Polymer Journal, Vol.33, No. 10, pp 741—745 (2001) Swelling Behavior of pH-Sensitive Hydrogels Containing Degradable Poly(1,3-dioxolane) Segments Juan DU, Yuxing PENG, Xiaobin DING, and Tingting ZHANG Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, P. R. China (Received March 8, 2001; Accepted July 18, 2001) ABSTRACT: pH-sensitive hydrogels were obtained by radical copolymerization of telechelic poly(1,3-dioxolane) (PDXLDA) with acrylic acid (AA). The copolymer networks of Poly(AA-block-DXL) showed pH sensitivity due to –COOH groups. They were insoluble in any solvents, but swelled in water or good solvents. The swelling behavior in both water and organic solvents is composition -dependent. The property that the hydrogel can swell in both water and organic solvents can be explained by a microphase-separated bicontinuous structure. The networks containing polyacetal segments (polyDXL) can be decrosslinked under acidic condition due to the low ceiling temperature of polyDXL. The networks structure, swelling behavior and degradation were characterized by Fourier transform infrared, differential scanning calorimetry, GC-MS analysis and swelling data. KEY WORDS pH-Sensitive Hydrogel / Degradation / Poly(1,3-dioxolane) / Polyacetals such as poly(1,3-dioxolane) (PDXL) tential applications in biosystem, such as intelligent provided with reactive endgroups have been used as drug delivery system. building blocks for the preparation of macromolecular compounds with various material properties.1 PDXL RESULTS AND DISCUSSION telechelic bismacromonomers can be synthesized by , several methods,2 3 but few polymer networks have Synthesis of PolyDXL Bismacromonomer been prepared so far. Since polyDXL has a low ceil- As reported earlier by Franta et al.,7 polyDXL with ing temperature, the networks contain segments which primary hydroxy endgroups (HO–PDXL–OH) are ob- are expected to degrade to monomer by treatment with tained if the cation polymerization is carried out in the a trace of an appropriate cationic initiator.4 Therefore, presence of a diol. This is due to a transfer reaction and such networks may be interesting from at least two /or to a propagation via an activated monomer mech- points: first, solubilization of cross-linked polymers anism.8 Ethylene glycol was used as the diol to pre- under mild conditions may be useful in itself; sec- pare the dihydroxylated polyDXL (macromonomer I). ond, the possibility to study the soluble residual lin- And the synthesis of polyDXL bismacromonomer was ear polymer chains may be used to obtain more insight based on acrylation of I with acrylic acid, as shown in in the copolymerization behavior. Among the reported scheme 1. networks containing polyDXL segments, including an This esterification reaction is carried out quantita- elastic material by copolymerization with styrene, and tively without side reaction.9 According to this reac- amphiphilic polymer networks by copolymerization tion, the polyDXL bismacromonomer II-polyDXL di- with methylmethacrylate (MMA) prepared by E. J. acrylate (PDXLDA) was produced. FT-IR spectra of Goethals group.5 However, no stimuli-sensitive such the original polyDXL and the resulting PDXLDA are as pH-sensitive polymer networks containing poly(1,3- shown in Figure 1. The difference appears in 1638 dioxolane) segment have been prepared. and 1720 cm−1 attributed to the streching of C=C and In this paper, novel pH-sensitive network was C=O of the ester structure, respectively. In polyDXL, prepared by copolymerization of poly(1,3-dioxolane) neither streching signals appear, while in PDXLDA, telechelic bismacromonomer with acrylic acid (AA). both signals appear due to AA end groups. Evi- As we know, polyacrylic acid (PAA) is characterized dently, PDXLDA was an ester product of polyDXL. as pH-sensitive.6 Networks containing homopolymeric By comparison of number average molecular weight of segments should give the polymer networks the same PDXLDA attained from hydroxy titration with a calcu- sensitivities. Due to the ease of degradation of PDXL lated result of HNMR analysis, we got confirmed result. in acid, the networks can be decrosslinked. Accord- According to this, the functionality of terminal groups ingly, these materials based on polyacetal segments is about 2.0. have some interesting physical properties and have po- 741 J. DU et al. Scheme 1. Figure 2. Swelling behavior of poly(AA-block-DXL) net- works with different constituents in pH = 1.5 water, Mn (PDXL) Figure 1. FT-IR-Spectrum of macromonomer 1: PDXL; = 2500, 1#: AA-block-95; 2#: AA-block-90; 3#: AA-block-65; 4#: 2: PDXDA; and networks 3: Poly(AA-block-DXL); Mn(PDXL) = AA-block-50. 