Unveiling the zwitterionic nature of an ethyl piperazine-based dithiocarbamate chain transfer agent for achieving high molar mass polyvinyl acetate: experimental and computational insights

Abstract

Vinyl acetate (VAc) is one of the least active unconjugated monomers among the less activated monomers (LAMs), resulting in highly reactive and unstable propagating radicals. A significant obstacle in achieving well-defined high molar mass polyvinyl acetate (PVAc) is the occurrence of chain transfer reactions during radical polymerization. Towards the controlled polymerization of VAc, we have employed two different dithiocarbamate-based chain transfer agents CTA-1 and CTA-2 bearing ethyl piperazine and pyrrole as Z stabilizing groups, respectively. In CTA-1, the lone pair of electrons on the nitrogen atom is not delocalized, reducing the double-bond character of the CS bond and enhancing its zwitterionic nature. Conversely, in the pyrrole-based CTA-2, the nitrogen's lone pair of electrons will be delocalized within the aromatic system of the Z group, resulting in a less zwitterionic nature. CTA-1 exhibited excellent control over VAc polymerization, yielding a high molar mass of 132 kDa with a low dispersity of 1.31. In contrast, CTA-2 showed poor control when attempting high molar mass VAc polymerization. This observable change in the polymerization behaviour with CTA-1 and CTA-2 can be attributed to the availability of a lone pair of electrons on the nitrogen of the stabilizing Z group, which deactivates the thiocarbonyl group by reducing its double bond character. To understand the underlying reasons, ab initio and DFT calculations were conducted to investigate the neutral or zwitterionic forms of CTA-1 and CTA-2. The Mulliken charges on each atom and the β-scission of the S–R bond in these RAFT-adduct radicals of both CTAs were measured. The results revealed that CTA-1 exists in a zwitterionic form, while CTA-2 remains in a neutral form, explaining the superior performance of CTA-1 in VAc polymerization. The other thermodynamic aspects of the S–R β-scission of RAFT-adduct radicals of CTA-1 and CTA-2 were also calculated to support the above hypothesis.