Photo-Induced Macromolecular Functionalization of Cellulose via Nitroxide Spin Trapping

Abstract

Cellulose is the most abundant organic raw material on the planet. Due to its renewability and biodegradability it is currently attracting much interest for the production of biofuels or platform chemicals. In addition, recent applications in the field of materials science have appeared, arising from the low density and the excellent thermal and mechanical properties of cellulose, particularly in the production of composites. However, there are still some major drawbacks to using cellulose including its water-absorbing nature and its poor compatibility with other materials, for example, synthetic polymers. To combat these problems, grafting synthetic polymers onto cellulose is the most straightforward method to alter its surface properties and thus to control the wettability, adhesion, or hydrophobicity of the biopolymer. Although grafting from methods such as surface-initiated nitroxide-mediated polymerization (NMP), atom transfer radical polymerization (ATRP),reversible addition–fragmentation transfer polymerization (RAFT), or ring-opening polymerization (ROP)are considered as the most efficient approaches, particularly in terms of grafting density, efficient grafting to methods have recently produced very good results. Cellulose has been successfully modified with preformed polymers by both hetero-Diels–Alderand 1,3-dipolar nitrile imine-ene cycloadditions. In the latter case, light was used as the grafting trigger. Importantly, employing light offers temporal and spatial control of the reaction. In the present contribution, we introduce a very facile grafting to protocol based on the generation of radicals at the surface of cellulose by mild UV irradiation (λmax ∼ 311 nm) of an immobilized photoinitiator, followed by radical trapping with a nitroxide-functionalized polymer (see Scheme 1). Previously, nitroxide radical coupling was employed to efficiently link polymer strands, however via a copper-catalyzed mechanism using ATRP-made polymers to generate reactive radicals. A rather similar philosophy based on spin capturing was reported, employing nitrones that after a first radical reaction generate a nitroxide able to undergo a second radical coupling.