“Nonstrained” γ-Butyrolactone-Based Copolyesters: Copolymerization Characteristics and Composition-Dependent (Thermal, Eutectic, Cocrystallization, and Degradation) Properties

Photochemical [2 + 2] Cycloaddition Enables the Synthesis of Highly Thermally Stable and Acid/Base-Resistant Polyesters from a Nonpolymerizable α,β-Conjugated Valerolactone

We report a simple strategy to transform a nonpolymerizable six-membered α,β-conjugated lactone, 5,6-dihydro-2H-pyran-2-one (DPO), into polymerizable bicyclic lactones via photochemical [2 + 2] cycloaddition. Two bicyclic lactones, M1 and M2, were obtained by the photochemical [2 + 2] cycloaddition of tetramethylethylene and DPO. Ring-opening polymerization (ROP) of M1 and M2 catalyzed by diphenyl phosphate (DPP), La[N(SiMe3)2]3, and 1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris (dimethylamino) phosphoranylide-namino]-2λ5, 4λ5-catenadi(phosphazene) (tBu-P4) were conducted. M1 is highly polymerizable, either DPP or La[N(SiMe3)2]3 could catalyze its living ROP under mild conditions, affording the well-defined PM1 with a predictable molar mass and low dispersity. M2 could only be polymerized with tBu-P4 as the catalyst, also generating the same polymer PM1. PM1 has high thermal stability, with a Td,5% being up to 376 °C. Ring-opening copolymerization (ROcP) of M1 and δ-valerolactone (δ-VL) catalyzed by La[N(SiMe3)2]3 afforded a series of random copolymers with enhanced thermal stabilities. Both PM1 and the copolymer containing 10 mol % M1 exhibited excellent resistance to acidic and basic hydrolysis. Our results demonstrate that direct photochemical [2 + 2] cycloaddition of α,β-conjugated valerolactone is not only a strategy to tune its polymerizability, but also allows for the synthesis of highly thermally stable aliphatic polyesters, inaccessible by other methods.

Achieving Exceptional Thermal and Hydrolytic Resistance in Chemically Circular Polyesters via In-Chain 1,3-Cyclobutane Rings

Polyesters, a highly promising class of circular polymers for achieving a closed-loop sustainable plastic economy, inherently exhibit material stability defects, especially in thermal and hydrolytic instability. Here, we introduce a class of polyesters, P(4R-BL) (R=Ph, Bu), featuring conformationally rigid 1,3-cyclobutane rings in the backbone. These polyesters not only exhibit superior thermostability (Td,5%=376-380 °C) but also demonstrate exceptional hydrolytic resistance with good integrity even after 1 year in basic and acidic aqueous solutions, distinguishing themselves from typical counterparts. Tailoring the flexibility of the side group R enables the controlled thermal and mechanical performance of P(4Ph-BL) and P(4Bu-BL) to rival durable syndiotactic polystyrene (SPS) and low-density polyethylene (LDPE), respectively. Significantly, despite their high stability, both polyesters can be effectively depolymerized into pristine monomers, establishing a circular life cycle.

Fully Bio-based Poly(ketal-ester)s by Ring-opening Polymerization of a Bicylcic Lactone from Glycerol and Levulinic Acid

A fully renewable bio-based bicyclic lactone containing a five-membered cyclic ketal moiety, 7-methyl-3,8,10-trioxabicyclo[5.2.1]decan-4-one (TOD), was synthesized through a two-step acid-catalyzed process from glycerol and levulinic acid. The ring-opening polymerization (ROP) of TOD at 30°C with benzyl alcohol (BnOH) as the initiator and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as the catalyst can afford high molar mass PTOD with a cis-2.4-disubstitued 2-methyl 1,3-dioxolane moiety in its repeating unit. PTOD is an amorphous polymer with a glass transition temperature (T_g ) of 13°C. It can be hydrolyzed into structurally defined small molecules under acidic or basic conditions by the selective cleavage of either the cyclic ketal or the ester linkage respectively. The TBD-catalyzed copolymerization of L-lactide (L-LA) and TOD at -20°C was investigated. It was confirmed that L-LA polymerized quickly with racemization to form PLA, followed by a slow incorporation of TOD into the formed PLA chains via transesterification. By varying the feed ratios of L-LA to TOD, a series of random copolymers (PLA-co-PTOD) with different TOD incorporation ratios and tunable T_g s were obtained. Under acidic conditions, PLA-co-PTOD degrades much faster than PLA via the selective cleavage of the cyclic ketal linkages. This work provides insights for the development of more sustainable and acid-accelerated degradable alternatives to aliphatic polyesters.

