Guanidine and Amidine Organocatalysts for Ring-Opening Polymerization of Cyclic Esters

Advancing H-Bonding Organocatalysis for Ring-Opening Polymerization: Intramolecular Activation of Initiator/Chain End

Organocatalytic ring-opening polymerization (ROP) of lactones is a green method for accessing renewable and biodegradable polyesters. Developing new organocatalysts with high activity and controllability is a major and challenging research topic in this field. Here, we report a series of organocatalysts to achieve a fast and controlled ROP of lactones. These catalysts incorporate (thio)urea and alkoxide in one molecule and act as initiators in the ROP. Such catalysts enable an effective intramolecular activation of initiator/chain end, as revealed by computational studies, resulting in higher activity and fewer (thio)urea loads than existing (thio)urea/alkoxide binary systems. These organocatalysts exhibit ultrahigh activity comparable to metal complexes, i.e., turnover number up to 900 and turnover of frequency up to 4860 min-1, affording polyesters with tailor-made structure, predicted molecular weights, narrow dispersity, less epimerization, and minimal transesterification. The catalyst synthesis is simple and scalable, allowing widely tuned activities of the ROP.

Polyesters and Polyester Nano- and Microcarriers for Drug Delivery

Many therapies require the transport of therapeutic compounds or substances encapsulated in carriers that reduce or, if possible, eliminate their direct contact with healthy tissue and components of the immune system, which may react to them as something foreign and dangerous to the patient's body. To date, inorganic nanoparticles, solid lipids, micelles and micellar aggregates, liposomes, polymeric micelles, and other polymer assemblies were tested as drug carriers. Specifically, using polymers creates a variety of options to prepare nanocarriers tailored to the chosen needs. Among polymers, aliphatic polyesters are a particularly important group. The review discusses controlled synthesis of poly(β-butyrolactone)s, polylactides, polyglycolide, poly(ε-caprolactone), and copolymers containing polymacrolactone units with double bonds suitable for preparation of functionalized nanoparticles. Discussed are syntheses of aliphatic polymers with controlled molar masses ranging from a few thousand to 106 and, in the case of polyesters with chiral centers in the chains, with controlled microstructure. The review presents also a collection of methods useful for the preparation of the drug-loaded nanocarriers: classical, developed and mastered more recently (e.g., nanoprecipitation), and forgotten but still with great potential (by the direct synthesis of the drug-loaded nanoparticles in the process comprising monomer and drug). The article describes also in-vitro and model in-vivo studies for the brain-targeted drugs based on polyester-containing nanocarriers and presents a brief update on the clinical studies and the polyester nanocarrier formulation approved for application in the clinics in South Korea for the treatment of breast, lung, and ovarian cancers.

Alternating Graft Copolymer Carrying PLA Graft Chains at Every Other Unit: Sequence Impacts on Crystallization Behaviors

Alternating graft copolymers were precisely synthesized via selective cyclopolymerization of pendant-transformable divinyl monomer (1), post-polymerization modification via aminolysis with alkylamine, and ring-opening polymerization of l-lactide (LLA) from the hydroxy pendant group in alternating sequence. The poly(LLA) (PLLA) graft chain on the alternating copolymer gave a higher crystallization degree on the isothermal treatment than that on the random counterpart likely because of the periodic sequence. The comonomer pendant group from alkylamine in the aminolysis reaction in the alternating sequence affected the crystallization behaviors, and the oligoethylene glycol (OEG) group promoted the crystallization thanks to the larger free volume effect. As for the stereocomplex formation of the racemic mixture of enantiomeric PLLA and poly(d-lactide) (PDLA) chains, the alternating graft copolymer gave a higher degree of stereocomplex crystallization in the mixture with the enantiomer homopolymer than the random analogue.

Highly Selective O-Phenylene Bisurea Catalysts for ROP: Stabilization of Oxyanion Transition State by a Semiflexible Hydrogen Bond Pocket

Organocatalyzed ring-opening polymerization (ROP) is a versatile technique for synthesizing biodegradable polymers, including polyesters and polycarbonates. We introduce o-phenylene bisurea (OPBU) (di)anions as a novel class of organocatalysts that are fast, easily tunable, mildly basic, and exceptionally selective. These catalysts surpass previous generations, such as thiourea, urea, and TBD, in selectivity (kp/ktr) by 8 to 120 times. OPBU catalysts facilitate the ROP of various monomers, achieving high conversions (>95%) in seconds to minutes, producing polymers with precise molecular weights and very low dispersities (Đ ≈ 1.01). This performance nearly matches the ideal distribution expected from living polymerization (Poisson distribution). Density functional theory (DFT) calculations reveal that the catalysts stabilize the oxyanion transition state via a hydrogen bond pocket similar to the "oxyanion hole" in enzymatic catalysis. Both experimental and theoretical analyses highlight the critical role of the semirigid o-phenylene linker in creating a hydrogen bond pocket that is tight yet flexible enough to accommodate the oxyanion transition state effectively. These new insights have provided a new class of organic catalysts whose accessibility, moderate basicity, excellent solubility, and unparalleled selectivity and tunability open up new opportunities for controlled polymer synthesis.

