Functional tailor-made polyesters via chemical and enzymatic catalysis and their applications as biomaterials

摘要

This thesis deals with the preparation of novel functional (co)polyesters and copolyester resins based on e-caprolactone and functional e-caprolactones as well as (co)polyesters based on w-pentadecalactone and functional macrocyclic lactones or functional e-caprolactones. The characteristics of these polyesters were evaluated with respect to their molecular weight, molecular weight distribution, composition, microstructure, and concentration of the functional groups. The degradation of polyester resins obtained via photo-crosslinking and via Michael addition was studied as a function of crosslink density. Microstructured materials with tunable degradation rate suitable for drug delivery carriers were prepared and characterized. All functional e-caprolactones used in this thesis are g-acyloxy-e-caprolactones which were prepared in two steps starting from 4-hydroxy-cyclohexanone. In the first step acylation of the hydroxyl group occurs and in the second step ring enlargement by Baeyer-Villiger oxidation. If the reaction sequence is inverted rearrangement occurs in the Baeyer-Villiger oxidation of 4-hydroxy cyclohexanone leading to g-hydroxyethyl-g-butyrolactone. Using the first procedure g-acetyloxy- (AcetCL), g-benzoyloxy- (BenzCL), g-acryloyloxy- (AcrCL), and g- methacryloyloxy-e-caprolactone (McrCL) were prepared. These monomers and for comparison reasons e-caprolactone and g-methyl-e-caprolactone were polymerized by means of chemical and enzymatic catalysis. The functional repeating units in copolyesters obtained from e-caprolactone and g-acyloxy-e-caprolactones change the crystallinity of the copolyester, in case of AcrCL and McrCL are used for the preparation of copolyester resins and the preparation of microstructured surfaces. It is expected that the rate of degradation of these polymers can be finely tuned by the nature of the acyl group and the concentration of the g-acyloxy-e-caprolactone repeating units. The expectation is based on the fact that the hydrolysis of the ester side chains or thermal treatment of the polymers releases the corresponding acid from the respective acyloxy side chain, which serves as a catalyst for further degradation (biotic acid generator). Copolymerization of e-caprolactone with g-acyloxy-e-caprolactones was performed using chemical and enzymatic catalysis. All monomers, except AcetCL, undergo controlled ringopening polymerization using chemical catalysts such as aluminium isopropoxide under selected reaction conditions. AcetCL, however, rearranges to a large extent under all polymerization conditions to give g-acetyloxyethyl-g-butyrolactone. In the presence of an enzyme (Novozyme 435, Lipase B from Candida antarctica (CALB) immobilized on a macroporous resin) all g-acyloxy-e-caprolactones, except BenzCL, partly rearrange to result in the corresponding g-acyloxy-g-butyrolactones, while e-caprolactone (CL) and g-methyl-e- caprolactone yield the corresponding polymers, the latter even in a stereoselective manner as reported earlier in the literature. This is the first time that rearrangement during polymerization was observed. A molecular dynamic study was performed with AcetCL and BenzCL as tetrahedral intermediates in the active-site of CALB to get information on the substrate recognition displayed by the enzyme. Based on the experimental results and the molecular dynamic studies a mechanism for the chemically and enzymatically catalyzed reactions of g-acyloxy-e-caprolactones was proposed. Novel biodegradable polyester resins were prepared via photo-crosslinking of functional polyesters obtained by copolymerization of CL with AcrCL and McrCL. Copolymers with different content of either acryloyloxy or methacryloyloxy functional groups were synthesized via ring-opening polymerization of g-acyloyloxy-e-caprolactones and CL using Al(OiPr)3 as catalyst and initiator. 2D- and 3D-micropatterning of the copolymers was performed via UVcrosslinking of polymer films on a suitable substrate by UV replica moulding on both rigid and elastic masters, showing the processability of these novel functional polyesters and their potential as substrates for biomedical devices. Degradation experiments on the polyester resins obtained via photo-crosslinking of methacryloyloxy and diamine-crosslinking of acryloyloxy pendant groups were performed. The presence of the functional groups affects the degradation rate of the copolymer: the degradation rate being faster with a higher degree of functionalization. Polyesters consisting of a poly(e-caprolactone) backbone bearing pendant acryloyloxy and methacryloyloxy groups in different amounts were successfully used also in preparation of biodegradable microparticle drug carriers. Stable microparticles can be prepared via an oil/water emulsion-solvent evaporation technique if polymers with up to 8 % acryloyloxy groups are used. If higher amounts of pendant groups are present the emulsion solvent evaporation technique is combined with a simultaneous crosslinking procedure to confer stability to the particles. We were successful in obtaining crosslinked particles with two different methods: UV irradiation in the presence of a photoinitiator and Michael type addition with diamines. Encapsulation of a hydrophobic fluorescent dye and a hydrophilic protein, as model drugs, were performed and confirmed by optical microscopy and Raman spectroscopy. The presence of the functional groups allow not only for tunable degradability, but also for further processing (e.g. crosslinking) and (bio)functionalization, broadening the potential use of polycaprolactones in biomedical applications. Poly(pentadecalactone) (PPDL) is a polyester of high crystallinity, with a melting point around 95°C. PPDL is nontoxic and although a polyester, shows no hydrolytic enzyme catalyzed degradation. This is ascribed to the high crystallinity and hydrophobicity of the materials. In order to enhance the degradability of PPDL copolymerization of PDL with g-acyloyloxy-e- caprolactones monomers were performed via enzymatic catalysis. It is expected that by integration of functional pendant groups, as acid generators, along the PPDL backbone degradability can be tailored. It was observed that by using AcetCL and AcrCL as comonomers, no copolymers were obtained; the result of the copolymerization was PPDL and rearranged AcetCL and AcrCL. Using McrCL and BenzCL as comonomers random copolyesters were obtained and no rearrangement was observed. Gas chromatographic analysis using chiral columns of non converted monomer during copolymerization of BenzCL and PDL revealed that one of the enantiomeric monomers is preferentially consumed. As a consequence optical active copolyester are obtained. Copolymerization of PDL with CL and BenzCL leads to nearly quantitative monomer conversion affording PDL based polyesters which are expected to show an enhanced degradation rate. Copolymerization of PDL with functional macrolactones represents another versatile and straightforward method to introduce functionalities along the PPDL backbone. Pentadecalactone based copolymers with C,C-double bonds, epoxide rings and amide functional groups in the backbone were obtained by copolymerizing PDL with ambrettolide, ambrettolide epoxide and a cyclic ester amide. It is expected that by reducing the crystallinity of the copolymers an enhanced degradation rate will be observed.