Optimization of N-carboxyanhydride (NCA) polymerization by variation of reaction temperature and pressure

Several new methods for the controlled ring-opening polymerization of N-carboxyanhydrides (NCA ROP) have been established in the recent past, all of which based on the normal amine mechanism. In this paper the optimal NAM polymerization conditions were investigated combining the high-vacuum and the low-temperature for the NCAs of [gamma]-benzyl-l-glutamate (BLG), N[varepsilon]-benzyloxycarbonyl-l-lysine (ZLL), l-alanine (Ala), [small beta]-benzyl-l-aspartate (BLA), O-benzyl-l-serine (BLS), and O-benzyl-l-threonine (BLT). The polymerizations were followed by FTIR, size exclusion chromatography (SEC) and MALDI-ToF-MS to provide information of the monomer conversion, polymer molecular weight and chain composition as a function of pressure and temperature. It was found that the studied NCAs could be divided into two groups: in the first group monomers of BLG, ZLL and Ala polymerized considerably faster when a lower pressure of 1 [times] 10-5 bar was applied. MALDI-ToF-MS analysis confirmed that the formation of side products for these monomers mostly started after full monomer conversion. The second group of monomers, i.e. BLA, BLS and BLT, polymerized considerably slower than the first group and no effect was observed from the lower pressure. On the other hand, the number of side reactions was significant at 20 [degree]C, so that the polymerizations for the latter monomers should preferably be done at 0 [degree]C. By combining both methods, multiblock polypeptides were synthesized including a tetrablock of PBLG-b-PAla-b-PZLL-b-PBLA with a polydispersity of 1.3.