Simultaneous reaction- and analytical model building using dynamic flow experiments to accelerate process development

In modern pharmaceutical research, the demand for expeditious development of synthetic routes to active pharmaceutical ingredients (APIs) has led to a paradigm shift towards data-rich process development. Conventional methodologies encompass prolonged timelines for the development of both a reaction model and analytical models. The development of both methods are often separated into different departments and can require an iterative optimization process. Addressing this issue, we introduce an innovative dual modeling approach, combining the development of a Process Analytical Technology (PAT) strategy with reaction optimization. This integrated approach is exemplified in diverse amidation reactions and the synthesis of the API benznidazole. The platform, characterized by a high degree of automation and minimal operator involvement, achieves PAT calibration through a "standard addition" approach. Dynamic experiments are executed to screen a broad process space and gather data for fitting kinetic parameters. Employing an open-source software program facilitates rapid kinetic parameter fitting and additional in silico optimization within minutes. This highly automated workflow not only expedites the understanding and optimization of chemical processes, but also holds significant promise for time and resource savings within the pharmaceutical industry.

Biodegradable biopolymers: Real impact to environment pollution

Biobased biodegradable polymers (BBP) derived from different renewable resources are commonly considered as attractive alternative to petroleum-based polymers, such as polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), etc. It is because they can address the issues of serious environmental problems resulted from accumulation of plastic wastes. In the review current methods of obtaining of most abundant BBP, polylactic acid (PLA) and polyhydroxybutyrate (PHB), have been studied with an emphasis on the toxicity of compounds used for their production and additives improving consumer characteristics of PLA and PHB based market products. Substantial part of additives was the same used for traditional polymers. Analysis of the data on the response of different organisms and plants on exposure to these materials and their degradation products confirmed the doubts about real safety of BBP. Studies of safer additives are scarce and are of vital importance. Meanwhile, technologies of recycling of traditional petroleum-based polymers were shown to be well-developed, which cannot be said about PLA or PHB based polymers, and their blends with petroleum-based polymers. Therefore, development of more environmentally friendly components and sustainable technologies of production are necessary before following market expansion of biobased biodegradable products.

Heteroatom Substitution Strategy Modulates Thermodynamics Towards Chemically Recyclable Polyesters and Monomeric Unit Sequence by Temperature Switching

In this paper, we proposed a heteroatom substitution strategy (HSS) in the δ-valerolactone (VL) system to modulate thermodynamics toward chemically recyclable polyesters. Three VL-based monomers containing different heteroatoms (M1 (N), M2 (S), and M3 (O)), instead of C-5 carbon, were designed and synthesized to verify our proposed HSS. All three monomers undergo organocatalytic living/controlled ROP and controllable depolymerization. Impressively, the resulting P(M1) achieved over 99 % monomer recovery under both mild solution depolymerization and high vacuum pyrolysis conditions without any side reactions, and the recycled monomers can be polymerized again forming new polymers. The systematic study of the relationship between heteroatom substitution and recyclability shows that introducing heteroatoms does change the thermodynamics of the monomers (ΔHp o, ΔSp o and Tc values), thereby adjusting the polymerizability and depolymerizability. DFT calculations found that the introduction of heteroatoms adjusts the ring strain by changing the angular strain of the monomers, and the order of their angular strain (M2>M1>M3) is consistent with the order of the experimentally obtained enthalpy change. Notably, the one-pot/one-step copolymerization of two of each of the three monomers enables the synthesis of sequence-controlled copolymers from gradient to random to block structures, by simply switching the copolymerization temperature.

Hybrid Photoiniferter and Ring-Opening Polymerization Yields One-Pot Anisotropic Nanorods

Polymerization-induced self-assembly (PISA) has emerged as a scalable one-pot technique to prepare block copolymer (BCP) nanoparticles. Recently, a PISA process, that results in poly(l-lactide)-b-poly(ethylene glycol) BCP nanoparticles coined ring-opening polymerization (ROP)-induced crystallization-driven self-assembly (ROPI-CDSA), was developed. The resulting nanorods demonstrate a strong propensity for aggregation, resulting in the formation of 2D sheets and 3D networks. This article reports the synthesis of poly(N,N-dimethyl acrylamide)-b-poly(l)-lactide BCP nanoparticles by ROPI-CDSA, utilizing a two-step, one-pot approach. A dual-functionalized photoiniferter is first used for controlled radical polymerization of the acrylamido-based monomer, and the resulting polymer serves as a macroinitiator for organocatalyzed ROP to form the solvophobic polyester block. The resulting nanorods are highly stable and display anisotropy at higher molecular weights (>12k Da) and concentrations (>20% solids) than the previous report. This development expands the chemical scope of ROPI-CDSA BCPs and provides readily accessible nanorods made with biocompatible materials.

