Metal-Free Removal of Polymer Chain Ends Using Light

Photoinduced Controlled/Living Polymerizations

The application of photochemistry in polymer synthesis is of interest due to the unique possibilities offered compared to thermochemistry, including topological and temporal control, rapid polymerization, sustainable low-energy processes, and environmentally benign features leading to established and emerging applications in adhesives, coatings, adaptive manufacturing, etc. In particular, the utilization of photochemistry in controlled/living polymerizations often offers the capability for precise control over the macromolecular structure and chain length in addition to the associated advantages of photochemistry. Herein, the latest developments in photocontrolled living radical and cationic polymerizations and their combinations for application in polymer syntheses are discussed. This Review summarizes and highlights recent studies in the emerging area of photoinduced controlled/living polymerizations. A discussion of mechanistic details highlights differences as well as parallels between different systems for different polymerization methods and monomer applicability.

End-functionalized polymers by controlled/living radical polymerizations: synthesis and applications

This review focuses on end-functionalized polymers synthesized by controlled/living radical polymerizations and the applications in fields including bioconjugate formation, surface modification, topology construction, and self-assembly.

Recent progress in creating complex and multiplexed surface-grafted macromolecular architectures

Surface-grafted macromolecules, including polymers, DNA, peptides, etc., are versatile modifications to tailor the interfacial functions in a wide range of fields. In this review, we aim to provide an overview of the most recent progress in engineering surface-grafted chains for the creation of complex and multiplexed surface architectures over micro- to macro-scopic areas. A brief introduction to surface grafting is given first. Then the fabrication of complex surface architectures is summarized with a focus on controlled chain conformations, grafting densities and three-dimensional structures. Furthermore, recent advances are highlighted for the generation of multiplexed arrays with designed chemical composition in both horizontal and vertical dimensions. The applications of such complicated macromolecular architectures are then briefly discussed. Finally, some perspective outlooks for future studies and challenges are suggested. We hope that this review will be helpful to those just entering this field and those in the field requiring quick access to useful reference information about the progress in the properties, processing, performance, and applications of functional surface-grafted architectures.

Mixed Polymer Brushes for "Smart" Surfaces

Mixed polymer brushes (MPBs) are composed of two or more disparate polymers covalently tethered to a substrate. The resulting phase segregated morphologies have been extensively studied as responsive "smart" materials, as they can be reversible tuned and switched by external stimuli. Both computational and experimental work has attempted to establish an understanding of the resulting nanostructures that vary as a function of many factors. This contribution highlights state-of-the-art MPBs studies, covering synthetic approaches, phase behavior, responsiveness to external stimuli as well as novel applications of MPBs. Current limitations are recognized and possible directions for future studies are identified.

RAFT dispersion polymerisation of lauryl methacrylate in ethanol–water binary mixtures: synthesis of diblock copolymer vesicles with deformable membranes

Diblock copolymer vesicles with deformable membranes are prepared via RAFT dispersion polymerisation of lauryl methacrylate in an 80 : 20 w/w ethanol–water mixture; visible light irradiation allows facile RAFT chain-end removal from these nano-objects.

Catalyst free removal of trithiocarbonate RAFT CTAs from poly(vinylpyridine)s using tris(trimethylsilyl)silane and light

Trithiocarbonate end groups on various polymers, including polyvinylpyridines, are reduced rapily and quantitatively using only tris(trimethylsilyl)silane and light.

Aminolysis induced functionalization of (RAFT) polymer-dithioester with thiols and disulfides

Efficient exchange of the polymer-dithioester end group by aminolysis/functionalization with thiol or disulfide under ambient atmospheric conditions.

Fast and Complete Neutralization of Thiocarbonylthio Compounds Using Trialkylborane and Oxygen: Application to Their Removal from RAFT-Synthesized Polymers

A rapid and efficient method to remove thiocarbonylthio end groups from polymers prepared by reversible addition-fragmentation chain transfer (RAFT) is described. The elimination process is obtained in less than 1 min by treating the solution of RAFT-synthesized polymers with 5 equiv of trialkylborane (TAB) in the presence of oxygen under an ambient temperature. The versatility of this method was checked on the most relevant families of thiocarbonylthio chain transfer agents (CTA), including dithioesters, trithiocarbonates, dithiocarbamates, and xanthates, carried by the corresponding RAFT-synthesized polymers. UV, NMR, and MALDI-TOF MS characterization results all confirm the complete removal of their terminal CTA groups.

Cooperative reduction of various RAFT polymer terminals using hydrosilane and thiol via polarity reversal catalysis

Cooperative reduction of thiocarbonylthio terminals of polymers obtained by RAFT polymerization was investigated using a catalytic amount of thiol as a polarity reversal catalyst in conjunction with hydrosilane as a reducing agent. A combination of C12H25SH and Ph3SiH enabled the complete removal of xanthate, dithiobenzoate, and trithiocarbonate groups from poly(vinyl acetate), polystyrene, and poly(methyl acrylate) under the radical conditions.

A photo-selective chain-end modification of polyacrylate-iodide and its application in patterned polymer brush synthesis

A photo-selective chain-end modification of polyacrylate-iodide (polymer-I) was developed. The method was used to generate chain-end patterned polymer brushes.

Photo-selective chain end transformation of polyacrylate-iodide using cysteamine and its application to facile single-step preparation of patterned polymer brushes

A novel photo-selective reaction for converting polymer-I to polymer-SH and polymer-H and its application to patterned functional polymer brushes.

A light-mediated metal-free atom transfer radical chain transfer reaction for the controlled hydrogenation of poly(vinylidene fluoride-chlorotrifluoroethylene)

A light-mediated atom transfer radical chain transfer process is proposed for the controlled hydrogenation of P(VDF-CTFE) using metal-free photocatalyst.

