Hyperbranched Poly(ether ketone) with Carboxylic Acid Terminal
Groups: Synthesis, Characterization, and Derivatization
Ching-Fong Shu* and Chi-Meng Leu
Department of Applied Chemistry, National Chiao Tung University, Hsin-Chu, Taiwan, 30035, Republic of China
Received July 14, 1998; Revised Manuscript Received September 21, 1998
ABSTRACT: A new procedure is described for the formation of a hyperbranched poly(ether ketone) with carboxylic acid terminal groups. This preparation was based on the one-step synthesis of an AB2-type monomer 5-phenoxyisophthalic acid using phosphorus pentoxide/methanesulfonic acid as the condensing agent and solvent. The electrophilic aromatic substitution reaction led to the formation of the aryl ketone linkage. With the help of model compounds,1H NMR studies revealed that the degree of branching of the poly(ether ketone) was about 55%. The terminated carboxylic acid groups were readily functionalized, yielding hyperbranched polymers with a variety of different functional chain ends. The nature of the chain ends was shown to dramatically affect physical properties of the hyperbranched macromolecules. The ammonium derivative was soluble in water and behaved as a unimolecular micelle.
Introduction
Recently, hyperbranched macromolecules have re-ceived considerable attention due to the expectation that their unique highly branched structure will impart unusual properties.1aHyperbranched polymers may be considered as irregular analogues of the dendritic macromolecules that have a well-defined and perfectly branched structure.1bAlthough dendrimers are built up by step-by-step sequences,1b requiring isolation and purification after each step, hyperbranched polymers are prepared by direct one-step polymerization of AB2 monomers.2,3The AB
2-type monomer is unique because there is double the amount of B functionality compared with the A functionality in the polymerization mixture. Despite this stoichiometric imbalance, a high molecular weight polymer is achieved because both functionalities are located on the same molecule. For each A function-ality reacted, a new A functionfunction-ality is connected to the polymer chain. As predicted theoretically by Flory,4 direct polymerization of this type of monomer would produce polymers with a highly branched, irregular structure possessing one unreacted A functional group and n + 1 number of unreacted B functional groups at the chain ends of the polymers, where n is the degree of the polymerization. In pronounced contrast to linear polymers, these hyperbranched polymers possess nu-merous branching points and end groups. The chain-end functional groups have been shown to dramatically affect the physical properties of the hyperbranched polymers.3,5
Intense research efforts have been devoted to the development of novel synthetic routes to hyperbranched polymers as well as the investigation of the physical properties of these polymers. Aromatic poly(ether ke-tone)s are a class of polymer that gained technical interest because they have outstanding thermal and mechanical properties.6The preparation of traditional linear poly(ether ketone)s has been carried out by two methods,7 the first of which is a synthesis involving nucleophilic aromatic substitution, resulting in the formation of an aryl ether linkage. The second method is a synthesis involving electrophilic aromatic substitu-tion in which an aryl ketone linkage is obtained. The
nucleophilic reaction has been applied to the synthesis of hyperbranched/dendritic poly(ether ketone)s using AB2monomers containing a phenolic group and two aryl fluorides which were activated toward nucleophilic displacement by carbonyl moieties.2i,3f,8
This report describes another synthetic approach for the preparation of hyperbranched poly(ether ketone)s. The synthetic procedure used here is derived from the linear aromatic poly(ether ketone)s.9In this procedure the polymeric linkages are formed via direct self-polycondensation of the substituted isophthalic acid containing phenyl ether structures using phosphorus pentoxide/methanesulfonic acid in a weight ratio of 1:12 (PPMA) as condensing agent and solvent.9,10This one-step synthesis led to the formation of a hyperbranched poly(ether ketone) possessing numerous terminal car-boxylic acid groups. By chemical modification reactions of the carboxylic chain ends, a variety of different functional groups were introduced into the hyper-branched poly(ether ketone). The effect of the chain-end functional group on the physical properties of these hyperbranched poly(ether ketone)s was investigated.
