Examination of Dynamic Processes in Living Ziegler-Natta Polymerization and New Polypropylene Architectures through Bimolecular Control

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The cationic cyclopentadienyl zirconium acetamidinate (CPZA) complex {CpZrMe[N(Et)C(Me)N(tBu)]}[B(Csub6 Fsub5)sub4], (Cp = ηsup5-Csub5Mesub5) generated through activation of CpZrMesub2[N(Et)C(Me)N(tBu)] via protonolysis of a methyl group with one equivalent of [PhNMesub2H][B(Csub6Fsub5)sub4], has been shown to be a highly active initiator for living alpha-olefin polymerization. Discrete cationic CPZA alkyl complexes of the general structure {CpZrR[N(Et)C(Me)N(tBu)]}[B(Csub6Fsub5)sub4] were studied as model complexes for living polymers derived from this system. Detailed analysis of models in which R = nPr, iPr, iBu, and 2-Et-Bu, as well as living isotactic poly(1-butene) and living isotactic poly(1-sup13C-decene) revealed significant differences with respect to isomerization and decomposition. Studies carried out with various isotopically labeled iso-butyl derivatives revealed isomerization concurrent with decomposition, while the only case which did not provide evidence for isomerization was an isotopically labeled n-propyl derivative. Products of decomposition in cases of R = iBu and 2-Et-Bu included not only the expected alkenes from isomerization/beta-hydride elimination, but significant quantities of alkane. This is proposed to arise from competitive intramolecular abstraction of a hydrogen from the Cp* ligand. During decomposition, all species mentioned above disappeared in a first order manner, and all products of decomposition/isomerization appeared in a zero order manner. This is consistent with slow beta-hydride elimination followed by rapid isomerization, decomposition or reinsertion. Possible mechanisms of isomerization are discussed.

The {Cp*ZrMe[N(Et)C(Me)N(tBu)]}[B(Csub6Fsub5)sub4] initiator was found to be an active initiator for the living isotactic polymerization of propylene ([mmmm] = 0.71). When substoichiometrically activated with only 0.5 equivalents of [PhNMesub2H][B(Csub6Fsub5)sub4], atactic polypropylene was produced due to the rapid degenerative transfer of methyl groups between active and dormant centers, which results in the racemization of active sites by virtue of the configurational instability of the dormant state. Successive additions of [PhNMesub2H][B(Csub6Fsub5)sub4] and a methylating agent were used to modulate the conditions of the polymerization system between degenerative transfer (at 50% activation) and fully activated conditions to produce the first discrete, homogeneous isotactic-atactic stereoblock polypropylene (sb-PP) materials. Preliminary tensile testing of three unique sb-PP materials displayed elastomeric properties that were heavily dependant on the stereoblock architecture. The synthesis of a wide range of polypropylene stereochemical microstructures between isotactic and atactic through bimolecular control by substoichiometric activation was also demonstrated, leading to the development of a fundamentally new architecture: stereogradient polypropylene.