Design and Synthesis of New Group (IV) Cyclopentadienyl Amidinate and Guanidinate Initiators for Controlling the Microstructure of Poly(α-olefins) During Living Coordinative Chain Transfer Polymerizations

Abstract

C1-symmetric, cationic group 4 metal (Zr and Hf) mono-methyl complexes, {(η5-C5Me5)M[N(tBu)C(Me)N(Et)](Me)}[B(C6F5)4], are highly active initiators for the living and stereo-selective (isotactic) coordinative polymerization of propene and longer-chain α-olefins. Utilizing technology previously discovered but not yet fully utilized, it is possible to demonstrate the remarkable ability to stereo-engineer poly(α-olefins) with the use of a single initiator. A two-state living coordination polymerization process can be engaged by controlling the relative populations of the active and dormant species as a function of time to incorporate stereo-errors in a programmed fashion. Secondly, in the presence of excess equivalents of a main group metal alkyl such as diethylzinc (DEZ), rapid and reversible chain transfer between the active propagating species and the `surrogate' main group metal alkyl, which occurs at a rate that is significantly greater than propagation, serves as a work-around solution to the `one-chain-per-metal-site' limitation of a living polymerization. Successful adaptation of this reversible group transfer technology can include the rapid and reversible transfer of a polymeric group between `tight' and `loose' propagating ion pairs that mediated by excess DEZ to precisely control co-monomer incorporation.

While this process of living coordinative chain transfer polymerization (LCCTP) can provide practical quantities of precision polyolefins, the exchange process results in loss of stereo-regularity in the final polymer microstructure. Strategies for achieving a high degree of stereo-regularity during LCCTP include the synthesis of new classes of configurationally stable and optically pure cyclopentadienyl, amidinate and guanidinate initiators that incorporate a distal, chiral substituent. A second strategy to create enantiomerically pure propagating species involves the adaptation of hydrozirconation to create a new class of terpene substituted cyclopentadienyl-amidinate complexes via insertion of an olefin into a Zr-H bond. The last attempt to impart stereocontrol under LCCTP conditions involves the addition of an enantiomerically substituent to the N-amidinate to ensure the same enantiofacial insertion.