Experimental Section
General Directions. Tetrahydrofuran (THF) was distilled from a sodium diphenyl ketyl solution just before use. The reagent PPMA (Eaton’s reagent) was obtained from Aldrich and used as received. Other starting materials and reagents were used as obtained from the suppliers. NMR spectra were recorded on a Varian Unity 300 MHz spectrometer. Dif-ferential scanning calorimetry (DSC) was performed on a SEIKO SSC 5200 DSC using a heating/cooling rate of 10 °C min-1. Thermogravimetric analysis (TGA) was made on a SEIKO TG/DTA 200 using a heating rate of 10 °C min-1in nitrogen. Size-exclusion chromatography (SEC) was carried out on a Waters chromatography connected to a Waters 410 differential refractometer with dimethylformamide (DMF) as the solvent. UV-vis absorption spectra were taken on a HP 8453 UV/vis spectrometer. Mass spectra were obtained on a JEOL JMS-HX 110 with EI ionization. Analytical TLC was performed on commercial Merck plated coated with silica gel GF254. Silica gel for column chromatography was Merck kieselgel 60 (70-230 mesh).
1,3-Dimethyl-5-phenoxybenzene, 1. A mixture of phenol (6.1 g, 64.8 mmol), toluene (6 mL), and KOH (3.03 g, 54 mmol) 10.1021/ma981114m CCC: $18.00 © 1999 American Chemical Society
was heated at 145 °C for 2.5 h with the water collected in a Dean-Stark trap. Then, excess phenol and water were re-moved under reduced pressure at 160 °C for 3 h. Copper powder (0.1 g), 5-bromo-m-xylene (2 g, 10.8 mmol), and phenol (3 mL) were added to the dry salt. The mixture was stirred under nitrogen at 220 °C for 3 h. The reaction mixture was poured slowly into water (150 mL), and aqueous 5 wt % NaOH solution was added to dissolve the excess phenol. The mixture was extracted with ethyl acetate (3× 70 mL). The combined extract was dried and evaporated to dryness. The crude product was purified by column chromatography, eluting with CH2Cl2/hexane 1:7 to give 1 (1.99 g, 93%) as a colorless liquid. 1H NMR (acetone-d 6)δ 2.25 (s, 6 H), 6.61 (s, 2 H), 6.77 (s, 1 H), 6.97 (d, 2 H, J ) 8.7 Hz), 7.10 (t, 1 H, J ) 7.4 Hz), 7.35 (dd, 2 H, J ) 8.7, 7.4 Hz);13C NMR (CDCl 3) δ 21.3, 116.6, 118.8, 122.9, 125.0, 129.6, 139.5, 157.1, 157.4; MS (m/e) 198.1062, Calcd 198.1045 for C14H14O.
5-Phenoxyisophthalic Acid, 2. KMnO4(80 g, 0.51 mol) was added in small portions to a mixture of 1 (10 g, 50.4 mmol), water (70 mL), and pyridine (185 mL) heated at 100 °C during 6 h. The reaction mixture was stirred at 120 °C for 36 h. The manganese dioxide was filtered and extracted with hot water, and the filtrate was acidified with 4 N HCl. The product was collected by filtration (11.2 g, 86%).1H NMR (DMSO-d
6)δ 7.13 (d, 2 H, J ) 8.1 Hz), 7.24 (t, 1 H, J ) 7.2 Hz), 7.46 (dd, 2 H, J ) 8.1, 7.2 Hz), 7.64 (d, 2 H, J ) 0.9 Hz), 8.19 (s, 1 H);13C NMR (DMSO-d6)δ 119.9, 122.1, 124.5, 124.8, 130.5, 133.2, 155.5, 157.7, 166.1; MS (m/e) 258. 0522, Calcd 258.0528 for C14H10O5.
