Mapping poly(butyl acrylate) product distributions by mass Spectrometry in a wide temperature range: Suppression of midchain radical side reactions

The polymer product spectra of poly(butyl acrylate), polyBA, samples obtained by full conversion bulk polymerization in the temperature range of 60-140 degrees C in presence of the chain transfer agent (CTA) octylthiol is analyzed via electrospray ionization-mass spectrometry (ESI-MS) and quantitative C-13 NMR. The combination of both these analytical techniques allows for very detailed information on the reaction products associated with the formation of midchain radicals to be obtained. At 60 degrees C, highly uniform polyBA is generated that consists of the structure alkyl-S-(BA)(n)-H. With increasing temperature, several other peaks emerge that can be assigned to either conventional termination product or to the three different beta-scission products of the so-called midchain radicals that are known to be formed in acrylate polymerization via transfer to polymer reactions. Because of the formation of beta-scission products that were formed by scission to either side of the radical functionality, intermolecular transfer to polymer presumably plays an important role in the present system. For the 140 degrees C sample, 1.37 mol % of quaternary carbons on the polymer backbone are identified with only a small fraction of the chains carrying an unsaturated end group. As much higher levels of branching and unsaturation are expected under the same conditions from conventional free radical polymerization, it is concluded that the chain transfer agent is capable of suppressing the midchain radical side reactions to a large extent.