Hydrolysis-improved thermosensitive polyorganophosphazenes with alpha-amino-omega-methoxy-poly(ethylene glycol) and amino acid esters as side groups

Abstract

A series of hydrolysis-improved thermosensitive polyorganophosphazenes with alpha-amino-omega-methoxy-poly(ethylene glycol) (AMPEG) and amino acid esters (AAEs) of 'N,N-systems' was synthesized, and their properties were evaluated in comparison with the thermosensitive polyorganophosphazenes with methoxy-poly(ethylene glycol) (MPEG) and AAEs of 'O,N-systems', by means of P-31 NMR spectroscopy, gel permeation chromatography (GPC) and differential scanning calorimetry (DSC). Most of the present polymers showed a lower critical solution temperature (LCST) in the range 32.0-79.0 degrees C, depending on the kinds of AAE, length of AMPEG and the mol ratio of the two substituents. These polymers exhibited higher LCSTs and faster degradation rates than the MPEG-based polymers. The aqueous solution of poly(ethyl glycinate phosphazene)-graft-poly(ethylene glycol) [NP(GlyEt)(0.94)(AMPEG350)(1.06)](n) did not show an LCST, which is presumed to be due to its high hydrophilicity, in contrast to [NP(GlyEt)(1.01)(MPEG350)(0.99)](n) which showing an LCST at 77.5 degrees C. On the other hand, the polymers with a high content of AAE or with hydrophobic amino acids such as L-aspartic acid and L-glutamic acid, have shown a similar LCST to those of the MPEG-based polymers. The half-lives (t(1/2)) for hydrolysis of [NP(AMPEG350)(1.06)(GlyEt)(0.94)](n) at pH 5, 7.4 and 10 were 9, 16, and 5 days, respectively, which are almost 2.5 to 4 times faster than that of the MPEG-based polymers. The LCST of the present N,N-polymers has been shown to be more influenced by salts such as NaCl ('salting-out' effect) and tetrapropylammonium bromide (TPAB) ('salting-in' effect) compared with the 'O,N-system'. Such differences of the 'N,N-systems' from the 'O,N-systems' in thermosensitivity, hydrolysis behavior and salt effect seem to be due to the higher hydrophilicity of the amino group in AMPEG. (c) 2005 Society of Chemical Industry.