Synthesis and Characterization of Photoresponsive N

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Synthesis and Characterization of Photoresponsive N-Isopropylacrylamide Cotelomers Arnaud Desponds and Ruth Freitag* Center of Biotechnology, Faculty of Basic Sciences, Swiss Federal Institute of Technology Lausanne, 1015 Ecublens, Switzerland Received November 29, 2002. In Final Form: April 30, 2003 Free radical chain transfer polymerization of N-isopropylacrylamide (NIPAM) and three different succinimide-bearing comonomers was used to synthesize semitelechelic cotelomers characterized by a statistical distribution of the comonomers, low polydispersity, and controlled molar mass. Upon temperature increase, aqueous solutions of the cotelomers underwent a reversible phase transition (precipitation) once the critical solution temperature was surpassed. The critical solution temperature of the cotelomers showed characteristic differences to the one observed for PNIPAM homopolymers. A polymer analogous route exploiting the N-hydroxysuccinimide groups of the cotelomers was then used to introduce light-responsive azobenzene groups into the molecules. By irradiation (305 nm), the azo group is switched from the more hydrophobic trans to the more hydrophilic cis state. The half-life of the cis state in the dark was approximately 13 h. The influence of the temperature, the pH, and irradiation on the temperature-induced phase transition was studied by turbidimetry, UV-vis spectroscopy, and microcalorimetry. The observed shifts of the critical solution temperature were determined not only by the hydrophobic/hydrophilic balance and the chemical microstructure of the cotelomers but also by their conformation in solution. Whereas most cotelomers showed the expected increase of the critical solution temperature upon irradiation, at least one cotelomer was found with a lowered critical solution temperature in the irradiated state.

Introduction Stimulus-responsive polymers show abrupt property changes in response to small changes in external parameters such as the temperature, the pH, or the presence of certain chemicals. Stimulus-responsive polymers that show extreme changes in their water solubility have received much attention in recent years, because of their application potential ranging from drug delivery to artificial muscles and “smart” paints.1,2 The best known example for a reversibly water-soluble polymer is polyN-isopropylacrylamide (PNIPAM), which undergoes heatinduced phase transition at approximately 32 °C in pure water.3,4 Below the critical solution temperature, the polymer is soluble in water because of the strong hydrogen bonding between the water molecules and the N-isopropylacrylamide groups. An increase in temperature disrupts the hydrogen bonds and favors hydrophobic interactions.4 When the solution reaches the critical solution temperature, the individual polymer chains change from the well-dissolved coil state to the less soluble globular state,5 and on the macroscopic scale the aqueous solution becomes turbid. While the temperature and also the addition of certain chemicals including protons (pH-shift) are well-established triggers for inducing the phase separation, light, perhaps the most “noninvasive” trigger of all, is to date not used. For some putative applications, this is perhaps due to the ubiquitousness of “light” in our natural surrounding. However, the lack of suitable photoresponsive molecules (reproducible, well-characterized, homogeneous, easily activated/conjugated), but also the fact that irradiation * To whom correspondence may be addressed: phone, +41 21 693 6108; fax, +41 21 693 6030; e-mail, ruth.freitag@ uni-bayreuth.de. (1) Hoffman, A. S. In Controlled Drug Delivery; Park, K., Ed.; American Chemical Society: Washington DC, 1997; p 485. (2) Galaev, I. Y.; Mattiasson, B. Trends Biotechnol. 1999, 17, 335. (3) Heskins, M.; Guillet, J. E. J. Macromol. Sci. Chem. 1969, 2, 1441. (4) Schild, H. G. Prog. Polym. Sci. 1992, 7, 163. (5) Wu, C.; Zhou, S. Macromolecules 1995, 28, 8381.

renders most photoresponsive molecules more soluble, while for most applications one would prefer irradiationinduced precipitation, certainly contribute to the current lack of use. A few possible applications of photoresponsive polymers are nevertheless discussed in the pertinent literature, for instance, reversible optical storage,6 polymer viscosity control,7 enforced conformational changes in polypeptides,8 and modulation of the antibody-antigen recognition.9 The photoresponsive polymers used in such cases are generally based on the type of molecule introduced by the group of Irie10 more than 10 years ago, i.e., copolymers of N-isopropylacrylamide and comonomers containing azobenzene groups. Upon UV irradiation the azobenzene groups in these polymers switch from the normal trans configuration to a more polar cis configuration.11 As a result, the critical solution temperature of the photoresponsive polymer changes (increases) depending on the degree of isomerization.12 While the concept proposed by Irie et al. is intriguing, the molecules originally proposed by them have also some drawbacks, for instance, the irreproducibility and pronounced polydispersity typically found in the polymers produced by direct copolymerization of NIPAM with azobenzene-containing monomers.10,13 The limited number of azobenzene-containing molecules that are suitable for direct copolymerization is also problematic. Most importantly, since many applications require the (6) Natansohn, A.; Rochon, P. Can. J. Chem. 2001, 79, 1093. (7) Waddington, J. C. B.; Lovrien, R. J. Am. Chem. Soc. 1964, 86, 2315. (8) Pieroni, O.; Fissi, A.; Ciardelli, F. React. Funct. Polym. 1995, 26, 185. (9) Konak, C.; Kopeckova, P.; Kopecek, J. Pure Appl. Chem. 1997, A34, 2103. (10) Irie, M.; Kungwatchakun, D. Makromol. Chem., Rapid Commun. 1988, 9, 243. (11) Rau, H. In Photochemistry and photophysics; Rabek, J. K., Ed.; CRC Press: Boca Raton, FL, 1990; Vol. II, p 119. (12) Irie, M. Adv. Polym. Sci. 1990, 94, 28. (13) Ross, D. L.; Blanc, J. In Photochromism; Brown, G. H., Ed.; Wiley-Interscience: New York, 1971; Chapter 5.

