Radical Reduction of Polymer Chain-End Functionality by Stoichiometric N-Heterocyclic Carbene Boranes
Ning Li, Shicheng Yang, Zhujun Huang, Xiangcheng Pan
Abstract
Controlled radical polymerization (CRP) has emerged as a spotlight of polymer chemistry in both academic and industrial fields. CRP relies on a reversible dissociation and combination of the carbon–heteroatom bond to construct an activation–deactivation equilibrium, carrying out a linear increase of Mn with the conversion. Heteroatoms serve as polymer chain ends after the CRP process, and they reinitiate polymerization to form block polymers and other topological architectures. However, heteroatom chain ends also undermine the stability of polymers under some extreme external conditions, such as heating and ultraviolet (UV) irradiation. Herein, we develop a radical strategy for removing heteroatom chain ends by stoichiometric N-heterocyclic carbene borane (NHC-BH3). NHC-BH3 abstracts a heteroatom and concurrently supplies a hydrogen atom without additional catalysts and hydrogen sources. This strategy is applied for polymers synthesized from various CRP approaches (including atom transfer radical polymerization (ATRP), reversible addition–fragmentation transfer (RAFT), nitroxide-mediated polymerization (NMP), and iodine transfer polymerization (ITP)) with a scalable ability (>15 g) in both organic and aqueous conditions. After chain-end removal, the polymers show improved thermal and UV stability.