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Here, we describe protocols for the preparation of trans-cyclobutane fused cyclooctenes (tCBCO), 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. 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.
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 (ΔHp, which is typically negative and favors polymerization) and the entropy of polymerization (ΔSp, which is also typically negative but disfavors polymerization), with the ceiling temperature (Tc) 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 ΔHp and ΔSp 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, ΔHp 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·mol−1, as calculated using DFT). Thermodynamic studies revealed a low ΔHp (−1.7 kcal·mol−1 to −2.8 kcal·mol−1), which was offset by a low ΔSp (−3.6 kcal·mol−1·K−1 to −4.9 kcal·mol−1·K−1), allowing the preparation of high molecular weight polymers (at high monomer concentrations) and near quantitative depolymerization (>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 (≥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.
NOTE: The protocols outlined below are detailed forms of experimental procedures reported previously15,18,19. Characterization of the small molecules and polymers has been reported previously15,18. Additionally, syntheses of monomers and polymers and depolymerization of polymers should be performed inside a fume hood with appropriate personal protective equipment (PPE), including nitrile gloves, safety glasses, and a lab coat.
1. tCBCO monomer preparation15
2. Column chromatography
NOTE: The following is a general procedure for column chromatography as performed for the compounds described herein.
3. Photochemical Isomerization18
NOTE: The photoisomerization was adapted from a literature procedure22.
4. Polymer synthesis
5. Depolymerization
6. Preparation of tensile testing specimens for P315
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 with G1/PPh3 (black)18
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-c...
A patent application (PCT/US2021/050044) has been filed for this work.
We acknowledge funding support from the University of Akron and the National Science Foundation under grant DMR-2042494.
Name | Company | Catalog Number | Comments |
1 and 3 dram vials | VWR | 66011-041, 66011-100 | |
1,4-butanediol | Sigma-Aldrich | 240559-100G | |
1,5-cyclooctadiene | ACROS | AC297120010 | |
1-butanol | Fisher | A399-1 | |
20 mL scintillation vials | VWR | 66022-081 | |
Acetic Anhydride | Alfa-Aesar | AAL042950B | |
Acetone | Fisher | A18-20 | |
Aluminum backed TLC plates | Silicycle | TLA-R10011B-323 | |
Ammonium hydroxide | Fisher | A669-212 | |
Aniline | TCI | A0463500G | |
BD precisionglide (18 G) | Fisher | ||
Chloroform | Fisher | C298-4 | |
Column for circulation (to be packed with silver nitrate treated silica gel) | Approximately 1 cm radius and 25 cm long, with inner thread on either end | ||
d-Chloroform | Cambridge Isotopes | DLM-7-100 | |
Dichloromethane | VWR | BDH1113-19L | |
EDC.HCl; 3-(3-dimethylaminopropyl)-1-ethyl-carbodiimide hydrochloride | Chemimpex | 00050 | |
Ethyl Acetate | Fisher | E145-20 | |
Ethyl Vinyl Ether | Sigma-Aldrich | 422177-250ML | |
Glass chromatography columns | Fabricated in-house | D = 20 mm, L= 450 mm and D = 40 mm, L = 450 mm | 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. |
Grubbs Catalyst 1st Generation (M102) | Sigma-Aldrich | 579726-1G | |
Grubbs Catalyst 2nd Generation (M204) | Sigma-Aldrich | 569747-100MG | |
Hexanes | Fisher | H292-20 | |
Hydraulic press | Carver Instruments | #3912 | Coupled with temperature control modules (see below) |
Hydrochloric acid | Fisher | AA87617K4 | |
Maleic Anhydride | ACROS | AC125240010 | |
Methanol | Fisher | A412-20 | |
Micro essential Hydrion pH paper (1-13 pH) | Fisher | 14-850-120 | |
Normject Luer Lock syringes (1, 3 and 10 mL) | VWR | 89174-491, 53547-014 and 53547-010 | |
Photoreactor chamber | Rayonet | RPR-100 | |
QuadraPure TU (catalyst scavenger) | Sigma-Aldrich | 655422-5G | |
Quartz tubes | Favricated in-house | D=2", L=12.5" and D=1.5", L=10.5" | |
Rotavap | Buchi | ||
SciLog Accu Digital Metering Pump MP- 40 | Parker | 500 mL capacity | |
Siliaflash Irregular Silica, F60 | Silicycle | R10030B-25KG | |
Silver Nitrate | ACROS | AC197680050 | |
Sodium hydroxide | VWR | BDH9292-2.5KG | |
Steel Mold | Fabricated in-house | Overall dimensions of mold cavity: length 20 mm, width 7 mm and depth 1 mm; gauge dimensions: length 10 mm, width 3 mm) | |
Steel Plates | Fabricated in-house | 100 mm x 150 mm x 1 mm | |
Teflon Mold (6-cavities) | Fabricated in-house | Overall cavity dimensions: length 25 mm, width 8.35 mm and depth 0.8 mm; gauge dimensions: length 5 mm, width 2 mm) | |
Teflon Sheets (0.005" thick) | McMaster-Carr | 8569K61 | |
Temperature Control Modules | Omega | C9000A and C9000 | °C units (two modules, one for top and one for bottom) |
Triphenyl Phosphine | TCI | T0519500G | |
UV lamps | Rayonet | RPR2537A and RPR3000A | |
Vacuum pump | Welch Duoseal | ||
Whatman Filter Paper (grade 2) | VWR | 09-810F | filter paper |
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