Ubiquitous Nature of Rate Retardation in Reversible Addition-Fragmentation Chain Transfer Polymerization

Photo-RAFT Polymerization Under Microwatt Irradiation via Unimolecular Photoinduced Electron Transfer

Photoinduced radical addition fragmentation chain transfer (PET-RAFT) polymerization typically requires high light intensity (>5 mW cm- 2), limiting energy efficiency, and scalability. We demonstrate that adding a base to PET-RAFT systems drastically enhances the reactivity of acidic chain transfer agents (CTAs) with Zn-based photocatalysts (Zn porphyrin and Zn phthalocyanine). This approach enables complete polymerization under microwatt light intensity (0.25 mW cm- 2), a significant improvement over traditional PET-RAFT, which showed no conversion under the same conditions. Both acrylates and methacrylates polymerized efficiently with excellent chain-end fidelity. Reactivity was triggered chemically (via base addition) or electrochemically (via electrolytic reduction). Mechanistic studies reveal that base addition promotes a CTA-Zn photocatalyst complex, shifting the activation from bimolecular to more efficient unimolecular PET-RAFT.

Temperature Effects in Conventional and RAFT Photopolymerization

Photochemical processes are often thought to be temperature-independent. However, photochemical polymerization involves photochemical processes such as light-driven radical generation coupled with thermal-driven reactions such as monomer propagation. The apparent activation energy of propagation, E A(R p), of a series of three monomers, methyl acrylate (MA), methyl methacrylate (MMA), and styrene (STY), are deduced from Arrhenius analysis of conventional and RAFT photopolymerization of these monomers across a range of corresponding temperatures. The deduced E A(R p) was compared with the benchmarked E A(k p) derived from pulse laser polymerizations coupled with size exclusion chromatography (PLP-SEC). For conventional photopolymerization of MA, MMA and STY, the relatively small discrepancy between the photopolymerization-derived E A(R p) and the E A(k p) from PLP-SEC was rationalized due to temperature-induced changes in termination. The deviation between the E A(R p) measured in RAFT photopolymerization and E A(k p) from PLP-SEC depends on the retardation strength in RAFT polymerizations. MMA and STY monomers are characterized with minimal retardation and recorded excellent agreement in PLP-SEC and RAFT-derived E p values. However, the RAFT photopolymerization of MA, which is subject to strong retardation, had a much larger E A(R p) than the E A(k p) from PLP-SEC. The high apparent E A(R p) in RAFT polymerization of MA is likely due to the added influence of temperature-induced changes in the RAFT equilibrium. Overall, these results rationalize temperature-dependent effects in photochemical reactions.

RAFT Polymerization for Advanced Morphological Control: From Individual Polymer Chains to Bulk Materials

Control of the morphology of polymer systems is achieved through reversible-deactivation radical polymerization techniques such as Reversible Addition-Fragmentation chain Transfer (RAFT). Advanced RAFT techniques offer much more than just "living" polymerization - the RAFT toolkit now enables morphological control of polymer systems across many decades of length-scale. Morphological control is explored at the molecular-level in the context of syntheses where individual monomer unit insertion provides sequence-defined polymers (single unit monomer insertion, SUMI). By being able to define polymer architectures, the synthesis of bespoke shapes and sizes of nanostructures becomes possible by leveraging self-assembly (polymerization induced self-assembly, PISA). Finally, it is seen that macroscopic materials can be produced with nanoscale detail, based on phase-separated nanostructures (polymerization induced microphase separation, PIMS) and microscale detail based on 3D-printing technologies. RAFT control of morphology is seen to cross from molecular level to additive manufacturing length-scales, with complete morphological control over all length-scales.

