Efficient Synthesis and Self-Assembly of Segmented Hyperbranched Block Copolymers via RAFT-Mediated Dispersion Polymerization Using Segmented Hyperbranched Macro-RAFT Agents

Microphase Separation and Gelation through Polymerization-Induced Self-Assembly Using Star Polyethylene Glycols

Polymerization-induced self-assembly (PISA) during the synthesis of diblock copolymers has garnered considerable interest; however, architectures beyond diblock copolymers have scarcely been explored. Here, we studied PISA using 4- and 8-arm star polyethylene glycol (PEG), as well as 2-arm (linear) PEG, wherein each terminus of PEG was functionalized with a chain-transfer agent, holding a constant molar mass for each arm. Styrene was polymerized from each PEG terminus through reversible addition-fragmentation chain-transfer (RAFT) polymerization in an ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate, [BMIM][PF6]), with a total solute concentration of 40 wt %. While the styrene monomer is soluble in [BMIM][PF6], polystyrene is not; thus, self-assembly and cross-linking (gelation) occur. Structural analysis by small-angle X-ray scattering revealed that a relatively ordered microphase-separated structure for PISA was observed. Two-arm PEG-PS formed hexagonally packed cylinders, whereas 4- and 8-arm PEG-PS exhibited hexagonal close-packed spheres and disordered spheres. The dynamics, studied by oscillatory rheology, were also influenced by the number of arms; the 4-arm star block copolymers showed the highest plateau modulus. This study demonstrates that the topology is an important factor in controlling the microphase-separated structure and mechanical properties when preparing gels through PISA.

Aqueous RAFT Dispersion Polymerization Mediated by an ω,ω-Macromolecular Chain Transfer Monomer: An Efficient Approach for Amphiphilic Branched Block Copolymers and the Assemblies

Herein, an ω,ω-macromolecular chain transfer monomer (macro-CTM) containing a RAFT (reversible addition-fragmentation chain transfer) group and a methacryloyl group was synthesized and used to mediate photoinitiated RAFT dispersion polymerization of hydroxypropyl methacrylate (HPMA) in water. The macro-CTM undergoes a self-condensing vinyl polymerization (SCVP) mechanism under RAFT dispersion polymerization conditions, leading to the formation of amphiphilic branched block copolymers and the assemblies. Compared with RAFT solution polymerization, it was found that the SCVP process was promoted under RAFT dispersion polymerization conditions. Morphologies of branched block copolymer assemblies could be controlled by varying the monomer concentration and the [HPMA]/[macro-CTM] ratio. The branched block copolymer vesicles could be used as seeds for seeded RAFT emulsion polymerization, and framboidal vesicles were successfully obtained. Finally, degrees of branching of branched block copolymers could be further controlled by using a binary mixture of the macro-CTM and a linear macro-RAFT agent or a small molecule CTM. We believe that this study not only provides a versatile strategy for the preparation of branched block copolymer assemblies but also offers important insights into polymer synthesis via heterogeneous RAFT polymerization.

Efficient Synthesis of μ-A(BC)C Miktoarm Star Polymer Assemblies via Aqueous Photoinitiated Polymerization-Induced Self-Assembly

In this study, green light-activated photoiniferter reversible addition-fragmentation chain transfer (RAFT) polymerization of glycerol methacrylate was performed using an ω,ω-heterodifunctional macro-RAFT agent. Because of the different RAFT controllability of two RAFT groups toward methacrylic monomers, only one RAFT group was activated under green light irradiation, leading to the formation of a diblock copolymer macro-RAFT agent with one RAFT group located at the chain end and the other RAFT group located between two blocks. The obtained diblock copolymer macro-RAFT agent was then used to mediate aqueous photoinitiated RAFT dispersion polymerization of diacetone acrylamide (DAAM), which formed μ-A(BC)C miktoarm star polymer assemblies with a diverse set of morphologies. Comparing with the ABC triblock copolymer, it was found that the architecture of the μ-A(BC)C miktoarm star polymer facilitated the formation of higher-order morphologies. Kinetic studies indicated that the aqueous photoinitiated RAFT dispersion polymerization exhibited ultrafast polymerization behavior, with quantitative monomer conversion being achieved within 5 min. Size exclusion chromatography analysis confirmed that good RAFT control was maintained during the polymerization. A morphological phase diagram for μ-A(BC)C miktoarm star polymer assemblies was constructed by varying the monomer concentration and the [DAAM]/[Macro-RAFT] ratio. We expect that this study not only develops an approach for the preparation of miktoarm star polymer assemblies but also provides mechanistic insights into the polymerization-induced self-assembly of nonlinear polymers.

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.

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.

Exploiting the Monomer-Feeding Mechanism of RAFT Emulsion Polymerization for Polymerization-Induced Self-Assembly of Asymmetric Divinyl Monomers

We exploited the monomer-feeding mechanism of reversible addition-fragmentation chain transfer (RAFT) emulsion polymerization to achieve the successful polymerization-induced self-assembly (PISA) of asymmetric divinyl monomers. Colloidally stable cross-linked block copolymer nanoparticles with various morphologies, such as vesicles, were directly prepared at high solids. Morphologies of the cross-linked block copolymer nanoparticles could be controlled by varying the monomer concentration, degree of polymerization (DP) of the core-forming block, and length of the macro-RAFT agent. X-ray photoelectron spectroscopy (XPS) characterization confirmed the presence of unreacted vinyl groups within the obtained block copolymer nanoparticles, providing a landscape for further functionalization via thiol-ene chemistry. Finally, the obtained block copolymer nanoparticles were employed as additives to tune the mechanical properties of hydrogels. We expect that this study not only offers considerable opportunities for the preparation of well-defined cross-linked block copolymer nanoparticles, but also provides important insights into the controlled polymerization of multivinyl monomers.

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.

A3B-type miktoarm star polymer nanoassemblies prepared by reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization

The investigation on a series of A3B-type miktoarm star polymer assemblies by RAFT PISA has revealed the role of A3B architecture in delaying morphological transitions, and the formation of larger vesicles as well as other interesting morphologies.