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Thermal and pH Responsive High Molecular Weight Poly(urethane-amine) with High Urethane Content Lin Gu,1,2 Xianhong Wang,1 Xuesi Chen,1 Xiaojiang Zhao,1 Fosong Wang1 1Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Renmin Street 5625, Changchun 130022, People’s Republic of China 2Graduate University of Chinese Academy of Science, Beijing 100039, People’s Republic of China Correspondence to: X. Wang (E-mail: xhwang@ciac.jl.cn) Received 21 June 2011; accepted 27 August 2011; published online 15 September 2011 DOI: 10.1002/pola.24981 ABSTRACT: Aliphatic poly(urethane-amine) (PUA) was synthe- sized from copolymerization of CO2 and 2-methylaziridine (MAZ) using Y(CCl3COO)3-ZnEt2-glycerine coordination catalyst, the urethane content of PUA was over 80%, and its yield could reach 90%. PUA with molecular weight as high as 31.0 kg/mol was obtained when the copolymerization reaction was carried out in N,N-dimethylacetamide (DMAc), mainly due to the good solubility of PUA in DMAc. PUA exhibited reversible thermo-re- sponsive property in deionized water, and the lower critical so- lution temperature (LCST) was highly sensitive to its urethane content and molecular weight, which was observed in a broad window from 37 to 90 �C. Furthermore, the phase transition behavior could also be controlled by change of pH value. When the pH value of the PUA aqueous solution changed from 9.2 to 13, the LCST value of the solution decreased from 48.4 �C to 30 �C. Therefore, the PUA showed thermo- and pH- dual responsive performance in water. VC 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 5162–5168, 2011 KEYWORDS: carbon dioxide; LCST; pH-responsive; poly(ur- ethane-amine); polyurethanes; stimuli-sensitive polymers; thermo-responsive; transitions; water-soluble polymers INTRODUCTION Stimuli-responsive polymers, especially for polymers undergoing phase transition in response to envi- ronmental stimuli such as temperature and pH value, have attracted much attention,1 mainly due to their potential applications in drug delivery systems,2 microactuators,3 sen- sors,4 gene transfection agent,5 etc. In addition to the single- responsive polymers,6–9 polymers that can respond to more than one stimulus are very interesting owing to their physio- logical and biological applications.10–14 Typical dual-respon- sive polymers are thermo- and pH-responsive.15–20 As far as CO2 based copolymer is concerned, Ikariya first reported a double stimuli-responsive polymer by copolymerization of CO2 and 2-methylaziridine (MAZ) in 2005, which showed sharp and rapid phase transition in response to both tem- perature and pH value.21 Since the pioneering work of Inoue in 1969,22,23 metal-cata- lyzed copolymerization of CO2 and epoxides to aliphatic pol- ycarbonates has been extensively investigated in the past four decades.24–30 Analogously, distorted three-membered cyclic amine like 2-methylaziridines (MAZ) has been reported to react with CO2 to give cyclic urethanes and poly- mers consisting of urethane and amine moieties even in the absence of a metal catalyst (Scheme 1).31,32 However, the urethane content of the poly(urethane-amine) (PUA) was generally below 0.35, since the successive ring- opening reaction of aziridines to form polyamines unit is preferred to the copolymerization between MAZ and CO2. 