using an excess amount of PPh3 with respect to G1, allowing polymerization to proceed to high conversions while maintaining low &#272; (&#60;1.2). The polymerization shows a living character and can be applied for the synthesis of block copolymers18. The technique is fairly straightforward and robust enough that it can be conducted under ambient conditions by the simple addition of stock solutions. One important note, however, is that PPh3 must be purified (to remove oxidized PPh3 and other impurities) and stored under nitrogen (the purification may be done by recrystallization from ethyl acetate); additionally, care should be taken to dry the glassware before performing this polymerization.
The depolymerization of linear and crosslinked polymers based on this system under mild conditions is also demonstrated. It is interesting that this depolymerization is not restricted to linear polymers only-polymer networks prepared with this system can also be readily depolymerized. This is likely because, while the local concentrations of olefinic groups in the swollen network may be high, chain scission events in the presence of catalyst aid in the degradation and dissolution of the network, following which the fragments further undergo depolymerization. It is critical to quench the catalyst with ethyl vinyl ether after depolymerization prior to evaporating the solvent since the extent of depolymerization may be affected if the active catalyst is still present in the system.
The versatility of this system is further cemented by the range of properties accessible. Here, the preparation of a soft rubbery network, as well as a rigid glassy plastic with the same depolymerizable core, is demonstrated. The preparation of network PN1 may be challenging since it is rather fragile in the swollen state, requiring careful handling when removing it from the mold. Additionally, when performing Soxhlet extraction, highly volatile solvents (like dichloromethane) should be avoided since the rapid evaporation of such solvents may lead to warping and fracture of the sample. Additionally, to avoid such fracture, the swollen network should be allowed to dry in a covered container to slow the evaporation of the solvent. If the dissolution of P3 in DCM during the preparation of dogbone samples proves difficult, an additional solvent may be added in small increments. Further, to avoid defects while preparing dogbone samples with P3, the underfilling of mold cavities should be avoided. High-temperature processing of P3 can also lead to oxidative degradation due to the presence of olefinic groups in the backbone. To prevent this, butylated hydroxytoluene (BHT) may be added to the polymer.
The versatile nature of the tCBCO system lends itself to a diverse range of thermomechanical properties through facile functionalization, which can facilitate the introduction of chemical recyclability to areas where it has been as yet limited, like high-performance thermosets and composites. Additionally, the ability to access living polymerization with this system drastically expands the scope of depolymerizable polymer architectures that can be prepared, including block copolymers and bottlebrush and graft polymers.

