Using RAFT Polymerization Methodologies to Create Branched and Nanogel-Type Copolymers

This review aims to highlight the most recent advances in the field of the synthesis of branched copolymers and nanogels using reversible addition-fragmentation chain transfer (RAFT) polymerization. RAFT polymerization is a reversible deactivation radical polymerization technique (RDRP) that has gained tremendous attention due to its versatility, compatibility with a plethora of functional monomers, and mild polymerization conditions. These parameters lead to final polymers with good control over the molar mass and narrow molar mass distributions. Branched polymers can be defined as the incorporation of secondary polymer chains to a primary backbone, resulting in a wide range of complex macromolecular architectures, like star-shaped, graft, and hyperbranched polymers and nanogels. These subcategories will be discussed in detail in this review in terms of synthesis routes and properties, mainly in solutions.

Navigating the Expansive Landscapes of Soft Materials: A User Guide for High-Throughput Workflows

Synthetic polymers are highly customizable with tailored structures and functionality, yet this versatility generates challenges in the design of advanced materials due to the size and complexity of the design space. Thus, exploration and optimization of polymer properties using combinatorial libraries has become increasingly common, which requires careful selection of synthetic strategies, characterization techniques, and rapid processing workflows to obtain fundamental principles from these large data sets. Herein, we provide guidelines for strategic design of macromolecule libraries and workflows to efficiently navigate these high-dimensional design spaces. We describe synthetic methods for multiple library sizes and structures as well as characterization methods to rapidly generate data sets, including tools that can be adapted from biological workflows. We further highlight relevant insights from statistics and machine learning to aid in data featurization, representation, and analysis. This Perspective acts as a "user guide" for researchers interested in leveraging high-throughput screening toward the design of multifunctional polymers and predictive modeling of structure-property relationships in soft materials.

Exploring the role of polymer hydrophobicity in polymer-metal binding thermodynamics

Metal-chelating polymers play a key role in rare-earth element (REE) extraction and separation processes. Often, these processes occur in aqueous solution, but the interactions among water, polymer, and REE are largely under-investigated in these applications. To probe these interactions, we synthesized a series of poly(amino acid acrylamide)s with systematically varied hydrophobicity around a consistent chelating group (carboxylate). We then measured the ΔH of Eu³⁺ chelation as a function of temperature across the polymer series using isothermal titration calorimetry (ITC) to give the change in heat capacity (ΔC_P). We observed an order of magnitude variation in ΔC_P (39-471 J mol¹ K^-1) with changes in the hydrophobicity of the polymer. Atomistic simulations of the polymer-metal-water interactions revealed greater Eu³⁺ and polymer desolvation when binding to the more hydrophobic polymers. These combined experimental and computational results demonstrate that metal binding in aqueous solution can be modulated not only by directly modifying the chelating groups, but also by altering the molecular environment around the chelating site, thus suggesting a new design principle for developing increasingly effective metal-chelating materials.

Synthesis and Visualization of bottlebrush-shaped segmented hyperbranched polymers

Visualization of single molecular morphology provides an intuitive evidence to understand the relationships of molecular structure-synthetic method. Herein, by combining the architectural features of molecular bottlebrush (MBB) and segmented hyperbranched...

Facile synthesis and self-assembly behaviors of biodegradable amphiphilic hyperbranched copolymers with reducible poly(caprolactone) grafts

A reducible hydrophobic macromonomer, HEMA-g-PCL, developed herein provides a facile yet robust strategy for biodegradable amphiphilic hyperbranched copolymers.

Multi-responsive poly(oligo(ethylene glycol)methyl methacrylate)-co-poly(2-(diisopropylamino)ethyl methacrylate) hyperbranched copolymers via reversible addition fragmentation chain transfer polymerization

Multi-responsive P(OEGMA-co-DIPAEMA) hyperbranched copolymers are synthesized via RAFT polymerization. The copolymers form different aggregates in aqueous media depending on solution pH, temperature and copolymer composition.

