Mapping the multi-step mechanism of a photoredox catalyzed atom-transfer radical polymerization reaction by direct observation of the reactive intermediates

The rapid development of new applications of photoredox catalysis has so far outpaced the mechanistic studies important for rational design of new classes of catalysts. Here, we report the use of ultrafast transient absorption spectroscopic methods to reveal both mechanistic and kinetic details of multiple sequential steps involved in an organocatalyzed atom transfer radical polymerization reaction. The polymerization system studied involves a N,N-diaryl dihydrophenazine photocatalyst, a radical initiator (methyl 2-bromopropionate) and a monomer (isoprene). Time-resolved spectroscopic measurements spanning sub-picosecond to microseconds (i.e., almost 8 orders of magnitude of time) track the formation and loss of key reactive intermediates. These measurements identify both the excited state of the photocatalyst responsible for electron transfer and the radical intermediates participating in propagation reactions, as well as quantifying their lifetimes. The outcomes connect the properties of N,N-diaryl dihydrophenazine organic photocatalysts with the rates of sequential steps in the catalytic cycle.

Short-lived intermediates are tracked in real-time by transient absorption spectroscopy during a multi-step photoredox catalysed polymerization reaction.     

N‐alkoxypyridinium salts as coinitiators in radical polymerization: Synthesis and Photochemical Properties

A few N‐alkoxypyridinium salts are developed as photoinitiators for efficient polymerization reactions. They are characterized by absorption properties below 300 nm, and generate alkoxy radicals on UV‐Vis light exposure. The squarylium dye was used as a blue‐light photosensitizer. Polymerization results are correlated with the photochemistry of N‐alkoxypyridinium salts. The quenching of the excited singlet state of squarylium dye by pyridinium salt and the formation of the semioxidized species of squaraine suggests an electron transfer from an excited dye to a coinitiator, and that the resulting oxygen‐centered radical initiates the polymerization process. The chemical mechanism was investigated by steady state photolysis and nanosecond laser flash photolysis experiments. Photoinitiating activity of new photoinitiators for initiation of polymerization of trimethylolpropane triacrylate in the UV‐blue light region was compared with photoinitiating ability of selected commercially available initiators. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2840–2850     

New Squaraine‐based two‐component initiation systems for UV‐blue light induced radical polymerization: Kinetic and time‐resolved laser spectroscopy studies

In this article, the ability of two‐component photoinitiator systems for efficient polymerization of 2‐ethyl‐2‐(hydroxymethyl)?1,3‐propanediol triacrylate was presented. The photophysics and photochemistry of squaraine dyes in the presence of an electron donor as well as an electron acceptor was investigated, and it was found that the photosensitizer in an excited state might act as an electron acceptor or an electron donor. The excited states of squaraines may be quenched by tetramethylammonium n‐butyltriphenylborate ( B2 ), diphenyliodonium chloride ( I1 ), and N‐methoxy‐4‐phenylpyridinium tetrafluoroborate ( NO ). © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 471–484     

Reverse atom transfer radical polymerization of methyl methacrylate initiated by p‐chlorobenzenediazonium tetrafluoroborate

The controlled polymerization of methyl methacrylate (MMA) in bulk was initiated with p‐chlorobenzenediazonium tetrafluoroborate ( 1 ) and Cu(II) or Cu(I)/Cu(II)/N,N,N′,N″,N″‐pentamethyldietylene triamine (PMDETA) complex system at various temperatures (20, 60, and 90 °C). The proposed polymerization mechanism is based on the Meerwein‐type arylation reaction followed by a reverse atom transfer radical polymerization. In this mechanism, aryl radicals formed by the reaction with 1 and Cu(I) and/or PMDETA initiated the polymerization of MMA. The polymerization is controlled up to a molecular weight of 46,000 at 90 °C. Chain extension was carried out to confirm the controlled manner of the polymerization system. In all polymerization systems, the polydispersity index and initiator efficiency ranged from 1.10–1.57 to 0.10–0.21, respectively. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2019–2025, 2003     

Theoretical description of solvent effects on fluorescence spectra of bulky charge transfer compound DMA-DMPP

