Isospecificity

Y. V. Kissin, Isospecific Polymerisation of Olefins with Heterogeneous Ziegler-Natta Catalysts, Springer-Vedag, New York, 1985. 

Polymerization Processes. Isotactic PB and PMP are produced commercially in slurry processes in Hquid monomers or monomer mixtures (optionally diluted with light inert hydrocarbons) at 50—70°C. The first commercial process for PB production used a highly isospecific... 

Zirconocene dichloride 121 derived from (l-phenylethyl)cyclopentadienyl ligand is formed as a mixture of diastereomers from which the racemic form can be isolated by fractional crystallization. This complex was studied by X-ray diffraction methods and revealed a virtually chiral C2-symmetrical conformation in which the chiral ring-substituents are arranged in a synclinal position relative to the five-membered ring. It was proposed that this conformation is preserved in solution. Using 121 as catalyst the influence of double stereodifferentiation during isospecific polymerization of propylene (Eq. 32) was demonstrated for the first time [142],... 

Kissin YV (1985) Isospecific polymerization of olefins. Springer, Berlin Heidelberg New York... 

Several isospecific Ci-symmetry catalysts have also been described including (12-15). When activated with [Ph3C]+ [B(C6F5)4]-, (12) affords highly regioregular i-PP (mmmm = 95%) with the stereochemical defects predominantly being isolated rr triads, consistent with a self-correcting enantiomorphic site-control pathway. 2,73 The isospecificity was therefore explained by a mechanism... 

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... 

Elastomeric PP has also been synthesized using Ti, Zr and Hf ansa-metallocenes, (27). An alternative explanation for stereoblock formation was proposed, in which epimerization between isospecific and aspecific sites is rapid, affording predominantly atactic PP with short isotactic-rich sequences.103-105... 

Isospecific Polymerization by Ansa-Type (.Bridged) Metallocenes... 

The isotacticities and activities achieved with nonbridged metallocene catalyst precursors were low. Partially isotactic polypropylene has been obtained by using a catalyst system of unbridged (non-ansa type) metallocenes at low temperatures [65]. A chiral zirconocene complex such as rac-ZrCl2(C5H4 CHMePh)2 (125) is the catalyst component for the isospecific polymerization of propylene (mmmm 0.60, 35% of type 1 and 65% of type 2 in Scheme Y) [161]. More bulky metallocene such as bis(l-methylfluorenyl)zirconium dichloride (126) together with MAO polymerized propylene to isotactic polypropylene in a temperature range between 40 and 70°C [162]. 

The Tj, and the isospecificity obtained with these FI catalysts represent some of the highest values for iPPs ever synthesized. The isospecific propylene polymerization proceeds via a 1,2-insertion with a site-control mechanism. 

As stated above, we postulated that fast, reversible chain transfer between two different catalysts would be an excellent way to make block copolymers catalytically. While CCTP is well established, the use of main-group metals to exchange polymer chains between two different catalysts has much less precedent. Chien and coworkers reported propylene polymerizations with a dual catalyst system comprising either of two isospecific metallocenes 5 and 6 with an aspecific metallocene 7 [20], They reported that the combinations gave polypropylene (PP) alloys composed of isotactic polypropylene (iPP), atactic polypropylene (aPP), and a small fraction (7-10%) claimed by 13C NMR to have a stereoblock structure. Chien later reported a product made from mixtures of isospecific and syndiospecific polypropylene precatalysts 5 and 8 [21] (detailed analysis using WAXS, NMR, SEC/FT-IR, and AFM were said to be done and details to be published in Makromolecular Chemistry... 

Lieber and Brintzinger reported a detailed study using mixtures of precatalysts with different stereospecificities [25], Isospecific precatalyst 10 undergoes facile chain transfer to TMA, as revealed by an increase in isopropyl end groups and a sharp reduction in molecular weight in the presence of TMA (there was no exchange with TiBA). An aspecific precatalyst 7 was affected similarly by TMA... 

