Polymers are ubiquitous in modern society, playing important roles in providing convenience and function through plastics and other advanced materials. Concerns regarding the environmental impact of fossil fuel-derived polymers have prompted a quest for sustainable alternatives. Biopolymers such as proteins offer a promising solution due to their renewability and biodegradability. Proteins are glassy and stiff when dry due to extensive intermolecular interactions, which can be leveraged as reinforcing domains in elastomers. Previous efforts have showcased the potential of protein copolymers, where proteins conjugated to rubbery polyacrylates as the soft domains were shown to demonstrate high mechanical strength and toughness. However, these materials are often crosslinked due to the numerous reactive functional groups on proteins, limiting their processability. We address this challenge by introducing strategies to enable thermal processing of protein-based copolymers, thereby expanding their applicability in materials manufacturing.
We present two approaches: site-selective modification for fabricating linear block copolymers, and the introduction of dynamic covalent chemistries into covalently crosslinked protein-polymer networks. Through precise functionalization of the protein N-terminus with a chain transfer agent, followed by grafting-from polymerization, we achieve site-specific attachment of polymer chains. This approach ensures that polymers are tethered at single sites, resulting in materials exhibiting thermoplastic behavior. We also show that incorporation of a dynamic covalent bond capable of reversible bond exchanges enable repeated melt moldability even in crosslinked networks. We demonstrate this in copolymers prepared using protein macrocrosslinkers, and we incorporate dynamic covalent bonds into the network through copolymerization with comonomers bearing dynamic disulfide groups. Overall, this work mitigates challenges with crosslinked protein copolymers through rational design of protein functionalization and comonomers.