Photoinduced metal-free atom transfer radical polymerizations: state-of-the-art, mechanistic aspects and applications

Photoinduced atom transfer radical polymerization has recently been the center of intensive research in synthetic polymer chemistry because of the unique possibility of topological and temporal control in addition to precise control of macromolecular structure offered by conventional ATRP.

Desulfurization-bromination: direct chain-end modification of RAFT polymers

We report a simple and efficient transformation of thiol and thiocarbonylthio functional groups to bromides using stable and commercially available brominating reagents. This procedure allows for the quantitative conversion of a range of small molecule thiols (including primary, secondary and tertiary) to the corresponding bromides under mild conditions, as well as the facile chain-end modification of polystyrene (PS) homopolymers and block copolymers prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization. Specifically, the direct chain-end bromination of PS prepared by RAFT was achieved, where the introduced terminal bromide remained active for subsequent modification or chain-extension using classical atom transfer radical polymerization (ATRP). This transformation sets the foundation for bridging RAFT and ATRP, two of the most widely used controlled radical polymerization (CRP) strategies, and enables the preparation of chain-end functionalized block copolymers not directly accessible using a single CRP technique.

Comparison of photo- and thermally initiated polymerization-induced self-assembly: a lack of end group fidelity drives the formation of higher order morphologies

Polymerization-induced self-assembly (PISA) is an emerging industrially relevant technology, which allows the preparation of defined and predictable polymer self-assemblies with a wide range of morphologies. In recent years, interest has turned to photoinitiated PISA processes, which show markedly accelerated reaction kinetics and milder conditions, thereby making it an attractive alternative to thermally initiated PISA. Herein, we attempt to elucidate the differences between these two initiation methods using isothermally derived phase diagrams of a well-documented poly(ethylene glycol)-b-(2-hydroxypropyl methacrylate) (PEG-b-HPMA) PISA system. By studying the influence of the intensity of the light source used, as well as an investigation into the thermodynamically favorable morphologies, the factors dictating differences in the obtained morphologies when comparing photo- and thermally initiated PISA were explored. Our findings indicate that differences in a combination of both reaction kinetics and end group fidelity led to the observed discrepencies between the two techniques. We find that the loss of the end group in photoinitiated PISA drives the formation of higher order structures and that a morphological transition from worms to unilamellar vesicles could be induced by extended periods of light and heat irradiation. Our findings demonstrate that PISA of identical block copolymers by the two different initiation methods can lead to structures that are both chemically and morphologically distinct.

Light-Mediated Atom Transfer Radical Polymerization of Semi-Fluorinated (Meth)acrylates: Facile Access to Functional Materials

A highly efficient photomediated atom transfer radical polymerization protocol is reported for semi-fluorinated acrylates and methacrylates. Use of the commercially available solvent, 2-trifluoromethyl-2-propanol, optimally balances monomer, polymer, and catalyst solubility while eliminating transesterification as a detrimental side reaction. In the presence of UV irradiation and ppm concentrations of copper(II) bromide and Me₆-TREN (TREN = tris(2-aminoethyl amine)), semi-fluorinated monomers with side chains containing between three and 21 fluorine atoms readily polymerize under controlled conditions. The resulting polymers exhibit narrow molar mass distributions (Đ ≈ 1.1) and high end group fidelity, even at conversions greater than 95%. This level of control permits the in situ generation of chain-end functional homopolymers and diblock copolymers, providing facile access to semi-fluorinated macromolecules using a single methodology with unprecedented monomer scope. The results disclosed herein should create opportunities across a variety of fields that exploit fluorine-containing polymers for tailored bulk, interfacial, and solution properties.

Catalyst-Free Photoinduced End-Group Removal of Thiocarbonylthio Functionality

An initiator- and catalyst-free method for polymer end-group modification has been designed. Under long-wave ultraviolet irradiation, polymers with thiocarbonylthio end groups undergo photolytic cleavage to reveal an active macroradical capable of irreversible termination with a suitable hydrogen source. This straightforward method was successfully demonstrated by the removal of a range of end groups that commonly result from reversible addition-fragmentation chain transfer or photoiniferter polymerizations, including trithiocarbonate, dithiobenzoate, xanthate, and dithiocarbamate mediating agents. This strategy proved efficient for polymers derived from acrylamido, acrylic, methacrylic, styrenic, and vinylpyrrolidone monomers.

Dual-pathway chain-end modification of RAFT polymers using visible light and metal-free conditions

We report a metal-free strategy for the chain-end modification of RAFT polymers utilizing visible light. By turning the light source on or off, the reaction pathway in one pot can be switched between either complete desulfurization (hydrogen chain-end) or simple cleavage (thiol chain-end), respectively. The versatility of this process is exemplified by application to a wide range of polymer backbones under mild, quantitative conditions using commercial reagents.

H2O2 Enables Convenient Removal of RAFT End-Groups from Block Copolymer Nano-Objects Prepared via Polymerization-Induced Self-Assembly in Water

RAFT-synthesized polymers are typically colored and malodorous due to the presence of the sulfur-based RAFT end-group(s). In principle, RAFT end-groups can be removed by treating molecularly dissolved copolymer chains with excess free radical initiators, amines, or oxidants. Herein we report a convenient method for the removal of RAFT end-groups from aqueous dispersions of diblock copolymer nano-objects using H2O2. This oxidant is relatively cheap, has minimal impact on the copolymer morphology, and produces benign side products that can be readily removed via dialysis. We investigate the efficiency of end-group removal for various diblock copolymer nano-objects prepared with either dithiobenzoate- or trithiocarbonate-based RAFT chain transfer agents. The advantage of using UV GPC rather than UV spectroscopy is demonstrated for assessing both the kinetics and extent of end-group removal.