32 Ikariya reported that copolymerization of MAZ and CO2 under supercritical condition (100 �C, 22 MPa) gave aliphatic PUA with a high content of urethane units (0.62), but the conversion of MAZ was below 0.35.21,33,34 Earlier in 1979, Kuran reported fully alternating copolymerization of CO2 and MAZ by organozinc coordination catalyst consisting of dieth- ylzinc (ZnEt2) and a compound having multiple active hydro- gens, but the molecular weight was low in the range of 400�600.35 Aliphatic PUA shows a thermally induced reversible transi- tion property in aqueous solution at lower critical solution temperature (LCST).21,33,34 It is soluble in water below LCST, however, when the temperature increases above LCST, the polymer becomes insoluble and precipitates out from its aqueous solution, and the system becomes turbid. Its LCST can be adjusted by changing the hydrophilicity/hydrophobic- ity balance in the copolymer. A decrease in amine content of the polymer leads to decrease in LCST due to the decrease in the hydrophilicity of the polymer.21 Additional Supporting Information may be found in the online version of this article. VC 2011 Wiley Periodicals, Inc. 5162 JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2011, 49, 5162–5168 ARTICLE WWW.POLYMERCHEMISTRY.ORG It is interesting if highly alternating PUA with high molecular weight can be synthesized, which may show special thermo- and pH- dual responsive property. It has been confirmed that the rare-earth compound plays an important role in raising the yield and molecular weight of poly(propylene carbonate) in addition to maintaining a high alternating ra- tio.36 Therefore, in this work, copolymerization of MAZ and CO2 was catalyzed using the Y(CCl3COO)3-ZnEt2-glycerin rare earth ternary catalyst, aiming at preparing aliphatic PUA with high urethane content and high molecular weight. As a result, aliphatic PUA with high urethane content (>80%) and molecular weight (3 � 103 � 3 � 104) was obtained in high yield (>90%). The LCST window of PUA can be tunable in a range of 37�90 �C by controlling the urethane content and molecular weight via rare earth ter- nary catalyst and adjustment of copolymerization condi- tions. Furthermore, the phase transition temperature was highly sensitive to pH changes. EXPERIMENTAL Materials 2-Methylaziridine (MAZ) was purchased from Shanghai Zeal- chem Co. Ltd., and purified by distillation from calcium hydride in argon atmosphere. Glycerin was analytically pure and distilled under reduced pressure before use. Yttrium tri- chloroacetate [Y(CCl3COO)3] and diethyl zinc (ZnEt2) were synthesized according to the literatures.37–39 Commercial CO2 (99.99% pure) was used without further purification. 1,3-Dioxane and N,N-dimethylacetamide (DMAc) were dried over calcium hydride and distilled before use. Copolymerization Procedure The Y(CCl3COO)3-ZnEt2-glycerin coordination catalyst was prepared in argon atmosphere according to the litera- tures.25,36,39,40 The freshly produced rare-earth ternary cata- lyst suspension, MAZ and 1,3-dioxane were introduced into a 500-mL autoclave free of oxygen and water, and the auto- clave was pressurized to 4.5 MPa by CO2. The reaction was carried out at 70 �C for 12 hour, and then cooled down to room temperature, followed by slow release of CO2. The polymeric product was purified by reprecipitation from dichloromethane/diethyl ether to remove low molecular weight products, and then dried at 35 �C in vacuum till con- stant weight. Measurements The NMR spectra of the copolymers were recorded at room temperature on a Unity-400 NMR spectrometer in deuter- ated chloroform with tetramethylsilane as internal reference. Elemental analysis was performed on Elementar VaroEL ana- lyzer. Urethane content (UC) was calculated from the ele- mental analysis of the copolymers containing urethane and amine moieties. The Infrared spectra were obtained by cast- ing the methanol solution of the copolymers onto a KBr disk with a Bruker TENSOR-27 spectrophotometer. The molar mass distributions of the polymers were deter- mined by size-exclusion chromatography coupled with multi- angle laser light scattering (SEC-MALLS), which was carried out at 30 �C on Shodex OHpak SB-804HQ columns using 0.2 M NaNO3/0.5 M CH3COOH aqueous solution as eluent. The LCST was defined as the onset point in the absorbance curve of the polymer solution during the heating process, which was measured by monitoring the transparency at 500 nm of a 5.0 wt % of aqueous solution at a heating rate of 1 �C/min on a variable-temperature