Dimethyl 5-Phenoxyisophthalate, 3. A mixture of 2 (2 g, 7.75 mmol), methanol (20 mL), and concentrated H2SO4(4 mL) was heated at reflux for 4 h. The reaction mixture was poured into water (300 mL) and extracted with ethyl acetate (3× 70 mL). The combined extract was washed with aqueous sodium bicarbonate solution and dried over anhydrous sodium sulfate. The solvent was evaporated to give 3 (2.17 g, 98%). 1H NMR (CDCl 3)δ 3.90(s, 6 H), 7.00(d, 2 H, J ) 8.4 Hz), 7.16 (t, 1 H, J ) 7.2 Hz), 7.36 (dd, 2 H, J ) 8.4, 7.2 Hz), 7.82 (d, 2 H, J ) 1.5 Hz), 8.39 (t, 1 H, J ) 1.5 Hz);13C NMR (CDCl 3)δ 52.4, 119.2, 123.5, 124.2, 125.1, 130.1, 132.2, 156.1, 157.8, 165.6; MS (m/e) 286.0839, Calcd 286.0841 for C16H14O5.
Dimethyl 5-(4-Benzoylphenoxy)isophthalate, 4. A mix-ture of 3 (2 g, 6.99 mmol), benzoic acid (1.71 g, 14 mmol), and PPMA (20 mL) was stirred under nitrogen at 60 °C for 2 h. The reaction mixture was poured into water (500 mL), neutralized with sodium bicarbonate, and extracted with ethyl acetate (3× 70 mL). The combined extract was dried, and the solvent was evaporated to give 4 (2.65 g, 97%). 1H NMR (CDCl3) 3.93 (s, 6 H), 7.04 (d, 2 H, J ) 9.3 Hz), 7,47 (dd, 2 H, J ) 7.5, 7.2 Hz), 7.57 (t, 1 H, J ) 7.2 Hz), 7.78 (d, 2 H, J ) 7.5 Hz), 7.84 (d, 2 H, J ) 9.3 Hz), 7.91 (d, 2 H, J ) 1.5 Hz), 8.49 (d, 1 H, J ) 1.5 Hz);13C NMR (CDCl 3)δ 52.5, 117.7, 124.8, 126.3, 128.2, 129.8, 132.2, 132.6, 132.9, 137.6, 156.1, 160.2, 165.4, 195.3; MS (m/e) 390.1112, Calcd 390.1103 for C23H18O6. 5-(4-Benzoylphenoxy)isophthalic Acid, 5. A mixture of 4 (2.5 g, 6.4 mmol), methanol (25 mL), and 40 wt % NaOH (aq) (3 mL) was heated at reflux for 2 h. The solvent was removed by evaporation and poured into water (40 mL). The solution was acidified with 4 N HCl. The precipitated solid was isolated by filtration and recrystallized from ethyl acetate/ methanol to give 5 (2.15 g, 93%).1H NMR (DMSO-d
6) 7.21 (d, 2 H, J ) 8.1 Hz), 7.55 (dd, 2 H, J ) 8.1, 7.5 Hz), 7.66 (t, 1 H, J ) 7.5 Hz), 7.73 (d, 2 H, J ) 8.1 Hz), 7.79 (d, 2 H, J ) 1.5 Hz), 7.82 (d, 2 H, J ) 8.1 Hz), 8.30 (s, 1 H);13C NMR (DMSO-d6) 118.2, 123.6, 125.8, 128.6, 129.5, 132.4, 132.5, 134.2, 137.3, 155.8, 160.0, 166.1, 194.5; MS (m/e) 362.0792, Calcd 362.0790 for C21H14O6. 3,5-Bis-(4-phenoxybenzoyl)-(4-benzoylphenoxy)ben-zene, 6, and 3-(4-Benzoylphenoxy)-5-(4-phenoxybenz-oyl)benzoic acid, 7. A mixture of 5 (2.1 g, 5.8 mmol), diphenyl ether (1.48 g, 8.7 mmol), and PPMA (20 mL) was stirred under nitrogen at 60 °C for 4 h. The reaction mixture was poured into water (500 mL), and sodium bicarbonate was added to the solution to adjust pH∼5. The solution was then extracted with ethyl acetate (3× 80 mL). The combined extract was
