Synthesis and characterization of tailorable biodegradable thermoresponsive methacryloylamide polymers based on l-serine and l-threonine alkyl esters

A series of monomers based on the methyl, ethyl, and isopropyl esters of Nα-(methacryloyl)-serine and -threonine were synthesized, and used in an AIBN-initiated radical polymerization reaction to yield polymers with an Mn ranging between 6.6 and 23.8 kDa. The newly synthesized polymers showed LCST behavior in aqueous solution that could be tailored by subtle variations of the hydrophobicity of the monomers to obtain a broad range of cloud points between 1.5 and >100°C. According to HPLC, the hydrolytic t1/2-values (pH 7.4 at 37°C) of the monomers were found to be 5, 12, and 40 days of the methyl, ethyl, and isopropyl esters, respectively, while the hydrolysis rate of poly[Nα-(methacryloyl)-Ser-OMe] and poly[Nα-(methacryloyl)-Thr-OMe] was found to be significantly lower compared to the corresponding monomers. In order to obtain thermoresponsive nanoparticles, Nα-(methacryloyl)-Thr-OEt was polymerized with (PEG monomethyl ether 5000)2-ABCPA as macroinitiator to yield an amphiphilic block co-polymer, poly[Nα-(methacryloyl)-Thr-OEt]-b-(PEG monomethyl ether 5000), which forms particles of 300 nm at a temperature higher than its cloud point of 24°C. Incubation at physiological conditions induced ester hydrolysis resulting in a destabilization of the particles making these particles suitable for drug delivery purposes.