Polymer Functionalization
Controlled Chitosan Molecular Weight Reduction
Chitosan is typically produced by the deacetylation of biopolymer chitin. Its molecular weight (Mw) is a function of the chitin source and the deacetylation conditions. It remains a challenge to both control the Mw of chitosan and obtain good polydispersity, essential for optimal function of chitosan in different applications. We explored the use of mechanochemistry and aging as effective methods towards the controlled reduction of the Mw of commercial high Mw chitosan (HMwChs) with phosphoric acid where Mw was measured by gel permeation chromatography (GPC). In comparison to conventional solution-based methods, this new mechanochemical process by varying our conditions can target Mw in the range of 20 to 200 kDa with PDI of 4 or under. We investigated the kinetics of this process during the mechanochemical and the aging steps to better understand the controlled chitosan Mw reduction. We investigated reaction parameters including milling time, aging time, and quantity of H3PO4. We also compared two sources of phosphoric acids, 85% H3PO4 and crystalline H3PO4 in order to gain insight on the role of water during the hydrolysis reaction. Finally, we demonstrate that our process features an improved process mass intensity (PMI) as compared to a comparable solution state method. (Yang, ACS Sustainable Chem. Eng. 2023)
Substituded Functional Biopolymers via Reductive Amination
Chitosan is readily available from various biomass waste streams including crustaceans, cephalopods, insects, and fungus. The polymer possesses primary amine groups which are great handles for functionalization. Yet efficient functionalization with high degree of substitution is challenging to achieve via solvothermal methods due to limitations in chitosan solvation properties. Herein we reported a mechanochemical and aging-based method of alkylating Chs via reductive amination, with a wide scope of functionalities to directly address this point. The resulting materials feature excellent stability. We investigated the kinetics of the solid-state SB formation and reduction by sodium borohydride (NaBH4) via aging. We also investigated the versatility of this method with a range of carbonyl compounds from aromatic aldehydes to aliphatic aldehydes. The method also allows for the production of Chs derivatives inaccessible by solvothermal methods, affording a novel pathway to the green functionalization of chitosan by reductive alkylation, with unprecedentedly high degrees of substitution. We characterized the special properties of some Chs derivatives and suggested potential applications for the materials. More importantly, we evaluated this method with green metrics and demonstrated that our process features improved process mass intensity (PMI) of 36 as compared to comparable solution-based methods. This work is also a proof of concept of the deployment of reductive amination methodology through mechanochemistry and aging. (Yang, Green Chem., 2024)