The versatile and robust nature of synthetic polymers has made them a ubiquitous fixture of modern human existence. On the flip side, the same robust and environmentally resistant properties make polymer waste exceedingly persistent. This, together with the fact that a large fraction of all synthetic polymers ever made has ended up in landfills1, has raised legitimate concerns about their environmental effects2. Additionally, the open-loop nature of the traditional polymer economy has caused a steady consumption of petrochemical resources and a mounting carbon footprint3. Promising routes to a closed-loop polymer economy are, thus, highly sought after.
Chemical recycling to monomer (CRM) is one such route. The advantage of CRM over traditional recycling is that it leads to the regeneration of monomers that can be used to manufacture pristine polymers, as opposed to mechanical recycling of materials with deteriorating properties over multiple processing cycles. Polymers based on ring-opening polymerizations have appeared as especially attractive routes to CRM materials4. The thermodynamics of polymerization is typically an interplay between two opposing factors: the enthalpy of polymerization (&#916;Hp, which is typically negative and favors polymerization) and the entropy of polymerization (&#916;Sp, which is also typically negative but disfavors polymerization), with the ceiling temperature (Tc)&#160;being the temperature at which these two factors balance each other out5. For a polymer to be capable of CRM under practical and economically beneficial conditions, the right balance of &#916;Hp and &#916;Sp must be achieved. Cyclic monomers allow a convenient means to tune these factors via the selection of the appropriate ring size and geometry, since here, &#916;Hp is primarily determined by the ring strain of the cyclic monomers4,5. As a result, CRM polymers with a wide variety of monomers have been reported of late6,7,8,9,10,11. Out of these systems, ROMP polymers prepared from cyclopentenes are particularly promising due to the rather cheap starting material required and the hydrolytic and thermal stability of the polymers. Additionally, in the absence of a metathesis catalyst, the depolymerization is kinetically unfeasible, affording high thermal stability despite a low Tc12. However, cyclopentenes (and other monomers based on small cyclic structures) pose a key challenge-they cannot be readily functionalized, as the presence of functional groups on the backbone can affect the thermodynamics of polymerization in drastic, and sometimes unpredictable, ways13,14.
Recently, we reported a system that overcomes some of these challenges15. Inspired by examples of low-strain fused ring cyclooctenes in the literature16,17, a new CRM system was designed based on ROMP polymers of trans-cyclobutane fused cyclooctenes (tCBCO) (Figure 1A). The tCBCO monomers could be prepared at a gram scale from the [2+2] photo cycloadduct of maleic anhydride and 1,5-cyclooctadiene, which could be readily functionalized to achieve a diverse set of substituents (Figure 1B). The resulting monomers had ring strains comparable to cyclopentene (~5 kcal&#183;mol&#8722;1, as calculated using DFT). Thermodynamic studies revealed a low &#916;Hp (&#8722;1.7 kcal&#183;mol&#8722;1 to &#8722;2.8 kcal&#183;mol&#8722;1), which was offset by a low &#916;Sp (&#8722;3.6 kcal&#183;mol&#8722;1&#183;K&#8722;1 to &#8722;4.9 kcal&#183;mol&#8722;1&#183;K&#8722;1), allowing the preparation of high molecular weight polymers (at high monomer concentrations) and near quantitative depolymerization (&#62;90%, under dilute conditions) at ambient temperatures in the presence of Grubbs II catalyst (G2). It was also demonstrated that materials with diverse thermomechanical properties could be obtained while preserving the ease of polymerization/depolymerization. This ability was further exploited to prepare a soft elastomeric network (which could also be readily depolymerized), as well as a rigid thermoplastic (with tensile properties comparable to polystyrene).
One drawback with this system was the need for high monomer concentrations to access high molecular weight polymers. At the same time, due to extensive chain transfer and cyclization reactions, the polymerization was uncontrolled in nature. This was addressed in a subsequent work via photochemical isomerization of the Z-alkene in the tCBCO monomers to prepare highly strained E-alkene tCBCO monomers18. These monomers could be rapidly polymerized in a living manner at low initial monomer concentrations (&#8805;25 mM) in the presence of Grubbs I catalyst (G1) and excess triphenylphosphine (PPh3). The polymers could then be depolymerized to yield the Z-alkene form of the monomers. This has created opportunities to access new depolymerizable polymer architectures, including block copolymers and graft/bottlebrush copolymers.
In this work, detailed protocols are outlined for the synthesis of tCBCO monomers with different functional groups and their polymerization, as well as the depolymerization of the resulting polymers. Additionally, protocols for the preparation of dogbone samples of a soft elastomeric network and their depolymerization, as well as compression molding of the N-phenylimide substituted rigid thermoplastic polymer, are also described. Finally, protocols for the photoisomerization of a tCBCO monomer to its strained E-alkene tCBCO form and its subsequent living ROMP are also discussed.