Microfluidic Assembly of Degradable, Stereocomplexed Hydrogel Microparticles

Hydrogel microparticles (HMPs) have been investigated widely for their use in tissue engineering and drug delivery applications. However, translation of these highly tunable systems has been hindered by covalent cross-linking methods within microparticles. Stereocomplexation, a stereospecific form of physical cross-linking, provides a robust yet degradable alternative for creating translationally relevant HMPs. Herein, 4-arm polyethylene glycol (PEG) stars were used as macromolecular initiators from which oligomeric poly(l-lactic acid) (PLLA) was polymerized with a degree of polymerization (DPn) of 20 on each arm. Similarly, complementary propargyl-containing ABA cross-linkers with enantiomeric poly(d-lactic acid) (PDLA) segments (DPn = 20) on each arm. Droplets of these gel precursors were formed via a microfluidic organic-in-oil-in-water system where microparticles self-assembled via stereocomplexation and were stabilized after precipitation in deionized water. By varying the flow rate of the dispersed phase, well-defined microparticles with diameters of 33.7 ± 0.5, 62.4 ± 0.6, and 105.7 ± 0.8 μm were fabricated. Gelation due to stereocomplexation was confirmed via wide-angle X-ray scattering in which HMPs exhibited the signature diffraction pattern of stereocomplexed PLA at 2θ = 12.2, 21.2, 24.2°. Differential scanning calorimetry also confirmed stereocomplexation by the appearance of a crystallization exotherm (Tc = 37 °C) and a high-temperature endotherm (Tm = 159 °C) that does not appear in the homocrystallization of PLLA or the hydrogel precursors. Additionally, the propargyl handle present on the cross-linker allows for pre- or post-assembly thiol-yne "click" functionalization as demonstrated by the addition of thiol-containing fluorophores to the HMPs.

Recyclable and (Bio)degradable Polyesters in a Circular Plastics Economy

Polyesters carrying polar main-chain ester linkages exhibit distinct material properties for diverse applications and thus play an important role in today's plastics economy. It is anticipated that they will play an even greater role in tomorrow's circular plastics economy that focuses on sustainability, thanks to the abundant availability of their biosourced building blocks and the presence of the main-chain ester bonds that can be chemically or biologically cleaved on demand by multiple methods and thus bring about more desired end-of-life plastic waste management options. Because of this potential and promise, there have been intense research activities directed at addressing recycling, upcycling or biodegradation of existing legacy polyesters, designing their biorenewable alternatives, and redesigning future polyesters with intrinsic chemical recyclability and tailored performance that can rival today's commodity plastics that are either petroleum based and/or hard to recycle. This review captures these exciting recent developments and outlines future challenges and opportunities. Case studies on the legacy polyesters, poly(lactic acid), poly(3-hydroxyalkanoate)s, poly(ethylene terephthalate), poly(butylene succinate), and poly(butylene-adipate terephthalate), are presented, and emerging chemically recyclable polyesters are comprehensively reviewed.

Synergetic binary organocatalyzed ring opening polymerization for the precision synthesis of polysiloxanes

Organocatalytic ring-opening polymerization (ROP) is a versatile method for synthesizing well-defined polymers with controlled molecular weights, dispersities, and nonlinear macromolecular architectures. Despite spectacular advances in organocatalytic ROP, precision synthesis of polysiloxanes remains challenging due to the mismatch in polarity between highly polar initiators and nonpolar monomers and polymers and the difficulty in suppressing the formation of scrambling products via transetherification reactions during ROP of cyclic siloxanes. Here, we describe a binary organocatalytic ROP (BOROP) of hexamethylcyclotrisiloxane (D3) employing organic bases as catalysts and (thio)ureas as cocatalysts. The BOROP of D3 using triazabicyclodecene (TBD) and (thio)ureas generates polydimethylsiloxanes (PDMSs) with narrow dispersity (Mw/Mn < 1.1). Despite the similar basicities of TBD and 1,8-bis(tetramethylguanidino)naphthalene (TMGN), which is known as a proton sponge, a unitary organocatalytic system using TMGN was inactive for the ROP of D3. When the TMGN was paired with acidic urea, the BOROP of D3 yielded PDMSs with narrow dispersity (Mw/Mn < 1.1). Data suggest that the synergetic effect of TMGN and urea is results in an unprecedented activation-deactivation equilibrium between dormant and propagating species. The benefits of the present BOROP system are demonstrated by the formation of PDMS elastomers with more uniform network structures that are highly stretchy and have excellent mechanical properties.

Al(III)/K(I) Heterodinuclear Polymerization Catalysts Showing Fast Rates and High Selectivity for Polyester Polyols

Low molar mass, hydroxyl end-capped polymers, often termed "polyols," are widely used to make polyurethanes, resins, and coatings and as surfactants in liquid formulations. Epoxide/anhydride ring-opening copolymerization (ROCOP) is a controlled polymerization route to make them, and its viability depends upon catalyst selection. In the catalysis, the polyester polyol molar masses and end-groups are controlled by adding specific but excess quantities of diols (vs catalyst), known as the chain transfer agent (CTA), to the polymerizations, but many of the best current catalysts are inhibited or even deactivated by alcohols. Herein, a series of air-stable Al(III)/K(I) heterodinuclear polymerization catalysts show rates and selectivity at the upper end of the field. They also show remarkable increases in activity, with good selectivity and control, as quantities of diol are increased from 10-400 equiv. The reactions are accelerated by alcohols, and simultaneously, their use allows for the production of hydroxy telechelic poly/oligoesters (400 < Mn (g mol-1) < 20,400, Đ < 1.19). For example, cyclohexene oxide (CHO)/phthalic anhydride (PA) ROCOP, using the best Al(III)/K(I) catalyst with 200 equiv of diol, shows a turnover frequency (TOF) of 1890 h-1, which is 4.4× higher than equivalent reactions without any diol (Catalyst/Diol/PA/CHO = 1:10-400:400:2000, 100 °C). In all cases, the catalysis is well controlled and highly ester linkage selective (ester linkages >99%) and operates effectively using bicyclic and/or biobased anhydrides with bicyclic or flexible alkylene epoxides. These catalysts are recommended for future production and application development using polyester polyols.

