Investigation Into The Ring-Opening Insertion Polymerization Of Cyclic Esters

Poly(ε-caprolactone) (PCL), poly(lactic acid) (PLA) and polycarbonate (PC) have been widely used as biomedical materials because of their good biodegradability and biocompatability. In this paper, the developments of the ring-opening polymerization (ROP) of cyclic esters for producing PCL, PLA and PC, are introduced.For bio-security considerations, metal-free ROP of cyclic esters has become a research focus. This dissertation focuses on the ROP ofε-caprolactone (ε-CL), lactide and cyclic carbonates in the presence of phosphrous compounds.The ring opening insertion polymerization (ROIP) ofε-CL initiated by diisopropyl hydrogen phosphonate (diiPro-HP) was investigated. The ROIP occurred by a coordination-insertion mechanism containing two steps. First, the carbonyl carbon was attacked by the the phosphorus atom of the H-phosphonate tautomerization (a phosphine-like structure) and the acyl-oxygen bond was broken. An intermediate was formed by the coordination of the former carbonyl carbon and acyl oxygen ofε-CL to phosphorus atom. Then the phosphorus-alkoxide of H-phosphonate was cleavaged to form acyl-alkoxide bond. Poly(ε-caprolactone) (PCL)-inserted H-phosphonates (PCL-HPs), which was not only the product of the occurred ROIP but also the initiator for the next ROIP, were produced. The number average molecular weight (Mn) of resultant polymer had a linear growth with monomer conversion. The kinetics was first order for diiPro-HP and the monomer conversion fitted very well with diiPro-HP conversion in an index scale.Macromolecular hydrogen phosphonates (macro-HPs, containing PEG2000-HP, PCL400-HP and PCL2200-HP) were produced and the ROIPs of CL initiated by macro-HPs were investigated. Block copolymer of PCL-PEG was successfully prepared through the PEG2000-HP-initiated ROIP of CL. The copolymer could self-assemble into micells in water and the critical micelle concentration (CMC) was 22.05 mg/L. Transmission electron microscopy (TEM) showed that the micelles exhibited nanospheric morphology with a size range of 20-35 nm. Cytotoxicity test showed that the PCL-PEG copolymer has good biocompatability. PCL with Mn=4500 g/mol and monomer conversion of 99.44% was obtained by the ROIP of CL initiated by PCL400 under microwaves (molar ratio of PCL400 to CL:1:100,340 W,60 minutes). Compared with the diiPro-HP system, PCL2200-HP brought higher monomer conversion in the early stage but slower chain propagation and broader polydispersity index (PDI) in the later stage.Phenylphosphinic acid (Ph(O)P(H)OH, PPA)-initiated ROIP of CL was investigated. PCL with Mn=7330 g/mol was obtained after 40 minutes microwave heating (molar ratio of PPA to CL:1:50). The ROIP also occurred by a coordination-insertion mechanism:the carbonyl carbon was attacked by the the phosphorus atom of the PPA tautomerization (a phosphine-like structure) and the ring opened by the broken of acyl-oxygen bond. The chain propagated through the insertion of CL segments into phosphrous-oxgen bond but not the phosphrous-phenyl bond. By comparing the ROPs with different initiator concentrations (IC), higher IC brought higher monomer conversion in the early stage but was not good for chain propagation in the later stage. And the ROIP under conventional heating showed faster monomer initiation and steadier polymer chain growth.The ROP of lactide in the presence of PPA or diiPro-HP was inverstigated. PPA initiated the ROP of lactide by coordination-insertion mechanism and PLA-inserted PPA was produced in the process. The product of lactide ROP in the presence of diiPro-HP was only PLA terminated by hydroxyl and without phosphrous atom in polymer chain. Compared with the ROP of lower initiator concentrations (IC), higher IC brought higher monomer conversion in the early stage but more side reactions and faster polymer degradation in the later stage.PPA-initiated ROPs of trimethylene carbonate (TMC) and 2,2-dimethyl trimethylene carbonate (DTC) were investigated. PTMC and PDTC with Mn=7100 g/mol and 3230 g/mol, monomer conversion of 66.12% and 23.05% were obtained respectively by the ROP under 70 minutes conventional heating (160℃salt bath, molar ratio of PPA to monomer:1:50). Both of the ROPs occurred by coordination-insertion mechanism and polycarbonate-inserted PPA was produced. The two methyls in DTC monomer, to some extent, hindered the insertion of DTC segments into PPA.