Tetraacylgermanes: Highly Efficient Photoinitiators for Visible‐Light‐Induced Free‐Radical Polymerization

In this contribution a convenient synthetic method to obtain tetraacylgermanes Ge[C(O)R]₄ (R=mesityl ( 1 a ), phenyl ( 1 b )), a previously unknown class of highly efficient Ge‐based photoinitiators, is described. Tetraacylgermanes are easily accessible via a one‐pot synthetic protocol in >85 % yield, as confirmed by NMR spectroscopy, mass spectrometry, and X‐ray crystallography. The efficiency of 1 a , b as photoinitiators is demonstrated in photobleaching (UV/Vis), time‐resolved EPR (CIDEP), and NMR/CIDNP investigations as well as by photo‐DSC studies. Remarkably, the tetraacylgermanes exceed the performance of currently known long‐wavelength visible‐light photoinitiators for free‐radical polymerization     

Red‐Light‐Induced Cationic Photopolymerization: Perylene Derivatives as Efficient Photoinitiators

Nine different perylene derivatives are prepared and their ability to initiate, when combined with an iodonium salt (and optionally N‐vinylcarbazole), a ring‐opening cationic photopolymerization of epoxides under very soft halogen lamp irradiation is investigated. One of them is particularly efficient under a red laser diode exposure at 635 nm and belongs now to the very few systems available at this wavelength. The photochemical mechanisms are studied by steady‐state photolysis, electron spin resonance spin trapping, fluorescence, cyclic voltammetry, and laser flash photolysis techniques.

     

Enantioselective Visible‐Light‐Induced Radical‐Addition Reactions to 3‐Alkylidene Indolin‐2‐ones

The title compounds underwent a facile and high‐yielding addition reaction (19 examples, 66–99 % yield) with various N‐(trimethylsilyl)methyl‐substituted amines upon irradiation with visible light and catalysis by a metal complex. If the alkylidene substituent is non‐symmetric and if the reaction is performed in the presence of a chiral hydrogen‐bonding template, products are obtained with significant enantioselectivity (58–72 % ee) as a mixture of diastereoisomers. Mechanistic studies suggest a closed catalytic cycle for the photoactive metal complex. However, the silyl transfer from the amine occurs not only to the product, but also to the substrate, and interferes with the desired chirality transfer.     

Abilities of 2‐arylmethyloxy‐6‐cyanonaphthalene derivatives to initiate radical and cationic photopolymerizations

An investigation was undertaken to shed much light on the bond‐cleavage mode (heterolysis vs homolysis) of bichromophoric photoinitiators in styrene (St) and 1,2‐epoxycyclohexane (EpC) and then to develop a highly efficient hybrid‐type photoinitiator. Excited‐state 2‐(9‐anthrylmethyloxy)‐6‐cyanonaphthalene in toluene and 1,2‐dimethoxyethane, which were used instead of St and EpC, respectively, underwent both homolytic Ar? OCH₂ bond cleavage and heterolytic ArO? CH₂ bond cleavage to give the corresponding radical‐pair and ion‐pair intermediates. The charge‐separated state characteristic of the naphthol chromophore in the singlet excited state was found to play a pivotal role in this heterolytic bond‐cleavage mode. An inspection of the radical (St) and cationic (EpC) photopolymerization behavior observed in the presence of some 2‐arylmethyloxy‐6‐cyanonaphthalenes led us to conclude that the aforementioned 9‐anthrylmethyloxy‐substituted naphthalene derivative is a candidate for a functional hybrid‐type photoinitiator containing no halogen and/or metal. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 25–31, 2006     

Simple One‐Pot Syntheses of Water‐Soluble Bis(acyl)phosphane Oxide Photoinitiators and Their Application in Surfactant‐Free Emulsion Polymerization

The sodium salt of the new bis(mesitoyl)phosphinic acid (BAPO‐OH) can be prepared in a very efficient one‐pot synthesis. It is well soluble in water and hydrolytically stable for at least several weeks. Remarkably, it acts as an initiating agent for the surfactant‐free emulsion polymerization (SFEP) of styrene to yield monodisperse, spherical nanoparticles. Time‐resolved electron paramagnetic resonance (TR‐EPR) and chemically induced electron polarisation (CIDEP) indicate preliminary mechanistic insights.

