Biobased Polymers: Synthesis of Graft Copolymers and Comb

Sep 18, 2012 - With stirring, 3 mL of DIW containing KPS (0.10 g) was added to the ...... Synthesis and Characterization of Comb and Centipede Multigr...
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Biobased Polymers: Synthesis of Graft Copolymers and Comb Polymers Using Lactic Acid Macromonomer and Properties of the Product Polymers Kiyoaki Ishimoto,† Maho Arimoto,† Tomoya Okuda,† Syuhei Yamaguchi,† Yuji Aso,† Hitomi Ohara,*,† Shiro Kobayashi,*,‡ Masahiko Ishii,§ Koji Morita,∥ Hirofumi Yamashita,∥ and Naoya Yabuuchi∥ †

Department of Biobased Materials Science and ‡Center for Nanomaterials and Devices, Kyoto Institute of Technology, Matsugasaki, Kyoto 606-8585, Japan § Vehicle Engineering Group, Toyota, Paint & Finishing Design Department, Toyota Motor Co., Aichi 471-8572, Japan ∥ Basic Technologies Division, Nippon Bee Chemical Co., Shodai-Ohtani, Hirakata, Osaka 573-1153, Japan S Supporting Information *

ABSTRACT: For developing broader application of biobased polymers, graft copolymers and comb polymers having poly(lactic acid) (PLA) side chains have been synthesized by using a macromonomer technique. PLA macromonomers (MMm) having a methacryloyl polymerizable group with different PLA chain length with an average length m = 4, 6, 8, 12, 18, and 30 were prepared via ring-opening polymerization of L-lactide using hydroxyethyl methacrylate (HEMA) initiator catalyzed by Sn(Oct)2. Radical polymerization behaviors of these macromonomers were examined. Radical copolymerization of MMm (m = 4, 6, and 8), with vinyl monomers like n-butyl methacrylate (BMA) and n-butyl acrylate (BA) in water as the reaction medium, gave stable miniemulsions of poly[n-butyl (meth)acrylate-graf t-lactic acid]s [PB(M)A-g-PLAm]. MMm with m value higher than 12, however, gave aggregate products in a minor amount besides miniemulsions in a major amount, producing not a stable emulsion system of graft copolymers. The solution copolymerization, on the other hand, produced a wider variety of the graft copolymers, where a wider range of MMm (even m ≥ 12) can be employed. In a 1,4-dioxane solution, the radical copolymerization of MMm with BMA and methyl methacrylate (MMA) gave various graft copolymers [PB(M)MA-g-PLAm]. A new type of comb polymers (PMMm) having PLAm as pendant side chains were obtained by radical homopolymerization of MMm in a 1,4-dioxane solution. The graft copolymers and comb polymers obtained here are amorphous. Physical properties of the polymers from miniemulsions suggested them to be applicable for coatings or elastic materials which are environmentally desirable as a new class of biobased polymers. In addition, the present approach provided fundamental information on relationships between the length of PLA side chain and the bulk properties of the product polymers.

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well as in an organic solvent system using itaconic anhydride,14 and the lipase- and protease-catalyzed enantioselective oligomerization of alkyl lactates.15,16 Aliphatic polyesters like poly(ε-caprolactone) (PCL), poly(butylene succinate) (PBS), and poly(hydroxyalkanoate)s (PHA)s are practically much used. An aromatic polyester of poly(ethylene terephthalate)

INTRODUCTION It is demanded today that the environmental problem and fossil resource shortage problem are to be seriously coped with. The former is related to rapid climate change due partly to the increasing carbon dioxide in the air. Possible directions to mitigate and contribute to solving these problems are to employ renewable resources as starting materials1 and to conduct green polymer chemistry.2−12 For the contribution, we recently reported the preparation of polymeric materials from lactic acid (LA) in a water solvent miniemulsion system13 as © 2012 American Chemical Society

Received: July 31, 2012 Revised: September 12, 2012 Published: September 18, 2012 3757

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contents of the present paper: the synthesis of (meth)acryloylpolymerizable macromonomers having various PLA chain lengths, the possibility of miniemulsion system formation in graft copolymer synthesis from PLA macromonomers having different chain lengths, the synthesis of PLA graft copolymers in an organic solvent, comb polymer synthesis by homopolymerization of PLA macromonomers, and some properties of the typical product polymer samples. According to the definition of Japan BioPlastics Association proposed in 2006, “biomass plastics” denote the plastics containing biomass content higher than 25 wt %. In this line, all the present polymers are in the scope of biomass plastics.

