Olefins optically active polymers

When a chiral ansa-type zirconocene/MAO system was used as the catalyst precursor for polymerization of 1,5-hexadiene, an main-chain optically active polymer (68% trans rings) was obtained84-86. The enantioselectivity for this cyclopolymerization can be explained by the fact that the same prochiral face of the olefins was selected by the chiral zirconium center (Eq. 12) [209-211]. Asymmetric hydrogenation, as well as C-C bond formation catalyzed by chiral ansa-metallocene 144, has recently been developed to achieve high enantioselectivity88-90. This parallels to the high stereoselectivity in the polymerization. 

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). 

The third method (106), which has been so far adopted only to produce optically active polymers from poly-a-olefins obtained from racemic monomers, is of great interest, owing to its simplicity and to the relatively high optical purity attained in one step. 

II. 2.1.4. Cyclic Olefin Polymers Benzofuran (16) gives an optically active polymer by cationic polymerization with AlEtCl2 or A1C13 in the presence of an optically active cocatalyst such as P-phenylalanine and 10-camphorsulfonic acid [12,48-50], The optically active polymer is considered to have an erythro- or threodiisotactic structure with no plane of symmetry. Initiator systems of AlCl3/(-)-menthoxytriethyltin, -germanium, and -silicon also give an optically active polymer [51,52],... 

In the early stage of helical polymer stereochemistry, a few polymers were known to retain a helical main chain with a predominantly single screw sense in solution at room temperature. For example, in cases of poly( f-bulyl isocyanides) [22], poly(triphenylmethyl methacrylate) [23], polyisocyanate [24], and poly-a-olefins [19], helical structures are kept through side group interactions. Since these pioneering works, many synthetic optically active polymers with a chromophoric main chain bearing chiral and/or bulky side... 

A fruitful approach to obtain asymmetric polymer synthesis was proposed by Overberger (126) who suggested that propagation could be influenced and optically active polymers could be synthesized by using optically active gegen-ions. Schmidt and Schuerch (127) followed up this suggestion and used boron trifluoride in conjunction with asymmetric Lewis bases (1-a-methyl benzyl alcohol, tosyl L-valine, camphor) to polymerize certain cyclic olefins. However, in spite of careful work and various modifications in reaction conditions, no optical activity was obtained in the polymers in this first attempt to test this ingenious hypothesis. 

An interesting behavior of the polymerization would be the formation of optically active polymers from racemic olefin oxides (81, 153, 154, 162a, 208,212-214,215a, 250,274,275,316,393,500,504,507,508,510, 511). The catalyst systems consist of dialkylzinc and optically active alcohol or amino acid. In the polymerization, one enantiomer of the racemic monomer is selectively introduced in the polymer. 

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. 

In addition to propylene, other nonconjugated olefins have been copolymerized with CO using enantiopure palladium catalysts. Allylbenzene, 1-butene, 1-heptene, 4-methyl-l-pentene, and cis-2-butene [84,85] as well as hydroxy- and carboxylic acid-functionalized monomers [87] have been polymerized to give optically active polymers. Waymouth, Takaya and Nozaki have recently reported the enantioselective cyclocopolymerization of 1,5-hexadiene and CO [88,89]. 

Achiral polymers synthesized from achiral monomers have been modified after polymerization using asymmetric catalysts to yield optically active polymers. For example, enantioselective ketone reduction, hydrogenation, olefin epoxida-tion, and olefin hydroxylation have been carried on the functional groups of achiral polymers [111, 112]. Such functionalizations, however, are often incomplete or occur with a low degree of asymmetric control. 

Aromatic compounds have not only been of academic interest ever since organic chemistry became a scientific discipline in the first half of the nineteenth century but they are also important products in numerous hydrocarbon technologies, e.g. the catalytic hydrocracking of petroleum to produce gasoline, pyrolytic processes used in the formation of lower olefins and soot or the carbonization of coal in coke production [1]. The structures of benzene and polycyclic aromatic hydrocarbons (PAHs) can be found in many industrial products such as polymers [2], specialized dyes and luminescence materials [3], liquid crystals and other mesogenic materials [4]. Furthermore, the intrinsic (electronic) properties of aromatic compounds promoted their use in the design of organic conductors [5], solar cells [6],photo- and electroluminescent devices [3,7], optically active polymers [8], non-linear optical (NLO) materials [9], and in many other fields of research. 

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. °... 

Finally, it may be noted that some polymers have been obtained in which optical activity is ascribed mainly to conformational asymmetry. In these cases there is a predominance of either right-handed or left-handed enantiomorphs of helical polymer molecules, in contrast to the more usual situation wherein equal amounts of the two enantiomorphs are produced and there is no resultant optical activity. Optically active polymers of this type have been obtained from a-olefins possessing optically active side chains, e.g., 3-methylpent-l-ene, 4-methylhex-l-ene and 5-methylhept-l-ene. Isotactic polymers from these monomers have greatly enhanced optical activity compared to the monomer. Since these polymers are vinyl polymers this optical activity cannot be associated with the asymmetry of the carbon atom in the polymer backbone (for the reasons given above). Thus it is supposed that the presence of optically active side groups favours a particular screw sense of the helix so that the resultant polymer shows a large optical rotation. Optical activity of this type has not been observed when the side groups are not asymmetric. 

In addition to this structural problem, there is also a stereochemical problem. Unlike vinyl monomers or a-olefins, propylene oxide has an asymmetric carbon atom before polymerization. Thus, it is possible to obtain optically active polymers if the polymerization proceeds with either complete retention or complete inversion at the asymmetric center. Four different dimer units in the main chain are therefore possible [eq.(l)]. 

The crystal structures of polyesters based on o-hydroxy and 3-hydroxyacids have been extensively studied since the realization that a family of high melting polymers was achievable based on these structures (29,30). By suitable substitution of the monomers, optically active polymers can be prepared and the deliberate adjustment of the relative optical antipode content and distribution leads to steric copolymers which can present all the characteristics of isotactic, syndiotactic and stereoblock structures now familiar in the poly-a-olefin series. 

In recent years there has been a great number of studies on optically active polymers especially related to the synthesis of polypeptides and to vinyl polymerization. A large number of studies have been devoted to the synthesis of crystalline polymers with stereoregular arrangement of the type produced by Schildknecht [2] with vinylethers, such as isobutyl and methyl in 1947, and Natta [3] with -olefins, such as propylene and styrene. 

In contrast to homo-polymerization of vinyl monomers, where the main chain becomes pseudo-asymmetric and optically inactive, in the homo-polymerization of diene monomers and a, 3-substituted olefins asymmetry can be introduced into the main chain, thus leading to optically active polymers... 

Progress in macromolecular chemistry have permitted the synthesis of completely different optically active polymers (poly-a-olefins, polysubstituted heterocyles, polycondensates of different types...) which will be named synthetic polymers in what follows. 

Finally, it is worth noting that optically active polymers containing pendant carbazole units have been prepared by attachment of the carbazole group to chemically activated, optically active macromolecules, such as coisotactic copolymers of chiral a-olefins and p-chloromethyl styrene... 

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