Ziegler-Natta olefin polymerization studies

Studies of the reactions of propylene and alpha-olefins show that the Ziegler-Natta isotactic polymerizations are between the highly cationic and mildly anionic catalysts. 

On the technical scale, olefin polymerization is based on the activation of a suitable zirconocene precursor, typically Cp2ZrMe2 or Cp2ZrCl2, by a strong Lewis acid as co-catalyst. Mechanistic studies on Ziegler-Natta-like polymerization have long focused on the role of cationic alkylzirconocene species in the catalysis. There have been previous mass spectrometric studies of isolated metallocene ions in the gas phase, most notably by Richardson and co-workers. The failure to observe olefin oligomerization in this study was attributed to incomplete thermaliza-... 

The strict connection between chiral structure of the active site and polymer stereochemistry, pointed out by the investigations on the heterogeneous Ziegler-Natta catalytic polymerization, has been fully confirmed and extended by the studies concerning polymerization of prochiral a-olefins in the presence of metallocene catalysts. 

Olefin polymerization by catalysts based on transition metal halogenides is usually designated as coordinated anionic, after Natta (194). It is believed that the active metal-carbon bond in Ziegler-Natta catalysts is polarized following the type M+ - C. The polarization of the active metal-carbon bond should influence the route of its decomposition by some compounds ( polar-type inhibitors), e.g. by alcohols. When studying polymerization by Ziegler-Natta catalysts tritiated alcohols were used in many works to determine the number of metal-polymer bonds. However, as it was noted above (see Section IV), in two-component systems the polarization of the active bond cannot be judged by the results of the treatment of the system by alcohol, as the radioactivity of the polymer thus obtained results mainly from the decomposition of the aluminum-polymer bonds. 

The study of catalytic polymerization of olefins performed up to the present time is certain to hold a particular influence over the progress of the concepts of the coordination mechanism of heterogeneous catalysis. With such an approach the elementary acts of catalytic reaction are considered to proceed in the coordination sphere of one ion of the transition element and, to a first approximation, the collective features of solids are not taken into account. It is not surprising that polymerization by Ziegler-Natta catalysts is often considered together with the processes of homogeneous catalysis. 

Heterogeneous Ziegler-Natta catalysts used to polymerize olefins exhibit phenomena characteristic of active site heterogeneity (1- 5). Complex kinetic models which account for this likelihood have been developed and used only in simulation studies (6-7). 

Titanocene and zirconocene dichlorides (Cp2MtCl2 with Mt = Ti, Zr) were the first metallocenes studied [Breslow and Newburg, 1957 Natta et al., 1957a], The metallocene initiators, like the traditional Ziegler-Natta initiators, require activation by a Lewis acid coinitiator, sometimes called an activator. AIRCI2 and A1R3 were used initially, but the result was initiator systems with low activity for ethylene polymerization and no activity in a-olefin polymerization. The use of methylaluminoxane (MAO), [A1(CH3)0] , resulted in greatly improved activity for ethylene polymerization [Sinn and Kaminsky, 1980], The properties of MAO are discussed in Sec. 8-5g. MAO has two functions alkylation of a transition metal-chloride bond followed by abstraction of the second chloride to yield a metallocenium... 

The reactions between TiCl4 and AlMe3 are of particular interest in studies of olefin-polymerization catalysts as a Ziegler-Natta catalyst system.14 The reaction between a surface TiClx group and AlMe3 gas is known to give an active olefin polymerization catalyst.1... 

A patent by A. W. Anderson et al. (Du Pont) disclosed the first transition metal catalyzed polymerization of a cyclic olefin in 1955 [1]. Subsequent to the discovery of Ziegler-Natta polymerization [2] norbornene was found to polymerize in the presence of the catalyst systems TiCl /EtMgBr. Eleuterio [3 a] and Truett et al. [3 b] obtained polynorbornene by using transition metal catalysts based on Mo and Ti, respectively. IR-studies and ozonolysis of the polymer revealed the presence of carbon-carbon double bonds, indicating that polymerization had occured by unexpected ring opening Eq. 1 [3]. 

Switching from olefin metathesis to Ziegler Natta polymerization is of interest in order to prepare block copolymers and to establish the relationship between these two related modes of olefin polymerization. Model studies for this purpose included the conversion of titancyclobutanes 10 and 7a into titanium alkyl compounds Eq. (30a and b) by the addition of 1 equivalent of ethanol [43],... 

Zam belli and Tosi have extensively studied the stereochemistry of the propagation step in propylene polymerization on Ziegler-Natta catalysts. Specific features of this process are shown in Table 4. Cis-addition of the olefin to the active metal-carbon bond has been observed both in isospecific and syndiospecific polymerization. The olefin addition to the active bond proceeds with the participation of the primary (L,(Mt—CH2—CHR—P) and secondary (L,Mt—CHR—CH2—P) carbon atoms of the growing polymer chain using isospecific and syndiospecific catalysts, respectively. 

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