A Cascade Biodegradable Polymer Based on Alternating Cyclization and Elimination Reactions

Polymers that depolymerize by a cascade of intramolecular reactions in response to the removal of a stabilizing end-cap can allow for an unprecedented degree of control over the polymer degradation process. Described here is the development of polymers comprising N,N'-dimethylethylenediamine and 4-hydroxybenzyl alcohol linked by carbamate linkages. The polycarbamate backbone is stable in aqueous solution, but removal of a protective end-cap from the amine terminus allows the diamine to cyclize, forming N,N'-dimethylimidazolidinone and releasing the phenol, which undergoes a 1,6-elimination followed by the release of CO2 to reveal the next amine to continue the cascade. These polymers therefore degrade by alternating cyclization and elimination reactions. First, a tert-butylcarbamate (Boc) group was introduced as a cleavable end-cap, and the degradation kinetics and mechanism were studied by H-1 nuclear magnetic resonance (NMR) spectroscopy and size exclusion chromatography. Next, to demonstrate the degradability of these polymers under biologically relevant conditions, poly(ethylene oxide) was introduced as an end-cap via an ester linkage, to provide an amphiphilic block copolymer. This copolymer was found to assemble into cascade degradable nanoparticles that were capable of encapsulating and subsequently releasing a fluorescent dye in aqueous solution. This new class of polymers therefore provides highly promising materials that can be used for the development of medical devices, drug delivery vehicles, and tissue engineering scaffolds with unique biodegradation properties.