Synthesis of Hyperbranched Poly(l-lactide)s by Self-Polycondensation

Oct 12, 2012 - Soluble branched polymers can be prepared when the introduction of branching is not accompanied by an infinite network formation. Sever...
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Synthesis of Hyperbranched Poly(L‑lactide)s by SelfPolycondensation of AB2 Macromonomers and Their Structural Characterization by Light Scattering Measurements Mitsutoshi Jikei,*,† Maki Suzuki,† Kuniyuki Itoh,† Kazuya Matsumoto,† Yuta Saito,‡ and Seigou Kawaguchi*,‡ †

Department of Applied Chemistry, Akita University, Tegatagakuen-machi, Akita 010-8502, Japan Department of Polymer Science and Engineering, Graduate School of Science and Engineering, Yamagata University, 4-3-16, Jonan, Yonezawa, Yamagata 992-8510, Japan

‡

ABSTRACT: A series of hyperbranched polylactides (HB-PLLAs) with different branching densities were prepared by the selfpolycondensation of AB2-type macromonomers (degree of polymerization of polylactides (n) = 5.0, 8.4, 19, 29, and 42). The high-molecular-weight HB-PLLAs (Mw = 4.95 × 104−1.47 × 105) were prepared in good yields. Molecular characterizations of the HB-PLLAs were meticulously performed by size-exclusion chromatography at 25 °C using a system equipped with multiangle laser light scattering (SEC-MALS) and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) as the eluent. The z-averaged root-mean-square radius of gyrations (⟨S2⟩z1/2) of the HB-PLLAs in HFIP at 25 °C were much smaller than those of the linear PLLA with the corresponding weight-average molecular weight (Mw), supporting the presence of the branched architectures. The Mw dependence of ⟨S2⟩z1/2 of the HB-PLLAs was determined to follow the equations ⟨S2⟩z1/2 (nm) = 2.5 × 10−1Mw0.37 for n = 8.4, ⟨S2⟩z1/2 (nm) = 1.8 × 10−1Mw0.40 for n = 19, ⟨S2⟩z1/2 (nm) = 1.6 × 10−1Mw0.41 for n = 29, and ⟨S2⟩z1/2 (nm) = 1.1 × 10−1Mw0.44 for n = 42 in HFIP at 25 °C. A comparison of the experimental and theoretical results with respect to their dimensional properties indicated that the HB-PLLAs assume randomly branched architectures (i.e., a hyperbranched structure). The DSC and X-ray diffraction measurements suggested that the crystallization was remarkably suppressed by the introduction of the branched architecture.

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INTRODUCTION

formation of insoluble cross-linked materials. Soluble branched polymers can be prepared when the introduction of branching is not accompanied by an infinite network formation. Several papers reporting the preparation of star-branched PLLAs have mentioned that the introduction of branching reduces the crystallinity of the resulting PLLAs.7−13 Highly branched polymers, such as hyperbranched polymers, have also been reported in the literature.14−19 Ouchi and co-workers have reported a one-pot synthesis of hyperbranched PLLA copolymers by copolymerization of a lactide and mevalonolactone, which contains a hydroxy group as an initiation function.14 Frey and co-workers have reported the copolymerization of a lactide and an AB2-type monomer to form

Biodegradable aliphatic polyesters merit considerable attention for biomedical and environmental applications.1−3 Because of current environmental concerns, some consumer products made of biodegradable polyesters, such as plastic bags, dishes, and cases, are now commercially available.3 Among them, poly(L-lactide) (PLLA) is one of the most representative and important biodegradable and bioabsorbable materials and is commercially produced on a large scale.4−6 PLLA can be produced from biomass as a raw material and shows good mechanical and biodegradable properties. It is well-known that the introduction of branching points affects the properties of the resulting polymers. Branching influences the crystallinity and chain entanglement of the polymers in the bulk. Traditionally, branching points are introduced by the addition of multifunctional monomers during the production of the linear polymer, which results in the © 2012 American Chemical Society

Received: May 22, 2012 Revised: September 28, 2012 Published: October 12, 2012 8237

