Biobased Polymer System: Miniemulsion of Poly(alkyl methacrylate

October 2009

Published by the American Chemical Society

Volume 10, Number 10

 Copyright 2009 by the American Chemical Society

Communications Biobased Polymer System: Miniemulsion of Poly(alkyl methacrylate-graft-lactic acid)s Kiyoaki Ishimoto,† Maho Arimoto,† Hitomi Ohara,*,† Shiro Kobayashi,*,† Masahiko Ishii,‡ Kouji Morita,§ Hirofumi Yamashita,§ and Naoya Yabuuchi§ R&D Center for Bio-based Materials, Kyoto Institute of Technology, Matsugasaki, Kyoto 606-8585, Japan, Paint and Finishing Design Department, Vehicle Engineering Group, Toyota Motor Company, Toyota, Aichi 471-8572, Japan, and Basic Technologies Division, Nippon Bee Chemical Company, Shodai-Ohtani, Hirakata, Osaka 573-1153, Japan Received July 14, 2009; Revised Manuscript Received August 19, 2009

To broaden the application scope of lactic acid polymers, a new miniemulsion of poly(alkyl methacrylate-graftlactic acid)s has been developed. The graft copolymer synthesis was based on a poly(lactic acid) macromonomer having a methacryloyl polymerizable group. The macromonomer and a comonomer of n-butyl methacrylate together with a suitable surfactant formed a miniemulsion of the comonomers in water. A radical copolymerization of the comonomers took place to produce the graft copolymer as a stable miniemulsion. The copolymer showed elastic polymer properties. The miniemulsion system may find useful applications as a new biobased polymer material that is environmentally desirable.

Introduction Nowadays, our society is faced with environmental problems, such as fossil resource shortage for oil production, which is considered to be related to rapid climate changes due to increasing carbon dioxide levels in the air. One of the possible directions to contribute to rectifying these problems is to employ renewable resources as starting materials and to conduct green polymer chemistry.1 Aliphatic polyesters are well-known as exemplified by poly(ε-caprolactone) (PCL), poly(butylene succinate) (PBS), and poly(hydroxyalkanoate)s (PHA)s, which are used much less in comparison with aromatic polyesters like poly(ethylene terephthalate) (PET). Recent trends, however, clearly demonstrate that poly(lactic acid) (PLA), an aliphatic polyester, has gained a major role in this line. High molecular weight PLA is a representative of biobased plastics and is already used commercially as a green plastic, such as in electronic products and automobile parts and also in biomedical * To whom correspondence should be addressed. E-mail: [email protected] (H.O.); [email protected] (S.K.). † Kyoto Institute of Technology. ‡ Toyota Motor Company. § Nippon Bee Chemical Company.

applications.2 PLA is currently prepared by the ring-opening polymerization of lactide (a six-membered cyclic dimer of lactic acid) derived from lactic acid produced by fermentation of plant carbohydrate products like corn starch, sugar canes, and so on. Besides the usage for plastics, PLA has potential for many other applications. It is a possible extension for preparing a polymer having a PLA chain as a graft component. There are some examples of the reported synthesis of graft copolymers using a (meth)acryloyl-polymerizable PLA macromonomer as a copolymerization monomer. Such macromonomers were prepared from a dehydration between methacrylic acid and oligo-LA having an OH group at one end and a COOH group at the other end and using N,N′-dicyclohexylcarbodiimide as the dehydrating agent.3 Other syntheses of the macromonomers are the Sn-catalyzed ring-opening polymerization of lactide initiated by 2-hydroxyethyl methacrylate4 and the Sn-catalyzed ring-opening polymerization of lactide initiated with n-dodecanol followed by methacrylation of the terminal OH group of PLA via urethane bond formation with 2-isocyanate ethyl methacrylate.5 From an environmental viewpoint it is desirable to not use organic solvents for coatings. We therefore aimed to develop a

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water solvent system of PLA-based coating and film forming materials using the above macromonomers; to the best of our knowledge this paper reports the first example of a poly(alkyl methacrylate-graft-lactic acid) (PRMA-g-PLA) (mini)emulsion. Recently, there are vivid discussions available from official organizations concerning the biobased resource versus petroleumbased resource content. According to the definition of Japan BioPlastics Association proposed in 2006, “biomass plastics” denote the plastics containing the biomass content higher than 25 wt %. In this regard, the present polymeric materials are in a context of biomass plastics as seen below.