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further modification of the stimulus-responsive agent, e.g., the conjugation of a biologically active molecule, the intrinsic lack of reactive groups in polymers produced by direct copolymerization constitutes another drawback. In this paper, we introduce a new concept for the synthesis of polymers with light-controlled water solubility, which may help to overcome these difficulties. Our approach is based on free radical chain transfer copolymerization. Free radical chain transfer polymerization (telomerization) itself has elicited much interest, since this method permits production of homogeneous polymers of (controllable) low molar mass and polydispersity through the use of a chain transfer agent.14 In addition, a functional group is introduced at the end of the polymer chain, which can be used for further modification of the polymer (semitelechelic polymers). The application potential of such molecules is high and especially stimulusresponsive PNIPAM homotelomers have been studied in detail. However, to date only a very few publications exist that address the cotelomerization of N-isopropylacrylamide and a second monomer,15,16 and certainly none that deal with the preparation of photoresponsive PNIPAM cotelomers. Here telomerization was used for the first time for the synthesis of functionalized, photoresponsive telomers of N-isopropylacrylamide and N-hydroxysuccinimidoalkylacrylamides. Other than in the Irie approach, the lightresponsive group is introduced into these molecules by a separate step. As a consequence two monomers of nearly identical reactivity can be used to prepare the cotelomer, and hence a truly statistical monomer distribution is obtained. The dependency of the phase transition observed in aqueous solutions of the resulting photopolymers on the temperature, the pH, and the irradiation was subsequently studied in detail. Most cotelomers showed the expected increase of the critical solution temperature. However, the chain transfer approach allows an unusual control over the polymer structure, and by carefully balancing the contributions of the different components to the overall solubility, at least one cotelomer type could be produced, where the opposite was the case, i.e., a molecule that could be precipitated by irradiation. Experimental Section Chemicals. N-Isopropylacrylamide (NIPAM), N-acryloxysuccinimide (NAS), methyl 3-mercaptopropionate (MPP, chain transfer agent), p-aminoazobenzene, 4-phenylazophenol, 3aminopropylbromide, N-hydroxysuccinimide (NHS), and N,N′dicyclohexylcarbodiimide (DCC) were obtained from SigmaAldrich. 2,2′-Azobisisobutyronitrile (AIBN, initiator) and triethylamine were obtained from Fluka. Analytical grade diethyl ether, dimethylformamide (DMF), tetrahydrofuran (THF), methanol, hexane, and dioxane were obtained from Fluka and CarloErba. AIBN was recrystallized from diethyl ether prior to use. The chain transfer agent MPP was purified by distillation under reduced pressure. Dry organic solvents were directly taken from the continuous distillation apparatus in the laboratory. Water was purified with an Elix-3 water purification system (Millipore, USA). General Methods For the photoirradiation experiments, the polymer solutions were irradiated with an Oriel 100-W highdensity mercury lamp. The beam was collimated through an optic fiber and filtered with 305 nm (long-pass) and cutoff (>440 nm) Oriel filters. The light intensity at 305 nm was ca. 10 mW (14) Starks, C. M. Free Radical Telomerization; Academic Press Inc.: New York, 1974. (15) Okano, T.; Takei, Y. G.; Aoki, T.; Sanui, K.; Ogata, N.; Sakurai, Y. Bioconjugate Chem. 1993, 4, 341. (16) Bulmus, V.; Ding, Z.; Long, C. J.; Stayton, P. S.; Hoffman, A. S. Bioconjugate Chem. 2000, 11, 78.

Desponds and Freitag cm-2, the distance between polymer sample and irradiation source was approximately 2 cm. Synthesis Protocols. (3-Aminopropyloxy)azobenzene (Chromophore Group).17 4-Phenylazophenol (1.5 g, 7.6 mmol) and finely crushed sodium hydroxide (3.0 g, 0.075 mol) were mixed in 400 mL of dried dimethylformamide for 20 min under argon atmosphere. 3-Aminopropylbromide HBr (1.8 g, 8.2 mmol) was added, and the mixture was stirred at room temperature for 18 h. A second portion of 3-aminopropylbromide HBr was added, and the stirring was continued for another 2 h. The reaction progress was followed by thin-layer chromatography. After the solvent was removed by distillation, the crude product was dissolved in 300 mL of water and the aqueous phase extracted with three portions of ethyl acetate. The collected organic phases were dried over sodium sulfate and then evaporated in a vacuum. The residue was eluted over silica gel with a ethyl acetate/ methanol mixture to give 0.41 g (yield: 22%) of a red-orange solid. 1H NMR (CDCl3): 1.6 (s, 2H, NH2), 2.0 (q, 2H, CH2-CH2CH2), 2.9 (t, 2H, N-CH2-CH2), 4.14 (t, 2H, CH2-CH2-O), 7.03 (m, 2H, aromatic H), 7.43 (m, 3H, aromatic H), 7.9 (m, 4H, aromatic H). ESI-MS m/z: 256.147 (H+). N-Hydroxysuccinimide Ester of 2-Carboxyisopropylacrylamide (NHSIPAM). The precursor 2-carboxyisopropylacrylamide was synthesized by a three-step protocol following a procedure proposed by Okano et al.18 (total yield 30%). 1H NMR (CDCl3): 1.6 (s, 2H, NH2), 2.0 (q, 2H, CH2-CH2-CH2), 2.9 (t, 2H, N-CH2CH2), 4.14 (t, 2H, CH2-CH2-O), 7.03 (m, 2H, aromatic H), 7.43 (m, 3H, aromatic H), 7.9 (m, 4H, aromatic H). To produce the NHSIPAM, the 2-carboxyisopropylacrylamide (0.414 g, 2.64 mmol) and N-hydroxysuccinimide (0.334 g, 2.912 mmol) were dissolved in 35 mL of dry DMF and placed in a three-necked flask. After the solution was cooled in an ice bath, a solution of N,N′-dicyclohexylcarbodiimide (0.65 mL, 2.91 mmol) in DMF was added dropwise. After 40 min, the ice bath was removed and the solution stirred at room temperature for 24 h. After the urea precipitate was filtered off, the solvent was evaporated in a vacuum to yield a viscous oil. This crude oil was resuspended in 50 mL of dry tert-butyl methyl ether overnight. The solution was filtered, and the solvent was evaporated in a vacuum to yield 0.41 g of powder (1.61 mmol, yield 61%). 1H NMR (CDCl3): 1.15 (d, 3H, CH3-CH-CH2), 2.0 (m, 2H, CH3-CH-CH2), 2.7 (m, 4H, -CH2-CON), 4.1-4.4 (m, 1H, CH3-CH-CH2), 5.6, 5.9-6.5 (m, 3H, CHdCH2), 8.6 (s, 1H, CO-NH). ESI-MS m/z: 277.17 (Na+). N-Hydroxysuccinimide Ester of 10-Undecenoic Acid (NHSUDA). The same procedure as described above was used, albeit with 10-undecenoic acid as reactant and ethyl acetate as solvent. Yield: 50%. 1H NMR (CDCl3): 1.25 (m, 8H, (CH2)2-(CH2)4(CH2)2CO), 1.73 (m, 4H, (CH2-CH2-(CH2)4-CH2-CH2-COO), 2.03 (m, 2H, -CH2-CHdCH2), 2.6 (t, 2H, -CH2-CO), 2.8 (m, 4H, -CH2-CON), 5.0, 5.7-6.0 (m, 3H, CHdCH2). ESI-MS m/z: 304.16 (Na+). Cotelomer Synthesis. The molar ratios of NIPAM/NHSalkylacrylamide/initiator/chain transfer agent employed in the cotelomerizations are indicated in Table 1. A typical telomerization was carried out as follows: A solution of N-isopropylacrylamide and the respective N-hydroxysuccinimidoalkylacrylamide in dioxane was placed in a two-necked flask and purged with nitrogen for 30 min. The mixture was heated to 60 °C under positive argon pressure, and the indicated amount of AIBN in 1 mL of dioxane was added with a degassed syringe. The reactants were gently stirred for 18 h at 60 °C. The solution was cooled to room temperature, and the cotelomer precipitated in dry diethyl ether. It was further purified by reprecipitation from dioxane into diethyl ether and from acetone into hexane (all solvents being anhydrous). Cotelomer Derivatization with Photoresponsive Groups. The photoresponsive polymers were obtained by the catalyzed reaction of p-aminoazobenzene or alternatively (3-aminopropyloxy)azobenzene with the succinimide group of the cotelomers. An exemplary procedure would be the following: telo(NIPAM-coNAS) (Mw ) 2813 g/mol, 0.3 g, 0.13 mmol), triethylamine (60 µL, (17) Etemad-Moghadam, G.; Ding, L.; Tadj, F.; Meunier, B. Tetrahedron 1989, 45, 2641. (18) Aoyagi, T.; Ebara, M.; Sakai, K.; Sakurai, Y.; Okano, T. J. Biomater. Sci., Polym. Ed. 2000, 11, 101.