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hyperbranched PLLAs.15,16 Statistically controlled branched PLLA could also be prepared by copolymerization. However, for the copolymerization strategy, the opportunity for the propagating molecules to react with the branching component is low when the feed ratio of the branching component is low. As a result, the fabricated copolymer may be a mixture of linear (major) and branched (minor) components. This makes it difficult to investigate the effect of limited branching on the properties of the resulting polymers. Another unique approach to prepare branched polyesters was first reported by Hedrick and co-workers.20−22 Hyperbranched poly(ε-caprolactone)s and their copolymers were prepared by the self-polycondensation of AB2-type macromonomers. The benzyl ester of 2,2-bis(hydroxymethyl)propionic acid (bis MPA) was used as an initiator for the ring-opening polymerizations to form the AB2 type caprolactone and lactide macromonomers. It was reported that the thermal properties of the hyperbranched poly(ε-caprolactone)s were mainly dependent on the molecular weight of the AB2 macromonomers. Recently, Hutchings and co-workers have also reported the polycondensation of AB2 macromonomers to form highly branched polystyrenes, polybutadienes, and poly(methyl methacrylate)s.23−28 The distance between the branching points was controlled by a living anionic polymerization process. Moreover, Wu and co-workers recently reported hyperbranched polystyrenes prepared by the selfcondensation of long seesaw-type macromonomers via the copper-catalyzed click chemistry.29,30 The seesaw-type macromonomers were prepared by atom transfer radical polymerization. More recently, Frey and co-workers reported the preparation of hyperbranched PLLA by the self-polycondensation of the AB2 macromonomers which were prepared by the ring-opening polymerization of lactides initiated by 2,2bis(hydroxymethyl)butylic acid.31 The self-polycondensation of AB2 macromonomers allows one to control the branching density by changing the length of the linear segments. In principle, homogeneous branched polymers with an extremely high (hyperbranched) or low branching density can be prepared by this strategy. Perrier and co-workers have recently reviewed long-chain branched dendrimers and hyperbranched polymers.32 The degree of branching is one of the key factors for characterizing branched polymers. However, it is difficult to directly determine the degree of branching by spectroscopic measurements of long-branched polymers. On the other hand, dilute solution properties are sensitive to the presence or absence of long branching chains and afford an insight into the structural characterization of branched polymers. However, to date, limited studies on the structural characterizations of branched polymers resulting from the polymerization of AB2 type macromonomers have been reported. The ratio of the mean-square radius of gyration of a branched polymer (⟨S2⟩b) to that of its linear analogue (⟨S2⟩l), a measure of branching, denoted as the branching ratio (g) (g = ⟨S2⟩b/⟨S2⟩l), may afford information regarding molecular characterization.33 Kwak and co-worker reported the effect of the length of the linear segment in the AB2 macromonomer on the g values of the resulting long-chain branched poly(ε-caprolactone)s.34 Wu and co-workers reported g values in the range of 0.25−0.70 for fractionated hyperbranched polystyrenes, which is scaled to Mw by g = 2.02 × 101Mw−0.260.30 In the current study, a series of hyperbranched PLLAs (HBPLLAs) with different branching densities were prepared by the

self-polycondensation of AB2 type macromonomers. To our knowledge, this is the first report for the preparation of HBPLLA homopolymers with controlled branching densities, although the HB-PLLA copolymer has been reported in the literature.22 The AB2-type PLLA macromonomers were prepared by the ring-opening polymerization of a lactide initiated by 2,2-bis(hydroxymethyl)propionic acid (bis MPA) as the initiator, according to the method reported by Feijen and co-workers.35 The self-polycondensation of the AB2 macromonomer (degree of polymerization of the lactide (n) = 5.0, 8.4, 19, 29, and 42) was then preformed in the presence of condensation agents to form high-molecular-weight branched polylactides with different branching densities. We especially focus on the dimensional and structural characterization of the resulting branched polymers by light scattering experiments. The results were compared to those of the linear PLLA reported in a previous paper36 and analyzed from the standpoint of current polymer solution theories. The thermal properties and crystallization behavior of the HB-PLLAs were also investigated and discussed from a structural viewpoint.

