Isomerization Polymerizations with Coordination Catalysts

Polymerizations of many internal olefins, like 2-butene, 2-pentene, 3-heptene, 4-methyl-2-pentene, 4-phenyl-2-butene and others, with Ziegler-Natta catalysts are accompanied by monomer rearrangements. The isomerizations take place before insertions into the chains [311-318]  

The double bonds migrate from the internal to the a-positions  

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Isomerization polymerizations can be associated with coordination catalyst systems, ionic catalyst systems, and free radical systems. The cationic isomerization polymerization of 4-methyl-1-pentene is of interest because the product can be viewed as an alternating copolymer of ethylene and isobutylene. This structure cannot be obtained by conventional... 

This monomer is usually obtained as a mixture of the cis and trans isomers both of which have been polymerized with coordination type catalysts. Polymerization of the cis form is considered to be preceded by isomerization, since those catalysts which do not isomerize the cis monomer (e.g. cobalt salt—organo aluminium halide) selectively polymerize the trans isomer. A kinetic study of the polymerization of cis 1,3-pentadiene using Ti(OBu-n)4/AlEt3 (Al/Ti = 1.3—6) as catalyst has been published [267]. This gives a polymer containing ca. 73% cis 1,4 15—16% trans 1,4 and 11—12% 3,4 microstructure. 

Butadiene could be polymerized using free radical initiators or ionic or coordination catalysts. When butadiene is polymerized in emulsion using a free radical initiator such as cumene hydroperoxide, a random polymer is obtained with three isomeric configurations, the 1,4-addition configuration dominating ... 

The chemistry of polymerizations with monomer-isomerization-pre-ceeding-propagation is not confined to the catalysts of the Ziegler-Matta type, i.e., anionic coordinated mechanisms. For examjde, allylbenzene gives (among other products) poly-/3-methylst3 rene by a conventional cationic process, however, this s tem is not well defined because of disturbing side reactions (alkylations, etc.) also occur. 

The ability of complexes to catalyze several important types of reactions is of great importance, both economically and intellectually. For example, isomerization, hydrogenation, polymerization, and oxidation of olefins all can be carried out using coordination compounds as catalysts. Moreover, some of the reactions can be carried out at ambient temperature in aqueous solutions, as opposed to more severe conditions when the reactions are carried out in the gas phase. In many cases, the transient complex species during a catalytic process cannot be isolated and studied separately from the system in which they participate. Because of this, some of the details of the processes may not be known with certainty. 

To account for the effect of monomer structure on the tacticity of the polymer, Natta et al. (1963b) presented the following interpretation. Complexing of the monomer with catalyst was thought to occur with only one double bond, even for both isomeric 1,3-pentadienes which afford cis-1,4-polydiene. The idea of coordination of two double bonds was rejected for steric reasons. This cis isomer (LXXI) of 1,3-pentadiene should be too hindered in the cis conformation for double coordination. Furthermore, if double coordination of the tram isomer (LXXII) is evoked to explain the stereochemical results, then butadiene, which is even less hindered, should polymerize by 1,4-addition. 

A major consequence of this pathway, also known as migratory insertion, is that the growing chain sweeps from one side to another with every addition of monomer. This is a generalization, for some authors have explained the loss of stereoselection in metallocene polymerizations of propylene, for example, at low monomer concentrations to the action of a windshield wiper isomerization, in which the polymer chain and open coordination site switch places without the benefit of monomer insertion. At low insertion rates, this site inversion phenomenon may become competitive with insertion and thus render ineffective any substituent influences which differ between the two faces of the catalyst site. With appropriate ligand design, different or enantiomeric steric environments may be created for the two sides of the active site. This makes possible stereoselective polymerization of propylene and higher a-olefins, as will be seen below. 

Cugini, C. Rombola, O. A. Giarrusso, A. Poni, L. Ricci, G. Polymerization of 4-methyl-l,3-pentadiene with catalysts based on cyclopentadienyl titanium chlorides Effect of anti/syn isomerism of the allyUc group on the chemoselectivity and the role of backbiting coordination in 1,3-diene polymerization. Macromol. Chem. Phys. 2005, 206, 1684-1690. 

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