Bifunctional Carbanionic Synthesis of Fully Bio-Based Triblock Structures Derived from β-Farnesene and ll-Dilactide: Thermoplastic Elastomers

Current environmental challenges and the shrinking fossil-fuel feedstock are important criteria for the next generation of polymer materials. In this context, we present a fully bio-based material, which shows promise as a thermoplastic elastomer (TPE). Due to the use of β-farnesene and L-lactide as monomers, bio-based feedstocks, namely sugar cane and corn, can be used. A bifunctional initiator for the carbanionic polymerization was employed, to permit an efficient synthesis of ABA-type block structures. In addition, the "green" solvent MTBE (methyl tert-butyl ether) was used for the anionic polymerisation, enabling excellent solubility of the bifunctional anionic initiator. This afforded low dispersity (Đ=1.07 to 1.10) and telechelic polyfarnesene macroinitiators. These were employed for lactide polymerization to obtain H-shaped triblock copolymers. TEM and SAXS revealed clearly phase-separated morphologies, and tensile tests demonstrated elastic mechanical properties. The materials featured two glass transition temperatures, at - 66 °C and 51 °C as well as gyroid or cylindrical morphologies, resulting in soft elastic materials at room temperature.

Organocatalyzed ring-opening reactions of γ-carbonyl-substituted ε-caprolactones

Side-chain-functionalized aliphatic polyesters are promising as functional biodegradable polymers. We have investigated ring-opening reactions of γ-carbonyl-substituted ε-caprolactones (gCCLs) to obtain poly(ε-caprolactone) (PCL) analogues. Organic catalysts and Sn(Oct)2 often used for the ring-opening polymerization (ROP) of ε-caprolactone (CL) have been explored to find the conditions for the formation of polymeric products of gCCLs. We confirmed the consumption of gCCLs in all catalyzed reactions. However, chain propagation hardly occurs, as the propagating species are preferentially transformed to α-substituted five-membered lactones when the substituents are linked by ester or not sterically hindered. Intramolecular cyclization to form thermodynamically stable five-membered lactones releases alcohols and amines, serving as nucleophiles for the subsequent ring opening of other gCCLs. Thus, apparent chain reactions are realized for continuous consumption of gCCLs. The reaction preference remains unchanged independent of the catalysts, although the reactions of the amide-linked gCCLs by acidic catalysts are slightly mitigated. Finally, copolymerization of CL and a gCCL catalyzed by diphenyl phosphate has been investigated, which enables the chain propagation reaction to yield the linear oligomers of PCL analogues containing up to 16 mol% of gCCL units. This study contributes to understanding the chemistry of ring-opening reactions of substituted lactones for designing functional degradable polymers.

Efficient Synthesis of Hydrolytically Degradable Block Copolymer Nanoparticles via Reverse Sequence Polymerization-Induced Self-Assembly in Aqueous Media

Hydrolytically degradable block copolymer nanoparticles are prepared via reverse sequence polymerization-induced self-assembly (PISA) in aqueous media. This efficient protocol involves the reversible addition-fragmentation chain transfer (RAFT) polymerization of N,N'-dimethylacrylamide (DMAC) using a monofunctional or bifunctional trithiocarbonate-capped poly(ϵ-caprolactone) (PCL) precursor. DMAC monomer is employed as a co-solvent to solubilize the hydrophobic PCL chains. At an intermediate DMAC conversion of 20-60 %, the reaction mixture is diluted with water to 10-25 % w/w solids. The growing amphiphilic block copolymer chains undergo nucleation to form sterically-stabilized PCL-core nanoparticles with PDMAC coronas. 1 H NMR studies confirm more than 99 % DMAC conversion while gel permeation chromatography (GPC) studies indicate well-controlled RAFT polymerizations (Mw /Mn ≤1.30). Transmission electron microscopy (TEM) and dynamic light scattering (DLS) indicate spheres of 20-120 nm diameter. As expected, hydrolytic degradation occurs within days at 37 °C in either acidic or alkaline solution. Degradation is also observed in phosphate-buffered saline (PBS) (pH 7.4) at 37 °C. However, no degradation is detected over a three-month period when these nanoparticles are stored at 20 °C in deionized water (pH 6.7). Finally, PDMAC30 -PCL16 -PDMAC30 nanoparticles are briefly evaluated as a dispersant for an agrochemical formulation based on a broad-spectrum fungicide (azoxystrobin).

Recent Advances in Polycaprolactones for Anticancer Drug Delivery

Poly(ε-Caprolactone)s are biodegradable and biocompatible polyesters that have gained considerable attention for drug delivery applications due to their slow degradation and ease of functionalization. One of the significant advantages of polycaprolactone is its ability to attach various functionalities to its backbone, which is commonly accomplished through ring-opening polymerization (ROP) of functionalized caprolactone monomer. In this review, we aim to summarize some of the most recent advances in polycaprolactones and their potential application in drug delivery. We will discuss different types of polycaprolactone-based drug delivery systems and their behavior in response to different stimuli, their ability to target specific locations, morphology, as well as their drug loading and release capabilities.

