Cansu Aydogan

Shining a light on an adaptable photoinitiator: advances in photopolymerizations initiated by thioxanthones

Photochemistry plays a central role in synthetic polymer research. Aromatic ketones, examples of which include benzophenone, thioxanthone, camphorquinone, among others, are renowned for their excellent optical characteristics and have been extensively utilized for photochemical induction of polymerization processes. Of particular interest is thioxanthone due to its adaptability for bearing different functionalities and its applications in various modes of photopolymerization, in which it accomplishes photoinitiation in conjunction with other co-initiator compounds; a behavior that is referred to as bi-molecular photoinitiation. In this paper, we review the photochemistry of thioxanthone-based systems and their use in different modes of photoinitiated polymerizations. Citing examples from the literature, the development of various photoinitiating systems based on thioxanthones along with an understanding of their mechanistic behavior has been elucidated previously.

Conventional Type II photoinitiators as activators for photoinduced metal-free atom transfer radical polymerization

A novel methodology for photoinduced metal-free Atom Transfer Radical Polymerization (ATRP) by using conventional Type II photoinitiators such as benzophenone, thioxanthone, isopropyl thioxanthone and camphorquinone as sensitizers is presented. These sensitizers efficiently activate the ATRP of vinyl monomers when used in conjunction with co-initiators and alkyl halides under light irradiation. DFT calculations were performed to reveal mechanistic aspects and showed that the initiation of the controlled ATRP reaction pathway from the triplet state is energetically favoured for all sensitizers. In the case of benzophenone, however, the hydrogen abstraction pathway, which leads to free radical polymerization, is a possible side reaction. The strategy applied was proved to yield polymers with narrow molecular distribution. The chain-end fidelity of the polymer obtained was approved by chain extension and block copolymerization experiments, whereas the irradiation dependency of polymerization was confirmed by light on/off experiments.

Block copolymer synthesis in one shot: concurrent metal-free ATRP and ROP processes under sunlight

A completely metal-free strategy was developed by combining Atom Transfer Radical Polymerization (ATRP) and Ring Opening Polymerization (ROP) for the syntheses of block copolymers. These two different metal-free controlled/living polymerizations, which have no effect on each other, were concurrently realized in one reaction medium. Using a specifically designed bi-functional initiator, possessing primary hydroxyl and tertiary bromide functionalities, vinyl and lactone monomers were simultaneously polymerized under sunlight, by metal-free strategies. Spectral and chromatographic results reveal that the synthetic approach applied produces block copolymers in a controlled manner, free from homopolymers, and with narrow molecular weight distributions.

Hyperbranched Polymers by Type II Photoinitiated Self‐Condensing Vinyl Polymerization

Type II photoinitiated self-condensing vinyl polymerization for the preparation of hyperbranched polymers is explored using 2-hydroxyethyl methacrylate (HEMA) or 2-(dimethylamino)ethyl methacrylate (DMAEMA), and methyl methacrylate as hydrogen donating inimers and comonomer, respectively, in the presence of benzophenone and camphorquinone under UV and visible light. Upon irradiation at the corresponding wavelength, the excited photoinitiator abstracts hydrogen from HEMA or DMAEMA leading to the formation of initiating radicals. Depending on the concentration of inimers, type of the photoinitiator, and irradiation time, hyperbranched polymers with different branching densities and cross-linked polymers are formed.

Electrochemical manipulation of adhesion strength of polybenzoxazines on metal surfaces: from strong adhesion to dismantling

A novel electrochemical redox process for the manipulation of adhesion of polybenzoxazine thermosets on metal surfaces is reported. The method pertains to the electrochemically driven hydroquinone–quinone redox couple. Several antraquinone based bisbenzoxazines possessing phenyl, benzyl and methyl furfuryl substituents were synthesized and characterized. The antraquinone bisbenzoxazines were shown to readily undergo thermally activated ring-opening polymerization in the absence of a catalyst forming cross-linked networks on metal surfaces. The substituent effect on thermal curing behaviour and thermal stability of the cured polymers were investigated. The strong binding affinities of phenolic hydroxyl groups of the anthraquinone moiety in the cured polymers promote adhesion on the metal surface. Electrochemical oxidation converts hydroquinone groups into quinone moieties resulting in the dismantling of the coated films. The generality of this electrochemical method is demonstrated by initial results on platinum electrodes as well as steel plates.

Hyperbranched Polymers by Light-Induced Self-Condensing Vinyl Polymerization

Hyperbranched polymers (HBPs), a unique class of dendritic macromolecules, have received continuous interest from macromolecular scientists due to their inherent properties such as high level of functional terminal units, high solubility, and low viscosity. Despite enormous efforts devoted to the synthesis of HBPs by traditional methods such as single and double monomer strategies involving step-growth polymerization and self-condensing vinyl polymerization (SCVP) processes, there have been limited attempts to employ light-induced processes. Photochemical methods, however, exhibit distinct advantages not characteristically disclosed by traditional ones, such as spatial and temporal control, low energy, and site-specific activation. This review, after a brief summary of the conventional methods, presents the unique features and the key functionalities of the inimers for photoinduced SCVP and strategies for preparing HBPs.