dried and evaporated to dryness. The crude product was purified by column chromatography, eluting with ethyl acetate/ hexane 1:3 to give 6 (1.8 g, 47%).1H NMR (CDCl 3)δ 7.01 (d, 4 H, J ) 8.7 Hz), 7.07 (d, 4 H, J ) 8.4 Hz), 7.10 (d, 2 H, J ) 8.4 Hz), 7.19 (t, 2 H, J ) 7.2 Hz), 7.39 (dd, 4 H, J ) 8.7, 7.2 Hz), 7.46 (dd, 2 H, J ) 7.5, 7.2 Hz), 7.57 (t, 1 H, J ) 7.5 Hz), 7.67 (d, 2 H, J ) 0.9 Hz), 7.77 (d, 2 H, J ) 7.2 Hz), 7.81 (d, 4 H, J ) 8.4 Hz), 7.83 (s, 1 H), 7.87 (d, 2 H, J ) 8.4 Hz);13C NMR (CDCl3)δ 117.1, 117.9, 120.2, 123.5, 124.8, 126.3, 128.2, 129.8, 130.1, 130.7, 132.2, 132.4, 132.5, 133.0, 137.5, 140.0, 155.1, 156.0, 160.0, 162.2, 193.4, 195.2; MS (m/e) 666.2040, Calcd 666.2042 for C45H30O6. Further elution with ethyl acetate gave 7 (0.75 g, 25%).1H NMR (CDCl 3)δ 7.02 (d, 2 H, J ) 8.7 Hz), 7.09 (d, 4 H, J ) 8.4 Hz), 7.20 (t, 1 H, J ) 7.5 Hz), 7.40 (dd, 2 H, J ) 8.7, 7.5 Hz), 7.47 (dd, 2 H, J ) 7.8, 7.2 Hz), 7.57 (t, 1 H, J ) 7.2 Hz), 7.73 (s, 1 H), 7.78 (d, 2 H, J ) 7.8 Hz), 7.81 (d, 2 H, J ) 8.4 Hz), 7.85 (d, 2 H, J ) 8.4 Hz), 7.97 (s, 1 H), 8.24 (s, 1 H);13C NMR(CDCl 3)δ 117.1, 117.9, 120.3, 124.1, 124.8, 125.3, 126.8, 128.3, 129.9, 130.1, 130.6, 131.6, 132.4, 132.5, 132.7, 133.0, 137.5, 140.3, 155.1, 156.3, 160.6, 162.3, 169.9, 193.3, 195.5; MS (m/e) 514.1409, Calcd 514. 1416 for C33H22O6. Methyl 3-(4-benzoylphenoxy)-5-(4-phenoxybenzoyl) benzoate, 8. A mixture of 7 (0.35 g, 0.68 mmol), methanol (10 mL), and concentrated H2SO4(0.5 mL) was heated at reflux for 4 h. The reaction mixture was poured into water (100 mL) and extracted with ethyl acetate (3× 30 mL). The combined extract was washed with aqueous sodium bicarbonate solution, dried, and evaporated to dryness. The crude product was purified by column chromatography, eluting with ethyl acetate/ hexane 1:3 to give 8 (0.288 g, 80%).1H NMR (CDCl 3)δ 3.91 (s, 3 H), 7.02 (d, 2 H, J ) 8.7 Hz), 7.07 (d, 2 H, J ) 8.7 Hz), 7.09 (d, 2 H, J ) 8.7 Hz), 7.20 (t, 1 H, J ) 7.5 Hz), 7.40 (dd, 2 H, J ) 8.7, 7.5 Hz), 7.47 (dd, 2 H, J ) 7.2, 6.9 Hz), 7.57 (t, 1 H, J ) 7.2 Hz), 7.68 (m, 1 H), 7.78 (d, 2 H, J ) 6.9 Hz), 7.80 (d, 2 H, J ) 8.7 Hz), 7.84 (d, 2 H, J ) 8.7 Hz), 7.92 (m, 1 H), 8.18 (t, 1 H, J ) 1.5 Hz);13C NMR (CDCl 3)δ 52.5, 117.1, 117.8, 120.3, 123.6, 124.6, 124.8, 126.2, 128.2, 130.0, 130.7, 132.2, 132.4, 132.5, 133.0, 137.5, 140.2, 155.1, 156.1, 160.1, 162.2, 165.4, 193.3, 195.2; MS (m/e) 528.1580, Calcd 528.1573 for C34H24O6.