Elucidation of Substantial Differences in Ring-Opening Polymerization Outcomes from Subtle Variation of Glucose Carbonate-Based Monomer Substitution Patterns and Substituent Types

The substituents present upon five-membered bicyclic glucose carbonate monomers were found to greatly affect the reactivities and regioselectivities during ring-opening polymerization (ROP), which contrast in significant and interesting ways from previous studies on similar systems, while also leading to predictable effects on the thermal properties of the resulting polycarbonates. Polymerization behaviors were probed for a series of five five-membered bicyclic 2,3-glucose-carbonate monomers having 4,6-ether, -carbonate, or -sulfonyl urethane protecting groups, under catalysis with three different organobase catalysts. Irrespective of the organobase catalyst employed, regioregular polycarbonates were obtained via ROP of monomers with ether substituents, while the backbone connectivities of polymers derived from monomers with carbonate protecting groups suffered transcarbonylation reactions, resulting in irregular backbone connectivities and broad molar mass distributions. The sulfonyl urethane-protected monomers were unable to undergo organobase-catalyzed ROP, possibly due to the acidity of the proton in urethane functionality. The thermal behaviors of polycarbonates with ether and carbonate pendant groups were investigated in terms of thermal stability and glass transition temperature (Tg). A two-stage thermal decomposition was observed when tert-butyloxycarbonyl (BOC) groups were employed as protecting side chains, while all other polycarbonates presented high thermal stabilities with a single-stage thermal degradation. Tg was greatly affected by side-chain bulkiness, with values ranging from 39 to 139 °C. These fundamental findings of glucose-based polycarbonates may facilitate the development of next-generation sustainable highly functional materials.

Phosphazene Functionalized Silsesquioxane-Based Porous Polymer as Thermally Stable and Reusable Catalyst for Bulk Ring-Opening Polymerization of ε-Caprolactone

The bulk ring-opening polymerization (ROP) of ε-caprolactone using phosphazene-containing porous polymeric material (HPCP) has been studied at high reaction temperatures (130-150 °C). HPCP in conjunction with benzyl alcohol as an initiator induced the living ROP of ε-caprolactone, affording polyesters with a controlled molecular weight up to 6000 g mol-1 and moderate polydispersity (Ð~1.5) under optimized conditions ([BnOH]/[CL] = 50; HPCP: 0.63 mM; 150 °C). Poly(ε-caprolactone)s with higher molecular weight (up to Mn = 14,000 g mol-1, Ð~1.9) were obtained at a lower temperature, at 130 °C. Due to its high thermal and chemical stability, HPCP can be reused for at least three consecutive cycles without a significant decrease in the catalyst efficiency. The tentative mechanism of the HPCP-catalyzed ROP of ε-caprolactone, the key stage of which consists of the activation of the initiator through the basic sites of the catalyst, was proposed.

PEGylated and functionalized polylactide-based nanocapsules: An overview

Polymeric nanocapsules (NC) are versatile mixed vesicular nanocarriers, generally containing a lipid core with a polymeric wall. They have been first developed over four decades ago with outstanding applicability in the cosmetic and pharmaceutical fields. Biodegradable polyesters are frequently used in nanocapsule preparation and among them, polylactic acid (PLA) derivatives and copolymers, such as PLGA and amphiphilic block copolymers, are widely used and considered safe for different administration routes. PLA functionalization strategies have been developed to obtain more versatile polymers and to allow the conjugation with bioactive ligands for cell-targeted NC. This review intends to provide steps in the evolution of NC since its first report and the recent literature on PLA-based NC applications. PLA-based polymer synthesis and surface modifications are included, as well as the use of NC as a novel tool for combined treatment, diagnostics, and imaging in one delivery system. Furthermore, the use of NC to carry therapeutic and/or imaging agents for different diseases, mainly cancer, inflammation, and infections is presented and reviewed. Constraints that impair translation to the clinic are discussed to provide safe and reproducible PLA-based nanocapsules on the market. We reviewed the entire period in the literature where the term "nanocapsules" appears for the first time until the present day, selecting original scientific publications and the most relevant patent literature related to PLA-based NC. We presented to readers a historical overview of these Sui generis nanostructures.

Organocatalyzed Ring-Opening Polymerization of (S)-3-Benzylmorpholine-2,5-Dione

A 3-benzylmorpholine-2,5-dione monomer is synthesized from the natural amino acid l-phenylalanine and characterized by means of nuclear magnetic resonance and infrared spectroscopy, electrospray ionization mass spectrometry, and elemental analysis. Subsequent to preliminary polymerization studies, a well-defined poly(ester amide) homopolymer is synthesized via ring-opening polymerization using a binary catalyst system comprising 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and a 1-(3,5-bis(trifluoromethyl)phenyl)-3-cyclohexylthiourea (TU) cocatalyst with a feed ratio of M/I/DBU/TU = 100/1/1/10. Kinetic studies reveal high controllability of the dispersities and molar masses up to conversions of almost 80%. Analysis by mass spectrometry hints toward excellent end-group fidelity at these conditions. In consequence, utilization of hydroxyl-functionalized poly(ethylene glycol) and poly(2-ethyl-2-oxazoline) as macroinitiators results in amphiphilic block copolymers. Bulk miscibility of the building blocks is indicated by differential scanning calorimetry investigations. As more and more promising new drugs are based on hydrophobic molecules featuring aromatic moieties, the novel polyesteramides seem highly promising materials to be used as potential drug delivery vehicles.