Photoresponsive Polymers

Langmuir, Vol. 19, No. 15, 2003 6263 Table 1. Synthesis Conditions of the Telomers NHS comonomer (mol %)

polymer

in feed

in copolymera

[Mtot]/[I]/[S]b

Mn

Mw

DP

D

Psp

yield (%)

telo(NIPAM-co-NAS)-5 telo(NIPAM-co-NAS)-10 telo(NIPAM-co-NAS)-20 telo(NIPAM-co-NHSIPAM)-8 telo(NIPAM-co-NHSIPAM)-8 telo(NIPAM-co-NHSUDA)-5

5 10 20 8 8 5

4.4 11 25 9 7.7 1

100:1:3 100:1:3 100:1:3 100:1:3 100:1:1 100:1:3

3214 2737 2188 3576 4394 2733c

3482 2813 2362 3774 4747

28 22 17 31 38

1.08 1.03 1.08 1.06 1.08

13.6 18.2 21.2 31.2 44

45 34 36 39 58 21

a Determined by NMR. b Molar ratio of total monomer [M], chain transfer agent [S], and initiator [I]. c Evaluated by NMR by integration of the ester end group.

0.43 mmol), and (3-aminopropyloxy)azobenzene (66.0 mg, 0.26 mmol) were dissolved in 5 mL of dry DMF or THF. The mixture was stirred under argon atmosphere in the dark and at room temperature for 2-10 days. The polymer was precipitated into 200 mL of diethyl ether and reprecipitated once from the same solvent. To hydrolyze the remaining succinimide groups, the crude photoresponsive polymer was added to a solution of water/THF (50:50 v/v) containing 3% v/v of 5 M NaOH, and the mixture was stirred one night at 4 °C. The photoresponsive polymer was purified by dialysis (1000 Da cutoff) against cold water and freezedried to produce 0.24 g of final product (85% yield). Analytics. Telomerization Kinetics. The monomer conversion was determined by 1H NMR (Bruker 200 MHz). Samples were analyzed at a concentration of 10 mg mL-1 in CDCl3. The conversion was determined by a comparison of the integrated monomer peaks CdC-H (around 6 ppm) with the (CH3)2-CH peaks of the polymer plus monomer at 4 ppm for NIPAM, the -(CH2)2- peaks of the polymer plus monomer at 2.8-3.0 ppm for NAS, and the broad alkyl peaks (CH3 + CH2 + CH) observed between 0.9 and 2.4 ppm. The average of the two integration results was used to obtain the respective concentrations of the two monomers as a function of the time. The incorporation of the NAS monomer was also calculated by 1H NMR. The multiplet at 2.8 ppm assigned to the succinimide methylene protons was compared with the methylene proton of the NIPAM isopropyl group at 4 ppm and with the total protons of the spectra. The average value of the two integrations was taken as the NAS incorporation (in mol %). The incorporation of NHSIPAM was calculated in the same manner as described for NAS. The consumption of the chain transfer agent MPP was followed by the Ellman test19 in aliquots regularly taken of the reaction mixture (with a blank consisting of the same quantity of PNIPAM and reactant). The chain transfer coefficient CT was determined for the monomer NIPAM and methyl 3-mercaptopropionate as chain transfer agent by using the method of O’Brien et al.20 In this method, the advancement of the reaction is connected to the concentration of the chain transfer agent

ln

[RSH]0 [RSH]

) CT ln(1 - RM)

RM ) ([M]0 - [M])/[M]0 with [M]0, [RSH]0, [M], and [RSH] being the initial concentrations and the concentrations during the reaction of the monomer and the telogen (in our case methyl 3-mercaptopropionate), respectively. By plotting ln([RSH]0/[RSH]) as a function of ([M]/[M]0), a straight line is obtained with a slope equal to CT. The reactivity ratios for medium-high conversion were evaluated by the extended Kelen-Tu¨dos method21 based on the Mayo-Lewis terminal model.22 Spectrometry. The 1H NMR spectra were recorded with a WM 400 (400 MHz) and a WM 200 FT spectrometer (200 MHz, both Bruker). Unless indicated otherwise, CDCl3 was used as solvent. The mass spectra of the low molar mass compounds were acquired on a LCT mass spectrometer from Micromass (Man(19) Ellman, G. L. Arch. Biochem. Biophys. 1959, 82, 70. (20) O’Brien, J. L.; Gornick, F. J. Am. Chem. Soc. 1955, 77, 4757. (21) Tu¨dos, F.; Kelen, T.; Foldes-Berezsnich, T.; Turcsanyi, B. J. Macromol. Sci. Chem. 1975, A10, 1513. (22) Mayo, F. R.; Lewis, F. M. J. Am. Chem. Soc. 1944, 66, 1594.

chester, U.K.). This instrument combines an electrospray ionization source with a TOF (time-of-flight) mass analyzer. The LCT mass spectrometer was used in the positive ionization mode for all experiments. The ES+ Source parameters were adjusted as a function of the tested products. The molar mass of the telomers was determined by MALDIMS (matrix-assisted laser desorption/ionization mass spectrometry) using a PerSeptive Biosystems Voyager-DE STR (Atheris Laboratories, Geneva, Switzerland). 3,5-Dimethoxy-4-hydroxycinnamic acid was used as the matrix and insulin as external calibrant. The apparent weight average of the molar mass, Mw, and the apparent number average of the molar mass, Mn, were calculated from the mass spectra using the following formula23

