Racemo diads

Note 2 A syndiotactic macromolecule consists of racemo diads. 

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). 

Stereoregularity. Representative stereoregularities of the two adjacent constitutional units in the homopolymer and co-polymers of terminal epoxides and symmetrical m-epoxides are listed in Figure 1. A diad is described as meso m) if the absolute configurations of the two adjacent constitutional units are the same, and racemo (r) if they are the... 

Diads (triads, pentads) View of two (three, five) combined monomer units at any position in a polymer chain and investigating (e.g., by 13C NMR spectroscopy) the configuration of the tertiary carbon atoms. The term meso is used if they have the same configuration the term racemo is used if they have the opposite configuration. 

Figure 8. NMR spectrum in CDCI3 of poly(1,2-epoxypropane) prepared by the polymerization with chloroaluminum tetraphenyl-porphyrin (1c). m, r, I, S and H represent meso, racemo (diad) and isotactic, syndiotactic, and heterotactic (triad) sequences, respectively.

Although the analysis of [97] was not exhaustive, it succeeded in identifying extended conformations of minimum energy for stereoirregular model chains of PAN in which the torsion angles are close to 180° in racemo diads and may deviate significantly from 180° in meso diads. As an example, locally extended stretches of rmr tetrads and rmmr pentads in a minimum energy conformation are shown in Fig. 11. 

Fig. 20 (A,B) Meso- and (C,D) racemo- norbornene-ethylene-norbornene (N-E-N) sequences, in the lowest energy conformation. R and S indicate the configuration of the chiral carbon atoms of the norbornene units. Depending on the succession of the configurations R and S the meso diads may assume Z- or S-shapes (A,B)> whereas the racemic diads may assume a C shape with different orientation (C,D). (Reprinted with permission from [121], Copyright 2003 by the American Chemical Society)...

When pseudoasymmetric centers are used for the description of the stereochemical sequence, six distinct statistical weight matrices, Eqs. (3.18)-(3.20), are required. They can be replaced by a total of three statistical weight matrices, denoted by Up,Um, and Ur, if the stereochemical sequence is described instead as a sequence of meso and racemo diads. 

There are four possible combinations of regiochemistry and stereochemistry within diad units. The olefins can join in a head-to-head or head-to-tail fashion. Most polyolefins formed by early metal catalyts are formed by strict head-to-tail enchainment, but this head-to-tail enchainment can occur by a series of 1,2-insertions in which the a-olefin substituent is located P to the metal in tihe insertion product or 2,1-insertions in which the a-olefin substituent is located a to the metal in the insertion product. In addition, the olefins can join to give rise to a diad unit containing identical (meso, m) or opposite (racemo, r) stereochemical relationships to the last inserted monomer unit. Site control of polymerization to form isotactic polymer gives rise to rr-defects m the polymer from stereoerrors, but cham-end control of polymerization to form isotactic polymer gives rise to r-defects from stereoerrors. 

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... 

About the nomenclature of like and unlike The meso and racemo nomenclature commonly used for vinyl monomer diads, are not applicable to the head-to-tail polyketone because the junchon unit between the two stereocenters, -CH2-C(=0)-, is not symmetric. Poly(a-amino acid)s and poly(propylene oxide) are other examples of polymers with asymmetric (W. V. Metanomski, Compendium of Macromolecular Nomenclature, lUPAC Macromolecular division). Accordingly, the words like and unlike, which are used in organic chemistry, are applied like (/) is used for the diad consisting of the same configuration (analogous to meso) and unlike ( ) for the opposite (analogous to racemo). 

In syndiotactic or syntactic macromolecules, the substituents have alternate positions along the chain (Figure 20.1). The macromolecule consists of 100% racemo diads. Syndiotactic polystyrene (sPS), made by metallocene catalysis polymerization, is crystalline with a melting temperature of 161°C. 

A pair of consecutive, but not necessarily contiguous, tetrahedral stereoisomeric centers defines a diad [16]. Stereosequences terminating in tetrahedral stereoisomeric centers at both ends, and which comprise two, three, four, five, and so on, consecutive centers of that type, may be called diads, triads, tetrads, pentads, and so on, respectively. If the two units belonging to a diad have the same configuration, the diad is defined meso (m) and is characterized by a mirror plane of symmetry if the two units are enantiomorphous, the diad is defined racemo (r) and is characterized by a twofold rotation axis of symmetry (Fig. 2.2b). 

Statistical weight matrices generally applicable to the meso and racemo diads of a vinyl polymer have been developed by Flory and his collaborators. Adopting their notation, for PAVE, we have for the meso diad. 

Addition of monomer to a pm-meso conformation will lead to formation of a meso diad (m), while addition to a pro-racemo configuration forms a racemo (r) diad. The polymer terminus can interconvert between pro-meso and pro-racemo conformations via rotation of the terminal ( CH2—CHR ) sp —sp bond. This torsion does not dismpt stabilizing conjugation between the radical and terminal side-chain (R). Thus, it is anticipated that for most... 

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