<table><tbody><tr><td>1 and 3 dram vials</td><td>VWR</td><td>66011-041, 66011-100</td><td></td></tr><tr><td>1,4-butanediol</td><td>Sigma-Aldrich</td><td>240559-100G</td><td></td></tr><tr><td>1,5-cyclooctadiene</td><td>ACROS</td><td>AC297120010</td><td></td></tr><tr><td>1-butanol</td><td>Fisher</td><td>A399-1</td><td></td></tr><tr><td>20 mL scintillation vials</td><td>VWR</td><td>66022-081</td><td></td></tr><tr><td>Acetic Anhydride</td><td>Alfa-Aesar</td><td>AAL042950B</td><td></td></tr><tr><td>Acetone</td><td>Fisher</td><td>A18-20</td><td></td></tr><tr><td>Aluminum backed TLC plates</td><td>Silicycle</td><td>TLA-R10011B-323</td><td></td></tr><tr><td>Ammonium hydroxide</td><td>Fisher</td><td>A669-212</td><td></td></tr><tr><td>Aniline</td><td>TCI</td><td>A0463500G</td><td></td></tr><tr><td>BD precisionglide (18 G)</td><td>Fisher</td><td></td><td></td></tr><tr><td>Chloroform</td><td>Fisher</td><td>C298-4</td><td></td></tr><tr><td>Column for circulation (to be packed with silver nitrate treated silica gel)</td><td></td><td></td><td>Approximately 1 cm radius and 25 cm long, with inner thread on either end</td></tr><tr><td>d-Chloroform</td><td>Cambridge Isotopes</td><td>DLM-7-100</td><td></td></tr><tr><td>Dichloromethane</td><td>VWR</td><td>BDH1113-19L</td><td></td></tr><tr><td>EDC.HCl; 3-(3-dimethylaminopropyl)-1-ethyl-carbodiimide hydrochloride</td><td>Chemimpex</td><td>00050</td><td></td></tr><tr><td>Ethyl Acetate</td><td>Fisher</td><td>E145-20</td><td></td></tr><tr><td>Ethyl Vinyl Ether</td><td>Sigma-Aldrich</td><td>422177-250ML</td><td></td></tr><tr><td>Glass chromatography columns</td><td>Fabricated in-house</td><td>D = 20 mm, L= 450 mm and D = 40 mm, L = 450 mm</td><td>The columns are fitted with a teflon stopcock at one end and a 24/40 ground glass joint to accommodate a solvent reservoir if needed.</td></tr><tr><td>Grubbs Catalyst 1st Generation (M102)</td><td>Sigma-Aldrich</td><td>579726-1G</td><td></td></tr><tr><td>Grubbs Catalyst 2nd Generation (M204)</td><td>Sigma-Aldrich</td><td>569747-100MG</td><td></td></tr><tr><td>Hexanes</td><td>Fisher</td><td>H292-20</td><td></td></tr><tr><td>Hydraulic press</td><td>Carver Instruments</td><td>#3912</td><td>Coupled with temperature control modules (see below)</td></tr><tr><td>Hydrochloric