Preparation of Hyperpranched Poly(ether ketone), P1. A solution of 2 (0.6 g) in PPMA (3.6 mL) was stirred under nitrogen at 110 °C for 10 h. The resulting solution was poured into water (200 mL). The polymer was collected, washed with water, and stirred again in water (200 mL) at 70 °C for 12 h. The polymer was collected and dried in vacuo. The crude product was purified by precipitating from DMF into methanol to give P1 (0.48 g, 86%).
Preparation of Hyperbranched Poly(ether ketone), P2. A solution of P1 (0.2 g) and thionyl chloride (0.18 mL) in anhydrous THF (3 mL) was heated at reflux for 4 h. The excess thionyl chloride and solvent were removed under reduced pressure. Anhydrous THF (4 mL), methanol (0.1 mL), and triethylamine (0.25 mL) were added to the polymeric acid chloride. The mixture was stirred at 25 °C for 12 h. The precipitate was filtered off and washed with THF. The filtrate was concentrated to 2 mL and precipitated into water (200 mL). The precipitate was collected by filtration and dried to give P2 (0.2 g, 92%).
Preparation of Hyperbranched Poly(ether ketone), P3. A solution of P1 (0.2 g) and diphenyl ether (0.66 g) in PPMA (5 mL) was heated at 50 °C for 24 h. The resulting solution was poured into water (400 mL) and neutralized with sodium bicarbonate. The precipitate was filtered, washed with water and methanol, and reprecipitated from DMF into methanol to give P3 (0.24 g, 73%).
Preparation of Hyperbranched Poly(ether ketone), P4. P4 was prepared from P1 and toluene using the same procedure as for P3, except that the reaction temperature was 110 °C (86%).
Preparation of Hyperbranched Poly(ether ketone), P5. A slow stream of anhydrous ammonia was bubbled through a solution of P1 (0.35 g) in anhydrous THF (30 mL)
for 25 min. The precipitate was collected by filtration, washed with THF, and dried (0.33 g, 88%).
Results and Discussion
Polymer Synthesis. The synthesis of the AB2 mono-mer 5-phenoxyisophthalic acid, 2, was performed by the Ullmann reaction of potassium phenate with 5-bromo-m-xylene to form compound 1,11followed by oxidation of the methyl groups of 1 with KMnO4,12as outlined in Scheme 1. The general procedure developed by Ueda and Sato9 for the preparation of linear aromatic poly-(ether ketone)s was applied to the AB2monomer, using PPMA as a condensing agent and solvent. PPMA is expected to react with the carboxylic acid group to yield a highly activated mixed anhydride intermediate be-tween the carboxylic acid and methanesulfonic acid and to enable condensation to proceed under rather mild reaction conditions. We first studied the reaction of the phenoxy group of monomer 2 with benzoic acid in PPMA to demonstrate the feasibility of using the reaction for the formation of hyperbranched poly(ether ketone). The carboxylic groups of 2 were converted to ester groups, and then the reaction of the methyl ester 3 with benzoic acid in PPMA proceeded at 60 °C and yielded 4 quantitatively after 2 h. NMR spectrum of the acylated product 4 revealed that the benzoyl group was exclu-sively at the para-position of the phenoxy group. The model reaction demonstrated that the acylation had taken place clearly at the para-position and that it is suitable as a polymer-forming reaction.
