Solvent Programmable Polymers Based on Restricted Rotation Yagang Zhang, Judith M. Lavin, and Ken D. Shimizu* Department of Chemistry and Biochemistry, UniVersity of South Carolina, Columbia, South Carolina 29208 Received May 25, 2009; E-mail: shimizu@mail.chem.sc.edu The ability to control and manipulate recognition properties of polymer gels is important for many applications including chro- matography, drug delivery, and biosensors. 1 An attractive strategy has been to develop stimuli-responsive gels that can change their chemical properties in response to heat, solvent, and pH. 1d,f,2 A limitation of this strategy is that the stimuli-induced structural changes are typically fragile and unstable. Thus, the gels revert back to their equilibrium states upon removal of the stimuli. Reported, herein, is a new class of solvent programmable polymers (SPPs) based on restricted rotation with the ability to respond and remember their stimuli-induced properties (Scheme 1). 3 Like other stimuli-responsive polymers, the recognition properties can be modulated by heating in different solvents. At elevated temperatures, the carboxylic acid recognition groups have free rotation and can switch their relative orientations in response to the solvent. For example, heating the SPP in polar solvents increased the number of solvent accessible carboxylic groups. Conversely, heating in nonpolar solvents decreases the number of solvent accessible carboxylic acids. On cooling to rt, these solvent-induced changes were “saved” due to restricted rotation about the C aryl -N imide bonds. 4 Thus, the orientation of the carboxylic acid groups were maintained even when the imprinting solvent is removed or exchanged. The solvent-induced changes are also reversible, and the binding properties can be modulated by cycling between heating the polymer in a polar and nonpolar solvent. The SPP was prepared via ROMP polymerization of monomer 1 with restricted rotation and diimide cross-linker 2 (Scheme 2). 5 A high molar percentage of cross-linker (80%) was used to ensure that 1 was rigidly fixed within the polymer framework. The resulting polymer gel was not soluble in water or organic solvents. However, the interiors of the polymers were solvent accessible as they swelled to 30% v/v and 50% v/v in water and acetonitrile, respectively. A control polymer (CP) was prepared under the same conditions, using monomer 3 that has free rotation around its C aryl -N imide bond. To verify that the polymerized ring opened products of 1 would also have restricted rotation, the conformational stability of model compound 4, which has a similar cis-fused bicyclic framework as the ring-opened products of 1, was studied. 6 The rotamers of 4 were stable at rt. A rotational barrier of 27.7 kcal/mol was measured by following the kinetics of isomerization. This barrier equates to a half-life of 0.58 y at 24 °C and 90 min at 83 °C. The switching and memory properties of the SPP were initially tested by heating the polymer (83 °C, 26 h) in solvents of varying polarity from cyclohexane to water (Figure 1). After heating, the polymers were cooled to rt and the imprinting solvents were removed in Vacuo. The solvent-induced changes were assessed by measuring the binding capacities of the polymers for ethyl adenine- 9-acetate (EA9A) in acetonitrile. 7 This basic guest is known to form strong H-bonding interactions with carboxylic acids. 4a-c,8 Thus, an increase or decrease in the number of solvent accessible carboxylic acids would result in an increase or decrease in the binding capacity of the polymers for EA9A. The solvent-induced conformational changes were evident by the wide variation in binding properties of the polymers (Figure 1). The polymers heated in the most polar solvent (water) had a 50% higher binding capacity than the polymers heated in nonpolar solvent (cyclohexane). A sigmoidal correlation was observed between the polarities (E T N ) of the imprinting solvent and the binding capacities of the solvent- imprinted polymers. 9 These solvent studies demonstrate the ability to attenuate and tailor the binding capacities of the SPP simply by heating in a solvent of appropriate polarity. The solvent trends were also consistent with the carboxylic acids favoring orientations that maximized their contact with solvents of matching polarity. To establish that the changes in the binding capacity of the SPP were coupled to restricted rotation, the rates of the solvent imprinting processes were measured (Figure 2) and compared with the rotational barriers of atropisomer model system 4. A high- Scheme 1. A Solvent Programmable Polymer (SPP) That Modulates Its Recognition Properties When Heated in Different Polarity Solvents a a The solvent induced changes are maintained after cooling and removal of solvent due to the presence of a monomer with restricted rotation. Scheme 2. Synthesis of the SPP and Depictions of Control Monomer 3 Which Lacks Restricted Rotation and Atropisomeric Model Compound 4 Published on Web 08/05/2009 10.1021/ja904234w CCC: $40.75  2009 American Chemical Society 12062 9 J. AM. CHEM. SOC. 2009, 131, 12062–12063