acid</td><td>Fisher</td><td>AA87617K4</td><td></td></tr><tr><td>Maleic Anhydride</td><td>ACROS</td><td>AC125240010</td><td></td></tr><tr><td>Methanol</td><td>Fisher</td><td>A412-20</td><td></td></tr><tr><td>Micro essential Hydrion pH paper (1-13 pH)</td><td>Fisher</td><td>14-850-120</td><td></td></tr><tr><td>Normject Luer Lock syringes (1, 3 and 10 mL)</td><td>VWR</td><td>89174-491, 53547-014 and 53547-010</td><td></td></tr><tr><td>Photoreactor chamber</td><td>Rayonet</td><td>RPR-100</td><td></td></tr><tr><td>QuadraPure TU (catalyst scavenger)</td><td>Sigma-Aldrich</td><td>655422-5G</td><td></td></tr><tr><td>Quartz tubes</td><td>Favricated in-house</td><td>D=2&quot;, L=12.5&quot; and D=1.5&quot;, L=10.5&quot;</td><td></td></tr><tr><td>Rotavap</td><td>Buchi</td><td></td><td></td></tr><tr><td>SciLog Accu Digital Metering Pump MP- 40</td><td>Parker</td><td></td><td>500 mL capacity</td></tr><tr><td>Siliaflash Irregular Silica, F60</td><td>Silicycle</td><td>R10030B-25KG</td><td></td></tr><tr><td>Silver Nitrate</td><td>ACROS</td><td>AC197680050</td><td></td></tr><tr><td>Sodium hydroxide</td><td>VWR</td><td>BDH9292-2.5KG</td><td></td></tr><tr><td>Steel Mold</td><td>Fabricated in-house</td><td>Overall dimensions of mold cavity: length 20 mm, width 7 mm and depth 1 mm; gauge dimensions: length 10 mm, width 3 mm)</td><td></td></tr><tr><td>Steel Plates</td><td>Fabricated in-house</td><td>100 mm x 150 mm x 1 mm</td><td></td></tr><tr><td>Teflon Mold (6-cavities)</td><td>Fabricated in-house</td><td>Overall cavity dimensions: length 25 mm, width 8.35 mm and depth 0.8 mm; gauge dimensions: length 5 mm, width 2 mm)</td><td></td></tr><tr><td>Teflon Sheets (0.005&quot; thick)</td><td>McMaster-Carr</td><td>8569K61</td><td></td></tr><tr><td>Temperature Control Modules</td><td>Omega</td><td>C9000A and C9000</td><td>&amp;deg;C units (two modules, one for top and one for bottom)</td></tr><tr><td>Triphenyl Phosphine</td><td>TCI</td><td>T0519500G</td><td></td></tr><tr><td>UV lamps</td><td>Rayonet</td><td>RPR2537A and RPR3000A</td><td></td></tr><tr><td>Vacuum pump</td><td>Welch Duoseal</td><td></td><td></td></tr><tr><td>Whatman Filter Paper (grade 2)</td><td>VWR</td><td>09-810F</td><td>filter paper</td></tr></tbody></table>

