Polythioamides of High Refractive Index by Direct Polymerization of Aliphatic Primary Diamines in the Presence of Elemental Sulfur

Sulfur-containing polymers have renewed widespread attention due to their fascinating properties like high refractive index and semiconducting character. However, examples of direct polymerization involving elemental sulfur are limited. Herein, a new strategy to prepare polythioamide (PTA) by direct polymerization of aliphatic primary diamines in the presence of sulfur is reported. The polymerization of <i>p</i>-xylylenediamine (<b>1</b>) and sulfur at 110 °C in <i>N</i>-methyl-2-pyrrolidinone afforded PTA1 of high <i>M</i>_w and high yield when the feed ratio of [<b>1</b>]:[S] ranged from 1:2 to 1:3. ¹H NMR and ¹³C NMR spectra confirmed that there are three kinds of structural units among the PTA1 chains. Moreover, different diamines including <i>m</i>-xylylene­diamine (<b>2</b>), 1,6-hexane­diamine (<b>3</b>), ethylene­diamine (<b>4</b>), and 1,4-cyclohexane­diamine (<b>5</b>) were copolymerized with <b>1</b> in the presence of sulfur to obtain PTA copolymers. With an increase in <b>5</b> content, the copolymer PTA1/5 with an alternating sequence in the range of 56%–94% was prepared. Solubility and thermal properties of homopolymers and copolymers were studied. Meanwhile, the copolymers PTA1/3 and PTA1/5 possessed a high refractive index as high as 1.7