Polymerization of 2 in PPMA at 110 °C for 24 h gave the corresponding carboxylic acid-terminated hyper-branched poly(ether ketone), P1, in 86% yield. The structure of P1 and general reaction are shown in Scheme 2. The hyperbranched poly(ether ketone), P1, which had a high number of terminal carboxylic groups, could not be analyzed directly by SEC, because the polymer adsorbed to the column, resulting in incomplete elution. This problem was overcome by esterification of the carboxylic acids groups with methanol. By reacting with thionyl chloride, P1 was readily converted to the polymeric acid chloride, which was then reacted with methanol to form the methyl ester-terminated polymer P2. 1H NMR analysis confirmed that at least 95% of the carboxylic acids were esterified.
SEC analysis of a sample of P2 gave Mnof 4200 and
Mwof 14 300 (Mw/Mn) 3.40). The molecular weight of the polymer was calibrated against poly(ethylene glycol) standards. Because of the highly branched nature of hyperbranched macromolecules, SEC measurements tended to underestimate the true molecular weight.13 The polymerization was sensitive to reaction time and reaction temperature. The molecular weight distribu-tions of these poly(ether ketones) are broad, and broaden with increasing conversion. This observation is similar to previous reports of hyperbranched polymers and agrees with Flory’s predications on molecular weight
distribution behavior for highly branched systems.4 Because monomer 2 is only partially soluble in PPMA at 100 °C, polymerization below that temperature gave low conversion. Polymerization carried out at temper-atures greater than 120 °C also resulted in low molec-ular weight polymers. This might be caused by the decarboxylation of aromatic carboxylic acids in strong acids.9
Degree of Branching. Hyperbranched polymer P1 was formed by a sequence of condensation of AB2 monomer resulting in an irregular dendritic structure in which three different types of subunits may be present as shown in Figure 1. These include the terminal units, which have two carboxylic acid groups, the linear units, which have one carboxylic group and one benzoyl group, and the dendritic units, which have no free carboxylic acid group and two benzoyl groups. The degree of branching of hyperbranched polymers is given by
A hyperbranched polymer takes DB values between 0 and 1. The extent of branching will have a profound influence on the physical properties of the polymeric Scheme 1
Figure 1. Structure of repeat units in the hyperbranched poly(ether ketone)s.
Scheme 2
DB )
(no. of dendritic units) + (no. of terminal units) total no. of units
material. Hawker et al.2bused NMR spectroscopy and model compounds to quantify the different subunits appearing in their hyperbranched materials, and the same methodology was applicable to the hyperbranched poly(ether ketone) described here.
For the acid-terminated P1, the degree of branching could not be determined directly from its 1H NMR spectrum obtained in DMSO because the chemical shifts of the aromatic protons were not well enough resolved for this determination. However, the1H NMR spectrum of the methyl ester derivative P2 showed sufficient differences in the chemical shifts of the aromatic protons to allow the degree of branching to be determined. The preparation of the model compounds useful for NMR characterization is detailed in Scheme 3. Figure 2 shows the1H NMR spectra for the model compounds 4, 6, and 8.