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.

Discussed here are representative results previously published15,18,19. Figure 5 shows the GPC traces for polymer P1 prepared by conventional ROMP with G2 (red curve)15 and living ROMP of EM1&#160;with G1/PPh3 (black)18. The polymer prepared by living ROMP has a much narrower molecular weight distribution (Mn = 114.9 KDa, &#208; = 1.17) versus the rather broad distribution seen for the polymer prepared by conventional ROMP with G2 (Mn = 142 KDa, &#208; = 1.55).
1H NMR spectra for the depolymerization of linear (P1) and crosslinked (PN1) polymers are given in Figure 6. The extent of depolymerization of P1 is measured by calculating the ratio of the integral of the peaks corresponding to monomeric olefinic protons with respect to the sum of the peak integrals of the monomer and residual oligomer olefinic protons (as indicated in Figure 6A). Under the dilute conditions and in the presence of 1 mol% G2, P1 is depolymerized nearly quantitatively (~93%). The extent of depolymerization of PN1 is calculated similarly and amounts to ~94% (Figure 6B). It must be noted here that, for PN1, &#34;monomers&#34; refers to the mixture of monofunctional monomer and crosslinkers (M2 and XL, respectively) obtained after depolymerization.
Figure 7 shows the representative tensile curves (these data are from previously published work15) for polymer P3 and networks PN1. The presence of the flexible butyl chains in M2 causes PN1 to be a soft, elastomeric material with an ultimate tensile strain of ~0.64 MPa, modulus of ~ 0.76 MPa, and strain at break of ~226%.
On the other hand, polymer P3 with the rigid phenyl imide substituent behaves like a rigid glassy material with an ultimate tensile strength of ~41.4 MPa and strain at break of ~3.4%. Tensile testing was performed for P3 with an Instron Universal Testing Frame, while that for PN1 was performed with a homemade tensile tester, both at a crosshead speed of 5 mm&#183;min&#8722;1.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/64182/64182fig01v2.jpg" /> 
Figure 1: tCBCO monomers for depolymerizable olefinic polymers. (A) tCBCO monomers for chemically recyclable polymers. (B) Synthesis of tCBCO monomers. Photochemical [2 + 2] cycloaddition of 1,5-cyclooctadiene and maleic anhydride affords the anhydride 1, which can be readily converted to M1 and XL, M2, and M3 through conditions (i), (ii), and (iii), respectively. (i) M1: MeOH, reflux; MeOH, EDC, DMAP, DCM; XL:&#160;1,4-butanediol, EDC, DMAP, DCM. (ii) M2: NaOH, H2O, 60 &#176;C; 1-butanol, EDC, DMAP, DCM. (iii) M3: aniline, acetone; sodium acetate, acetic anhydride, 100 &#176;C. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/64182/64182fig01largev2.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/64182/64182fig02.jpg" /> 
Figure 2: Reaction schemes for small molecule and polymer synthesis outlined in this work. (A) Synthesis of tCBCO small molecules and monomers. (B) Synthesis of P1 by conventional ROMP. (C) Synthesis of P1 by living ROMP. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/64182/64182fig02large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/64182/64182fig03.jpg" /> 
Figure 3. Reaction setup for photochemical isomerization of M1. The photoisomerization of M1 to EM1 involves irradiation under flow conditions, and the setup consists of a photoreactor housing the quartz reaction tube, a column packed with AgNO3-impregnated silica (to trap the product), and a metering pump to enable the flow of the reaction mixture. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/64182/64182fig03large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/64182/64182fig04.jpg" /> 
Figure 4: Molds used for compression molding of P3 and preparation of PN1. (A) Steel mold for compression molding of P3 and (B) PTFE mold for curing elastomer network PN1. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/64182/64182fig04large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/64182/64182fig05.jpg" /> 
Figure 5: GPC traces for polymer. GPC traces for polymer P1 prepared by living ROMP in the presence of G1 and PPh3 (black) and conventional ROMP in the presence of G2 (red). This figure has been prepared from previously published data (red trace from Sathe&#160;et al.15, black trace from Chen et al.18). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/64182/64182fig05large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/64182/64182fig06.jpg" /> 
Figure 6: Depolymerization of tCBCO based polymers. (A) Depolymerization reaction scheme and stacked partial 1H NMR spectra of (B) polymer P1 after depolymerization (black), polymer P1 before depolymerization (blue), and monomer M1 (red) and (C) network PN1 after depolymerization (black), crosslinker XL (blue), and monomer M2 (red). This figure has been prepared from previously published data (data for B are from Sathe&#160;et al.19, data for C are from Sathe&#160;et al.15). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/64182/64182fig06large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/64182/64182fig07.jpg" /> 
Figure 7: Stress vs. strain curves. (A) Polymer network PN1 and (B) polymer P3. This figure has been prepared from previously published data from Sathe&#160;et al.15. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/64182/64182fig07large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Depolymerizable Olefinic Polymers Based on Fused-Ring Cyclooctene Monomers at JoVE.com

Depolymerizable Olefinic Polymers Based on Fused-Ring Cyclooctene Monomers

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. ...

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3.2K Views.  University of Akron. Dit protocol beschrijft de synthese, karakterisering en de chemische recycling van een chemisch recyclebaar polymeersysteem dat onze lamp ontwikkelt, waarvan we denken dat het het potentieel heeft om de uitdagingen aan te gaan die gepaard gaan met het duurzame gebruik van kunststoffen. Vergeleken met chemisch recyclebare polymeren die tot nu toe zijn ontwikkeld, is het voordeel van ons systeem dat het de chemische recycling van polymeren met koolwaterstofbackbone mogelijk maakt, die een uitst...

Video: Depolymerizable Olefinic Polymers Based on Fused-Ring Cyclooctene Monomers

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.

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 ...

