Ziegler-Natta polymerization schemes

The insertion of ethylene into a carbon-cobalt bond was carefully studied as part of work on the mechanism of Ziegler-Natta polymerization (Scheme 1.10). ... 

Three mechanisms have been proposed to explain metallocene-based homogeneous and Ziegler-Natta polymerization schemes. The Cossee-Arlman mechanism... 

The first example of homogeneous transition metal catalysis in an ionic liquid was the platinum-catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate (mp. 78 °C), described by Parshall in 1972 (Scheme 5.2-1, a)) [1]. In 1987, Knifton reported the ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [Bu4P]Br, a salt that falls under the now accepted definition for an ionic liquid (see Scheme 5.2-1, b)) [2]. The first applications of room-temperature ionic liquids in homogeneous transition metal catalysis were described in 1990 by Chauvin et al. and by Wilkes et ak. Wilkes et al. used weekly acidic chloroaluminate melts and studied ethylene polymerization in them with Ziegler-Natta catalysts (Scheme 5.2-1, c)) [3]. Chauvin s group dissolved nickel catalysts in weakly acidic chloroaluminate melts and investigated the resulting ionic catalyst solutions for the dimerization of propene (Scheme 5.2-1, d)) [4]. 

Protonation of the TMM complexes with [PhNMe2H][B(C6Fs)4] in chlorobenzene at —10 °C provided cationic methallyl complexes which are thermally robust in solution at elevated temperatures as determined by NMR spectroscopy. In contrast, addition of BfCgFsls to the neutral TMM precursors provided zwitterionic allyl complexes (Scheme 98). Surprisingly, it was found that neither the cationic nor the zwitterionic complexes are active initiators for the Ziegler-Natta polymerization of ethylene and a-olefins. °°... 

The Ziegler-Natta polymerization of ethylene and propylene is among the most significant industrial processes. Current processes use heterogeneous catalysts formed from Ti(IH)Cl3 or MgCl2-supported Ti(IV)Cl4 and some otganoaluminum compounds. The widely accepted Cossee mechanism of ethylene polymerization is illustrated in Scheme 62. 

Of greater relevance catalytically is that the combined use of l3C enrichment and 13C nutation NMR spectroscopy can distinguish between proposed rival mechanisms for the Ziegler-Natta catalyzed polymerization of acetylene. In the four-center insertion mechanism the enriched acetylene (HC =C H) is incorporated as shown in Scheme 6. It is to be noted that the, 3C—13C bond label is here incorporated into a carbon-carbon double bond, the length of which is significantly smaller than that of a carbon-carbon single bond, which is how the enriched acetylene would be incorporated in the two-center mechanism shown in Scheme 7. The results of nutation experiments leave little doubt that the Ziegler-Natta polymerization of acetylene proceeds by a four-center mechanism. 

In this instance, Ziegler-Natta polymerization yields a soluble, linear polymer 2, containing a six-membered cyclic ring fused at each repeat unit. Unfortunately, this polymer undergoes isomerization to form a non-conjugated polymer, disrupting the electronic properties of the backbone [31]. It was found that this isomerization could be prevented by the introduction of heteroatom functionality into the diyne architecture, as exemplified by the polymerization of propiolic anhydride 3, which yielded a stable polymer 4 as shown in Scheme 11 [32]. 

This model, of the Rideal type, is of general applicability and includes and explains the most relevant phenomena involved in Ziegler-Natta polymerization. Mathematical formulation of the proposed reaction scheme led to the following equations for polymerization rate Rp and number average molecular weight of polymer Mn, in the absence of hydrogen ... 

Fig. 27. Reaction scheme of a Ziegler—Natta polymerization process Cat, catalytic active centre ethylene molecule w, polymer chain w, polymer chain with a vinyl group, al, 1/3 Al R, alkyl group (for example, C2H5) H, H2, hydrogen atom or hydrogen molecule 8S). By permission of Butter-worth Scientific Ltd.

Ziegler-Natta polymerization is characterized by a series of elementary reactions which can be represented by suitable models. A scheme of such reactions, as proposed by Grieveson and including Natta s original hypotheses as well, shows (Table 2) besides the initiation and propagation steps, the various possible types of chain transfer and termination processes, both in the presence and in the absence of... 

Control of Distribution in Polyolefins with Catalytic Systems Table 2. Kinetic scheme for Ziegler-Natta polymerization (ethylene)... 

An analogous scheme can be advanced to rationalize the stereospecificity with which alkyl migration proceeds in metal alkyl-transition metal-catalyzed alkene oligomerization and polymerization reactions such as Ziegler-Natta polymerization and alkene metathesis. " ... 

Olefin metathesis was first discovered during research stemming from Ziegler-Natta polymerization catalysis in the late 1950s [13-15]. The term olefin metathesis was not coined until 1967 [16]. Olefin metathesis is the apparent exchange of the carbons of olefins to produce new olefins. Empirically, this process can swap the substituents of olefins to give aU possible products (Scheme 6.1). 

Scheme 4.2 Cossee-Arlman mechanism for (a) primary insertion and (b) secondary insertion in Ziegler-Natta polymerization of propene P = polymer chain.

According to the Cossee mechanism, the two key steps in Ziegler—Natta polymerizations are monomer coordination and migratory insertion into the metal-polymer chain bond (Scheme 10). In analogy to heterogeneous Ziegler—Natta catalysis, a first-order reaction rate with respect to monomer concentration is generally assumed also for metallocene-based catalysts, that is. 

Ziegler-Natta polymerization is well known to involve a two-stage process [148, 149]. In the first stage, an aluminum alkyl (such as trialkyl aluminum) is reacted with TiCU in order to produce active jS-TiCls. The alkyl radicals, which are also produced in this reaction, are terminated by coupling and create inert products. Subsequent alkylation of -TiCb then occurs to generate the titanium species that is capable of initiating the polymerization of olefins such as ethylene (Scheme 11.37). 

Scheme 11.37 General mechanism of Ziegler-Natta polymerization of ethylene.

Scheme 11.38 Synthesis of block copolymers by transformation of living anionic polymerization into Ziegler-Natta polymerization.

Figure 1 Reaction scheme of a Ziegler-Natta polymerization process Cat, catalytic active centre II ethylene molecule w, polymer chain ...

The mechanisms of Ziegler-Natta polymerization are not fully understood, but the heterogeneous nature of the catalysts appears to play an important role in controlling the stereochemistry of the polymer. The mechanism suggested in Scheme 5 therefore possibly reflects the process taking place on the (modified) surface of a titanium chloride crystal. 

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