Mn ) NiMi /Ni

Mw ) NiMi2/NiMi

with Mi ) mass of a given unimolecular oligomer species in a given sample and Ni ) number of molecules of that mass in the preparation. The degree of polydispersity, D, was calculated as D ) Mw/Mn. The content of azobenzene chromophore in the photoresponsive polymers but also the quantity of residual NHS-groups was determined by UV spectrometry in methanol using the absorption coefficient of N-(4-phenylazophenyl)propionamide and by 1H NMR in DMSO and CDCl3. Turbidity Measurements. The turbidity of aqueous cotelomer solutions as a function of the temperature was measured at 650 nm with a Lambda 20 spectrophotometer (Perkin-Elmer) equipped with a PTP 6 thermostat (Perkin-Elmer) and a magnetic stirrer for constant agitation of the contents in the measurement cells. Heating rates were 1 °C/min. The point of inflection of the S-shaped transmission curves (usually approximated as halfheight) was taken as the cloud point. The turbidity curves of the photopolymers were recorded in 0.05 M acetate or phosphatebuffered solutions (constant ionic strength). The polymer concentration in these experiments typically was 5-7 g L-1. Differential Scanning Calorimetry. Microcalorimetric measurements were performed with a 6100 Nano DSC II (Calorimetry Science Corporation Inc.) at an external pressure of 3.0 atm. The cell volume was 0.33 mL, and the heating rate was 1 °C/min. The aqueous solutions for the analysis (5 g L-1 telomer) were prepared the day before the measurement and stocked at 4 °C in the fridge.

Results and Discussion Irie et al. have proposed a first approach for the synthesis of photoresponsive polymers that called for the direct copolymerization of N-isopropylacrylamide with azobenzene-containing monomers.10 While successful, this procedure led to pronounced irreproducibility and polydispersity in the produced polymers. It has also been shown that the azobenzene content in the copolymerization mixture has a direct influence on the molar mass of the final product.24 To circumvent these difficulties, an alternative synthesis protocol is proposed by us in this contribution, which is based on a two-step approach. In the first step the principle of telomerization is used to (23) Bahr, U.; Deppe, A.; Karas, M.; Hillenkamp, F.; Giessman, U. Anal. Chem. 1992, 64, 2866. (24) Kro¨ger, R.; Menzel, H.; Hallensleben, M. L. Macromol. Chem. Phys. 1994, 195, 2291.

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Figure 1. Schematic presentation of the two-step approach to the synthesis of photoresponsive polymers taking the cotelomerization of NIPAM and NHSIPAM as an exemplary first and the coupling of the chromophore (3-aminopropyloxy)azobenzene as an exemplary second step.

prepare a homogeneous thermo- but not photoresponsive cotelomer backbone. In the second step the chromophore functions are coupled to this backbone by a catalyzed polymer-analogous reaction of the side chains. Figure 1 shows a general scheme of this approach taking the cotelomerization of NIPAM and NHSIPAM as an exemplary first and the coupling of the chromophore (3aminopropyloxy)azobenzene as an exemplary second step. Synthesis and Characterization of PNIPAM Cotelomers. Three succinimidoxy-containing molecules were chosen as NIPAM comonomers, namely, N-acryloxysuccinimide (NAS), the N-hydroxysuccinimide ester of 2-carboxyisopropylacrylamide (NHSIPAM), and the Nhydroxysuccinimide ester of 10-undecenoic acid (NHSUDA). Due to their chemical structure, these comonomers should have similar reactivities as NIPAM itself, which is prerequisite to the production of a randomly distributed cotelomer. The experimental conditions (Table 1) were designed to ensure the reproducible production of low molar mass, low polydispersity products. When the reactivity ratios for medium-high conversion were evaluated in the case of a NIPAM/NAS cotelomerization, values of r1 ) 0.76 and r2 ) 0.73 were obtained for NAS and NIPAM, respectively. Such values support our assumption that the cotelomerization of these monomers is likely to produce randomly distributed telomers with a small content of segments with alternating monomers. In the case of NIPAM and NHSIPAM, very similar reactivity ratios can be assumed due to the quasiidentical structure of the polymerizable groups in the two monomers. NHSUDA was expected to be markedly less reactive than NIPAM with its very reactive acrylamide group. Indeed, we find a much lower NHSUDA content in the telomers than in the initial reaction mixture, Table 1. The chain transfer coefficient CT for NIPAM and MPP was determined for three telomerizations performed with varying ratios of the monomer concentration to telogen

concentration (corresponding to [RSH]0/[M]0 ratios of 0.05, 0.1, and 0.2). These experiments yielded a mean value of CT ) 0.51 (correlation coefficient R2 ) 0.991). This value is reasonably close to the value of 0.32 found by Kopececk et al.25 for the telomerization of (2-hydroxypropyl)methacrylamide and fits well with the coefficient range of 0.6-1.7 calculated for telomerization of acrylamide with different thiols.26,27 This value, however, does differ markedly from the extremely low value of 0.00663 given by Okano et al.28 for the telomerization of NIPAM with mercaptopropionic acid as chain transfer agent. The molar mass, degree of polymerization, and polydispersity of the cotelomers were determined by MALDITOF MS, Table 1. This high-resolution technique allows determining directly the molar mass of individual polymer chains and by consequence their chemical composition.29 The technique is thus especially powerful for twocomponent copolymer analysis.30,31 The MALDI-TOF spectrum of the telo(NIPAM-co-NAS)-5 is shown as an example in Figure 2a. The mass spectrum is composed of several series of peaks stemming from the numerous statistically distributed species produced by the random cotelomerization. Despite its limited resolution, the spectrum was deconvoluted and a dominant peak series was found (peak series A) at m/z ) 113.16n + 169.1m + 120.1 + 1.0(H+). In this context, n is the number of NIPAM units, m the number of NAS units and 120.1 the molar (25) Kopecek, J.; Lu, Z. R.; Kopeckova, P.; Wu, Z. Bioconjugate Chem. 1998, 9, 793. (26) Pichot, C.; Pellicer, R.; Grossette, P.; Guillot, J. Makromol. Chem. 1984, 185, 113. (27) Boutevin, B.; Mouanda, J.; Pietrasanta, Y.; Taha, M. J. Polym. Sci., Part A 1986, 24, 2891. (28) Okano, T.; Takei, Y. G.; Aoki, T.; Sanui, K.; Ogata, N.; Sakurai, Y. Bioconjugate Chem. 1993, 4, 42. (29) Ra¨der, H. J.; Schrepp, W. Acta Polym. 1998, 49, 272. (30) Montaudo, G.; Garozzo, D.; Montaudo, M. S.; Puglisi, C.; Samperi, F. Macromolecules 1995, 28, 7983. (31) Montaudo, G.; Montaudo, M. S.; Puglisi, C.; Samperi, F. Rapid Commun. Mass Spectrom. 1998, 12, 519.


