Accelerated Continuous Flow Depolymerization of Poly(Methyl Methacrylate)

A continuous flow setup comprising an inline dialysis unit for immediate monomer removal is used for the depolymerization of poly(methyl methacrylate) (pMMA), synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization. The approach used allows one to carry out solution depolymerizations at much higher polymer content compared to batch processes while maintaining high depolymerization conversions. pMMA is efficiently depolymerized in the flow reactor, yielding up to 68% monomer recovery under catalyst-free reaction conditions at 160 °C, starting from a 1 molar repeat unit concentration, which is a 20-fold improvement compared to previous batch studies. This was achieved by using the inline dialysis to continuously remove monomer from the depolymerization solution and hence continuously shifting of the depolymerization equilibrium to the recycling side. Depolymerizations at various temperatures, starting polymer concentrations, and reactor setup modifications are investigated, clearly showing the highly advantageous effect of the monomer removal on the reaction. The employed approach represents a significant advancement toward the industrial feasibility of depolymerization of methacrylates by lowering the solvent use, expanding its temperature operation window, and bringing polymer contents to a practically relevant level.

Characteristic System Time Scales Can Influence the Collective Sequence Development of Nematically Ordered Copolymers

The sequence of copolymers is of significant importance to their material properties, yet controlling the copolymer sequence remains a challenge. Previously, we have shown that polymer chains with sufficient stiffness and intermolecular attractions can undergo an emergent, polymerization-driven nematic alignment of nascent oligomers during a step-growth polymerization process. Both the extent of alignment and the point in the reaction at which it occurs impact the kinetics and the sequence development of the growing polymer. Of particular interest is the emergence of a characteristic block length in the ensemble of sequences, resulting in unusually peaked block length distributions. Here we explore the emergence of this characteristic block length over time and investigate how changes in activation energy, solution viscosity, and monomer density influence the sequence and block length distributions of stiff copolymers undergoing step-growth polymerization. We find that emergent aggregation and nematic ordering restrict the availability of longer chains to form bonds, thereby altering the propensity of chains to react in a length dependent fashion, which changes as the reaction progresses, and promoting the formation of chains and blocks of a characteristic length. Further, we demonstrate that the characteristic length scale which emerges is sensitive to the relative time scales of reaction kinetics and reactant diffusion, shifting in response to changes in the activation energy of the reaction and the viscosity of the solvent. Our observations suggest the potential for biasing characteristic lengths of sequence repeats in stiff and semiflexible copolymer systems by targeting specific nonbonded interactions and reaction kinetics through the informed adjustment of reaction conditions and the selection or chemical modification of monomer species.

Solvent-Dependent Sequence-Controlled Copolymerization of Lactones: Tailoring Material Properties from Robust Plastics to Tough Elastomers

Copolymerization stands as a versatile and potent method for tailoring polymer properties by adjusting structural unit composition and sequence distribution. However, achieving sequence-controlled copolymerization in a one-step and one-pot process remains challenging. This study introduces a solvent-dependent sequence-controlled copolymerization strategy to produce block and statistical copolyesters from 4-phenyl-2-oxabicyclo[2.1.1]hexan-3-one (4Ph-BL) and ϵ-caprolactone (ϵ-CL). The distinct kinetics of the two monomers enable the facile synthesis of diblock and triblock copolyesters, PCL-b-P(4Ph-BL) and P(4Ph-BL)-b-PCL-b-P(4Ph-BL), in non-coordinating solvents, such as dichloromethane and toluene. Conversely, coordinating solvents like tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, 1,4-dioxane, and 1,2-dimethoxyethane facilitate frequent transesterifications, yielding statistical copolyesters P[CL-stat-(4Ph-BL)] with varying ratios of heterosequences. Density functional theory (DFT) calculations confirmed that coordinating solvents stabilize the transition state for transesterification, thereby validating their role in triggering this process. By varying the microstructures and compositions, the resultant copolyesters display tunable thermal and mechanical properties, evolving from robust plastics with an ultimate tensile strength of up to 46.3±3.1 MPa to tough elastomers with >99.3 % elastic recovery. All the copolyesters exhibit remarkable thermal stability (Td,5%=376 °C) and maintain desirable chemical circularity (>92 %), supporting a closed-loop life cycle for sustainable material economy.