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EXPERIMENTAL SECTION

Materials. L-Lactide (Musashino Chemical Laboratory, Ltd., LTDPharmH grade) was purified by recrystallization in toluene. 2,2Bis(hydroxymethyl)propionic acid (bis MPA) was purified by recrystallization in toluene. 4-(Dimethylamino)pyridine (DMAP) was purchased from Acros Organics and used without further purification. Tin(II) 2-ethylhexanoate (Sn(Oct)2) was purchased from Sigma-Aldrich and used without further purification. N,NDiisopropylcarbodiimide (DIPCI) was purchased from Kokusan Chemical Co., Ltd., and used without further purification. 4(Dimethylamino)pyridine 4-toluenesulfonate (DPTS) was prepared according to the literature procedure.37 1,1,2,2-Tetrachloroethane (Kanto Chemical Co., Inc.) was purified by vacuum distillation before use. HFIP (Central Glass Co., Inc.) was purified by distillation with calcium hydride just before use. All other solvents were purchased from Kanto Chemical Co., Inc., and used as received. Commercially available poly(L-lactide) (Unichika Ltd.) was used as a linear PLLA. Preparation of AB2 Macromonomers.35 The typical experimental procedure for the AB2 macromonomer is described as follows (n = 29). L-Lactide (5.0 g, 35 mmol), bis MPA (78 mg, 0.58 mmol), and Sn(Oct)2 (20 mg, 0.049 mmol) were added to a flask. The mixture was heated at 130 °C for 3 h. After cooling, dichloromethane (10 mL) and acetic acid (few drops) were added to the flask and stirred for 1 h to form a clear solution. The solution was poured into cold ether (150 mL), and the precipitate was recovered by filtration. The product was dried in vacuo at room temperature. The oligomer was isolated as a white powder in 91% yield. 1H NMR (CDCl3, ppm): 5.3−5.1 (m, methine), 4.4−4.3 (m, methine), 4.2 (q, methylene), 1.8−1.4 (m, methyl), 1.3 (s, methyl). 13C NMR (CDCl3, ppm): 175.1, 169.7, 69.1, 66.8, 65.8, 46.1, 20.5, 17.6, 16.8. Synthesis of HB-PLLAs. The experimental procedure for the selfpolycondensation of the AB2 macromonomer (n = 29) is described as follows. In a Schlenk flask, the AB2 macromonomer (n = 29) (0.50 g) was dissolved in tetrachloroethane (1 mL) under nitrogen. DPTS (30.8 mg) and DMAP (11.2 mg) were added and stirred until a clear solution was formed. DIPCI (91.4 μL) was added to the solution, and the solution was then stirred for 48 h at room temperature. The viscous solution was diluted with dichloromethane and poured into methanol. The precipitate was recovered by filtration and dried in vacuo at room temperature. The fibrous white product was isolated in 90% yield. 1H NMR (CDCl3, ppm): 5.3−5.1 (m, methine), 4.4−4.3 (m, methine), 1.8−1.4 (m, methyl), 1.3 (s, methyl). 13C NMR (CDCl3, ppm): 169.6, 69.1, 66.8, 65.7, 46.3, 20.6, 17.6, 16.8. Measurements. The 1H and 13C NMR spectra were recorded using a JEOL JNM-ECX 500 NMR spectrometer. The inherent viscosity was measured in chloroform at 30 °C at a concentration of 8238

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Scheme 1. Preparation of the AB2 Macromonomers

Figure 1. 1H NMR spectrum of the AB2 macromonomer ([M]/[I] = 10) in CDCl3.

