Vinyl Sulfonate Esters: Efficient Chain Transfer Agents for the 3D Printing of Tough Photopolymers without Retardation

From Unregulated Networks to Designed Microstructures: Introducing Heterogeneity at Different Length Scales in Photopolymers for Additive Manufacturing

Photopolymers have been optimized as protective and decorative coating materials for decades. However, with the rise of additive manufacturing technologies, vat photopolymerization has unlocked the use of photopolymers for three-dimensional objects with new material requirements. Thus, the originally highly cross-linked, amorphous architecture of photopolymers cannot match the expectations for modern materials anymore, revealing the largely unanswered question of how diverse properties can be achieved in photopolymers. Herein, we review how microstructural features in soft matter materials should be designed and implemented to obtain high performance materials. We then translate these findings into chemical design suggestions for enhanced printable photopolymers. Based on this analysis, we have found microstructural heterogenization to be the most powerful tool to tune photopolymer performance. By combining the chemical toolbox for photopolymerization and the analytical toolbox for microstructural characterization, we examine current strategies for physical heterogenization (fillers, inkjet printing) and chemical heterogenization (semicrystalline polymers, block copolymers, interpenetrating networks, photopolymerization induced phase separation) of photopolymers and put them into a material scientific context to develop a roadmap for improving and diversifying photopolymers' performance.

Exploiting Visible-Light Induced Radical to Cation Transformation Pathway for Reactivity Enhanced Electrophilic Aromatic Substitution Polymerization of Heteroaromatics

A straightforward approach is employed to synthesize methylene-bridged poly(hetero aromatic)s based on furan, pyrrole, thiophene, and thiophene derivatives. The process involves an electrophilic aromatic substitution reaction facilitated by a visible light-initiated system consisting of manganese decacarbonyl and an iodonium salt. The approach mainly relies on the formation of halomethylium cation, the attack of this cation to heteroaromatic, regeneration of methylium cation on the heteroaromatic, and reactivity differences between halomethylium and heteroaromatic methylium cations for successful polymerizations. This innovative synthetic strategy lead to the formation of polymers with relatively high molecular weights as the stoichiometric imbalance between the comonomers increased. Accordingly, these newly obtained polymers exhibit remarkable fluorescence properties, even at excitation wavelengths as low as 330 nm. Moreover, by harnessing the halogens at chain ends of homopolymers, block copolymers are successfully synthesized, offering opportunities for tailored applications in diverse fields.

Ethyl-2-(tosylmethyl)acrylate: A promising chain transfer agent for the development of low-shrinkage dental composites

OBJECTIVE

To evaluate the potential of ethyl-2-(tosylmethyl)acrylate (ASEE) as chain transfer agent for the development of low-shrinkage photopolymerizable dental composites.

METHODS

Composites containing 10, 20 and 30 mol% of ASEE in their organic matrix were formulated. Camphorquinone (CQ)/ethyl 4-(dimethylamino)benzoate (EDAB) (0.33 wt%/0.60 wt%), CQ/EDAB/Ivocerin® (0.33 wt%/0.60 wt%/0.10, 0.25 or 0.50 wt%), CQ/EDAB/SpeedCure 938 (SC-938) (0.33 wt%/0.60 wt%/0.30, 0.50 or 1.00 wt%) and Ivocerin® (0.50 wt%) were used as photoinitiator systems. The glass transition temperature (Tg) and the crosslink density were determined by DMTA measurements. The flexural strength/modulus and ambient light working time were assessed according to ISO 4049. The shrinkage force was evaluated using a universal testing machine. The double bond conversion (DBC) was determined by NIR spectroscopy. DBC, flexural strength and modulus were measured after the storage of the specimens in deionized water at 37 °C for 24 h. The DBC, flexural strength and modulus data were analyzed by one-way ANOVA with p = 0.05 as significance level.

