title: Mechanistic studies on initiation and propagation of rare earth metal-mediated group transfer polymerization of vinylphosphonates.
abstract: 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 α-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 SN2-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.
authors.affiliation: WACKER-Lehrstuhl für Makromolekulare Chemie, Technische Universität München, Lichtenbergstraße 4, 85748 Garching bei München, Germany.
authors.forename: Stephan
authors.initials: S
authors.lastname: Salzinger
authors.forename: Benedikt S
authors.initials: BS
authors.lastname: Soller
authors.forename: Andriy
authors.initials: A
authors.lastname: Plikhta
authors.forename: Uwe B
authors.initials: UB
authors.lastname: Seemann
authors.forename: Eberhardt
authors.initials: E
authors.lastname: Herdtweck
authors.forename: Bernhard
authors.initials: B
authors.lastname: Rieger
mesh_terms: D008674:Metals, Rare Earth / Q000737:chemistry*; D015394:Molecular Structure; D063065:Organophosphonates / Q000138:chemical synthesis* / Q000737:chemistry; D058105:Polymerization; D014753:Vinyl Compounds / Q000138:chemical synthesis* / Q000737:chemistry
chemical_list: D008674:Metals, Rare Earth; D063065:Organophosphonates; D014753:Vinyl Compounds; C433467:vinylphosphonic acid