Figure 2. Inverse gated-decoupling 13C NMR spectrum of the AB2 macromonomer ([M]/[I] = 10) in CDCl3. 0.5 g/dL. The molecular weight determinations of the HB-PLLAs were performed by size exclusion chromatography (SEC) (pump: Jasco PU-2080 Plus; degasser: Jasco 2080-53; column oven: Jasco CO2065 Plus; temperature of the column oven: 40 °C) using HFIP containing 10 mM CF3COONa as the eluent at the flow rate of 0.50 mL/min and room temperature of 25 °C.36 The SEC was equipped with two columns (Shodex KF-804L × 2), a refractive index detector (RI: Shodex RI-71), and a multiangle laser light scattering detector (MALS; Wyatt Technology DAWN-DSP, wavelength λ = 632.8 nm). The Rayleigh ratio at a scattered angle of 90°, R(90), was based on that of pure toluene at the wavelength of 632.8 nm at 25 °C. The corrections for sensitivity and scattering volume of 17 detectors at angles of other than 90° and the dead volume of each detector were performed using the scattering intensities of a 0.15 wt % PMMA standard (Mw = 2.87 × 104, Mw/Mn = 1.06). The values of the weightaveraged molecular weight, Mw, and z-averaged mean square of the radius of gyration (⟨S2⟩z) at each eluted fraction for an SEC-MALS measurement were determined via the root plots given by the following equations:

⎞1/2 ⎛ Kc ⎞1/2 ⎛ 1 ⎟ (1 + A 2 M w P(θ)c) ⎜ ⎟ =⎜ ⎝ R(θ) ⎠ ⎝ M w P(θ) ⎠

P(θ) = 1 −

q=

1 2 2 ⟨S ⟩z q + ... 3

4πn0 sin(θ /2) λ

(2)

(3)

where K is the optical constant (K = 4π2n02(dn/dc)2/(NAλ4)), n0 is the refractive index of HFIP, c is the mass concentration (g/mL), dn/dc is the specific refractive index increment, θ is the scattering angle, NA is Avogadro’s number, λ is the wavelength of the incident light, and A2 is the second virial coefficient. Polymer sample solutions with the mass concentration of about 1.0 mg/mL were injected using a sample loop of 100 μL into the SEC columns and diluted down to 10−103 times lower than the original c in the columns during the separation. Thus, the concentration effect on the Mw and ⟨S2⟩z values in eq 1 can be ignored. The specific refractive index increment (dn/dc) was measured using a differential refractometer (Otsuka Electronics DRM-1021, wavelength λ = 632.8 nm) at 25 °C. The dn/dc value of the HB-PLLAs in HFIP was in the range of 0.149 ± 0.004 mL/g. The Mw dependence of ⟨S2⟩z1/2 of the linear PLLA in HFIP at 25 °C was reported and given by the equation of ⟨S2⟩linear1/2 (nm) = 0.023 Mw0.60.36 In order to evaluate the molecular weight distribution of the AB2 oligomers, the SEC measurements (Shodex KF-806 M and KF-802.5 columns) in

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chloroform as the eluent were also carried out. The retention volume was calibrated using a series of polystyrene standards. The DSC measurements were carried out by a Rigaku Thermo plus DSC 8230. The heating and cooling rates were set at 10 °C/min. The X-ray diffraction measurements of the branched and linear PLLA films were carried out using a Rigaku Ultima IV at room temperature.

dichloromethane, and THF but insoluble in methanol, ether, and acetone. The AB2 macromonomers underwent further self-polycondensation to form the HB-PLLAs. Because the carboxyl group is located in the middle of the macromonomer, the propagation reaction is accompanied by branching. The self-polycondensation was carried out in the presence of diisopropyl carbodiimide (DIPCI), 4-(dimethylamino)pyridinum 4-toluenesulfonate (DPTS), and (N,N-dimethylamino)pyridine (DMAP) in tetrachloroethane at room temperature (Scheme 2). Tetrachloroethane was used as the solvent because it accommodated a high concentration of the macromonomer (ca. 50 wt %) for 48 h. A large excess of DIPCI with respect to the macromonomer (ca. 10-fold) was also a key to forming the high-molecular-weight HB-PLLAs. After optimization of the reaction conditions, the molecular weights of the HB-PLLAs were much higher than those of the long-chain branched poly(lactide)s reported in the literature.31 The results of the self-polycondensation are summarized in Table 2. A series of

