Abstract
2-(Dimethylamino)ethyl methacrylate/styrene statistical copolymers (poly(DMAEMA-stat-styrene)) with feed compositions fDMAEMA = 80–95 mol%, (number average molecular weights Mn = 9.5–11.2 kg mol−1) were synthesized using succinimidyl ester-functionalized BlocBuilder alkoxyamine initiator at 80 °C in bulk. Polymerization rate increased three-fold on increasing fDMAEMA = 80 to 95 mol%. Linear Mn increases with conversion were observed up to about 50% conversion and obtained copolymers possessed monomodal, relatively narrow molecular weight distributions (polydispersity = 1.32–1.59). Copolymers with fDMAEMA = 80 and 90 mol% were also cleanly chain-extended with DMAEMA/styrene mixtures of 95 and 90 mol% DMAEMA, respectively, confirming the livingness of the copolymers. Copolymer phase behavior in aqueous solutions was examined by dynamic light scattering and UV-Vis spectroscopy. All copolymers exhibited lower critical solution temperature (LCST)-type behavior. LCST decreased with increasing styrene content in the copolymer and with increasing solution concentration. All copolymers were completely water-soluble and temperature insensitive at pH 4 but were more hydrophobic at pH 10, particularly copolymers with fDMAEMA = 80 and 85 mol%, which were water-insoluble. At pH 10, LCST of copolymers with fDMAEMA = 90 and 95 mol% were more than 10 °C lower compared to their solutions in neutral, de-ionized water. Block copolymers with two statistical blocks with different DMAEMA compositions exhibited a single LCST, suggesting the block segments were not distinct enough to exhibit separate LCSTs in water.
Highlights
Stimuli-responsive polymers have sharp and reversible responses to small changes in environmental conditions such as temperature, pH, light, ionic strength, electric and magnetic fields, and they have emerged as a class of materials known as “smart” materials [1,2]
To fine-tune the transition conditions and polymer properties, DMAEMA has been incorporated into block copolymers with various counter blocks such as the pH-sensitive poly(2-vinyl pyrindine) to obtain pH-dependent micelles [8], a temperature-responsive block such as POEGMA [9] and PNIPAM [11] to alter the lower critical solution temperature (LCST), and a hydrophobic block such as poly(2-(N-carbazolyl)ethyl methacrylate) to make amphiphilic diblock copolymers with hole-transport properties [12]
Aside from the alkoxyamines synthesized for methacrylates [26,27], a commercially available SG1-based unimolecular alkoxyamine called BlocBuilder (N-(2-methylpropyl)-N-(1-diethylphosphono-2,2dimethylpropyl)-O-(2-carboxylprop-2-yl) hydroxylamine; molecular structure of SG1 and BlocBuilder shown in Scheme 1(a)) has achieved controlled polymerization of a range of methacrylates, including methyl [28,29], ethyl [30], butyl [31], benzyl [32] and poly(ethylene glycol) methyl ether methacrylates [20], with a small amount (4–10 mol%) of styrene in the feed and in the presence of about 10% excess of free SG1 relative to the BlocBuilder initiator
Summary
Stimuli-responsive polymers have sharp and reversible responses to small changes in environmental conditions such as temperature, pH, light, ionic strength, electric and magnetic fields, and they have emerged as a class of materials known as “smart” materials [1,2]. Aside from the alkoxyamines synthesized for methacrylates [26,27], a commercially available SG1-based unimolecular alkoxyamine called BlocBuilder (N-(2-methylpropyl)-N-(1-diethylphosphono-2,2dimethylpropyl)-O-(2-carboxylprop-2-yl) hydroxylamine; molecular structure of SG1 and BlocBuilder shown in Scheme 1(a)) has achieved controlled polymerization of a range of methacrylates, including methyl [28,29], ethyl [30], butyl [31], benzyl [32] and poly(ethylene glycol) methyl ether methacrylates [20], with a small amount (4–10 mol%) of styrene in the feed and in the presence of about 10% excess of free SG1 relative to the BlocBuilder initiator. Synthesis of statistical copolymers of 2-(dimethylamino)ethyl methacrylate (DMAEMA) and styrene using N-hydroxysuccinimidyl-functionalized BlocBuilder (NHS-BlocBuilder) as the initiator
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