Effects of Cyclic Glycopolymers Molecular Mobility on their Interactions with Lectins

Cyclic polymers, which are found in the field of biopolymers, exhibit unique physical properties such as suppressed molecular mobility. Considering thermodynamics, the suppressed molecular mobility of cyclic polymers is expected to prevent unfavorable entropy loss in molecular interactions. In this study, we synthesized cyclic glycopolymers carrying galactose units and investigated the effects of their molecular mobility on the interactions with a lectin (peanut agglutinin). The synthesized cyclic glycopolymers exhibited delayed elution time on size exclusion chromatography and a short spin-spin relaxation time, indicating typical characteristics of cyclic polymers, including smaller hydrodynamic size and suppressed molecular mobility. The hemagglutination inhibition assay revealed that the cyclic glycopolymers exhibited weakened interactions with peanut agglutinin compared to the linear counterparts, attributable to the suppressed molecular mobility. Although the results are contrary to our expectations, the impact of polymer topology on molecular recognition remains intriguing, particularly in the context of protein repellent activity in the biomedical field.

Streamlining the Generation of Advanced Polymer Materials Through the Marriage of Automation and Multiblock Copolymer Synthesis in Emulsion

Synthetic polymers are of paramount importance in modern life - an incredibly wide range of polymeric materials possessing an impressive variety of properties have been developed to date. The recent emergence of artificial intelligence and automation presents a great opportunity to significantly speed up discovery and development of the next generation of advanced polymeric materials. We have focused on the high-throughput automated synthesis of multiblock copolymers that comprise three or more distinct polymer segments of different monomer composition bonded in linear sequence. The present work has exploited automation to prepare high molar mass multiblock copolymers (typically>100,000 g mol-1) using reversible addition-fragmentation chain transfer (RAFT) polymerization in aqueous emulsion. A variety of original multiblock copolymers have been synthesised via a Chemspeed robot, exemplified by a multiblock copolymer comprising thirteen constituent blocks. Moreover, libraries of copolymers of randomized monomer compositions (acrylates, acrylamides, methacrylates, and styrenes), block orders, and block lengths were also generated, thereby demonstrating the robustness of our synthetic approach. One multiblock copolymer contained all four monomer families listed in the pool, which is unprecedented in the literature. The present work demonstrates that automation has the power to render complex and laborious syntheses of such unprecedented materials not just possible, but facile and straightforward, thus representing the way forward to the next generation of complex macromolecular architectures.

Tailoring Hierarchical Structure and Rare Earth Affinity of Compositionally Identical Polymers via Sequence Control

Macromolecule sequence, structure, and function are inherently intertwined. While well-established relationships exist in proteins, they are more challenging to define for synthetic polymer nanoparticles due to their molecular weight, sequence, and conformational dispersities. To explore the impact of sequence on nanoparticle structure, we synthesized a set of 16 compositionally identical, sequence-controlled polymers with distinct monomer patterning of dimethyl acrylamide and a bioinspired, structure-driving di(phenylalanine) acrylamide (FF). Sequence control was achieved through multiblock polymerizations, yielding unique ensembles of polymer sequences which were simulated by kinetic Monte Carlo simulations. Systematic analysis of the global (tertiary- and quaternary-like) structure in this amphiphilic copolymer series revealed the effect of multiple sequence descriptors: the number of domains, the hydropathy of terminal domains, and the patchiness (density) of FF within a domain, each of which impacted both chain collapse and the distribution of single- and multichain assemblies. Furthermore, both the conformational freedom of chain segments and local-scale, β-sheet-like interactions were sensitive to the patchiness of FF. To connect sequence, structure, and target function, we evaluated an additional series of nine sequence-controlled copolymers as sequestrants for rare earth elements (REEs) by incorporating a functional acrylic acid monomer into select polymer scaffolds. We identified key sequence variables that influence the binding affinity, capacity, and selectivity of the polymers for REEs. Collectively, these results highlight the potential of and boundaries of sequence control via multiblock polymerizations to drive primary sequence ensembles hierarchical structures, and ultimately the functionality of compositionally identical polymeric materials.

Chemical recycling of bromine-terminated polymers synthesized by ATRP

Chemical recycling of polymers is one of the biggest challenges in materials science. Recently, remarkable achievements have been made by utilizing polymers prepared by controlled radical polymerization to trigger low-temperature depolymerization. However, in the case of atom transfer radical polymerization (ATRP), depolymerization has nearly exclusively focused on chlorine-terminated polymers, even though the overwhelming majority of polymeric materials synthesized with this method possess a bromine end-group. Herein, we report an efficient depolymerization strategy for bromine-terminated polymethacrylates which employs an inexpensive and environmentally friendly iron catalyst (FeBr2/L). The effect of various solvents and the concentration of metal salt and ligand on the depolymerization are judiciously explored and optimized, allowing for a depolymerization efficiency of up to 86% to be achieved in just 3 minutes. Notably, the versatility of this depolymerization is exemplified by its compatibility with chlorinated and non-chlorinated solvents, and both Fe(ii) and Fe(iii) salts. This work significantly expands the scope of ATRP materials compatible with depolymerization and creates many future opportunities in applications where the depolymerization of bromine-terminated polymers is desired.