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RESULTS AND DISCUSSION The AB2-type PLLA macromonomers were prepared from bis MPA and a lactide in the presence of Sn(Oct)2 as described in the literature35 (Scheme 1). Oligomerization was carried out in bulk, and the reaction mixture was solidified during the final stage of the reaction. After precipitation from the dichloromethane solution by the addition of ether, the AB2 macromonomers were recovered in good yield (86−93%). Figure 1 shows the 1H NMR spectrum of the AB2 macromonomer ([M]/[I] = 10). All the peaks in Figure 1 were assigned to the proposed structure. The peaks at 4.2 ppm were assigned to the terminal methine protons of the polylactide chains. The integration ratio of these peaks to the methine protons in the main chain (5.1 ppm) enables a calculation of the numberaverage molecular weight. Figure 2 shows the inverse gateddecoupling 13C NMR spectrum of the AB2 macromonomer ([M]/[I] = 10). The peak at 65.7 ppm is attributed to the methylene carbon from bis MPA. The peak at 66.7 ppm is attributed to the methine carbon of the terminal group connected to the hydroxy group. The integration ratio of the peaks at 65.7 and 66.7 ppm was calculated to be 1.0. The 13C NMR measurement confirms the formation of the AB2-type macromonomer. Table 1 summarizes the results of the

Table 2. Self-Polycondensation of the AB2 Macromonomers polymer HB-PLLA (n = 5.0) HB-PLLA (n = 8.4) HB-PLLA (n = 19) HB-PLLA (n = 29) HB-PLLA (n = 42) PLLA

Table 1. Preparation of the AB2 Macromonomers [M]/[I]a 5 10 20 30 45

yield (%) 86 88 93 91 92

nb 5.0 8.4 19 29 42

Mnb 1580 2560 5610 8490 12200

Mw/Mnc

Mw/Mna

ηinhb (dL/g)

dn/dcc (mL/g)

DPwd

81

4

4.95 × 10

1.80

0.33

0.150

31

92

1.13 × 105

2.22

0.48

0.150

55

95

1.15 × 105

1.95

0.70

0.152

29

90

1.42 × 105

1.93

0.80

0.148

26

97

1.47 × 105

2.07

1.00

0.145

12

1.13 × 105

1.38

1.45

0.155

yield (%)

Mwa (g/mol)

Determined by SEC-MALS in HFIP. bChloroform, 30 °C, 0.5 dL/g. c Determined in HFIP at 25 °C. dWeight-averaged degree of polymerization, calculated by Mw of HB-PLLA/Mn of AB2 macromonomer. a

1.47 1.16 1.14 1.15 1.21

HB-PLLAs with different branching densities were prepared in good yields (81%−97%). The structure of the resulting polymers was confirmed by NMR measurements. 1H NMR spectra of the HB-PLLAs were very similar to those of the corresponding AB2 macromonomers. In the 13C NMR spectrum of HB-PLLA (n = 8.4), the peak at 175 ppm attributed to the carboxyl group of the AB2 macromonomer disappeared, supporting the successful self-polycondensation of the AB2 macromonomers. We have recently reported that the absolute Mw and ⟨S2⟩z values of linear PLLAs can be accurately estimated by SECMALS measurements in HFIP as the eluent.36 The characteristic ratio (C∞) was also determined to be 11.2 from the ⟨S2⟩ and intrinsic viscosity measurements, implying that PLLA behaves as a typical flexible chain polymer perturbed by the excluded volume effects in CHCl3 and HFIP. The absolute Mw

[M] = [lactide], [I] = [bis MPA] × 2. bCalculated from 1H NMR spectra. cDetermined by SEC in chloroform. a

oligomerization. The degree of polymerization (n) determined by the 1H NMR measurements was nearly consistent with the feed ratio of bis MPA and L-lactide. The molecular weight distribution determined by the SEC measurements was in the range of 1.14−1.47. However, it is not clear whether the n values of each of the chains in the AB2 macromonomer are equal. It seems reasonable that these values for each chain are similar because the polymerization should proceed without any severe steric effect, and thus, the terminal functional groups in each chain have an equal opportunity to react with the lactide. The resulting AB2 macromonomers were soluble in chloroform, Scheme 2. Synthesis of Hyperbranched Polylactides (HB-PLLAs)

