Charge-Shifting Polycations with Tunable Rates of Hydrolysis: Effect of Backbone Substituents on Poly[2-(dimethylamino)ethyl acrylates]

While polycations based on 2-(dimethylamino)ethyl methacrylate and 2-(dimethylamino)ethyl acrylate are used in applications ranging from biomaterials to wastewater treatment, few studies have considered the remarkable differences in the hydrolytic stabilities of the respective ester groups. Here, we describe how the nature of nonmethyl alpha-substituents affect the rates of ester hydrolysis of such polymers, with an emphasis on the resulting shift of net polymer charge from cationic toward anionic. We introduce 2(dimethylamino)ethyl 2-hydroxymethyl acrylate (DHMA) as a new, very hydrolytically labile, cationic monomer that can be used to form homopolymers as well as a means to tune copolymer hydrolysis. DHMA synthesis and free radical polymerization are described, including reactivity ratios for hydroxyl-protected derivatives of DHMA and 2-(dimethylamino)ethyl acrylate (DMAEA). Hydrolyses of PDHMA, P[DHMA-co-DMAEA], PDMAEA, and PDMAEMA in pH 5 and 7 buffer are reported. The presence of the hydroxymethyl alpha-substituent in PDHMA led to rates of hydrolysis 2-3 orders of magnitude faster than the already rapid hydrolysis of PDMAEA. Furthermore, hydrolysis rates of P[DHMA-co-DMAEA] copolymers were shown to increase as the DHMA mole fraction increased. As a result, a new route to adjusting the charge-shifting rates of such polycations in aqueous media is described.