Organocatalytic [2 + 2] Photopolymerization under Visible Light: Accessing Sustainable Polymers from Cinnamic Acids

Abstract

Herein, the successful development of a metal‐free, solution [2 + 2] photopolymerization of natural cinnamic acid‐derived bisolefinic monomers is reported, which is enabled by a strategy based on direct triplet state access via energy transfer catalysis. 2,2’‐Methoxythioxanthone has been identified as an effective organic photocatalyst for the [2 + 2] photopolymerization in solution, which can be excited by visible light and activate the biscinnamate monomers via triplet energy transfer. This method features its metal‐free conditions, visible light utilization, solution polymerization, and abundant biomass‐based feedstock, as well as processable polymer products, which is different from the rigid, insoluble products obtained from solid‐state photopolymerization. This solution polymerization method also shows a good compatibility to monomer structures; cinnamic acid‐derived bisolefinic monomers with different linkers, including diamine, natural diol, and bisphenol, can all readily undergo [2 + 2] photopolymerization, and be transformed into colorless, sustainable polymers.

Renewable Photopolymers: Transformation of Biomass Resources into Value-Added Products Under Light

“…Various building blocks can be incorporated into the linker of biscinnamate monomers to tune the properties of photodimerzied polymers with a high molecular weight (Scheme 23B). 141,203 Similarly, the [2 + 2] photocycloaddition of a disorbic ester monomer afforded a polyladderane via the head-to-tail packing in the solid state due to the electrostatic interactions. 204 The phototriggered [2 + 2] cycloaddition of caffeic acidderived polyesters was applied to cross-link the polymer into thermosets (Scheme 23C).…”

“…Both CBDA-1 and CBDA-4 can reverse back to trans -cinnamic acid by thermal cleavage and irradiation by deep UV, making it possible to prepare thermally or photorecyclable and/or degradable polymer materials. , With the assistance of iridium complexes or an organic catalyst, 2,2′-methoxythioxanthone, as the energy transfer catalyst, photodimerization in solution is possible even without preorganization, and the light irradiation can be red-shifted to the visible light region. Various building blocks can be incorporated into the linker of biscinnamate monomers to tune the properties of photodimerzied polymers with a high molecular weight (Scheme B). , Similarly, the [2 + 2] photocycloaddition of a disorbic ester monomer afforded a polyladderane via the head-to-tail packing in the solid state due to the electrostatic interactions …”

Accessing Cyclobutane Polymers: Overcoming Synthetic Challenges via Efficient Continuous Flow [2 + 2] Photopolymerization

“…Synthetic methods toward small molecule cyclobutanes via [2 + 2] photocycloadditions are established and provide insight for methodology optimization of cyclobutane polymers. In fact, [2 + 2] photopolymerizations are gaining increased attention today. ,, Solution state intramolecular [2 + 2] photocycloadditions utilize dilute substrate reaction conditions, employ electron-rich olefins such as cinnamic esters, ,− and heavily rely on olefin distances less than 4–5 Å to favor small molecule dimerization. For covalently tethered cinnamic esters, the linker defines the absolute configuration of the cyclobutane products, resulting in high diastereoselectivity .…”

“…Iridium-based photosensitizers are the most common; , however, thioxanthone-derived photosensitizers are attractive organic, low-cost alternatives, − which have been previously employed in [2 + 2] photopolymerizations of cinnamates. Liao et al describe a photopolymerization strategy utilizing thioxanthone derivates, all of which sufficiently promote the [2 + 2] photopolymerization . Building upon the prior small molecule findings and photopolymerization strategies, we present an efficient synthesis of a library of functionally diverse cinnamic ester monomers for novel cyclobutane polymers (Figure a).…”

“…Liao et al describe a photopolymerization strategy utilizing thioxanthone derivates, all of which sufficiently promote the [2 + 2] photopolymerization. 23 Building upon the prior small molecule findings and photopolymerization strategies, we present an efficient synthesis of a library of functionally diverse cinnamic ester monomers for novel cyclobutane polymers (Figure 2a). Specifically, we report a library of functionally diverse cyclobutane polyesters from both electronrich and electron-poor olefins, 36 of which have not been previously synthesized to the best of our knowledge.…”