A NOVEL WAY TO STUDY THE INITIAL-STAGES OF SOAP-FREE EMULSION POLYMERIZATIONS - THE INTERCALATION OF POLYSTYRENE OLIGOMERS INTO HYDROTALCITE

The dominant species in the early stages of an emulsifier-free emulsio n polymerization of styrene has been found to be an oligomer of two to three monomer units using a novel trapping technique. This involved t he intercalation of charged primary oligomers between the layers of a hydrotalcite, [Mg4Al2(OH)(12)](2+)[A](2-) (where A = dianion). Hydrota lcites are an important class of lamellar, inorganic compounds whose i nterlayer spacing can be modified by anion exchange. Our approach firs t involved preparing a hydrotalcite precursor in which the layers were propped apart by an organic dianion (terephthalate = TA). This materi al was then used to capture the negatively charged polystyrene oligome rs from the emulsion polymerization reaction mixture. We found that TA was rapidly ion-exchanged for the charged oligomers. The resulting pi llared hydrotalcite material was characterized using XRD and SEC. We f ound that the interlayer spacing between the hydroxide layers increase d to 23.2 Angstrom on exposure to the emulsion reaction mixture. This represents an interlayer expansion of 18.3 Angstrom (after subtraction of the hydroxide layer contribution), which is consistent with interc alation of oligomers with two to three monomer units arranged in a bil ayer. This size estimate was confirmed by the results of size exclusio n chromatography. (C) 1995 John Wiley and Sons, Inc.