Fundamental Molecular Design for Precise Control of

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Fundamental Molecular Design for Precise Control of ThermoResponsiveness of Organic Polymers by Using Ternary Systems Shogo Amemori, Kenta Kokado, and Kazuki Sada J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja301688u • Publication Date (Web): 30 Apr 2012 Downloaded from http://pubs.acs.org on May 2, 2012

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Journal of the American Chemical Society

Fundamental Molecular Design for Precise Control of ThermoResponsiveness of Organic Polymers by Using Ternary Systems Shogo Amemori,† Kenta Kokado,†,‡ and Kazuki Sada*,†,‡ †Department

of Chemical Sciences and Engineering, Graduate School of Chemical Sciences and Engineering, Hokkaido University, ‡Department of Chemistry, Graduate School of Science, Hokkaido University, Kita10nishi8, Kita-ku, Sapporo, Japan thermoresponsive polymers, lower critical solution temperature, upper critical solution temperature, hydrogen bonding, urea

Supporting Information Placeholder ABSTRACT: De novo design of thermosensitive polymer in solution has been achieved by using addition of small organic molecules (or “effectors”). Hydrogen bonding as attractive interaction for the polymer-polymer or polymer-effector substantially dominates the responsivity, causing facile switching between LCST-type and UCST-type phase transition, control of transition temperature, and further coincidence of both transitions. Small molecules having a high affinity for polymer induce UCST-type phase behavior, whereas they having a low affinity for polymer showed LCST-type phase behavior.

Thermo-sensitive polymers exhibiting drastic change of polymer solubility upon heating (LCST; lower critical solution temperature) or upon cooling (UCST; upper critical solution temperature) have been attracting continuous interest toward designing smart materials.1 According to general model of LCST behavior of amphiphilic polymers such as poly(N-isopropylacrylamide) (PNIPAM) in water, the phase transitions are generally induced by imbalance and competition between polymer-polymer interaction and hydration of polymer, which is substantially affected by heating or cooling.2 Supramolecular interactions between the unit structure of polymer chain and small molecules around polymers such as H2O should play a crucial role. The reverse solubility transition, i.e., UCST, is originated from breaking of interactions among polymer chains by heating.3 For examples, poly(acrylic acid)3a or polymers containing zwitter ionic groupsb shows UCST in water. Owing to the importance in development of stimuli-responsive materials, tuning the phase transition temperatures around ambient regions (0 °C to 100 °C) has been one of the interesting topics for supramolecular and polymer chemistry. A plethora of researches have focused on the control of phase transition of organic polymers including PNIPAM by variation of concentration, solvents, molecular weights, and copolymerization to achieve integrated control of these phase transition behavior.4 However, de novo design for LCST and UCST behavior still remained unclear. In this communication, we present that control

of intermolecular interaction broken by heating can easily attain LCST-type and UCST-type phase transitions, wide-range control of their transition temperatures, and further coincidence of both transitions, only using single polymeric component.

Figure 1. (a) Concept of the thermo-sensitive polymer controlled LCST type and UCST type by additives. (b) Synthetic route for urea polymer 1.

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Table 1. Thermal behavior of polymer 1 (25 mg/mL (0.11 M)) in the presence of small molecules in 1,2-dichloroehtane. Concentration of small molecules and phase transition temperature are in parentheses.

In order to realize a desirable thermoresponsivity at ambient temperature, we selected the following ternary system consisting of polymer, “effector” (small molecules effect on solubility), and solvent; (1) The polymer should contain strong hydrogen bonding functional groups as attractive interaction between the polymer chains. (2) Surrounding the polymers, small amount of effector, providing relatively weak hydrogen bonding functional groups interacting with the polymer chains. (3) The solvent as background needs to be inert toward the hydrogen bonding, meaning that it can rarely interfere with the hydrogen bonds of polymer chain-polymer chain or polymer chain-effector. In this report, we shed light on phase transition behavior of urea polymers by adding 510 equivalents of the hydrogen bonding guest molecules as an effector in aprotic nonpolar media. Moreover, the chemo-selective switching of thermal behavior was accomplished by changing the strength of hydrogen bonding of the functional group in effectors. As a platform polymer for the ternary system, ureamodified acrylate polymer 1 was designed (Figure 1a). A urea moiety was selected as a hydrogen bonding functional group which has attractive interaction because of its remarkable ability to form one-dimensional aggregates in aprotic organic solvents.5 Urea-modified monomer 3 was synthesized by condensation reaction of acryloyl chloride and urea-modified alcohol 2 in an acceptable yield (Figure 1b). The structure of monomer 3 was confirmed by 1H NMR, IR, electron spray ionization mass spectroscopies, and elemental analysis. Polymer 1 was obtained via a controlled radical polymerization (CRP) of monomer 3 using dithioester 4 as the chain transfer agent (CTA).6 The molecular weights of polymer 1 was determined by size exclusion chromatography (SEC, PEO standard) to be Mn = 2.47×104 and Mw/Mn = 1.92.

