RAFT synthesis of well-defined PVDF-b-PVAc block copolymers

Abstract

RAFT polymerization of vinylidene fluoride (VDF), leading to relatively well defined poly(vinylidene fluoride) (PVDF), is negatively affected by chain inversion resulting in less easily reactivatable PVDF_T-XA dormant chains (terminated with the tail end of an inversely added VDF unit; XA = xanthate moiety). Although slow reactivation of these chains by PVDF˙ radicals (in contrast to general belief) was recently demonstrated, slow radical exchange leads to progressive loss of chain growth control. This article deals with the possibility of synthesizing block copolymers from PVDF-XA macroCTAs by sequential addition. The investigations show that only PVDF_H-XA (chains terminated with the head end of regularly added VDF) can be reactivated by PNVP˙ (poly(N-vinylpyrrolidone)) radicals and that PVDF_T-XA chains are completely unreactive in the presence of PNVP˙, PB˙ (poly(butylacrylate)) or PDM˙ (poly(N,N′-dimethylacrylamide)). However, both PVDF_H-XA and PVDF_T-XA can be reactivated by PVAc˙ (poly(vinyl acetate)) radicals. The reactivation of the PVDF_T-XA, albeit slower than that of the PVDF_H-XA, is sufficiently fast to allow the synthesis of unprecedented well-defined PVDF-b-PVAc block copolymers with relatively high end-group fidelity. DFT calculations rationalize this behavior on the basis of faster radical exchange in the order PVDF_H-XA/VAc > PVDF_H-XA/NVP > PVDF_T-XA/VAc ≫ PVDF_T-XA/NVP. The success of the chain extension also relies on faster activation relative to homopropagation of the chain extending monomer, as well as fast addition of the released and to the monomer.