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Diblock copolymers of poly(N-isopropylacrylamide-co-N-(2-hydroxypropyl)methacrylamide lactate) (poly(NIPAAm-co-HPMAm-lactate)) as a thermosensitive block and poly(ethylene glycol) (PEG) as a hydrophilic block form polymeric micelles above the cloud point (CP) of the temperature-sensitive block. Destabilization of these micelles occurs upon hydrolysis of the lactate side chains. Here we report on the degradation kinetics of the HPMAm-mono(di)lactate monomers and their copolymers with NIPAAm. The degradation of the monomers and polymers in their soluble state (thus below their CP) followed normal ester hydrolysis behavior:  the degradation rate increased with temperature, pH (from pH 7.5 to 11), and dielectric constant of the medium. Above the CP, where the polymers are in a precipitated state, a significant retardation of the polymer degradation occurred due to a decrease of dielectric constant of the local environment of the precipitated polymer. This study shows that it is possible to predict the rate of formation of HPMAm in NIPAAm-co-HPMAm-lactate copolymers which results in an increase of the overall hydrophilicity of the polymers and destabilization of polymeric micelles based on poly(NIPAAm-co-HPMAm-lactate).

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SUMMARY: N-isopropylacrylamide (NIPAAm) was copolymerized with 2-hydroxyethyl methacrylatemonolactate (HEMA-monolactate). With increasing HEMA-monolactate content in the copolymer, the lower critical solution temperature (LCST) decreases. During incubation in an aqueous solution, the lactate groups are released by hydrolysis, by which the copolymer is converted into poly(NIPAAm-co-HEMA). By this process, the hydrophilicity of the copolymer increases, resulting in increased LCST values.

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