Tuning the Properties of Ester-Based Degradable Polymers by Inserting Epoxides into Poly(ϵ-caprolactone)

A series of ester-ether copolymers were obtained via the reaction between α,ω-dihydroxyl poly(ϵ-caprolactone) (PCL) and ethylene oxide (EO) or monosubstituted epoxides catalyzed by strong phosphazene bases. The two types of monomeric units were distributed in highly random manners due to the concurrence of epoxide ring-opening and fast transesterification reactions. The substituent of epoxide showed an interesting bidirectional effect on the enzymatic degradability of the copolymer. Compared with PCL, copolymers derived from EO exhibited enhanced hydrophilicity and decreased crystallinity which then resulted in higher degradability. For the copolymers derived from propylene oxide and 1,2-butylene oxide, the hydrophobic alkyl pendant groups also allowed lower crystallinity of the copolymers thus higher degradation rates. However, further enlarging the pendant groups by using styrene oxide or 2-ethylhexyl glycidyl ether caused a decrease in the degradation rate, which might be ascribed to the higher bulkiness hindering the contact of ester groups with lipase.

Alternating Isotactic Polyhydroxyalkanoates via Site- and Stereoselective Polymerization of Unsymmetrical Diolides

Naturally produced, biodegradable polyhydroxyalkanoates (PHAs) promise more sustainable alternatives to nonrenewable/degradable plastics, but biological PHA's stereomicrostructures are strictly confined to isotactic (R)-polymers or copolymers of random sequences. Chemical synthesis via catalyzed ring-opening polymerization (ROP) of cyclic (di)esters offers expedient access to diverse PHA microstructures, including those with defined comonomer sequences and tacticities. However, the synthesis of alternating isotactic PHAs has not been achieved by the existing methodologies. Here, we report the design of unsymmetrically disubstituted eight-membered diolides (rac-8DL^R1-R2) and their site- and stereoselective ROP with discrete chiral catalysts, enabling the synthesis of alternating isotactic PHAs, poly(3-hydroxybutyrate-alt-3-hydroxyvalerate) (alt-P3HBV) and poly(3-hydroxybutyrate-alt-3-hydroxyheptanoate) (alt-P3HBHp), with high to quantitative (>99%) alternation and isotacticity and M_n up to 113 kDa and Đ = 1.01. Physical properties of such PHAs are substantially determined by the degree of backbone sequence alternation and tacticity, ranging from amorphous to semi-crystalline materials. The alt-P3HBV shows significantly improved mechanical performance relative to the constituent homopolymers. Intriguingly, enantiomeric (R)-alt-P3HBV and (S)-alt-P3HBV, synthesized by kinetically resolved ROP of rac-8DL^Me-Et, form a stereocomplex with a significantly enhanced T_m (by 53 °C), while the enantiomeric homopolymers do not form a stereocomplex.

