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The hydrolytic properties of the novel biodegradable thermosensitive poly(organophosphazenes) with methoxypoly(ethylene glycol) (MPEG) and amino acid esters as side groups have been studied by means of gel permeation chromatography and 31 P and 1 H NMR spectroscopy and by identification of the hydrolysis products. The polymers substituted with R-amino acid esters were hydrolyzed faster than that with β-amino acid ester. The higher content of the amino acid ester in the polymer backbone caused enhanced hydrolysis. The rate of the polymer degradation decreased in the order of methyl > ethyl > benzyl esters. The polymer hydrolysis occurred more rapidly in both acidic and basic buffer solutions than in the neutral solution. The 31 P NMR spectra of the polymers with high content of glycine ethyl ester showed that the polyphosphazene backbone underwent fragmentation mostly to small molecules after incubation in the buffer solution of pH 10 for 26 days. Phosphates and ammonia were formed as hydrolysis products in most cases. The hydrolytic behaviors of the present thermosensitive polyphosphazenes are consistent with the conventional acid-catalyzed degradation mechanism, and a detailed pathway to their hydrolytic degradation is proposed. The salt and pH effects on the thermosensitivity of the polymers were also examined by measuring their lower critical solution temperature (LCST) in aqueous solutions containing various inorganic and organic salts. When various inorganic salts were added to aqueous solutions of the polymers, their salting-in and salting-out effects were found to be mainly dependent on the anions of the salts. On the other hand, in the case of tetraalkylammonium halides which are organic salts, cations seem to play an important role: the salting-in effect is stronger with increasing alkyl chain of the ammonium salt. The aqueous solutions of the polymers showed higher LCST in the acidic solution than in the neutral and basic buffer solutions.

Novel thermosensitive poly(organophosphazenes) bearing methoxy-poly(ethylene glycol) (MPEG) and amino acid esters as substituents have been synthesized, and their lower critical solution temperature (LCST) was investigated. Differential scanning calorimetry (DSC) has shown that some of the polymers exhibit crystallinity, which is probably induced by the MPEG side chain of the polymers. Most of the polymers show their LCSTs in the range of 25.0−98.5 °C, depending on several factors such as mole ratio of the substituents, molecular weight of the MPEG, and kinds of amino acids and esters. The more hydrophilic composition of the polymers offers the higher LCST. The LCST of the polymers exhibits almost concentration-independent behavior in the range of 3−30 wt % of the polymers in aqueous solution.