In the (Very) Long Run We Are All Dead: Activation and Termination in SET-LRP/SARA-ATRP

From Frank-Kasper, Quasicrystals, and Biological Membrane Mimics to Reprogramming In Vivo the Living Factory to Target the Delivery of mRNA with One-Component Amphiphilic Janus Dendrimers

This Perspective is dedicated to the 25th Anniversary of Biomacromolecules. It provides a personal view on the developing field of the polymer and biology interface over the 25 years since the journal was launched by the American Chemical Society (ACS). This Perspective is meant to bridge an article published in the first issue of the journal and recent bioinspired developments in the laboratory of the corresponding author. The discovery of supramolecular spherical helices self-organizing into Frank-Kasper and quasicrystals as models of icosahedral viruses, as well as of columnar helical assemblies that mimic rodlike viruses by supramolecular dendrimers, is briefly presented. The transplant of these assemblies from supramolecular dendrimers to block copolymers, giant surfactants, and other self-organized soft matter follows. Amphiphilic self-assembling Janus dendrimers and glycodendrimers as mimics of biological membranes and their glycans are discussed. New concepts derived from them that evolved in the in vivo targeted delivery of mRNA with the simplest one-component synthetic vector systems are introduced. Some synthetic methodologies employed during the synthesis and self-assembly are explained. Unraveling bioinspired applications of novel materials concludes this brief 25th Anniversary Perspective of Biomacromolecules.

Cu(0)-RDRP of acrylates using an alkyl iodide initiator

In the vast majority of atom transfer radical polymerizations, alkyl bromides or alkyl chlorides are commonly employed as initiators. Herein, alkyl iodides are demonstrated as ATRP initiators.

Dimensional Optimization for ZnO-Based Mechano-ATRP with Extraordinary Activity

Various piezoelectric nanomaterials were utilized in ultrasound-mediated atom transfer radical polymerization (ATRP), owing to their outstanding piezoelectric effect. However, the relationship between the morphology of those piezocatalysts and polymerization has not been clearly established. Herein, we employed different piezoelectric zinc oxide (ZnO) nanomaterials to achieve novel mechano-induced ATRP (mechano-ATRP). Based on the synergistic effect of piezoelectric properties and specific surface area, the catalytic activity of 1D ZnO nanorods (1D-ZnO NRs) with increased aspect ratio outperformed that of 0D ZnO nanoparticles (0D-ZnO NPs). Compared to the conventional ATRP system, this system exhibited extraordinary activity toward the less activated monomer acrylonitrile (67% conversion after 6 h), with a narrow molecular weight distribution (polydispersity index ∼ 1.19). Furthermore, implications of ZnO loading, copper salt amount, degree of polymerization, monomer, and solvent were also studied for the highly efficient mechano-ATRP.

Can We Push Rapid Reversible Deactivation Radical Polymerizations toward Immortality?

All reversible deactivation radical polymerization (RDRP) processes require a compromise between the rate of polymerization, which requires a high radical concentration, and retention of chain end functionality, which requires a low radical concentration. Here, we demonstrate that this compromise may be partially averted where fast deactivation of the propagating radical occurs. It is shown that, contrary to the predictions of classical reaction kinetics, when the probability density functions of the termination reactions are adjusted to take into account the time needed for radical diffusion, a reduction in the extent of termination can be expected if chain deactivation is rapid. We subsequently use this framework to explain experimental results in the copper(0)-mediated polymerization of acrylamide. The main concept put forward in the paper questions the commonly held assumptions of the limitations of RDRP processes and suggests the ability for a seemingly impossible level of control of radical reactions.