Using Nitroxides To Model the Ion Mobility Behavior of Nitroxide-Ended Oligomers: A Bottom-up Approach To Predict Mobility Separation

Block-truncated poly(phosphodiester)s are digital macromolecules storing binary information that can be decoded by MS/MS sequencing of individual blocks released as primary fragments of the entire polymer. As such, they are ideal species for the serial sequencing methodology enabled by MS-(CID)-IMS-(CID)-MS coupling, where two activation stages are combined in-line with ion mobility spectrometry (IMS) separation. Yet, implementation of this coupling still requires efforts to achieve IMS resolution of inner blocks, that can be considered as small oligomers with α termination composed of one nitroxide decorated with a different tag. As shown by molecular dynamics simulation, these oligomers adopt a conformation where the tag points out of the coil formed by the chain. Accordingly, the sole nitroxide termination was investigated here as a model to reduce the cost of calculation aimed at predicting the shift of collision cross-section (CCS) induced by new tag candidates and extrapolate this effect to nitroxide-terminated oligomers. A library of 10 nitroxides and 7 oligomers was used to validate our calculation methods by comparison with experimental IMS data as well as our working assumption. Based on conformation predicted by theoretical calculation, three new tag candidates could be proposed to achieve the +40 Å2 CCS shift required to ensure IMS separation of oligomers regardless of their coded sequence.

Toward the Rational Design of Organic Catalysts for Organocatalysed Atom Transfer Radical Polymerisation

Thanks to their diversity, organic photocatalysts (PCs) have been widely used in manufacturing polymeric products with well-defined molecular weights, block sequences, and architectures. Still, however, more universal property-performance relationships are needed to enable the rational design of such PCs. That is, a set of unique descriptors ought to be identified to represent key properties of the PCs relevant for polymerisation. Previously, the redox potentials of excited PCs (PC*) were used as a good descriptor for characterising very structurally similar PCs. However, it fails to elucidate PCs with diverse chromophore cores and ligands, among which those used for polymerisation are a good representative. As showcased by model systems of organocatalysed atom transfer radical polymerisation (O-ATRP), new universal descriptors accounting for additional factors, such as the binding and density overlap between the PC* and initiator, are proposed and proved to be successful in elucidating the experimental performances of PCs in polymerisation. While O-ATRP is exemplified here, the approach adopted is general for studying other photocatalytic systems.

Installing a Single Monomer within Acrylic Polymers Using Photoredox Catalysis

Incorporating exactly one monomer at a defined position during a chain polymerization is exceptionally challenging due to the statistical nature of monomer addition. Herein, photoinduced electron/energy transfer (PET) enables the incorporation of exactly one vinyl ether into polyacrylates synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization. Near-quantitative addition (>96%) of a single vinyl ether is achieved while retaining >99% of the thiocarbonylthio chain ends. Kinetic studies reveal that performing the reactions at 2 °C limits unwanted chain breaking events. Finally, the syntheses of diblock copolymers are reported where molecular weights and dispersities are well-controlled on either side of the vinyl ether. Overall, this report introduces an approach to access acrylic copolymers containing exactly one chemical handle at a defined position, enabling novel macromolecular architectures to probe structure-function properties, introduce sites for de/reconstruction, store information, etc.

Visible Light Photoiniferter Polymerization for Dispersity Control in High Molecular Weight Polymers

The synthesis of polymers with high molecular weights, controlled sequence, and tunable dispersities remains a challenge. A simple and effective visible-light controlled photoiniferter reversible addition-fragmentation chain transfer (RAFT) polymerization is reported here to realize this goal. Key to this strategy is the use of switchable RAFT agents (SRAs) to tune polymerization activities coupled with the inherent highly living nature of photoiniferter RAFT polymerization. The polymerization activities of SRAs were in situ adjusted by the addition of acid. In addition to a switchable chain-transfer coefficient, photolysis and polymerization kinetic studies revealed that neutral and protonated SRAs showed different photolysis and polymerization rates, which is unique to photoiniferter RAFT polymerization in terms of dispersity control. This strategy features no catalyst, no exogenous radical source, temporal regulation by visible light, and tunable dispersities in the unprecedented high molecular weight regime (up to 500 kg mol-1 ). Pentablock copolymers with three different dispersity combinations were also synthesized, highlighting that the highly living nature was maintained even for blocks with large dispersities. Tg was lowered for high-dispersity polymers of similar MWs due to the existence of more low-MW polymers. This strategy holds great potential for the synthesis of advanced materials with controlled molecular weight, dispersity and sequence.