Several theoretical models are compared to reproduce the spectroscopic fluorescence shift of 4-(4′-N,N-dimethylaminophenyl)-3,5-dimethyl-1,7-diphenyl-bis-pyrazolo-[3,4-b;4′,3′e]-pyridine (DMA-DMPP) in different solvents. DMA-DMPP is used as a model compound because it shows a large shift in emission energy for solvents of various polarities and dual fluorescence in polar protic solvents. Although the simple Onsager model is not able to reproduce the experimental results, the self-consistent reaction field (SCRF) model with extension to excited states based on the AM1 Hamiltonian yields excellent agreement. According to the latter model, the red-shifted emission band can be related to a highly polar charge transfer state without geometrical rearrangements, whereas the normal (short wavelength) emission is attributed to emission from an excited state with increased conjugation in a flattened geometry. A supramolecular approach with six molecules of water surrounding the solute can explain satisfactorily the two distinct fluorescence bands. In protic solvents, the emitting CT state shows additional stabilization of the locally excited state with a planar conformation. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1584–1595, 1998     

Solvent effect on the atom transfer radical polymerization of allyl methacrylate

The controlled radical polymerization of allyl methacrylate by atom transfer radical polymerization was carried out in solution at 70 °C, with ethyl 2‐bromoisobutyrate as the initiator and copper halide (CuX, where X is Cl or Br) with N,N,N′,N″,N″‐pentamethyldiethylenetriamine as the catalyst system. Kinetic analyses demonstrated that all the homopolymerization reactions showed a general behavior characterized by two clearly differentiated stages. Thus, in the early stage, the conversion increased continually with the time, independently of the solvent employed. In the second stage, a deceleration process took place, and a limit conversion was achieved, depending on the polarity and amount of the solvent used. The dependence of both the gel formation and limit conversion, as well as the molecular characteristics of poly(allyl methacrylate)s formed with different experimental parameters, such as the initial monomer concentration, the solvent employed, and the type of halide used as a catalyst, was also examined. The prepared polymers were characterized by size exclusion chromatography, Fourier transform infrared, and one‐ and two‐dimensional nuclear magnetic resonance spectroscopy. Moreover, chain‐growth experiments with butyl acrylate as the comonomer proved the living character of the poly(allyl methacrylate)s obtained, with these used as macroinitiators. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 6247–6261, 2005     

Charge transfer interactions in polyesters with a donor‐(σ‐bridge)‐acceptor moiety in the repeating unit

Two high molecular weight linear polyesters were investigated to gain insight in how the photophysics of electron donor‐(σ‐spacer)‐electron acceptor (DσA) compounds are affected by incorporation into a polymer. They were prepared by condensation of either adipoyl or sebacoyl chloride with a diol that was functionalized with an N,N‐dialkylaniline donor, a cyclohexyl type σ‐spacer, and a 1,1‐dicyanovinyl acceptor. The solubility, which is very low, and the thermal properties of the polyesters are dictated by physical crosslinking as a consequence of interchain donor‐acceptor interactions. Charge transfer (CT) absorption and emission are observed, which involve CT between DσA moieties of different chains rather than CT processes within a single DσA unit. As a result, the photophysics of the DσA units in the polyesters differs strongly from that of similar DσA compounds in solution. Upon swelling the polymers with THF, the CT fluorescence disappears partly. Analogous polymers containing only an N,N‐dialkylaniline donor display dual fluorescence; one band reflects local emission, while the other is attributed to excimer emission. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4775–4784, 2004     

Nitrogen Analogues of Thiele’s Hydrocarbon

A series of bis[N,N‐di‐(4‐methoxylphenyl)amino]arene dications 1 ²⁺– 3 ²⁺ have been synthesized and characterized. Their electronic structures were investigated by various experiments assisted by theoretical calculations. It was found that they are singlets in the ground state and that their diradical character is dependent on the bridging moiety. 3 ²⁺ has a smaller singlet–triplet energy gap and its excited triplet state is thermally readily accessible. The work provides a nitrogen analogue of Thiele’s hydrocarbon with considerable diradical character.     

Effect of phenol and derivatives on atom transfer radical polymerization in the presence of air

The various phenolic compounds in conjunction with Cu(II) or Cu(I)‐N,N,N′,N″,N″‐pentamethyl diethylenetriamine (PMDETA) complexes are used to initiate atom transfer radical polymerization (ATRP) of methyl methacrylate, styrene, and methyl acrylate in the presence of a limited amount of air at temperatures in the range of 80–110 °C. Meanwhile, an effort is directed toward the elucidation of the role of phenol and derivatives in ATRP catalyzed by Cu(II)/PMDETA. The catalytic sequence involves the formation of Cu(I) by electron transfer from phenol to Cu(II); Cu(I) so formed can then react in two distinctly different ways: with organic halide to form a propagating radical or with oxygen to form copper salt in its higher oxidation state; and regeneration of Cu(I) by excess phenol. Such regeneration of Cu(I) would be expected to lead to polymerization as a result of the consumption of oxygen and phenol as well. The phenols with electron releasing groups tended to increase the conversion of the polymerization. In this respect, sodium phenoxide, a more effective additive was found, whereas p‐nitro phenol was the least effective. The obtained polymers displayed the common features of a controlled polymerization such as molecular weight control and low polydispersity index value (M_w/M_n < 1.5). © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 351–359, 2004     

