Mechanistic Aspects of Initiation and Deactivation in N-Heterocyclic Olefin Mediated Polymerization of Acrylates with Alane as Activator

Abstract

Highly active catalyst systems based on N-heterocyclic olefin (NHO) as initiator were developed for polymerizing various polar monomers such as methyl methacrylate (MMA), n-butyl methacrylate (BMA), N,N-dimethylacrylamide (DMAA) and N,N-diphenylacrylamide (DPAA) at ambient temperature. The preactivation of these polar monomers by Lewis acid such as Al(C6F5)3 or AlCl3 is a prerequisite for their rapid transformation, since a stable adduct easily forms by the interaction of nucleophilic NHO and Lewis acidic activator. The formation of NHO/Al(C6F5)3 adduct was confirmed by 1H NMR spectroscopy and X-ray single crystal analysis. The length of AlAl(C6F5)3–CNHO bond is in the range of 2.002–2.018 Å, dependent on the substitute groups of N-heterocyclic ring. Highly molecular weight (Mn = 7.50 × 105 g/mol) and narrow molecular weight distribution (Mw/Mn = 1.04–1.27) were achieved with dissymmetry NHO as initiator and Al(C6F5)3 as activated agent. Although binary NHO/Al(C6F5)3 catalyst system could polymerize both MMA and BMA with high activity, the attempt to synthesize their block copolymers proved to be unsuccessful. Electrospray ionization time-of-flight mass spectrometry (ESI–TOF MS) study provided important information on the polymer-chain ends. It was found that NHO as the initiation group bounded to one end of a polymer chain and an unexpected six-membered lactone ring appeared at another chain end. The formation of lactone end is ascribed to the nucleophilic backbiting of the polymeric anion to the carboxyl carbon of the adjacent unit, in companion with the release of the methoxyl group. The low initiation efficiency of NHO is attributed to the formation of the stable NHO/alane adduct during the polymerization, while the production of lactone end results in complete deactivation in polymer chain propagation.