Figure 2. (a) MALDI-TOF mass spectrum of telo(NIPAMco-NAS)-5. (b) MALDI-TOF mass spectrum of telo(NIPAM-coNHSIPAM)-8.

mass of the chain transfer agent MPP. A second major peak series, dubbed series B, corresponded to m/z ) 113.16(n + 2) + 169.1(m + 1) + 120.1 + 1.0(H+). The MALDI-TOF spectrum of telo(NIPAM-co-NHSIPAM)-8 (Figure 2b) also yielded two main peak series: one with m/z ) 113.16n + 304.1p + 120.1 + 1.0(H+) (p corresponds to the number of NHSIPAM units) and a second one with m/z ) 113.16n + 304.1(p + 1) + 120.1 + 1.0(H+). The difference in resolution between the two types of cotelomer was probably due to the different solubility of the respective monomers in the matrix used for the MALDI experiments.29 Similar MALDI-TOF spectra were recorded for the other cotelomers and showed the expected peak series (data not shown). These results confirmed that the telomers were indeed mainly formed by the chain transfer reactions, i.e., initiation of each chain by the radical formed by the chain transfer agent and terminated by proton transfer. At the same time, the deconvolution of the MALDI-TOF spectra proved that the cotelomers were semitelechelic (ester end group). In addition, the presence of the ester end group was confirmed for all cotelomers by the appearance of the methoxy protons at 3.7 ppm on the 1H NMR spectra. In all cases the degree of polymerization of the cotelomers was below the theoretical one (calculated as suggested by Boutevin et al.32,33). The reasons of this discrepancy may be found in the dependence of the propagation and chain transfer rate constants on the chain length. In addition, methyl 3-mercaptopropionate is prone to some side (32) Boutevin, B.; Parisi, J. P.; Vaneechhoutte, P. Eur. Polym. J. 1990, 26, 1027. (33) Boutevin, B.; Parisi, J. P.; Vaneechhoutte, P. Eur. Polym. J. 1990, 27, 159.

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reactions such as disulfide bridge formation and Michaeltype thiol addition on the monomers’ double bonds.34 The polydispersity of the cotelomers was determined by calculating D as defined in the Experimental Section. However, this definition strictly applies only to homopolymers with narrow polymer mass distribution. As an alternative way to represent this important parameter in the case of the cotelomers, we chose to adapt a concept recently proposed by Tatro et al. for the MALDI analysis of the width of molar mass distributions.35 In this case the polymer spread (Psp), was calculated by dividing the width at half-height of the spectrum by the mass of the monomer(s). Whereas the D values were similar for all cotelomers, the Psp values differed and increased with the degree of polymerization, Table 1. Synthesis of the Photoresponsive Polymers. Photoresponsive polymers were prepared by polymer analogous modification of the cotelomers with azobenzenecontaining chromophores. By using already NHS-activated cotelomers and a highly reactive chromophore ((3-aminopropyloxy)azobenzene), we minimized the problems associated with the typical slowness of polymer analogue reactions.36 Three types of photoresponsive polymers were synthesized based on the three cotelomers described in the previous section, dubbed Photo-NAS, PhotoNHSIPAM, and Photo-NHSUDA. In all cases it was not possible to completely convert the NHS groups into the corresponding azobenzene side chains. After a reaction time of 2 weeks, there were still unreacted N-hydrosuccinimido groups present in the cotelomers as evidenced, e.g., by the persistence of a multiplet at 2.8 ppm in the corresponding 1H NMR spectra. A similar difficulty was also encountered by Winnik et al.37 which in their case was linked to the formation of a small quantity of pseudoaggregates of the polymers in the solvent DMF or THF. The existence of a solvent-mediated intermolecular association of N-isopropylacrylamide polymer chains in organic media has been reported a number of times.38,39 However, the exact cause is still not fully understood.40 In our case, remaining NHS groups were hydrolyzed after the coupling reaction, leaving a certain amount of carboxylic acid functions in the final photopolymers, which were consequently containing three types of monomeric units, NIPAM units, chromophore-bearing units, and carboxylic acid-bearing units, Figure 3 and Table 2. These photopolymers allowed us to study not only the effect of the azobenzene content per se but also a putative influence of the azobenzene group spacer length and structure on the photoresponsiveness of the ensuing polymer. Of the investigated photopolymers PhotoNHSIPAM comes closest to a PNIPAM homopolymer, since the backbone structure realized by these two comonomers is very similar. In the case of Photo-NAS the polymer backbone consists instead of NIPAM and (modified) acrylic acid moieties, while Photo-NHSUDA carries the azo group linked by a highly hydrophobic linear C8 alkyl chain. In the case of Photo-NAS and Photo(34) Griesbaum, K. Angew. Chem., Int. Ed. 1990, 9, 273. (35) Tatro, S. R.; Baker, G. R.; Fleming, R.; Harmon, J. P. Polymer 2002, 43, 2329. (36) Odian, G. Principles of Polymerization; Wiley-Interscience: New York, 1991. (37) Winnik, F. M.; Yamazaki, A.; Song, J. M.; Brash, J. L. Macromolecules 1998, 31, 109. (38) Ganachaud, F.; Monteiro, M. J.; Gilbert, R. G.; Dourges, M. A.; Thang, S. H.; Rizzardo, E. Macromolecules 2000, 33, 6738. (39) Zhou, S.; Fan, S.; Au-Yeung, S. C. F.; Wu, C. Polymer 1995, 36, 1341. (40) Smithenry, D. W.; Kang, M. S.; Gupta, V. K. Macromolecules 2001, 34, 8503.

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Figure 3. Chemical structure and composition of the photoresponsive telomers. Table 2. Physical Properties of the Photoresponsive Telomers

photopolymer

prepolymer

photo-NHSIPAMa photo-NHSIPAMb photo-NASa photo-NASb photo-NHSUDA

telo(NIPAM-co-NHSIPAM)-8 telo(NIPAM-co-NHSIPAM)-8 telo(NIPAM-co-NAS)-10 telo(NIPAM-co-NAS)-20 telo(/NIPAM-co-NHSUDA)-5

azo content (mol %) by UV by NMR 3.8 4.8 6.0 2.1 0.6

NHSIPAM, two polymers (a and b) were prepared that differed in azo and carboxylic acid group content. Solubility of the Photoresponsive Polymers in Aqueous Solution. PNIPAM homopolymers have a critical solution temperature in pure water of approximately 32 °C. The introduction of comonomers into a PNIPAM backbone is known to influence the critical solution temperature of the corresponding copolymers in a characteristic manner.41,42 Charged groups, for instance, hinder the intermolecular aggregation of the polymer chains during the coil-to-globule transition and thus increase the critical solution temperature, while hydrophobic groups have the opposite effect. The temperature dependency of the solubility of the photopolymers in water was subsequently studied as a function of the chemical structure of these molecules but also as a function of the (41) Deng, Y.; Pelton, R. Macromolecules 1995, 28, 9392. (42) Chen, G.; Hoffman, A. S. Nature 1995, 373, 49.

