Achiral Stereospecific Polymerizations

No optically active centers are produced in the polymerization of achiral monomers to dissymmetric polymers. Consequently, the smallest configurational unit is the configurational diad from two monomer units. 

Addition or insertion of a monomer leads to the formation of a new configurational diad. In general, there are different addition reaction possibilities, although, in polyinsertion, the monomer to be added on can only be added on in a quite specific position (see Chapter 19). In the simplest case, the addition is controlled by a single chemical species, for example, a free radical. In other cases, the growing chain end may exist in equilibrium with different species, for example, free ions in equilibrium with ion pairs. A distinction is made between one-way and multiway mechanisms. The following discussion is confined to one-way or one-path mechanisms. 

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Achiral monomers are converted to dissymmetric polymer molecules in achiral stereospecific polymerizations. An example of this is the polymerization of propylene to isotactic or syndiotactic poly(propylene). These kinds of polymerizations are commonly called stereospecific, although the two following classes are also stereospecific. 

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). 

Chiral polymers have been applied in many areas of research, including chiral separation of organic molecules, asymmetric induction in organic synthesis, and wave guiding in non-linear optics [ 146,147]. Two distinct classes of polymers represent these optically active materials those with induced chirality based on the catalyst and polymerization mechanism and those produced from chiral monomers. Achiral monomers like propylene have been polymerized stereoselectively using chiral initiators or catalysts yielding isotactic, helical polymers [148-150]. On the other hand, polymerization of chiral monomers such as diepoxides, dimethacrylates, diisocyanides, and vinyl ethers yields chiral polymers by incorporation of chirality into the main chain of the polymer or as a pedant side group [151-155]. A number of chiral metathesis catalysts have been made, and they have proven useful in asymmetric ROM as well as in stereospecific polymerization of norbornene and norbornadiene [ 156-159]. This section of the review will focus on the ADMET polymerization of chiral monomers as a method of chiral polymer synthesis. 

It is in the stereospecific polymerization of propylene that metallocene complexes display their astonishing versatility. Commercial Ziegler-Natta catalysts for isotactic polypropylene - based on combinations of TiCU, MgCl2, Lewis bases and aluminum alkyls - depend on a metal-centered chirality which exists at specific edge and defect sites on the crystal lattice to direct the incoming monomer in a particular orientation. These catalysts produce small amounts of undesirable atactic material due to the presence of achiral active sites. 

Stereospecific polymerizations can be divided into three groups according to whether they proceed from prochiral or chiral monomer molecules and whether they have chiral or achiral monomeric units. Those polymerizations classified according to the property of the groups can be further subdivided into classes according to the properties of the molecules produced and the molecular systems produced. 

STEREOSPECIFIC POLYMERIZATION USING ACHIRAL INITIATORS 2-1) Initiator systems... 

The attempts to obtain stereoelection with a chirally modified achiral stereospecific initiator such as ZnEt -MeOH generally failed except in the case of polymerization or diastereoisomeric mixtures of 3-methyl 1,2-epoxy pentane or 3-methyl 1,2-epithio-pentane (48). Yet in this last case, the observed election is rather due to different consumption rates for the two diastereo-isomers present in the mixture than to a real modification of the spectrum of catalytic sites. 

The basic mechanism of metallocene-based polymerization involves a catalytic cycle very similar to that of Fig. 6.5. The precatalysts 6.22 and 6.23, in combination with MAO, produce polypropylene of high isotacticity and syndiotac-ticity, respectively. As shown in Fig. 6.7, 6.22 has C2 symmetry and is chiral, while the symmetry of 6.23 is Cs and is therefore achiral. Two points need to be noted before we discuss the mechanism of stereospecific insertion of propylene. First, propylene is a planar molecule that has two potentially nonequivalent, prochiral faces (see Section 9.3.1). Second, the symmetry around the metal atom determines whether or not coordinations by the two faces of propylene are equivalent. 

In syndiospecific polymerization using soluble vanadium catalysts, defects of the chemical addition are noticeable, their number being consistent with the Markoff first — order distribution 59-6i.63> jjj jjjjg regioselectivity as well as stereospecificity are greatly influenced by the possibility of the secondary insertion of monomer The chiratity of the last monomeric unit of the growing polymer determines the stereocontrol of syndiotactic polymerization. The introduction of an achiral (ethylene) unit results in the loss of stereocontrol As active center of syndio-... 

Mechanisms of Stereocontrol. Stereochemistry of the olefin insertion step can be controlled by both the steric environment of the active site (enantiomorphic-site control) as well as the growing polymer chain (chain end control). In chain end stereocontrol, stereospecificity arises from the chiral )3-carbon atom of the last enchained monomer imit, which in turn influences the stereochemistry of monomer addition. Chain-end control is usually less effective than site control and has been observed for some achiral metallocenes at low polymerization temperatures. Partially iPP resulting from chain end stereocontrol has been obtained with Cp2TiPh2/MAO (56,272). The syndiospecific polymerization of 1-butene using the Cp 2MCl2/MAO (M = Zr, Hf) catalyst systems has been described (273). Predominantly sPP has been obtained under chain end control, using Brookhart s diimine nickel catalysts (274-277). 

Finally, optically active polymeric materials with a great variety of chemical and physical properties can be expected from the extension of the stereospecific copolymerization between easily available chiral monomers and achiral functional comonomers. Limits to this extension can arise from the difficulty of finding catalytic systems able to yield stere-ordered copolymers of the two comonomers which can be rather different in reactivity. 

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