Towards high-performance sustainable polymers via isomerization-driven irreversible ring-opening polymerization of five-membered thionolactones

The development of sustainable polymers that possess useful material properties competitive with existing petroleum-derived polymers is a crucial goal but remains a formidable challenge for polymer science. Here we demonstrate that irreversible ring-opening polymerization (IROP) of biomass-derived five-membered thionolactones is an effective and robust strategy for the polymerization of non-strained five-membered rings-these polymerizations are commonly thermodynamically forbidden under ambient conditions, at industrially relevant temperatures of 80-100 °C. Computational studies reveal that the selective IROP of these thionolactones is thermodynamically driven by S/O isomerization during the ring-opening process. IROP of γ-thionobutyrolactone, a representative non-strained thionolactone, affords a sustainable polymer from renewable resources that possesses external-stimuli-triggered degradability. This poly(thiolactone) also exhibits high performance, with its key thermal and mechanical properties comparing well to those of commercial petroleum-based low-density polyethylene. This IROP strategy will enable conversion of five-membered lactones, generally unachievable by other polymerization methods, into sustainable polymers with a range of potential applications.

Access to high-molecular-weight poly(γ-butyrolactone) by using simple commercial catalysts

The simple commercial organomagnesium catalysts were utilized for efficient access to high-molecular-weight poly(γ-butyrolactone) and facile manipulation of the reaction conditions enabled the polymer topology controlled.

Advances, Challenges, and Opportunities of Poly(γ-butyrolactone)-Based Recyclable Polymers

The discovery and prosperous growth of synthetic polymers have presented both significant advantages and daunting challenges in the last century. To address the issues of environmental pollution and fossil consumption, recyclable, degradable, and/or biobased polymers have been given much attention in the polymer science community. This viewpoint focuses on the emerging fully chemical recyclable poly(γ-butyrolactone)-based polymers. The breakthrough from nonpolymerizable to efficient polymerization is highlighted by the benefits of the development of a series of catalysis for ring-opening polymerization of γ-butyrolactone. Subsequently, the design of γ-butyrolactone derivatives and synthesis of more recyclable polymers are summarized together with the discussions about the structure and property relationship. Finally, the remaining challenges and promising opportunities are suggested in order to provide insights into the further direction for sustainable polymers.

γ-Butyrolactone Copolymerization with the Well-Documented Polymer Drug Carrier Poly(ethylene oxide)-block-poly(ε-caprolactone) to Fine-Tune Its Biorelevant Properties

Polymeric drug carriers exhibit excellent properties that advance drug delivery systems. In particular, carriers based on poly(ethylene oxide)-block-poly(ε-caprolactone) are very useful in pharmacokinetics. In addition to their proven biocompatibility, there are several requirements for the efficacy of the polymeric drug carriers after internalization, e.g., nanoparticle behavior, cellular uptake, the rate of degradation, and cellular localization. The introduction of γ-butyrolactone units into the hydrophobic block enables the tuning of the abovementioned properties over a wide range. In this study, a relatively high content of γ-butyrolactone units with a reasonable yield of ≈60% is achieved by anionic ring-opening copolymerization using 1,5,7-triazabicyclo[4.4.0]dec-5-ene as a very efficient catalyst in the nonpolar environment of toluene with an incorporated γ-butyrolactone content of ≈30%. The content of γ-butyrolactone units can be easily modulated according to the feed ratio of the monomers. This method enables control over the rate of degradation so that when the content of γ-butyrolactone increases, the rate of degradation increases. These findings broaden the application possibilities of polyester-polyether-based nanoparticles for biomedical applications, such as drug delivery systems.

Zwitterionic copolymerization of γ-butyrolactone with 3,3-bis(chloromethyl) oxacyclobutane catalyzed by scandium triflates

Zwitterionic copolymerization of γ-butyrolactone with 3,3-bis(chloromethyl) oxacyclobutane is catalyzed by scandium triflates generating random linear and cyclic copolymer.

Turning natural δ-lactones to thermodynamically stable polymers with triggered recyclability

Extending the use of natural δ-lactones in circular materials via a synthetic strategy yielding thermodynamically stable polyesters with triggered recyclability.

Fast, selective and metal-free ring-opening polymerization to synthesize polycarbonate/polyester copolymers with high incorporation of ethylene carbonate using an organocatalytic phosphazene base

Polycarbonate/polyester copolymers with high incorporation of EC were realized by a fast and selective process using a metal-free catalyst.