Effect of Film Thickness on the Self-Assembly of CBABC Symmetric Pentablock Terpolymer Melts under 1D Confinement: A Dissipative Particle Dynamic Study

The study investigates the impact of film thickness on the phase behavior of pentablock terpolymers, denoted as C3B3A6B3C3, when subjected to wall confinement by utilizing the dissipative particle dynamics method. Phase diagrams were constructed to elucidate how factors such as block-block interaction strength, film thickness, and wall properties affect the self-assembly structures. In cases where the wall exhibits no preference for any of the blocks, lamellae phases with orientations perpendicular to the wall are observed. The order-disorder transition (ODT) temperature is found to be influenced by the interaction between the polymer and the wall in thin confinement scenarios. When the wall displays a preference for specific blocks, the orientation of lamellae structures undergoes variations. Lamellae tend to align parallel to the wall when the wall favors A or C blocks, and they orient perpendicularly when B blocks are favored. Furthermore, the mechanical properties of the lamellae structures are related to the conformations of the polymer chains. Structures where chains predominantly adopt a loop conformation exhibit enhanced elastic properties. The ratio of looping to bridging conformations can be adjusted by altering the film thickness and wall selectivity.

Fate of the RAFT End-Group in the Thermal Depolymerization of Polymethacrylates

Thermal RAFT depolymerization has recently emerged as a promising methodology for the chemical recycling of polymers. However, while much attention has been given to the regeneration of monomers, the fate of the RAFT end-group after depolymerization has been unexplored. Herein, we identify the dominant small molecules derived from the RAFT end-group of polymethacrylates. The major product was found to be a unimer (DP = 1) RAFT agent, which is not only challenging to synthesize using conventional single-unit monomer insertion strategies, but also a highly active RAFT agent for methyl methacrylate, exhibiting faster consumption and yielding polymers with lower dispersities compared to the original, commercially available 2-cyano-2-propyl dithiobenzoate. Solvent-derived molecules were also identified predominantly at the beginning of the depolymerization, thus suggesting a significant mechanistic contribution from the solvent. Notably, the formation of both the unimer and the solvent-derived products remained consistent regardless of the RAFT agent, monomer, or solvent employed.

SI-PET-RAFT in flow: improved control over polymer brush growth

Surface-initiated photoinduced electron transfer-reversible addition-fragmentation chain transfer (SI-PET-RAFT) provides a light-dependent tool to synthesize polymer brushes on different surfaces that tolerates oxygen and water, and does not require a metal catalyst. Here we introduce improved control over SI-PET-RAFT polymerizations via continuous flow conditions. We confirm the composition and topological structure of the brushes by X-ray photoelectron spectroscopy, ellipsometry, and AFM. The improved control compared to no-flow conditions provides prolonged linear growth of the polymer brush (up to 250 nm, where no-flow polymerization maxed out <50 nm), and improved polymerization control of the polymer brush that allows the construction of diblock polymer brushes. We further show the linear correlation between the molecular weight of the polymer brush and its dry thickness by combining ellipsometry and single-molecule force spectroscopy.

Protein-Inspired Control over Synthetic Polymer Folding for Structured Functional Nanoparticles in Water

The folding of proteins into functional nanoparticles with defined 3D structures has inspired chemists to create simple synthetic systems mimicking protein properties. The folding of polymers into nanoparticles in water proceeds via different strategies, resulting in the global compaction of the polymer chain. Herein, we review the different methods available to control the conformation of synthetic polymers and collapse/fold them into structured, functional nanoparticles, such as hydrophobic collapse, supramolecular self-assembly, and covalent cross-linking. A comparison is made between the design principles of protein folding to synthetic polymer folding and the formation of structured nanocompartments in water, highlighting similarities and differences in design and function. We also focus on the importance of structure for functional stability and diverse applications in complex media and cellular environments.

Versatile Triphenylphosphine-Containing Polymeric Catalysts and Elucidation of Structure-Function Relationships

Synthetic polymers are a modular solution to bridging the two most common classes of catalysts: proteins and small molecules. Polymers offer the synthetic versatility of small-molecule catalysts while simultaneously having the ability to construct microenvironments mimicking those of natural proteins. We synthesized a panel of polymeric catalysts containing a novel triphenylphosphine acrylamide monomer and investigated how their properties impact the rate of a model Suzuki-Miyaura cross-coupling reaction. Systematic variation of polymer properties, such as the molecular weight, functional density, and comonomer identity, led to tunable reaction rates and solvent compatibility, including full conversion in an aqueous medium. Studies with bulkier substrates revealed connections between polymer parameters and reaction conditions that were further elucidated with a regression analysis. Some connections were substrate-specific, highlighting the value of the rapidly tunable polymer catalyst. Collectively, these results aid in building structure-function relationships to guide the development of polymer catalysts with tunable substrates and environmental compatibility.