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noted. First, the ⟨S2⟩z1/2 values of the HB-PLLAs are much lower than those of linear PLLA with the corresponding Mw. This clearly proves that the HB-PLLAs synthesized in the present study have certain branched architectures, as expected. In addition, the ⟨S2⟩z1/2 values decreases with a decreasing degree of polymerization (n) of the AB2 macromonomer. This is due to the fact that at a constant Mw the number of branching points increases with decreasing n values. The second noteworthy observation is the slope in these plots. The Mw dependence of ⟨S2⟩z1/2 for HB-PLLAs within the region examined can be given by the following equations:

of the linear PLLA was no more than one-half of that determined by conventional SEC measurements. HB-PLLAs (n = 5.0, 8.4, 19, 29, and 42) were thus characterized by the SECMALS measurements in HFIP. A typical SEC curve is shown in Figure 3. The SEC curve based on a 90° light scattering

Figure 3. SEC curves of HB-PLLA (n = 29) measured in HFIP.

⟨S2⟩z

1/2

(nm) = 2.5 × 10−1M w 0.37

for n = 8.4

(4)

⟨S2⟩z

1/2

(nm) = 1.8 × 10−1M w 0.40

for n = 19

(5)

⟨S2⟩z

1/2

(nm) = 1.6 × 10−1M w 0.41

for n = 29

(6)

⟨S2⟩z

1/2

(nm) = 1.1 × 10−1M w 0.44

for n = 42

(7)

The values of the power law exponent for the HB-PLLAs are less than 0.50, which is a lower limit for a linear flexible polymer chain in solution. This result also supports the formation of the branched polymer. The slight increase of the power law exponent in eqs 4−7 with increasing n values is most likely responsible for the increase of the molecular weight between branching points. Figure 5 shows a comparison of theoretical results with the experimental data for the HB-PLLAs (n = 8.4, 19, 29, and 42). Two theoretical models are the randomly branched model by Zimm−Stockmayer33 and the cascade theory for polycondensates by Kajiwara.41 According to Zimm and Stockmayer, the shrinking factor (g) for a randomly branched polymer with trifunctional branching units can be given by

detector (LS in Figure 3) is composed of a major peak with a minor shoulder in the high-molecular-weight region. The highmolecular-weight components were not detected by the RI detector. The SEC curve based on the RI detector showed a minor shoulder in the low-molecular-weight region. The Mw values of the HB-PLLAs determined by SEC-MALS in HFIP were in the range of 4.95 × 104−1.47 × 105, as seen in Table 2. The molecular weight distribution (Mw/Mn) was ∼2.0, which seems reasonable for the polycondensations. The weightaveraged degree of polymerization (DPw), calculated by dividing the Mw of an HB-PLLA by the Mn of the AB2 macromonomer, was in the range of 12−55, which implies successful self-polycondensation of the AB2 macromonomers. The inherent viscosities of the HB-PLLAs in chloroform were lower than that of the linear PLLA, as listed in Table 2, indicating the presence of a branched architecture. The inherent viscosity of the HB-PLLAs increased as the degree of polymerization (n) of the polylactide chain in the AB2 macromonomer increased. Figure 4 shows double-logarithmic plots of ⟨S2⟩z1/2 with Mw for the HB-PLLAs in HFIP at 25 °C in which the experimental data for the linear PLLA36 are also shown for comparison. The data for the HB-PLLA (n = 5) are excluded owing to a low molecular weight. There are two interesting observations to be

−0.5 ⎡⎛ γMM ⎞0.5 ⎛ 4γMM ⎞⎤ ⎥ ⎢ g = ⎜1 + ⎟ +⎜ ⎟ ⎝ 9π ⎠⎥⎦ ⎢⎣⎝ 7 ⎠

(8)

where M is the molecular weight and γM is the average number of branching points per M and in this study is exactly equal to the reciprocal of the weight-averaged molecular weight (see Table 1). Another model is the cascade theory for the polycondensates of the monomers with functionality f. According to Kajiwara,41 the shrinking factor (g) for the polycondensate is given by 3 (m − 1)! g= 2 m [(f − 1)m]!

m−2

∑ (f − 1)n n=0

[(f − 1)m − (n + 1)]! (m − n − 2)!