     

Visible‐Light Emulsion Photopolymerization of Styrene

The photopolymerization of styrene in emulsion is achieved in a conventional double‐wall reactor equipped with a LED ribbon coiled around the external glass wall. Styrene mixed to acridine orange is added to the water phase containing sodium dodecyl sulfate, a water‐soluble N‐heterocyclic carbene–borane and disulfide, and irradiated. Highly stable latexes are obtained, with particles up to a diameter of 300 nm. The ability to reach such large particle sizes via a photochemical process in a dispersed medium is due to the use of visible light: the photons in the visible range are less scattered by larger objects and thus penetrate and initiate better the polymerizations. They are also greener and cheaper to produce via LEDs, and much safer than UVs. The method presented does not require any specific glassware; it works at lower temperature and delivers larger particles compared to thermal processes at similar solids contents and surfactant concentrations.     

Novel Synthetic Pathways for Bis(acyl)phosphine Oxide Photoinitiators

Photoactive: A cheap, safe, and widely functional-group-tolerant synthetic protocol to an important class of photoinitiators, bis(acyl)phosphine oxides (BAPOs), has been disclosed, together with examples of application to stain-proof fabrics and photoactive polymeric films.     

Photocatalyst‐Free and Blue Light‐Induced RAFT Polymerization of Vinyl Acetate at Ambient Temperature

Vinyl acetate is polymerized in the living way under the irradiation of blue light‐emitting diodes (LEDs) or sunlight without photocatalyst at ambient temperature. 2‐(Ethoxycarbonothioyl)sulfanyl propanoate is exclusively added and acts as initiator and chain transfer agent simultaneously in the current system. Poly(vinyl acetate) with well‐regulated molecular weight and narrow molecular weight distribution (Đ < 1.30) is synthesized. Near quantitative end group fidelity of polymer is demonstrated by nuclear magnetic resonance (NMR) and matrix‐assisteed laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOF MS).

     

Light‐Regulated Polymerization under Near‐Infrared/Far‐Red Irradiation Catalyzed by Bacteriochlorophyll a

Photoregulated polymerizations are typically conducted using high‐energy (UV and blue) light, which may lead to undesired side reactions. Furthermore, as the penetration of visible light is rather limited, the range of applications with such wavelengths is likewise limited. We herein report the first living radical polymerization that can be activated and deactivated by irradiation with near‐infrared (NIR) and far‐red light. Bacteriochlorophyll a (Bachl a) was employed as a photoredox catalyst for photoinduced electron transfer/reversible addition–fragmentation chain transfer (PET‐RAFT) polymerization. Well‐defined polymers were thus synthesized within a few hours under NIR (λ=850 nm) and far‐red (λ=780 nm) irradiation with excellent control over the molecular weight (M_n/M_w<1.25). Taking advantage of the good penetration of NIR light, we showed that the polymerization also proceeded smoothly when a translucent barrier was placed between light source and reaction vessel.     

Visible‐Light‐Mediated Decarboxylative Radical Additions to Vinyl Boronic Esters: Rapid Access to γ‐Amino Boronic Esters

The synthesis of alkyl boronic esters by direct decarboxylative radical addition of carboxylic acids to vinyl boronic esters is described. The reaction proceeds under mild photoredox catalysis and involves an unprecedented single‐electron reduction of an α‐boryl radical intermediate to the corresponding anion. The reaction is amenable to a diverse range of substrates, including α‐amino, α‐oxy, and alkyl carboxylic acids, thus providing a novel method to rapidly access boron‐containing molecules of potential biological importance.     

Isoprene‐Assisted Radical Coupling of (Co)polymers Prepared by Cobalt‐Mediated Radical Polymerization

Tackling blocks : The isoprene‐assisted radical coupling (I‐ARC) of polymers prepared by cobalt‐mediated radical polymerization (see picture) is the first efficient radical coupling method that is not restricted to short chains. When applied to AB diblock copolymers, I‐ARC constitutes a straightforward approach to the preparation of novel symmetrical ABA triblock copolymers.

     

Photoinitiators with functional groups. VII. Covalently bonded camphorquinone—amines

Camphorquinone (CQ), a widely used photoinitiator (PI) in dental applications, was covalently bonded to aromatic amines to enhance the rate of electron and proton transfer effect due to the close vicinity of the diketone and the amine group. 10‐bromocamphorquinone and 10‐bromomethylcamphorquinone were selected as suitable precursors for esterification with the carboxyl group containing aromatic amines based on 4‐dimethylaminobenzoic acid. Properties of the new photoinitiating systems were investigated by UV spectroscopy and differential scanning photocalorimetry in lauryl acrylate. Compared to physical mixtures, in all cases similar or even better performance was obtained. Surprisingly, 10‐acetyl derivatives 7 – 9 and 18 especially, were found to be highly reactive. Compared to CQ/ethyl 4‐dimethylaminobenzoate, the rate of photopolymerization was increased by a factor of up to 2. Intramolecular reaction was confirmed by photo‐differential scanning calorimetry experiments with varying PI concentrations. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4948–4963, 2004     

