Schematic of a polymer-grafted lignin nanoparticle based on a discrete lignin core with a polymer corona.
There have been numerous approaches to preparing hybrid materials based on synthetic polymers and lignin, but these generally have relied on free radical polymerization (FRP) chemistries or other conventional polymerization methods, which produce crosslinked products either as nanoscale aggregates or bulk materials. Living polymerizations, such as anionic polymerization or controlled radical polymerization (CRP), offer numerous advantages in preparing polymer-grafted lignin nanoparticles. These include site-specific initiation, control over molecular weight distribution, and control of chain-end functionality.
Comparison of polymer grafting architectures based on CRP (left) and FRP (right). The FRP product is crosslinked due to a lack of control over the propagation step, but the CRP product is based on discrete units having a lignin core and a polymer corona. Our hypothesis is that the CRP architecture more effectively leverages the native functions of lignin.
ATRP and other CRP methods are well suited for grafting a broad range of hydrophilic and thermoplastic polymers from lignin. The group of Prof. John Kadla was the first to report the use of atom transfer radical polymerization (ATRP) in the preparation of lignin grafted with poly(N-isopropylacrylamide), a polymer with unusual temperature-dependent solubility in water. This pointed to a broader range of polymer-grafted lignin nanoparticles that could be prepared using CRP to graft both hydrophilic polymers for the preparation of surfactants and thermoplastic polymers for the preparation of nanocomposites. In the Washburn Lab, we make extensive use of ATRP and related techniques, such as reversible addition-fragmentation chain-transfer (RAFT) polymerization, to explore a broad range of lignin-based nanomaterials.
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Chemistry of lignin-based materials. H. Chung and N. R. Washburn. Green Materials, 2013; 1: 137–160.