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

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

Supramolecular Cross-Links in Poly(alkyl methacrylate) Copolymers and Their Impact on the Mechanical and Reversible Adhesive Properties

Hydrogen-bonded, side-chain-functionalized supramolecular poly­(alkyl methacrylate)­s were investigated as light- and temperature-responsive reversible adhesives that are useful for bonding and debonding on demand applications. Here, 2-hydroxyethyl methacrylate (HEMA) was functionalized with 2-ureido-4­[1<i>H</i>]­pyrimidinone (UPy) via a hexamethylenediisocyanate (HMDI) linker, to create a monomer (UPy-HMDI-HEMA) that serves to form supramolecular cross-links by way of forming quadruple hydrogen bonded dimers. UPy-HMDI-HEMA was copolymerized with either hexyl methacrylate or butyl methacrylate to create copolymers comprising 2.5, 5, or 10 mol % of the cross-linker. The mechanical properties of all (co)­polymers were investigated with stress–strain experiments and dynamic mechanical analysis. Furthermore, the adhesive properties were studied at temperatures between 20 and 60 °C by testing single lap joints formed with stainless steel substrates. It was found that increasing the concentration of the UPy-HMDI-HEMA cross-linker leads to improved mechanical and adhesive properties at elevated temperatures. Concurrently, the reversibility of the bond formation remained unaffected, where rebonded samples displayed the same adhesive strength as regularly bonded samples. Debonding on demand abilities were also tested exemplarily for one copolymer, which for light-induced debonding experiments was blended with a UV-absorber that served as light–heat converter. Single lap joints were subjected to a constant force and heated or irradiated with UV light until debonding occurred. The necessary debonding temperature was comparable for direct heating and UV irradiation and varied between 28 and 82 °C, depending on the applied force. The latter also influenced the debonding time, which under the chosen conditions ranged from 30 s to 12 min

An In-Depth Mechanistic Investigation of the Radical Initiation Behavior of Monoacylgermanes

Five <i>para</i>-substituted monoacyltrimethylgermane derivatives, i.e., <i>p</i>-fluorobenzoyl­trimethylgermane (pFBG, λ_max = 405 nm), <i>p</i>-methoxy­benzoyltrimethyl­germane (pMBG, λ_max = 397 nm), benzoyltrimethyl­germane (pHBG, λ_max = 409 nm), <i>p</i>-cyanobenzoyl­trimethylgermane (pCBG, λ_max = 425 nm), and <i>p</i>-nitrobenzoyl­trimethylgermane (pNBG, λ_max = 429 nm) are investigated via a combination of pulsed laser polymerization with subsequent electrospray ionization and mass spectrometry (PLP-ESI-MS) as well as femtosecond transient absorption spectroscopy. The relative initiation efficiencies of the initiating benzoyl radical fragments of pFBG, pMBG, and pHBG are determined using PLP-ESI-MS. The <i>para</i>-substituted derivatives with the electron-donating groups, pFBG and pMBG, display a factor 1.5 and 1.3, respectively, superior overall initiation efficiency compared to the unsubstituted pHBG. In contrast, the derivatives pCBG and pNBG carrying electron-withdrawing groups display only weak initiation behavior at a factor 4 higher total energy of ∼112 J (∼28 J for typical PLP experiments with pMBG, pFBG, and pHBG at ∼320 J and 90 000 pulses). The differences in the initiation efficiencies are representative for two classes of monoacyltrimethyl­germane initiators, i.e., efficient initiators and weak initiators, each distinct in their specific radical cleavage mechanism. The efficient initiators pMBG, pFBG, and pHBG show an ultrafast intersystem crossing within 2–4 ps after pulse irradiation and subsequent formation of benzoyl and trimethylgermyl radical fragments. In contrast, the weak initiators pCBG and pNBG relax to the ground state after photoexcitation via a dominating ultrafast internal conversion (IC) within 13 and 2 ps, respectively, disallowing effective initiation under typical PLP conditions (∼320 J/pulse with 90 000 pulses resulting in ∼28 J total energy per sample). pCBG features weak initiation behavior additionally forming methyl and <i>p</i>-cyanobenzoyl­dimethylgermyl radicals at a factor 4 higher total energy of ∼112 J. Consistent with a considerably faster IC relaxation, pNBG features a factor 10 weaker monomer conversion than pCBG