1 4 5 2.5 1

COOH content (mol %) 5 4.5 5 22.5 4

critical solution temperature (°C) pH 4 pH 6 UV 29.3 27.3 17 39.1 33.7

33.6 31.0 24.1 35.0

28.6 27.6 18.8 35.8 33

λ max (nm) MeOH H 2O 350 351 346 344 379

347 349 345 347 330

pH (pH 4, 6, and 7, Figure 4), since the presence of carboxylic acid groups in the photopolymers was expected to render the solubility of these molecules pH-sensitive. At pH 4 the carboxylic groups are protonated (uncharged). At higher pH, the carboxylate groups release their proton and as a consequence electrostatic repulsion between the polymer chains may become significant. In addition, complexation has been reported between PNIPAM homopolymers and polyacids such as poly(acrylic acid).43,44 Since both groups are present in the photopolymers, attractive inter- and intramolecular interactions could a priori not be ruled out, although they are somewhat unlikely to dominate the solubility behavior under the investigated conditions (pH range of 4-7, low salt). For the measurements, all molecules were in the dark photostationary state; i.e., the azobenzene groups were (43) Dautzenberg, H.; Gao, Y. B.; Hahn, M. Langmuir 2000, 16, 9070. (44) Kratz, K.; Hellweg, T.; Eimer W. Colloid Surf., A 2000, 170, 137.


Figure 4. Influence of pH on the phase transition profiles of Photo-NHSIPAMb (pH 4, 9; pH 6, b; pH 7, 2) and Photo-NASa (pH 4, 0; pH 6, O; pH 7, 4).

mainly (ca. 80%) in the stable trans conformation. The hydrophobicity of the original cotelomers was found to increase markedly with increasing azobenzene content, and as a result, the critical solution temperature decreased to a point where the photopolymers were not soluble anymore in water. According to these findings (data not shown), the azobenzene content in a given photopolymer should not exceed 5 mol % in order to maintain a critical solution temperature in pure water that is equal or superior to room temperature. The presence of carboxylic acid groups counterbalanced this hydrophobicity increase to some extent. As a result the photopolymers differed significantly in their solubility as a function of the comonomer chemistry and content, Table 2. For example, at pH 4 the critical solution temperature of Photo-NASa (5 mol % azo, 5 mol % COOH groups) is very much decreased (17 °C) compared to the PNIPAM homopolymer, whereas the critical solution temperature of Photo-NASb (2.5 mol % azo, 22.5 mol % COOH groups) is increased (39.1 °C). The pronounced influence of the microstructure of the carboxylic comonomer on the thermoprecipitation behavior of the photopolymers was further corroborated by a comparison with the behavior of Photo-NHSIPAM. In this case the cotelomer backbone is composed of almost identical repeating isopropylamide moieties, mimicking the backbone of the PNIPAM homopolymer to a high degree. For a similar composition (4.8 mol % azo, 5 mol % COOH groups) as Photo-NASa, the critical solution temperature of Photo-NHSIPAM is only lowered to 27 °C. The phase transition is very sharp, and a shift by only 4 °C is observed when the pH is increased from 4 to 6. At pH 7, the onset of the turbidity curve occurred at roughly the same temperature as for pH 6; however, the final turbidity of the solution remains somewhat lower. In the case of Photo-NASa, the phase transition was more strongly influenced by the pH and a shift of 6-7 °C was observed in the critical solution temperature when the pH was increased from 4 to 6. Above a pH of 6, no phase transition was observable. With 33.7 °C, the cloud point measured for PhotoNHSUDA was almost identical to that of the PNIPAM homopolymer. Moreover, only a 1-2 °C shift of the critical solution temperature was observed upon changing the pH from 4 to 6. This was surprising, since normally, even a small quantity of hydrophobic moieties is sufficient to promote the aggregation in water, i.e., to lower the critical solution temperature. Typically, large hydrated clusters are formed in such cases by random association of interpolymeric hydrophobic domains and/or by pointlike contacts.9 However, it has been shown that as long as the hydrophobic segments can aggregate into an isolated

Langmuir, Vol. 19, No. 15, 2003 6267

Figure 5. Absorption spectra of Photo-NHSIPAMb in water (s) and methanol (- - -) before (dark state) and after (photostationary state) UV irradiation. The spectra were normalized with the π-π* band surface for better comparison.

hydrophobic core, e.g., small micelle-like clusters with the hydrophobic moieties in the core and extended hydrated PNIPAM chains forming the micellar hydrophilic shell,37,45 the thermoresponsive properties of the outer mobile PNIPAM chains remain unaltered.46,47 Below the cloud point, the aggregation of the micellar structure is prevented by steric repulsion of the extended hydrated PNIPAM chains. Only when these chains collapse upon heating does aggregation of the micelles and the macroscopic phase transition take place. The temperature of the phase transition in such cases is therefore very similar to that of PNIPAM homopolymers. A similar phenomenon could in our case be responsible for the observed solubility behavior of Photo-NHSUDA. Photocontrol of the Polymer Phase Transition in Aqueous Solution. A photoeffect on the solubility behavior was expected as a result of the trans to cis isomerization of the azobenzene groups in the side chains of the cotelomers upon irradiation. The photoinduced isomerization of azobenzene causes large structural changes in the molecules, which are reflected by changes in the physicochemical behavior. In particular the distance between the para carbon atoms in azobenzene changes from ca. 9 Å in the trans to 5.5 Å in the cis form. Likewise, trans-azobenzene has no dipole moment, while the dipole moment of cis-azobenzene is 3.0 D.48 At room temperature and in the dark, the azo chromophores are predominantly in the more stable trans configuration. The UV-vis absorption spectra of the solutions showed two main bands, a major one at 350 nm and a smaller one at 430 nm. Figure 5 shows as a typical example the spectra recorded for Photo-NHSIPAMb. Upon irradiation at 305 nm, the major band progressively decreased and shifted to slightly shorter wavelengths (ca. 335 nm). A state, in which the side chain azo groups are mainly in the cis configuration (no further changes in the UV-vis spectra), was reached within 2 h in water for all investigated photopolymers. The kinetics of the cis-trans thermal back isomerization were shown to fit the relation for first-order reaction kinetics. The relaxation times had a typical half-time period of t1/2 ≈ 13 h. The half-life of the cis photostationary state is therefore sufficient to allow, e.g., the measure(45) Winnik, F. M.; Ringsdorf, H.; Simon, J. Macromolecules 1992, 25, 7306. (46) Principi, T.; Goh, C. E. E.; Liu, C. W.; Winnik, F. M. Macromolecules 2000, 33, 2958. (47) Okano, T.; Chung, J. E.; Yokoyama, M.; Aoyagi, Y.; Sakurai, Y. J. Controlled Release 1998, 53, 119. (48) Delang, J. L.; Robertson, J. M.; Woodward, I. Proc. R. Soc. London, Sect. A 1939, 171, 398

