Mechanistic aspects of the cationic polymerization of vinylformamide

The electron density distributions of the four possible configurational iso mers of the monomer vinylformamide (VFA) and of the cationically propagatin g species were examined by means of quantum chemical calculations. They sho w that the electrophilic attack of the propagating chain at the carbonyl ox ygen of the VFA monomer is kinetically controlled whereas the attack at the C=C bond of the VFA is thermodynamically controlled. For experimental prov ing of this theoretical argument, VFA is cationically polymerized in toluen e at different reaction temperatures by means of iodine, trifluoromethanesu lfonic acid (HOTf), and trimethylsiliyl triflate (TMST) as initiators. Head group functionality and molecular weight distribution (MWD) of the oligo(v inylformamide) (OVFA) were investigated by means of H-1 NMR spectroscopy, m atrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS), gel permeation chromatography (GPC), and combustion analys is. In all cases, OVFAs with narrow MWD were obtained in moderate yield (5- 50%) in the temperature range from 253 to 313 K. A reaction temperature bel ow 253 K is not suitable for the cationic polymerization of VFA because no oligomeric products are obtained. The properties and chemical constitution of the OVFAs, such as the average molecular weight, yield, and the chain st ructure are strongly determined by the reaction temperature used. With incr easing reaction temperature the head group functionality decreases and the yield of OVFA increases. The correspondence between the results of quantum chemical calculations with those of the experiments is discussed.