Prochiral monomer

According to an early model.13 1 there are two adjacent accessible positions at the catalytic site, each favoring the coordination of the prochiral monomer with one of its two faces if the growing polymer chain alternates between the two positions at each insertion step, syndiotactic propagation is ensured. Due to the successive finding of a chain-end stereocontrol, this model has to be rejected. 

In principle, each enchainment of a prochiral monomer such as propene generates a chiral carbon center. The relative orientation of two such centers can therefore give rise to meso-diads or racemo-diads. Three monomers consecutively enchained in meso-fashion give rise to a mm triad, and so forth (Scheme 8.37). 

Asymmetric polymerization of prochiral monomers, if feasible, would be an important class of synthetic reactions. The first trial was reported by Marvel who, in 1943, attempted asymmetric vinyl polymerization of styrene, methyl methacrylate, and acrylonitrile by using optically active acyl peroxides (32). All attempts, however, were unsuccessful. 

Asymmetric synthesis polymerization is the reaction that produces the polymers with configurational chirality of the main chain from an optically inactive, prochiral monomer or a prochiral... 

Topochemical Polymerization The chiral crystalline environment of a monomer itself can be a source of asymmetric induction in solid-state polymerization [69-72], Prochiral monomers such as 37 give enantiomorphic crystals, one of which can be preferentially formed by recrystallization with a trace amount of optically active compounds. Photoir-... 

Taking into account the above basic definitions, let us consider the structure of polymers formed in the polymerisation of prochiral monomers such as propylene or acetaldehyde ... 

Figure 22-11 Stereochemistry of successive propane insertion steps into M—R bonds to give isotactic polypropene (a) and syndiotactic polypropene (b). In the absence of a stereocontrol mechanism atactic polypropene is formed. Note that in (a) the prochiral monomers coordinate to the metal with the same ff-face, so that an isotactic polymer is formed. This stereochemistry is favored by C2-symmetric ligands of type (22-XXVIII). In reaction (b) the second monomer coordinates with the opposite w-face to the first this stereochemistry is enforced by metallocenes of Cs-symmetry (22-XXX, R = H).

The effect of the ligands is to sterically control the stereochemistry of monomer coordination prior to insertion, as indicated in Fig. 22-11. For C2-symmetric complexes the coordination pocket may be represented by two hindered and two open quadrants (c/. scheme 22-XI). A prochiral monomer such as propene will adopt the orientation where repulsive interactions between the propene-methyl group, the ligand framework, and the growing polymer chain are minimized. With rac-bis(indenyl)metallocene complexes this mechanism necessarily results in an isotactic polymer. 

A host of further issues complicate catalyst performance for ROMP reactions. Intrinsic polymer characteristics are not just dependent on the nature of the monomer and/or comonomer, but are also highly dependent on the cis, Irons sequence of double bonds along the polymer chain, as well as on the tacticity of the polymer if a chiral or prochiral monomer is used, since the latter reflects the stereochemical sequence by which the chiral centres are linked. [See Chapter 7 and J. G. Hamilton in Handbook of Metathesis, Volume 3 , R. H. Grubbs ed., Wiley-VCH, Weinheim, 2003]. 

For stereospecific polymerization of a-olefms such as propene, a chiral active center is needed, giving rise to diastereotopic transition states when combined with the prochiral monomer and thereby different activation energies for the insertion (see Figure 2). Stereospecificity may arise form the chiral /0-carbon atom at the terminal monomer unit of the growing chain - chain end control - or from a chiral catalyst site - enantiomorphic site control . The microstructure of the polymer produced depends on the mechanism of stereocontrol as well as on the metallocene used [42-44]. 

Due to the fact that the polymer chain migrates during insertion, the symmetry of the metallocene is of fundamental importance to the tacticity of the polymer produced. C2-symmetric metallocenes such as the bridged bis(indenyl) compounds mentioned above have homotopic coordination sites and thereby always favor the same orientation of the prochiral monomer during the approach. This leads to the formation of an isotactic polymer (Figure 3). 

