Synthesis, Characterization And Enzymatic Degradation Of PCL-PLLA Block Copolymers

In the past decades,biodegradable materials have been receiving increasingly more attention due to their potential applications in the fields such as biomedicals, pharmaceuticals and environmental protection.Among them,aliphatic polyesters are most attractive because of their excellent biocompatibility,processing properties and nontoxic degradation products which are finally converted to water and carbon dioxide.PCL-PLLA block copolymers are especially attractive due to their defined chain structure,tunable degradation rate and appropriate mechanical properties. These materials may provide many potential applications in biomedical,tissue engineering and pharmaceutical areas.The adsorption of proteinase K on poly(L-lactide)(PLLA) and poly(D-lactide) (PDLA) films was studied by contact angle,surface tension and microscopic measurements.Environmental scanning electron microscopic clearly shows that proteinase K can irreversibly adsorb on PLLA film.In contrast,no enzyme adsorption was detected on PDLA film under the same conditions.The contact angle of PLLA film rapidly decreases after immersion in Tris buffer containing proteinase K,whereas that of PDLA remains unchanged.These findings indicate that enzyme adsorption may be a prerequisite for enzymatic degradation of polylactide substrates.Surface tension measurements allow calculating the average area occupied per proteinase K molecule(ApM).The ApM value was found to be about 46(?)~2 at 25℃, and 29(?)~2 at 37℃,suggesting that at higher temperature,enzyme molecules exhibit a more compact conformation.copolymers ofε-caprolactone and L-lactide with different molar ratios were prepared via sequential ring opening polymerization(ROP) of both monomers.The resulting PCL-PLLA-PCL triblock copolymers were characterized by using NMR, SEC,DSC and XRD.One melting peak corresponding to the PCL block was detected, but the presence of PLLA decreased the crystallinity of PCL.Enzyme-catalyzed biodegradation of solution cast films was investigated at 37℃in the presence of pseudomonas lipase.It was observed that the PLLA component retarded the degradation of the block copolymer as compared to PCL homopolymer.Therefore,the enzymatic degradation rate can be adjusted by varying the composition of the copolymers.~1H NMR and SEC data showed no significant chemical composition or molecular weight changes during degradation,indicating that the degradation proceeded according to surface erosion mechanism. In another work,poly(L-lactide)/poly(ε-caprolactone) diblock,triblock and four-armed copolymers with the same monomer feed ratio(50/50) were synthesized by ring opening polymerization of successively addedε-caprolactone and L-lactide, using isopropanol,ethylene glycol,or pentaerythritol as initiator and nontoxic zinc lactate as catalyst.The resulting copolymers were characterized by ~1HNMR,SEC, FT-IR and XRD,which confirmed the blocky characteristic of the copolymers. Various solution cast films were allowed to degrade at 37℃in the presence of proteinase K,and the degradation was monitored by DSC,SEC,~1H NMR and ESEM (Environmental Scanning Electronic Microscopy).The effects of chain structure, block length and phase dispersion on degradation were discussed.No obvious thermal property changes and compositional changes were detected during degradation. ESEM was used to follow the morphology evolution of solution cast films prepared from these block copolymers.