Ring Formation in Linear Stepwise Polymerization

We have found the formation of small rings in stepwise polymerization of a vinyl-end-capped bifunctional poly(dimethylsiloxane) (PDMS) and 2-, 3-, and 4-functional siloxanes. Low molecular weight reactants (approximately 200) were used to enhance ring formation. We have shown the number of rings increases in dilute solutions and with equimolar concentrations of reactants. The amount of each lower molecular weight species in the product distribution was determined by gas chromatography (GC) and actual identification of the products was performed by nuclear magnetic resonance (NMR) and mass spectroscopy-gas chromatography (MS-GC). Results compare favorably with a simple kinetic model. A Gaussian model overpredicts the concentration of larger rings. We believe this is the first study to actually measure rings in a nonequilibrium polymerization.