Multiblock copolymer synthesis via RAFT emulsion polymerization

A multiblock copolymer is a polymer of a specific structure that consists of multiple covalently linked segments, each comprising a different monomer type. The control of the monomer sequence has often been described as the "holy grail" of synthetic polymer chemistry, with the ultimate goal being synthetic access to polymers of a "perfect" structure, where each monomeric building block is placed at a desired position along the polymer chain. Given that polymer properties are intimately linked to the microstructure and monomer distribution along the constituent chains, it goes without saying that there exist seemingly endless opportunities in terms of fine-tuning the properties of such materials by careful consideration of the length of each block, the number and order of blocks, and the inclusion of monomers with specific functional groups. The area of multiblock copolymer synthesis remains relatively unexplored, in particular with regard to structure-property relationships, and there are currently significant opportunities for the design and synthesis of advanced materials. The present review focuses on the synthesis of multiblock copolymers via reversible addition-fragmentation chain transfer (RAFT) polymerization implemented as aqueous emulsion polymerization. RAFT emulsion polymerization offers intriguing opportunities not only for the advanced synthesis of multiblock copolymers, but also provides access to polymeric nanoparticles of specific morphologies. Precise multiblock copolymer synthesis coupled with self-assembly offers material morphology control on length scales ranging from a few nanometers to a micrometer. It is imperative that polymer chemists interact with physicists and material scientists to maximize the impact of these materials of the future.

A Transformable and Bulky Methacrylate Monomer That Enables the Synthesis of an MMA-nBA Alternating Copolymer: Sequence-Dependent Self-Healing Properties

In this study, we designed a methacrylate molecule with an alkyl-substituted trichloro salicylic acid pendant as a transformable bulky monomer to enable the synthesis of an alternating copolymer of methyl methacrylate (MMA) and n-butyl acrylate (nBA). The adamantyl-substituted methacrylate monomer (1-Ad) showed very low homopolymerization propensity in radical polymerizations, but afforded the alternating copolymer with nBA via copolymerization. The 1-Ad units in the resultant copolymer were quantitatively and selectively transformed into MMA via transesterification with methanol to yield the alternating copolymer of MMA and nBA. Its alternating sequence was clearly demonstrated by a structural analysis via ¹³ C NMR spectroscopy as well as the low reactivity ratios for the 1-Ad and nBA pair. Finally, we verified the superior self-healing ability of the alternating copolymer compared to that of the corresponding 1 : 1 statistical copolymer.

Embedding Peptides into Synthetic Polymers: Radical Ring-Opening Copolymerization of Cyclic Peptides

Biopolymers such as proteins and nucleic acids are the key building blocks of life. Synthetic polymers have nevertheless revolutionized our everyday life through their robust synthetic accessibility. Combining the unmatched functionality of biopolymers with the robustness of tailorable synthetic polymers holds the promise to create materials that can be designed ad hoc for a wide array of applications. Radical polymerization is the most widely applied polymerization technique in both fundamental science and industrial polymer production. While this polymerization technique is robust and well controlled, it generally yields unfunctional all-carbon backbones. Combinations of natural polymers such as peptides, with synthetic polymers, are thus limited to tethering peptides onto the side chains or chain ends of the latter. This synthetic limitation is a critical restraint, considering that the function of biopolymers is programmed into the sequence of their main chain (i.e., primary structure). Here, we report the radical copolymerization of peptides and synthetic comonomers yielding synthetic polymers with defined peptide sequences embedded into their main chain. Key was the development of a solid-phase peptide synthesis (SPPS) approach to generate synthetic access to peptide conjugates containing allylic sulfides. Following cyclization, the obtained peptide monomers can be readily copolymerized with N,N-dimethylacrylamide (DMA)─controlled by reversible addition-fragmentation chain transfer (RAFT). Importantly, the developed synthetic strategy is compatible with all 20 standard amino acids and uses exclusively standard SPPS chemicals or chemicals accessible in one-step synthesis─prerequisite for widespread and universal application.

Development of Sequence-Controlled, Degradable, and Cytocompatible Oligomers with Explicit Fragmentation Pathways

Sequence-defined and degradable polymers can mimic biopolymers, such as peptides and DNA, to undertake life-supporting functions in a chemical way. The design and development of well-structured oligomers/polymers is the most concern for the public, even to further uncover their degradation process illustrating the degraded products and their properties. However, seldom investigation has been reported on the aforementioned aspects. In this work, the alternating photo-reversible addition-fragmentation chain-transfer (photo-RAFT) single unit monomer insertion (SUMI) of different N-substituted maleimides and thermal radical ring-opening SUMI of a cyclic ketene acetal monomer (i.e., 5,6-benzo-2-methylene-1,3-dioxepane (BMDO)) is adopted, to produce two degradable pentamers owing to the conversion of the exo-methylene group of BMDO into ester bonds along the main chains of the prepared products. Moreover, the possible degraded approach of pentamers is studied by combining high-resolution mass spectrometry (HRMS) and liquid chromatography-mass spectrometry (LC-MS) for the first time. This work also sheds light on the precise structures and cytotoxicity of SUMI products and their degraded compounds, proposing a detailed and credible outlook for biomedical applications.

