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.

中文翻译：

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揭示基于乙基哌嗪的二硫代氨基甲酸酯链转移剂的两性离子性质，以实现高摩尔质量的聚乙酸乙烯酯：实验和计算见解


醋酸乙烯酯 （VAc） 是活化程度较低的单体 （LAM） 中活性最低的未共轭单体之一，导致高反应性和不稳定的传播自由基。获得明确定义的高摩尔质量聚醋酸乙烯酯 （PVAc） 的一个重大障碍是自由基聚合过程中发生的链转移反应。为了实现 VAc 的受控聚合，我们采用了两种不同的基于二硫代氨基甲酸酯的链转移剂 CTA-1 和 CTA-2，分别含有乙基哌嗪和吡咯作为 Z 稳定基团。在 CTA-1 中，氮原子上的孤对电子没有离域，降低了 CS 键的双键特性并增强了其两性离子性质。相反，在基于吡咯的 CTA-2 中，氮的孤对电子将在 Z 族的芳香族体系内离域，从而导致两性离子性质较低。CTA-1 对 VAc 聚合表现出优异的控制，产生 132 kDa 的高摩尔质量和 1.31 的低分散度。相比之下，CTA-2 在尝试高摩尔质量 VAc 聚合时表现出较差的控制。CTA-1 和 CTA-2 聚合行为的这种可观察到的变化可归因于稳定 Z 基团的氮上孤对电子的可用性，它通过降低其双键特性使硫代羰基失活。为了了解根本原因，进行了从头计算和 DFT 计算，以研究 CTA-1 和 CTA-2 的中性或两性离子形式。 测量了两个 CTA 的这些 RAFT 加合自由基中每个原子上的 Mulliken 电荷和 S-R 键的β断裂。结果表明，CTA-1 以两性离子形式存在，而 CTA-2 保持中性形式，这解释了 CTA-1 在 VAc 聚合中的优异性能。还计算了 CTA-1 和 CTA-2 的 RAFT 加合自由基的 S-R β切割的其他热力学方面以支持上述假设。