Mechanistic Studies on Initiation and Propagation of Rare Earth Metal-Mediated Group Transfer Polymerization of Vinylphosphonates

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• 作者：Stephan Salzinger ; Benedikt S. Soller ; Andriy Plikhta ; Uwe B. Seemann ; Eberhardt Herdtweck ; Bernhard Rieger
• 刊名：Journal of the American Chemical Society
• 出版年：2013
• 出版时间：September 4, 2013
• 年：2013
• 卷：135
• 期：35
• 页码：13030-13040
• 全文大小：628K
• 年卷期：v.135,no.35(September 4, 2013)
• ISSN：1520-5126

文摘

Initiation of rare earth metal-mediated vinylphosphonate polymerization with unbridged rare earth metallocenes (Cp₂LnX) follows a complex reaction pathway. Depending on the nature of X, initiation can proceed either via abstraction of the acidic 伪-CH of the vinylphosphonate (e.g., for X = Me, CH₂TMS), via nucleophilic transfer of X to a coordinated monomer (e.g., for X = Cp, SR) or via a monomer (i.e., donor)-induced ligand-exchange reaction forming Cp₃Ln in equilibrium (e.g., for X = Cl, OR), which serves as the active initiating species. As determined by mass spectrometric end group analysis, different initiations may also occur simultaneously (e.g., for X = N(SiMe₂H)₂). A general differential approach for the kinetic analysis of living polymerizations with fast propagation and comparatively slow initiation is presented. Time-resolved analysis of monomer conversion and molecular weights of the formed polymers allow the determination of the initiator efficiency throughout the whole reaction. Using this normalization method, rare earth metal-mediated vinylphosphonate GTP is shown to follow a Yasuda-type monometallic propagation mechanism, with an S_N2-type associative displacement of the polymer phosphonate ester by a monomer as the rate-determining step. The propagation rate of vinylphosphonate GTP is mainly determined by the activation entropy, i.e. the change of rotational and vibrational restrictions within the eight-membered metallacycle in the rate-determining step as a function of the steric demand of the metallacycle side chains and the steric crowding at the metal center.