Trimer/Tetramer
Side group replacement reactions carried out on high polymers (macromolecular substitution) are more complex than their counterparts at the small molecule level. For example, an average of 30,000 chlorine atoms must be replaced by organic groups in a typical polyphosphazene synthesis reaction. For this reason, it is our normal practice to develop new substitution reactions with the use of small molecule model compounds such as those shown below. This provides a means to study the effects of, for example, steric hindrance on reaction patterns and side reactions. Once these small molecule reactions are understood they can be transposed to the high polymer level. The organic-substituted small molecules can be studied by NMR, mass spectrometry and X-ray crystallography to obtain structural information that is difficult or impossible to obtain for the high polymers.
Cyclic Trimer, Cylic Tetramer and Linear Short Chain
The following example illustrates this procedure. One objective of our materials program is the synthesis of transparent polymeric glasses with high refractive indices for use in electro-optical devices. An approach explored in our laboratory is to link fused ring aromatic organic side groups to the polyphosphazene chain to generate macromolecules with a high loading of delocalized electrons. However, it was not known if such bulky groups could replace all the chlorine atoms along the phosphazene polymer chain, or if those side groups would pack together to form crystalline domains, and thus give rise to opacity.
Naphthoxyphosphazene Cyclic Trimer
The conditions needed to carry out the synthesis of the naphhthyloxyphosphazene cyclic trimer were determined. Crystals were grown and the X-ray crystal structure obtained. This yielded bond angles, bond lengths and packing data that were later used to understand the behaviour of a high polymer with the same side groups. Other side units, such as 1-naphthoxy, 1-naphthylmethoxy, 9-anthraceneoxy, and 9-phenanthreneoxy were studied in the same way. This work led eventually to the synthesis of the corresponding high polymers and to the study of their as high refractive index properties.