Replacing poly(ethylene glycol) with RAFT lipopolymers in mRNA lipid nanoparticle systems for effective gene delivery

Lipid nanoparticles (LNPs) have emerged as promising carriers to efficiently transport mRNA into cells for protein translation, as seen with the mRNA vaccines used against COVID-19. However, they contain a widely used polymer - poly(ethylene glycol) (PEG) - which lacks the functionality to be easily modified (which could effectively control the physicochemical properties of the LNPs such as its charge), and is also known to be immunogenic. Thus, it is desirable to explore alternative polymers which can replace the PEG component in mRNA LNP vaccines and therapeutics, while still maintaining their efficacy. Herein, we employed reversible addition-fragmentation chain transfer (RAFT) polymerisation to synthesise five PEG-lipid alternatives that could stabilise LNPs encapsulating mRNA or pDNA molecules. Importantly, the resultant RAFT lipopolymer LNPs exhibit analogous or higher in vivo gene expression and antigen-specific antibody production compared to traditional PEG-based formulations. Our synthesis strategy which allows the introduction of positive charges along the lipopolymer backbone also significantly improved the in vivo gene expression. This work expands the potential of RAFT polymer-conjugated LNPs as promising mRNA carriers and offers an innovative strategy for the development of PEG-free mRNA vaccines and therapeutics.

Vinyl Ether Maleic Acid Polymers: Tunable Polymers for Self-Assembled Lipid Nanodiscs and Environments for Membrane Proteins

Native lipid bilayer mimetics, including those that use amphiphilic polymers, are important for the effective study of membrane-bound peptides and proteins. Copolymers of vinyl ether monomers and maleic anhydride were developed with controlled molecular weights and hydrophobicity through reversible addition-fragmentation chain-transfer polymerization. After polymerization, the maleic anhydride units can be hydrolyzed, giving dicarboxylates. The vinyl ether and maleic anhydride copolymerized in a close to alternating manner, giving essentially alternating hydrophilic maleic acid units and hydrophobic vinyl ether units along the backbone after hydrolysis. The vinyl ether monomers and maleic acid polymers self-assembled with lipids, giving vinyl ether maleic acid lipid particles (VEMALPs) with tunable sizes controlled by either the vinyl ether hydrophobicity or the polymer molecular weight. These VEMALPs were able to support membrane-bound proteins and peptides, creating a new class of lipid bilayer mimetics.

Tuning the Mechanical Properties of 3D-Printed Objects by Mixing Chain Transfer Agents in Norrish Type I Photoinitiated RAFT Polymerization

Photoinduced 3D printing via photocontrolled reversible-deactivation radical polymerization (photoRDRP) techniques has emerged as a robust technique for creating polymeric materials. However, methods for precisely adjusting the mechanical properties of these materials remain limited. In this study, we present a facile approach for adjusting the mechanical properties of 3D-printed objects by adjusting the polymer dispersity within a Norrish type I photoinitiated reversible addition-fragmentation chain transfer (NTI-RAFT) polymerization-based 3D printing process. We investigated the effects of varying the concentrations and molar ratios of trithiocarbonate (BTPA) and xanthate (EXEP) on the mechanical properties of the printed materials. Our findings demonstrate that increased concentrations of RAFT agents or higher proportions of the more active BTPA lead to a decrease in Young's modulus and glass transition temperatures, along with an increase in elongation at break, which can be attributed to the enhanced homogeneity of the polymer network. Using a commercial LCD printer, the NTI-RAFT-based 3D printing system effectively produced materials with tailored mechanical properties, highlighting its potential for practical applications.

Pyrazole carbodithiolate-driven iterative RAFT single-additions

In this Communication, we comprehensively investigated substituent effects relevant to iterative reversible activation fragmentation chain transfer (RAFT) single unit monomer insertion (SUMI) reactions. Through the use of the pyrazole carbodithiolate (PCDT) "Z-group" as the chain transfer component in RAFT SUMI, we show the importance of "Z-group" effects and its interplay with "R-group" (the carbon-centred radical precursor) effects. We also expanded the scope of RAFT SUMI to new monomer types and sequences thereof. As such, the C-S bond dissocation/reformation steps were found to be crucial factors in SUMI, and it was found that general substituent effects must be wholistically examined for every step of this reaction. This stands in contrast with conventional knowledge of RAFT polymerisation, where the main consideration is often centred around the propagation stage, i.e., the key C-C bond formation step. Indeed, contrary to SUMI, the latter characteristic was observed in the analogous alternating copolymerisation.