UV-2401PC UV/Vis spectrometer. The responsive time was defined as the time from 100% transmittance decreased to 50% transmittance. RESULTS AND DISCUSSION Copolymerization of MAZ and CO2 The copolymerization of MAZ and CO2 was carried out at 70 �C under 4.5 MPa for 6 hour using Y(CCl3COO)3-ZnEt2-gly- cerine as catalyst. The polymeric product was purified by reprecipitation of its dichloromethane solution using diethyl ether to give a powder in 91.1% yield. Figure 1 shows FTIR spectrum of the obtained PUA. The existence of sharp absorption peaks at 1703 and 1537 cm�1 characteristic of carbonyl group, in combination with the peaks at 1245 and FIGURE 1 FTIR spectrum of PUA. SCHEME 1 Thermo- and pH-responsive poly(urethane-amine). WWW.POLYMERCHEMISTRY.ORG ARTICLE WWW.MATERIALSVIEWS.COM JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2011, 49, 5162–5168 5163 1,066 cm�1 due to the CAOAC symmetric and asymmetric bending vibrations, indicated the existence of urethane moi- ety in the polymer. Figure 2 shows the 1H NMR spectrum of PUA. Formation of urethane units by incorporation of CO2 into the polymer can be confirmed by the signals at 3.6�4.3 ppm assigned to pro- ton on the carbon adjacent to the oxygen in urethane unit. Furthermore, the signals corresponding to the methylene ad- jacent to the amine nitrogen and urethane oxygen were observed at 2.3�2.6 ppm and 3.8�4.3 ppm, respectively. The change of yield and urethane content with copolymer- ization time was summarized in Table 1. The yield as well as urethane content increased slightly with increasing time. After 6 hour of polymerization, the yield reached 91.1%, and the urethane content of the obtained PUA was 82.6%. For all the tests, the weight-average molecular weight Mw of PUA was above 3.7 kg/mol. The effect of CO2 pressure on the copolymerization of MAZ and CO2 was summarized in Table 2, where the copolymer- ization was carried out for 2 hour. When the CO2 pressure increased from 1.0 to 4.5 MPa, though the molecular weight maintained around 4.0 kg/mol, the urethane content in PUA increased from 64.9 to 78.4%, while the yield increased slightly from 87.0 to 90.5%. The influence of reaction temperature on the copolymeriza- tion of MAZ and CO2 was shown in Table 3. When the reac- tion temperature increased from 50 to 90 �C, the yield of PUA increased from 92.9 to 96.1%, and the urethane con- tent increased from 63.3 to 85.2%, while the molecular weight decreased from 4.5 kg/mol to 3.5 kg/mol. The rea- son may lie in that higher reaction temperature may raise the alternative reaction rate as well as chain transfer rate, leading to higher urethane content and lower molecular weight. To overcome the phase separation problem during polymer- ization due to the lower solubility of the product in reaction media,33,34,41 N,N-dimethylacetamide (DMAc) was chosen as solvent for the copolymerization. As shown in Table 4, PUA with molecular weight as high as 31.0 kg/mol was obtained when copolymerization was carried out in DMAc, mainly due to the good solubility of PUA in DMAc, where a homogene- ous reaction system was realized, which was beneficial for higher molecular weight PUA. Possible Explanation of the Copolymerization Currently the active center of rare-earth complex/ ZnEt2/ glycerine system is still not very clear due to the compli- cated heterogeneous system. As far as the copolymerization of CO2 and propylene oxide was concerned, Shen and Tan had pointed out that a bimetallic active center between rare- earth metal and the reaction product between ZnEt2 and glycerin was formed.29,30,42 Earlier work in this lab FIGURE 2 The 1H NMR spectrum of PUA. TABLE 1 Effect of Reaction Time on the Copolymerization of CO2 and MAZ a Time (h) Yield (%) Urethane content (%) Mw (kg/mol) 2 90.5 78.4 3.9 6 91.1 82.6 4.4 8 91.4 83.0 3.7 10 92.6 83.5 4.5 12 94.6 84.4 3.8 a Polymerization conditions: Pco2: 4.5 MPa; Temperature: 70 �C; MAZ: 80 mL; Solvent: 