In the hunt for new building blocks for biodegradable polymers, researchers have turned their attention to carotenoids. Could these common and widely distributed pigments give us more than good night vision?
Degradability is an important attribute when developing new polymer materials. To ensure new polymers are fully degradable, scientists will often add alkyl side chains to the molecular design. Although these design strategies are common, they have not previously been explored with carotenoids as polymer building blocks...until now.
Carotenoids are biobased compounds that come in a variety of colors, including including yellow, orange, red, and purple. They are common pigments, with more than 850 naturally occurring types distributed across a wide range of life forms, from photosynthetic bacteria and species of fungi and algae to higher-order plants (such as the well-known carrot) and animals.
Carotenoids are susceptible to light, enzymatic, or chemical oxidation damage, offering alternative triggers for on-demand degradation when integrated into polymer systems. And interestingly, carotenoid molecules such as β-carotene have a similar substructure to polyacetylene, a well-known conductive but insoluble polymer.
To explore this further, researchers from the University of Toronto, Canada, developed and tested a series of carotenoid-based polymers with varying side chain lengths to tune their solubility. Using several spectroscopic techniques, the team quantitatively determined maximum solubilities based on varying alkyl chain lengths, and they also investigated the effects of acidic and artificial sunlight-promoted degradation.
The results showed the carotenoid-based polymer system had two modes of on-demand degradation, with acid hydrolysis accelerating the rate of polymer degradation and artificial sunlight generating additional degradation products.
This work highlights carotenoid monomers as viable candidates in the design of biobased, degradable, and conjugated polymers. Unsurprisingly, it's not the first time that researchers have looked to make polymers and plastics from natural, biodegradable ingredients, since success with such products would be highly desirable from both a consumer and environmental standpoint. Previous studies with indigo, vanillin, and melanin have resulted in biobased polymers with electrically conductive properties that are promising for energy storage, biomedical, and sensor applications. Effective antibacterial and tack-free coatings have also been synthesized by incorporating eugenol—a natural antibacterial agent—in a polymer network made from a β-carotene derivative.
Future research into the potential for carotenoids in the field will include improvements to the synthesis to yield polymers with higher molecular weight, as well as evaluating conductivity and investigating the potential for monomer recovery. For degradation studies, the authors note that mimicking nature using enzymatic degradation pathways would also be of interest. Overall, this work lays the foundation for a new class of fully degradable conjugated poly(azomethine)s that use building blocks from nature.
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