Preparation of porous PCL-PEG-PCL scaffolds using supercritical carbon dioxide

In this study, the Supercritical Carbon Dioxide (scCO₂) gas foaming procedure was used in the preparation of scaffolds containing the model drug dexamethasone (DXMT). The method used did not include an organic solvent thus making it a safe method. The ring-opening polymerization of PCL-PEG-PCL (PCEC) triblock was conducted using an organocatalyst [1,8 diazabicyclo [5.4.0] undec-7-ene (DBU)]. After mixing 5.0 g of DXMT with 50.0 g of PCEC, hydraulic pressure was applied to compress the mixed powder into disc-like tablets. The tablet-like scaffold of the triblock containing DXMT was inserted into a scCO₂ gas-foaming device. The peak porosity percentage of the synthesized triblock was found to be 55.58 %. Pressure, temperature, soaking time and the time required to depressurize were recorded as 198 bar, 50 °C, 2.0 h, and 28 min respectively. After treatment with scCO₂, the scaffolds experienced an almost full release of DXMT in vitro after 30 days (83.74 ± 1.54 % vs 52.24 ± 2.03 % before scCO₂ treatment). In conclusion, the results proved that the scCO₂ gas foaming procedure could be employed for constructing modifiable PCEC scaffolds with plausible porosity and structural and morphological features which can manipulate drug release.

Organocatalytic Ring-Opening Polymerization of ϵ-Caprolactone with Phosphoramidimidates (PADIs) as a Bifunctional Brønsted Acid Catalyst

In this study, an organocatalytic ring-opening polymerization (ROP) of ϵ-caprolactone (ϵ-CL) has been developed by employing PADIs as a novel and efficient acid/base bifunctional organocatalyst, which could afford metal-free poly(ϵ-caprolactone) with predictable molecular weight and narrow dispersity at a low catalyst loading under mild conditions. NMR and kinetic studies indicate that the ring-opening polymerizations of lactones catalyzed by PADIs proceed in a living and well controlled manner. Moreover, this organic Brønsted acid catalytic system could allow the synthesis of PCL with molecular weight above 60 kg/mol, as well as well-defined star polymers.

Gaining Structural Control by Modification of Polymerization Rate in Ring-Opening Polymerization-Induced Crystallization-Driven Self-Assembly

Polymerization-induced self-assembly (PISA) has become an important one pot method for the preparation of well-defined block copolymer nanoparticles. In PISA, morphology is typically controlled by changing molecular architecture and polymer concentration. However, several computational and experimental studies have suggested that changes in polymerization rate can lead to morphological differences. Here, we demonstrate that catalyst selection can be used to control morphology independent of polymer structure and concentration in ring-opening polymerization-induced crystallization-driven self-assembly (ROPI-CDSA). Slower rates of polymerization give rise to slower rates of self-assembly, resulting in denser lamellae and more 3D structures when compared to faster rates of polymerization. Our explanation for this is that the fast samples transiently exist in a nonequilibrium state as self-assembly starts at a higher solvophobic block length when compared to the slow polymerization. We expect that subsequent examples of rate variation in PISA will allow for greater control over morphological outcome.

Development of Biodegradable Polyesters: Study of Variations in Their Morphological and Thermal Properties through Changes in Composition of Alkyl-Substituted (ε-DL) and Non-Substituted (ε-CL, EB, L-LA) Monomers

Three series of polyesters based on monomer combinations of ε-caprolactone (ε-CL), ethylene brassylate (EB), and l-Lactide (LLA) with the alkyl substituted lactone ε-decalactone (ε-DL) were synthesized at different molar ratios. Copolymers were obtained via ring opening polymerization (ROP) employing TBD (1,5,7-triazabicyclo-[4.4.0]-dec-5-ene), an organic catalyst which can be handled under normal conditions, avoiding the use of glove box equipment. The molar monomer composition of resulting copolymers differed from theoretical values due to lower ε-DL reactivity; their M_n and M_w values were up to 14 kDa and 22.8 kDa, respectively, and distributions were (Ɖ) ≤ 2.57. The thermal stability of these materials suffered due to variations in their ε-DL molar content. Thermal transitions such as melting (T_m) and crystallization (T_c) showed a decreasing tendency as ε-DL molar content increased, while glass transition (T_g) exhibited minor changes. It is worth mentioning that changes in monomer composition in these polyesters have a strong impact on their thermal performance, as well as in their crystallization degree. Consequently, variations in their chemical structure may have an effect on hydrolyic degradation rates. It should be noted that, in future research, some of these copolymers will be exposed to hydrolytic degradation experiments, including characterizations of their mechanical properties, to determine their adequacy in potential use in the development of soft medical devices.

