Preparation, Characterization And Properties Of Biodegradable Aliphatic Polyesters

Aliphatic polyesters such as polylactide(PLA),poly-3-hydroxybutyrates(P3HB), poly(ε-caprolactone)(PCL)and their copolymers are the most intensively studies and widely used as biodegradable medical materials.Continuous efforts have been devoted to the development of new catalysts and initiators for the ring opening polymerization of lactide and lactones.In the present thesis,ring opening polymerizations of DL-lactide,L-lactide,glycolide, (R,S)-β-butyrolactone andε-caprolactone have been carried out by using stannous octoate, cyclic tin alkoxides,dibutylmagnesium and magnesium octoate as catalysts or initiators.The structure and properties of homopolymers and copolymers were characterized and analyzed by ¹HNMR,¹³CNMR,FT-IR,GPC,DSC and XRD.The detailed results and conclusions are as follows:1.Two well-designed cyclic tin alkoxides(SnA and SnB)were used as initiators for the ring opening polymerization of(R,S)β-butyrolactone(β-BL)to prepare high molecular weight P3HB.Polymerizations ofβ-BL were successfully carried out using high vacuum technique, being molar ratio of monomer to initiator(M/I)above 5 000 or around 20℃.Molecular weight of the polymer is largely dependent on an amount of initiator and inversely proportional to that.The kinetics ofβ-BL polymerization initiated with SnB was investigated at various M/I using dilatometer.The results showed that chain propagation rate forβ-BL polymerization was first order in monomer concentration and 0.56 order in initiator concentration,respectively.Analysis of diad sequence distributions at various polymerization temperatures for SnA and SnB showed the activation energy of ca.-11 and -13 kJ/mol for syndiotactic versus isotactic diad formation.Triad stereosequence distributions of P3HB samples agreed well with the Bernoullian statistical model of chain-end control.2.The kinetics ofβ-BL polymerization imitated by dibutylmagnesium(Bu₂Mg)was examined and showed a first order both in monomer concentration and initiator concentration. The value of activation energy obtained for Bu₂Mg-initiatedβ-BL polymerization is ca.30 KJ/mol.The end group analysis suggested that monomer inserted into the growing chains proceeding through the coordination-insertion mechanism based on the acyl-oxygen bond scission rather than the alkyl-oxygen bond cleavage of theβ-BL ring.A possible mechanism for the initiation and propagation procedures ofβ-BL polymerization with Bu₂Mg was proposed.3.Two series of poly(ε-caprolactone)-poly(ethylene glycol)-poly(ε-caprolactone)(PCL-PEG-PCL)triblock copolymers with different lengths of each block were synthesized successfully by ring opening polymerization ofε-CL using Bu₂Mg as catalyst and PEG as macroinitiator in 1,4-dioxane solution at 70℃.The triblock structure and molecular weight of the copolymers were analyzed and confirmed by ¹H NMR,¹³C NMR,FT-IR and GPC.The crystallization and thermal properties of the copolymers were investigated by WAXD and DSC.The results illustrated that the crystallization and melting behaviors of the copolymers were depended on the copolymer composition and the relative length of each block in copolymers.The crystallization of PEG block was strongly restricted by the crystallization of the lateral PCL blocks,whereas the crystallization of PCL blocks was not suppressed by the middle PEG block.4.Ring opening copolymerization of L- or DL-LA withε-CL catalyzed by magnesium octoate were investigated in bulk.The reactivity ratios of L-LA andε-CL were measured to be r_LA= 23 and r_CL= 0.22,respectively,using the Fineman-Ross method.The results(r_LA＞1 and r_CL＜1)imply that L-LA monomer added preferentially into the copolymer chain end in the first step of the reaction,resulting in the formation of long blocks of lactidyl unit in the polymer chain.Two modes of transesterification occurred during the copolymerization process and played an important role in the redistribution of comonomer sequences.The repeat unit sequence distribution of the copolymers was varied more or less toward a random sequence distribution by variation in the comonomer feed ratio,reaction time and temperature. The copolymer composition has a profound influence on their average block length, increasing as the relative proportions of the corresponding ective monomers increased.The effect of lactide configuration on the transesterification and randomization of the copolymer chain microstructure was determined.The DL-lactide-based copolymers have a more blocky structure and lower transesterification coefficient of the second mode than those containing L-lactide.It is less sensitive to transesterification reactions in the case of DL-LA. The thermal properties and crystallization of the copolymers were investigated by DSC and XRD.The results showed that a close relationship between copolymer crystallinity and the length of the copolymer sequences.All copolymers exhibited a single T_g determined from the second heating scan.The experimental T_gs were in agreement with the calculated values from Fox equation.It can be concluded that the copolymers are random,or comprise a mixture of compatible block copolymers.5.A series of poly(DL-lactide)(PDLLA),poly(L-lactide)(PLLA)and poly(L-lactide-co-glycolide)(PLGA)with various molecular weights and various copolymer composition were prepared via ring-opening polymerization of DL-lactide or L-lactide and glycolide catalyzed by stannous octoate in vacuum.The molecular weight of these polymers was controlled by using a molecular weight controller,lauryl alcohol.The number-average molecular weight of PDLLA,ranging from 14800 to 63300 Da,was controlled by the amount of molecular weight controller used.The effect of the amount of molecular weight controller on the polymer molecular weight was studied.The results indicated that the polymer molecular weight has a log-log linear relationship with the amount of lauryl alcohol.6.The resulting PDLLA,PLLA and PLGA were further investigated with respect to their degradation behaviors.The biodegradation rate of polymers films in Hank's solution were monitored by the changes of weight loss,inherent viscosity and morphology of the samples.PLGA containing a higher content of glycolide degraded faster than those containing a lower content of glycolide.The results demonstrated that the degradation rate of PLGA could be adjusted by changing the compositions and molecular weight of the copolymer and enhanced with the glycolide content increasing.The results indicated that the PLGA copolymer containing glycolide from 30 to 40%is a potential material as coating on drug elusion stents.