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Figure 6. Influence of UV irradiation on the phase transition profiles of Photo-NASa (before, 0; after, 4), Photo-NHSIPAMb (before, +; after, ×) and Photo-NASb (before, 9; after, 2).

ments of the critical solution temperature of the photopolymers in the irradiated cis state. When methanol was used as solvent instead of water, a small increase of the percentage of cis isomers upon UV irradiation and of trans isomers in the dark state was observed. The cloud points of the photopolymers in aqueous solution were measured before and after UV irradiation, Table 2, Figure 6. A pH value of 4 was chosen for these studies, because at this pH the carboxylic acid groups in the side chains were mostly protonated and therefore an influence of their charges on the solubility behavior of the photopolymers could be excluded. For PhotoNHSIPAM and Photo-NHSUDA, only small changes of the critical solution temperature were detected upon UV irradiation. For all practical purposes these differences fall within the margin of experimental error ((0.2 °C). Moreover, no clear trend was observed for an increase of the molar content of azobenzene moieties from 1% (PhotoNHSIPAMa) to 5% (Photo-NHSIPAMb). The only difference was the broader profile of the phase transition in the case of Photo-NHSIPAMb. These results confirmed the pH-related observations for these polymers; i.e., the phase transition behavior is dominated by the cooperative interactions of the NIPAM moieties of the polymer backbone irrespective of the conformation of the pendant azobenzene groups. In the case of Photo-NASa, the critical solution temperature was 2 °C higher when the azo groups were in the cis conformation compared to the same molecule with the side chains in the trans conformation. This was the expected effect of an irradiation upon the solubility of an azobenzene group containing photopolymer,10 since the cis isomer has a higher polarity due to its higher dipole moment.13 Other than expected and in contrast to PhotoNASa, however, the critical solution temperature for Photo-NASb was 3 °C lower in the predominately cis compared to the trans state, Figure 6. Photo-NASb could hence be precipitated by irradiation. This is especially surprising, since Photo-NASa and Photo-NASb have the same polymer microstructure. The two polymers differ only in the amounts of azobenzene and carboxylic acid groups in their side chains (5 mol % azo and 5 mol % COOH groups for Photo-NASa versus 2.5 mol % azo and 22.5 mol % COOH groups for Photo-NASb). Due to the higher content of hydrophilic groups and the lower content of azobenzene moieties, the Photo-NASb chains could be expected to dissolve in a more extended form than the Photo-NASa ones. The observed difference in the photo-


responsive behavior could therefore be due to different molecular assemblies of the two photopolymers in water. A similar inversion in the trend of the critical solution temperature has been observed by Kro¨ger49 for azobenzene-containing copolymers of alkylacrylamides. The formation of micelles and their disruption upon UV irradiation was proposed in that case to explain the phenomenon. However, the fact that both Photo-NASa and Photo-NASb show a significantly different critical solution temperature compared to PNIPAM homopolymer argues against the formation of micelles. Hence the argument put forward by Kro¨ger cannot be used to explain the behavior of Photo-NASb. Azobenzene-containing amphiphiles are well-known to form aggregates when dispersed in aqueous media.50,51 Recent investigations have shown that these azo aggregates are stabilized by strong noncovalent aromatic-aromatic interactions and that the stacking of azobenzene groups is favored in their trans planar configuration.52 Shimomura has shown that in the case of azobenzene-containing amphiphiles, the absorption spectrum in water reflected the aggregate structure as follows: H-aggregate (λ max ≈ 300 nm), dimeric chromophore type (330-340 nm), the isolated chromophore type (ca. 355 nm), and J-aggregate (350-390 nm).53,54 To study the possible aggregation of the azo photopolymers, the absorbance spectra of their aqueous solutions and in methanol were measured between 250 < λ < 600 nm with the azo groups in the dark photostationary state (almost 80% of trans isomers). The absorption spectra of all photopolymers save Photo-NASb showed a small blue shift of the λ max in water compared to the value measured in methanol, Table 2. The blue shift of the λmax in water is especially pronounced for Photo-NHSUDA. This incidentally supports the hypothesis of a formation of micellar assemblies with a stable hydrophobic inner core in aqueous solution for this polymer. A blue shift in water as compared to methanol is in this case easily accounted for by the assumption of a partial stacking of the azobenzene groups mainly in the form of dimers in water as opposed to methanol (mainly monomers). For Photo-NASb on the other hand, a λ max red shift was observed in water. This λ max red shift argues for an even greater tendency of this particular molecule to present isolated (nonstacked) chromophore groups in water than in methanol. This corroborates the hypothesis that the formation of micelles (or other aggregates) is not responsible for the observed inversion of the change in critical solution temperature upon irradiation in the case of PhotoNASb. The opposite trends observed for the critical solution temperature upon irradiation of aqueous solutions of Photo-NASb may instead be related to a difference in the phase transition mechanism itself. It was shown that the “coil-to-globule” transition of PNIPAM homopolymers in dilute aqueous solution is not an “all-or-none” process. The collapse of the PNIPAM chains involves a cooperative collapse of independent chain segments with a length in the range of ca. 90-500 NIPAM units.55-57 Our photoresponsive cotelomers have a molar mass, Mw, below 5000 g/mol and thus do not require (49) Kro¨ger, R. In Polymere mit lichtgesteuerter Wasserlo¨schlichkeit. Ph.D. Thesis, Hannover University, Hannover, 1995; Chapter 5. (50) Heesemann, J. J. Am. Chem. Soc. 1980, 102, 2167. (51) Konak, C.; Kopeckova, P.; Kopecek, J. Macromolecules 1992, 25, 5, 5451. (52) Song, X.; Perlstein, J.; Fleming, R.; Whitten, D. G. J. Am. Chem. Soc. 1997, 119, 9144. (53) Shimomura, M.; Kunitake, T. Chem. Lett. 1981, 1001. (54) Shimomura, M.; Ando, R.; Kunitake, T. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 1134.


