Degenerative Transfer Living Ziegler−Natta Polymerization: Application to the Synthesis of Monomodal Stereoblock Polyolefins of Narrow Polydispersity and Tunable Block Length

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Abstract
The neutral dimethyl pentamethylcyclopentadienylzirconium acetamidinate, (η5-C5Me5)ZrMe2[N(t-Bu)C(Me)N(Et)], can serve as a highly active initiator for the living Ziegler−Natta polymerization of α-olefins to produce polyolefins of narrow polydispersity (D ≤ 1.05) when “activated” through mono demethylation by a substoichiometric amount of the borate, [PhNMe2H][B(C6F5)4]. The mechanism by which this living polymerization proceeds is through a process of degenerative transfer involving rapid and reversible methyl group exchange between cationic (active) zirconium propagating centers and neutral (dormant) methyl, polymeryl zirconium end groups. Facile metal-centered epimerization of the dormant species is responsible for a loss of stereocontrol during propagation that produces iso-rich material in contrast to the pure isotactic polymer microstructure obtained when degenerative transfer is not present. By turning degenerative transfer “on” and “off” between successive monomer polymerizations, a successful strategy for the production of monomodal stereoblock polyolefins of narrow polydispersity and tunable block length has been demonstrated.

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