Network Polymer Properties Engineered Through Polymer Backbone Dispersity and Structure

Dispersity (Ð or Mw/Mn) is an important parameter in material design and as such can significantly impact the properties of polymers. Here, polymer networks with independent control over the molecular weight and dispersity of the linear chains that form the material are developed. Using a RAFT polymerization approach, a library of polymers with dispersity ranging from 1.2-1.9 for backbone chain-length (DP) 100, and 1.4-3.1 for backbone chain-length 200 were developed and transformed to networks through post-polymerization crosslinking to form disulfide linkers. The tensile, swelling, and adhesive properties were explored, finding that both at DP 100 and DP 200 the swelling ratio, tensile strength, and extensibility were superior at intermediate dispersity (1.3-1.5 for DP 100 and 1.6-2.1 for DP 200) compared to materials with either substantially higher or lower dispersity. Furthermore, adhesive properties for materials with chains of intermediate dispersity at DP 200 revealed enhanced performance compared to the very low or high dispersity chains.

A Versatile Comonomer Additive for Radically Recyclable Vinyl-derived Polymers

Radically-formed, vinyl-derived polymers account for over 30 % of polymer production. Connected through stable carbon-carbon bonds, these materials are notoriously challenging to chemically recycle. Herein, we report universal copolymerization of a cyclic allyl sulfide (CAS) additive with multiple monomers under free-radical conditions, to introduce main-chain dynamic motifs. Backbone allyl sulfides undergo post-polymerization radical rearrangement via addition-fragmentation-transfer (AFT) that fosters both chain scission and extension. Scission is selectively induced through allyl sulfide exchange with small molecule thiyl radicals, resulting in oligomers as low as 14 % of the initial molar mass. Crucially, oligomers retain allyl sulfide end groups, enabling their extension with monomer under radical conditions. Extended, i.e., recycled, product molar mass is tunable through the ratio of monomer to oligomer, and can surpass that of the initial copolymer. Two scission-extension cycles are demonstrated in copolymers with methyl methacrylate and styrene without escalation in dispersity. In illustration of forming higher-value products, i.e., upcycling, we synthesized block copolymers through the extension of oligomers with a different vinyl monomer. Collectively, our approach to chemical recycling is unparalleled in its ability to 1) function in a variety of vinyl-derived polymers, 2) complete radical closed-loop cycling, and 3) upcycle waste material.

The Nanostructured Self-Assembly and Thermoresponsiveness in Water of Amphiphilic Copolymers Carrying Oligoethylene Glycol and Polysiloxane Side Chains

Amphiphilic copolymer self-assembly is a straightforward approach to obtain responsive micelles, nanoparticles, and vesicles that are particularly attractive for biomedicine, i.e., for the delivery of functional molecules. Here, amphiphilic copolymers of hydrophobic polysiloxane methacrylate and hydrophilic oligo (ethylene glycol) methyl ether methacrylate with different lengths of oxyethylenic side chains were synthesized via controlled RAFT radical polymerization and characterized both thermally and in solution. In particular, the thermoresponsive and self-assembling behavior of the water-soluble copolymers in water was investigated via complementary techniques such as light transmittance, dynamic light scattering (DLS), and small-angle X-ray scattering (SAXS) measurements. All the copolymers synthesized were thermoresponsive, displaying a cloud point temperature (T_cp) strongly dependent on macromolecular parameters such as the length of the oligo(ethylene glycol) side chains and the content of the SiMA counits, as well as the concentration of the copolymer in water, which is consistent with a lower critical solution temperature (LCST)-type behavior. SAXS analysis revealed that the copolymers formed nanostructures in water below T_cp, whose dimension and shape depended on the content of the hydrophobic components in the copolymer. The hydrodynamic diameter (D_h) determined by DLS increased with the amount of SiMA and the associated morphology at higher SiMA contents was found to be pearl-necklace-micelle-like, composed of connected hydrophobic cores. These novel amphiphilic copolymers were able to modulate thermoresponsiveness in water in a wide range of temperatures, including the physiological temperature, as well as the dimension and shape of their nanostructured assemblies, simply by varying their chemical composition and the length of the hydrophilic side chains.

