Fusing Light-Induced Step-Growth Processes with RAFT Chemistry for Segmented Copolymer Synthesis: A Synergetic Experimental and Kinetic Modeling Study

Linear Cyclobutane-Containing Polymer Synthesis via [2 + 2] Photopolymerization in an Unconfined Environment under Visible Light

The [2 + 2] photopolymerization of diolefinic monomers is an appealing approach for the construction of polymeric materials. Herein, we demonstrate that the establishment of an effective donor-acceptor conjugation by introducing electron-donating alkoxy groups at appropriate positions of the benzene ring could activate p-phenylenediacrylate (PDA), thus enabling the development of the first solution [2 + 2] photopolymerization of such monomers under the irradiation of visible light. Variation on the alkoxy groups and the ester parts could allow access to a series of linear cyclobutane-containing polymer products with high molecular weight (up to 140 kDa) and good solubility in common solvents. Further, temporal control and postpolymerization modification with preinstalled pendant C═C bonds via thiol-ene click reaction are also demonstrated with this [2 + 2] photopolymerization system.

Catalyst-Free, Visible-Light-Induced Step-Growth Polymerization by a Photo-RAFT Single-Unit Monomer Insertion Reaction

Photoinduced polymerization is an attractive technique with the advantages of easy operation, mild conditions, and excellent temporospatial controllability. However, the application of this technique in step-growth polymerization is highly challenging. Here, we present a catalyst-free, visible-light-induced step-growth polymerization method utilizing a photo-RAFT single-unit monomer insertion reaction between the xanthate and vinyl ether groups. Benefitting from this reaction, a pendant cationic RAFT agent can be generated in each repeating unit of the polymer backbone. Both cationic and radical side chain extensions were successfully realized, providing a facile approach for the postpolymerization of step-growth polymers for the development of various functional polymeric materials.

Differences and similarities between mono-, bi- or tetrafunctional initiated cationic ring-opening polymerization of 2-oxazolines

Cationic ring-opening polymerization (CROP) is an interesting synthesis technique to obtain well-defined polymers with narrow molar mass distribution (MMD).

A unified kinetic Monte Carlo approach to evaluate (a)symmetric block and gradient copolymers with linear and branched chains illustrated for poly(2-oxazoline)s

The synthesis of well-defined gradient, block-gradient and di-block copolymers with both asymmetric and symmetric compositions considering hydrophilic and hydrophobic monomer units is relevant for application fields, such as drug/gene delivery...

Going Beyond the Carothers, Flory and Stockmayer Equation by Including Cyclization Reactions and Mobility Constraints

A challenge in the field of polymer network synthesis by a step-growth mechanism is the quantification of the relative importance of inter- vs. intramolecular reactions. Here we use a matrix-based kinetic Monte Carlo (kMC) framework to demonstrate that the variation of the chain length distribution and its averages (e.g., number average chain length x_n), are largely affected by intramolecular reactions, as mostly ignored in theoretical studies. We showcase that a conventional approach based on equations derived by Carothers, Flory and Stockmayer, assuming constant reactivities and ignoring intramolecular reactions, is very approximate, and the use of asymptotic limits is biased. Intramolecular reactions stretch the functional group (FG) conversion range and reduce the average chain lengths. In the likely case of restricted mobilities due to diffusional limitations because of a viscosity increase during polymerization, a complex x_n profile with possible plateau formation may arise. The joint consideration of stoichiometric and non-stoichiometric conditions allows the validation of hypotheses for both the intrinsic and apparent reactivities of inter- and intramolecular reactions. The kMC framework is also utilized for reverse engineering purposes, aiming at the identification of advanced (pseudo-)analytical equations, dimensionless numbers and mechanistic insights. We highlight that assuming average molecules by equally distributing A and B FGs is unsuited, and the number of AB intramolecular combinations is affected by the number of monomer units in the molecules, specifically at high FG conversions. In the absence of mobility constraints, dimensionless numbers can be considered to map the time variation of the fraction of intramolecular reactions, but still, a complex solution results, making a kMC approach overall most elegant.

