Highly Living Stars via Core-First Photo-RAFT Polymerization: Exploitation for Ultra-High Molecular Weight Star Synthesis

Organocatalyzed Photo-Controlled Synthesis of Ultrahigh-Molecular-Weight Fluorinated Alternating Copolymers

Ultrahigh-molecular-weight (UHMW) polymers with tailored structures are highly desirable for the outstanding properties. In this work, we developed a novel photoorganocatalyzed controlled radical alternating copolymerizations of fluoroalkyl maleimide and diverse vinyl comonomers, enabling efficient preparation of fluorinated copolymers of predetermined UHMWs and well-defined structures at high conversions. Versatility of this method was demonstrated by expanding to controlled terpolymerization, which allows facial access toward fluorinated terpolymers of UHMWs and functional pendants. The obtained copolymers exhibited attractive physical properties and furnished thermoplastic, anticorrosive and (super)hydrophobic attributes as coatings on different substrates. Molecular simulations provided insights into the coating morphology, which unveiled a fluorous protective layer on the top surface with polar groups attached to the bottom substrate, resulting in good adhesion and hydrophobicity, simultaneously. This synthetic method and customized copolymers shed light on the design of high-performance coatings by macromolecular engineering.

Ultra-High-Molecular-Weight, Narrow-Polydispersity Polyacrylamides Synthesized Using Photoiniferter Polymerization to Generate High-Performance Flocculants

Ultra-high-molecular-weight, water-soluble polyelectrolytes are commonly employed as flocculants for solid-liquid separation via colloidal destabilization, enabling the rapid and efficient removal of particulate matter from wastewater streams. A drive toward more sustainable and less polluting industrial practices, coupled with the desire to reduce freshwater usage and improve closed-loop systems, demands the development of flocculants with ever-higher dewatering dose performance. Herein, the use of trithiocarbonate-mediated reversible addition-fragmentation chain transfer (RAFT) polymerization under either blue LED (λmax = 470 nm) or UV (λmax = 365 nm) irradiation, known as photoiniferter polymerization, was successfully utilized to generate ultra-high-molecular-weight (Mn > 1,000,000 g mol-1) polyelectrolyte copolymer flocculants with narrow molecular weight distributions (Mw/Mn < 1.2). Cationic and anionic polyelectrolyte flocculants were synthesized containing various monomer compositions of acrylamide (AM), dimethylacrylamide (DMA), 3-(acryloyloxyethyll)trimethylammonium chloride (DMAEAq), 3-(acrylamidopropyl)trimethylammonium chloride (APTAC), sodium acrylate (NaAA), and sodium 2-(acrylamido)-2-methylpropylsulfonate (NaATBS) with high monomer conversion using simple experimental apparatus. The narrow molecular weight distribution cationic polyelectrolytes showed improved flocculation efficiency in the clarification of kaolin suspensions of up to 50% in comparison to a broad polydispersity (Mw/Mn > 5.0) commercial benchmark with an equivalent number average molecular weight. The improved performance of the narrow-polydispersity copolymers is attributed to the reduction in the content of the lower-molecular-weight polymer chains, which impart lower flocculation performance.

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.

Designing polymers for cartilage uptake: effects of architecture and molar mass

Osteoarthritis (OA) is a progressive disease, involving the progressive breakdown of cartilage, as well as changes to the synovium and bone. There are currently no disease-modifying treatments available clinically. An increasing understanding of the disease pathophysiology is leading to new potential therapeutics, but improved approaches are needed to deliver these drugs, particularly to cartilage tissue, which is avascular and contains a dense matrix of collagens and negatively charged aggrecan proteoglycans. Cationic delivery vehicles have been shown to effectively penetrate cartilage, but these studies have thus far largely focused on proteins or nanoparticles, and the effects of macromolecular architectures have not yet been explored. Described here is the synthesis of a small library of polycations composed of N-(2-hydroxypropyl)methacrylamide (HPMA) and N-(3-aminopropyl)methacrylamide (APMA) with linear, 4-arm, or 8-arm structures and varying degrees of polymerization (DP) by reversible addition fragmentation chain-transfer (RAFT) polymerization. Uptake and retention of the polycations in bovine articular cartilage was assessed. While all polycations penetrated cartilage, uptake and retention generally increased with DP before decreasing for the highest DP. In addition, uptake and retention were higher for the linear polycations compared to the 4-arm and 8-arm polycations. In general, the polycations were well tolerated by bovine chondrocytes, but the highest DP polycations imparted greater cytotoxicity. Overall, this study reveals that linear polymer architectures may be more favorable for binding to the cartilage matrix and that the DP can be tuned to maximize uptake while minimizing cytotoxicity.

