N-Carboxyanhydride ring opening polymerization (NCA ROP) is a method to prepare polypeptides with a high degree of polymerization in large quantities. The living polymerization technique of NCA ROP gave the opportunity to synthesize many polymer architectures with well-defined blocks and copolymers with a well-controllable composition. By combining other polymerization techniques, biohybrid polymers have been prepared. Although the polypeptides prepared by NCA ROP have a random amino acid order and a polydispersity, which is uncommon for natural peptides and proteins, they still can be considered as natural polymers and still have some of the features of natural peptides. For example, they can form secondary structures and can be degraded enzymatically. This provides opportunities for biomedical applications such as drug-delivery and hydrogels for the polypeptides and the hybrid polymers prepared by NCA ROP. The goals of this thesis were to study the living ROP of the NCAs and to make use of the versatility of the polymerization technique to obtain polypeptide and hybrid polymer architectures. Finally, the functionality of the polypeptide products was investigated for biomimetic crystallization, self-organization and enzymatic degradation. In the field of NCA ROP there are several methods known for living polymerizations. These can be classified as methods were the mechanism is altered to ensure that no side reactions can occur at the reactive polypeptide chain end and methods in which the reaction conditions are optimized to obtain living polymerizations. A lower temperature and a decreased pressure have both been claimed by separate groups to give the best results. In a systematic study for several different NCA monomers the monomer conversion, molecular weight distribution and chain composition were studied for reactions performed at different temperatures and different pressures. Depending on the monomer species, different side reactions were identified; these were found to be temperature dependent. Monomer conversion studies identified two groups of monomer. The first group of the NCA monomers (¿-benzyl-L-glutamate, Ne-benzyloxycarbonyl-L-lysine and L-alanine) showed fast monomer conversion and responded to the low pressure, showing an increase in the speed of propagation at room temperature. The number of side reactions was low, so the optimal reaction conditions for this group of monomers is under high vacuum and at room temperature. The second group (ß-benzyl-L-aspartate, O-benzyl-Lserine and O-benzyl-L-threonine) showed a lower rate of monomer conversion and no beneficial effect was observed at low pressures. For this second group of monomers, the number of side reactions was also much higher. The best results for a living polymerization of this group of NCAs were obtained at 0 ºC under atmospheric pressure. Using the previously mentioned living ring opening polymerization techniques, different polypeptide architectures have been synthesized. Copolypeptides, graft copolypeptides and block copolypeptides have been synthesized. Although NCA ROP is known as a living polymerization the solubility and the formation of secondary structures can decrease the solubility of the reaction products, resulting in less well-defined block copolypeptides upon macroinitiation. Therefore, the block copolypeptides have been intensively studied to identify the optimal block order synthesis. Improved and quicker reaction conditions were found for tetrablock copolypeptides by combining the optimal solubility and reaction conditions. Biohybrid block and graft copolymers were synthesized by combining radical polymerizations with NCA ROP. Grafted structures were obtained from the free radical chain transfer reaction or thiol-ene reaction with the thiols of poly(¿-benzyl-L-glutamate-co-L-cysteine). Biohybrid block copolymers were obtained by using amine-functionalized bifunctional initiators for atomic transfer radical polymerization (ATRP) and nitroxide mediated radical polymerization (NMRP). The functionality of the copolypeptides was investigated for the biomimetic crystallization of calcium carbonate. Due to the random distribution of amino acids in copolypeptides, this enabled a facile understanding of the function of amino acid species in natural peptides in biomineralization. Fluorescein-labeled copolypeptides of L-glutamic acid, L-aspartic acid and L-alanine were prepared and used in the crystallization of calcium carbonate. The crystal morphology was highly altered by the addition of the independent copolypeptides. An elongated crystal was found for the crystallization in the presence of poly(L-aspartic acid-co-L-alanine) and a crystal with round features was found for the crystallization with poly(L-glutamic acid-co-L-alanine). The fluorescent-labeled polypeptides were incorporated in the crystals. The enzymatic degradation of the polypeptides and biohybrid block copolymers containing L-glutamic acid and L-alanine was also studied. The enzymes elastase and thermolysin were used for this study, since these are known to be selective towards L-alanine-containing peptide bonds. First, biohybrid block copolymers were prepared by using NMRP in combination with NCA ROP. In the hydrophilic polypeptide block the quantity of the L-alanine was altered to direct enzymatic degradation. The hydrophobic block was either polystyrene with a Tg of 100 ºC or poly(n-butylacrylate) with a Tg of -49 ºC. In phosphate buffer solutions these biohybrid block copolymers formed micelles and vesicles. Upon addition of the enzymes, the poly(n-butylacrylate)-containing polymers with a 50% L-alanine content in the hydrophilic block did give an enzymatic response, manifesting itself as an increased particle size and precipitation. For the polystyrene biohybrid block copolymers no response was found for the same polypeptide composition, due to the stability of the high Tg core or membrane material. Block copolypeptides of L-glutamic acid and L-alanine were prepared by living NCA ROP and were found to self-assemble into vesicles in water. A first attempt was made to make vesicles with cellmembrane recognition combined with an enzymatic release trigger for targeted delivery.
|Qualification||Doctor of Philosophy|
|Award date||24 May 2011|
|Place of Publication||Eindhoven|
|Publication status||Published - 2011|