While ethylene insertion can occur in a single mode, insertion of a-olefins can occur in the four geometrically different modes represented in Scheme 6. Thus, polymerization of prochiral monomers requires the definition of a few terms. 

It is well accepted that two mechanisms of stereocontrol (the chiral induction responsible for selecting the monomer enantioface) are operative in stereoselective a-olefm polymerizations. In the simpler cases, the discrimination between the two faces of the prochiral monomer may be dictated either by the configuration of the asymmetric tertiary C atom of the last inserted monomer unit or by the chirality of the catalytic site. These two different mechanisms of stereocontrol are named chain-end stereocontrol and enantiomorphic-site or site stereocontrol. In the case of chain-end stereocontrol, the selection between the two enantiofaces of the incoming monomer is operated by the chiral environment provided by the last inserted tertiary C atom of the growing chain, whereas in the case of site stereocontrol this selection is operated by the chirality of the catalytic site. The origin of stereocontrol in olefin polymerization has been reviewed extensively.162,172-178... 

Stereoregular polymerization requires that the faces of the prochiral monomer must have a different reactivity toward one given chiral reactive site. By using 3c NMR to examine the stereochemical sequences of the configurations of the monomer units of polypropyl-... 

Introduction of relatively weak functional groups, such as carbonyl, hydroxyl, nitro, amide, etc., in the nanochannels of PCPs would affect the monomer alignment, which may lead to precision control of stereoselectivity and regioselectivity of the resulting polymers. In particular, PCPs with either helical or chiral structures on the pore surface are of intense interest in chemistry and such porous solids are potentially useful to find applications in enantioselective sorption/separation and catalysis [34, 38 0, 42, 45]. Of considerable interest is the use of the chiral channels to affect asymmetric polymerizations such as asymmetric selective polymerization of racemic monomers as well as asymmetric polymerization of prochiral monomers, which may give helical polymer conformations. 

A prochiral monomer molecule is converted to a polymer with chiral groups in prochiral stereospecific or chirality-producing stereospecific polymerizations. An example of this is the polymerization converting benzo-furan to optically active polymers with the catalyst RAlCl2/optically active phenyl alanine. 

As far as polymer stereochemistry is concerned, a controversial issue is what should be defined as stereoblock-isotactic Isotactic polypropylene is usually obtained as a result of site control (i.e., the preference of an intrinsically chiral transition metal active species to react with one of the two enantiofaces of the prochiral monomer). In the case of a simple C2-symmetric single-center catalyst with homotopic active sites, if we denote as o the probability that the monomer inserts with a given enantioface at an active site of given chirotopicity, the fractions [m] and [r] of meso and racemo diads in the polymer are given by Equations 8.1 and 8.2... 

A prochiral monomer such as propylene offers two faces for coordination to a metal center. The steric environment at the active site, formed by the coordinated ligands and the growing polymer chain after activation with a cocatalyst, determines the orientation of the incoming monomer. In this case, the mechanism of stereoselection is referred to as enantiomorphic site control. The stereochemistry of the polymer is thus determined by the chirality relationship of the two coordination sites of the catalyst. However, every monomer insertion generates a new stereogenic center. As a consequence, chiral induction (enantioface preference) arises from the last-inserted monomer unit in the growing polymer chain. This mechanism is referred to as chain-end control (see Chapter 1 for an introduction to chain-end and enantiomorphic site control mechanisms in iPP synthesis). 

Complexes with C -symmetry (e.g., 2, Figure 9.6) have an internal vertical mirror plane bisecting the ligand from back to front. The two coordination sites formed after activation are mirror images, and therefore show opposite selectivity for the coordination and insertion of the prochiral monomer. This means that the preferred propylene face for coordination changes after every insertion step, which affords a syndiotactic polypropylene microstructure. 

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