Wavelength-optimized two-photon polymerization using initiators based on multipolar aminostyryl-1,3,5-triazines

Two-photon induced polymerization (2PP) based 3D printing is a powerful microfabrication tool. Specialized two-photon initiators (2PIs) are critical components of the employed photosensitive polymerizable formulations. This work investigates the cooperative enhancement of two-photon absorption cross sections (σ2PA) in a series of 1,3,5-triazine-derivatives bearing 1-3 aminostyryl-donor arms, creating dipolar, quadrupolar and octupolar push-pull systems. The multipolar 2PIs were successfully prepared and characterized, σ2PA were determined using z-scan at 800 nm as well as spectrally resolved two-photon excited fluorescence measurements, and the results were compared to high-level ab initio computations. Modern tunable femtosecond lasers allow 2PP-processing at optimum wavelengths tailored to the absorption behavior of the 2PI. 2PP structuring tests revealed that while performance at 800 nm is similar, at their respective σ2PA-maxima the octupolar triazine-derivative outperforms a well-established ketone-based quadrupolar reference 2PI, with significantly lower fabrication threshold at exceedingly high writing speeds up to 200 mm/s and a broader window for ideal processing parameters

Benzoyl Phenyltelluride as Highly Reactive Visible-Light TERP-Reagent for Controlled Radical Polymerization

Benzoyl phenyltelluride (<b>BPT</b>) is a highly reactive TERP-reagent for visible-light-induced (400–500 nm) controlled radical polymerization. The compound can be easily prepared in one step from diphenyl ditelluride and benzoyl chloride. It shows a strong absorption at 407 nm that tails out to 473 nm and provides PDIs (1.2 to 1.3) among the lowest reported in literature for photoiniferters in general, to which our compound was compared. PDIs obtained with <b>BPT</b> are much lower than those for benzyl dithiocarbamte (<b>BDC</b>) (1.7 to 1.8), which was used as a reference compound. Choice of <b>BDC</b> as reference is based on its property as UV-photoiniferter and on a similar initiation/control mechanism. However, <b>BDC</b> does not allow living radical polymerization under visible light. The newly discovered compound <b>BPT</b> provides best results with acrylamides and acrylates. Photoinitiation with styrene was ineffective, and reaction with methacrylates is not considered living

UV-Initiated Bubble-Free Frontal Polymerization in Aqueous Conditions

From an industrial point of view the most interesting method of bulk curing of acrylate-based monomers is a thermal frontal reaction initiated by the application of UV-light. In water-based systems, UV triggering of bubble-free thermal front reactions is difficult to realize as solvent boiling and decomposition of the thermal initiator lead to uncontrollable heat loss, porous polymer samples, and expansion of the formulation. Especially bubble formation in the light-exposed area causes diffuse light scattering and as a result nonreproducible investigation of the UV initiation. Within this work a new thermal initiator has been synthesized, allowing us to report a bubble-free steady-state thermal front reaction in water. The new system allowed us to achieve a well-observable front reaction and provided the basis for an accurate and reproducible, systematic analysis of UV initiation in water-based frontal polymerizations, considering the ratios of all reactive components

UV-Initiated Bubble-Free Frontal Polymerization in Aqueous Conditions

From an industrial point of view the most interesting method of bulk curing of acrylate-based monomers is a thermal frontal reaction initiated by the application of UV-light. In water-based systems, UV triggering of bubble-free thermal front reactions is difficult to realize as solvent boiling and decomposition of the thermal initiator lead to uncontrollable heat loss, porous polymer samples, and expansion of the formulation. Especially bubble formation in the light-exposed area causes diffuse light scattering and as a result nonreproducible investigation of the UV initiation. Within this work a new thermal initiator has been synthesized, allowing us to report a bubble-free steady-state thermal front reaction in water. The new system allowed us to achieve a well-observable front reaction and provided the basis for an accurate and reproducible, systematic analysis of UV initiation in water-based frontal polymerizations, considering the ratios of all reactive components

Debonding on Demand with Highly Cross-Linked Photopolymers: A Combination of Network Regulation and Thermally Induced Gas Formation

