Synthesis, Characterization Of Alkali Metal Complexes And Applications In The Ring-Opening Polymerization Of Lactide

Poly-lactide(PLA) is one of the important polyesters. It has similar characters to petro polymer materials. The discarded PLA products can be degraded to H2 O and CO2 by microbes, which are eco-friendly. PLA is biocompatible which confirmed by a lot of experiments. Most importantly, as one of polyesters, the mechanical properties of PLA make it a suitable candidate for most applications. Therefore, PLA is known as one of the most potential and developable economical materials in 21 st century, which is now widely used as packing materials, disposable dishware, household apparatus shell, fibers, 3D materials, medical devices and so on. It is a very important subject to design and synthesis complexes which can be applied to efficient and stereoselective catalysis of lactide ring-opening polymerization to obtain polyesters with controllable molecular weights and narrow PDI. In order to solve the current problems in the synthesis of PLA, three series of phenol alkali metal complexes were designed, synthesized, characterized, and applied in the ring-opening polymerization of lactide.The paper consists of four Chapters.Chapter 1: A review of the applications of PLA, the catalytic mechanism of PLA synthesis and metal complexes applied to the ROP of LA.Chapter 2: Ten alkali metal complexes supported by two bulky tetraphenols were synthesized and characterized. Complexes 1-10 were all characterized by NMR, elemental analysis, and single-crystal X-ray diffraction techniques and all of the complexes were applied in the ring-opening polymerization of lactide. Bimetallic complexes 1, 2 and 3 are comparatively more efficient than others because of their symmetric structures, in which complex 3 presents as a rare highly active potassium catalyst for the ring-opening polymerization of LA and polymers with good molecular weight control and narrow molecular weight distributions were obtained. This catalytic process can be conducted with a high benzyl alcohol to metal complex ratio(2, 4, 8, 16 and 20 equiv.), and is active after a second feeding of L-LA. The effects of metal center, ligand steric hindrance and complex structure on the ring-opening polymerization were discussed.Chapter 3: Nine sodium and potassium complexes coordinated by a bulky monophenoxy ligand with one xanthenyl group at ortho-position were synthesized and characterized in this chapter. Complexes 11-18 were characterized by single-crystal X-ray diffraction techniques, NMR, elemental analysis. VT-NMR, DOSY, DFT calculations are used to characterize their solution behaviors and solid structures, and all nine complexes’ activity and stereo-selectivity on ROP are studied. The complexes 17 and 18 are highly iso-selective and active catalysts for the controlled ring-opening polymerization of rac-LA. Excellent isotacticity was achieved(Pm = 0.86), which is a breakthrough in the field of alkali metal complex catalyzed selective rac-LA ring-opening polymerization to obtain high isotacticity PLA. Furthermore, the ROP of rac-LA catalyzed by complexes 17 and 18 at low temperatures is investigated, and the results suggest that this is an anti-Arrhenius reaction, in which the reaction rate is slower at high temperature than that at low temperature.Chapter 4: Based on the previous achievements, simple and commercial monophenols were chosen as ligands to synthesize alkali metal complexes(20, 21 a, 21b), complexes 20-22 were all characterized by single-crystal X-ray diffraction techniques, NMR, elemental analysis, and the alkali metal complexes were applied to the ROP of rac-lactide. The results show that all of the three complexes can catalyze iso-selective ROP of rac-lactide(Pm = 0.81-0.83). Compared with other catalysts which have been reported by our group recently, we found that the sandwich structure of catalysis center surrounded by the plane of a crown and a steric hindrance of phenol at ortho position is vital for highly iso-selective catalysis rac-LA ROP. In the process of the experiments, we found that para-benzylic C(sp~3)-H phenol potassium complex can be easily oxyfunctionalized to carbonyl via a benzoquinon reaction intermediate in the air/O2 atmosphere, so it is inadvisable to choose para-benzylic C(sp~3)-H phenol as ligand to synthesize similar catalysts. However the oxyfunctionalization is useful for the remote oxidation of the phenol’s para-benzylic C(sp~3)-H to carbonyl.