Synthesis and pyrolysis of ABC type miktoarm star copolymers with polystyrene, poly(lactic acid) and poly(ethylene glycol) arms

An ABC type miktoarm star copolymer possessing polystyrene (PS), poly(lactic acid) (PLA) and poly(ethylene glycol) (PEG) arms was synthesized by combining Atom Transfer Radical Polymerization (ATRP) and Ring Opening Polymerization (ROP) with two click chemistries, namely thiol-ene and copper catalyzed azide-alkyne cycloaddition (CuAAC). For this purpose, a core 1-(allyloxy)-3-azidopropan-2-ol with allyl and azide functionalities was synthesized in two steps. Then, clickable polymers, polystyrene with thiol functionality (PS-SH) and poly(ethylene glycol) with alkyne functionality (PEG-acetylene) were independently prepared. As the first step of the grafting onto process, PS-SH was thiol-ene clicked onto the core to yield PS-N ₃-OH. The second arm was then incorporated onto the core by the Ring Opening Polymerization (ROP) of l-(-)-Lactide (LA) using as PS-N ₃-OH initiator and tin(II) 2-ethylhexanoate as catalyst. Finally, alkyne-PEG-acetylene was bonded to the resulting PLA-PS-N ₃ using CuAAC click reaction. All intermediates, related polymers at different stages and final PS-PLA-PEG miktoarm star copolymer were characterized by ¹H NMR, FT-IR, SEC and DP-MS analyses. Direct pyrolysis mass spectrometry, (DP-MS) analyses of PS-PLA-PEG and all intermediate polymers indicated that the decomposition of PS and PEG chains occurred almost independently, following the degradation mechanisms of the corresponding homopolymers. On the other hand, during the pyrolysis of PS-PLA-PEG, elimination of H ₂O during the decomposition of PEG chains at the early stages of pyrolysis caused hydrolysis of PLA chains and increased the yields of CO ₂, CO and units involving unsaturation and/or crosslinked structure.