Thermal/Blue Light Induced Cross-Linking of Acrylic Coatings with Diazo Compounds.

The thermal curing of industrial coatings (e.g., car painting and metal coil coatings) is accompanied by a substantial energy consumption due to the intrinsically high temperatures required during the curing process. Therefore, the development of new photochemical curing processes-preferably using visible light-is in high demand. This work describes new diazo-based cross-linkers that can be used to photocure acrylic coatings using blue light. This work demonstrates that the structure of the tethered diazo compounds influences the cross-linking efficiency, finding that side reactions are suppressed upon engineering greater molecular flexibility. Importantly, this work shows that these diazo compounds can be employed as either thermal or photochemical cross-linkers, exhibiting identical crosslinking performances. The performance of diazo-cross-linked coatings is evaluated to reveal excellent water resistance and demonstrably similar material properties to UV-cured acrylates. These studies pave the way for further usage of diazo-functionalized cross-linkers in the curing of paints and coatings.

A highly efficient and sustainable catalyst system for terminal epoxy-carboxylic acid ring opening reactions.

The nucleophilic ring opening of epoxides by carboxylic acids is an indispensable transformation for materials science and coating technologies. Due to this industrial significance, improvements in operational energy consumption and catalyst sustainability are highly desirable for this transformation. Herein, an efficient, environmentally benign and non-toxic halide free cooperative catalyst system based on an iron(iii) benzoate complex and guanidinium carbonate is reported. The novel catalyst system shows improved activity over onium halide catalysts under neat conditions and in several solvents, including anisole and nBuOAc. Detailed mechanistic studies using FeCl3/DMAP as a catalyst revealed the importance of a carboxylate bridged cationic trinuclear µ3-oxo iron cluster and guanidinium carbonate or DMAP as a carboxylate reservoir due to its superior activity.

Diazirine-Functionalized Polyurethane Crosslinkers for Isocyanate-Free Curing of Polyol-Based Coatings.

Polyurethane coatings have strong material properties due to the hydrogen bonding inherent to the urethane groups. However, installing this urethane moiety usually requires curing through difficult-to-handle isocyanates. In this work, we show the development of a polyurethane-based crosslinker that can be used to formulate a one-component polyurethane coating with material properties similar to those of isocyanate-based polyurethane coatings. To achieve this, we used diazirine functionalities that generate carbenes upon heating, which react with alcohol functionalities in a polyol to generate a crosslinked network with a high storage modulus.

Activation of alkyl hydroperoxides by manganese complexes of tmtacn for initiation of radical polymerisation of alkenes.

The activation of alkyl hydroperoxides to generate radicals is a key step in the initiation of radical polymerisations in many industrial applications, not least protective coatings. Cobalt soaps (Co(ii) alkyl carboxylates) are highly effective catalysts under ambient conditions but viable alternatives based on less scarce catalysts are desirable, with especially iron and manganese catalysts showing potential. Manganese complexes of the ligand N,N',N″-trimethyl-1,4,7-triazacyclononane (tmtacn) are long established as catalysts for organic oxidations with H2O2, however their reactivity with alkyl hydroperoxides is less studied especially in apolar solvents. Here we show that this family of complexes can be employed as catalysts for the decomposition of alkyl hydroperoxides in apolar solvents such as styrene/methyl methacrylate mixtures and resins based on styrene/bisphenol-A based diglycidyl ether bismethacrylate (BADGE-MA). The progress of alkene polymerisation in crosslinking resins is followed by Raman spectroscopy to establish its dependence on the oxidation state of the manganese catalyst used, as gelation time and onset of autoacceleration are of particular interest for many applications. We show, through reaction progress monitoring with UV/vis absorption and Raman spectroscopy, that the stability of the manganese complexes in the resin mixtures has a substantial effect on curing progress and that the oxidation state of the resting state of the catalyst is most likely Mn(ii), in contrast to reactions with H2O2 as oxidant in which the oxidation state of the resting state of catalyst is Mn(iii). Manganese complexes of tmtacn are shown to be capable initiators of alkene radical polymerisations, and their rich coordination and redox chemistry means that resin curing kinetics can potentially be tuned more readily than with cobalt alkyl carboxylates.

Formation of substituted dioxanes in the oxidation of gum arabic with periodate.

Renewable polysaccharide feedstocks are of interest in bio-based food packaging, coatings and hydrogels. Their physical properties often need to be tuned by chemical modification, e.g. by oxidation using periodate, to introduce carboxylic acid, ketone or aldehyde functional groups. The reproducibility required for application on an industrial scale, however, is challenged by uncertainty about the composition of product mixtures obtained and of the precise structural changes that the reaction with periodate induces. Here, we show that despite the structural diversity of gum arabic, primarily rhamnose and arabinose subunits undergo oxidation, whereas (in-chain) galacturonic acids are unreactive towards periodate. Using model sugars, we show that periodate preferentially oxidises the anti 1,2-diols in the rhamnopyranoside monosaccharides present as terminal groups in the biopolymer. While formally oxidation of vicinal diols results in the formation of two aldehyde groups, only traces of aldehydes are observed in solution, with the main final products obtained being substituted dioxanes, both in solution and in the solid state. The substituted dioxanes form most likely by the intramolecular reaction of one aldehyde with a nearby hydroxyl group, followed by hydration of the remaining aldehyde to form a geminal diol. The absence of significant amounts of aldehyde functional groups in the modified polymer impacts crosslinking strategies currently attempted in the preparation of renewable polysaccharide-based materials.