An Arbitrarily Regulated Monomer Sequence in Multi-Block Copolymer Synthesis by Frustrated Lewis Pairs

Rapid access to sequence-controlled multi-block copolymers (multi-BCPs) remains as a challenging task in the polymer synthesis. Here we employ a Lewis pair (LP) composed of organophosphorus superbase and bulky organoaluminum to effectively copolymerize the mixture of methacrylate, cyclic acrylate, and two acrylates, into well-defined di-, tri-, tetra- and even a hepta-BCP in one-pot one-step manner. The combined livingness, dual-initiation and CSC feature of Lewis pair polymerization enable us to achieve not only a trihexaconta-BCP with the highest record in 8 steps by using four-component monomer mixture as building blocks, but also the arbitrarily-regulated monomer sequence in multi-BCP, simply by changing the composition and adding order of the monomer mixtures, thus demonstrating the powerful capability of our strategy in improving the efficiency and enriching the composition of multi-BCP synthesis.

Oxygen-Enhanced Atom Transfer Radical Polymerization through the Formation of a Copper Superoxido Complex

In controlled radical polymerization, oxygen is typically regarded as an undesirable component resulting in terminated polymer chains, deactivated catalysts, and subsequent cessation of the polymerization. Here, we report an unusual atom transfer radical polymerization whereby oxygen favors the polymerization by triggering the in situ transformation of CuBr/L to reactive superoxido species at room temperature. Through a superoxido ARGET-ATRP mechanism, an order of magnitude faster polymerization rate and a rapid and complete initiator consumption can be achieved as opposed to when unoxidized CuBr/L was instead employed. Very high end-group fidelity has been demonstrated by mass-spectrometry and one-pot synthesis of block and multiblock copolymers while pushing the reactions to reach near-quantitative conversions in all steps. A high molecular weight polymer could also be targeted (DP_n = 6400) without compromising the control over the molar mass distributions (Đ < 1.20), even at an extremely low copper concentration (4.5 ppm). The versatility of the technique was demonstrated by the polymerization of various monomers in a controlled fashion. Notably, the efficiency of our methodology is unaffected by the purity of the starting CuBr, and even a brown highly-oxidized 15-year-old CuBr reagent enabled a rapid and controlled polymerization with a final dispersity of 1.07, thus not only reducing associated costs but also omitting the need for rigorous catalyst purification prior to polymerization.

Effect of backbone flexibility on covalent template-directed synthesis of linear oligomers

Covalent template-directed synthesis can be used to replicate synthetic oligomers, but success depends critically on the conformational properties of the backbone. Here we investigate how the choice of monomer building block affects the flexibility of the backbone and in turn the efficiency of the replication process for a series of different triazole oligomers. Two competing reaction pathways were identified for monomers attached to a template, resulting in the formation of either macrocyclic or linear products. For flexible backbones, macrocycles and linear oligomers are formed at similar rates, but a more rigid backbone gave exclusively the linear product. The experimental results are consistent with ring strain calculations using molecular mechanics: products with low ring strain (20-30 kJ mol^-1) formed rapidly, and products with high ring strain (>100 kJ mol^-1) were not observed. Template-directed replication of linear oligomers requires monomers that rigid enough to prevent the formation of undesired macrocycles, but not so rigid that the linear templating pathway leading to the duplex is inhibited. Molecular mechanics calculations of ring strain provide a straightforward tool for assessing the flexibility of potential backbones and the viability different monomer designs before embarking on synthesis.

Investigating the Effect of End-Group, Molecular Weight, and Solvents on the Catalyst-Free Depolymerization of RAFT Polymers: Possibility to Reverse the Polymerization of Heat-Sensitive Polymers

Reversing reversible deactivation radical polymerization (RDRP) to regenerate the original monomer is an attractive prospect for both fundamental research and industry. However, current depolymerization strategies are often applied to highly heat-tolerant polymers with a specific end-group and can only be performed in a specific solvent. Herein, we depolymerize a variety of poly(methyl methacrylate) materials made by reversible addition-fragmentation chain-transfer (RAFT) polymerization and terminated by various end groups (dithiobenzoate, trithiocarbonate, and pyrazole carbodithioate). The effect of the nature of the solvent on the depolymerization conversion was also investigated, and key solvents such as dioxane, xylene, toluene, and dimethylformamide were shown to facilitate efficient depolymerization reactions. Notably, our approach could selectively regenerate pure heat-sensitive monomers (e.g., tert-butyl methacrylate and glycidyl methacrylate) in the absence of previously reported side reactions. This work pushes the boundaries of reversing RAFT polymerization and considerably expands the chemical toolbox for recovering starting materials under relatively mild conditions.