Surface-Initiated Cu(0)-Mediated CRP for the Rapid and Controlled Synthesis of Quasi-3D Structured Polymer Brushes

Surface-initiated controlled radical polymerization mediated by Cu(0) plate (SI-Cu(0)_plate-CRP) is an extremely effective and versatile technique for the synthesis of functional polymer brushes from vinyl monomers on planar substrates. The advantages of SI-Cu(0)_plate-CRP in comparison to "classical" SI-CRP methods not only rely on the easy accessibility, handling, and recycling of the catalyst source, but also on the faster brush growth rates, and exceptionally high reinitiation efficiencies and grafting densities for the obtained brushes. The confined geometry of the SI-Cu(0)_plate-CRP reaction setup, with a Cu(0) plate placed in close proximity to the initiator bearing substrate, considerably simplifies the preparation of polymer brushes over large areas, and the fabrication of gradient, patterned and arrayed polymer brushes. In this viewpoint we summarize the recent developments and applications of SI-Cu(0)_plate-CRP, emphasizing its mechanism, advantages, and standing challenges.

Copper mediated RDRP of thioacrylates and their combination with acrylates and acrylamides

Ethyl thioacrylate was polymerised via Cu-RDRP and subjected to amidation to obtain the first “all-acrylic” copolymer.

Cu(0)-RDRP of methacrylates in DMSO: importance of the initiator

The controlled radical polymerization of methacrylates via Cu(0)-mediated RDRP is challenging in comparison to acrylates with most reports illustrating higher dispersities, lower monomer conversions and poorer end group fidelity relative to the acrylic analogues.

The downside of dispersity: why the standard deviation is a better measure of dispersion in precision polymerization

Dispersity gives a deceptively rosy picture of the extent of dispersion in molecular weight distributions. For complex structures or relatively narrow molecular weight distributions, the standard deviation of the number distribution is a better choice.

Surface-Initiated Controlled Radical Polymerization: State-of-the-Art, Opportunities, and Challenges in Surface and Interface Engineering with Polymer Brushes

The generation of polymer brushes by surface-initiated controlled radical polymerization (SI-CRP) techniques has become a powerful approach to tailor the chemical and physical properties of interfaces and has given rise to great advances in surface and interface engineering. Polymer brushes are defined as thin polymer films in which the individual polymer chains are tethered by one chain end to a solid interface. Significant advances have been made over the past years in the field of polymer brushes. This includes novel developments in SI-CRP, as well as the emergence of novel applications such as catalysis, electronics, nanomaterial synthesis and biosensing. Additionally, polymer brushes prepared via SI-CRP have been utilized to modify the surface of novel substrates such as natural fibers, polymer nanofibers, mesoporous materials, graphene, viruses and protein nanoparticles. The last years have also seen exciting advances in the chemical and physical characterization of polymer brushes, as well as an ever increasing set of computational and simulation tools that allow understanding and predictions of these surface-grafted polymer architectures. The aim of this contribution is to provide a comprehensive review that critically assesses recent advances in the field and highlights the opportunities and challenges for future work.

Mechanism of Cu(I)/Cu(0)-Mediated Reductive Coupling Reactions of Bromine-Terminated Polyacrylates, Polymethacrylates, and Polystyrene

The mechanism of the Cu(I)/Cu(0)-mediated reductive coupling reactions of bromine-terminated polymer chain-end radicals, so-called atom-transfer radical coupling (ATRC), is studied. Poly(methyl acrylate) (PMA), poly(methyl methacrylate) (PMMA), and polystyrene (PSt), prepared by atom-transfer radical polymerization (ATRP), were activated by an excess amount of Cu(I)Br and Cu(0) in the presence of tris[2-(dimethylamino)ethyl]amine (Me₆TREN), and the structural analyses of the resulting polymer products and deuterium-labeling experiments unambiguously determined the mechanism. While PMMA and PSt reacted by a radical mechanism as expected, PMA-bromide was reduced to an anionic species, which was most likely an organocopper species. Trapping experiments with TEMPO suggested that the polymer chain-end radicals were generated in all cases by the reduction of the bromine-terminated polymers by low-valent Cu species. However, the PMA chain-end radical was further reduced to the anionic species from which coupling products formed in low yield.

