Controlled radical polymerization by use of half-sandwich chromium complexes

Abstract

This last decade, controlled/living radical polymerization (CRP) processes have seen a considerable surge of interest due, in part, to their relevance to access to a variety of well-defined polymer structures (eg predetermined molecular mass, narrow molecular weight distribution). The available methods, to date, may be separated in two general categories, those involving continuous and reversible trapping of the growing radical chain to yield an energetically more stable “dormant” species [eg stable free radical polymerization (SFRP),[1] nitroxide-mediated polymerization (NMP),[2] atom transfer radical polymerization (ATRP),[3] organometallic radical polymerization (OMRP)[4]] and those involving a rapid degenerate exchange between the growing chain and a stabilized dormant chain [eg alkyl iodides,[5] organotellurides,[6] organostibines,[7] dithiocarboxylates (RAFT),[8] dithiocarbamates,[9] xanthates (MADIX)[10] and transition metal based systems [11]]. Controlled radical polymerization of monomers such as styrene, acrylates, methylacryaltes, etc. have been readily available through the techniques described above; however, the possibility to control the radical polymerization of “nucleophilic” monomers (eg. vinyl chloride, vinylidene dichloride, vinyl acetate, etc.) remains a challenge. In search for transition metal complexes capable of controlling the polymerization of “nucleophilic” monomers, we have argued [4] that OMRP is likely to provide easier solutions relative to ATRP, because the energetics of the activation/deactivation process depends only on an easily tunable (by appropriate choice of the ligands) metal-carbon bond strength, whereas the energetics of the corresponding ATRP equilibrium is the balance of two bond strengths (radical-halogen and halogen-metal). Herein, we report on the level of control for the radical polymerization of vinyl acetate (VAc) using a half-sandwich β-ketiminato system of CrII (Figure 1).