Langmuir, Vol. 19, No. 15, 2003 6269

Figure 7. Schematic representation of the different mechanisms involved in the precipitation of Photo-NASa and Photo-NASb.

cooperative chain movement during phase transition. The chain of such a low molar mass polymer may thus collapse (intrachain collapse) before interchain aggregation takes place, whereas for high molar mass polymer chains, the transition is always a mixture of interchain aggregation and intrachain collapse as demonstrated by Wu et al. for poly(N-isopropylacrylamide-co-acrylic acid) chains.58 By analogy, we propose a two-step mechanism for the photopolymer phase transition. The scheme of the possible precipitation mechanism is shown in Figure 7, where for the sake of clarity individual polymer chains are drawn although the phenomenon takes place in more concentrated aqueous solutions and, as a result, the entanglement of chains should be more pronounced. In the case of Photo-NASb a given extended and therefore flexible hydrated polymer chain upon heating first collapses into a small assembly with the hydrophobic NIPAM segments and the planar (trans) azobenzene moieties stacked together in the center, while thesmanys hydrophilic carboxylic moieties form an outer shell. Subsequently, the collapsed chains aggregate into large particles and produce the macroscopically observed phase separation. By comparison, a solution of Photo-NASa below the critical solution temperature is characterized by partially entangled photopolymer chains. Inter- and intrachain stacking of the azobenzene groups (higher content of azo groups in Photo-NASa than in Photo-NASb) in the side chains reduces the mobility of the single chains and therefore upon heating the intrachain collapse and (55) Tiktopulo, E. I.; Bychkova, V. E.; Ricka, J.; Ptitsyn, O. B. Macromolecules 1994, 27, 2879. (56) Tiktopulo, E. I.; Bychkova, V. E.; Uversky, V. N.; Klenin, S. I.; Ptitsyn, O. B.; Lushchik, V. B. Macromolecules 1995, 28, 7519. (57) Wu, C.; Zhou, S. Phys. Rev. Lett. 1996, 14, 3053. (58) Qiu, X.; Kwan, C. M.; Wu, C. Macromolecules 1997, 30, 6090.

the interchain aggregation take place simultaneously, very much like in the case of larger PNIPAM-polymers. Upon UV irradiation, the trans-cis isomerization of the azobenzene groups will result in an increase of the polarity of the interpolymeric microdomains and at the same time the overall hydrophilicity of the polymer chains will increase. In the case of Photo-NASa the result is an increase in the critical solution temperature. In the case of Photo-NASb irradiation also changes the planar geometry of the azobenzene moieties and increases markedly the polarity of the azo side chains. However, since this polymer is dissolved predominantly in the form of individual chains, the more polar cis azo segments serve mainly to destabilize the hydrophobic inner core formed during the intrachain collapse of the NIPAM units. The density of the hydrophobic core therefore decreases and the temperature at which the phase transition occurs is reduced. A similar destabilization upon UV irradiation of aqueous aggregates of azobenzene-containing N-(2hydroxypropyl)methacrylamide copolymers formed just below the limit for molecular solubility in water was observed by Kopecek et al.51 The change of phase transition behavior upon UV irradiation is therefore not only dependent on the hydrophobic/hydrophilic balance of the photoresponsive telomers but also on the assembly structure of the photopolymer in aqueous solution before and during the heat-induced phase transition. To validate the proposed mechanism, a more thorough analysis of the photoresponsive properties of the telomers is at present performed in our laboratory. Microcalorimetric Study of the Phase Transition of Photopolymers. Microcalorimetry permits determination of the thermodynamic data of the phase transition

6270



moieties. For the three photopolymers, a decrease of the heat capacity at high temperature was observed after the phase transition. This heat capacity change is attributed to the decrease of polymer-water contacts after the polymer precipitation.55 The smaller decrease of heat capacity observed for Photo-NASb is probably due to the extensive hydration of the phototelomer chains. Conclusion

Figure 8. Endotherms of Photo-NHSIPAMa (- ‚ -), PhotoNASa (- - -), and Photo-NASb (s) (in their dark photostationary state). Table 3. Microcalorimetry Analysis of the Photoresponsive Telomers DSC

Photo-NHSIPAMa Photo-NASa Photo-NASb

turbidimetry LCST (°C)

LCST (°C)

∆H (kJ/mol)/unit

29.3 17.0 39.1

27.5 15.7 34.3

5.357 5.362 5.011

of polymers in dilute aqueous solutions.59 In addition, this thermal analytical technique allows the measurement of the true transition temperature thus overcoming some uncertainties inherent to the cloud-point determination by turbidimetry. The changes with temperature of the molar heat capacities of Photo-NHSIPAMa, Photo-NASa, and Photo-NASb in buffered solutions were measured at pH 4. The phase transition temperatures and the corresponding enthalpies are given in Table 3. The measured endotherms are shown in Figure 8. The transition enthalpy was ca. 5 kJ/mol of monomeric unit for the different photopolymers. This value corresponds to the loss of one hydrogen bond per monomeric unit during the phase transition.60 It confirms that the triggering of the photopolymer chain collapse is mostly due to the NIPAM (59) Ladbury, J. E.; Chowdhry, B. Z. Biocalorimetry; John Wiley & Sons Ltd.: Chichester, U.K., 1997.

As a first conclusion of these studies, we would like to propose an improved strategy for the synthesis of photoresponsive polymers, which minimize the problems associated with the well-known slowness of polymer analogue reactions. In a first step, the photopolymer precursor was prepared as a cotelomer of NIPAM and highly reactive succinimide-containing comonomers. Second, a highly reactive chromophore derivative was synthesized for the coupling reaction, namely, (3-aminopropyloxy)azobenzene. This compound carries the reactive primary amine group at the end of a linear alkyl chain and therefore the influence (steric hindrance) of the aromatic rings of the azobenzene group was greatly reduced during the coupling reaction. The solution properties of the photopolymers in water were subsequently studied by turbidimetry and microcalorimetry. The conclusion of this part of the study is that the temperature-induced phase transition of the azobenzene-containing telomers can be efficiently controlled by electromagnetic irradiation. The photoisomerization of the incorporated chromophores results in a shift of up to 3 °C of the critical solution temperature of the telomers. This change of phase transition behavior upon UV-vis irradiation is largely dependent on the hydrophobic/hydrophilic content of the telomers and on the conformation of the photopolymer chains in solution. The physics of the phase transition play an important role in determining the actual critical solution temperature of the telomers. Acknowledgment. This work was supported by the Swiss National Science Foundation (Grant No. 2063530.00 to R.F.). Special thanks is due to Stephane Canarelli for technical assistance and help with interpreting the mass spectroscopy data. LA020944X (60) Fujishige, S.; Kubota, K.; Ando, I. J. Phys. Chem. 1989, 93, 3311.

Synthesis and Characterization of Photoresponsive N

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