Lignin-Based Graft Copolymers via ATRP and Click Chemistry

Abstract

New lignin-based graft copolymers have been developed by atom transfer radical polymerization (ATRP) and click chemistry. These hybrid materials have a lignin center and poly(n-butyl acrylate) or polystyrene grafts. Poly(n-butyl acrylate) represents a typical elastomeric polymer, and polystyrene is used as a representative glassy polymer, and grafting these onto lignin was pursued as a method to integrate a renewable resource into polymers commonly used in a broad range of applications. Two different graft copolymerization methods were used: "graft from" and "graft onto". The "graft from" method utilized ATRP to polymerize vinyl monomers from a lignin-based macroinitiator. Kinetic data showed that the ATRP graft copolymerization occurred in controlled manner for both polystyrene and poly(n-butyl acrylate) with a conversion up to 25%. In "graft onto" method, the backbone lignin was linked to polystyrene graft via click chemistry. In the "graft onto" method, a polystyrene homopolymer was prepared by ATRP followed by end-group functionalization to obtain azide functionality at polymer chain terminals. As a counterpart of azide group, lignin hydroxyl groups were functionalized to alkyne groups for the click reaction, copper-catalyzed Huisgen cycloaddition. The click chemistry was conducted with high efficiency to form lignin-graft-polystyrene under mild conditions. Grafting efficiency was monitored by ^1H NMR and GPC. The ^1H NMR data demonstrated a complete functionalization of polystyrene terminal groups and following successful click reaction without remaining lignin or polystyrene fragments. In these experiments, GPC characterization indicated that click coupling completed within 5 hours. Unlike fully crosslinked lignin-based network polymers such as lignin-based polyurethane, these lignin-based graft copolymers may show high flexibility in processing as a thermoplastic polymer.

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