Stereochemistry polymer conformation

High-resolution proton NMR spectroscopy can furnish valuable information about the chemical structure, regiochemistry, stereochemistry, and conformation of a polymer. Model compounds are helpful for assigning peaks in the NMR spectra of polymers. Although data from the monomer and dimer models can be presumed to be transferable to the polymers, some unique features in the proton spectra of the polymers can give insight into the polymer structure. 

We have designed PBUILD, a new CHEMLAB module, for easy construction of random copolymers. A library of monomers has been developed from which the chemists can select a particular sequence to generate a polymeric model. PBUILD takes care of all the atom numbering, three dimensional coordinates, and knows about stereochemistry (tacticity) as well as positional isomerism (head to tail versus head to head attachment). The result is a model of the selected polymer (or more likely a polymer fragment) in an all trans conformation, inserted into the CHEMLAB molecular workspace in literally a few minutes. 

The transition state was shown to have a four-centered nonplanar structure and the product showed a strong jS-agostic interaction.59 Molecular-mechanics (MM) calculations based on the structure of the transition state indicated that the regioselectivity is in good agreement with the steric energy of the transition state rather than the stability of the 7r-complex. The MM study also indicated that the substituents on the Cp rings determine the conformation of the polymer chain end, and the fixed polymer chain end conformation in turn determines the stereochemistry of olefin insertion at the transition state.59... 

The butadiene polymers represent another cornerstone of macromolecular stereochemistry. Butadiene gives rise to four different types of stereoregular polymers two with 1,2 linkage and two with 1,4. The first two, isotactic (62) and syndiotactic (25), conform to the definitions given for vinyl polymers, while the latter have, for eveiy monomer unit, a disubstituted double bond that can exist in the two different, cis and trans, configurations (these terms are defined with reference to the polymer chain). If the monomer units all have the same cis or trans configuration the polymers are called cis- or trans-tactic (30 and 31). The first examples of these stereoisomers were cited in the patent literature as early as 1955-1956 (63). Structural and mechanistic studies in the field have been made by Natta, Porri, Corradini, and associates (65-68). 

All this is true both for low molecular weight compounds and for polymers. However, relevant differences are found not only in specific structural features, but also in the different intent with which the analysis is carried out. In common otganic stereochemistry, attention is essentially on reactivity Many aspects of regio- or stereoselectivity, as weU as reaction rate, can be explained or predicted in terms of conformation. In contrast, in polymers, the focus of interest is almost always on the physical (or mechanical) properties. It should not be forgotten that polymers are principally used as materials (plastics, fibers, elastomers, etc.) and that their properties are sometimes modified during fabrication. The fabri-cation process may induce the formation of particular structures at the molecular and supetmolecular level, which can often be traced back to conformational factors. 

Numerous articles relevant to polymer stereochemistry have appeared since this chapter was completed, most of them dealing with conformational analysis, spectroscopy, and chirality. In this addendum I shall discuss only a few items pertaining to the optical activity of rigid polymers. This matter has recently received a lot of attention and merits a more detailed discussion than was presented earlier. 

Starch exists in two major forms, amylose and amylopecttn. Although amylose is a linear polymer like cellulose, the a stereochemistry enables the chains to twist and to assume a helical shape. Small molecules such as l2 fit inside the helix and form a complex. This is the basis for the deep blue color in the starch-iodine test. This non-straight or bent conformation is sug-... 

The structure of ligands in metallocene complexes determines activity, stereoselectivity, and molecular weight of 1-alkene polymerizations, by controlling the preferential conformation of the growing polymer chain which in turn controls the stereochemistry of monomer coordination ( enantiomorphic site control ). The difference between this and the chain-end control mechanism mentioned earlier is that stereo errors due to misinsertions can be repaired.101,106... 

For trans-2,3-butene oxide and trans-l,4-dichloro-23-butere oxide, in order to understand the formation of disyndtotactic polymers, it is sufficient to assume a stereoselective process, i. e. that the active species selects a moncmia with a stereochemistry resembling its own Two different notattons are used in (xrder to conform with original papers... 

An isotactic structure is one in which the optically active centers of the repeat units all have the same absolute stereochemistry (G. Natta, P. Pino, P. Corradini, F. Danusso, E. Mantica, G. Mazzanti, G. Moraglio, J. Am. Chem. Soc. 1955, 77, 1708) in a syndiotactic polymer, neighboring units have opposite stereochemistry. If an isotactic polyolefin is drawn in its extended conformation, it will have all its substituents pointing in the same direction. If an isotactic polyketone is drawn in its extended conformation, the substituents will alternately point up and down. 

For polymeric chains, the act of coordination almost invariably requires a change in the shape of the chain as a consequence of satisfying the demands of the metal ion for a preferred stereochemistry and a set of donors with bond distances within a limited range. Thus, coordinate bond formation has consequences that clearly alter the local environment around the metal ion, but may also alter polymer chain conformation over an extended range. Since three-dimensional shape in biopolymers plays a role in function, natural complexation evolved by Nature usually plays a positive role, whereas unnatural complexation through the addition of foreign metal ions may be deleterious to function. 

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