Isotactic catalyst sites

PP synthesized using TiCl4/Et3Al is mostly isotactic, but two minor fractions are also produced. One is a soluble, atactic PP, whilst the other fraction is a partially crystalline, elastomeric stereoblock of iso- and a-tactic PP sequences.98 Elastomeric PP may also be prepared using the ansa-titanoccnc complex, (26), (although this catalyst does undergo rapid deactivation).99 Stereoblock formation was attributed to an equilibrium mixture of slowly interconverting isospecific and aspecific catalyst sites. Other stereoblock PP materials have been prepared via chain transfer between two catalysts of different stereoselectivities.101,102... 

The probabilistic aspect of error propagation in isotactic polypropylene was treated both as a second-order Markov chain (in terms of m and r dyads) (408) and, in terms of a model of enantiomorphic catalyst sites, as asymmetric Ber-... 

The term on the left is extremely sensitive, and this criterion should be used only with sufficiently accurate triad data. This is especially important if the polymer is very highly isotactic or syndiotactic, that is, with very small value of either (rr) or (mm). The term 4(mm)(rr)/ (mr)2 is considerably larger than one for the Markov and catalyst site control models. 

We now turn to the actual polymerization process and we will try to present a series of pictures that clarifies how chain-end control can be used to obtain either syndiotactic or isotactic polymers. Subsequently we will see how a chiral site can influence the production of syndiotactic or isotactic polymers. Finally, after the separate stories of chain-end control and site control, the reader will be confused by introducing the following elements (1) pure chain-end control can truly occur when the catalyst site does not contain chirality (2) but since we are making chiral chain ends in all instances, pure site control does not exist. In a polymerization governed by site control there will potentially always be the influence of chain-end control. This does not change our story fundamentally all we want to show is that stereoregular polymers can indeed be made, and which factors play a role but their relative importance remains hard to predict. 

Present-day Ziegler-Natta catalysts are supremely suitable for the production of linear polyethylene and of highly isotactic polypropylene. They are also used to produce the softer ethylene-propylene copolymers, used for packaging and related purposes. Due to the presence of distinct catalyst sites in typical Ziegler-Natta catalysts, these copolymers suffer from non-uniformity however, and copolymers which contain increased amounts of higher ot-olefins, desirable for certain applications, cannot easily be made with these catalysts. 

Recently, Doi152) speculated on the presence of two types of bimetallic active centers, based on 13C NMR analysis of the structure and stereochemistry of polypropylene fractions obtained with different Ziegler-Natta catalyst systems (see Fig. 44). Site A produces highly isotactic polypropylene, site B atactic polypropylene consisting of isotactic and syndiotactic stereoblocks. The formation of the latter fraction would be due to the reversible migration of the aluminum alkyl, made... 

When racemic methyloxirane is polymerized with zinc dimethoxide, D-and L-monomers are separately incorporated into growing chains to form an isotactic polymer consisting of poly(D-methyl-oxirane) and poly(L-methyloxirane). This stereoselective polymerization can be satisfactorily explained in terms of the enantio-morphic catalyst sites model (1 ). The d -sites accept D-methyl-oxirane in preference to the L-monomer, resulting in the formation of -DDDD- isotactic sequences. The same situation is valid for the l -catalyst sites. 

The insertion reaction has both cationic and anionic features. There is a concerted nucleophilic attack by the incipient carbanion polymer chain end on the a-carbon of the double bond of the monomer together with an electrophilic attack by the cationic counterion (G) on the alkene tt-electrons. The catalyst fragment acts essentially as a template or mold for the orientation and isotactic placement of incoming successive monomer units. Isotactic placement occurs because the Initiator fragment forces each monomer unit to approach the propagating center with the same face. This mechanism is referred to as catalyst site control or enantiomorphic site control. 

That is, the structure of isotactic active site is not always stable during polymerization period, because it is supposed that Lewis base makes isotactic active site by co-ordinating to Ti atoms in the catalyst, while, AlEt3 attacks Lewis base and reduces its electron donating ability. 

Stereospecific behavior of the catalyst site is related to the chirality of the surface sites of the solid TiCls. Models by Corradini can explain a number of observations—the type of tacticity errors along a predominantly isotactic chain, stereospecificity of the initiation reaction, and the maintenance of isotacticity after the insertion of ethylene monomer in the chain (241). Furthermore, since violet TiCls and MgCl2 solid-state crystal structures are similar, Corradini s model relates well to both TiCls and MgCl2-supported catalysts. 

Summarizing, isotactic and syndiotactic propagation highlights the catalyst site as the entity controlling stereoregularity for iPP and the last monomer unit of growing chain for syndiotactic propagation with vanadium catalysts. The monomer insertion type is primary (1,2) for isotactic and secondary (2,1) for this latter syndiotactic catalyst. 

On the basis of this experiment, Pino and coworkers were able to determine that catalysts derived from the (/ )-ethylenebis(tetrahydroindenyl)zirconium binaptholate preferentially selected the Re enantioface of propylene. These results led to a model for the transition state where the polymer chain is forced into an open region of the metallocene, thereby relaying the chirality of the metallocene to the incoming monomer through the orientation of the p-carbon of the aUcyl chain (Scheme IIA).43 Here, the role of the C2-symmetry of the catalyst site can be readily appreciated since as the polymer chain migrates to the coordinated olefin, the coordination site available for binding of the olefin alternates between two coordination sites (A -> B -> C). Because these two sites are related by a C2-symmetry axis, they are homotopic and therefore selective for the same olefin enantioface. The result is polymerization to yield an isotactic polyolefin. 

Amer and van Reenen [39] fractionated isotactic polypropylenes by TREE to get fractions with different molar masses but similar tacticities. The DSC results of the fractions indicated that the crystallization behaviour is strongly affected by the configuration (tacticity) and the molar mass of the PP. Soares et al. [40] proposed a new approach for identifying the number of active catalyst sites and the polymer chain microstructural parameters produced at each active site for ethylene/l-olefin copolymers synthesized with multiple-site catalysts. This method is based on the simultaneous deconvolution of bivariate MMD/CCD, which can be obtained by cross-fractionation techniques like SEC/TREE or TREE/SEC. The proposed approach was validated successfully with model ethylene/1-butene and ethylene/ 1-octene copolymers. Alamo and co-workers [41] studied the effects of molar mass and branching distribution on mechanical properties of ethylene/1-hexene copolymer film grade resins produced by a metallocene catalyst Molar mass fractions were obtained by solvent/non-solvent techniques while P-TREE was used for fractionation according to the 1-hexene content. 

If there is propagation through metallacarbenes of octahedral symmetry with a vacant alternating ligand position such as described above, these species may be chiral, with the formation of tactic polymer. Furthermore, cis double bond formation will be associated with syndiotactic junctions and tram double bonds with isotactic junctions, as in Scheme 12. It however, the catalyst site is achiral, or... 

Changing the symmetry of the complex to C2 (14-17, Rg. 7.3) results in the switch of the stereoselectivity of the polymerization from SPS to isotactic PS along with switch from chain end to catalyst site control [14],... 

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