Synthesis and reactions of polyamines with living cationic end groups

Abstract

The living character (expressed by the ratio k0/kt) of the cationic ring-opening polymerization of N-substituted aziridines is strongly influenced by the number and the bulkiness of the substituents on the monomer. Thus, N-tert.butyl aziridine (TBA) has a k/kt value of iO' l.mol1 compared with 80 for N-isopropyl aziridine and 4 for N-ethylaziridine. P0lymerization of TBA initiated by methyl trifluoromethylsulfonate (methyltriflate) in a mixture of THF and HMPT allows to prepare poly-TBA with well defined molecular weight (up to 100,000) and of low dispersity. The thus obtained polymers giye a slow termination reaction which obeys first order kinetics and during which the molecular weight of the polymer remains unchanged. It is therefore assumed to be an intramolecular attack of the growing species by an amino function of its own polymer chain. Because the termination is very slow compared with propagation, it is possible to react the aziridinium ions which are the active chain ends, with a nucleophile before the termination becomes important. This reaction was first checked with a model compound (N-methyl-N-tert.butyl aziridinium tnflate) and was then carried out with living poly-TBA. In this way a number of poly-TBA's with different functional end groups were synthesized. They were characterized by 360 MHz 'H NMR spectroscopy and by GPC. If the nucleophile is part of another polymer chain, block or graft copolymers containing poly-TBA segments are obtained. Other aziridines showing a high living character are N-substituted propylenimines. Also with these monomers, polymers with predictable molecular weight and well defined end groups can be synthesized. This kind of chemistry allows to form a great variety of blockand graft copolymers containing polyamine segments.