Cp2Zr(13CH3)2 (1) has been used as a probe for the reactivity of metallocene−methylaluminoxane catalysts for olefin polymerization. A 1H and 13C NMR study of the reaction equilibria between Cp2Zr(13CH3)2 and Lewis acids such as AlMe3 (2), B(C6F5)3 (3), and methylaluminoxane (MAO) (4) has been performed. AlMe3 is always present in MAO solutions, and B(C6F5)3 is a relatively strong Lewis acid, which has a capacity to form and stabilize ion pairs comparable to that of MAO. The use of isotopically 13C-enriched dimethylzirconocene has permitted the study of these systems by 13C NMR in conditions as close as possible to usual polymerization conditions, which require large excesses of MAO for reaching high activities. The comparisons of the reactivity of Cp2Zr(13CH3)2 with B(C6F5)3 and with MAO have provided the first direct evidence of the formation in solution of monomeric [Cp2Zr(13CH3)]+[Me·MAO]- (8), of dimeric [Cp2Zr(13CH3)]2(μ-13CH3)+[Me·MAO]- (7), and of the [(Cp2Zr(μ-Me)2AlMe2]+[Me·MAO]- (9) cationic species, having MeMAO- counterions. The influence of temperature, Al/Zr mole ratio, and zirconium concentration on the equilibria of ion pair formation has been elucidated.

The new cobalt (II) phosphine complex CoCl2(P i PrPh2)2 was synthesized by reacting CoCl2 with isopropyldiphenylphosphine in ethyl alcohol as solvent. The molecular structure of the complex was determined by the X-ray diffraction method. CoCl2(P i PrPh2)2 was then used in combination with methylaluminoxane for the polymerization of 1,3-butadiene:  it was found to be highly active and stereospecific for the preparation of 1,2 syndiotactic polybutadiene. The same system was also able to polymerize substituted butadienes giving highly stereoregular 1,2 polymers from E-1,3-pentadiene, 1,3-hexadiene, and 3-methyl-1,3-pentadiene. Some of these polymers are completely new and were never prepared before.

Three pairs of alkoxysilanes, (a) Me(EtO)sSi, Ph(EtO),Si, (b) PhAMeO)zSi, Phz(EtO)zSi, (c) Phz(EtO)& Ph(EtO)sSi, differing only in one parameter, have been used as external donors with the catalysts MgCldTiCb and MgCIdDIBP/TlCb (DIBP = diisobutyl phthalate), using selectively %enriched AlEt, as cocatalyst. The heptane-insoluble fractions of all the samples have been characterized by 13C NMR analysisand gel permeation chromatography. The catalyst chemical modifications due to the external donors have been studied by GC analysis of the base content of the solid catalyst under the polymerization conditions. On the basis of all these data we can conclude that (i) all the alkoxy silane donors used, even those that do not produce any evident increase of isotactic productivity, interact with the isospecific centers making them more stereospecific and more active and (ii) it is possible to modulate the activation of the isospecific centers and the poisoning of the nonstereospecific ones by varying the number and the relative bulkiness of both hydrocarbon and alkoxy substituents. The mechanism of the activation effect is discussed.