Experimental Section Materials. L-Lactide (Purac Biochem bv, Gorinchem, Holland), 2-hydroxyethyl methacrylate (HEMA; Sigma-Aldrich Inc., St. Louis), n-butyl methacrylate (BMA), and tin octoate (Sn(Oct)2; both are from Nacalai Tesque Inc., Kyoto) were commercial reagents and used as received. Two surfactants, sodium dodecyl sulfate (SDS; Nacalai Tesque Inc., Kyoto) and sodium dialkyl sulfosuccinate (PEREX; PEREX OTP, Kao Co., Japan), were purchased. A radical initiator, potassium peroxodisulfate (KPS; Nacalai Tesque Inc., Kyoto), was a commercial reagent. 1,4-Dioxane, deuterochloroform (CDCl3), and other solvents were commercially available and used without further purification. Synthesis of Macromonomer. A macromonomer having a methacryloyl-polymerizable group, MM6.0, was synthesized as follows. L-Lactide (35.9 g) was placed in a 200 mL flask with three necks and dried under vacuum for 1 h. Into the flask filled with dry nitrogen gas were added HEMA (10.8 g) and Sn(Oct)2 (0.20 g), and the mixture was heated at 110 °C, using an oil bath, for 3 h with stirring. MM6.0 was almost quantitatively produced. Synthesis of Graft Copolymer via Miniemulsion Copolymerization. To a 150 mL flask were added SDS (0.17 g, 1.0 wt % for the total comonomers) and deionized water (DIW, 47 mL). With stirring, a mixed solution of comonomers, MM6.0 (7.3 g) and BMA (9.2 g), was added dropwise to the surfactant solution, and the mixture was further subjected to ultrasound sonication for 6 min by using an ultrasonic generator (UD-200, Tomy Digital Biology Co., Japan). The mixture gave a stable miniemulsion system. Then, the miniemulsion was transferred to a 100 mL three-necked flask under nitrogen and heated to 70 °C using a water bath. With stirring, 3 mL of DIW containing KPS (0.10 g) was added to the miniemulsion, which was further heated at 85 °C for 0.5 h. The conversion of the comonomers was completed, indicating the quantitative yield of poly(BMA-coMM6.0), that is, PBMA-g-PLA. Isolation of Graft Copolymer. To DIW (100 mL) containing 31.5 g NaCl, 5.0 mL of the copolymerized miniemulsion solution was added dropwise with vigorous stirring to precipitate the graft copolymer. Further, 200 mL of DIW was added with stirring. The graft copolymer was isolated by filtration under vacuum and washed further with DIW. The white solids were isolated via vacuum filtration. The graft copolymer obtained after drying was 1.05 g (calculated amount; 1.38 g). Preparation of Cast Film. The copolymer miniemulsion (5.0 mL) was poured into a teflon shale (d ) 75 mm) and dried at room temperature under atmospheric pressure for 48 h to give a cast film of the copolymer. Preparation of PBMA Homopolymer. Poly(n-butyl methacrylate) (PBMA) was obtained by a radical homopolymerization of BMA in an emulsion using PEREX as surfactant and KPS as an initiator at 85 °C for 30 min. The molecular weight of the isolated polymer was Mn ) 1.4 × 105 (Mw 4.4 × 105) determined by GPC measurement. Analytical Methods. 1H NMR measurements were recorded on a spectrometer ARX-500 (500 MHz, Bruker Co.). ESI-TOF-MS analysis was performed by using a microTOF instrument (ESI-TOF MS; BRUKER DALTONICS, Germany). The molecular weight of polymers was measured by a gel permeation chromatography (GPC) instrument

Communications Scheme 1

(GL-7400 Series, GL Science Inc., Japan) with a refractive index (RI) detector using chloroform eluent at a column temperature 40 °C, in which polystyrene standards (molecular weight ) 2.2 × 103 to 6.5 × 105) were employed. Particle size measurements were conducted on a dynamic light scattering instrument DLS-7000 (Otsuka Electronic Co., Japan) under a He-Ne atmosphere. Differential scanning calorimetric (DSC) analysis was carried out using a DSC-50 (Shimadzu Co. Kyoto) under a N2 flow (20 mL/min), while increasing the temperature at a rate of 20 °C/min. The physical properties of the film were measured on an Autograph AG-IS (Shimazdu Co., Kyoto) with a strain application change rate of 25 mm/min for the film length of 50 mm.

