In this study, the effect of three aliphatic diisocyanate linkers, L-lysine diisocyanate ethyl ester (LDI), hexamethylene diisocyanate (HDI), and racemic 2,2,4-/2,4,4-trimethyl hexamethylene diisocyanate (TMDI), on the degradation of oligo[(rac-lactide)-co-glycolide] (64:36 mol%) based polyester urethanes (PEU) was examined. Samples were characterized for their molecular weight, mass loss, water uptake, sequence structure, and thermal and mechanical properties. Compared to non-segmented PLGA, the PEU showed higher water uptake and generally degraded faster. Interestingly, the rate of degradation was not directly correlating with the hydrophilicity of the diisocyanate moieties; instead, competing intra-/intermolecular hydrogen bonds in between urethane moieties appear to substantially decrease the rate of degradation for LDI-derived PEU. By comparing microparticles (μm) and films (mm) as matrices of different dimensions, it was shown that autocatalysis remains a contributor to degradation of the larger-sized PEU matrices as it is typical for non-segmented lactide/glycolide copolymers. The shown capacity of lactide/glycolide-based multiblock copolymers to degrade faster than PLGA and exhibit improved elastic properties could be of interest for medical implants and drug release systems.
The pore structures of microparticulate drug carriers are important diffusion pathways, which are not a static property but rather may be changing in the case of degradable matrix polymers such as poly[(rac‐lactide)‐co‐glycolide] (PLGA). In this study, the mutual impacts of dynamic changes in microparticle porosity and polymer degradation were analyzed for PLGA with different molecular weights and end groups as well as PLGA‐based triblock copolymers. In selected cases, particularly for PLGA with hydrophilic end groups and low initial number average molecular weight of 5 kDa, pore opening/pore closing phenomena were detected during incubation in phosphate buffer at 37 °C. Initially, pore closing was induced by water‐induced plasticization and the reduction of interfacial tension. The pattern of molecular weight decrease and mass loss suggested that pore closing did not result in undesired autocatalytic acceleration of degradation or delayed mass loss due to trapped acidic degradation products.
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