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 p-xylylenediamine (1) and sulfur at 110 °C in N-methyl-2-pyrrolidinone afforded PTA1 of high Mw and high yield when the feed ratio of [1]:[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 m-xylylenediamine (2), 1,6-hexanediamine (3), ethylenediamine (4), and 1,4-cyclohexanediamine (5) were copolymerized with 1 in the presence of sulfur to obtain PTA copolymers. With an increase in 5 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.