1,3-dioxane; The rare earth ternary catalyst consisted of ZnEt2, Y(CCl3COO)3 and glycerin in molar ratio of 20/1/10, where 1.0 mL of ZnEt2 was used. TABLE 2 Effect of CO2 Pressure on the Copolymerization of CO2 and MAZ a Pco2 (MPa) Yield (%) Urethane content (%) Mw (kg/mol) 1 87.0 64.9 4.0 2 88.4 65.9 4.4 3.5 93.4 76.3 3.9 4.5 90.5 78.4 3.9 a Polymerization conditions: Reaction temperature: 70 �C; MAZ: 80 mL; Solvent: 1,3-dioxane; The rare earth ternary catalyst consisted of ZnEt2, Y(CCl3COO)3 and glycerin in molar ratio of 20/1/10, where 1.0 mL of ZnEt2 was used. TABLE 3 Effect of Reaction Temperature on the Copolymerization of CO2 and MAZ a T (�C) Yield (%) Urethane content (%) Mw (kg/mol) 50 92.9 63.3 4.5 70 94.6 84.4 3.8 90 96.1 85.2 3.5 a Polymerization conditions: Reaction time: 12 h; Pco2: 4.5 MPa; MAZ: 80 mL; Solvent: 1,3-dioxane; The rare earth ternary catalyst consisted of ZnEt2, Y(CCl3COO)3 and glycerin in molar ratio of 20/1/10, where 1.0 mL of ZnEt2 was used. ARTICLE WWW.POLYMERCHEMISTRY.ORG 5164 JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2011, 49, 5162–5168 confirmed that the rare-earth metal compound did coordi- nate with zinc-oxygen bond in the ternary catalyst by ‘‘double metal bridged coordinating’’ mode as shown in Scheme 2,43 and the introduction of rare earth complex Y(CCl3COO)3 dramatically raised the yield as well as the molecular weight of the copolymer.36 It is well-known that primary and secondary amines can react with CO2 to form corresponding carbamic acid and derivatives.44,45 Kuran35 noted that the UV-visible spectro- scopic absorption of 2-methylaziridine saturated with CO2 shifted toward a longer wavelength in comparison with pure aziridine, suggesting that cyclic amine like MAZ may form ammonium carbamate with CO2 according to eq 1 in Scheme 3. In the ternary rare metal coordination catalysts, electrons around Zn partially flow to Y via coordinating, resulting in a decrease of the electron cloud density and a increase of the Lewis acidity around catalytic center.43 Hence, the addition of the rare-earth Y(CCl3COO)3 can facilitate the formation of a complex CO2 with MAZ (eq 2 in Scheme 3). Furthermore, the increase of the ligand’s pKb in the rare-earth compound would enhance the Lewis acidity around catalytic centre (see Supporting Information Table S1). Such a zwitterion complex I should be highly reactive in the propagation step of the reaction, considering the existence of possible mesomeric structures as shown in Scheme 4.35 The reactivity of complex I toward itself or complex Z would be greater than toward uncomplexed MAZ. The propagation reaction mainly proceeded according to eq 3 in Scheme 3, which was the reason why the copolymers had such high urethane contents. Furthermore, the higher reaction pressure facilitated the formation of the complex I, resulting in higher urethane content, which was consistent with the data in Table 2. Thermo-Responsive Property The above prepared PUA was soluble in water at low tem- perature. The transparent solution turned turbid when tem- perature increased to certain degree, and became clear again as the temperature decreased, indicating the occurrence of phase transition and reversible thermo-responsiveness. Fig- ure 3 shows the UV-vis light transmittance spectra at k ¼ 500 nm for 5.0 wt % aqueous solutions of PUA with differ- ent urethane content (UC) and molecular weight (Mw). All of them showed rapid and reversible phase transition with the responsive time of 24�120 s. For PUA with Mw of 31.0 kg/ mol and UC of 82.3%, the transmittance of the solution decreased sharply above 37 �C (see curve e in Fig. 3). More- over, small hysteresis during the heating and cooling cycle was observed, which might be induced by the hydrogen bonding among PUA.18,46 Similar thermal responsive phe- nomenon was observed for PUA with Mw of around 4.0 kg/ mol and UC from 65.9 to 84.4%, as shown in curves a-d in Figure 3. For PUA with similar Mw