Distinct resonance for the terminal model compound, 4, appears at 8.49 ppm (Ha), whereas the corresponding proton for the linear model compound, 8 (Hb), is ob-served at 8.19 ppm. The other resonances due to proton Hcof the linear, 8, and protons Hdof the dendritric, 6, are observed at 7.68 and 7.67 ppm, respectively. Good correlation is observed in the comparison of the1H NMR spectra of these model compounds with that of the methyl ester-terminated P2. The resonances at 8.48 and 8.14 ppm are attributed to protons Haof the terminal subunit and Hb of the linear subunit, whereas the resonance at 7.70 ppm is due to protons Hcof the linear subunit and protons Hdof the dendritic subunit. Inte-gration of these well-resolved resonances allows the relative percentage of each subunit to be determined. The percentage estimated for the terminal subunit is approximately equal to that for the dendritic subunit. The result is consistent with the theoretical prediction that the number of dendritic units is equal to the number of terminal units for an AB2-type hyper-branched polymer possessing high molecular weight.4 This consistency provides additional support for the assignments of the 1H NMR spectra. With these for-mulas, the degree of branching for the hyperbranched poly(ether ketone) calculated was approximately 55%, independent of molecular weight. This indicates that branching does not change as a function of conversion. Chemical Modification of Hyperbranched Poly-(ether ketone) P1. Hyperbranched polymers based on AB2monomers are characterized by a large number of chain-end groups, the number of which is equal to the degree of polymerization plus one. As shown in Scheme 4, a variety of different functional groups could be introduced into P1 by reactions of the terminated carboxylic acid groups.1H NMR integration was used to estimate the degree of functionalization in examples where the derivatives contain a functional group that exhibits a 1H NMR peak well separated from the aromatic peaks. In another case, elemental analysis was used to determine the conversion yields. For all the modification reactions above, the use of excess reagents resulted in complete (95%) functionalization, indicating that the carboxylic acid groups at the chain ends are readily accessible to reagents in solution. By reacting with thionyl chloride, the carboxylic acid group of P1 was readily converted to acid chloride, which was then reacted with methanol to give the ester derivative P2. In PPMA the carboxylic acid groups of P1 reacted Figure 2. The 300-MHz1H NMR spectra in CDCl
3of model compounds 4, 6, and 8 compared with the hyperbranched poly(ether ketone) P2.
Scheme 3
smoothly with aromatic electrophilics such as diphenyl ether and toluene to give the corresponding polymers with ketone chain ends P3 and P4, respectively.
Properties such as the glass transition temperature (Tg) and the solubility of hyperbranched polymer are dramatically affected by the functionality of the termi-nal groups.3The T
gvalues determined by differential scanning calorimetry (DSC) of these poly(ether ketone)s are also very dependent on the nature of chain ends, with increases in Tg following increases in chain-end polarities (Table 1). The Tg of P1, which has polar carboxylic acid terminal groups, is 226 °C. The Tgvalues of P2, P3, and P4, which have less polar terminal groups such as ester and ketone groups, are 113, 135, and 140 °C, respectively.
The thermal stability of the hyperbranched poly(ether ketone)s was examined by thermogravimetric analysis (TGA). P1 lost about 5 wt % at 170 °C and an additional 5 wt % at 324 °C, and P2 lost 5 wt % at 246 °C and an additional 5 wt % at 278 °C. Weight losses observed for P1 and P2 may be caused by the thermal decomposition of the carboxylic groups and the methyl ester groups. When the functionality of the chain ends was changed to the ketone, the thermal stability increased. For example, P3 was stable to 420 °C, with a 10 wt % loss occurring over 470 °C.
Because of their highly branched structures, these hyperbranched poly(ether ketone)s have enhanced solu-bility in organic solvents and are highly soluble in typical solvents such as THF, DMF, and N-methylpyr-rolidone (NMP) (Table 1). However, the different chain ends resulted in differences of solubility in very polar and in relatively nonpolar solvents. P1 was sparingly soluble in DMSO, whereas P2, P3, and P4 were totally insoluble. Conversely, in solvents such as chloroform and dichloromethane P1 was totally insoluble, whereas P2, P3, and P4 were extremely soluble.
