Investigation of Dynamic Intra- and Intermolecular Processes within a Tether-Length Dependent Series of Group 4 Bimetallic Initiators for Stereomodulated Degenerative Transfer Living Ziegler-Natta Propene Polymerization

• Published in: Advanced synthesis & catalysis 2008-02, Vol.350 (3), p.439-447
• Main Authors: Zhang, Wei, Sita, Lawrence R.
• Format: Article
• Language: English
• Subjects: bimetallic catalyst; living polymerization; polypropene; stereoselectivity; Ziegler-Natta catalysis
• ISSN: 1615-4150; 1615-4169
• DOI: 10.1002/adsc.200700506

The series of bimetallic complexes, [(η5‐C5Me5)Zr(Me)2]2 [N(t‐Bu)C(Me)N(CH2)nNC(Me)N(t‐Bu)] 3 (n=8), 4 (n=6), and 5 (n=4) were prepared in high yield through a simple, one‐pot synthesis involving 2 equiv. of in situ generated (η5‐C5Me5)Zr(Me)3 and the corresponding bis‐carbodiimide, (t‐Bu)NCN(CH2)nNCN(t‐Bu). Compounds 3–5 were found to be highly isoselective for the living Ziegler–Natta polymerization of propene upon 100% activation using 2 equiv. of the borate co‐initiator, [PhNHMe2] [B(C6F5)4] (2), with the degree of stereoselectivity decreasing slightly as the two metal centers are brought closer together [cf., 3 (σ=0.92)>4 (σ=0.91)>5 (σ=0.89)]. Under conditions of sub‐stoichiometric activation by 2, all three bimetallic initiators, 3–5, were found to engage in degenerative transfer living Ziegler–Natta polymerization involving rapid and reversible methyl group transfer between active, (cationic) and dormant, (neutral) methyl, polymeryl zirconium centers. Under these conditions, the frequency of mr triad stereoerror incorporation into the polypropene (PP) microstructure decreases as the two metal centers are brought closer together as a result of increasing barriers for metal‐centered epimerization within the neutral metal site due to correspondingly greater non‐bonded steric interactions vis‐à‐vis mononuclear 1.