Ultrafast SET-LRP of hydrophobic acrylates in multiphase alcohol–water mixtures

A multiphase SET-LRP system using thein situgenerated Cu(0) by reduction of Cu(ii)Br₂with NaBH₄in various alcohols containing water was developed and used for SET-LRP of hydrophobic acrylates. From left to right: a solution of methanol/water containing Cu(ii)Br₂, the same solution after addition of BA and its SET-LRP.

Pentablock star shaped polymers in less than 90 minutes via aqueous SET-LRP

The synthesis of core-first multi-block star-shaped copolymers via aqueous SET-LRP has been reported for the first time.

Insights into relevant mechanistic aspects about the induction period of Cu0/Me6TREN-mediated reversible-deactivation radical polymerization

The induction period and subsequent autoaccelerated polymerization of a Cu⁰/Me₆TREN-catalyzed system originate from the accumulation of soluble copper species.

Comment on “Aqueous SET-LRP catalyzed with in situ generated Cu(0) demonstrates surface mediated activation and bimolecular termination” by S. Samanta, et al., Polym. Chem., 2015, 6, 2084

A recent paper proposed a new polymerization mechanism for aqueous SET-LRP, but experimental results are consistent with conventional radical propagation and termination.

Sequence-controlled multi-block copolymerization of acrylamides via aqueous SET-LRP at 0 °C

Aqueous single electron transfer living radical polymerization (SET-LRP) has been employed to synthesize multi-block homopolymers and copolymers of a range of acrylamide monomers including N-isopropylacrylamide (NIPAM), 2-hydroxyethyl acrylamide (HEAA), N,N-dimethyl acrylamide (DMA) and N,N-diethylacrylamide (DEA).

Aperiodic Copolymers

Current polymer terminology only describes very simple copolymer structures such as block, graft, alternating periodic, or statistical copolymers. This restricted vocabulary implies that copolymers exhibit either segregated (i.e., block and graft), regular (i.e., alternating and periodic), or uncontrolled (i.e., statistical or random) comonomer sequence distributions. This standard classification does not include many new types of sequence-controlled copolymers that have been reported in recent years. In this context, the present viewpoint describes a new category of copolymers: aperiodic copolymers. Such structures can be defined as copolymers in which monomer sequence distribution is not regular but follows the same arrangement in all chains. The term aperiodic can be used to describe encoded comonomer sequences in monodisperse sequence-defined copolymers but also the block sequence of some multiblock copolymers. These new types of copolymers open up very interesting perspectives for the design of complex materials. Some recent relevant literature on the topic is discussed herein.

How are radicals (re)generated in photochemical ATRP?

The polymerization mechanism of photochemically mediated Cu-based atom-transfer radical polymerization (ATRP) was investigated using both experimental and kinetic modeling techniques. There are several distinct pathways that can lead to photochemical (re)generation of Cu(I) activator species or formation of radicals. These (re)generation pathways include direct photochemical reduction of the Cu(II) complexes by excess free amine moieties and unimolecular reduction of the Cu(II) complex, similar to activators regenerated by electron-transfer (ARGET) ATRP processes. Another pathway is photochemical radical generation either directly from the alkyl halide, ligand, or via interaction of ligand with either monomer or with alkyl halides. These photochemical radical generation processes are similar to initiators for continuous activator regeneration (ICAR) ATRP processes. A series of model experiments, ATRP reactions, and kinetic simulations were performed to evaluate the contribution of these reactions to the photochemical ATRP process. The results of these studies indicate that the dominant radical (re)generation reaction is the photochemical reduction of Cu(II) complexes by free amines moieties (from amine containing ligands). The unimolecular reduction of the Cu(II) deactivator complex is not significant, however, there is some contribution from ICAR ATRP reactions involving the interaction of alkyl halides and ligand, ligand with monomer, and the photochemical cleavage of the alkyl halide. Therefore, the mechanism of photochemically mediated ATRP is consistent with a photochemical ARGET ATRP reaction dominating the radical (re)generation.