Controlled synthesis of PLA-series environment-friendly polymers with guanidine catalysts

Controlled polymerization is of great significance for the tailoring of macromolecular structure and synthesis of polymers with excellent properties. Several non-toxic organic guanidinines were prepared/selected in our laboratory in recent years and used as the catalysts/initiators to realize controlled melt polycondensation (MP), ring-opening polymerization (ROP), and depolymerization (DEP). Isotactic melt polycondensation (Iso-MP) of L-lactic acid/D-lactic acid (LLa/DLa) was carried out for the first time with biogenic creatinine (CR) catalyst and poly(L-lactic acid)/poly(D-lactic acid) (PLLA/PDLA) with high isotacticity (Iso. > 98%) was obtained; Combined MP with solid-state polycondensation (MP-SSP) of the medium molecular weight PLLA/PDLA (M_w = 2.2 × 10⁴ Iso. = 98.2%), prepared by the Iso-MP with CR, was carried out, and high molecular weight PLLA/PDLA (M_w > 1.0 × 10⁵) with high isotacticity (Iso. > 98%) was obtained. The initial decomposition temperature (T_d,i) of the synthesized PLLA reached up to 324.5 °C, which was 120 °C higher than that of PLLA synthesized by SnCl₂·2H₂O catalyst; Optical pure (e.e. 100%) L-lactide/D-lactide (OP-LLA/OP-DLA) were synthesized via Iso-MP and the subsequent depolymerization (DEP) of LMW-PLLA/LMW-PDLA (M_w = 8.0 × 10² - 9.0 × 10²) with CR catalyst. The polymer residues (r-PLLA/r-PDLA), produced accompanying with the lactides formation, were reused as the raw material for LLA/DLA synthesis after being hydrolyzed. The total yield of OP-LLA/OP-DLA reached up to 98%; Several sterically hindered guanidine carboxylates (HBG·OAc, CRA, CRG, CRL, TBDA) were synthesized and used to realize living ROP of LLA. An optical pure morpholine-2,5-dione (3S,6S-BMMD), derived from LLa and L-serine, was synthesized. Living ROP of 3S,6S-BMMD was carried out with CRA catalyst for the first time. After debenzylation of the synthesized P(3S,6S-BMMD), an amphiphillic copolymer P(LLa-co-Ser) was obtained, which is a useful support material in biomedical areas. The fine molecular structures and isotacticities of growing polymeric species were characterized by in situ monitoring with¹H-NMR,¹³C-NMR. And the mechanisms of the above mentioned controlled polymerizations were proposed.