More than expected: Overall initiation efficiencies of mono-, bis-, and tetraacylgermane radical initiators

We introduce a quantitative comparison of the overall initiation efficiency for a library of eight mono-, bis-, and tetraacylgermane visible light photoinitiators with p-methoxy and fluorine substitution patterns. Specifically, cocktail experiments were carried out, fusing pulsed laser polymerization (PLP) of methyl methacrylate (MMA) with subsequent high-resolution electrospray mass spectrometry (ESI-MS) analysis. The overall initiation efficiency reflects all critical events leading to macromolecular growth, i.e., from light absorption and cleavage of the initiator to radical initiation. Importantly, we show that the obtained overall initiation efficiencies are self-consistent when going from lower to higher substituted systems, evidencing the validity of the derived overall initiation efficiencies within the error margins of the experiment. Remarkably, the comparison of mono-, bis-, and tetraacylgermanes reveals a nonstoichiometric increase of 40-90% in the overall initiation efficiency with increasing number of benzoyl moieties per initiator (size dependence), e.g., a maximum value of 7.6 ± 1.8 for tetrabenzoylgermane versus benzoyltrimethylgermane. In addition, the size dependence observed for acylgermanes scales with the nπ∗ extinction coefficients with the number of benzoyl moieties of the respective photoinitiator. Finally, with increasing system size of the acylgermanes, a more complex channel branching - as suggested from time-dependent density functional theory (TDDFT) calculations - results in substitution-dependent intersystem crossing (ISC) and cleavage quantum yields

Synthesis of bis(3-{[2-(allyloxy)ethoxy]methyl}-2,4,6-trimethylbenzoyl)(phenyl)phosphine oxide – a tailor-made photoinitiator for dental adhesives

Because of the poor solubility of the commercially available bisacylphosphine oxides in dental acidic aqueous primer formulations, bis(3-{[2-(allyloxy)ethoxy]methyl}-2,4,6-trimethylbenzoyl)(phenyl)phosphine oxide (WBAPO) was synthesized starting from 3-(chloromethyl)-2,4,6-trimethylbenzoic acid by the dichlorophosphine route. The substituent was introduced by etherification with 2-(allyloxy)ethanol. In the second step, 3-{[2-(allyloxy)ethoxy]methyl}-2,4,6-trimethylbenzoic acid was chlorinated. The formed acid chloride showed an unexpected low thermal stability. Its thermal rearrangement at 180 °C resulted in a fast formation of 3-(chloromethyl)-2,4,6-trimethylbenzoic acid 2-(allyloxy)ethyl ester. In the third step, the acid chloride was reacted with phenylphosphine dilithium with the formation of bis(3-{[2-(allyloxy)ethoxy]methyl}-2,4,6-trimethylbenzoyl)(phenyl)phosphine, which was oxidized to WBAPO. The structure of WBAPO was confirmed by 1H NMR, 13C NMR, 31P NMR, and IR spectroscopy, as well as elemental analysis. WBAPO, a yellow liquid, possesses improved solubility in polar solvents and shows UV–vis absorption, and a high photoreactivity comparable with the commercially available bisacylphosphine oxides. A sufficient storage stability was found in dental acidic aqueous primer formulations

Supramolecular Polymer Networks Made by Solvent-Free Copolymerization of a Liquid 2‑Ureido-4[1<i>H</i>]‑pyrimidinone Methacrylamide

Supramolecular polymer networks based on polyacrylates with hydrogen bonding 2-ureido-4­[1<i>H</i>]-pyrimidinone (UPy) side chains are of considerable interest due to the stimuli-responsive nature imparted by the reversible cross-links formed by dimerized UPy groups. Previously reported UPy-containing acrylic monomers are solid and show limited miscibility with comonomers, and this has stifled their (co)­polymerization in bulk. We here report the synthesis of a liquid 2-ureido-4­[1<i>H</i>]-pyrimidinone methacrylamide (UPy-OPG-MAA), which was made by connecting the UPy motif and methacrylamide (MAA) via an amine-terminated oligo­(propylene glycol) (OPG) linker. The new monomer was miscible with conventional methacrylates. This permitted the photoinitiated free-radical bulk copolymerization with hexyl methacrylate (HMA) to afford a series of copolymers (poly­(UPy-OPG-MAA-<i>co</i>-HMA)) in which the UPy-OPG-MAA content was varied between 0 and 20 mol %. The investigation of the mechanical properties of these copolymers by dynamic mechanical analysis and adhesion tests revealed that the introduction of the UPy groups leads to an increase of the stiffness in the glassy state, the formation of a rubbery plateau above the glass transition temperature, and a significant increase of the adhesive strength. Joints bonded with poly­(UPy-OPG-MAA-<i>co</i>-HMA) could be debonded on demand using light or heat

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