Generation and Rearrangement of Thioethers for Self-Reinforcing Thermosets and Semi-crystalline Thermoplastic Photopolymers

Abstract

In an effort toward long-term sustainability of plastics, the development of robust polymeric materials with longer lifespans is required. The life cycle of polymeric materials can be expanded by programming polymers with chemical functionalities that afford recyclability or the ability to self-heal and self-reinforce. The versatile nature of thioether chemistry enables facile coupling and rearrangement of covalent bonds due to the reactivity of thiolate anions and thiyl radicals. In this thesis, I will detail two approaches using the versatile thioether chemistry to improve sustainability of polymers: the design of stimuli-responsive self-reinforcing thiol-ene thermosets and the development of recyclable thermoplastic cyclic allylic sulfide resins for stereolithographic additive manufacturing. To produce sustainable polymers that can respond to their environment, two-stage self-reinforcing films incorporating hexaarylbimidazole (HABI) as a mechanophore were investigated. Using orthogonal thiol-ene chemistries, we first generated thioethers through selective polymerization of the thiol/acrylate using nucleophilic thiol-Michael addition. Then, the network-bound HABI was activated to initiate the in situ thiol/allyl radical thiol-ene addition to yield cross-linking via locally generated thioethers. This two-stage network was demonstrated first using light as a stimulus with HABI acting as a thiol-ene photoinitiator. Then demonstrated the mechanically labile nature of HABI for use as a mechanophore in the two-stage polymer. The modification of monomer types and stoichiometric ratios affords almost limitless possibilities for the properties of the first and second stage polymer which can be tailored to a wide variety of applications. Facile generation of thioethers is possible through thiol-ene chemistry but thioethers also possess the ability to be rearranged using addition-fragmentation chain transfer in allyl sulfide functionalities. Here, we evaluated cyclic allylic sulfide (CAS) monomers in thermoplastic stereolithography for their ability to use rearrangeable thioethers to enhance layer adhesion in the light-based additive manufacturing technique. The 7- and 8-membered CAS ring-opening monomers were demonstrated as being effective photopolymers and subsequently crystallized in tens of seconds, making them capable monomers for thermoplastic stereolithography. The advantageous properties of CAS-based resins were demonstrated in stereolithography with a range of complex geometries printed at high resolution using a standard DLP stereolithographic 3D printer. To counteract the brittleness of the homopolymers, copolymers of different sized CAS monomers were produced and assessed. Initially between the 7- and 8-membered, the copolymers were found to produce overall tougher polymers. These copolymer combinations were expanded through the synthesis of 11- and 15-membered semi-crystalline CAS monomers, affording a wider variety of copolymer properties including extremely tough copolymers made from 8- and 11-membered CAS comonomers. To increase the overall print speed, CAS monomers were evaluated for their compatibility with two color photoinitiation/photoinhibition systems used in continuous stereolithographic additive manufacturing. However, the visible light photoinitiators that were evaluated, camphorquinone and H-Nu-640, produced lower quality films than near-UV photoinitiator, Irgacure 819. Additionally, the photoinhibition of CAS was unable to be achieved with readily attainable UV-active photoinhibitors; hexaarylbimidazole, butyl nitrite, and tetraethylthiuram disulfide.