Polymerisation stereochemistry

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. 

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... 

Although thiacyclic monomers do not play such an important role in coordination polymerisation as oxacyclic monomers, some polymerisation of tiiranes with coordination catalysts deserves attention, especially considering the polymerisation stereochemistry ... 

Steric hindrance may hamper the correct stereochemistry required for 13-elimination, and perhaps this can be used to stabilise our metal alkyl complex. In the modem polymerisation catalysts for polypropene this feature has actually been observed, which leads to higher molecular weight polymers. This now forms part of the design of new catalysts. 

The interaction may not be quite as strong as in the case of 2,1 insertion discussed above, but there will always be a tendency of the growing chain to arrive at an isotactic stereochemistry when 1,2 insertion occurs. One example of chain-end control leading to isotactic polymer was reported by Ewen [13] using Cp2TiPh2/alumoxane as the catalyst. The stereoregularity increased with lower temperatures at -45 °C the isotactic index as measured on pentads amounted to 52 %. The polymer contains stereoblocks of isotactic polymer. At 25 °C the polymerisation gives almost random 1,2 insertion and an atactic polymer is formed. 

To start the polymerisation the X atoms in Fig 10.13 are replaced by a growing alkyl chain and a co-ordinating propene molecule. Co-ordination of propene introduces a second chiral element and several diastereomers can be envisaged. The step-by-step regulation of the stereochemistry by the site can be most clearly depicted by drawing the molecule as shown in Figure 10.14, a... 

The data discussed in Sections 8.5 and 8.6 make it clear that in the low-dielectric media typically employed for polymerisation reactions, the counteranions in metallocene ion pair catalysts are closely associated with the cationic complex as either inner-sphere or outer-sphere ligands. If anions are coordinated in the transition state, they must be expected to exert a significant influence on the stereochemistry of alkene polymerisation, even though the formation of syndiotactic and isotactic 1-alkenes have been readily explained by considering only the cationic metallo-cenium species and their ligand structure [21, 23, 122, 132, 133]. 

Recently we discovered [11] that photolysis of the diphenylallyl carbanion with white light causes isomerisation of the trans,trans conformer to the cis,trans (scheme 1). When the source of the illumination is removed, conformational relaxation proceeds at a rate which is markedly sensitive to the nature of the ion pairing, being much slower for loose than for tight ion pairs. The results of an extension of this work will be presented in this paper together with an outline of the possible relevance of this photochemical phenomenon to the stereochemistry of the polymerisation of dienes. 

In marked contrast to this behaviour, monomer 11, l-benzyl-2-methyl-aziridine, gives a relatively slow reaction with Et30+BF4. However, polymerisation is quantitative and all the indications are that the system is living . Why this would be so is difficult to explain. Electronic factors would seem to be elimi-minated since this monomer is intermediate as far as basicity is concerned. Presumably the explanation lies in the stereochemistry of the system since this particular monomer is the only carbon substituted species. Even more strangely, however, the closely related molecule l-benzyl-2-ethyl-aziridine is reported (141) to behave like 8,9, and 10 when treated with boron trifluoride. 

A few years later, in 1993, Akermark et al. reported the preparation of imprinted polymers capable of reducing selectively a 3-17-steroidal diketone (21) to the corresponding alcohol with high control of the stereochemistry [13]. The polymers were prepared attaching a polymerisable unit via an ester linkage to the desired steroidal product in position 17 (Fig. 3). The resulting compound was then used as a... 

We refer the readers to a useful body of books and reviews in the bibliography which will prove helpful to investigators determining the mechanism of radical reactions. The early two-volume compendium edited by Kochi has much valuable information, even though 30 years old, and most modern texts on radicals provide excellent guidance to radical synthesis and mechanism. We shall not discuss stereochemistry explicitly which now forms an important part of the mechanisms of radical reactions except to note that excellent stereoselectivities can be obtained in radical reactions with a clear understanding of the mechanisms involved. Many concepts in radical polymerisations are equally applicable to small molecule reactions and we refer the reader to an excellent account on the subject by Moad and Solomon. 

Table 3.3 Stereochemistry of propylene polymerisation with stereospecific Ziegler-Natta catalysts...

Zambelli, A., Proto, A. and Longo, P., Stereochemistry of Polymerisation of Some a-olefins in the Presence of Ziegler-type Catalysts , in Ziegler Catalysts, Springer-Verlag, Berlin, 1995, pp. 217-235. 

Figure 5.6 Stereochemistry of the formation of cis-, A-threo monomeric units in the polymerisation of terminally symmetrically disubstituted conjugated dienes such as ( , )-CHR=CH-CH=CHR...

It was proved that metal carbynes are sources of metal carbenes [e.g. scheme (9) in Chapter 6] promoting the polymerisation of acetylenic monomers. Therefore, related metal carbynes and carbenes appeared to catalyse the polymerisation of alkynes in the same way as regards the identity of the products, in particular as regards stereochemistry. For the terminal and internal alkynes, the Fischer carbyne acts much like the Casey and Fischer metal carbenes. The Fischer carbyne also promotes acetylene polymerisation, and it does this where the Fischer carbene fails and the Casey carbene is much less effective [22,143]. 

Models of the Active Sites, Epoxide Polymerisation Mechanism and Stereochemistry... 

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