Triple Functional AB2 Unit-Modulated Facile Preparation of Bioreducible Hyperbranched Copolymers

Facile preparation of hyperbranched polymers (HPs) has been advanced tremendously by the use of either various multifunctional agent-mediated controlled living radical polymerizations or a highly reactive AB_x unit-modulated self-stepwise polymerizations. However, it remains, to our knowledge, a significant challenge to prepare HPs with simultaneously precisely controlled degree of branching (DB) and biorelevant signal-triggered degradation property for controlled release applications due to the respective limitations of the aforementioned two strategies. For this purpose, a triple functional AB₂ unit, A-SS-B₂ chain transfer agent (AB₂ CTA), that integrates the merits of both multifunctional agents and highly reactive AB_x units was designed and synthesized successfully to include a disulfide bond for reduction-triggered polymer degradation toward promoted intracellular release of encapsulated cargoes, a trithiocarbonate group for a universal reversible addition-fragmentation chain transfer (RAFT) polymerization of any vinyl-based monomer, and three terminal groups consisting of one azide and two alkyne functions for the generation of hyperbranched topology via a self-click coupling-based polymerization. A subsequent self-click polymerization of the resulting AB₂ CTA by click coupling in the presence of CuSO₄·5H₂O and sodium ascorbate (NaVc) generated a hyperbranched polymer template (HPT) with precisely modulated DB and a plurality of CTA units for a universal reversible addition-fragmentation chain transfer (RAFT) polymerization of any vinyl-containing monomer. The HPT was next used as a multimacro-CTA for RAFT polymerization of a typical hydrophilic monomer, oligo(ethylene glycol) monomethyl ether methacrylate (OEGMA), to demonstrate the potential of this HPT for a robust and facile production of bioreducible hyperbranched polymers for controlled release applications. The synthesized HPT-4-POEGMA can form unimolecular micelles with enhanced stability due to the hyperbranched structure, and the size of micelles varied in the range from 82.4 to 140.3 nm by a modulation of the molar feed ratio of monomer to HPT and polymerization time. More importantly, HPT-POEGMA micelles incubated with 10 mM glutathione (GSH) showed reduction-triggered cleavage of the disulfide links and polymer degradation for promoted intracellular doxorubicin (DOX) release and enhanced therapeutic efficiency. Taken together, this triple functional AB₂ CTA provided a powerful means for the facile preparation of bioreducible hyperbranched polymers with precisely controlled DB for controlled release applications.

Photoreversible Covalent Hydrogels for Soft-Matter Additive Manufacturing

Reversible covalent chemistry provides access to robust materials with the ability to be degraded and reformed upon exposure to an appropriate stimulus. Photoresponsive units are attractive for this purpose, as the spatial and temporal application of light is easily controlled. Coumarin derivatives undergo a [2 + 2] cycloaddition upon exposure to long-wave UV irradiation (365 nm), and this process can be reversed using short-wave UV light (254 nm). Therefore, polymers cross-linked by coumarin groups are excellent candidates as reversible covalent gels. In this work, copolymerization of coumarin-containing monomers with the hydrophilic comonomer N, N-dimethylacrylamide yielded water-soluble, linear polymers that could be cured with long-wave UV light into free-standing hydrogels, even in the absence of a photoinitiator. Importantly, the gels were reverted back to soluble copolymers upon short-wave UV irradiation. This process could be cycled, allowing for recycling and remolding of the hydrogel into additional shapes. Further, this hydrogel can be imprinted with patterns through a mask-based, post-gelation photoetching method. Traditional limitations of this technique, such as the requirement for uniform etching in one direction, have been overcome by combining these materials with a soft-matter additive manufacturing methodology. In a representative application of this approach, we printed solid structures in which the interior coumarin-cross-linked gel is surrounded by a nondegradable gel. Upon exposure to short-wave UV irradiation, the coumarin-cross-linked gel was reverted to soluble prepolymers that were washed away to yield hollow hydrogel objects.

Main-chain degradable star polymers comprised of pH-responsive hyperbranched cores and thermoresponsive polyethylene glycol-based coronas

Dual stimuliresponsive main-chain degradable star hyperbranched polymers have been synthesized via cyclic ketene acetal radical ring-opening and RAFT-based methacrylate copolymerization.

Synthetic upcycling of polyacrylates through organocatalyzed post-polymerization modification

The direct transformation of commercially available commodity polyacrylates into value-added materials was achieved. We demonstrate how 1,5,7-triazabicyclo[4.4.0]dec-5-ene, serving as a nucleophilic catalyst, can be used to catalyze acyl substitution reactions of acrylic polymers in the presence of alcohol and amine nucleophiles. Furthermore, we found that organocatalytic transesterification exhibits high selectivity towards sterically unhindered esters, thus providing a new route towards site-selective acyl substitution of macromolecular materials. Combining this methodology with reversible-deactivation radical polymerization (RDRP) techniques such as reversible addition-fragmentation chain-transfer (RAFT) polymerization allowed for the precise functionalization of sterically-differentiated acrylic copolymers and polymeric chain ends. We envision this approach to expedite functional polymer synthesis and provide access to functional macromolecules prepared from inexpensive, hydrolytically-stable polymeric precursors.