RESULTS

ASEE-based composites containing the classical initiator system CQ/EDAB exhibited low mechanical properties (flexural strength/modulus) and DBC. The screening of various photoinitiator systems showed that composites based on CQ/EDAB/Ivocerin® (0.33 wt%/0.60 wt%/0.50 wt%), Ivocerin® (0.50 wt%) or CQ/EDAB/SC-938 (0.33 wt%/0.60 wt%/1.00 wt%) were particularly attractive. Indeed, the use of these photoinitiator systems enabled the formulation of composites containing up to 30 mol% ASEE exhibiting excellent mechanical properties, high DBC, good network homogeneity and low shrinkage force values. Interestingly, the addition of SC-938 did not impair the ambient light working time of the uncured composites, whereas the incorporation of 0.50 wt% Ivocerin® resulted in a strong decrease of this value.

SIGNIFICANCE

The addition of the allyl sulfone ASEE in combination with the initiator system CQ/EDAB/SC-938 (0.33 wt%/ 0.60 wt%/ 1.00 wt%) is a promising strategy to develop low-shrinkage dental composites which exhibit excellent mechanical properties, low shrinkage force, high DBC and suitable ambient light working time.

Visible Light Induced High Resolution and Swift 3D Printing System by Halogen Atom Transfer

3D printing technology offers solutions for numerous needs in industry and the daily life of individuals. In recent years, most research efforts have focused on this technology as the market share has grown and requirements have become specified in their related fields. In this work, a novel visible light induced 3D printing system with high resolution and short printing time using dimanganese decacarbonyl (Mn₂ (CO)₁₀ ) in combination with organic halides is reported. The radicals formed through halogen abstraction by photochemically generated manganese pentacarbonyl from organic halides with high quantum efficiency initiate the polymerization of acrylic resins. The kinetics of the process using various halide-containing molecules in the photoinitiaiting system are investigated with real-time fourrier transform infrared spectroscopy and photo-differential scanning calorimetry analyses, and the characteristics of 3D printouts are presented and compared with that of the commercial photoinitiator, 2,4,6-trimethylbenzoyl)phosphine oxide without Mn₂ (CO)₁₀ . The results obtained confirm that the combination of Mn₂ (CO)₁₀ and structurally diverse organic halides is a class of promising 3D system for various applications.

Visible-light-induced one-pot synthesis of sulfonic esters via multicomponent reaction of arylazo sulfones and alcohols

Sulfonic ester is a chemical structure common to many organic molecules, including biologically active compounds. Herein, a visible-light-induced synthetic method to prepare aryl sulfonic ester from arylazo sulfones was developed. In the present study, a one-pot reaction was carried out using arylazo sulfones, DABSO (DABCO·(SO2)2), and alcohols in the presence of CuI as a coupling catalyst and HCl as an additive to yield sulfonic esters via multicomponent reaction. This synthetic method afforded a wide range of sulfonic esters with high yields under mild conditions.

Thermal expansion of free volume in "classic" and regulated dimethacrylates: photocured directly and via a mask to study pillar formation

The temperature dependence of free volume in dimethacrylates (poly2M), cured by direct irradiation (poly2M-A) or via a mask (poly2M-B), and in a thiol-based 2M sample (poly2M-co-EDDT), was investigated by positron annihilation lifetime spectroscopy (PALS) and dilatometry (DIL) to study the influence of thiol regulation on the microstructure via free volume characteristics. It was found that the free volume fraction as determined from experimental data by using the standard spherical approach for the hole shapes showed systematic differences from the analogous quantity as evaluated from the lattice-hole theory. Much better results were obtained for cylindrical holes, which expand 'anisotropically' in poly2M samples and 'isotropically' in the poly2M-co-EDDT resin. In addition, the hydrogen bond changes and the conversion of monomers in cured samples studied by near infra-red spectroscopy (NIR) revealed spectrum-structure correlations for the final cured thermosets.

