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

摘要

The controlled ring-opening polymerization of L-lactide and D-lactide using 2-aminopyridine (AP), isocytosine (IC), undine (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 "H-1 NMR titration", the association constant (K-a), 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 (M-n approximate to 70000 g/mol) compared :with the molar mass of the starting dimer (UPy-PLA-OH, M-n approximate to 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 PIA enantiomers, on the stereocomplexation phenomenon was analyzed. It was observed that stereocomplexes prepared from PLAs with UPy on 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.