Chirality cyclic olefin polymers

With metallocene catalysts, not only homopolymers such as polyethylene or polypropylene can be synthesized but also many kinds of copolymers and elastomers, copolymers of cyclic olefins, polyolefin covered metal powders and inorganic fillers, oligomeric optically active hydrocarbons [20-25]. In addition, metallocene complexes represent a new class of catalysts for the cyclopolymerization of 1,5- and 1,6-dienes [26]. The enantio-selective cyclopolymerization of 1,5-hexadiene yields an optically active polymer whose chirality derives from its main chain stereochemistry. 

The stereosequences indicate site stereochemical control with chain migratory insertions, which result in site isomerization and occasional reversal in diastereoface selectivity. With the zirconocene, the activity was found to be 56 kg PP/mmol Zr. Even cyclic olefins such as cyclobutene, cyclopentene, or norbomene could be polymerized with the chiral catalysts to high melting polymers (mp > 400°C) . ... 

Using metallocene catalysts it has proved possible to tailor the microstructure of the polymers by fine-tuning of the ligands. Besides polyethylene, it is possible to co-polymerize ethylene with a-olefins such as propylene, but-l-ene, pent-l-ene, hex-l-ene, and oct-l-ene, in order to produce LLDPE. In addition, many kinds of co-polymers and elastomers, and new structures of polypropylenes, polymers and co-polymers of cyclic olefins can be obtained. Furthermore, catalysts with chiral centers can be beneficial in stereospecific polymerization to build the desired isotactic products. 

Cyclic olefins afford optically active polymers by asymmetric synthesis polymerization. The first example was the polymerization of benzofuran (265) with AlEtCl2 or AICI3 in the presence of optically active cocatalysts such as p-phenylalanine and 10-CSA the polymer is considered to possess the chiral erythro- or threodiisotactic structure (266a or 266b)... 

Various types of copolymers of cyclic olefins and other monomers have been prepared by asymmetric synthesis polymerizations using monomers with optically active side groups, ° optically active additives, " cata-lysts, or solvents.Among these, the synthesis of a copolymer of maleic anhydride and (S)-(-)-a-methylbenzyl methacrylate (MBMA, 269) is the first example of preparation of an optically active polymer consisting of a C-C backbone with chiral induction to the main chain. °... 

Cyclic olefins like cyclobutene, cyclopentene, and norbomene can be polymerized to give isotactic polymers with chiral metallo-cene/alumlnoxane catalysts, especially with et(blsindenyl)zir-conlumdichloride/methylalumoxane. 

From Table 3 it is obvious that the activity of the chiral catalyst is much higher (by a factor of 10 at 25 C, by a factor of 100 at SO C) than that of the simple biscyclopentadienylzirconium compound. Moreover under comparable conditions, the Incorporation of the cyclic olefin is improved with the chiral catalyst. The same was previously found for the polymerization of other a-oleflns. Again we found a random distribution of the norbornene units in the polymer chain using C-NMR measurements.. 

Cyclic monomers are a special class of prochiral monomer from the viewpoint of asymmetric polymerization [4, 13]. Symmetrically substituted cyclic olefins can give erythro cfz-isotactic polymers which are nonchiral because they possess a mirror glide plane on the contrary the threo-di-isotdictic polymer can be optically active due to the lack of the above symmetry elements. Unsymmetrically disubstituted cyclic olefins give both erythro- and threo-di-hoi2iCi c polymers which are chiral and which can then be obtained in optically active form by asymmetric induction polymerization (Scheme 11). 

In the case of chiral cyclic monomers the prepared polymer is isotactic, while this is not a necessity in principle in the case of chiral olefins. In most of the cases however, the choice is not as perfect as defined and, generally, one speaks of stereoelective process when there is a preferential polymerization of one type of enantiomer from a mixture. Moreover the sites controlling the polymerization could be more or less stereospecific, so that heterotactic polymers could in principle be obtained with the predominance of one type of enantiomeric unit. 

In the polymer field, reactions of this type are subject to several limitations related to the structure and symmetry of the resultant polymers. In effect, the stereospecific polymerization of propylene is in itself an enantioface-diflferen-tiating reaction, but the polymer lacks chirality. As already seen in Sect. V-A there are few intrinsically chiral stractures (254) and even fewer that can be obtained from achiral monomers. With two exceptions, which will be dealt with at the end of this section, optically active polymers have been obtained only from 1- or 1,4-substituted butadienes, fiom unsaturated cyclic monomers, fiom substituted benzalacetone, or by copolymerization of mono- and disubstituted olefins. The corresponding polymer stmctures are shown as formulas 32 and 33, 53, 77-79 and 82-89. These processes are called asymmetric polymerizations (254, 257) the name enantiogenic polymerization has been recently proposed (301). 

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