[n(f − 2) + 2(f − 1)](n + 1)

(9)

where m is the degree of polymerization of the polycondensate. As seen in Figure 5, the slopes of the plots for the HB-PLLAs (n = 8.4, 19, 29, and 42) were similar to the theoretical curves. The experimental ⟨S2⟩z1/2 values of the HB-PLLAs (n = 8.4 and 19) are intermediate between both theories. The experimental ⟨S2⟩z1/2 values of the HB-PLLA (n = 29) are relatively well described by the randomly branching model. In addition, the values of ⟨S2⟩z1/2 for the HB-PLLA (n = 42) deviate downward from the theories. Judging from all the experimental data for the HB-PLLAs, their dimensional properties are apparently better described in terms of the model for a randomly branched polymer than the cascade model. However, it should be noted that the present branched polymers with regular branching

Figure 4. Double-logarithmic plots of ⟨S2⟩z1/2 with Mw of the HBPLLAs in HFIP at 25 °C; n = 8.4 (△), n = 19 (□), n = 29 (◇), and n = 42 (○). The data (●) of linear PLLA36 are also shown for comparison. 8241

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Figure 5. Comparison of the experimental ⟨S2⟩z1/2 of the HB-PLLAs with the theories for random branching model (solid line) and for cascade theory (broken lines) with f = 3. (a) n = 8.4, (b) n = 19, (c) n = 29, and (d) n = 42. Plots for linear PLLA were also shown for comparison.

Figure 6. DSC curves of PLLA and HB-PLLAs: (a) the second heating traces and (b) the second cooling traces.

solvent for PLLA. It is clear that the solubility of the HB-PLLAs (n = 19, 29, and 42) is similar to PLLA. The number of terminal groups is automatically increased by the introduction of branching points. Therefore, the number of hydroxy groups located at the linear and terminal units increases in HB-PLLA when n is small. The relatively low solubility in chloroform and the good solubility in THF of the HB-PLLAs (n = 5.0 and 8.4) can be attributed to the increased number of hydroxy groups. The thermal properties of the HB-PLLAs were determined by DSC measurements. It is known that the linear PLLA is a crystalline polymer. Heating scans of the HB-PLLAs are shown in Figure 6a. The DSC curve of the linear PLLA is also included as a reference. The linear PLLA showed a glass transition at 60 °C, a broad crystallization peak at 111 °C, and a large melting peak at 165 °C. All of the HB-PLLAs showed a glass transition at 54−60 °C. The HB-PLLA (n = 8.4) showed a very small endothermic peak at 140 °C attributed to the melt transition. The crystallization peak was not observed for HB-PLLA (n = 8.4). It is clear that the crystallization of the polylactide chains is strongly suppressed for HB-PLLA (n = 8.4). The melting peak was clearly observed at 147 °C for the HB-PLLA (n = 19).

distances between branching points are not randomly branched polymers. From a theoretical point of view, the cascade theory for polycondensates of monomers with well-defined structures and functionality is more appropriate than the random branching model. The experimental ⟨S2⟩z1/2 values of the HBPLLA (n = 8.4) are relatively well described by the cascade theory. The extent of deviation from the cascade theory increases with increasing n. A logical explanation is that the probability of cyclization increases with increasing n, leading to the smaller ⟨S2⟩z1/2 value. All of the HB-PLLAs can be dissolved in THF and chloroform when the sample is stirred for several minutes. The time period required to completely dissolve the sample depends on the distance between the branching points. The HB-PLLAs (n = 5.0 and 8.4) is immediately dissolved in THF at room temperature. On the other hand, more than 30 s is required to dissolve the HB-PLLAs (n = 29 and 42) in THF. The HB-PLLA (n = 19) is immediately dissolved in chloroform. The period to dissolve HB-PLLAs (n = 5.0 and 8.4) in chloroform is clearly longer than the one for HB-PLLAs (n = 19, 29, and 42). It is known that chloroform is a good 8242

