Chain propagation Ziegler-Natta polymerization

As in Ziegler-Natta polymerization, steric control of the propagation step may involve either the interaction of the monomer with a chiral metal centre (enantiomorphic sites model), or the interaction of the monomer with the chiral centres in the repeating unit(s) adjacent to the metal centre (chain-end model). (The relationship to Ziegler-Natta polymerization will be considered further in Section VIII.C.3.)... 

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... 

Despite passage of more than 57 years since the basic discoveries, the mechanism of Ziegler-Natta polymerization is still not fully understood. As in all chain-growth polymerizations (12), the basic steps are initiation, propagation and termination (chain transfer). 

Ziegler-Natta polymerizations have the characteristics of living polymerization with regard to catalyst active sites but not individual propagating chains. Thus the propagating chains have lifetimes of seconds or minutes at most, while active sites have lifetimes of the order of hours or days. Each active site produces many polymer molecules. The termination of a polymer chain growing at an active center may occur by various reactions, as shown below with propylene as an example. 

While most authors consider that the initiation and propagation of chain growth in the Ziegler-Natta polymerization occur by the same type of mechanism, there is a considerable amount of evidence that the rates of these two processes are very... 

Burger, B. J. Thompson, M. E. Cotter, W. D. Bercaw, J. E. Ethylene insertion and -hydrogen elimination for permethylscandocene alkyl complexes. A study of the chain propagation and termination steps in Ziegler-Natta polymerization of ethylene. J. Am. Chem. Soc. 1990,112, 1566-1577. 

Very many kinetic models have been proposed for both heterogeneous and homogeneous Ziegler-Natta polymerization (c.f. 2,8,18) but for the purposes of this paper only the recent models proposed by Tait et al (26,29f30), Yermakov et al (31) and Bohm (39) will be considered. These three kinetic treatments have the common features that chain propagation is treated as a two-stage reaction, monomer... 

More detailed studies have revealed that the stereochemical propagation can occur by several different mechanisms. Two general kinds of mechanisms can be identified one is chain-end controlling and the other is catalytic-site controlling. Free radical, anionic and most cationic polymerizations are chain end controlled, whereas many Ziegler-Natta polymerizations are catalytic-site controlled [19]. 

Despite the difference in composition of various olefin polymerization catalysts the problems of the mechanism of their action have much in common. The difference between one-component and traditional Ziegler-Natta two-component catalysts seems to exist only at the stage of genesis of the propagation centers, while the mechanism of the formation of a polymer chain on the propagation center formed has many common basic features for all the catalytic systems based on transition metal compounds. 

On the basis of the nature of the initiation step, polymerization reactions of unsaturated hydrocarbons can be classified as cationic, anionic, and free-radical polymerization. Ziegler-Natta or coordination polymerization, though, which may be considered as an anionic polymerization, usually is treated separately. The further steps of the polymerization process (propagation, chain transfer, termination) similarly are characteristic of each type of polymerization. Since most unsaturated hydrocarbons capable of polymerization are of the structure of CH2=CHR, vinyl polymerization as a general term is often used. 

The coordination polymerization of ethylene and a-olefins with Ziegler-Natta catalysts involves, in general, many elementary reactions, such as initiation (formation of active centers), chain propagation, chain transfers and chain terminations. The length of growing polyolefin chains is limited by the chain-terminating processes, as schematically represented (for ethylene) by 21,49 51)... 

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