Chemically Recyclable CO2 -Based Solid Polyesters with Facile Property Tunability

Bifunctional and recyclable polyesters by chemoselective ring-opening polymerization of a δ-lactone derived from CO2 and butadiene

When aiming at the direct use of CO2 for the preparation of advanced/value-added materials, the synthesis of CO2/olefin copolymers is very appealing but challenging. The δ-lactone 3-ethylidene-6-vinyltetrahydro-2H-pyran-2-one (EVP), synthesized by telomerization of CO2 with 1,3-butadiene, is a promising monomer. However, its chemoselective ring-opening polymerization (ROP) is hampered by unfavorable thermodynamics and the competitive polymerization of highly reactive C=C double bonds under usual conditions. Herein, we report the chemoselective ROP of EVP using a phosphazene/urea binary catalyst, affording exclusively a linear unsaturated polyester poly(EVP)ROP, with a molar mass (Mn) up to 16.1 kg·mol-1 and a narrow distribution (Ð < 1.6), which can be fully recycled back to the pristine monomer, thus establishing a monomer-polymer-monomer closed-loop life cycle. In these polyesters, the CO2 content reaches 33 mol% (29 wt%). The reasons for the unexpected chemoselectivity were investigated by Density-functional theory (DFT) calculations. The poly(EVP)ROP features two pendent C=C double bonds per repeating unit, which show distinct reactivity and thus can be properly engaged in sequential functionalizations towards the synthesis of bifunctional polyesters. We disclose here a methodology providing a facile access to bifunctional and recyclable polyesters from readily available feedstocks.

Monomer-Recyclable Polyester from CO2 and 1,3-Butadiene

Synthesis of monomer-recyclable polyesters solely from CO2 and bulk olefins holds great potential in significantly reducing CO2 emissions and addressing the issue of plastic pollution. Due to the kinetic disadvantage of direct copolymerization of CO2 and bulk olefins compared to homopolymerization of bulk olefins, considerable research attention has been devoted to synthesis of polyester via the ring-opening polymerization (ROP) of a six-membered disubstituted lactone intermediate, 1,2-ethylidene-6-vinyl-tetrahydro-2H-pyran-2-one (𝜹-L), obtained from telomerization of CO2 and 1,3-butadiene. However, the conjugate olefin on the six-membered ring of 𝜹-L leads to serious Michael addition side reactions. Thus, the selective ROP of 𝜹-L, which can precisely control the repeating unit for the production of polyesters potentially amenable to efficient monomer recycling, remains an unresolved challenge. Herein, the first example of selective ROP of 𝜹-L is reported using a combination of organobase and N,N'-Bis[3,5-bis(trifluoromethyl)phenyl]urea as the catalytic system. Systematic modifications of the substituent of the urea show that the presence of electron-deficient 3,5-bis(trifluoromethyl)-phenyl groups is the key to the extraordinary selectivity of ring opening over Michael addition. Efficient monomer recovery of oligo(𝜹-L) is also achieved under mild catalytic conditions.

Sustainable Copolymer Synthesis from Carbon Dioxide and Butadiene

Carbon dioxide (CO2) has long been recognized as an ideal C1 feedstock comonomer for producing sustainable materials because it is renewable, abundant, and cost-effective. However, activating CO2 presents a significant challenge because it is highly oxidized and stable. A CO2/butadiene-derived δ-valerolactone (EVP), generated via palladium-catalyzed telomerization between CO2 and butadiene, has emerged as an attractive intermediate for producing sustainable copolymers from CO2 and butadiene. Owing to the presence of two active carbon-carbon double bonds and a lactone unit, EVP serves as a versatile intermediate for creating sustainable copolymers with a CO2 content of up to 29 wt % (33 mol %). In this Review, advances in the synthesis of copolymers from CO2 and butadiene with divergent structures through various polymerization protocols have been summarized. Achievements made in homo- and copolymerization of EVP or its derivatives are comprehensively reviewed, while the postmodification of the obtained copolymers to access new polymers are also discussed. Meanwhile, potential applications of the obtained copolymers are also discussed. The literature references were sorted into sections based on polymerization strategies and mechanisms, facilitating readers in gaining a comprehensive view of the present chemistry landscape and inspiring innovative approaches to synthesizing novel CO2-derived copolymers.

Functional and Degradable Polyester-co-polyethers from CO2, Butadiene, and Epoxides

Carbon dioxide (CO2), as a renewable and nontoxic C1 feedstock, has been recognized as an ideal comonomer to prepare sustainable materials. In this regard, substantial focus has been dedicated to the ring-opening copolymerization of CO2 and epoxides, which results in the creation of aliphatic polycarbonates in most cases. Here, we report an unprecedented strategy to synthesize functional and degradable polyester-co-polyethers from CO2, butadiene, and epoxides via a CO2/butadiene-derived δ-valerolactone intermediate (EVP). Utilizing a chromium salen complex as the catalyst, the copolymerization of EVP and epoxides was successfully achieved to produce CO2/butadiene/epoxide terpolymers. The obtained polyester-co-polyethers with varied 39-93 mol % EVP content (equal to 18-28 wt % CO2 incorporation) show high thermal stability, tunable glass-transition temperatures, on-demand functionality, and good chemical degradability. This method extends the potential to access functional CO2-based polymers.

Advances in the Synthesis of Chemically Recyclable Polymers

The development of modern society is closely related to polymer materials. However, the accumulation of polymer materials and their evolution in the environment causes not only serious environmental problems, but also waste of resources. Although physical processing can be used to reuse polymers, the properties of the resulting polymers are significantly degraded. Chemically recyclable polymers, a type of polymer that degrades into monomers, can be an effective solution to the degradation of polymer properties caused by physical recycling of polymers. The ideal chemical recycling of polymers, i. e., quantitative conversion of the polymer to monomers at low energy consumption and repolymerization of the formed monomers into polymers with comparable properties to the original, is an attractive research goal. In recent years, significant progress has been made in the design of recyclable polymers, enabling the regulation of the "polymerization-depolymerization" equilibrium and closed-loop recycling under mild conditions. This review will focus on the following aspects of closed-loop recycling of poly(sulfur) esters, polycarbonates, polyacetals, polyolefins, and poly(disulfide) polymer, illustrate the challenges in this area, and provide an outlook on future directions.