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43433-GB7
Tuning Lower Critical Solution Temperatures of Smart Polymers

Chunmei Li, Stephen F. Austin State University

Smart copolymers based on poly (N-isopropylacrylamide) (PNIPAM) whose solubilities in water respond to temperature changes were prepared. These polymers were soluble at room temperature but precipitated when heated to certain temperatures. This project describes tuning the precipitation temperatures of these polymers for a desired application by changing the polymer compositions. The resulting polymers can be applied to systems in which there are needs to regulate solubility with temperature, for example, to turn on/off catalytic activity according to reaction temperature. A “thermometer” was made from these polymers to demonstrate the concept. According to the solubility of a series of copolymers used in the thermometer, a temperature range can be determined if it falls within 31.4 oC and 46.5 oC. PNIPAM has a LCST at 31 oC and therefore has limited application due to its fixed precipitation temperature. A comonomer was incorporated in order to tune the LCST, taking advantage of the hydrophilicity of the second monomer. Both acrylic acid and acrylamide were tested. Random copolymers of the N-isopropylacrylamide and acrylic acid were made by free radical polymerization, with the ratio of the two monomers being 10:1, 20:1 and 40:1. To our surprise, all these copolymers have LCSTs even lower than that of the homopolymer PNIPAM. The other comonomer tested, acrylamide, did give the desired results. Random copolymers of the N-isopropylacrylamide and acrylamide were made. Different ratio of the two monomers were used for these copolymers (1:1, 2:1, 3:1, 5:1 and 10:1) in order for the LCST to fall in the desired range. The 1:1 copolymer was too soluble in water and did not precipitate even when heated to 100 oC. The other four copolymers have LCSTs of 46.5, 41.0, 34.6 and 31.4 oC, respectively. These thermoresponsive “smart polymers” have applications in many areas, for example, in serving as supports for catalysts when the reaction rates must be regulated. At low temperatures, when the reaction rates are low, the copolymer is soluble in the aqueous solution so that the catalyst supported by this copolymer can catalyze the reaction homogeneously. Once the reactions goes faster than desired and the temperature goes above the LCST, the copolymer precipitates and the catalyst falls out of solution, thus slowing down the reaction rate. Now with our approach, we will be able to systematically design these “smart catalysts” according to the desired temperatures to shut down the catalytic activity. To help the undergraduate students easily visualize the applications of these polymers, a thermometer was made using the series of poly (N-isopropylacrylamide/ acrylamide) copolymers we have made. In this thermometer, several 1 mL vials were glued together, each filled with 1 mL of water solutions of the copolymers, in the order of increasing LCST, 31.4 oC, 34.6 oC, 41.0 oC and 46.5 oC. This polymer thermometer was then used to give a temperature range when a liquid has a temperature between 31.4 oC and 46.5 oC. For example, as shown in Figure 1, when the polymer thermometer was placed in a liquid of 43 oC, a mercury thermometer gave a reading of 43 oC; in the top vial, a clear solution was maintained since 43 oC is below the LCST of the copolymer in that vial; in the second, third and bottom vials, precipitation occurred since 43 oC is above the LCST of the copolymers in those vials. From the precipitation pattern, the temperature of the liquid could be decided to be between 41.0 oC and 46.5 oC, without a mercury thermometer. Figure 1. Polymer thermometer in a solution of 43 oC Labeling the copolymers with a dye molecule, such as methyl red, to make the phase change easier to observe will also be investigated. This PRF grant and a faculty research grant from Stephen F. Austin State University were both used to support this project. The PRF grant has enabled the PI to get her research started and going. Basic lab equipments, supplies and chemicals were purchased and a functioning organic research lab was set up using the fund. More than ten undergraduate students were involved in the proposed project. All of them learnt a great amount of techniques and skills during the training, which reinforced what they learned in organic laboratory course and extended their skills far beyond that. After participating in these projects, they can follow established procedures well to set up organic reactions, isolate and purify the products obtained, including air sensitive reactions that need to be protected in inert atmosphere, and polymerizations. Their research experience greatly helped them to further understand what they learnt in sophomore organic chemistry classes. This research experience inspired some of them to keep pursuing careers in Organic Chemistry. For example, one undergraduate student went to graduate school in Ohio State University, and another one went to University of Texas, Austin.

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