Extreme Extensibility in Physically Cross-Linked Nanocomposite Hydrogels Leveraging Dynamic Polymer-Nanoparticle Interactions

Designing yield stress fluids to exhibit desired functional properties is an integral challenge in many applications such as 3D printing, drilling, food formulation, fiber spinning, adhesives, and injectable biomaterials. Extensibility in particular has been found to be a highly beneficial characteristic for materials in these applications; however, few highly extensible, high water content materials have been reported to date. Herein we engineer a class of high water content nanocomposite hydrogel materials leveraging multivalent, noncovalent, polymer-nanoparticle (PNP) interactions between modified cellulose polymers and biodegradable nanoparticles. We show that modulation of the chemical composition of the PNP hydrogels controls the dynamic cross-linking interactions within the polymer network and directly impacts yielding and viscoelastic responses. These materials can be engineered to stretch up to 2000% strain and occupy an unprecedented property regime for extensible yield stress fluids. Moreover, a dimensional analysis of the relationships between extensibility and the relaxation and recovery time scales of these nanocomposite hydrogels uncovers generalizable design criteria that will be critical for future development of extensible materials.

Lignin as a Renewable Building Block for Sustainable Polyurethanes

Currently, the pulp and paper industry generates around 50-70 million tons of lignin annually, which is mainly burned for energy recovery. Lignin, being a natural aromatic polymer rich in functional hydroxyl groups, has been drawing the interest of academia and industry for its valorization, especially for the development of polymeric materials. Among the different types of polymers that can be derived from lignin, polyurethanes (PUs) are amid the most important ones, especially due to their wide range of applications. This review encompasses available technologies to isolate lignin from pulping processes, the main approaches to convert solid lignin into a liquid polyol to produce bio-based polyurethanes, the challenges involving its characterization, and the current technology assessment. Despite the fact that PUs derived from bio-based polyols, such as lignin, are important in contributing to the circular economy, the use of isocyanate is a major environmental hot spot. Therefore, the main strategies that have been used to replace isocyanates to produce non-isocyanate polyurethanes (NIPUs) derived from lignin are also discussed.

Spotting Trends in Organocatalyzed and Other Organomediated (De)polymerizations and Polymer Functionalizations

Organocatalysis has evolved into an effective complement to metal- or enzyme-based catalysis in polymerization, polymer functionalization, and depolymerization. The ease of removal and greater sustainability of organocatalysts relative to transition-metal-based ones has spurred development in specialty applications, e.g., medical devices, drug delivery, optoelectronics. Despite this, the use of organocatalysis and other organomediated reactions in polymer chemistry is still rapidly developing, and we envisage their rapidly growing application in nascent areas such as controlled radical polymerization, additive manufacturing, and chemical recycling in the coming years. In this Review, we describe ten trending areas where we anticipate paradigm shifts resulting from novel organocatalysts and other transition-metal-free conditions. We highlight opportunities and challenges and detail how new discoveries could lead to previously inaccessible functional materials and a potentially circular plastics economy.

3-Ethyl-6-vinyltetrahydro-2H-pyran-2-one (EVP): a versatile CO2-derived lactone platform for polymer synthesis

3-Ethyl-6-vinyltetrahydro-2H-pyran-2-one (EVP) is a CO₂-derived lactone synthesized via Pd-catalyzed telomerization of butadiene. As EVP is 28.9% by weight CO₂, it has received significant recent attention as an intermediary for the synthesis of high CO₂-content polymers. This article provides an overview of strategies for the polymerization of EVP to a wide variety of polymer structures, ranging from radical polymerizations to ring-opening polymerizations, that each take unique advantage of the highly functionalized lactone.

Concentration-Driven Self-Assembly of PS-b-PLA Bottlebrush Diblock Copolymers in Solution

Bottlebrush polymers are a class of semiflexible, hierarchical macromolecules with unique potential for shape-, architecture-, and composition-based structure–property design. It is now well-established that in dilute to semidilute solution, bottlebrush homopolymers adopt a wormlike conformation, which decreases in extension (persistence length) as the concentration and molecular overlap increase. By comparison, the solution phase self-assembly of bottlebrush diblock copolymers (BBCP) in a good solvent remains poorly understood, despite critical relevance for solution processing of ordered phases and photonic crystals. In this work, we combine small-angle X-ray scattering, coarse-grained simulation, and polymer synthesis to map the equilibrium phase behavior and conformation of a set of large, nearly symmetric PS-b-PLA bottlebrush diblock copolymers in toluene. Three BBCP are synthesized, with side chains of number-averaged molecular weights of 4500 (PS) and 4200 g/mol (PLA) and total backbone degrees of polymerization of 100, 255, and 400 repeat units. The grafting density is one side chain per backbone repeat unit. With increasing concentration in solution, all three polymers progress through a similar structural transition: from dispersed, wormlike chains with concentration-dependent (decreasing) extension, through the onset of disordered PS/PLA compositional fluctuations, to the formation of a long-range ordered lamellar phase. With increasing concentration in the microphase-separated regimes, the domain spacing increases as individual chains partially re-extend due to block immiscibility. Increases in the backbone degree of polymerization lead to changes in the scattering profiles which are consistent with the increased segregation strength. Coarse-grained simulations using an implicit side-chain model are performed, and concentration-dependent self-assembly behavior is qualitatively matched to experiments. Finally, using the polymer with the largest backbone length, we demonstrate that lamellar phases develop a well-defined photonic band gap in solution, which can be tuned across the visible spectrum by varying polymer concentration.

