Metallocenes propene polymerization

Resconi L, Cavallo L, Fait A, Pietmontesi F (2000) Selectivity in propene polymerization with metallocene catalysts. Chem Rev 100 1253-1345... 

Possible Back-Skip of Growing Chain. Several experimental facts relative to propene polymerization behavior of different metallocene-based catalytic systems can be rationalized by considering a disturbance of the chain migratory insertion mechanism due to a kinetic competition between the monomer coordination in the alkene-free state and a back-skip of the growing chain to the other possible coordination position (see Scheme 1.3). 

Traditional Ziegler-Natta and metallocene initiators polymerize a variety of monomers, including ethylene and a-olefins such as propene, 1-butene, 4-methyl-1-pentene, vinylcyclo-hexane, and styrene. 1,1-Disubstituted alkenes such as isobutylene are polymerized by some metallocene initiators, but the reaction proceeds by a cationic polymerization [Baird, 2000]. Polymerizations of styrene, 1,2-disubstituted alkenes, and alkynes are discussed in this section polymerization of 1,3-dienes is discussed in Sec. 8-10. The polymerization of polar monomers is discussed in Sec. 8-12. 

Two communications on propene polymerization by non-metallocene catalysts that include DFT/MM calculations have been recently published [60, 61]. They deal with group 4 bidentate non-cyclopentadienyl complexes. In the first communication [60], the topic addressed is the fact that a C2-symmetric precatalyst of titanium leads to a syndiotactic polymer, contrary to observations of metallocene catalysts. The chirality at the metal center is found to play a key role in the stereocontrol of the process. The second communication [61] addresses the fact that a C2-symmetric precatalyst of zirconium very similar to the previous one produces an isotactic polymer, finds out that it is due to a complicated concourse of synergic steric and electronic effects, and emphasizes the key role that serendipity still plays in the design of new catalysts. 

Jiingling, S., Mulhaupt, R., Stehling, U., Brintzinger, H.-H., Fischer, D. and Langhau-ser, F., The Role of Dormant Sites in Propene Polymerization using Methylalumox-ane Activated Metallocene Catalysts , Macromol. Symp., 97, 205-216 (1995). 

Table 3. Propene polymerization with metallocene/methylaluminoxane catalysts.

For some applications of metallocenes, especially for making i-PP, only the racemic isomers formed in the transmetallation step are of interest. In some cases the meso isomer formed with the rac isomer in a 50 50 mixture is not active in propene polymerization, while in other cases it produces undesired a-PP causing unacceptably high soluble polymer fractions. 

The classical heterogeneously catalyzed propene polymerization as discovered hy Natta is a stereospecific reaction forming a polymer with isotactic microstructure. During the development of single-site polymerization catalysts it was found that C2-symmetric chiral metallocene complexes own the same stereospecificity. An analysis of the polymer microstructure hy means of NMR spectroscopy revealed that misinsertions are mostly corrected in the next insertion step, which suggests stereocontrol (Figure 6) hy the coordination site, as opposed to an inversion of stereospecificity hy control from the previous insertion steps (chain-end control). In addition, it was found that Cs-symmetric metallocene catalysts lead to syndio-tactic polymer since the Cosee-Arlmann chain flip mechanism induces an inversion of the stereospecificity at every insertion step. This type of polymer was inaccessible by classical heterogeneous systems. 

Corradini, P. Gavallo, L. Guerra, G. Molecular Modeling Studies on Stereospecificity and Regiospecificity of Propene Polymerization by Metallocenes. In Metallocene-based Polyolefins Preparation, Properties and Technology, Scheirs, J., Kaminsky, W., Eds. Wiley Chichester, 2000 Vol. 2, p 3. 

Propene Polymerization with Silica-Supported Metallocene/MAO Catalysts... 

The kinetics of a propene polymerization which is promoted by Si02-supported metallocene catalysts depends on various factors (1) On the applied reaction engineering (gas-phase, bulk, and slurry polymerization) ° (2) On the degree of catalyst/ cocatalyst distribution on the support and (3) On the chosen reaction conditions and parameters (Figure... 

Figure 10. Propene polymerization profiles of a silica-supported metallocene/MAO catalyst prepared by suspension impregnation (a) depending on polymerization time and polymerization temperature and (b) depending on particle size and polymerization time, (c) Comparison of the activity profiles between an 1-octene prepolymerized catalyst and an untreated system, (d) Comparison of the activity profiles between a catalyst system employing 2 vol % hydrogen and the not activated system.

Figure 18. Schematic particle growth model for the propene polymerization of a silica-supported metallocene/MAO catalyst.

Selectivity in Propene Polymerization with Metallocene Catalysts... 

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