Synthesis of Cyclic Polymers and Characterization of Their Diffusive Motion in the Melt State at the Single Molecule Level

We demonstrate a method for the synthesis of cyclic polymers and a protocol for characterizing their diffusive motion in a melt state at the single molecule level. An electrostatic self-assembly and covalent fixation (ESA-CF) process is used for the synthesis of the cyclic poly(tetrahydrofuran) (poly(THF)). The diffusive motion of individual cyclic polymer chains in a melt state is visualized using single molecule fluorescence imaging by incorporating a fluorophore unit in the cyclic chains. The diffusive motion of the chains is quantitatively characterized by means of a combination of mean-squared displacement (MSD) analysis and a cumulative distribution function (CDF) analysis. The cyclic polymer exhibits multiple-mode diffusion which is distinct from its linear counterpart. The results demonstrate that the diffusional heterogeneity of polymers that is often hidden behind ensemble averaging can be revealed by the efficient synthesis of the cyclic polymers using the ESA-CF process and the quantitative analysis of the diffusive motion at the single molecule level using the MSD and CDF analyses.

A protocol for the synthesis and characterization of diffusive motion of cyclic polymers at the single molecule level is presented.

We demonstrate a method for the synthesis of cyclic polymers and a protocol for characterizing their diffusive motion in a melt state at the single ...

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

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

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

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

Self-assembling Morphologies Obtained from Helical Polycarbodiimide Copolymers and Their Triazole Derivatives

A facile method for the preparation of polycarbodiimide-based secondary structures (e.g., nano-rings, &#34;craters,&#34; fibers, looped fibers, fibrous networks, ribbons, worm-like aggregates, toroidal structures, and spherical particles) is described. These aggregates are morphologically influenced by extensive hydrophobic side chain-side chain interactions of the singular polycarbodiimide strands, as inferred by atomic force microscopy (AFM) and scanning electron microscopy (SEM) techniques. Polycarbodiimide-g-polystyrene copolymers (PS-PCDs) were prepared by a combination of synthetic methods, including coordination-insertion polymerization, copper(I)-catalyzed azide alkyne cycloaddition (CuAAC) &#34;click&#34; chemistry, and atom transfer radical polymerization (ATRP). PS-PCDs were found to form specific toroidal architectures at low concentrations in CHCl3. To determine the influence of a more polar solvent medium (i.e., THF and THF/EtOH) on polymer aggregation behavior, a number of representative PS-PCD composites have been tested to show discrete concentration-dependent spherical particles. These fundamental studies are of practical interest to the development of experimental procedures for desirable architectures by directed self-assembly in thin film. These architectures may be exploited as drug carriers, whereas other morphological findings represent certain interest in the area of novel functional materials.

Here, we present a protocol to prepare and visualize secondary structures (e.g., fibers, toroidal architectures, and nano-spheres) derived from helical polycarbodiimides. The morphology characterized by both atomic force microscopy (AFM) and scanning electron microscopy (SEM) was shown to depend on molecular structure, concentration, and the solvent of choice.

A facile method for the preparation of polycarbodiimide-based secondary structures (e.g., nano-rings, &#34;craters,&#34; fibers, looped fibers, fibrous ...

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, ...

Designed for Molecular Recycling: A Lignin-Derived Semi-aromatic Biobased Polymer

The development of chemically recyclable biopolymers offers opportunities within the pursuit of a circular economy. Chemically recyclable biopolymers make a positive effort to solve the issue of polymer materials in the disposal phase after the use phase. In this paper, the production of biobased semi-aromatic polyesters, which can be extracted entirely from biomass such as lignin, is described and visualized. The polymer poly-S described in this paper has thermal properties similar to certain commonly used plastics, such as PET. We developed a Green Knoevenagel reaction, which can efficiently produce monomers from aromatic aldehydes and malonic acid. This reaction has been proven to be scalable and has a remarkably low calculated E-factor. These polyesters with ligno-phytochemicals as a starting point show an efficient molecular recycling with minimal losses. The polyester poly(dihydrosinapinic acid) (poly-S) is presented as an example of these semi-aromatic polyesters, and the polymerization, depolymerization, and re-polymerization are described.

