Ziegler-Natta polymerization chain transfer

Hydrogen is the most used chain transfer agent in Ziegler-Natta polymerization. However, information on the influence of hydrogen during polymerization with Mg/Ti catalysts is scarce. 

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). 

Chain termination occurs primarily by j8-elimination with hydride transfer to chromium and by /3-elimination with hydride transfer to monomer. These terminations are analogous to those previously shown for Ziegler-Natta polymerizations (see eq 3.8 and 3.9 in Chapter 3). In some cases, supported chromium catalysts, e.g., chromocene on SiO and Cr on AlPO, are responsive to hydrogen... 

The compositions of EPDM elastomers are controlled by using the appropriate monomer feed ratio (see Eq. (2.38)) to obtain the desired composition in a continuous polymerization process. In general the excess propylene required is recycled. The molecular weights of EPDM polymers are controlled primarily by chain transfer reactions with added molecular hydrogen (Eqs. (2.86) and (2.87)), as is common with other Ziegler-Natta polymerizations (Boor, 1979). 

Chain transfer processes with aluminium alkyl compounds in Ziegler-Natta polymerization means that there can exist at least two types of polymer chain both of idiich will react with the quenching agent, i.e. 

Emulsion High molecular weight polymers made at fast polymerization rates Heat easily dispersed Low viscosity May be used directly as emulsion Contamination by surfactants or other polymerization adjuvants Washing/drying needed Cannot be used for condensation, ionic or Ziegler-Natta polymerization, although some exceptions are known. Chain transfer agents needed to control the relative molar mass of the products... 

Consider a Ziegler-Natta polymerization in which chains are terminated only by chain transfer with hydrogen. The rate of transfer is given by rtr = kffC [H2 -... 

In the Ziegler - Natta polymerization takes also place the transfer reactions of the chain activity as demonstrated by the fact that the molecular weight reaches a relatively constant value in a short time after the initiation of reaction. These reactions include ... 

It was discovered by Ziegler in Germany and Natta in Italy in the 1950s that metal alkyls were very efficient catalysts to promote ethylene polymerization at low pressures and low temperatures, where free-radical polymerization is very slow. They further found that the polymer they produced had fewer side chairrs because there were fewer growth mistakes caused by chain transfer and radical recombination. Therefore, this polymer was more crystalline and had a higher density than polymer prepared by free-radical processes. Thus were discovered linear and high-density polymers. 

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. 

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