P1 was insoluble in water, but its ammonium deriva-tive P5 was water-soluble. Hyperbranched/dendritic macromolecules possessing terminate carboxylate groups resemble the environment of a micellar structure and are capable of enhancing the solubility of hydrophobic compounds in water.14Those macromolecules behaved as unimolecular micelles that do not suffer from the critical micelle concentration, and an increased solubi-lization of hydrophobic molecules in aqueous solutions was observed even at extremely low concentrations.14 The micellar behavior of the carboxylate-terminated hyperbranched poly(ether ketone), P5, was studied using the same methodology reported previously by Hawker et al.14dThe hydrophobic compound 1,4-diami-noanthraquinone, which has an strong absorption at 585 nm, was used as a probe to exam the solubilizing ability of P5. As it has been shown that within experimental error there is no change in the absorption coefficient
with changes in solvent,15the value of
max(1.28× 104 M-1cm-1) obtained in a mixture of methanol/water 9:1 was used to give a saturated concentration of 1,4-diaminoanthraquinone in pure water of 1.92× 10-6M, which agrees with values in the literature.3fUnder the same conditions, the saturation concentrations of the hydrophobic compound in a series of aqueous solutions with changes in the concentration of P5 were deter-mined. In an aqueous solution of P5 (3.37× 10-4M), the saturation concentration of 1,4-diaminoanthraquino-ne determi1,4-diaminoanthraquino-ned was 3.72× 10-5M, approximately a 20-fold increase when compared with pure water. On the average a single P5 molecule with molecular weight Mw of 15 200 dissolves 0.11 molecule of 1,4-diaminoan-thraquinone. As shown in Figure 3, increasing the concentration of P5 resulted in a linear increase in saturation concentration of 1,4-diaminoanthraquinone. The solubility enhancement of 1,4-diaminoanthraquino-ne in P5 solutions was even observed at the concentra-tions of P5 in the micromolar range, which is below the critical micelle concentration of the traditional micelle. This result demonstrates unimolecular micelle behavior of the hyperbranched polymer P5, which contrasts with traditional micelle behavior where essentially no solu-bility enhancement occurs below the critical micelle concentration.
Summary
A hyperbranched poly(ether ketone) with terminal carboxylic acid groups was prepared by the one-step synthesis of an AB2monomer 5-phenoxyisophthalic acid using PPMA as condensing agent and solvent. This synthesis involved electrophilic aromatic substitution, resulting in the formation of the aryl ketone linkage. The degree of branching characterized by1H NMR was about 55%. The carboxylic acid groups at the chain ends were readily accessible to reagent in solution and were converted to a variety of functional groups. Physical properties, such as the glass transition temperature and the solubility of the hyperbranched poly(ether ketone)s depended heavily on the nature of the chain ends. The hyperbranched poly(ether ketone) with ammonium
car-Table 1. Effect of the Functionality of the Chain Ends on the Thermal and Solution Properties of the
Hyperbranched Poly(ether ketone)s
solubilityain polymer Tg(°C) CH2Cl2 CHCl3 THF DMF NMP DMSO H2O P1 226 - - + + + +- -P2 113 + + + + + - -P3 135 + + + + + - -P4 140 + + + + + - -P5 236 - - - +-b +-b +-b +
aSolubility: +, soluble; +-, partially soluble; -, insoluble.bIn
hot solvent.
Figure 3. Saturation concentration of 1,4-diaminoanthraquino-ne as a function of the concentration of the carboxylate-terminated hyperbranched poly(ether ketone), P5, in water. The concentration of P5 was estimated based on a molecular weight, Mw, of 15 200.
boxylate terminal groups was soluble in water, and its unimolecular micellar behavior was demonstrated.
Acknowledgment. We thank the National Science Council (ROC) (NSC 87-2113-M009-003) for financial support.
References and Notes
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Advances in Dendritic Molecules, JAI Press: Greenwich, CT,
1994; Vol. 1.
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