Harnessing Macromolecular NHC-Boryl radicals for the Construction of Boron-Hinged Reversible Comb- and Bottlebrush-Copolymers

The synthesis of polymer polystyrene- g -(meth)acrylates combs and bottlebrushes linked by N-Heterocyclic carbene (NHC)–borane Lewis pairs is reported. The radical polymerization proceeds from a polystyrene-based macro-coinitiator with strategically-positioned NHC•BH 3 functions. Initial formation of an NHC-boryl macroradical from H-atom transfer to the excited benzophenone photoinitiator is followed by polymerization until a chain transfer event to another NHC-borane on the initial backbone generates a new site for polymerization. When MMA is the second monomer then g-BH 2 -PMMA “hairs” are obtained, yielding comb-like structures. In contrast, butyl acrylate leads to double grafting at boron, delivering copolymer bottlebrushes macro-architectures selectively. DOSY, SEC and multinuclear liquid/solid-state NMR spectroscopies suggest that a high grafting density generates too much steric strain, resulting in some of the "hairs" to detach from the main chain. When the macrocoinitiator is itself a comb copolymer, secondary grafting is possible, but not at boron. The structures obtained resemble hyperbranched copolymers. The NHC-borane Lewis pairing exhibits a dynamic behavior, from complete release of the poly(meth)acrylate hairs to more balanced exchanges, either with small molecules or with another Lewis-pair containing polymer.