An example of a closed-loop approach towards a circular materials economy is described here. A whole sustainable cycle is presented where biobased semi-aromatic polyesters are designed by polymerization, depolymerization, and then re-polymerized with only slight changes in their yields or final properties.

The development of chemically recyclable biopolymers offers opportunities within the pursuit of a circular economy. Chemically recyclable biopolymers make ...

Kinetics of Addition Polymerization to Polydimethylsiloxane

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

Synthesis of Polydimethylsiloxane via Addition Polymerization

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

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Free-Radical Chain Reaction and Polymerization of Alkenes

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

Free-radical Polymerization Reaction of Alkenes

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

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Olefin Metathesis Polymerization: Overview

Recently, the development of olefin metathesis polymerization advanced the field of polymer synthesis. Simply put, the reorganization of substituents on their double&#160;bonds between two olefins in the presence of a catalyst is known as the olefin metathesis reaction. The use of metathesis reaction for polymer synthesis is called olefin metathesis polymerization.
Ruthenium-based Grubbs catalyst is the most commonly used catalyst for olefin metathesis polymerization. Grubbs catalyst consists of a carbon-metal double bond, also known as carbene.
The olefin metathesis reaction follows the reversible mechanism as shown in the figure below:
<img alt="Figure1" src="/files/ftp_upload/12934/12934_PT_Figure_1.svg" style="text-align: center; height: 300px;" />
The general mechanism involves the reaction of Grubbs catalyst with an alkene in a [2+2] cycloaddition giving a four-membered metallacyclobutane intermediate which immediately undergoes a ring-opening reaction in a reverse manner to give back the starting material or by breaking different bonds to form a different carbene or a catalyst and a different alkene product.
The new catalyst undergoes [2+2] cycloaddition with a second alkene to form a new metallacyclobutane, giving the metathesis product and a new carbene complex. This new carbene complex is then ready to attack another molecule of starting material, and the cycle is repeated.
There are several ways the olefin metathesis has been implemented for polymer synthesis. However, ring-opening metathesis polymerization (ROMP) and acyclic diene metathesis (ADMET) are the most widely used for polymerization. ROMP is a chain growth, addition polymerization reaction, whereas ADMET is a step-growth, condensation polymerization reaction.

Recently, the development of olefin metathesis polymerization advanced the field of polymer synthesis. Simply put, the reorganization of substituents on ...

Synthetic Polymers

Olefin Metathesis Polymerization: Ring-Opening Metathesis Polymerization (ROMP)

Ring-opening metathesis polymerization or ROMP involves strained cycloalkenes as starting materials. The mechanism of ROMP proceeds by reacting cycloalkene with Grubbs catalyst to give metallacyclobutane intermediate which undergoes a ring-opening reaction to form new carbene. The new carbene reacts with another molecule of cycloalkene. Repetition of these steps leads to the formation of an unsaturated open-chain polymer product. All these steps are reversible, however, relieving the ring strain drives the reaction forward.
The characteristic feature of the ROMP is that the unsaturation present in the reactants is conserved in the products. The reactivity of cycloalkenes towards ROMP decreases with an increase in the ring size.
Poly(phenylene vinylene) (PPV), an important polymer having alternating phenyl and vinyl groups used for electro-optical applications, is prepared by ROMP reaction. Acetyl-substituted bicyclooctadiene undergoes ROMP in the presence of Grubbs catalyst and gives processable polymer which upon heating loses two moles of acetic acid and aromatizes to form poly(phenylene vinylene) polymer.
<img alt="Figure1" src="/files/ftp_upload/12935/12935_PT_Figure_1.svg" style="text-align: center; height: 95px;" />
Figure 1. ROMP synthesis of poly(phenylene vinylene).

Depolymerizable Olefinic Polymers Based on Fused-Ring Cyclooctene Monomers

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