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Macromolecules

Article

A broad exothermic peak was also observed at 122 °C, which is attributed to crystallization. Clear endothermic and exothermic peaks were both observed for the HB-PLLAs (n = 29 and 42). The endothermic peak attributed to the melting transition shifted to a higher temperature and probably approaches the melting peak for the linear PLLA. A shoulder peak was also observed with the endothermic peaks of the HB-PLLAs (n = 19, 29, and 42). The double melting behavior has also been reported in the literature.35,38 We assume that the origin of the shoulder peak is attributable to the melt-recrystallization process reported in the literature.38 Considering the cooling DSC experiments, the distinct exothermic peak attributed to crystallization was only observed for the linear PLLA (Figure 6b). A small exothermic peak was observed for the HB-PLLA (n = 42). None of the other HB-PLLAs (n = 8.9, 19, and 42) showed an exothermic peak. The cooling scans indicated that crystallization is inhibited by the introduction of branching points. Table 3 summarizes the thermal properties of the HB-

Figure 7. X-ray diffraction patterns of PLLA and HB-PLLAs.

hampers the crystallization of the polylactide chains, and the trend in the X-ray data is consistent with the DSC measurements.

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Table 3. Thermal Properties of PLLA and HB-PLLAs Tga

Tc (°C)

ΔH1 (J/g)

Tm (°C)

ΔH2a

60 54

111

12.8

57

122

58 60

(°C) PLLA HB-PLLA (n = 8.4) HB-PLLA (n = 19) HB-PLLA (n = 29) HB-PLLA (n = 42) a

(J/g)

Tcb

(°C)

ΔH3 (°C)

165 140

20.1 0

98

4.1

13.1

147

19.1

120

18.5

155

22.6

117

23.6

159

28.4

99

0.7

a

a

a

CONCLUSION A series of HB-PLLAs were prepared by self-polycondensation of AB2 macromonomers (n = 5.0, 8.4, 19, 29, and 42). Structural characterizations of the HB-PLLAs by light scattering measurements in HFIP were performed. The dimensional properties of the HB-PLLAs were compared to the linear PLLA and analyzed in terms of current polymer solution theory. A comparison of the experimental and theoretical results with respect to their dimensional properties indicated that the HBPLLAs in this study assume randomly branched architectures (i.e., a hyperbranched structure). The DSC and X-ray diffraction measurements suggested that crystallization was strongly suppressed by the introduction of the branched architecture. It should be mentioned that the degree of polymerization of the AB2 macromonomers in this study is much lower than that required for chain entanglements of the polylactides chains.39 Consequently, poor mechanical properties were observed in a film fabricated from one of the HBPLLA (n = 29), consistent with an insufficiently entangled state of the HB-PLLA. The preparation of such long-branched polylactides is now in progress.

b

Second heating at 10 °C/min. bSecond cooling at 10 °C/min.

PLLAs. As discussed above, the glass transition temperatures were observed in the same temperature range, and the melting temperatures shifted to a higher level with an increasing distance between the branching points. The enthalpy data calculated from the area of the peaks are also listed in Table 3. It was observed that ΔH1 and ΔH2, calculated from the exothermic and endothermic peaks, respectively, were similar for the HB-PLLAs (n = 29 and 42). This suggests that the endotherm is caused by melting of the crystalline component formed during the heating scan. It appears that the HB-PLLAs (n = 29 and 42) are almost amorphous at the beginning of the heating scans. On the other hand, ΔH1 was noticeably lower than ΔH2 for the linear PLLA. It is clear that the linear PLLA contains a crystalline component at the beginning of the heating scan. The DSC cooling scan also supports this conclusion. A transparent film of the HB-PLLA (n = 29) was prepared by casting the chloroform solution onto a glass plate. The film was brittle in comparison to the linear PLLA film. The reduced mechanical properties might be caused by the lower chain entanglement of the HB-PLLA. The entanglement molecular weight of polylactide was reported to be 8000, which is much greater than the molecular weight of the linear segment in this study.39 The X-ray diffraction patterns of the HB-PLLA films are shown in Figure 7. Diffraction peaks were clearly observed for the linear PLLA. The peak intensity decreased with an increasing degree of polymerization (n) of the AB2 macromonomers. A very small peak was observed for the HB-PLLA (n = 8.4). Thus, it is clear that the introduction of branching

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (m.j.); skawagu@yz. yamagata-u.ac.jp (s.k.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by a Grant-in-Aid for Scientific Research (C) from the Ministry of Education, Culture, Sports, Science and Technology (No. 22550106) and the Yamada Science Foundation.

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