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Polarity‐reversal catalysts (PRCs) for hydrogen‐atom transfer reactions have been known in radical chemistry for more than 60 years but are rarely described and utilized in the field of photopolymerization up to now. Herein, we present the use of thiols in a unique dual function as thiol‐ene click reagents and as polarity‐reversal catalyst (PRC) for the radical‐mediated redox rearrangements of benzylidene acetals. During the rearrangement reaction, cyclic benzylidene acetals are transformed into benzoate esters leading to a significant volumetric expansion to reduce thermoset shrinkage. We were able to show that this expansion on a molecular level reduces shrinkage and polymerization stress but does not significantly affect the (thermo−)mechanical properties of the cross‐linked networks. One of the key advantages of this process lies in its simplicity. No additives like sensitizers or combinations of different initiators (radical and cationic) are needed. Furthermore, the same light source can be used for both the polymerization reaction and expansion through rearrangement. Additionally, the applied photoinitiator enables spatial and temporal control of the polymerization; thus, the developed system can be an excellent platform for additive manufacturing processes. [ABSTRACT FROM AUTHOR]

Vanillyl alcohol has emerged as a widely used building block for the development of biobased monomers. More specifically, the cationic (photo‐)polymerization of the respective diglycidyl ether (DGEVA) is known to produce materials of outstanding thermomechanical performance. Generally, chain transfer agents (CTAs) are of interest in cationic resins not only because they lead to more homogeneous polymer networks but also because they strikingly improve the polymerization speed. Herein, the aim is to compare the cationic chain‐growth photopolymerization with the thermally initiated anionic step‐growth polymerization, with and without the addition of CTAs. Indeed, CTAs lead to faster polymerization reactions as well as the formation of more homogeneous networks, especially in the case of the thermal anionic step‐growth polymerization. Resulting from curing above the TG of the respective anionic step‐growth polymer, materials with outstanding tensile toughness (>5 MJ cm−3) are obtained that result in the manufacture of potential shape‐memory polymers. [ABSTRACT FROM AUTHOR]

Biodegradable thermoplastic polyurethanes (TPUs) are promising materials for vascular grafts, due to their excellent mechanical properties and the possibility to incorporate degradable moieties. Especially degradability is important in the field of tissue engineering (TE) or regenerative medicine, because the synthetic scaffold should be replaced by human tissue after a certain time. We synthesized thermoplastic polythiourethanes (TPTUs) based on degradable chain extenders and tested their mechanical, thermal and degradation characteristics. As soft‐block we used polytetrahydrofuran (pTHF) and as hard‐block the aliphatic hexamethylenediisocyanate (HMDI) in combination with a variety of chain extenders. The focus was set on the boronic acid‐based dithiol 2,2′‐(1,4‐phenylene)‐bis[4‐mercaptan‐1,3,2‐dioxaborolane] (BDB). As dithiol‐based reference 1,2‐bis(2‐mercaptoethoxy)ethane (BMEE) was used and bis(2‐hydroxyethyl)‐terephthalate (BHET) as well‐known ester‐based reference chain extender. By using BDB as degradable chain extender we could accelerate the degradation rate in terms of mass erosion at 37 and 90°C compared to BHET‐based TPUs. However, a strong increase in molecular weight was observed under degradation conditions. Further investigation by ATR‐FTIR revealed a release of benzene‐1,4‐diboronic acid (BDBA) in the polymer. This leads to the formation of a hydroxy‐terminated polyol, which then undergoes an intramolecular functional group metamorphosis with the thiourethane group to form a crosslinked polymer. BMEE‐based TPTUs in contrast show excellent mechanical properties, even better than BHET‐TPUs but a slightly lower mass erosion at 37 and 90°C. [ABSTRACT FROM AUTHOR]