The authors conducted a similar investigation of precatalysts 7 and 11 using TiBA and trityl tetrakis(pentafluorophenyl)borate as the cocatalyst. They concluded that this material contained no fraction that could be characterized as blocky. It was therefore proposed that reversible chain transfer occurred only with MAO or TMA and not with TiBA. This stands in contrast to the work of Chien et al. [20] and Przybyla and Fink [22] (vida supra), who claim reversible chain transfer with TiBA in similar catalyst systems. Lieber and Brintzinger also investigated a mixture of isospecific 11 and syndiospecific 12 in attempts to prepare iPP/sPP block copolymers. Extraction of such similar polymers was acknowledged to be difficult and even preparative temperature rising elution fractionation (TREF) [26, 27] was only partially successful. 

Section 3 will deal with catalytic systems whose stereospecificity is mainly controlled by the chirality of the environment of the transition metal, independently of the possible chirality of the growing chain (chiral site stereocontrol). In particular, in Section 3.1 the chirality and stereospecificity of homogeneous catalytic systems based on metallocenes of different symmetries and in different experimental conditions will be reviewed. In Section 3.2 the chirality of model catalytic sites, which have been supposed for isospecific first-generation TiCl3-based and high-yield MgC -supported catalysts, is described. In Section 3.3 we will present a comparison between model catalytic sites proposed for heterogeneous and homogeneous stereospecific site-controlled catalysts. 

Section 4 will deal with catalytic systems whose stereospecificity is controlled principally by the chirality of the closest tertiary carbon atom of the growing chain (chain-end stereocontrol). In Section 4.1 possible mechanisms for chain-end controlled isospecific and syndiospecific propene polymerizations will be reviewed. In Section 4.2 informations relative to the mechanism of chain-end controlled syndiospecific polymerization of styrene and substituted styrenes will be reviewed. In Section 4.3 chain-end controlled mechanisms for the isospecific and syndiospecific cis-1,4 and 1,2 polymerizations of dienes will be presented. 

Figure 1.8 Approximated transition states for primary propene insertion for model complexes with (a) isopropyl-bis(l-indenyl) ligand (isospecific) for the (R, R) coordination and (b) iso-propyl(cyclopentadienyl-9-fluorenyl) ligand (syndiospecific) for the R chirality at metal atom. Corresponding preinsertion intermediates, labeled a in Figures 1.7a,b, are sketched in Figures 1.4 and 1.6a, respectively.

Of course, the stereoselectivity (and hence the isospecificity) of the catalytic models strongly depends on the encumbrance of the jt-ligand, increasing along the following series 3-methyl-cyclopentadienyl (e.g., 3.7 kcal/mol for 10), indenyl (e.g., 4.9 kcal/mol for 13), 4,7-dimethyl-indenyl (e.g., 5.3 kcal/mol for 17), and tetrahydroindenyl (e.g., 5.9 kcal/mol for 18). This is in good qualitative agreement, for instance, with the percent of mmmm pentads evaluated for polypropylene samples obtained for different catalytic systems in strictly similar conditions by Resconi and co-workers.58... 

The calculation results listed in Table 1.1 are also able to account for the influence of the bridge on the isospecificity. In fact, in agreement with the experimental results,58 the calculated stereoselectivities for a given jt-ligand are generally smaller for the case of the -C(CH3)2- (2-6) than for the -Si(CH3)2- bridge (10-14), while for the case of the -CH2-CH2- bridge the stereoselectivities are often intermediates (compare, e.g., 5, 13, and 23). 

In general, these stereochemical events are completely independent. For instance, isospecific catalysts like the heterogeneous ones (based on TiCl3 or on TiCU supported on MgCI2) as well as C2 symmetric metallocenes [e.g., based on rac-C2H4(H4-l-Ind)2ZrCl2] can present extremely poor cis trans... 

Figure 1.10 Preinsertion intermediates for secondary propene insertion into primary polypropylene chain for (a) isospecific model complex based on (R, R)-coordinatedisopropyl-bis(l-indenyl) ligand and (b) syndiospecific model complex based on isopropyl(cyclopentadienyl-9-fluorenyl) ligand for R chirality at metal atom. Stereoselectivity of isospecific model site is in favor of opposite monomer prochiral faces for primary and secondary insertions (cf. Figures 1.4 and 1.10a). Stereoselectivity of syndiospecific model site is in favor of same monomer prochiral face for primary and secondary insertions (cf. Figures 1.6a and 1.1 Ob).

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