Supramolecular Polylactides by the Cooperative Interaction of the End Groups and Stereocomplexation

The controlled ring-opening polymerization of l-lactide and d-lactide using 2-aminopyridine (AP), isocytosine (IC), uridine (U), and 2-ureido-4[1H]-pyrimidinone (UPy-OH) initiators with stannous(II) octanoate as catalyst leads to polylactides (AP-PLA-OH, IC-PLA-OH, U-PLA-OH, and UPy-PLA-OH), which are capable of forming strong hydrogen bonds and consequently self-assemble. By means of "1H NMR titration", the association constant (Ka) was determined for chosen model system composed of polylactides with uridine and aminopyridine end groups. The coupling reaction of UPy-PLA-OH with diisocyanates was used to obtain PLLA and PDLA with UPy end groups on both sides of the chain: UPy-PLLA-UPy and UPy-PDLA-UPy. The SEC analysis of the telechelic UPy-PLA-UPy revealed the presence of a supramolecular polymer with a high-molecular-weight fraction (Mn ≈ 70 000 g/mol) compared with the molar mass of the starting dimer (UPy-PLA-OH, Mn ≈ 7000 g/mol), which confirms the strong complementary interactions of the end groups. Moreover, the influence of the end groups of enantiomeric PLAs on their thermal properties and morphology was investigated. Additionally, the influence of the modified PLA enantiomers on the stereocomplexation phenomenon was analyzed. It was observed that stereocomplexes prepared from PLAs with UPy or U end groups on one chain end lead to the formation of microspheres during precipitation from N-methylpyrrolidone into methanol. The microscopic analysis of the telechelic UPy-PLLA-UPy/UPy-PDLA-UPy stereocomplex resulting from the precipitation from chloroform into methanol revealed a fibrous morphology. The probable mechanism of formation of the hierarchical structure of this stereocomplex material, which can be related to the specific geometry of the enantiomeric chains (mainly in a parallel arrangement), was proposed.