On the Mechanism of the Inverse Vulcanization of Elemental Sulfur: Structural Characterization of Poly(sulfur-random-(1,3-diisopropenylbenzene))

Organosulfur polymers, such as those derived from elementalsulfur,are an important new class of macromolecules that have recently emergedvia the inverse vulcanization process. Since the launching of thisnew field in 2013, the development of new monomers and organopolysulfidematerials based on the inverse vulcanization process is now an activearea in polymer chemistry. While numerous advances have been madeover the last decade concerning this polymerization process, insightsinto the mechanism of inverse vulcanization and structural characterizationof the high-sulfur-content copolymers that are produced remain challengingdue to the increasing insolubility of the materials with a highersulfur content. Furthermore, the high temperatures used in this processcan result in side reactions and complex microstructures of the copolymerbackbone, complicating detailed characterization. The most widelystudied case of inverse vulcanization to date remains the reactionbetween S-8 and 1,3-diisopropenylbenzene (DIB) to form poly-(sulfur-random-1,3-diisopropenylbenzene)-(poly-(S-r-DIB)). Here, to determine the correct microstructure of poly-(S-r-DIB), we performed comprehensive structural characterizationsof poly-(S-r-DIB) using nuclear magnetic resonancespectroscopy (solid state and solution) and analysis of sulfuratedDIB units using designer S-S cleavage polymer degradation approaches,along with complementary de novo synthesis of thesulfurated DIB fragments. These studies reveal that the previouslyproposed repeating units for poly-(S-r-DIB) were incorrectand that the polymerization mechanism of this process is significantlymore complex than initially proposed. Density functional theory calculationswere also conducted to provide mechanistic insights into the formationof the derived nonintuitive microstructure of poly-(S-r-DIB).