Coordination catalysts stereospecific chain polymerization

IX. Stereospecific Chain Polymerization and Copolymerization by Coordination Catalysts... 

IX. STEREOSPECIFIC CHAIN POLYMERIZATION AND COPOLYMERIZATION BY COORDINATION CATALYSTS... 

The different behavior of the catalysts apparently arises from the nature of the transition metal of the catalyst. It seems reasonable to treat the mechanism of stereospecific olefin polymerization in terms of coordination ionic catalysts, regarding the valence state, coordination number, and nature of ligands of the transition metal as a matter of primary importance. In such an approach the polymerization mechanism is based on the character of metal—carbon bond by which a growing polymer chain is linked to the transition metal. 

Polymerization Ziegler-Natta, supported metal oxides such as PhiUps, Unipol and metallocene Stereospecific chain formation required weak interaction/complexation with any initiators/catalysts metal coordination complexes required... 

A third type of chain reaction is based on coordination catalysts. A most common example of such initiation is given in the Ziegler-Natta catalyst described in Fig. 3.24. A coordination complex may be formed on a crystal surface or in solution. The monomer is inserted into the active site, as is illustrated in the reaction sequence of propylene polymerization on a TiClj crystal surface, as described on Figs. 3.26 and 27, below. The geometry of the various ligands of the complex may direct the incoming monomer in such a way that a stereospecific polymerization is possible. 

In the coordinated cationic mechanism the polymerization is initiated by a proton formed by interaction of the catalyst-active hydrogen-containing compounds. The metal atom coordinates the monomer and the growing polymer chain, thus leading to some degree of stereospecificity during the polymerization. 

Polypropylene (PP) is a semicrystalline commodity thermoplastic produced by coordination addition polymerization of propylene monomer [197]. Most frequently, stereospecific Ziegler-Natta catalysts are used in industrial processes to produce highly stereospecific crystalline isotactic (iPP) and syndiotactic (sPP) polymer with a small portion of amorphous atactic PP as a side product. Polymerization of non-symmetrical propylene monomer yields three possible sequences however, the steric effect related to the methyl side group highly favors the head-to-tail sequence. The occurence of head-to-head and tail-to-tail sequences produces defects along the PP chain [198]. Presence of such defects affects the overall degree of crystallinity of PP. 

The most outstanding feature of alkyl metal catalysts is their stereospecificity. Viewing the entire field in this respect, it appears that a single, common feature is beginning to emerge. This is the coordination of monomer with one part of the catalyst prior to the addition of a partially stabilized polymer chain end. The coordination takes place between jr-electrons or lone-pair electrons of the monomer with vacant orbitals of a metal component. The polymer chain end is fixed in position and partially stabilized by either simple or complex gegen-ions. Such polymerizations are referred to as coordination or coordinated polymerizations to emphasize coordination of monomer. It should be noted that prior usage of these terms frequently implied either coordination of catalyst components or a concerted polymerization mechanism. 

The vacant sixth coordination site of these Ti centres can take up an olefin molecule to form the reaction complex required for the initiation and subsequent growth of polyolefin chains. Due to their octahedral dichelate-type structure, these Ti(III) centres are chiral and thus able to steer each incoming molecule into a preferred enantiofacial orientation. The stereospecificity with which subsequent propylene units insert into the growing polymer chain is most likely based on a mechanism analogous to that determined for soluble polymerization catalysts (Section 7.4.3). 

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. 

The coordinative (stereospecific) polymerization mechanism differs from the previous ones. It caused a real revolution in the polymer world when developed in the 1950s by the scientists Ziegler (in Germany) and Natta (in Italy)—both Nobel Prize Laureates in 1963. The principle here is the use of specific catalysts that orient the mers in the chain into a highly ordered configuration. By this mechanism, ethylene forms a linear (branchless) chain, so-called high density polyethylene (specific mass 0.95 to 0.96) which was developed by Ziegler. 

Several reaction mechanisms were also proposed to explain stereospecific placement with insoluble catalysts. Furukawa [46] suggested that here the mechanism for cationic polymerization of vinyl ethers depends upon multicentered coordinations. He felt that coordinations of the polymeric chains and monomers with the catalysts are possible if the complexed counteranions have electrically positive centers. This can take place in the case of aluminum alkyl and boron fluoride ... 

The polymerization mechanism involves repeated insertion of olefin into the C—M bond of the growing alkyl chain but many details of the process remain unclear because the catalyst systems are extraordinarily difficult to study. The stereoregularity of the Natta polymers may result from stereospecific coordination of the olefin to an asymmetric site on the TiCls crystal adjacent to the growing polymer chain 184a). 

---