Tetra-n-butylammonium Fluoride–Catalyzed Eschenmoser–Claisen [3,3]-Sigmatropic Rearrangement

Condensation of dibenzofuran and dibenzothiophene carboxaldehydes with various aryl acetic acids followed by esterification of the acid intermediate afforded the 2,3-diaryl acrylates in good overall yields. Reduction of the esters with diisobutylaluminium hydride afforded the allylic alcohols, which underwent a smooth Eschenmoser–Claisen [3,3]-sigmatropic rearrangement on exposure to N,N-dimethylacetamide dimethylacetal in the presence of tetra n-butylammonium fluoride as catalyst to give 3,4-diaryl γ,δ-unstaturated amides in excellent yields.     

Photophysics,Excited‐state Double‐Proton Transfer and Hydrogen‐bonding Properties of 5‐Deazaalloxazines

The photophysical properties of 5‐deazaalloxazine and 1,3‐dimethyl‐5‐deazaalloxazine were studied in different solvents. These compounds have higher values of fluorescence quantum yields and longer fluorescence lifetimes, compared to those obtained for their alloxazine analogs. Electronic structure and S₀–S_i transitions were investigated using the ab initio methods [MP2, CIS(D), EOM‐CCSD] with the correlation‐consistent basis sets. Also the time‐dependent density functional theory (TD‐DFT) has been employed. The lowest singlet excited states of 5‐deazaalloxazine and 1,3‐dimethyl‐5‐deazaalloxazine are predicted to have the π, π* character, whereas similar alloxazines have two close‐lying π, π* and n, π* transitions. Experimental steady‐state and time‐resolved spectral studies indicate formation of an isoalloxazinic excited state via excited‐state double‐proton transfer (ESDPT) catalyzed by an acetic acid molecule that forms a hydrogen bond complex with the 5‐deazaalloxazine molecule. Solvatochromism of both 5‐deazaalloxazine and its 1,3‐dimethyl substituted derivative was analyzed using the Kamlet–Taft scale and four‐parameter Catalán solvent scale. The most significant result of our studies is that the both scales show a strong influence of solvent acidity (hydrogen bond donating ability) on the emission properties of these compounds, indicating the importance of intermolecular solute–solvent hydrogen‐bonding interactions in their excited state.     

Chain transfer to solvent in the radical polymerization of N‐isopropylacrylamide

Chain transfer to solvent has been investigated in the conventional radical polymerization and nitroxide‐mediated radical polymerization (NMP) of N‐isopropylacrylamide (NIPAM) in N,N‐dimethylformamide (DMF) at 120 °C. The extent of chain transfer to DMF can significantly impact the maximum attainable molecular weight in both systems. Based on a theoretical treatment, it has been shown that the same value of chain transfer to solvent constant, C_tr,S, in DMF at 120 °C (within experimental error) can account for experimental molecular weight data for both conventional radical polymerization and NMP under conditions where chain transfer to solvent is a significant end‐forming event. In NMP (and other controlled/living radical polymerization systems), chain transfer to solvent is manifested as the number‐average molecular weight (M_n) going through a maximum value with increasing monomer conversion. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Palladium-catalyzed amination in the synthesis of macrocyclic compounds containing 1,3-disubstituted adamantane fragments

Palladium-catalyzed amination of 1,3-dibromobenzene, 2,6-dibromopyridine, 3,3′-dibromobiphenyl, 2,7-dibromonaphthalene, and 1,8-dichloroanthracene with an equimolar amount of 2,2′-(adamantane-1,3-diyl)diethanamine resulted in the formation of macrocyclic compounds containing one or several adamantane and one or several aromatic fragments. The reactions of 2,2′-(adamantane-1,3-diyl)diethanamine with excess 1,3-dibromobenzene, 2,6-dibromopyridine, 1,8-dichloroanthracene, and 1,8-dichloroanthraquinone gave the corresponding N,N′-diaryl derivatives. Polyaza macrocycles incorporating adamantane, aromatic, and 4,7,10-trioxatridecane-1,13-diamine fragments, were obtained by palladium-catalyzed amination of the N,N′-diaryl derivatives with 4,7,10-trioxatridecane-1,13-diamine.     

