Kinetic resolution of racemic α-olefins with ansa-zirconocene polymerization catalysts: Enantiomorphic site vs. chain end control

doi:10.1073/pnas.0603071103

Kinetic resolution of racemic α-olefins with ansa-zirconocene polymerization catalysts: Enantiomorphic site vs. chain end control

Jeffery A. Byers and
John E. Bercaw^†

Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, 1200 East California Avenue, Mail Code 127-72, Pasadena, CA 91125

Edited by Tobin J. Marks, Northwestern University, Evanston, IL, and approved July 21, 2006 (received for review April 18, 2006)

Abstract

Copolymerization of racemic α-olefins with ethylene and propylene was carried out in the presence of enantiopure C₁-symmetric ansa metallocene, {1,2-(SiMe₂)₂(η⁵-C₅H-3,5-(CHMe₂)₂)(η⁵-C₅H₃)}ZrCl₂ to probe the effect of the polymer chain end on enantioselection for the R- or S-α-olefin during the kinetic resolution by polymerization catalysis. Copolymerizations with ethylene revealed that the polymer chain end is an important factor in the enantioselection of the reaction and that for homopolymerization, chain end control generally works cooperatively with enantiomorphic site control. Results from propylene copolymerizations suggested that chain end control arising from a methyl group at the β carbon along the main chain can drastically affect selectivity, but its importance as a stereo-directing element depends on the identity of the olefin.

Footnotes

^†To whom correspondence should be addressed. E-mail: bercaw{at}caltech.edu
Author contributions: J.A.B. and J.E.B. designed research; J.A.B. performed research; J.A.B. and J.E.B. analyzed data; and J.A.B. and J.E.B. wrote the paper.
Conflict of interest statement: No conflicts declared.
This article is a PNAS direct submission.
↵ ‡ We ignore the main and side chain chirality that is farther from the catalyst site. Although these stereochemical elements are likely less important, they probably exert some influence on selectivity as well.
↵ § Molecular weight distributions for these polymers were broad (6 > PDI > 3). Although multiple polymerization sites cannot be definitively ruled out, broad molecular weight distributions are believed to be due to mass transport issues. This conclusion was reached considering the narrow molecular weight distributions found for polypropylene copolymers (see supporting information) and a significant portion (85 wt%) of the ethylene copolymers containing the desired microstructure established by solvent fractionation.
↵ ¶ The homopolymerization vs. copolymerization comparisons from Table 1 under the same polymerization conditions are: for 3-methyl-1-pentene, entry 1; for 3-methyl-1-hexene, entry 3; for 3,5,5-trimethyl-1-hexene, entry 4. Entries 5 and 7 were for runs under somewhat different polymerization conditions.
↵ ‖ DSC for isotactic poly(3-methyl-1-pentene) displayed no melting point at all and decomposition at 250°C as evidenced by DSC/thermogravimetric analysis. Therefore, the lack of a high temperature melting point in these copolymers does not necessitate an absence of consecutive chiral repeat units in the copolymer.
†† ¹³C{¹H} NMR spectrum for polyethylene synthesized under the same reaction conditions showed no evidence for incorporation of long chain branches.
↵ ‡‡ A more thorough statistical analysis was carried out for the origin of the enantiofacial selectivity, but results from this analysis do not provide any more profound insight than the simple triad tests discussed.
Abbreviations:

MAO,

methylaluminoxane;

DSC,

dynamic scanning calorimetry.