Color stability is crucial for dental materials to ensure they perfectly match a patient's tooth color. This is particularly challenging in photoresist-based additive manufacturing. Although some studies have addressed this issue, the exact causes of discoloration and ways to minimize it remain unclear. In this study, the intrinsic causes of discoloration in materials intended for 3D printing are investigated by examining thin-film samples (1200 µm) of various compositions, which are stored under different conditions. The samples are evaluated by measuring the UV-Vis absorption spectra at regular intervals to monitor changes. The findings reveal that both the composition of the formulations and the storage conditions significantly influence the discoloration behavior. Furthermore, methods have been developed to reduce or completely prevent discoloration. The use of photoinitiators with sterically demanding benzoyl moieties, as well as the addition of stabilizers, effectively decreases the intensity of emerging discoloration. Furthermore, incorporating the oxidizing agent cumene hydroperoxide (CHP) results in materials that maintain color stability. [ABSTRACT FROM AUTHOR]

Nonlinear (NL) absorption and refraction are crucial optical phenomena in the development and application of optical materials. The Z-scan technique, first proposed by Bahae, is widely used to accurately measure the coefficients of NL absorption and refraction. This review presents a comprehensive overview of the most established versions of the Z-scan technique, including the transmittance closed aperture (CA) and open aperture (OA) Z-scan, reflecting CA and OA Z-scan, eclipsing Z-scan, and white light Z-scan. In each version, different sources with varying spatial and temporal intensity distributions are usually utilized. Numerical and analytical calculation results are provided for each version considering both continuous wave (CW) and pulsed laser sources. Unlike conventional reviews, which often summarize previously published results, this review primarily focuses on reproducing the findings from the literature. The analytical results, which are typically derived under specific approximations, are compared with numerical calculations to highlight the limitations of analytically derived relations. Furthermore, the necessary criteria and conditions for applying each Z-scan version are discussed. [ABSTRACT FROM AUTHOR]

Radiation curing is an excellent option for the economical implementation of coatings while simultaneously producing technically advanced products. Nevertheless, the number of materials used is mainly limited to pure (meth)acrylate and epoxy systems. Polyisocyanurates could be an excellent addition to this range, especially due to their thermal and chemical stability. In this work, through the use of a transition metal‐based photocatalyst, as well as the use of 1,5‐diazabicyclo(4.3.0)non‐5‐en as a co‐catalyst, formulations are presented, which enable pure polyisocyanurates to be used in radiation‐based coating technology. Trimerization with this system using any primary isocyanates worked and consequently thus opened the possibility for aromatic and aliphatic polyisocyanurates. Using monofunctional isocyanates, the theoretical background of trimerization as well as the theoretical enthalpy is evaluated and the possibilities as well as the limitations for poly‐trimerization of polyisocyanurates are analyzed. Finally, photo‐induced catalytic poly‐trimerization of aliphatic isocyanates is used to obtain samples with a Tg in the range of 100 °C and onset of degradation above 400 °C. Thus, for the first time, a framework for the use of isocyanates in the field of photopolymerization and, in particular, in the field of radiation‐based coating of polyisocyanurates is provided for further development. [ABSTRACT FROM AUTHOR]

Group transfer polymerization (GTP) is a polymerization method developed to obtain targeted (meth)acrylic polymers in solution at ambient temperatures. In this work, it is employed with methacrylic monomers to obtain low Ð polymers in bulk. In this regard, different initiator systems that exhibit different mechanisms are compared concerning the molecular weight, the polydispersity and double bond conversion of the resulting bulk polymers. The respective systems were chosen carefully to give a broad overview of earlier developed initiator‐catalyst combinations 1‐methoxy‐1‐(trimethylsiloxy)‐2‐methylprop‐1‐ene (MTS) & tetrabutylammonium cyanide (TBACN) and more recently investigated initiating systems MTS & trityl tetrakis(pentafluorophenyl)borate (TTPB) or dimethyl phenyl silane (DMPS) & tris(pentafluorophenyl)borate (BCF). The described initiating systems are applied as a two‐component (2K) system to ensure a homogeneous distribution of the respective initiator and catalyst in the bulk monomer. In addition to the 2K experiments, photochemical initiation is also applied to bulk formulations. Therefore, a photoacid generator (PAG) and MTS is used to trigger the polymerization reaction by irradiation with UV light. A highly controlled photopolymerization method in bulk was developed that way achieving a low polydispersity polymer with high double bond conversion. [ABSTRACT FROM AUTHOR]