Figure 2. Transmittance measurement of 1 (25 mg·cm-3, 0.11 M) in the presence of 1-dodecanol (0.57 M) in 1,2dichloroethane. Recorded at 700 nm. Heating and cooling rate: 1 °C·min-1.

First, we checked the solubility of the polymer 1 in various organic solvents. In aprotic solvent such as acetonitrile, THF, 1,2-dichloroethane, and hexane, the polymer 1 was practically insoluble because of the strong hydrogen bonds between the urea moieties on the polymer chain. On the contrary in methanol, ethanol and hexanol, the polymer showed high solubility (>100 mg/mL) because these protic polar solvents can form hydrogen bonds with the urea groups and cleave the association of polymers. This result further prompted us to investigate control of LCST and UCST behavior of 1 in the presence of “effectors” in non-polar media with low solubility for 1 by perturbation of hydrogen bonds between the urea groups.


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The solubility changes of the polymer 1 upon heating was investigated in 1,2-dichloroethane as the solvent in the presence of various small molecules such as alcohols, amides, ureas and carboxylic acids (Table 1). By gradually increasing equivalent of small molecules, 25 mg·cm-3 (0.11 M of the urea units in the polymer) of 1 in 1,2dichloroethane was changed from insoluble to soluble. Moreover at the boundary state of insoluble and soluble, thermo-sensitive behavior, i.e., LCST or UCST was observed. For an example, Figure 2 shows the transmittance change of polymer 1 with 5 equivalents of 1dodecanol in 1,2-dichloroethane upon heating, and LCST was observed at 49 °C (The clouding point was determined as the temperature with 90% transmittance). UCST-type phase separation was observed upon addition of aliphatic carboxylic acids, dialkylureas and tetrahexylammonium bromide as shown in Table 1. In contrast, addition of alcohols such as 1-dodecanol, pyrenebutanol and cholesterol resulted in LCST-type phase separation. Until now, few examples of LCST polymer in organic solvents have been reported,7 due to the lack of de novo design, and they were accidentally found out through laborious trial an error. For a series of aliphatic amides with different alkyl chain length, shorter and longer alky amides exhibited UCST-type and LCST-type type phase separation, respectively. Furthermore, polyhydroxy compounds such as neopentyl glycol and triethanolamine exhibited UCST-type phase separation, indicating that increase of the number of hydroxyl groups changed from LCST-type phase separation to UCST-type. These results presumably indicate that the hydrogen bonding abilities between the urea groups and the effectors play a key role for control of the thermo-sensitivity. The effectors that can interact strongly to the urea groups tended to induce UCST-type phase separation and those weakly did LCSTtype one.

Figure 3. Transmittance measurement of 1 (25 mg·cm-3) in the presence of 1-dodecanol (0.39 M) and N,N’butyloctylurea (0.10 M) in 1,2-dichloroethane. Recorded at 700 nm. Heating and cooling rate: 1 °C·min-1.

In order to estimate the strength of interaction between effectors and urea moieties, 1H NMR spectra of monomer 3 were measured in 1,2-dichloroethane-d4 with