Visible Light Induced Conventional Step-Growth and Chain-Growth Condensation Polymerizations by Electrophilic Aromatic Substitution

A novel visible light induced step-growth polymerization by electrophilic aromatic substitution between photochemically generated carbocations and dimethoxybenzene nucleophile is described. Conventional step-growth polymerization and chain-growth condensation polymerization (CCP) mechanisms are presented. It is found that by changing the molar ratios of the monomers slightly, the CCP mechanism becomes operative and relatively higher molecular weight polymers are obtained because of the higher reactivity of the end groups of the intermediates and oligomers than that of the monomers. The possibility of grafting onto polymers containing epoxide at their side chains by photoinduced chain end activation of poly(dimethoxyphenylene methylene) is demonstrated. This study is expected to promote potential applications of the combination of photoinduced electron transfer reactions and CCP in macromolecular synthesis and material science.

Visible light induced step-growth polymerization by electrophilic aromatic substitution reactions

A novel visible light induced step-growth polymerization to form poly(phenylene methylene) by electrophilic aromatic substitution reactions is described. The effect of different nucleophilic aromatic molecules on polymerization has been investigated. The possibility of combining step-growth polymerization with conventional free radical and free radical promoted cationic polymerizations through photoinduced chain-end activation has been demonstrated. Highly fluorescent fibers of the resulting block copolymers were obtained using the electrospinning technique. The versatile photoinduced step-growth polymerization process reported herein paves the way for a new generation of polycondensates and their combination with chain polymers that cannot be obtained by conventional methods.

Efficient 3D printing via photooxidation of ketocoumarin based photopolymerization

Photopolymerization-based three-dimensional (3D) printing can enable customized manufacturing that is difficult to achieve through other traditional means. Nevertheless, it remains challenging to achieve efficient 3D printing due to the compromise between print speed and resolution. Herein, we report an efficient 3D printing approach based on the photooxidation of ketocoumarin that functions as the photosensitizer during photopolymerization, which can simultaneously deliver high print speed (5.1 cm h-1) and high print resolution (23 μm) on a common 3D printer. Mechanistically, the initiating radical and deethylated ketocoumarin are both generated upon visible light exposure, with the former giving rise to rapid photopolymerization and high print speed while the latter ensuring high print resolution by confining the light penetration. By comparison, the printed feature is hard to identify when the ketocoumarin encounters photoreduction due to the increased lateral photopolymerization. The proposed approach here provides a viable solution towards efficient additive manufacturing by controlling the photoreaction of photosensitizers during photopolymerization.

Catalyst-Free Spontaneous Polymerization with 100% Atom Economy: Facile Synthesis of Photoresponsive Polysulfonates with Multifunctionalities

Photoresponsive polymers have attracted extensive attention due to their tunable functionalities and advanced applications; thus, it is significant to develop facile in situ synthesis strategies, extend polymers family, and establish various applications for photoresponsive polymers. Herein, we develop a catalyst-free spontaneous polymerization of dihaloalkynes and disulfonic acids without photosensitive monomers for the in situ synthesis of photoresponsive polysulfonates at room temperature in air with 100% atom economy in high yields. The resulting polysulfonates could undergo visible photodegradation with strong photoacid generation, leading to various applications including dual-emissive or 3D photopatterning, and practical broad-spectrum antibacterial activity. The halogen-rich polysulfonates also exhibit a high and photoswitched refractive index and could undergo efficient postfunctionalizations to further expand the variety and functionality of photoresponsive heteroatom-containing polyesters.