Step-growth polymerization by the RAFT process

Reversible Addition-Fragmentation Chain Transfer (RAFT) step-growth polymerization is an emerging method that synergistically combines the benefits of RAFT polymerization (functional group and user-friendly nature) and step-growth polymerization (versatility of the polymer backbone). This new polymerization method is generally achieved by using bifunctional reagents of monomer and Chain Transfer Agent (CTA), that efficiently yield Single Monomer Unit Insertion (SUMI) adducts under stoichiometrically balanced conditions. This review covers a brief history of the RAFT-SUMI process and its transformation into RAFT step-growth polymerization, followed by a comprehensive discussion of various RAFT step-growth systems. Furthermore, characterizing the molecular weight evolution of step-growth polymerization is elaborated based on the Flory model. Finally, a formula is introduced to describe the efficiency of the RAFT-SUMI process, assuming rapid chain transfer equilibrium. Examples of reported RAFT step-growth and SUMI systems are then categorized based on the driving force.

Design, Synthesis, and Biomedical Application of Multifunctional Fluorescent Polymer Nanomaterials

Luminescent polymer nanomaterials not only have the characteristics of various types of luminescent functional materials and a wide range of applications, but also have the characteristics of good biocompatibility and easy functionalization of polymer nanomaterials. They are widely used in biomedical fields such as bioimaging, biosensing, and drug delivery. Designing and constructing new controllable synthesis methods for multifunctional fluorescent polymer nanomaterials with good water solubility and excellent biocompatibility is of great significance. Exploring efficient functionalization methods for luminescent materials is still one of the core issues in the design and development of new fluorescent materials. With this in mind, this review first introduces the structures, properties, and synthetic methods regarding fluorescent polymeric nanomaterials. Then, the functionalization strategies of fluorescent polymer nanomaterials are summarized. In addition, the research progress of multifunctional fluorescent polymer nanomaterials for bioimaging is also discussed. Finally, the synthesis, development, and application fields of fluorescent polymeric nanomaterials, as well as the challenges and opportunities of structure-property correlations, are comprehensively summarized and the corresponding perspectives are well illustrated.

Development and Experimental Validation of a Dispersity Model for In Silico RAFT Polymerization

The exploitation of computational techniques to predict the outcome of chemical reactions is becoming commonplace, enabling a reduction in the number of physical experiments required to optimize a reaction. Here, we adapt and combine models for polymerization kinetics and molar mass dispersity as a function of conversion for reversible addition fragmentation chain transfer (RAFT) solution polymerization, including the introduction of a novel expression accounting for termination. A flow reactor operating under isothermal conditions was used to experimentally validate the models for the RAFT polymerization of dimethyl acrylamide with an additional term to accommodate the effect of residence time distribution. Further validation is conducted in a batch reactor, where a previously recorded in situ temperature monitoring provides the ability to model the system under more representative batch conditions, accounting for slow heat transfer and the observed exotherm. The model also shows agreement with several literature examples of the RAFT polymerization of acrylamide and acrylate monomers in batch reactors. In principle, the model not only provides a tool for polymer chemists to estimate ideal conditions for a polymerization, but it can also automatically define the initial parameter space for exploration by computationally controlled reactor platforms provided a reliable estimation of rate constants is available. The model is compiled into an easily accessible application to enable simulation of RAFT polymerization of several monomers.

