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)