Reversible Deactivation Radical Polymerization: From Polymer Network Synthesis to 3D Printing

3D printing has changed the fabrication of advanced materials as it can provide customized and on-demand 3D networks. However, 3D printing of polymer materials with the capacity to be transformed after printing remains a great challenge for engineers, material, and polymer scientists. Radical polymerization has been conventionally used in photopolymerization-based 3D printing, as in the broader context of crosslinked polymer networks. Although this reaction pathway has shown great promise, it offers limited control over chain growth, chain architecture, and thus the final properties of the polymer networks. More fundamentally, radical polymerization produces dead polymer chains incapable of postpolymerization transformations. Alternatively, the application of reversible deactivation radical polymerization (RDRP) to polymer networks allows the tuning of network homogeneity and more importantly, enables the production of advanced materials containing dormant reactivatable species that can be used for subsequent processes in a postsynthetic stage. Consequently, the opportunities that (photoactivated) RDRP-based networks offer have been leveraged through the novel concepts of structurally tailored and engineered macromolecular gels, living additive manufacturing and photoexpandable/transformable-polymer networks. Herein, the advantages of RDRP-based networks over irreversibly formed conventional networks are discussed.

Resins for Frontal Photopolymerization: Combining Depth-Cure and Tunable Mechanical Properties

Photopolymerization has undergone significant development in recent years. It enables fast and easy processing of materials with customized properties and allows precise printing of complex surface geometries. Nevertheless, photopolymerization is mainly applied to cure thin films since the low curing depth limits the fast production of large volumes. Frontal photopolymerization (FPP) is suitable to overcome these limitations so that curing of centimeter-thick (meth)acrylic layers can be accomplished within minutes by light induction only. Prerequisites, however, are the low optical density of the resin and bleaching ability of the photoinitiator. To date, tailored FPP-resins are not commercially available. This study discusses the potential of long-chain polyether dimethacrylates, offering high-temperature resistance and low optical density, as crosslinkers in photobleaching resins and investigates the mechanical properties of photofrontally-cured copolymers. Characteristics ranging from ductile to hard and brittle are observed in tensile tests, demonstrating that deep curing and versatile material properties are achieved with FPP. Analyzed components display uniform polymerization over a depth of four centimeters in Fourier transform infrared spectroscopy and swelling tests.

Effects of network structures on the tensile toughness of copper-catalyzed azide-alkyne cycloaddition (CuAAC)-based photopolymers

In the present study, the photo-initiated copper-catalyzed azide-alkyne cycloaddition (CuAAC) polymerization was utilized to form structurally diverse glassy polymer networks. Systematic alterations in the monomer backbone rigidity (e.g., cyclic or aliphatic groups with a different length of backbone) and the reactive functional group density (e.g., tetra-, tri-, di-, and mono-functional azide and alkyne monomers) were used to provide readily tailorable network structures with crosslink densities (estimated from the rubbery modulus) varying by a factor of over 20. All eight of the resultant networks exhibited glass transition temperatures (Tg) between 50 and 80 °C with tensile toughness ranging from 28 to 61 MJ m-3. A nearly linear dependence of yield stress and elongation at break (broadly defined as strength and ductility, respectively) on the Tg and rubbery modulus was established in these triazole networks. When a flexible di-alkyne monomer (5 carbon spacing between alkynes) was incorporated in a network composed of a tri-alkyne and di-azide monomer, the elongation at break was improved from 166 to 300 %, while the yield stress was reduced from 36 to 23 MPa. Additionally, the polymer ductility was also varied by incorporating mono-functional azides as chain ends in the network - replacing a sterically hindered stiff mono-azide with a more flexible mono-azide increased the elongation at break from 24 to 185 % and the tensile toughness from 6 to 28 MJ m-3.

