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Zhang, Yonghui
Sita, Lawrence R.
A degenerative transfer (DT) living Ziegler-Natta polymerization system based on the cyclopentadienyl amidinate zirconium complex, (h5-C5Me5)ZrMe2[N(t-Bu)C(Me)N(Et)] (19), has been studied in detail. Compound 19 is an active pre-catalyst for the living and stereospecific polymerization of a-olefins, upon activation by a stoichiometric amount of [PhNHMe2][B(C6F5)4] (18). With a substoichiometric amount of 18, degenerative transfer living Ziegler-Natta polymerization occurs. The extremely fast interconversion between an active cationic species and a dormant neutral species via a methyl (MeDeT) or chloride (ChloDeT) group is essential for degenerative transfer to occur (Rex>>Rp). Under MeDeT conditions, atactic polyolefins are produced due to the configurational instability of the amidinate ligand in its dormant state, with the rate of epimerization being much faster than the rate of propagation (Repi >>Rp). The configurational stability of all the zirconium species involved in ChloDeT ensures that this degenerative transfer living polymerization is stereospecific. Methylating reagents (h5-C5Me5)ZrMe2[N(t-Bu)C(t-Bu)N(Et)] (34) and (h5-C5Me5)ZrMe2[NCH2(t-Bu)C(Me)N(Et)] (39) were synthesized to provide a means by which a fully activated polymerization system could be brought under degenerative transfer conditions. Adding 18 and 34 or 39 alternatively to a living polymerization system produces well-defined atactic-isotactic stereoblock polyolefins. This allows, for the first time, the production of stereoblock polyolefins with controlled block length, block tacticity and block placement within a polymer backbone. Living stereoselective propylene polymerizations were carried out by {(h5-C5Me5)ZrMe[N(t-Bu)C(Me)N(Et)]}[B(C6F5)4] (32). A mmmm of 73% and a s value of 94% were determined, which is the highest among living propylene polymerizations reported to date. Under degenerative transfer conditions, atactic PP was produced. Using the developed methodology, a family of different atactic-isotactic stereoblock polypropylene (sb-PP) was synthesized with controlled block length, block tacticity for the first time. Preliminary data shows excellent elastomeric properties for the sb-PPs that are synthesized. The amidinate zirconium initiator was chemically immobilized to polystyrene-based solid support via deprotonation and nucleophilic addition reactions at the distal position of (h5-C5Me5)ZrCl2[N(t-Bu)C(Me)N(Et)] (40). Heterogeneous initiator (58) polymerizes a-olefins in a living and isospecific fashion. Also, a long shelf time was observed for 58 at room temperature. This is the first heterogeneous living Ziegler-Natta stereospecific catalyst reported to date.