Boron-based poly(asymmetric substituted glycolide) nanospheres†

Boron-based asymmetrically structured poly(substituted glycolide) homopolymers (BF₂I-PASG) were prepared from a library of asymmetric substituted glycolide (ASG, ) monomers (linear alkyl (propyl methyl- and propyl ethyl-) or branched (isobutyl methyl-, isobutyl ethyl-, isoleucine methyl-, and isoleucine ethyl-) glycolide), which have an analogous structure to lactide (LA) and glycolide (GA), using BF₂dbmOEtOH (BF₂I) as the initiator in the presence of the Sn(Oct)₂ catalyst in a solvent-free environment. The presence of amorphous BF₂I-PASG, with a glass transition temperature (T_g) ranging from 4.1 to 31.9 °C determined by DSC analysis, suggested its suitability for short-term drug release. This amorphous character is attributed to the asymmetric structure of monomers , which incorporate various substituted side groups on the GA rings. The thermal stability (i.e., T_d) of BF₂I-PASG was examined by TGA analysis, revealing a single-step degradation that shifted towards higher T_d values, from 346 °C to 377 °C, as the length of the alkyl side group on the polymer backbone increased. BF₂I-PASGs exhibit high extinction coefficients of up to 53 750 M⁻¹ cm⁻¹, demonstrating strong π–π* transitions and intense blue fluorescence, with fluorescence lifetimes following a single-exponential decay as high as 1.96 ns and good quantum yields of up to 75%. Moreover, the nanoparticles (NPs) of these polymers possess a smooth surface and regular spherical shape, with average diameters as small as 236 nm, low and narrow polydispersity indices (PDI) ranging from 0.062 to 0.136, and a drug loading capacity of approximately ≥5%. For example, the NPs exhibited a rapid release profile for the anticancer drug paclitaxel, with around 82.1–96.2 of the drug released within approximately 5 days, depending on the NP composition.