Nontoxic N-Heterocyclic Olefin Catalyst Systems for Well-Defined Polymerization of Biocompatible Aliphatic Polycarbonates

Herein, N-heterocyclic olefins (NHOs) are utilized as catalysts for the ring-opening polymerization (ROP) of functional aliphatic carbonates. This emerging class of catalysts provides high reactivity and rapid conversion. Aiming for the polymerization of monomers with high side chain functionality, six-membered carbonates derived from 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) served as model compounds. Tuning the reactivity of NHO from predominant side chain transesterification at room temperature toward ring-opening at lowered temperatures (-40 °C) enables controlled ROP. These refined conditions give narrowly distributed polymers of the hydrophobic carbonate 5-methyl-5-benzyloxycarbonyl-1,3-dioxan-2-one (MTC-OBn) (Đ < 1.30) at (pseudo)first-order kinetic polymerization progression. End group definition of these polymers demonstrated by mass spectrometry underlines the absence of side reactions. For the active ester monomer 5-methyl-5-pentafluorophenyloxycarbonyl-1,3-dioxane-2-one (MTC-PFP) with elevated side chain reactivity, a cocatalysis system consisting of NHO and the Lewis acid magnesium iodide is required to retune the reactivity from side chains toward controlled ROP. Excellent definition of the products (Đ < 1.30) and mass spectrometry data demonstrate the feasibility of this cocatalyst approach, since MTC-PFP has thus far only been polymerized successfully using acidic catalysts with moderate control. The broad feasibility of our findings was further demonstrated by the synthesis of block copolymers for bioapplications and their successful nanoparticular assembly. High tolerability of NHO in vitro with concentrations ranging up to 400 μM (equivalent to 0.056 mg/mL) further emphasize the suitability as a catalyst for the synthesis of bioapplicable materials. The polycarbonate block copolymer mPEG₄₄-b-poly(MTC-OBn) enables physical entrapment of hydrophobic dyes in sub-20 nm micelles, whereas the active ester block copolymer mPEG₄₄-b-poly(MTC-PFP) is postfunctionalizable by covalent dye attachment. Both block copolymers thereby serve as platforms for physical or covalent modification of nanocarriers for drug delivery.

Introducing SuFEx click chemistry into aliphatic polycarbonates: a novel toolbox/platform for post-modification as biomaterials

As a biodegradable and biocompatible biomaterial, aliphatic polycarbonates (APCs) have attracted substantial attention in terms of post-polymerization modification (PPM) for functionalization. A strategy for the introduction of sulfur(VI)-fluoride exchange (SuFEx) click chemistry into APCs for PPM is proposed for the first time in this work. 4'-(Fluorosulfonyl)benzyl 5-methyl-2-oxo-1,3-dioxane-5-carboxylate (FMC) was designed as a SuFEx clickable cyclic carbonate for APCs via ring-opening polymerization (ROP), and an operational and nontoxic synthetic route was achieved. FMC managed to undergo both ROP and PPM through the SuFEx click chemistry organocatalytically without constraining or antagonizing each other, using 1,5,7-triazabicyclo[4,4,0]dec-5-ene (TBD) as a co-organocatalyst here. Its ROP was systematically investigated, and density functional theory (DFT) calculations were performed to understand the acid-base catalytic mechanism in the anionic ROP. Exploratory investigations into PPM by SuFEx of poly(FMC) were conducted as biomaterials, and the one-pot strategies to achieve both ROP and SuFEx were confirmed.

Preparing Well-Defined Polyacrylamide-b-Polycarbonate by Integrating Photoiniferter Polymerization and TBD-Catalyzed ROP

The dual-initiator technique allows the polymerization of different monomers from orthogonal polymerization mechanisms to obtain block copolymers (BCPs). In this study, it is attempted to combine photoiniferter living free radical polymerization and organocatalytic ring-opening polymerization (ROP) to design a hydroxyl-functionalized carbamodithioate, i.e., 4-(hydroxymethyl)benzyl diethylcarbamodithioate (HBDC), which can integrate photoiniferter polymerization of acrylamide monomers and ROP of cyclic carbonates. As a proof of concept, the monomer applicability is further extended to acrylates and lactones. The results confirm that the two polymerization systems are experimentally compatible in a stepwise sequence as well as in a simultaneous one-pot process to synthesize BCPs. It is reasonable to assume that HBDC can allow for simple and efficient one-pot access to well-defined BCPs from a larger range of monomers, which is more advantageous from the operational, economical, and environmental points of view.

Highly Active Heterogeneous Double Metal Cyanide Catalysts for Ring-Opening Polymerization of Cyclic Monomers

A series of heterogeneous Zn-Co double metal cyanide (DMC) catalysts were investigated for ring-opening polymerization (ROP) of various cyclic monomers. Notably, inexpensive and commonly used organic solvents such as acetone, N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, nitromethane, and 1-methylpyrrolidin-2-one were very effective complexing agents for the preparation of DMC catalysts, showing high catalytic activity for the ROP of propylene oxide, ε-caprolactone, and δ-valerolactone. The chemical structures and compositions of the resultant catalysts were determined using various techniques such as FT-IR, X-ray photoelectron spectroscopy, powder X-ray diffraction, and elemental analysis. α,ω-Hydroxyl-functionalized polyether and polyester polyols with high yields and tunable molecular weights were synthesized in the presence of various initiators to control functionality. Kinetic studies of the ROP of δ-valerolactone were also performed to confirm the reaction mechanism.