Inverse Miniemulsion Enables the Continuous-Flow Synthesis of Controlled Ultra-High Molecular Weight Polymers

We report the controlled synthesis of ultra-high molecular weight (UHMW) polymers (Mn ≥ 106 g/mol) via continuous flow in a tubular reactor. At high monomer conversion, UHMW polymers in homogeneous batch polymerization exhibit high viscosities that pose challenges for employing continuous flow reactors. However, under heterogeneous inverse miniemulsion (IME) conditions, UHMW polymers can be produced within the dispersed phase, while the viscosity of the heterogeneous mixture remains approximately the same as the viscosity of the continuous phase. Conducting such IME polymerizations in flow results in a faster rate of polymerization compared to batch IME polymerizations while still providing excellent control over molecular weight up to 106 g/mol. Crucial emulsion parameters, such as particle size and stability under continuous flow conditions, were examined using dynamic light scattering. A range of poly(N,N-dimethylacrylamide) and poly(4-acryloylmorpholine) polymers with molecular weights of 104-106 g/mol (Đ ≤ 1.31) were produced by this method using water-soluble trithiocarbonates as photoiniferters.

Synthetic exploration of electrophilic xanthylation via powerful N-xanthylphthalimides

Organic xanthates are broadly applied as synthetic intermediates and bioactive molecules in synthetic chemistry. Electrophilic xanthylation represents a promising approach but has rarely been explored mainly due to the lack of powerful electrophilic reagents. Herein, synthetic exploration of electrophilic xanthylation via powerful N-xanthylphthalimides was investigated. This strategy might provide a new avenue to less-concerned but meaningful electrophilic xanthylation in organic synthesis. With the help of these powerful reagents, electrophilic xanthylation of a wide range of substrates including aryl/alkenyl boronic acids, β-keto esters, 2-oxindole, and alkyl amines, as well as previously inaccessible phenols (first report) was achieved under mild reaction conditions. Notably, this simple electrophilic xanthylation of alkyl amine substrates will occur in the desulfuration reaction, consistent with the previously reported methods. Similarly, xanthamide and thioxanthate groups could also be transformed into desired nucleophiles via this electrophilic reagent strategy. The broad substrate scope, excellent functional group compatibility and late-stage functionalization of bioactive or functional molecules made them very attractive as general reagents which will allow rapid incorporation of SC(S)R (R = OEt, Oalkyl, NEt2 and SEt) into the target molecules.

Visible Light-Accelerated Photoiniferter Polymerization in Ionic Liquid

The effect of ionic liquids on the reversible addition-fragmentation chain transfer (RAFT) polymerization mediated by a visible-light-induced photoiniferter mechanism was investigated. N,N-Dimethyl acrylamide was polymerized by photoiniferter polymerization in 1-ethyl-3-methylimidazolium ethylsulfate [EMIM][EtSO4] ionic liquid. We observed a considerable increase in the polymerization rate constants in ionic liquids (ILs), as well as in the mixed solvent of water and the IL, compared to those observed with water alone as the solvent. To demonstrate the robustness of the process, block copolymers with varying block ratios were synthesized with precise control over their molecular weight and mass dispersity (Đ). The very high chain-end fidelity provided by the photoiniferter polymerization in IL was described by using MALDI-ToF MS analysis.

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.

Solvent-Free Templated Synthesis of Core-Crosslinked Star-Shaped Polymers in Supramolecular Body-Centered Cubic Phase

This study reports the exploration of a solvent-free supramolecular templated synthesis strategy toward highly core-cross-linked star-shaped polymers (CSPs). To achieve this, a kind of cross-linkable giant surfactant, based on a functionalized polyhedral oligomeric silsesquioxanes (POSS) head tethered with a diblock copolymer tail containing reactive benzocyclobutene groups, is designed and prepared. By varying the volume fraction of linear block copolymer tail, these giant surfactants can self-assemble into a body-centered cubic (BCC) structure in bulk, in which the supramolecular spheres are composed of a core of POSS cages, a middle shell of crosslinkable poly(4-vinylbenzocyclobutene) (PBCB) blocks, and a corona of inert polystyrene (PS) blocks. The solvent-free thermally induced cross-linking reaction of the benzocyclobutene groups can be finished in 5 min upon heating, resulting in well-defined polymeric spheres with over 90 linear chains surrounding the cross-linked cores. The outer PS blocks serve as the protection corona to ensure that cross-linking of giant surfactants occurs in each supramolecular spherical domain. Given the modular design and diversity of the POSS-based giant surfactants, it is believed that the strategy may enable access to a wide range of CSPs.

Crosslinked polypeptide films via RAFT mediated continuous assembly of polymers

Polypeptide coatings are a cornerstone in the field of surface modification due to their widespread biological potential. As their properties are dictated by their structural features, subsequent control thereof using unique fabrication strategies is important. Herein, we report a facile method of precisely creating densely crosslinked polypeptide films with unusually high random coil conformations through continuous assembly polymerization via reversible addition-fragmentation chain transfer (CAP-RAFT). CAP-RAFT was fundamentally investigated using methacrylated poly- L -lysine (PLLMA) and methacrylated poly- L -glutamic acid (PLGMA). Careful technique refinement resulted in films up to 36.1 ± 1.1 nm thick which could be increased to 94.9 ± 8.2 nm after using this strategy multiple times. PLLMA and PLGMA films were found to have 30-50% random coil conformations. Degradation by enzymes present during wound healing reveals potential for applications in drug delivery and tissue engineering.