Organic Photocatalytic Cyclization of Polyenes: A Visible‐Light‐Mediated Radical Cascade Approach

A visible‐light‐mediated, organic photocatalytic stereoselective radical cascade cyclization of polyprenoids is described. The desired cascade cyclization products are achieved in good yields and high stereoselectivities with eosin Y as photocatalyst in hexafluoro‐2‐propanol. The catalyst system is also suitable for 1,3‐dicarbonyl compounds, which require only catalytic amounts of LiBr to promote the formation of the corresponding enols.     

Controlled Light‐Mediated Preparation of Gold Nanoparticles by a Norrish Type I Reaction of Photoactive Polymers

Gold nanoparticles (AuNPs) are subjects of broad interest in scientific community due to their promising physicochemical properties. Herein we report the facile and controlled light‐mediated preparation of gold nanoparticles through a Norrish type I reaction of photoactive polymers. These carefully designed polymers act as reagents for the photochemical reduction of gold ions, as well as stabilizers for the in situ generated AuNPs. Manipulating the length and composition of the photoactive polymers allows for control of AuNP size. Nanoparticle diameter can be controlled from 1.5 nm to 9.6 nm.     

Controlled Bimodal Molecular‐Weight‐Distribution Polymers: Facile Synthesis by RAFT Polymerization

The RAFT agents RAFT‐1 and RAFT‐2 were used for RAFT polymerization to synthesize well‐defined bimodal molecular‐weight‐distribution (MWD) polymers. The system showed excellent controllability and “living” characteristics toward both the higher‐ and lower‐molecular‐weight fractions. It is important that bimodal higher‐molecular‐weight (HMW) polymers and block copolymers with both well‐controlled molecular weight (MW) and MWD could be prepared easily due to the “living” features of RAFT polymerization. The strategy realized a mixture of higher/lower‐molecular‐weight polymers at the molecular level but also preserved the features of living radical polymerization (LRP) of the RAFT polymerization.     

Control of Molar Mass Distribution by Polymerization in the Analytical Ultracentrifuge

Molar mass distributions are of high interest in macromolecular chemistry because they directly determine the physical and chemical properties of polymers. A principal approach to obtain and control the shape of broad molar mass distributions is adjusting the initiator concentration in free radical polymerizations. A controlled gradient of the initiator concentration should potentially lead to tailored molar mass distributions. Here we use analytical ultracentrifugation (AUC) to adjust and measure a macroinitiator's concentration gradient. Subsequent photopolymerization of a uniformly distributed monomer leads to desired chain length distributions. Resulting distributions are described and calculated by a Schulz–Flory approach. The desired concentration profiles are simulated in advance and can be detected anytime by the optical systems in the centrifuge. Therefore, tailored broad molar mass distributions can now be produced using predictions from simulations using the established theory of AUC.     

Generating Hydrated Electrons for Chemical Syntheses by Using a Green Light‐Emitting Diode (LED)

We present the first working system for accessing and utilizing laboratory‐scale concentrations of hydrated electrons by photoredox catalysis with a green light‐emitting diode (LED). Decisive are micellar compartmentalization and photon pooling in an intermediate that decays with second‐order kinetics. The only consumable is the nontoxic and bioavailable vitamin C. A turnover number of 1380 shows the LED method to be on par with electron generation by high‐power pulsed lasers, but at a fraction of the cost. The extreme reducing power of the electron and its long unquenched life as a ground‐state species are synergistic. We demonstrate the applicability to the dechlorination, defluorination, and hydrogenation of compounds that are inert towards all other visible‐light photoredox catalysts known to date. A comprehensive mechanistic investigation from microseconds to hours yields results of general validity for photoredox catalysis with photon pooling, allowing optimization and upscaling.     

Poly(paraphenylene sulfide) and Poly(metaphenylene sulfide) via Light‐Initiated SRN1‐Type Polymerization of Halogenated Thiophenols

In this work, the synthesis of various halogenated thiophenol derivatives is presented. These thiophenols are used as monomers in light‐initiated S_RN1‐type radical polymerization reactions. The method provides easy access to industrially relevant poly(paraphenylene sulfide) and poly(metaphenylene sulfide). The influence of the halide leaving group and of other substituents in the thiophenol monomer on the polymerization process is investigated.