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Published March 4, 2002 | Supplemental Material
Journal Article Open

Isotactic−Hemiisotactic Polypropylene from C_1-Symmetric ansa-Metallocene Catalysts: A New Strategy for the Synthesis of Elastomeric Polypropylene


Control of "isotactic−hemiisotactic" alignment, a polypropylene microstructure in which every other stereocenter is of the same configuration and the intervening stereocenters tend to align with their neighbors, has been achieved by R substituent selection in the ansa-metallocene catalyst system R'_2C(η^5-3-R-C_5H_3)(η^5-C_(13)H_8)MCl_2−MAO (M = Zr, Hf; η^5-C_(13)H_8 = fluorenyl; MAO = methylaluminoxane). For R = 2-adamantyl, R' = Ph, and M = Zr (4), α, a parameter equal to the m diad fraction, is approximately 0.58, with M_n = 204 000−463 000, and the isotactic−hemiisotactic polypropylene obtained is elastomeric. Typical elongations are between 7 and 14 times the original length, and the residual elongation following a 200% strain is between 1.5% and 9.3%. The elastomeric properties are rationalized by the statistical existence of isotactic stereoblocks among otherwise amorphous hemiisotactic connecting segments. A two-parameter (α and M_n) statistical model has been derived which calculates the isotactic block length distribution and the percent crystallinity, assuming isotactic blocks of 21 monomer units and longer participate in crystallites. The best elastomers are found to have a calculated percent crystallinity in the range of 1.50−4.45% (α = 0.557−0.630).

Additional Information

© 2002 American Chemical Society. Received 29 August 2001. Published online 2 February 2002. Published in print 1 March 2002. This work has been funded by the USDOE Office of Basic Energy Sciences (Grant No. DE-FG03-88ER13431) and by a DOD National Defense Science and Engineering Fellowship awarded to S.M. We thank Dr. Michael Day and Mr. Lawrence Henling for obtaining the X-ray crystal structure. We also thank Dr. Andreas Ernst at BP-Amoco and Dr. Terry Burkhardt at ExxonMobil for assistance in obtaining molecular weight data. Stress-strain behavior was obtained by Dr. Eric Moore at BP-Amoco and by Dr. Andrew Peacock at ExxonMobil.

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