Photo-Iniferter RAFT Polymerization

Light-mediated polymerization techniques offer distinct advantages over polymerization reactions fueled by thermal energy, such as high spatial and temporal control as well as the possibility to work under mild reaction conditions. Reversible addition-fragmentation chain-transfer (RAFT) polymerization is a highly versatile radical polymerization method that can be utilized to control a variety of monomers and produce a vast number of complex macromolecular structures. The use of light to drive a RAFT-polymerization is possible via multiple routes. Besides the use of photo-initiators, or photo-catalysts, the direct activation of the chain transfer agent controlling the RAFT process in a photo-iniferter (PI) process is an elegant way to initiate and control polymerization reactions. Within this review, PI-RAFT polymerization and its advantages over the conventional RAFT process are discussed in detail.

The synthesis of thermoresponsive POSS-based eight-arm star poly(N-isopropylacrylamide): A comparison between Z-RAFT and R-RAFT strategies

Z-Type POSS-based eight-arm star poly(N-isopropylacrylamide), POSS-(PNIPAM)₈-Z, is synthesized and demonstrated to be a thermoresponsive switchable emulsifier.

Achieving Ultrahigh Molecular Weights with Diverse Architectures for Unconjugated Monomers through Oxygen-Tolerant Photoenzymatic RAFT Polymerization

Achieving well-defined polymers with ultrahigh molecular weight (UHMW) is an enduring pursuit in the field of reversible deactivation radical polymerization. Synthetic protocols have been successfully developed to achieve UHMWs with low dispersities exclusively from conjugated monomers while no polymerization of unconjugated monomers has provided the same level of control. Herein, an oxygen-tolerant photoenzymatic RAFT (reversible addition-fragmentation chain transfer) polymerization was exploited to tackle this challenge for unconjugated monomers at 10 °C, enabling facile synthesis of well-defined, linear and star polymers with near-quantitative conversions, unprecedented UHMWs and low dispersities. The exquisite level of control over composition, MW and architecture, coupled with operational ease, mild conditions and environmental friendliness, broadens the monomer scope to include unconjugated monomers, and to achieve previously inaccessible low-dispersity UHMWs.

Progress and Perspectives Beyond Traditional RAFT Polymerization

The development of advanced materials based on well-defined polymeric architectures is proving to be a highly prosperous research direction across both industry and academia. Controlled radical polymerization techniques are receiving unprecedented attention, with reversible-deactivation chain growth procedures now routinely leveraged to prepare exquisitely precise polymer products. Reversible addition-fragmentation chain transfer (RAFT) polymerization is a powerful protocol within this domain, where the unique chemistry of thiocarbonylthio (TCT) compounds can be harnessed to control radical chain growth of vinyl polymers. With the intense recent focus on RAFT, new strategies for initiation and external control have emerged that are paving the way for preparing well-defined polymers for demanding applications. In this work, the cutting-edge innovations in RAFT that are opening up this technique to a broader suite of materials researchers are explored. Emerging strategies for activating TCTs are surveyed, which are providing access into traditionally challenging environments for reversible-deactivation radical polymerization. The latest advances and future perspectives in applying RAFT-derived polymers are also shared, with the goal to convey the rich potential of RAFT for an ever-expanding range of high-performance applications.

100th Anniversary of Macromolecular Science Viewpoint: Achieving Ultrahigh Molecular Weights with Reversible Deactivation Radical Polymerization

Synthetic strategies for achieving ultrahigh molecular weights via reversible deactivation radical polymerization are discussed from the mechanistic, kinetic, and experimental aspects, and their applications as high-performance materials are highlighted. Further development of this field requires continuous effort to improve livingness and polymerization efficiency under greener conditions.

Parallel evolution of polymer chemistry and immunology: Integrating mechanistic biology with materials design

To develop new therapeutics involves the interaction of multiple disciplines to yield safe, functional devices and formulations. Regardless of drug function and potency, administration with controlled timing, dosing, and targeting is required to properly treat or regulate health and disease. Delivery approaches can be optimized through advances in materials science, clinical testing, and basic biology and immunology. Presently, laboratories focused on developing these technologies are composed of, or collaborate with, chemists, biologists, materials scientists, engineers, and physicians to understand the way our body interacts with drug delivery devices, and how to synthesize new, rationally designed materials to improve targeted and controlled drug delivery. In this review, we discuss both device-based and micro/nanoparticle-based materials in the clinic, our biologic understanding of how our immune system interacts with these materials, how this diverse set of immune cells has become a target and variable in drug delivery design, and new directions in polymer chemistry to address these interactions and further our advances in medical therapeutics.