The active oxidation state of Ti in the cyclopentadienyltributoxyltitanium methylaluminoxane (CpTi(OBu>3/MAO) catalyst has been determined by electron paramagnetic resonance (EPR) and redox titration. The results, together with previously reported active species concentrations measured by radiolabeling techniques, showed that all the catalytic species are trivalent Ti. The syndiospecific species (78% Ti) is characterized by a sharp EPR resonance at g = 1.989 (Ho = 7 G) which corresponds to a titanium(III) hydride. The nonspecific species (13% Ti) has a g value at 1.995.
SYNOPSISSyndioselective propylene polymerization has been promoted by rac-2,2-dimethylpropylidene ( 1-q5-cyclopentadienyl) ( 1-q5-fluoreny1)dichlorozirconium ( 1 ) . The active catalytic species were generated using either triphenylcarbenium tetrakis (pentafluorophenyl) borate ( 2 ) (Zr' method) or methylaluminoxane ( M A 0 method). The former exhibited much higher activity than the latter, especially at low polymerization temperatures (T,). Syndiotactic poly(propy1ene) (s-PP) obtained a t T, = -20°C has T, approaching 16OoC, [ r r r r ] pentad fraction of 0.92 to 0.95, and 45% crystallinity (X,). It crystallized in two antichiral unit cells B and C. The C structure is favored by low temperature of polymerization, slow crystallization from melt, and annealing. The s-PP has M w / M n ranging from 3.6 to 4.4, which can be separated into stereoregular fractions soluble in heptane and hexane and stereoirregular fractions soluble in pentane, ether, and acetone. Therefore, this system cannot he considered to be a single-site catalyst. A parallel study was made on the isopropylidene ( l-~5-cyclopentadieny1) ( l-q5-fluorenyl) dichlorozirconium (3 ) / M A 0 catalyst. Molecular mechanics calculations were performed for all combinations of the configuration of asymmetric centers. The steric energy favors syndiotactic enchainment for both catalysts 1 and 3, with 1 forming the more syndioselective catalyst.
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