Limits of Vinylidene Fluoride RAFT Polymerization
Résumé
The investigations reported in this article probe the behavior of the RAFT polymerization of vinylidene fluoride (VDF) when degrees of polymerization higher than 50 are targeted: they demonstrate that higher molar mass PVDF (11 000 g mol–1) can indeed be prepared by RAFT polymerization, but only at rather low monomer conversions (<33%). This study more carefully examines the behavior of the reputedly nonreactive −CF2CH−XA chain ends (where XA designates the xanthate group) formed by inverse VDF addition and known to accumulate in the reaction medium during the polymerization. A combination of 1H and 19F NMR spectroscopic monitoring and comprehensive DFT calculations of the various exchange and propagation reactions at work explains the unexpected behavior of this polymerization. The present study disproves entirely the generally accepted belief that −CF2CH2–XA-terminated PVDF chains are “dead” and shows how these chains are reactivated, albeit slowly, throughout the polymerization. This activation occurs prevalently and counterintuitively through degenerative exchange by the minority PVDF–CF2CH2• radicals. The resulting kinetic scheme rationalizes the experimentally observed absence, after conversion of all the dormant chains into the less reactive −CF2CH2–XA end-group, of the longer polymer chains expected from a free radical polymerization mechanism.
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