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

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.