Site control syndiotactic polymers

The formal view. The fluorenyl-cyclopentadienyl complex contains a mirror plane. The two sites are therefore mirror images. One site co-ordinates to the re-face of propene, the other site to the si-face of propene. One site will therefore be enantiospecific for making R configured carbon atoms and the other site for S configured carbon atoms. This alternation of configuration leads to a syndiotactic polymer. 

When the chain migrates from one site to the other without insertion we obtain two stereocentres of the same absolute configuration and our pentad analysis will show one meso (m) in the series of (r) relationships. 

If one 1,2-insertion occurs with the wrong stereochemistry we obtain two m relationships in the series of (r) relationships. Thus several types of mistakes can be distinguished from the 13C NMR analysis. 

Errors. Errors can be introduced in several ways, for the bis-indenyl as well as for the fluorenyl-cyclopentadienyl complexes  

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CEM a route of syndiotactic polymerization that is based on the chain-end control mechanism, where the last stereocenter of the growing polymer dominates the stereochemical outcome during monomer addition process SCM a recent strategy related to application of Cs-symmetric catalysts, where regularly alternating monomer insertion on enantiotopic coordination sites forms syndiotactic polymers depending on the site control mechanism. 

Some chiral initiators have structures such that alternate monomer placements occur with opposite faces of the monomer to yield the syndiotactic polymer. This is syndioselective polymerization proceeding with catalyst site control and is usually observed only with some homogeneous initiators, both traditional Ziegler-Natta and metallocene. 

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. 

Soluble catalysts again may exhibit unique selectivities. Hafnium and zirconium fluorenyl metallocenes with methylaluminoxanes give syndiotactic polymers in high yields. The microstructure of the products indicates site stereochemical control with chain migratory insertion resulting in site isomerization with each monomer... 

Fig. 7 Polymer 1, chain end control polymer 2, enantiomeric site control, where i and s are the relative stereochemistry of a pairwise addition of lactide units, i isotactic enchainment, s syndiotactic enchainment...

As regards higher 7-olefins, their polymerisation with metallocene-based catalysts of class IV with Cs symmetry affords highly syndiotactic polymers as in the case of propylene [117]. This is a consequence of enantiomorphic site control over the polymerisation stereochemistry. 

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. 

Ewen and Razavi [52] have shown that stereoselective Cs-symmetric metallocenes (18-20) with their enantiotopic vacancies form syndiotactic polymers. The rrrr stereosequences indicate enantiomorphic site control with chain migratory insertion errors arise from site isomerization without insertion and occasional reversal in diastereoface selectivity. 

A series of elegant experiments which support this mechanism involve the use of isopropyl(l-fluorenyl-cyclopentadienyl) ligands [51]. This complex is not chiral (i.e the dichloride precursor of Fig. 6.20), but has a plane of symmetry instead. The catalyst was found to give syndiotactic polymer. Coordination of propene at either site now leads to mirror images. Migration of the alkyl chain will create carbon atoms with the opposite absolute configuration (i.e syndiotactic polypropene). This is an important result, since hitherto it was thought that syndiotactic polymers could only be obtained via 2,1-insertion, controlled by the stereochemistry of the chain end. 

This brings us to double stereoselection and reinforcement of the mechanisms. If the site (a)symmetry were to control the orientation of the chain, and if, then, the orientation of the incoming propene is controlled by both the chain and the site, the highest stereoselection is obtained when the two influences reinforce one another. For 1,2-insertion this can be done most effectively for isotactic polymerization, since chain-end control naturally leads to isotactic polymer and this we can reinforce by site control with ligands of the bis(indenyl)ethane type. The chain-end influence of short chains is smaller than that of longer polymer chain and therefore short chain ends lead to lower selectivities. It may also be irrferred that making syndiotactic polymer via a 1,2-insertion mechanism on Ti or Zr complexes is indeed more difficult than making an isotactic polymer, because the two mechanisms now play a counterproductive role. 

The ring-opening polymerization of a simple cyclic olefin such as cyclooctene yields two structures of maximum order, which are distinguished by the configuration (cis or irans) of their main-chain olefins. In contrast, polymers made from bicyclic olefins such as norbornene are inherently more complicated and have four structures of maximum order (Scheme 24). In addition to cis- and trans-olefins, the polymers can also be isotactic or syndiotactic. The stereochemistry of these polymers becomes even more complicated when the monomer is asymmetric, since head—head, head—tail, and tail—tail regioisomers are possible. Nevertheless, single-site metathesis catalysts have been developed that can control polymer stereochemistry to an impressive degree by both chain-end and site-control mechanisms. ° ° ... 

If the enantiomorphic site control is operative (top-half view), stereoerrors do not propagate, and the corresponding iso- and syndiotactic polymers are characterized by the presence of rrand mm triads, respectively. If chain-end control is operative (bottom-half view), stereoerrors propagate, and the corresponding iso- and syndiotactic polymers are characterized by the presence of Isolated r and m dlads, respectively. Reprinted from ref 115. Copyright 1992 American Chemical Society. 

Syndiotactic Control. In prochiral Cs-symmetric metallocenes (Chart 2), one site favors the insertion of an olefin enantioface and the other preferably inserts the opposite one and a syndiotactic polymer is obtained. 

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