Spatiotemporal patterning of photoresponsive DNA-based hydrogels to tune local cell responses

Understanding the spatiotemporal effects of surface topographies and modulated stiffness and anisotropic stresses of hydrogels on cell growth remains a biophysical challenge. Here we introduce the photolithographic patterning or two-photon laser scanning confocal microscopy patterning of a series of o-nitrobenzylphosphate ester nucleic acid-based polyacrylamide hydrogel films generating periodically-spaced circular patterned domains surrounded by continuous hydrogel matrices. The patterning processes lead to guided modulated stiffness differences between the patterned domains and the surrounding hydrogel matrices, and to the selective functionalization of sub-regions of the films with nucleic acid anchoring tethers. HeLa cells are deposited on the circularly-shaped domains functionalized with the MUC-1 aptamers. Initiation of the hybridization chain reaction by nucleic acid tethers associated with the continuous hydrogel matrix results in stress-induced ordered orthogonal shape-changes on the patterned domains, leading to ordered shapes of cell aggregates bound to the patterns.

Multisegmented polymers via step-growth and RAFT miniemulsion polymerization

We report a method to efficiently prepare multisegmented polymers via a combination of step-growth (SG) and reversible addition-fragmentation chain transfer (RAFT) polymerization.

Role of External Field in Polymerization: Mechanism and Kinetics

The past decades have witnessed an increasing interest in developing advanced polymerization techniques subjected to external fields. Various physical modulations, such as temperature, light, electricity, magnetic field, ultrasound, and microwave irradiation, are noninvasive means, having superb but distinct abilities to regulate polymerizations in terms of process intensification and spatial and temporal controls. Gas as an emerging regulator plays a distinctive role in controlling polymerization and resembles a physical regulator in some cases. This review provides a systematic overview of seven types of external-field-regulated polymerizations, ranging from chain-growth to step-growth polymerization. A detailed account of the relevant mechanism and kinetics is provided to better understand the role of each external field in polymerization. In addition, given the crucial role of modeling and simulation in mechanisms and kinetics investigation, an overview of model construction and typical numerical methods used in this field as well as highlights of the interaction between experiment and simulation toward kinetics in the existing systems are given. At the end, limitations and future perspectives for this field are critically discussed. This state-of-the-art research progress not only provides the fundamental principles underlying external-field-regulated polymerizations but also stimulates new development of advanced polymerization methods.

Visible Light Catalyzed Step-Growth Polymerization through Mizoroki-Heck Coupling Reaction

The results of polymer synthesis via visible light (blue light λ = 465 nm) and a Pd-catalyzed Mizoroki-Heck coupling reaction at ambient temperature are reported. The kinetic study demonstrates that the polymerization rate is faster under light irradiation than that in the dark, affording larger molecular weight of polymer product in the former. A mechanistic study using ¹⁹ F NMR indicates that light can activate the oxidative addition step, increasing the rate of formation of the oxidative addition intermediate. The present work not only reveals a new mechanism of light's effect on Mizoroki-Heck coupling reaction in the absence of sensitizer, but also represents the first example of its application in step-growth polymerization.

Recent advances in light-regulated non-radical polymerisations

This review summarises recent advances in light-regulated non-radical polymerisations as well as the applications in materials science.

Fully independent photochemical reactivity in one molecule

We introduce a chemically λ-orthogonal bichromophore triggered simply by different colours of light, enabling two different photochemical reactions in one molecule. Uniquely, the short wavelength (λ = 314 nm) does not trigger the red-shifted reaction system (λ = 416 nm), opening possibilities for the light controlled gating of specific molecular sites independent of wavelength.