Photopolymerizable glues and cements that offer debonding on demand (DoD) through an external stimulus are of great interest for the fields of recycling and repair. State-of-the-art DoD solutions often require a high-energy impulse (e.g., >200 °C, strong force), which is due to the typical glassy nature of such photopolymer networks. Herein, various blocked isocyanates (BICs) that enable thermally induced gas formation at temperatures far below 200 °C are studied. Thermally induced gas bubble formation is accomplished within a linear, thermoplastic poly­(<i>N</i>-acryloyl­morpholine) matrix above glass transition temperature, introducing porosity. The resulting porosity within the material then causes mechanical failure. However, highly cross-linked photopolymer networks remain unchanged due to their glassy nature at temperatures well above 150 °C. A BIC-based thermolabile photopolymerizable cross-linker is prepared in order to create a polymer network with cleavable cross-link. Additionally, a β-allyl sulfone-based chain transfer reagent is used to tune the final cross-linking density and thermomechanical properties of the material. Above the resulting sharp glass transition (>60 °C), plastic deformation becomes possible, thus allowing formation of porosity. This introduces a covalently cross-linked, thermolabile photopolymer with a tailored network architecture as potential glue for DoD at ∼150 °C

β‑Allyl Sulfones as Addition–Fragmentation Chain Transfer Reagents: A Tool for Adjusting Thermal and Mechanical Properties of Dimethacrylate Networks

Dimethacrylates are known to have good photoreactivity, but their radical polymerization usually leads to irregular, highly cross-linked, and brittle polymer networks with broad thermal polymer phase transitions. Here, the synthesis of mono- and difunctional β-allyl sulfones is described, and those substances are introduced as potent addition–fragmentation chain transfer (AFCT) reagents leading to dimethacrylate networks with tunable properties. By controlling the content and functionality of said AFCT reagents, it is possible to achieve more homogeneous networks with a narrow glass transition and an adjustable glass transition temperature (<i>T</i>_g), rubber modulus of elasticity (<i>E</i>_r), and network density. In contrast to dimethacrylate networks containing monomethacrylates as reactive diluents, the network architecture of the β-allyl sulfone-based dimethacrylate networks is more homogeneous and the tunability of thermal and mechanical properties is much more enhanced. The reactivity and polymerization were investigated using laser flash photolysis, photo-DSC, and NMR, while DMTA and swellability tests were performed to characterize the polymer

Strategies to Reduce Oxygen Inhibition in Photoinduced Polymerization

Strategies to Reduce Oxygen Inhibition in Photoinduced Polymerizatio

Evaluation of Sulfonium Borate Initiators for Cationic Photopolymerization and Their Application in Hot Lithography

Sulfonium hexafluorophosphates and perfluorinated tetraphenylborates are well established initiators in the field of cationic photopolymerization and have already been applied in 3D-printing at elevated temperatures, named hot lithography. The cyanide-ligated borane-based photoinitiator B2 shows lower molecular weight compared to state-of-the-art borates and is less expensive to synthesize from commercial starting materials. By comparing reactivity in the epoxy monomer BADGE, B2 outperforms commercial initiators in a broad range of temperatures with outstanding epoxy group conversions of up to 99%. Besides B2’s excellent performance as a cationic initiator, formulations containing B2 are thermally and storage stable and can be sensitized by anthracene derivatives. Hot lithography of B2 was carried out successfully at 90 °C and compared to existing sulfonium initiators. Layer quality and coloration of the fabricated and postcured parts are superior for B2. Overall, the cyanide-ligated sulfonium salt B2 represents an excellent initiator for cationic photopolymerization and application in hot lithography

Initiators Based on Benzaldoximes: Bimolecular and Covalently Bound Systems

Typical bimolecular photoinitiators (PIs) for radical polymerization of acrylates show only poor photoreactivity because of the undesired effect of back electron transfer. To overcome this limitation, PIs consisting of a benzaldoxime ester and various sensitizers based on aromatic ketones were introduced. The core of the photoinduced reactivity was established by laser flash photolysis, photo-CIDNP, and EPR experiments at short time scales. According to these results, the primarily formed iminyl radicals are not particularly active. The polymerization is predominantly initiated by C-centered radicals. Photo-DSC experiments show reactivities comparable to that of classical monomolecular type I PIs like Darocur 1173

Photoinitiators with β-Phenylogous Cleavage: An Evaluation of Reaction Mechanisms and Performance

Bimolecular photoinitiators based on benzophenone and <i>N</i>-phenylglycine ideally overcome limitations of classical two-component systems, such as the possibility of deactivation by a back electron transfer or the solvent cage effect. Furthermore, if they are covalently linked, loss of reactivity by diffusion limitation could be reduced. Here we show that such an initiator displays unusually high photoreactivity. This is established by photo-DSC experiments and mechanistic investigations based on laser flash photolysis, TR-EPR, and photo-CIDNP. The β-phenylogous scission of the C–N bond is highly efficient and leads to the production of reactive initiating radicals at a short time scale