Carbon Dots as a Promising Green Photocatalyst for Free Radical and ATRP-Based Radical Photopolymerization with Blue LEDs

Carbon dots (CDs) have been used for the first time as a sensitizer to initiate and activate free radical and controlled radical polymerization, respectively, based on an ATRP protocol with blue LEDs. Consideration of diverse heteroatom-doped CDs indicated that N-doped CDs could serve as an effective photocatalyst and photosensitizer in combination with LEDs emitting either at 405 nm or 470 nm. Free radical polymerization was initiated by combining the CDs with an iodonium or sulfonium salt in tri(propylene glycol) diacrylate. Polymerization of methyl methacrylate (MMA) by photo-induced ATRP was achieved with CDs and ethyl α-bromophenylacetate using CuII as catalyst in the ppm range. The polymers obtained showed temporal control, narrower dispersity ≲1.5, and chain-end fidelity. The first-order kinetics and ON/OFF experiments additionally gave evidence of the constant concentration of polymer radicals. No remarkable cytotoxic activity was observed for the CDs, underlining their biocompatibility.

Highly Durable and Thermally Conductive Shell-Coated Phase-Change Capsule as a Thermal Energy Battery

Robust and thermally conductive phase-change capsules (PCCs) can be effectively used as dispersoids for heat transfer fluids (HTFs) to utilize waste heat. Here, we demonstrate PCCs encapsulated with a cross-linked poly(2-hydroxyethyl methacrylate) shell that showed high durability and low thermal hysteresis for effective heat uptake and release. The circulation system was manufactured by mimicking the 4th Generation District Heating (4GDH) system to confirm the heat delivery efficiencies of PCC-dispersed slurries (PCSs) as the HTFs. The enthalpy change of water after it received heat from the PCS improved by up to 41.1% on increasing the amount of PCCs in the PCS. Furthermore, a high PCC recovery of 92 wt % was achieved after 1500 cycles, which accompanied a phase transition. The PCC developed by us can thus enable effective storage/delivery of waste heat-driven energy for zero-energy buildings and a 4GDH system, as well as thermal management of electronics.

3D printing of polymeric materials based on photo-RAFT polymerization

For the first time, we report 3D printing of RAFT-based formulations to fabricate functional objects in a layer-by-layer fashion.

Seeing the Light: Advancing Materials Chemistry through Photopolymerization

The application of photochemistry to polymer and material science has led to the development of complex yet efficient systems for polymerization, polymer post-functionalization, and advanced materials production. Using light to activate chemical reaction pathways in these systems not only leads to exquisite control over reaction dynamics, but also allows complex synthetic protocols to be easily achieved. Compared to polymerization systems mediated by thermal, chemical, or electrochemical means, photoinduced polymerization systems can potentially offer more versatile methods for macromolecular synthesis. We highlight the utility of light as an energy source for mediating photopolymerization, and present some promising examples of systems which are advancing materials production through their exploitation of photochemistry.

Enhanced reduction of polymerization-induced shrinkage stress via combination of radical ring opening and addition fragmentation chain transfer

The combination of vinylcyclopropanes with an ester-activated vinyl sulfonate ester in a light-induced radical polymerization shows high reactivity accompanied by a significant increase in conversion and it leads to an enhanced reduction of polymerization-induced shrinkage stress.

In situinvestigation of the kinetics and microstructure during photopolymerization by positron annihilation technique and NIR-photorheology

The microstructural free volume evolution during a photopolymerization process was studied on a commercial photopolymer (SPOT LV) in situ by positron annihilation lifetime spectroscopy (PALS) and concomitant NIR-photorheology. Analysis of the positron lifetime spectra revealed a high sensitivity of the PALS technique to the different phases of photopolymerization associated with different reaction rates as well as to the evolution of microstructural free-volume shrinkage, which was described at the molecular level by the Kohlrausch–Williams–Watts equation. The in situ PALS study of microstructural changes in photopolymerization was related to the vitrification (gel point) accompanied by shrinkage stress registered via NIR-photorheology. The simultaneous NIR measurements yield information on the monomer conversion of SPOT LV, which can be correlated to the occurrence of the gel point and the evolution of the microstructural free volume. This combined study allows us to see deeper into the crosslinking process and its influence on the resulting material characteristics.

Positron annihilation lifetime spectroscopy is a sensitive tool for the in situ study of the microstructural evolution during photopolymerization.