Ziegler-Natta mechanism, block

The first example of Iiving polyolefin with a uniform chain length was found in the low-temperature polymerization of propylene with the soluble catalyst composed of V(acac)3 and Al(C1Hi)2Cl. The mechanism of the living coordination polymerization is discussed on the basis of the kinetic and stereochemical data. Subsequently, some applications of living polypropylene are introduced to prepare tailor-made polymers such as terminally functionalized polymers and block copolymers which exhibit new characteristic properties. Finally, new types of soluble Ziegler-Natta catalysts are briefly surveyed in connection with the synthesis of living polyolefins. 

The first example of a living polyolefin with a uniform chain length was disclosed in 1979 by Doi, Ueki and Keii 47,48) who used the soluble Ziegler-Natta catalyst composed of V(acac)3 (acac = acetylacetonate anion) and A1(C2H5)2C1 for the polymerization of propylene. In this review, we deal with the kinetics and mechanism of living coordination polymerization of a-olefins with soluble Ziegler-Natta catalysts and the synthesis of well-defined block copolymers by the use of living polyolefins. 

After activation, the catalyst is intrcxiuced into the polymerization reactor as slurry in a saturated hydrocarbon such as isobutane. The precise mechanism of initiation is not known, but is believed to involve oxidation-reduction reactions between ethylene and chromium, resulting in formation of chromium (II) which is the precursor for the active center. Polymerization is initially slow, possibly because oxidation products coordinate with (and block) active centers. Consequently, standard Phillips catalysts typically exhibit an induction period. The typical kinetic profile for a Phillips catalyst is shown in curve C of Figure 3.1. If the catalyst is pre-reduced by carbon monoxide, the induction period is not observed. Unlike Ziegler-Natta and most single site catalysts, no cocatalyst is required for standard Phillips catalysts. Molecular weight distribution of the polymer is broad because of the variety of active centers. 

Block Copolymers by Ziegler-Natta (Insertion) Mechanism... 

Copolymerizations of aldehydes take place by both anionic and cationic mechanisms. An elastic copolymer of formaldehyde and acetaldehyde forms with triisobutylaluminum. The rate of copolymerization is veiy rapid at 78°C. The reaction is complete within 30 min [364]. The product, however, is crosslinked. Aldehydes also copolymerize with some vinyl monomers [386]. An acetone block copolymer forms [387] with propylene when Ziegler-Natta catalysts are used at —78 C. Copolymers of acetone with other olefins and with formaldehyde were also prepared [388, 389]. Many initiators are effective in copolymerizations of aldehydes, ketones, and epoxies [387, 390]. 

Block copolymers can be obtained by copolymerization of cycloolefins of entirely different reactivities or by applying adequate sequential addition of the monomer. They also arise from cycloolefins and vinylic monomers, including linear olefins, in the presence of Ziegler-Natta catalysts [5] [Eq. (3)] or of metathesis catalysts. In the latter case it is usual to change the reaction mechanism to Ziegler Natta [6] and group transfer polymerization [7] or from anionic-coordina-tive to metathesis polymerization [8] [Eq. (4)]. 

Block copolymers from cyclooolelins have been prepared by various experimental techniques [52]. Some interesting methods use living ROMP catalysts, which allow ready synthesis of new products having controllable structures and properties. Other methods apply cross-metathesis between unsaturated polymers and/or polyalkenamers [3], polymerization of cycloolelins in the presence of unsaturated polymers [4], polymerization of two or more cycloolelins of quite different reactivities with classical ROMP catalysts [4], and copolymerization of cycloolefins with other monomers, effected by changing the polymerization mechanism from ROMP to anionic, cationic, Ziegler Natta, and group transfer, and vice versa [6-8, 52]. 

Another evidence confirming the coordinate - anionic mechanism of reaction is the relatively long life of growing chains. It makes possible obtaining of block copolymers by adding a second monomer after the reaction of the first. The process of polymerization in the presence of Ziegler-Natta catalysts is not affected by the typical radical transfer reactions. It means that the process is not a free radical one. 

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