Direct Synthesis of Well-Defined Quaternized Homopolymers and Diblock Copolymers via ATRP in Protic Media

The direct synthesis of well-defined cationic homopolymers and block copolymers based on methyl chloride-quaternized 2-(dimethylamino)ethyl methacrylate [MeDMA] by ATRP in protic media at 20 °C is described. Homopolymerization of MeDMA in purely aqueous media was fast and poorly controlled, leading to a relatively high polydispersity of 1.37 and low initiation efficiency. Addition of Cu(II)Br2 led to slower polymerizations but only slightly lower polydispersities. Addition of methanol also reduced the rate of polymerization and produced narrower molecular weight distributions. Unfortunately, 1H NMR studies indicated that transesterification of MeDMA with methanol produced significant quantities of methyl methacrylate (MMA) on the time scale of the polymerization; this side reaction led to the unwanted production of MeDMA−MMA statistical copolymers. This problem was alleviated by replacing the methanol cosolvent with 2-propanol, since the secondary alcohol was much less prone to transesterification. High conversions were obtained with a 1:1 2-propanol/water composition within a few hours at 20 °C, but partial phase separation occurred toward the end of the polymerization, particularly at higher monomer concentration. Although nonlinear kinetic plots were observed, final polydispersities were relatively low (ranging from 1.19 to 1.27, according to aqueous GPC studies), and good self-blocking efficiencies were demonstrated in chain extension experiments. A range of new cationic diblock copolymers were prepared either by using a poly(ethylene oxide)-based macroinitiator or via sequential monomer addition with various hydrophilic methacrylates such as glycerol monomethacrylate, [2-(methacryloyloxy)ethyl]phosphorylcholine, the benzyl chloride-quaternized analogue of MeDMA, and the sulfobetaine adduct of the reaction of 2-(dimethylamino)ethyl methacrylate with 1,3-propane sultone, [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide. Potential applications for these cationic diblock copolymers include novel gene/oligonucleotide transfer agents and also polymeric templates for the catalytic formation of silica in aqueous solution under mild conditions.