Alkyne Polymers from Stable Butatriene Homologues: Controlled Radical Polymerization of Vinylidenecyclopropanes

Controlled polymerization of cumulenic monomers represents a promising yet underdeveloped strategy toward well-defined alkyne polymers. Here we report a stereoelectronic effect-inspired approach using simple vinylidenecyclopropanes (VDCPs) as butatriene homologues in controlled radical ring-opening polymerizations. While being thermally stable, VDCPs mimic butatrienes via conjugation of the cyclopropane ring. This leads to exclusive terminal-selective propagation that affords a highly structurally regular alkyne-based backbone, featuring complete ring-opening and no backbiting regardless of polymerization conditions.

Fast Living 3D Printing via Free Radical Promoted Cationic RAFT Polymerization

The application of reversible deactivation radical polymerization techniques in 3D printing is emerging as a powerful method to build "living" polymer networks, which can be easily postmodified with various functionalities. However, the building speed of these systems is still limited compared to commercial systems. Herein, a digital light processing (DLP)-based 3D printing system via photoinduced free radical-promoted cationic reversible addition-fragmentation chain transfer polymerization of vinyl ethers, which can build "living" objects by a commercial DLP 3D printer at a relatively fast building speed (12.99 cm h^-1 ), is reported. The polymerization behavior and printing conditions are studied in detail. The livingness of the printed objects is demonstrated by spatially controlled postmodification with a fluorescent monomer.

Observation and Modeling of a Sharp Oxygen Threshold in Aqueous Free Radical and RAFT Polymerization

It is known that oxygen (O₂) stops radical polymerization (RP). Here, it was found that the reaction turn-off occurs abruptly at a threshold concentration of O₂, [O₂]_t, for both free RP and reversible addition-fragmentation chain-transfer polymerization (RAFT). In some reactions, there was a spontaneous re-start of conversion. Three cases were investigated: RP of (i) acrylamide (Am) and (ii) sodium styrene sulfonate (SS) and (iii) Am RAFT polymerization. A controlled flow of O₂ into the reactor was employed. An abrupt turn-off was observed in all cases, where polymerization stops sharply at [O₂]_t and remains stopped when [O₂] > [O₂]_t. In (i), Am acts as a catalytic radical-transfer agent during conversion plateau, eliminating excess [O₂], and polymerization spontaneously resumes at [O₂]_t. In no reaction, the initiator alone was capable of eliminating O₂. N₂ purge was needed to re-start reactions (ii) and (iii). For (i) and (ii), while [O₂] < [O₂]_t, O₂ acts a chain termination agent, reducing the molecular weight (M_w) and reduced viscosity (RV). O₂ acts as an inhibitor for [O₂] > [O₂]_t in all cases. The radical-transfer rates from Am* and SS* to O₂ are >10,000× higher than the initial chain propagation step rates for Am and SS, which causes [O₂]_t at very low [O₂].

Evolution of Molar Mass Distributions Using a Method of Partial Moments: Initiation of RAFT Polymerization

