Site isotactic polymers

Triethyl aluminum, complexed with another electron donor, typically ethyl -anisate [94-30-4J, was used as cocatalyst with the FT-1 catalyst and acted to reduce and stabilize the active titanium-containing catalytic site. The early versions of the FT-1 catalyst required extremely high molar ratios (>400 1) of aluminum to titanium to obtain satisfactory activity and selectivity to isotactic polymer. This resulted in excessively high aluminum residues in the polymer. Later versions of the FT-1 catalyst attained much higher activity. 

Kim and Somorjai have associated the different tacticity of the polymer with the variation of adsorption sites for the two systems as titrated by mesitylene TPD experiments. As discussed above, the TiCl >,/Au system shows just one mesitylene desorption peak which was associated with desorption from low coordinated sites, while the TiCl c/MgClx exhibits two peaks assigned to regular and low coordinated sites, respectively [23]. Based on this coincidence, Kim and Somorjai claim that isotactic polymer is produced at the low-coordinated site while atactic polymer is produced at the regular surface site. One has to bear in mind, however, that a variety of assumptions enter this interpretation, which may or may not be vahd. Nonetheless it is an interesting and important observation which should be confirmed by further experiments, e.g., structural investigations of the activated catalyst. From these experiments it is clear that the degree of tacticity depends on catalyst preparation and most probably on the surface structure of the catalyst however, the atomistic correlation between structure and tacticity remains to be clarified. 

By the time the concentration of monomer is low, the back-skip of the polymer chain to the less-hindered site is faster than the formation of the high-energy alkene coordinated intermediate (IV). For this reason, at low propene concentrations and elevated temperatures isotactic sequences are formed. The probability of monomer coordination at the aspecific site (IV) is enhanced when the propene concentration increases. The consequence is that single stereoerrors [mrrm] are introduced in the isotactic polymer chain. 13C-NMR was able to prove the mechanism because a... 

On the basis of the microstructure of the prevailing isotactic polymer chains, it is well established that the steric control of the heterogeneous Ziegler-Natta catalysts is due to the chirality of the catalytic site and not to the configuration of the last inserted monomer unit.28,29,95... 

The formal view. The formal view is much simpler. The racemic catalysts have a twofold axis and therefore C2-symmetry. Both sites of the catalysts will therefore preferentially co-ordinate to the same face (be it re or si) of propene. Both sites will show the same enantiospecificity the twofold axis converts one site in the other one. Subsequently, insertion will lead to the same enantiomer. According to the definition of Natta, this means that isotactic polymer will be formed. If the chain would move from one site to the other without insertion of a next molecule of propene, it will continue making the same absolute configuration at the branched carbon atom. Hence, no mistake occurs when this happens. 

For the polymer -[-CH(COOR)CH(CH3)-hp, if only the ester-bearing main-chain site in each constitutional repeating unit has defined stereochemistry, the configurational repeating unit is (7) and the corresponding isotactic polymer is (8). 

The driving force for isoselective propagation results from steric and electrostatic interactions between the substituent of the incoming monomer and the ligands of the transition metal. The chirality of the active site dictates that monomer coordinate to the transition metal vacancy primarily through one of the two enantiofaces. Actives sites XXI and XXII each yield isotactic polymer molecules through nearly exclusive coordination with the re and si monomer enantioface, respectively, or vice versa. That is, we may not know which enantio-face will coordinate with XXI and which enantioface with XXII, but it is clear that only one of the enantiofaces will coordinate with XXI while the opposite enantioface will coordinate with XXn. This is the catalyst (initiator) site control or enantiomorphic site control model for isoselective polymerization. 

The C2-symmetric ansa metallocenes possess a C2 axis of symmetry, are chiral, and their two active sites are both chiral. The two sites are equivalent (homotopic) and enantioselective for the same monomer enantioface. The result is isoselective polymerization. C2 ansa metallocenes are one of two classes of initiators that produce highly isotactic polymer, the other class being the C ansa metallocenes (Sec. 8-5e). C2 ansa metallocenes generally produce the most isoselective polymerizations. 

The formation of an isotactic polymer requires that insertion always occur at the same prochiral face of the propylene molecule. Theoretically, both a chiral catalytic site (enantiomorphic site control) and the newly formed asymmetric center of the last monomeric unit in the growing polymer chain (chain end control) may... 

An isotactic stereospecific polymerization arises essentially from the favored complexation of one prochiral face of the a-olefin, followed by a stereospecific process. The stereospecific insertion process and the stereospecific polymerization of racemic a-olefins giving isotactic polymers may be expected to be stereoselective whenever the asymmetric carbon atom is in an a- or /3-position relative to the double bond, and when the interaction between the chirality center of the olefin and the chiral catalytic site is negligible. 

The main elements of chirality possibly present in the intermediates and transition states that can be hypothesised within this framework are as follows [1]. Firstly, a prochiral a-olefin molecule, e.g. propylene, coordinating via its two faces at the catalyst active site gives rise to non-superpo sable re and si diastereoisomeric complexes (Figure 3.24) [362, 363]. According to the considered mechanisms, an isotactic polymer is generated by a long series of... 

Steric defects in isotactic polypropylene, which involve the appearance of isolated r diads or pairs of r diads, may be considered on a pentad level (Figures 3.45a and b respectively). The 13C NMR signals associated with occasional stereoerrors in the propylene isotactic polymers produced by chiral metallocene-based catalysts (pairs of r diads) indicate that the polymerisation stereochemistry is governed by the enantiomorphism of catalytic sites an error pentad distribution close to mmmr.mmrr.mmrm-.mrrm = 2 2 0 1 is observed... 

In research with Ziegler catalysts, Cossee (11) and Arlmann and Cossee (12) hypothesized that the insertion of propylene monomer takes place in a cis conformation into a titanium-carbon bond. Natta et al. (8) postulated that in the stereospecific polymerization, chiral centers on the surface are needed to produce isotactic polymers. These and other issues regarding the nature of the active sites have helped to increase the interest in investigations of homogeneous metallocene catalysis. 

Modifications of Cs-symmetric metallocenes may lead to Ci-symmetric metallocenes (Fig. 8). If a methyl group is introduced at position 3 of the cyclopentadienyl ring, stereospecificity is disturbed at one of the reaction sites so that every second insertion is random a hemiisotactic polymer is produced (276, 277). If steric hindrance is greater (e.g., if a /-butyl group replaces the methyl group), stereoselectivity is inverted, and the metallocene catalyzes the production of isotactic polymers (178-180). 

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. 

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