Expanding the Scope of Poly(β-amino ester)s by Investigating Degradation Behavior and Enhancing Functionality

Degradable polymers offer creative solutions to a host of environmental and human health challenges. This thesis describes an investigation of a rising class of degradable polymers, poly(β-amino ester)s (PBAEs), which boast an impressively facile synthesis, a diverse monomer library, and tunable lifetimes. Owing to a protonatable tertiary amine in each repeating unit imparting cationic character and enabling complexation of anionic cargo (e.g., DNA, RNA), PBAEs were originally developed as a non-viral gene therapy vectors, but are now being explored for increasingly diverse applications from shape memory thermosets to degradable hydrogel linkers. However, thus far, predictive degradation models, which would enable user-defined degradation rates, remain elusive due, in part, to the complex and interconnected phenomena surrounding pH-dependent PBAE degradation. Additionally, while the PBAE monomer library is extensive, one notably absent functionality, net anionic charge, could enable PBAEs to encapsulate and deliver cationic therapeutics such as antimicrobial peptides to fight antibiotic-resistant bacteria. Therefore, the specific goals of the work in this thesis were to a) further our understanding of the critical factors controlling PBAE degradation and b) expand the functional design space of these polymers by developing methods to incorporate net anionic charge.
In Chapter 2 we investigated degradation behavior in different pH environments, finding acidic environments to promote PBAE solubility, while basic conditions accelerate hydrolysis. Yet, without sufficient buffer, solution pH could be controlled by PBAE backbone amines and the carboxylic acids generated during hydrolysis, sometimes causing large swings of several pH units, impacting degradation. Then, to understand how the pH-dependent protonatable PBAE amines themselves influence degradation, in Chapter 3 we studied the effect of amine charge state on degradation by synthesizing permanently charged PBAE analogs, poly(-quaternary ammonium ester)s (PBQAEs). Similarly to PBAEs, PBQAEs degrade faster in basic environments. However, at a given pH, PBAEs consistently degrade faster than PBQAEs. The difference in degradation rates of PBAEs vs. PBQAEs increases with pH, suggesting the basic deprotonated PBAE amines, increasingly present in alkaline solutions, accelerate hydrolysis. Together, these studies expand our understanding of PBAE degradation, including the effect of pH, how polymers can control pH, and the role of amine charge state.
To expand the utility and scope of PBAEs, we developed a synthetic method to access net anionic charge in PBAEs (Chapter 4). These polymers, comprising two anionic groups and one cationic group in each repeating unit, display non-monotonic solution behavior as a function of pH. We observed them to be highly soluble in acidic and basic solutions, but to assemble and even crash out at neutral pH, a property that could be particularly useful for therapeutic delivery. In Chapter 5, we studied the complexation of net anionic PBAEs with a model cationic peptide, (glycine-arginine)10, using both high-throughput turbidity measurements alongside diffusion ordered spectroscopy (DOSY). These complexes fall apart as the polymer degrades (over ca. 5 h) and therefore, we envision these polymers will be useful to protect a variety of emerging sensitive cationic therapeutics during storage and delivery. Together, these studies considerably expand our understanding of the role of pH on PBAE degradation and the functional design space of PBAEs. Going forward, we are optimistic that these findings and methods will accelerate the use of PBAEs and other emerging degradable polymers to address a wide-range of societal challenges.