13 C NMR spectroscopic studies of the in situ polymerization of 13 C-enriched ethylene ( 13 C2H4) in the presence of Cp2Zr( 13 CH3)2 and methylaluminoxane or B(C6F5)3 as cocatalysts were carried out. The first direct observation of Cp2Zr-polymeryl species was made. The in situ polymerization experiments in the presence of different concentrations of dinuclear [(Cp2ZrMe)2(µ-Me)] + and mononuclear cation species [Cp2ZrMe] + , having [MeB(C6F5)3] -or [MeMAO] -counterions, were performed. These comparisons made it possible to make the assignments of the zirconocene complexes bearing the polymeryl chain as ligands [Cp2Zr 13 CH2P] + [ 13 CH3MAO] -, [Cp2Zr 13 CH2P] + [ 13 CH3B(C6F5)3] -, and Cp2Zr( 13 CH2P)( 13 CH3). They are in equilibrium with each other. Mononuclear Zr-polymeryl ion pairs such as [Cp2Zr 13 CH2P] + [ 13 CH3B(C6F5)3] -and [Cp2Zr 13 CH2P] + [ 13 CH3MAO] -either are the propagating active species or are intermediates, closely related to the active species, in the polymer propagation. IntroductionThe past decades have seen enormous advances in the design and synthesis of "well-defined" group 4 metallocenes for R-olefin polymerization. Metallocenes need to be activated by Lewis acid cocatalysts. 1 Methylaluminoxane (MAO) has been the first and is the most frequently used cocatalyst for these new "single-site" catalysts. 2 These catalytic systems, which are going to be employed in industrial processes, require the use of large excesses of MAO. Due to multiple equilibria present in MAO solutions, metallocene-MAO systems have been considered far too complex to allow identification of the species produced. One of the roles of MAO was presumed to be the formation of 14-electron metal complexes such as [Cp 2 MR] + . This assumption, based on early studies 3 and on the synthesis of stabilized group 4 metallocene ionic complexes, 4 was supported by the direct synthesis of [Cp 2 MMe] + [X] -, in which X -is a "noncoordinating" counterion such as [B(C 6 F 5 ) 4 ] -, 5 and by the observation of the interactions between Cp 2 Zr-( 13 CH 3 ) 2 and solid MAO. 6 Our interest in directly understanding the role of MAO in the polymerization activity in these homogeneous systems prompted us to make efforts to study titanocene-MAO catalysts by 13 C NMR spectroscopy. 7 More recently, we have undertaken the study of zirconocenes-MAO systems which because of their greater stability are more commonly used in this catalysis. Isotopically 13 C-enriched Cp 2 Zr( 13 CH 3 ) 2 (1) has been used as a probe for the reactivity of metallocenes with Lewis acids such as AlMe 3 , B(C 6 F 5 ) 3 , and MAO. 8 Direct evidence of the formation in solution of the mononuclear [Cp 2 ZrMe] + [MeMAO] -(2), the dinuclear [(Cp 2 ZrMe) 2 (µ-Me)] + [MeMAO] -(3), and the [Cp 2 Zr(µ-Me) 2 AlMe 2 ] + [MeMAO] -(4) cationic species, having [MeMAO] -counterions, has been provided. 3 and 4 have been proposed 1d as possible dormant states for the active sites for olefin polymerization and, thus, as possibly responsible for catalyst deactivations according to Scheme...

Two series of solution polymerizations of propene were conducted:  (i) using three different methylaluminoxane (MAO)-activated zirconocenes, rac-Et(Ind)2ZrCl2 (EI), rac-Me2Si(Ind)2ZrCl2 (MI), and rac-Me2Si(Benz-[e]-Ind)2ZrCl2 (MBI), at the same temperature (30 °C) and varying propene pressure from 0.15 to 1.1 bar and (ii) using rac-Me2Si(Benz-[e]-Ind)2ZrCl2 (MBI) at the propene partial pressure 1.1 bar and at a range of temperatures from 30 to 100 °C (Ind = Indenyl). 1H and 13C NMR investigation of the unsaturated chain end groups in these samples was performed, with particular attention being paid to (i) a detailed re-examination of the NMR assignments reported in the literature, (ii) the distinction between unsaturated species formed during the polymerization reactions and those formed afterward due to thermal treatment (i.e. during the NMR experiments), (iii) determination of the dependence of the quantities of different types of terminals on the polymerization conditions (temperature and monomer partial pressure). It was shown that four unsaturated chain end groups are formed during polymerization, that is, vinylidene, 2-butenyl, allyl, and the previously unidentified 4-butenyl terminals. In the oligomeric fraction of samples, no further types of unsaturated terminals were observed. Two further types of unsaturated groups observed, isobutenyl and the unidentified species X, were not formed during polymerization but, at least prevalently, afterward during the high-temperature NMR experiments. A previously unidentified regiomisinsertion, lateral n-butyl, is also described.