Ambient‐temperature copper‐catalyzed atom transfer radical polymerization of methacrylates in ethylene glycol solvents

The use of ethylene glycol solvents in the room‐temperature atom transfer radical polymerization (ATRP) of various hydrophobic and hydrophilic methacrylates is demonstrated. Unlike many of the very polar solvents described in the literature for room‐temperature ATRP, these solvents have good solvency for a wide range of polymers and monomers and are cheap and relatively nontoxic. Ethylene glycols with one hydroxyl and one methoxy group, such as tri(ethylene glycol) monomethyl ether (TEGMME), provide optimal results. The polymerization of methyl methacrylate in TEGMME with CuBr/N,N,N′N′,N″‐pentamethyldiethylenetriamine as the catalyst requires the addition of CuCl₂ at the beginning of the reaction to produce well‐controlled polymerizations. This leads to polymers with predictable molecular weights and relatively narrow polydispersities. Polymerization in solvents that are fully methoxy‐capped terminate prematurely because of catalyst precipitation. The electrochemical behavior of copper complexes in selected solvents is examined to determine why these solvents provide good rates at room temperature. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1588–1598, 2005     

UV-visible absorption spectra and luminescence of the pesticide fenarimol

The UV-visible absorption and emission spectra have been measured of the pesticide fenarimol ((±)-2,4′-dichloro-α-(pyrimidin-5-yl)-benzhydryl alcohol) in solution. From comparison with the spectra of chlorotoluenes and pyrimidine, and from the effect of solvent polarity on the absorption spectrum, it is shown that the lowest excited singlet state is localized on the pyrimidine ring, and has n,π* character. Higher excited π,π* states are localized on both chlorotoluene and pyrimidine rings. Fenarimol shows a weak, fluorescence from the n,π* state, with a quantum yield which is strongly dependent on solvent. It is shown that this is due to changes in the nonradiative decay rate, particularly in protic solvents, due to increased intersystem crossing. Phosphorescence is observed in low temperature glasses. Although this shows two decay components, it is suggested that only one triplet state is involved, and that this has predominantly π,π* character.     

High molecular weight polyacrylamides by atom transfer radical polymerization: Enabling advancements in water‐based applications

Through the use of copper (I) chloride (CuCl) and tris(2‐dimethylaminoethyl)amine (Me₆‐TREN) as a metal/ligand pair, conditions for the robust, fast, and controlled radical polymerization of high molecular weight N‐hydroxyethylacrylamide (HEAm),N‐isopropylacrylamide (NIPAm), N,N′‐dimethylacrylamide (DMAm), and acrylamide (Am) at ambient temperature are reported. Linear evolution of molecular weight and narrow molecular weight distribution was observed for all monomers with degrees of polymerization ranging from 50 to 5000. Random copolymers of several acrylamide‐based monomers are also reported with excellent control over molecular weight and polydispersity. Characterization of high molecular weight poly (NIPAm) demonstrated large changes in the lower critical solution temperature observed on heating and cooling, and this hysteresis was exploited for the controlled release of doxorubicin from poly(NIPAm) spheres. This study represents the first example of preparation of high molecular weight acrylamide polymers by a metal‐mediated controlled radical polymerization technique. Access to these materials, as well as to NIPAm polymers in particular, opens new doors for interesting applications in a variety of fields including tissue engineering, drug delivery, and controlled solution viscosity. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Charge transfer excitations in TDDFT: A ghost‐hunter index

This work presents a new index, M _AC, enabling the on‐the‐fly detection of ghost charge transfer (CT) states, a major problem in time‐dependent density‐functional theory calculations. This computationally inexpensive index, derived as a modification of the Mulliken estimation of transition energy for CT excitations, relies on two basic ingredients: an effective CT distance, computed using our density‐based index (D _CT), and an orbital weighted estimation of the Ionization Potential and Electron Affinity. Some model systems, representative of both intermolecular and intramolecular CT excitations, were chosen as test cases. The robustness of our approach was verified by analyzing the behavior of functionals belonging to different classes (GGA, global hybrids and range separated hybrids). The results obtained show that ghost states are correctly spotted, also in the delicate case of intramolecular excitations displaying substantial donor‐bridge‐acceptor delocalization, in a regime for which the standard Mulliken formulation attends its limits. © 2017 Wiley Periodicals, Inc.     