Precisely synthesized segmented polyurethanes toward block sequence-controlled drug delivery

The construction of polyurethanes (PUs) with sequence-controlled block structures remains a serious challenge. Here, we report the precise synthesis of PUs with desirable molecular weight, narrow molecular weight distribution, and controlled block sequences from commercially available monomers. The synthetic procedure is derived from a liquid-phase synthetic methodology, which involves diisocyanate-based iterative protocols in combination with a convergent strategy. Furthermore, a pair of multifunctional PUs with different sequence orders of cationic and anion segments were prepared. We show that the sequence order of functional segments presents an impact on the self-assembly behavior and results in unexpected surface charges of assembled micelles, thereby affecting the protein absorption, cell internalization, biodistribution and antitumor effect of the nanocarriers in vitro and in vivo. This work provides a versatile platform for the development of precise multiblock PUs with structural complexity and functional diversity, and will greatly facilitate the clinical translation of PUs in biomedicine.

Reversible On/Off Switching of Lactide Cyclopolymerization with a Redox-Active Formazanate Ligand

Redox-switching of a formazanate zinc catalyst in ring-opening polymerization (ROP) of lactide is described. Using a redox-active ligand bound to an inert metal ion (Zn²⁺) allows modulation of the catalytic activity by reversible reduction/oxidation chemistry at a purely organic fragment. A combination of kinetic and spectroscopic studies, together with mass spectrometry of the catalysis mixture, provides insight in the nature of the active species and the initiation of lactide ring-opening polymerization. The mechanistic data highlight the key role of the redox-active ligand and provide a rationale for the formation of cyclic polymer.

Reversing RAFT Polymerization: Near-Quantitative Monomer Generation Via a Catalyst-Free Depolymerization Approach

The ability to reverse controlled radical polymerization and regenerate the monomer would be highly beneficial for both fundamental research and applications, yet this has remained very challenging to achieve. Herein, we report a near-quantitative (up to 92%) and catalyst-free depolymerization of various linear, bulky, cross-linked, and functional polymethacrylates made by reversible addition-fragmentation chain-transfer (RAFT) polymerization. Key to our approach is to exploit the high end-group fidelity of RAFT polymers to generate chain-end radicals at 120 °C. These radicals trigger a rapid unzipping of both conventional (e.g., poly(methyl methacrylate)) and bulky (e.g., poly(oligo(ethylene glycol) methyl ether methacrylate)) polymers. Importantly, the depolymerization product can be utilized to either reconstruct the linear polymer or create an entirely new insoluble gel that can also be subjected to depolymerization. This work expands the potential of polymers made by controlled radical polymerization, pushes the boundaries of depolymerization, offers intriguing mechanistic aspects, and enables new applications.

Selective solvent conditions influence sequence development and supramolecular assembly in step-growth copolymerization

Sequence control in synthetic copolymers remains a tantalizing objective in polymer science due to the influence of sequence on material properties and self-organization. A greater understanding of sequence development throughout the polymerization process will aid the design of simple, generalizable methods to control sequence and tune supramolecular assembly. In previous simulations of solution-based step-growth copolymerizations, we have shown that weak, non-bonding attractions between monomers of the same type can produce a microphase separation among the lengthening nascent oligomers and thereby alter sequence. This work explores the phenomenon further, examining how effective attractive interactions, mediated by a solvent selective for one of the reacting species, impact the development of sequence and the supramolecular assembly in a simple A-B copolymerization. We find that as the effective attractions between monomers increase, an emergent self-organization of the reactants causes a shift in reaction kinetics and sequence development. When the solvent-mediated interactions are selective enough, the simple mixture of A and B monomers oligomerize and self-assemble into structures characteristic of amphiphilic copolymers. The composition and morphology of these structures and the sequences of their chains are sensitive to the relative balance of affinities between the comonomer species. Our results demonstrate the impact of differing A-B monomer-solvent affinities on sequence development in solution-based copolymerizations and are of consequence to the informed design of synthetic methods for sequence controlled amphiphilic copolymers and their aggregates.

The challenges of controlling polymer synthesis at the molecular and macromolecular level

In this Perspective, we outline advances and challenges in controlling the structure of polymers at various size regimes in the context of structural features such as molecular weight distribution, end groups, architecture, composition and sequence.