effectors such as tetrahexylammonium bromide, 1dodecanoic acid , N-butylpropylamide and Noctylpropylamide for UCST, and N-dodecylpropylamide, N-hexadecylpropylamide and 1-dodecanol for LCST (Figure S1). In all cases, with increasing amount of effectors, the N-H signal of urea shifted to lower magnetic field, whereas the shift values for each effector widely differed. For deeper understanding, the association constants (Ka) for the 3/effector pairs were determined by using Benesi-Hildebrand plots of 1/∆δobs versus 1/[E]0, where ∆δobs is the variation of chemical shift, [E]0 is initial concentration of the effector, respectively (Table S1). The larger Ka (>50 M-1) was observed in the presence of tetrabutylammonium bromide and 1-dodecanoic acid, which induced UCST behavior. On the other hand, the addition of 1-dodecanol showed smaller Ka (< 1M-1), which induced LCST behavior. In the case of dialkyl substituted amides which have middle Ka (1-3 M-1), the thermal behavior was depended on the alkyl chain length. Probably, the balance between hydrophobicity and hydrophilicity are valuable in determining thermosensitivity of an effector which has middle Ka. On the basis of these findings, we envision the following mechanism; with the weaker interaction between polymer and effector than that among polymers (in the case of using the alcohols and amides with short alkyl chain), heating preferentially breaks the interaction between polymer and effector, and then the polymer shows insolubility. In the case of using the effector which interacts with polymer enough (ureas, carboxylic acids, and halides), interaction among the polymers is cleaved by heating thus the polymer show UCST behavior.

Figure 4. Distribution of solubility of 1 in the presence of 1dodecanol and N,N’-butyloctylurea between 5 °C and 80 °C. The dashed line presents assumed boundary of each thermoresponsivity.

To accomplish the desirable thermoresponsive system, we adopted a quaternary system using two kinds of effectors inducing LCST and UCST, respectively. The solubility of the polymer 1 was investigated in the presence of 1dodecanol (LCST type) and N,N’-butyloctylurea (UCST type) in 1,2-dichloethane. As a result, with increasing of



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ratio of N,N’-butyloctylurea UCST-type phase separation was easily observed, and LCST-type phase separation was caused by increasing of ratio of 1-dodecanol. Amazingly at appropriate amount of 1-dodecanol and N,N’butyloctylurea, both LCST-type and UCST-type phase separation were achieved in one sample (Figure 3). Furthermore, the system exhibited clear reproducibility upon cooling and additional heating, indicating that the certain balance of hydrogen bonding between the polymer 1 and two additives should be responsible for the result. To provide an in-depth understanding for the quaternary system, a systematic study was carried out for samples including a broad range of concentration of N,N’-butyloctylurea and 1-dodecanol. The distribution of solubility of 1 shown in Figure 4 distinctly reflects a trend of LCST-type, UCST-type, and LCST+UCST-type phase transition in the quaternary system. Additionally, the transition temperature of LCST+UCST was easily controlled over wide range of temperature (LCST: 35 °C → 66 °C, UCST: 24 °C → 9 °C, respectively) by slightly varying the amount of two effectors (1-dodecanol: 0.34 M → 0.37 M, N,N’-butyloctylurea: 0.090 M → 0.098 M, respectively, Figure 5, see also Figure S2 and Table S2). It is well-known to be difficult to induce both LCST and UCST behavior at ambient temperature.4f,g Nevertheless, these data suggested that our fundamental molecular design is applicable to the coincidence of both transitions.

Figure 5. Temperature dependence of transmittance of polymer 1 upon addition of two kinds of small molecules in 1,2-dichloroethane at various concentration of small molecules (1: 25 mg·cm-3, scan rate: 1 °C·min-1).

In conclusion, we demonstrated simple example and fundamental molecular design for precise control of thermoresponsiveness by employing single polymer 1 in 1,2-dichloroethane with addition of the effectors which are competitive to polymer-polymer interaction. Hydrogen bonding ability of the effectors substantially governs the responsivity of 1, resulting in facile switching between LCST-type and UCST-type phase transition, and further additivity of them, i.e., the coincidence. More generally, effectors having high affinities for polymers induce UCST-type phase behavior because of breaking interaction between polymers, whereas those having low affinities cannot interact with polymers at high temperature then LCST-type phase behavior was observed. In addition, we pointed out the additivity of those phase

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transitions, i.e., the coincidence of UCST and LCST-type transitions. Further work is currently underway to design smart thermosensitive systems performing by employing various interactions.

ASSOCIATED CONTENT Supporting Information. Experimental details and spectrum data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]

Funding Sources This work was supported by MEXT Global COE Program “Catalysis as the Basis for the Innovation in Materials Science” and Grant-in-Aid for Scientific Research (23350048).

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Fundamental Molecular Design for Precise Control of Thermo-Responsiveness of Organic Polymers by Using Ternary Systems Shogo Amemori, Kenta Kokado, and Kazuki Sada*


Fundamental Molecular Design for Precise Control of

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