Stereochemical configuration, studies

Another important application of NMR to polymer systems is the elucidation of the stereochemical configurations of Polymer chains. Poly (methyl methacrylate) was first studied by Bovey in 1960. It is now possible to analyse for the statistical frequency of occurrence of all possible combinations of up to four successive pairs of units (dyads) capable of occurring with either the same (meso) or opposite (racemic) configurations. 

In the last decade, the advent of ligand-field theory has given a tremendous impetus to the study of transition metals, and of these, nickel has been among the most studied. This element can have a variety of stereochemical configurations under various conditions, and the aim of this chapter is to summarize the position of our knowledge up to the early part of 1961. It is necessary to give such a date because the field is very active at the time of writing, and new developments are to be anticipated. Palladium and platinum have not received quite so much attention as nickel from the structural point of view the trans-effect should be mentioned, however, and has been reviewed by Basolo in Volume 3 of this series. 

Very recently, Miller, Brey, and Butler (40) have carried out a very similar study of the stereochemical configuration of polymetha-crylic anhydride and have reported results which are at. considerable variance with ours. In particular, they have reported a very much more marked increase in the proportion of isotactic triads with increasing temperature of polymerization, a reaching values as high as 0.7—0.8. The source of this discrepancy remains to be discovered. 

We have carried out a preliminary study (61) of methyl methacrylate-styrene and methyl methacrylate-a-methylstyrene copolymers which indicates that high resolution NMR spectroscopy of solutions of copolymers can yield valuable information concerning both sequence lengths and stereochemical configurations of the monomer units. At the present stage of this work, however, it is not possible to disentangle sequence and configurational effects satisfactorily, and very much remains to be done. 

The obvious advantage of our photochemical procedure is that it allows the formation of 3 under mild conditions at very low temperature. Thereby, thermally rather unstable derivatives of 3 may be generated and their reactions studied.- This enabled us, for example, to show that stereochemical configurations at the diene termini are retained during photolysis, but may be lost by subsequent, rather rapid thermally induced intramolecular rearrangement steps. For example, irradiation of 10 in the presence of pure (Z,Z)-2,4-hexadiene at -60°C yields exclusively s-trans-Tj -(Z,Z)-2,4-hexadiene)zirconocene [3h-(Z,Z)]. At -40°C, the stereochemical information at one terminal diene carbon center is lost. A clean... 

Mechanistic studies on these four rearrangements have revealed that (i) these are concerted intramolecular rearrangements (ii) stereochemical configurations of the migrating groups are completely retained (iii) the kinetics are first order and (iv) neither free carbonium ions nor radical intermediates are formed during the rearrangements. 

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