Preparation of block copolymers of polystyrene and poly (t‐butyl acrylate) of various molecular weights and architectures by atom transfer radical polymerization

Block copolymers of polystyrene and poly(t‐butyl acrylate) were prepared using atom transfer radical polymerization techniques. These polymers were synthesized with a CuBr/N,N,N′,N″,N″‐pentamethyldiethylenetriamine catalyst system and had predictable molecular weights based on the degree of polymerization, as calculated from the initial ratio of monomer to initiator. The final polydispersities were low (1.10 < M_w /M_n < 1.3) for all the homopolymers and block copolymers. Polymers of various chain architectures were prepared, ranging from linear AB diblocks to three‐armed stars composed of AB diblocks on each arm. The key to controlled synthesis with this catalyst system was the choice of the solvent, temperature, and concentrations of catalyst and deactivator. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2274–2283, 2000     

The involvement of metal-to-CO charge transfer and ligand-field excited states in the spectroscopy and photochemistry of mixed-ligand metal carbonyls. A theoretical and spectroscopic study of [W(CO)(4)(1,2-ethylenediamine)] and [W(CO)(4)(N,N'-bis-alkyl-1,4-diazabutadiene)

A new interpretation of the electronic spectroscopy, photochemistry, and photophysics of group 6 metal cis-tetracarbonyls [M(CO)(4)L(2)] is proposed, that is based on an interplay between M --> L and M --> CO MLCT excited states. TD-DFT and resonance Raman spectroscopy show that the lowest allowed electronic transition of [W(CO)(4)(en)] (en = 1,2-ethylenediamine) has a W(CO(eq))(2) --> CO(ax) charge-transfer character, whereby the electron density is transferred from the equatorial W(CO(eq))(2) moiety to pi orbitals of the axial CO ligands, with a net decrease of electron density on the W atom. The lowest, emissive excited state of [W(CO)(4)(en)] was identified as a spin-triplet W(CO(eq))(2) --> CO(ax) CT excited state both computationally and by picosecond time-resolved IR spectroscopy. This state undergoes 1.5 ps vibrational relaxation/solvation and decays to the ground state with a approximately 160 ps lifetime. The nu(CO) wavenumbers and IR intensity pattern calculated by DFT for the triplet W(CO(eq))(2) --> CO(ax) CT excited state match well the experimental time-resolved spectrum. For [W(CO)(4)(R-DAB)] (R-DAB = N,N'-bis-alkyl-1,4-diazabutadiene), the W(CO(eq))(2) --> CO(ax) CT transition follows in energy the W --> DAB MLCT transition, and the emissive W(CO(eq))(2) --> CO(ax) CT triplet state occurs just above the manifold of triplet W --> DAB MLCT states. No LF electronic transitions were calculated to occur in a relevant energetic range for either complex. Molecular orbitals of both complexes are highly delocalized. The 5d(W) character is distributed over many molecular orbitals, while neither of them contains a predominant metal-ligand sigma 5d(W) component, contrary to predictions of the traditional ligand-field approach. The important spectroscopic, photochemical, and photophysical roles of M(CO(eq))(2) --> CO(ax) CT excited states and the limited validity of ligand field arguments can be generalized to other mixed-ligand carbonyl complexes.     

The triplet state of 4-nitro-N,N-dimethynaphthylamine

Nanosecond laser photolytic studies of 4-nitro-N,N-dimethylnaphthylamine (4-NDMNA) in nonpolar and polar solvents at room temperature show a transient species with an absorption maximum in the 500-510-nm range. This species is assigned to the lowest triplet excited state of 4-NDMNA. The absorption maximum of this state is independent of solvent polarity, and its lifetime is a function of the hydrogen donor efficiency of the solvent. In n-hexane the lifetime 1/k of the triplet state is 9.1 × 10^?6 sec, while in acetonitrile 1/k is 2.0 × 10^?7 sec. The hydrogen abstraction rate constant k_H of the triplet state with tributyl tin hydride (Bu₃SnH) in n-hexane is 1.7 × 10⁷M^?1·sec^?1, while in the case of isopropyl alcohol as hydrogen donor, k_H is 4.0 × 10⁷M^?1·sec^?1. The activation energy for the hydrogen abstraction by the triplet state from Bu₃SnH in deaerated n-hexane is 0.6 kcal/mol. The lack of spectral shift with increasing solvent polarity, and the appreciable hydrogen abstraction reactivity of the triplet state, also independent of solvent polarity, seem to indicate that this excited state is an n-π* state which retains its n-π* character even in polar media.