Light-Controlled Orthogonal Covalent Bond Formation at Two Different Wavelengths

We report light-induced reactions in a two-chromophore system capable of sequence-independent λ-orthogonal reactivity relying solely on the choice of wavelength and solvent. In a solution of water and acetonitrile, LED irradiation at λ_max =285 nm leads to full conversion of 2,5-diphenyltetrazoles with N-ethylmaleimide to the pyrazoline ligation products. Simultaneously present o-methylbenzaldehyde thioethers are retained. Conversely, LED irradiation at λ_max =382 nm is used to induce ligation of the o-methylbenzaldehydes in acetonitrile with N-ethylmaleimide via o-quinodimethanes, while 2,5-diphenyltetrazoles also present are retained. This unprecedented photochemical selectivity is achieved through control of the number and wavelength of incident photons as well as favorable optical properties and quantum yields of the reactants in their environment.

Visible-Light-Induced Passerini Multicomponent Polymerization

Herein, we introduce an additive-free visible-light-induced Passerini multicomponent polymerization (MCP) for the generation of high molar mass chains. In place of classical aldehydes (or ketones), highly reactive, in situ photogenerated thioaldehydes are exploited along with isocyanides and carboxylic acids. Prone to side reactions, the thioaldehyde moieties create a complex reaction environment which can be tamed by optimizing the synthetic conditions utilizing stochastic reaction path analysis, highlighting the potential of semi-batch procedures. Once the complex MCP environment is understood, step-growth polymers can be synthesized under mild reaction conditions which-after a Mumm rearrangement-result in the incorporation of thioester moieties directly into the polymer backbone, leading to soft matter materials that can be degraded by straightforward aminolysis or chain expanded by thiirane insertion.

Synthesis and Aqueous Solution Properties of an Amino Bisphosphonate Methacrylate Homopolymer via RAFT Polymerization

The present contribution reports on the synthesis via reversible addition-fragmentation chain transfer (RAFT) polymerization of a methacrylate derivative bearing an aminobisphosponate moiety as a pendant group, namely, ethyl N,N-tetramethylbis(phosphonate)-bis(methylene) amine methacrylate (MAC₂NP₂). The polymerization was performed by the use of cyanoisopropyl dithiobenzoate as chain transfer agent at 70 °C in various solvents with different polarities including N,N-dimethylformamide, acetonitrile, tetrahydrofuran, and in bulk. Best results were obtained in N,N-dimethylformamide where higher conversions and polymerization rates were noticed. The successful hydrolysis of the phosphonate ester groups was performed using bromotrimethylsilane with excellent yields leading to the formation of highly water soluble and pH-responsive polymers. Finally, a preliminary solution behavior study was carried out by investigating the aqueous solution properties of synthesized amino bisphosphonate methacrylate homopolymers and their phosphonic acid analogs via potentiometric titration and zeta potential measurements.

Light-Induced Step-Growth Polymerization of AB-Type Photo-Monomers at Ambient Temperature

We introduce two AB-type monomers able to undergo a facile catalyst-free photoinduced polycycloaddition of photocaged dienes, enabling rapid Diels-Alder ligations under UV-irradiation (λ_max = 350 nm) at ambient temperature, closely adhering to Carother's equation established by a careful kinetic study (17800 g mol^-1 < M_w < 24700 g mol^-1). The resulting macromolecules were in-depth analyzed via size exclusion chromatography (SEC) and nuclear magnetic resonance (NMR) spectroscopy. Additionally, SEC hyphenated to high resolution-electrospray ionization-mass spectrometry (HR-ESI-MS) enabled the careful mapping of the end group structure of the generated polymers. Furthermore, we demonstrate that both monomer systems can be readily copolymerized. The study thus demonstrates that Diels-Alder ligation resting upon photocaged dienes is a powerful tool for accessing step-growth polymers.

A facile route to segmented copolymers by fusing ambient temperature step-growth and RAFT polymerization

We introduce the facile synthesis of segmented copolymers by a catalyst-free Diels-Alder (DA) reaction at ambient temperature via step-growth and subsequent reversible addition fragmentation chain transfer (RAFT) polymerization. High molecular weight step-growth polymers are readily obtained (M_w = 40 000 g mol^-1), featuring trithiocarbonate moieties in their chain, which allow monomer insertion via RAFT polymerization yielding high molecular weight species.