Recent advances in the self-assembly of sparsely grafted amphiphilic copolymers in aqueous solution

This review describes the self-assembly of sparsely grafted amphiphilic copolymers and highlights the effects of structural factors and solvents on their self-assembly behaviour.

Unraveling Sequence Effect on Glass Transition Temperatures of Discrete Unconjugated Oligomers

Sequence plays a critical role in enabling unique properties and functions of natural biomolecules, which has promoted the rapid advancement of synthetic sequence-defined polymers in recent decades. Particularly, investigation of short chain sequence-defined oligomers (also called discrete oligomers) on their properties has become a hot topic. However, most studies have focused on discrete oligomers with conjugated structures. In contrast, unconjugated oligomers remain relatively underexplored. In this study, three pairs of discrete oligomers with the same composition but different sequence for each pair are employed for investigating their glass transition temperatures (T_g s). The resultant T_g s of sequenced oligomers in each pair are found to be significantly different (up to 11.6 °C), attributable to variations in molecular packing as demonstrated by molecular dynamics and density function theory simulations. Intermolecular interaction is demonstrated to have less impact on T_g s than intramolecular interaction. The mechanistic investigation into two model dimers suggests that monomer sequence caused the difference in intramolecular rotational flexibility of the sequenced oligomers. In addition, despite having different monomer sequence and T_g s, the oligomers have very similar solubility parameters, which supports their potential use as effective oligomeric plasticizers to tune the T_g s of bulk polymer materials.

Concurrent control over sequence and dispersity in multiblock copolymers

Controlling monomer sequence and dispersity in synthetic macromolecules is a major goal in polymer science as both parameters determine materials' properties and functions. However, synthetic approaches that can simultaneously control both sequence and dispersity remain experimentally unattainable. Here we report a simple, one pot and rapid synthesis of sequence-controlled multiblocks with on-demand control over dispersity while maintaining a high livingness, and good agreement between theoretical and experimental molecular weights and quantitative yields. Key to our approach is the regulation in the activity of the chain transfer agent during a controlled radical polymerization that enables the preparation of multiblocks with gradually ascending (Ɖ = 1.16 → 1.60), descending (Ɖ = 1.66 → 1.22), alternating low and high dispersity values (Ɖ = 1.17 → 1.61 → 1.24 → 1.70 → 1.26) or any combination thereof. We further demonstrate the potential of our methodology through the synthesis of highly ordered pentablock, octablock and decablock copolymers, which yield multiblocks with concurrent control over both sequence and dispersity.

PET-RAFT single unit monomer insertion of β-methylstyrene derivatives: RAFT degradation and reaction selectivity

Reversible addition-fragmentation chain transfer (RAFT) single unit monomer insertion (SUMI) of β-methylstyrene derivatives into diverse RAFT agents presented fast reaction kinetics, but significant degradation of the SUMI products occurred due to a hydrogen abstraction reaction. Fortunately, such degradation can be suppressed through appropriate design of initial RAFT agents attributed to effective chain transfer and selective photoactivation.

Hierarchical Nanomaterials Assembled from Peptoids and Other Sequence-Defined Synthetic Polymers

In nature, the self-assembly of sequence-specific biopolymers into hierarchical structures plays an essential role in the construction of functional biomaterials. To develop synthetic materials that can mimic and surpass the function of these natural counterparts, various sequence-defined bio- and biomimetic polymers have been developed and exploited as building blocks for hierarchical self-assembly. This review summarizes the recent advances in the molecular self-assembly of hierarchical nanomaterials based on peptoids (or poly-N-substituted glycines) and other sequence-defined synthetic polymers. Modern techniques to monitor the assembly mechanisms and characterize the physicochemical properties of these self-assembly systems are highlighted. In addition, discussions about their potential applications in biomedical sciences and renewable energy are also included. This review aims to highlight essential features of sequence-defined synthetic polymers (e.g., high stability and protein-like high-information content) and how these unique features enable the construction of robust biomimetic functional materials with high programmability and predictability, with an emphasis on peptoids and their self-assembled nanomaterials.

Glycomacromolecules: Addressing challenges in drug delivery and therapeutic development

Carbohydrate-based materials offer exciting opportunities for drug delivery. They present readily available, biocompatible components for the construction of macromolecular systems which can be loaded with cargo, and can enable targeting of a payload to particular cell types through carbohydrate recognition events established in biological systems. These systems can additionally be engineered to respond to environmental stimuli, enabling triggered release of payload, to encompass multiple modes of therapeutic action, or to simultaneously fulfil a secondary function such as enabling imaging of target tissue. Here, we will explore the use of glycomacromolecules to deliver therapeutic benefits to address key health challenges, and suggest future directions for development of next-generation systems.