Selective Initiation from Unprotected Aminoalcohols for the N-Heterocyclic Carbene-Organocatalyzed Ring-Opening Polymerization of 2-Methyl-N-tosyl Aziridine: Telechelic and Block Copolymer Synthesis

摘要

Commercial aminoalcohols, namely, 2-(methyl amino)ethanol (1) and diethanolamine (2), are investigated as direct initiators, i.e., with no need of protection of the hydroxyl groups, for the N-heterocyclic carbene-organocatalyzed ring-opening polymerization (NHC-DROP) of 2-methyl-N-p-toluenesulfonyl aziridine. NHC-OROP's are performed at 50 degrees C in tetrahydrofuran, in the presence of 1,3-bis(isopropy1)-4,5(dimethyl)imidazol-2-ylidene ((Me)5-IPr) as organocatalyst. Thus, nonprotected and nonactivated aminoalcohol initiators 1 and 2 provide a direct access to metal-free alpha-hydroxy-omega-amino- and alpha,alpha'-bis-hydroxy-omega-amino telechelics on the basis of polyaziridine (PAz), respectively. Excellent control over molar masses, narrow dispersities (D = 1.20), and high chain-end fidelity are evidenced by combined analyses, including NMR spectroscopy, size exclusion chromatography, and MALDI ToF mass spectrometry. The amino-initiated NHC-OROP is therefore tolerant to the presence of nonprotected hydroxyl group(s). The as obtained hydroxyl-ended PAz can be further derivatized in reaction with phenyl isocyanate, highlighting the accessibility of the hydroxyl groups in alpha-position. Moreover, block copolymer synthesis can be readily achieved by sequential NHC-OROP of 2-methyl-N-p-toluenesulfonyl aziridine and L-lactide, from 1 used in this case as a double-headed initiator. Remarkably, each of the two NHC-OROP steps proves highly chemoselective, with PAz and poly(L-lactide) (PLLA) segments being grown from the secondary amino- and the primary hydroxy- function, respectively. In this way, a well-defined PAz-b-PLLA diblock copolymer is synthesized in the presence of the same (Me)5-IPr organocatalyst, i.e., following a completely metal-free strategy.