of 4.5 kg/mol, increase in UC from 65.9 to 83.5% resulted in decrease of LCST from 87.5 �C to 82 �C, as shown in curve a and b in Figure 3. Since the thermally induced phase transition behavior in the aqueous solution was attributed to the cleavage of the hydrogen-bonding network between water and PUA and hydrophobic aggregation of PUA,33,34,47 the increase in urethane content led to decrease of the LCST due to reduc- tion of the hydrophilicity of PUA.18,21 Therefore, the thermo- responsive property should be adjustable by the ratio of the hydrophilic amine moiety in PUA to the relative hydrophobic urethane moiety. Furthermore, the molecular weight of PUA also affected the LCST.33,48 For PUA with similar UC value, as shown in curve a and e in Figure 3, increasing Mw from 4.5 to 31.0 kg/mol resulted in decrease of LCST from 82 �C to 37 �C. An inverse dependence of LCST on molecular weight was also reported in other polymer systems.33,48 In general, the LCST of PUA can be adjusted by its UC and MW, which can be tunable between 37 and 90 �C. pH-Responsive Behavior As listed in Table 5, the LCST of the aqueous solution was strongly influenced by the PUA concentration. If PUA with Mw of 31 kg/mol and UC of 82.3% was concerned, the LCST TABLE 4 Effect of Solvent on the Copolymerization of CO2 and MAZ a Solvent(50 mL) Yield (%) Urethane content (%) Mw (kg/mol) 1,3-dioxane 95.4 80.1 4.2 DMAc 94.9 82.3 31.0 a Polymerization conditions: Pco2: 4.0 MPa; MAZ: 20ml; Reaction time: 10 h; Temperature: 70 �C; The rare earth ternary catalyst consisted of ZnEt2, Y(CCl3COO)3 and glycerin in molar ratio of 20/1/10, where 0.25 mL of ZnEt2 was used. SCHEME 2 Schematic double-metal-bridge structure in rare-earth ternary catalyst. WWW.POLYMERCHEMISTRY.ORG ARTICLE WWW.MATERIALSVIEWS.COM JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2011, 49, 5162–5168 5165 decreased from 55.6 �C to 37.0 �C when its concentration increased from 0.5 to 5 wt %. Correspondingly, the pH value of the solution increased from 9.1 to 9.5 when the PUA con- centration increased from 0.5 to 5 wt %, perhaps due to the existence of basic amine moiety in PUA. To understand the pH responsive property of PUA, the pH value of PUA solution was adjusted to 10.0 by standard solu- tion of NaHCO3-Na2CO3 (0.025 M: 0.025 M). When PUA con- centration in the solution increased from 0.5 to 5 wt %, the LCST decreased from 41.2 �C to 30 �C, indicating that phase transition behavior could be controlled by pH value. Figure 4 shows the pH responsive behavior of PUA solution, where the pH value of the solution was adjusted from 9.2 to 13.0 by addition of dilute NaOH or HCl. The LCST value of the solution decreased with increasing pH value, indicating that the thermo-responsive property of the solution was tun- able within a wide temperature range from 30 to 50 �C by changing its pH value. Moreover, the thermo-responsive property of PUA in the above given pH conditions was sus- tainable (see Supporting Information Fig. S3). SCHEME 3 Possible mechanism for the copolymerization of MAZ and CO2. SCHEME 4 Possible mesomeric structures of the zwitterion complex. FIGURE 3 UV-vis light transmittance spectra (left) of 5.0 wt % aqueous solutions of the PUA with various urethane content (UC) and molecular weight. The LCSTs and responsive time of various solutions were (a) 82 �C; 48 s, (b) 87.5�C; 24 s, (c) 86.5 �C; 72 s, (d) 90 �C; 72 s, and (e) 37 �C; 120 s, respectively. The photographs in right of this figure showed the aqueous solu- tions above LCST (A) and below LCST (B). ARTICLE WWW.POLYMERCHEMISTRY.ORG 5166 JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2011, 49, 5162–5168 The phase transition behavior mentioned above was not observed in the acidic region below pH 7. Since the hydro- philicity of PUA was due to an equilibrium between the amine and the ammonium form, enhancing acidity resulted in dominating ammonium form and better solubility of PUA in water, which