Accessing Increased Sustainability in Commodity Polymers: Post-Polymerization Modification of Polybutadiene and Mechanochemical Synthesis of Polyacrylates

Abstract

As plastics and polymer materials are integral to modern life, wasted polymerproducts continue to accumulate and cause environmental damage. Further harm is done through the resource-intensive synthesis of virgin materials from petroleum feedstocks under harsh conditions. Through this work I show two parallel efforts to increase the sustainability of commodity polymers; firstly, I focus addressing the end-of-life accumulation polymer waste by the post-polymerization modification (PPM) of polybutadiene (PBD) with the goal of adding value to and prolonging the lifespan of post-consumer rubber waste, as well as creating reprocessable, crosslinked rubber. Secondly, I demonstrate the use of piezoelectrically mediated mechanochemistry to generate diverse polyacrylates by reversible addition-fragmentation chain-transfer (RAFT) polymerizations. The PPM of PBD occurs through the addition of sulfonamides and sulfamates; these aminations proceed via selenium- catalyzed, one-pot, room-temperature reactions. A myriad of functionalities can be imparted by varying both the R group of the sulfonamides or sulfamates and the mole percent functionalization of the polymer backbone. I present initial proof-of-concept work, where PBD is modified with a family of sulfonamides, generating polymers with tunable thermal and surface wetting properties. Further work explores applications of allylic amination by generating reversibly crosslinked rubber. Current crosslinking of PBD is commonly executed through di- and tri-sulfide linkages, creating vulcanized rubber; these materials are energy- intensive to de-crosslink and in the process generate toxic byproducts, including sulfur dioxide and hydrocarbons. I present a means of crosslinking PBD by amination with 1,1,1,3,3,3-hexafluoroisopropyl sulfamate followed by transesterification with a variety of diols. Further substitution with phenol regenerates a thermoplastic with free amines on the backbone, which can undergo crosslinking again. The second part of this work addresses the need for more sustainable synthetic methods to create existing polymers. Utilizing mechanochemistry provides a green chemistry alternative to traditional syntheses as minimal solvent and less energy is required, and immiscible monomers can be combined without excessive heating or exotic solvents. Herein we show the synthesis of random-co-polymers from immiscible monomers, ABA and ABC triblock-co-polymers, and ultrahigh molecular weight (UHMW) polymers by using ball-milling in the presence of piezoelectric nanoparticles to drive the reactions. We see comparable control of polymer length and dispersity compared to solution-state RAFT polymerizations, with significantly lower solvent and energy requirements. Both aims of this work address increasing sustainability in areas of polymer synthesis and processing by focusing on the PPM of PBD for valorization of polymer waste, generating a circular means to crosslink rubber, and minimizing resources needed to polymerize polyacrylates.