Results and Discussion The whole synthesis route is given in Scheme 1. First, ringopening polymerization of L-lactide was carried out, initiated by 2-hydroxyethyl methacrylate (HEMA), with a Sn(Oct)2 catalyst, to produce methacryloyl-polymerizable PLA macromonomer (MMm). Then, radical copolymerization of MMm with n-butyl methacrylate (BMA) as an alkyl methacrylate was performed in water by using an anionic surfactant to give a PBMA-g-PLA miniemulsion. Synthesis of Macromonomer (MMm). The Sn-catalyzed ring-opening polymerization of L-lactide initiated by HEMA has been reported.4 The reaction to prepare a macromonomer (MMm) was carried out at 110 °C in bulk for 3 h. The chain length (m value) could be controlled by the feed ratio of L-lactide/HEMA. Figure 1A shows the 1H NMR (500 MHz) spectrum of the product MMm, which was obtained by the reaction of L-lactide/HEMA ) 3.0/1.0, so that the lactide units might become statistically 6.0 and, hence, designated as MM6.0. The monomer conversion was almost quantitative (>98% from the NMR analysis). In comparison with spectra of (B) lactide and (C) HEMA, the spectrum (A) clearly supports the structure of MM6.0, in which the plausible assignments of peaks are given. From the peaks integration ratio due to methine protons and also to methyl protons, the average m value of MMm was obtained to be m ) 6.0, with the value being equal to the feed ratio. Further, from the integration ratio of the olefin proton and the methine proton, the methacryloyl-group content (functionality) of MM6.0 was calculated as higher than 96%. Figure 2 demonstrates the ESI-TOF-MS chart of the macromonomer (MM6.0), indicating that MM6.0 contains a mixture of 2-10 lactic acid units (peak top at 6) linked to HEMA. The peak top value of m/e 585.2 corresponds to the sum of the mass due to the structure of MM6.0 of 562 (130 of HEMA + 6 × 72, the repeat unit mass, plus 23 of Na+). In addition to the major peak, the minor peak observed at 601 is due to the mass of 562 plus 39 (K+). The major and minor peaks situation is similar for other 8 peaks, which appear at every 72 intervals. MM6.0 is composed of not only even numbered repeat units but odd numbered ones as well. According to the accepted Sncatalyzed reaction mechanism of the lactide ring-opening

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Figure 3. Size distributions measured by DLS: (A) the comonomer mixture solution with 1.0 wt % SDS (d ) 231 nm), (B) the radically copolymerized product solution with 1.0 wt % SDS (d ) 223 nm), (C) the comonomer mixture solution with 1.0 wt % PEREX (d ) 220 nm), and (D) the radically copolymerized product solution with 1.0 wt % PEREX (d ) 333 nm). Figure 1. 1H NMR (500 MHz) spectra (CDCl3 with TMS) of (A) MM6.0, (B) lactide, and (C) HEMA.

Figure 4. 1H NMR (500 MHz) spectra (CDCl3 with TMS) of (A) PBMA-g-PLA [poly(BMA-co-MM6.0)] and (B) homopolymer of BMA. Figure 2. ESI-TOF-MS chart of MM6.0.

polymerization, only even numbered units are to be produced.6 Therefore, the present reaction suggests that the transesterification reaction between lactide-ester bonds took place extensively. Following the macromonomer preparation reaction by ESI-TOF-MS measurements revealed that at the beginning of the reaction only the even numbered products were formed. With progress of the reaction, production of the odd numbered ones was also observed, indicating that the transesterification started. However, there was no product observed derived from the ester bond cleavage of HEMA under the present reaction conditions. This is a quite interesting aspect and will be a future problem to be examined in more detail. Miniemulsion Copolymerization. To prepare a stable emulsion system, a hydrophobic monomer MM6.0 was copolymerized with a relatively hydrophobic monomer of BMA. It was

essential to select an appropriate surfactant. Among surfactants examined, sodium dodecyl sulfate (SDS) and sodium dioctyl sulfosuccinate (PEREX), both anionic, showed effective results. Prior to the copolymerization, a monomer emulsion system was prepared. A mixed solution of the feed comonomers, MM6.0 (7.3 g) and BMA (9.2 g), which correspond to the molar ratio of 1.0/4.9 (17/83), was poured into deionized water (47 mL) containing SDS (0.17 g, 1.0 wt % for the total comonomers) with stirring at room temperature and further subjected to an ultrasound sonication for 6 min to form a stable small droplets dispersion solution. Figure 3A shows a size distribution of the monomer droplets measured by dynamic light scattering (DLS) method, showing an average particle diameter d ) 231 nm. This particle size belongs to a miniemulsion criterion as to the diameter size around 50-500 nm.7

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Table 1. Miniemulsion Radical Copolymerization between MM6.0 and BMA code surfactanta (wt %) [MM6.0]/[BMA] in feed (mol/mol) avg particle diameterb (nm) Mnc (× 10-5) Mwc (× 10-5) biomass content (wt %) 1 2 a

SDS: 1.0 PEREX: 1.0 1.0 wt % for the total monomers.