We describe a method of partial moments devised for accurate simulation of the time/conversion evolution of polymer composition and molar mass. Expressions were derived that enable rigorous evaluation of the complete molar mass and composition distribution for shorter chain lengths (e.g., degree of polymerization, Xn = N < 200 units) while longer chains (Xn ≥ 200 units) are not neglected, rather they are explicitly considered in terms of partial moments of the molar mass distribution, μxN(P)=∑n=N+1∞nx[Pn] (where P is a polymeric species and n is its’ chain length). The methodology provides the exact molar mass distribution for chains Xn < N, allows accurate calculation of the overall molar mass averages, the molar mass dispersity and standard deviations of the distributions, provides closure to what would otherwise be an infinite series of differential equations, and reduces the stiffness of the system. The method also allows for the inclusion of the chain length dependence of the rate coefficients associated with the various reaction steps (in particular, termination and propagation) and the various side reactions that may complicate initiation or initialization. The method is particularly suited for the detailed analysis of the low molar mass portion of molar mass distributions of polymers formed by radical polymerization with reversible addition-fragmentation chain transfer (RAFT) and is relevant to designing the RAFT-synthesis of sequence-defined polymers. In this paper, we successfully apply the method to compare the behavior of thermally initiated (with an added dialkyldiazene initiator) and photo-initiated (with a RAFT agent as a direct photo-iniferter) RAFT-single-unit monomer insertion (RAFT-SUMI) and oligomerization of N,N-dimethylacrylamide (DMAm).

RAFT Step-Growth Polymerization of Diacrylates

RAFT step-growth polymerization was previously demonstrated with monomers that bear low rate of homopropagation to favor the chain transfer process; by contrast, acrylates are known to be fast homopropagating monomers, thereby posing serious challenges for RAFT step-growth. Here, we identified a chain transfer agent (CTA) that rapidly yields single unit monomer inserted (SUMI) CTA adducts with a model acrylate monomer. Using a bifunctional reagent of this CTA, we successfully demonstrated RAFT step-growth polymerization with diacrylates, yielding linear polymer backbones. Furthermore, we achieved inclusion of functionality (i.e., disulfide) into RAFT step-growth polymer via a disulfide incorporated bifunctional CTA. Grafting from this backbone resulted in molecular brush polymers with cleavable functionality in each repeat unit of the backbone, allowing selective degradation to afford well-defined unimolecular species of two polymeric side chains. Given the wide selection of commercially available diacrylates, RAFT step-growth polymerization of diacrylates will further enable facile synthesis of complex architectures with modular backbones.

Selective Photoactivation of Trithiocarbonates Mediated by Metal Naphthalocyanines and Overcoming Activation Barriers Using Thermal Energy

Metal naphthalocyanines (MNcs) were demonstrated to be efficient photocatalysts to activate photoinduced electron-transfer reversible addition-fragmentation chain transfer (PET-RAFT) polymerization, enabling well-controlled polymerization of (meth)acrylates under near-infrared (λ = 780 nm) light. Owing to their lower redox potential compared to previously explored photocatalysts, the activation of trithiocarbonate RAFT agents exhibited a unique selectivity that was dependent on the nature of the R group. Specifically, MNcs were capable in activating tertiary R group trithiocarbonates, whereas no activation of the trithiocarbonate possessing a secondary R group was observed. The combination of density functional theory calculations and experimental studies have revealed new mechanistic insights into the factors governing a PET-RAFT mechanism and explained this unique selectivity of MNcs toward tertiary carbon trithiocarbonates. Interestingly, by increasing the reaction temperature moderately (i.e., ∼15 °C), the energy barrier prohibiting the photoactivation of the trithiocarbonate with a secondary R group was overcome, enabling their successful activation.

A general model for the ideal chain length distributions of polymers made with reversible deactivation

A general model is developed for the distribution of polymers made with reversible deactivation. The model is applied to a range of experimental systems including RAFT, cationic and ATRP.

Radical–radical coupling effects in the direct-growth grafting-through synthesis of bottlebrush polymers using RAFT and ROMP

The direct-growth technique was used to synthesize macromonomers from four classes of vinyl monomers, and the influence of monomer type and conversion on coupling reactions was followed in grafting-through ring-opening metathesis polymerization.

Reverse sequence polymerization-induced self-assembly in aqueous media: a counter-intuitive approach to sterically-stabilized diblock copolymer nano-objects

A 500 nm charge-stabilized latex is converted into 40 nm sterically-stabilized nanoparticles via reverse sequence polymerization-induced self-assembly (PISA).