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

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• 作者：Yin-Bao Jia ; Yan-Bo Wang ; Wei-Min Ren ; Tieqi Xu ; Jing Wang ; Xiao-Bing Lu
• 刊名：Macromolecules
• 出版年：2014
• 出版时间：March 25, 2014
• 年：2014
• 卷：47
• 期：6
• 页码：1966-1972
• 全文大小：354K
• 年卷期：v.47,no.6(March 25, 2014)
• ISSN：1520-5835

文摘

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 Al_Al(C6F5)3鈥揅_NHO bond is in the range of 2.002鈥?.018 脜, dependent on the substitute groups of N-heterocyclic ring. Highly molecular weight (M_n = 7.50 脳 10⁵ g/mol) and narrow molecular weight distribution (M_w/M_n = 1.04鈥?.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鈥揟OF 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.