Photoactive Fe Catalyst for Light-Triggered Alkyd Paint Curing.

Herein, we show that the photoactive complexes [(Cp)Fe(arene)]+ (Cp = cyclopentadienyl; arene = C6H6, C6H5Me) act as latent catalysts that allow for photochemical control over the onset of alkyd paint curing, without the need for antiskinning agents such as the volatile 2-butanone oxime normally used to prevent curing during paint storage. The highly soluble neutral complexes [(Cp)Fe(Ch)] and [(Cp)Fe(Ch')] (Ch = cyclohexadienyl, Ch' = methylcyclohexadienyl) readily convert to the photoactive complexes [(Cp)Fe(arene)]+ upon oxidation in alkyd, allowing the latter to be dosed in a wide range of concentrations. Infrared and Raman studies show similar spectral changes of the alkyd paint matrix as have been observed in alkyd curing mediated by well-known, industrially applied cobalt- and manganese-based catalyst Co(neodecanoate)2 and [(Me3TACN)2Mn2(µ-OOCR)3](OOCR). The [(Cp)Fe(Ch)]/[(Cp)Fe(arene)]+ system performs equally well as these cobalt- and manganese-based catalysts in terms of drying time and outperform the manganese catalyst by showing a hardness development (increase) similar to that of the cobalt-based catalyst. Based on electron paramagnetic resonance and light-activity studies, we propose that photolysis of [(Cp)Fe(arene)]+ generates short-lived active FeII species, explaining the desired latency. The [(Cp)Fe(Ch)]/[(Cp)Fe(arene)]+ alkyd curing systems presented herein are unique examples of intrinsically latent paint curing catalysts that (1) are based on an abundant and harmless transition metal (Fe), (2) do not require any antiskinning agents, and (3) show favorable performance in terms of drying times and hardness development.

In situ EPR and Raman spectroscopy in the curing of bis-methacrylate-styrene resins.

The curing of bis-methacrylate-styrene resins initiated by the cobalt catalyzed decomposition of cumyl hydroperoxide is monitored at ambient temperatures in situ by EPR and Raman spectroscopy. EPR spectroscopy shows the appearance of organic radicals after ca. 1 h from initiation with an increase in intensity from both polystyrene and methacrylate based radical species over a further ca. 2 h period to reach a maximum spin concentration of ca. 2-3 mM. Alkene conversion to polymer was monitored by Raman spectroscopy in real time in situ with EPR spectroscopy and reveals that the appearance of the radical signals is first observed only as the conversion approaches its maximum extent (70% at room temperature), i.e., the resin reaches a glass-like state. The radicals persist for several months on standing at room temperature. Flash frozen samples (77 K) did not show EPR signals within 1 h of initiation. The nature of the radicals responsible for the EPR spectra observed were explored by DFT methods and isotope labelling experiments (D8-styrene) and correspond to radicals of both methacrylate and polystyrene. Combined temperature dependent EPR and Raman spectroscopy shows that conversion increases rapidly upon heating of a cured sample, reaching full conversion at 80 °C with initially little effect on the EPR spectrum. Over time (i.e. subsequent to reaching full conversion of alkene) there was a small but clear increase in the EPR signal due to the methacrylate based radicals and minor decrease in the signal due to the polystyrene based radicals. The appearance of the radical signals as the reaction reaches completion and their absence in samples flash frozen before polymerization has halted, indicate that the observed radicals are non-propagating. The formation of the radicals due to stress within the samples is excluded. Hence, the observed radicals are a representative of the steady state concentration of radicals present in the resin over the entire timespan of the polymerization. The data indicate that the lack of EPR signals is most likely due to experimental aspects, in particular spin saturation, rather than low steady state concentrations of propagating radicals during polymerization.

Isocyanate-Free Polyurea Synthesis via Ru-Catalyzed Carbene Insertion into the N-H Bonds of Urea.

Polyureas have widespread applications due to their unique material properties. Because of the toxicity of isocyanates, sustainable isocyanate-free routes to prepare polyureas are a field of active research. Current routes to isocyanate-free polyureas focus on constructing the urea moiety in the final polymerizing step. In this study we present a new isocyanate-free method to produce polyureas by Ru-catalyzed carbene insertion into the N-H bonds of urea itself in combination with a series of bis-diazo compounds as carbene precursors. The mechanism was investigated by kinetics and DFT studies, revealing the rate-determining step to be nucleophilic attack on a Ru-carbene moiety by urea. This study paves the way to use transition-metal-catalyzed reactions in alternative routes to polyureas.

Photo-crosslinking polymers by dynamic covalent disulfide bonds.

A simple and general strategy to construct photo-crosslinkable polymers by introducing sidechain 1,2-dithiolanes based on natural thioctic acid is presented. The disulfide five-membered rings act both as light-absorbing and dynamic covalent crosslinking units, enabling efficient photo-crosslinking and reversible chemical decrosslinking of polydimethylsiloxane polymers.