A Priori Prediction of Mass Spectrometric Product Patterns of Photoinitiated Polymerizations

We introduce a method for the a priori prediction of mass spectra of complex poly­(methyl methacrylate)­s initiated by photoinitiators featuring multiple cleavage points. The method is based on permutation mathematics using multinomial coefficients to predict the probability of each poly­(methyl methacrylate) species’ isotopic pattern contribution to the overall mass spectrum. The method assumes a statistical behavior for the cleavage of the photoinitiator. The excellent agreement of the predicted mass spectrum based on multinomial coefficients with the experimental mass spectrum confirms a multipoint cleavage mechanism of the assessed photoinitiators. We exemplify our method for the prediction of mass spectra of poly­(methyl methacrylate)­s initiated by four tetraacylgermane derivates and one bisacylgermane, recorded after visible light pulsed-laser polymerization by high resolution Orbitrap electrospray ionization mass spectrometry (ESI-MS). The excellent agreement of our approach with experimental data suggests that a wide array of polymer mass spectra of polymers initiated by initiators capable of multiple cleavage events can be quantitatively predicted

Synthesis, Spectroscopic Behavior, and Photoinduced Reactivity of Tetraacylgermanes

Acylgermanes have been subject of great interest recently because of their low toxicity and the applicability as sources for germanium-centered radicals for visible-light induced free radical polymerization processes. We report on a novel and versatile method for the synthesis of tetraacylgermanes allowing the preparation of various tetra-substituted acylgermanes <b>1a</b>–<b>m</b>. The formation of these derivatives was confirmed by NMR spectroscopy, mass spectrometry, and X-ray crystallography. UV–vis absorption spectra of the prepared compounds reveal absorption in the visible region. This transition was assigned by TD-DFT calculations. It enabled a general screening of the influence of different substitution patterns on the absorption properties. The radical formation upon irradiation was confirmed by TR-EPR spectroscopy

Synthesis, Spectroscopic Behavior, and Photoinduced Reactivity of Tetraacylgermanes

Acylgermanes have been subject of great interest recently because of their low toxicity and the applicability as sources for germanium-centered radicals for visible-light induced free radical polymerization processes. We report on a novel and versatile method for the synthesis of tetraacylgermanes allowing the preparation of various tetra-substituted acylgermanes <b>1a</b>–<b>m</b>. The formation of these derivatives was confirmed by NMR spectroscopy, mass spectrometry, and X-ray crystallography. UV–vis absorption spectra of the prepared compounds reveal absorption in the visible region. This transition was assigned by TD-DFT calculations. It enabled a general screening of the influence of different substitution patterns on the absorption properties. The radical formation upon irradiation was confirmed by TR-EPR spectroscopy

Acylgermanes: Photoinitiators and Sources for Ge-Centered Radicals. Insights into their Reactivity

Acylgermanes have been shown to act as efficient photoinitiators. In this investigation we show how dibenzoyldiethylgermane <b>1</b> reacts upon photoexcitation. Our real-time investigation utilizes femto- and nanosecond transient absorption, time-resolved EPR (50 ns), photo-chemically induced dynamic nuclear polarization, DFT calculations, and GC-MS analysis. The benzoyldiethylgermyl radical <b>G</b>• is formed via the triplet state of parent <b>1</b>. On the nanosecond time scale this radical can recombine or undergo hydrogen-transfer reactions. Radical <b>G</b>• reacts with butyl acrylate at a rate of 1.2 ± 0.1 × 10⁸ and 3.2 ± 0.2 × 10⁸ M^–1 s^–1, in toluene and acetonitrile, respectively. This is ∼1 order of magnitude faster than related phosphorus-based radicals. The initial germyl and benzoyl radicals undergo follow-up reactions leading to oligomers comprising Ge–O bonds. LC-NMR analysis of photocured mixtures containing <b>1</b> and the sterically hindered acrylate 3,3-dimethyl-2-methylenebutanoate reveals that the products formed in the course of a polymerization are consistent with the intermediates established at short time scales