2500. for 1 h to remove the unreactive monomer or residues Synthesis and Characterization of pH-Sensitive and dried to constant weight and weighed again. The Poly(AA-block-DXL) Networks extraction ratio was about no more than 1%. PDXLDA can copolymer with different monomers Polyacrylic acid (PAA) is characterized as pH- which can determine the properties of networks. If sensitive hydrogel which is due to the –COOH groups. the comonomer is hydrophobic, amphiphilic networks Similarly, the polymer networks with homopolymeric are formed. Hydrophilic comonomer acrylic acid (AA) segments of acrylic acid should also be expected to was selected to prepare polyacetal-based networks with have pH-sensitivity. Figures 2–4 testify this result. Fig- some interesting physical properties. Radical copoly- ures 2–4 show that the swelling degree of the networks ◦ merization was all conducted in toluene at 70 C with in high pH values is always higher than that of in low 2,2 -azobis(2-methylpropionitrile) as initiator. The end pH value under the same constituents. The swelling products were transparent networks (poly(AA-block- behavior of the poly(AA-block-DXL) networks is sen- DXL)) with high elasticity. From Figure 1 follows sitive to pH values of water. the IR-Spectrum of polymer networks (poly(AA-block- The swelling degree is calculated from: DXL)) in which the streching intensity of C=C bond sd = (ws − wd)/wd × 100 was weakened and C=O bond was greatly strengthened ws: swelling sample; wd: dried gel sample. by comparison with PDXLDA. The difference of the The swelling degree of the hydrogels is mainly influ- fingerprints between the PDXLDA and polymer net- enced by the ionization of –COOH functions in water. works also shows poly(AA-block-DXL) is a copoly- In pH = 1.5 (Figure 2), –COOH can hardly be ionized, merizaton product of PDXLDA with AA instead of a thus formed strong hydrogen bonds among carbonyl simple blend of these two constituents. The radical groups prevent water to permeate into the networks. So copolymerization of PDXLDA with AA does occur. it swells slowly and has the lowest equilibrium swelling The polymer network was dried to constant weight un- degree. As shown in pH = 1.5 curve, the networks (AA- der vacuum and weighed and extracted with CH2Cl2 block-90) began to degrade in 10 h, and after 24 h the 742 Polym. J., Vol.33, No. 10, 2001 pH-Sensitive Hydrogels Containing Poly(1,3-dioxolane) Segments Figure 3. Swelling behavior of poly(AA-block-DXL) net- Figure 5. Swelling behavior of networks with different con- works with different constituents in pH = 7.0 water, Mn (PDXL) = 2500, 1#: AA-block-95; 2#: AA-block-90; 3#: AA-block-65; 4#: stituents in CH2Cl2, 1#: AA-block-95; 2#: AA-block-65; 3#: AA- AA-block-50. block-50; Mn(pDXL) = 2500. = 1.5), with decrease of PDXL content in the networks, the swelling degree decreases. The networks with high PDXL content degrade more easily and faster than net- works with low PDXL content. Figure 4 shows the swelling kinetics of the hydrogels with different ratio of the two constituents in pH = 10.0. The equilibrium swelling degree increases with the AA content, but the swelling rate decreases with AA content. PDXL seg- ments thus play a role in the swelling rate of the net- works in water. This means that PDXL contributes more than PAA segments to swelling behavior in wa- ter. The networks prepared swell in organic solvents. In CH2Cl2, the networks swell fast and has the high- est equilibrium swelling degree, then in H2O, THF, Figure 4. Swelling behavior of poly(AA-block-DXL) net- CH3OH in turn. With increase of PAA content in net- works with different constituents in pH = 10 water, Mn(PDXL) = works, the swelling degree in organic solvents is also 2500, 1#: AA-block-95; 2#: AA-block-90; 3#: AA-block-65; 4#: greatly influenced. Figure 5 support this. In CH Cl , AA-block-50. 2 2 the swelling degree decreases with increase of AA con- networks dissolved completely in water. At pH > 7.0 tent contrary to the situation of in CH3OH in which (present = 10.0), the –COOH functions can be easily the swelling degree increases with AA content. The ionized by reaction with OH−. The networks swell fast swelling behavior of the networks depends on a com- and then reach the highest equilibrium swelling degree. prehensive effect of polarity, solubility parameters of While at pH ≥ 7.0, the networks can only swell untill the solvents. As a result, the swelling behavior of reaching equilibrium swelling, but not be decrosslinked the networks is solvent-dependent and composition- in water. With higher basicity, the networks can reach dependent. higher equilibrium swelling degree. That the networks swell in water and organic sol- In the networks of poly(AA-block-DXL), AA ho- vents is interesting because it behaved like an am- mopolymeric segments are connected by the polyDXL phiphilic networks. This behavior can be explained segments. Figures 2–4 also show that the swelling de- by a microphase-separated bicontinuous structure.10 gree of the hydrogel is dependent on the ratio of the two DSC analysis gives further support for a microphase- constituents.