17/83 17/83 b

223 333

1.59 1.64

2.11 1.98

34 34

Determined by DLS. c Determined by GPC.

Then, the comonomer miniemulsion solution was transferred into a three-necked flask equipped with a reflux cooler filled with N2 gas. To the flask, which was kept at 70 °C, was added potassium peroxodisulfate (KPS, 0.1 g). The flask was then heated up to 85 °C and the solution was stirred for an additional 0.5 h to complete the radical copolymerization, resulting in the production of a miniemulsion of poly(BMAco-MM6.0), which corresponds to PBMA-g-PLA. Figure 4A shows the 1H NMR (500 MHz) spectrum of the graft polymer PBMA-g-PLA obtained from the miniemulsion. In the spectrum, the rational assignment of peaks is indicated. For reference, the spectrum of BMA homopolymer is shown in (B). In the spectrum (A), the olefinic protons due to both monomers disappeared, and it is considered that a radical copolymerization between BMA and MM6.0 took place randomly to produce quantitatively the copolymer of poly(BMA-co-MM6.0). The integral ratio of peak d due to BMA over peak b due to MM6.0 was 4.9, which coincides with the initial feed molar ratio of the comonomers. Figure 3B shows a DSL chart of the miniemulsion system of copolymerization between MM6.0 and BMA, indicating an average particle diameter d ) 223 nm, which is almost the same as that of the monomer miniemulsion. However, the particle size distribution became somewhat broadened after the copolymerization. The copolymerization results are given in Table 1. According to the measurement by gel permeation chromatography (GPC), molecular weight values of the isolated copolymer were Mn ) 1.59 × 105 and Mw ) 2.11 × 105, respectively. The present copolymer contains 34 wt % of LA component for the total polymer weight. According to the above-mentioned regulation, a plastics containing more than 25 wt % of biomassderived components is categorized as “biomass-plastics”. In this context the copolymer belongs to a biomass-polymer, demonstrating an example of conducting green polymer chemistry in the present study.1 Similarly, the emulsion copolymerization was performed by using PEREX (1.0 wt %) as surfactant to give again a miniemulsion showing values of the PBMA-g-PLA copolymer, Mn ) 1.64 × 105 and Mw ) 1.98 × 105, respectively (Table 1). In both surfactant cases, the polydispersity (Mw/ Mn) values obtained by GPC are relatively low (1.32 and 1.21, respectively), which seems indicative of having a branched structure of the copolymers. The average particle size was d ) 220 nm before the polymerization and that of the copolymer became somewhat larger as d ) 333 nm (C and D, respectively, in Figure 3). Conducting the polymerization in miniemulsion provided with a low viscosity aqueous dispersion, and the combination of PLA and PBMA seemed to give suitable film forming properties so that the materials could have applications as coatings.7 Actually, the copolymer obtained from SDS surfactant formed a transparent film with a cast method, and its Tg value was 30 °C, determined by a DSC measurement. Other physical properties together with those of the copolymer obtained from PEREX surfactant are given in Table 2. These values show that the copolymer is a typical elastic material.

Table 2. Physical Properties of PBMA-g-PLA [Poly(BMA-co-MM6.0)] Film Formed from the Miniemulsion code

Tg (°C)

Young’s modulus (kgf/cm2)

tensile strength (kgf/cm2)

elongation at break (%)

1 2

30 32

2390 1104

92 53

265 95

Conclusion We have shown the first example of a miniemulsion system of PBMA-g-PLA copolymer having lactic acid graft chains. The graft copolymer synthesis was achieved by the radical copolymerization between macromonomer MM6.0 and BMA. The film of PBMA-g-PLA showed elastic properties. The miniemulsion is a stable system without using organic solvents and can be conveniently employed for coatings and others as a “biomass-polymer” material. Properties of the graft copolymers will be tuned further by using a macromonomer with different lactic acid chain length and another comonomer, as well as comonomer composition, and by controlling the copolymer molecular weight. These strategies are now under investigation in our laboratories and will be published elsewhere.

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