Stereochemical sequences

In the previous section, the adaptation of the RIS model was based on the distance between next-nearest neighbor beads. This approach is obviously inadequate for CH3-CHX-CH2-CHX-CH3, because it necessarily abandons the ability to attribute different conformational characteristics to the meso and racemo stereoisomers. Therefore a more robust adaption of the RIS model to the 2nnd lattice is necessary if one wants to investigate the influence of stereochemical composition and stereochemical sequence on vinyl polymers [156]. Here we describe a method that has this capability. Of course, this method retains the ability to treat chains such as PE in which the bonds are subject to symmetric torsion potential energy functions. 

For a simulation of PP, the relationship between the 2nnd lattice and its underlying diamond lattice must be established at the beginning of the simulation, in order to preserve the stereochemical sequence and its influence on the conformations of the chains. The half of the equilateral triangles of area L2/2 that produce local collapsed beads is therefore known at the start, and the simulation can be performed in a manner which avoids the formation of these unphysical structures [158]. 

Organoiron complexes (7) are converted in high yield into ammonium salts (8) these in turn undergo oxidatively induced ligand transfer and cyclization to give azetidinones (9) in moderate yields (Scheme 9). Formation of the trans product (9b) indicates a stereochemical sequence of trans addition to the olefin complex followed by carboxamidation with retention of configuration at the C—Fe bond. 

The spectrum of 6, 20 and 40 wt. % PEMA solutions at 34 C are shown in Figure 1. All of the resonances are easily discernible except for the backbone methylene at 40%. At low concentration the poljimer a-CH3, quaternary carbon, and backbone methylene carbon exhibit resolved or partially resolved chemical shifts due to the various stereochemical sequences since the pol3rmer was not stereoregular. A rough estimate indicates the polymer is essentially atactic. 

Statistical analysis of the stereochemical sequence distributions (Table 8-3 and Sec. 8-16) also supports the enantiomorphic site control model. 

The polymer chain end control model is supported by the observation that highly syndiotactic polypropene is obtained only at low temperatures (about —78°C). Syndiotacticity is significantly decreased by raising the temperature to —40°C [Boor, 1979]. The polymer is atactic when polymerization is carried out above 0°C. 13C NMR analysis of the stereoerrors and stereochemical sequence distributions (Table 8-3 and Sec. 8-16) also support the polymer chain end control model [Zambelli et al., 2001], Analysis of propene-ethylene copolymers of low ethylene content produced by vanadium initiators indicates that a syndiotactic block formed after an ethylene unit enters the polymer chain is just as likely to start with an S- placement as with an R-placement of the first propene unit in that block [Bovey et al., 1974 Zambelli et al., 1971, 1978, 1979]. Stereocontrol is not exerted by chiral sites as in isotactic placement, which favors only one type of placement (either S- or R-, depending on the chirality of the active site). Stereocontrol is exerted by the chain end. An ethylene terminal unit has no preference for either placement, since there are no differences in repulsive interactions. 

As with organic polymers, inorganic polymers can be crystalline, amorphous, or glassy. The ease with which macromolecules can pack together into a regular array wUl depend on the stereochemical sequence of the backbone s monomeric units. This is worth looking at a little more closely with the more familiar organic polymers. In an... 

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]. 

PMPS, PMHS, PMDS, and PDHS. Individual resonances are observed for each carbon type, except for the dodecyl polymer in which C-4 to C-9 in the side chain are not completely resolved. The carbon resonances can also be assigned without the use of model compounds by using standard two-dimensional NMR techniques (2). C-1 and, in some cases, C-1, C-2, or both in the three asymmetrically substituted polymers show shift dispersion, which results from the many different stereochemical sequences along the polymer chain. The methyl carbon in PMPS is resolved to at least the pentad level of stereosequences. This chemical shift information can be analyzed to provide a description of the chain statistics resulting from a particular polymerization. 

Spectra of model compounds are also utilized to assign stereochemical sequences in polymer.182 However, model compounds with defined stereochemical configurations are rather difficult to obtain. Zambelli et a/.183 reported that heptad models for polypropylene, 3(5), 5(R), 1 (RS), 9(RS),... 

The properties of a copolymer depend on its composition, monomer sequence and stereochemical structure. Although compositional analysis can be achieved by several methods other than NMR spectroscopy, quantitative data on monomer sequence distribution can only be obtained from NMR spectroscopy. I3C NMR chemical shifts of C=0 carbons of PMMA are sensitive to pentad to heptad stereochemical sequences. The C=0 carbon signals for the copolymers of methacrylates are also sensitive to triad comonomer sequence. Thus it should be difficult to assign both tactic and comonomer sequence signals, especially in the case of copolymers with low stereoregularity. 

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-... 

When the method was applied to one-component and two-component"" melts of polypropylene chains of specific stereochemical sequence, reptation was included along with the single-bead moves, in order to achieve equilibration of the melts on an acceptable time scale. The polypropylenes (especially syndiotactic polypropylene) equilibrate slowly if the simulation uses single bead moves only. 

Vinyl polymers, for which pol5rpropylene serves as a prototype, present some additional issues not encountered in chains with symmetric torsions. The physical properties of these chains depend on the stereochemical composition and stereochemical sequence of the chain, and this dependence must be reflected in Z. Two equivalent methods have been used for description of the stereochemistry of vinyl polymers. One approach uses pseudoasymmetric centers [67]. Although the fragment denoted by -CH2-CHR-CH2- does not contain a chiral center, it can be treated as though it were chiral if one CH2 group is distinguished from the other. This distinction is drawn when the bonds in the... 

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. 

Literature citations for RIS models for several vinyl polymers, as well as related polymers for which stereochemical compositions and stereochemical sequences are issues, are summarized in Table 3.4. 

Table 3.4. RIS models for selected vinyl polymers and related polymers for which stereochemical composition and stereochemical sequence are Issues. Chains are listed in the order of the molecular weight of their repeat unit.

The stereochemical composition of vinyl polymers after epimerization to stereochemical composition can be determined from the information contained in a more elaborate form of Z that takes account of the conformations of all stereochemical sequences, with all sequences weighted with respect to the same definition for the zero point of the conformational energy [136,137]. 

This chain contains a truly chiral centre and a chain with only a single configuration would be optically active. As far as NMR is concerned, it is the relative configuration that is important, and as for vinyl polymers, the stereochemical sequence structure may be described in terms of m (same configuration) and r (opposite configuration) dyad building blocks repre-... 

Expressions for the mole fractions of longer sequences can be built up in a similar fashion. Expressions for first-order Markov dyad and triad distributions are given in Table 2.3, and can be compared with the analogous expressions for stereochemical sequence distributions in chapter 1. 

---