(PET) is much more utilized conveniently. Recently, poly(lactic acid) (PLA), an aliphatic polyester, has gained a major expectation among the polyesters. High molecular weight PLA is a representative of biobased plastics and used already as green plastics for electronic products and automobile parts, and also as biomedical applications.17−25 PLA is mainly prepared by the ring-opening polymerization of lactide derived from lactic acid produced by fermentation of plant carbohydrate products like corn starch, sugar cane, and others. Furthermore, PLA can be utilized for many other applications. However, PLA has a drawback in properties, which is due to hydrolysis of the ester bond to break the PLA chain. So far, various efforts have been devoted to decrease the bond breaking; however, it has been very difficult to suppress the hydrolysis completely. One possible extension to mitigate the bond breaking is not to use PLA as a main chain, but to employ PLA as side chains to reduce the property damage, the concept image of which is shown in Figure 1.

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EXPERIMENTAL SECTION

Materials. L-Lactide (Purac Biochem bv, Gorinchem, Holland), 2hydroxyethyl methacrylate (HEMA) (Sigma-Aldrich Inc., St. Louis), nbutyl methacrylate (BMA), n-butyl acrylate (BA), methyl methacrylate (MMA), and tin octoate (Sn(Oct)2) (these four, Nacalai Tesque Inc., Kyoto, Japan) were commercial reagents and used as received. A surfactant sodium dodecyl sulfate (SDS), radical initiators of potassium peroxodisulfate (KPS), and α,α′-azobisisobutyronitrile (AIBN) were purchased from Nacalai Tesque Inc. A surfactant sodium dialkyl sulfosuccinate (PEREX OT-P, Kao Co., Japan) was a commercial reagent. 1,4-Dioxane (DON), deuterochloroform (CDCl3), and other solvents were commercially available and used without further purification. Synthesis of Macromonomer. As a typical run, macromonomer having a methacryloyl-polymerizable group MM8 was synthesized as follows (code 3 in Table 1). L-Lactide (33.5 g) was placed in a 200 mL separable flask with three necks and dried under vacuum for 1 h. Into the flask filled with dry nitrogen gas, HEMA (7.56 g) and Sn(Oct)2 (0.14 g) were added, and the mixture was heated at 110 °C with using an oil bath for 3 h with stirring. The product MM8 was dissolved in chloroform, washed with aqueous 1 N HCl solution, and further washed three times with deionized water (DIW). The organic layer was separated, the solvent was evaporated in vacuo, and MM8 was dried at room temperature for 12 h under reduced pressure to give 35.3 g (86% yields). For the isolation of MM30, the reaction mixture was dissolved in chloroform and poured into methanol to precipitate the product, which was dried at 40 °C under vacuum for 12 h to give MM30. Synthesis of Graft Copolymer by Miniemulsion Copolymerization. A typical example of code 6 in Table 2 is shown. In a 150 mL flask, SDS (0.50 g, 3.0 wt % for the total comonomers) and DIW (47 mL) were placed. With stirring, a mixed solution of comonomers, MM8 (6.9 g) and BMA (9.6 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 by 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-co-MM8), that is, PBMA-g-PLA8. Isolation of Graft Copolymer. To DIW (100 mL) containing 31.5 g of 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 0.92 g (calculated amount; 1.42 g). Synthesis of Graft Copolymer by Solution Copolymerization. Procedures for codes 1−3 in Table 4 are given. As a typical run of reaction code 1, a mixture of BMA and MM6 (the total 1.0 g in mol/mol = 83/17 in order for the biomass content to be 34 wt %) and

Figure 1. Image of PLA polymer properties. (A) A polymer having PLA main chain causes large damage of properties via hydrolysis. (B) A vinyl polymer main chain having PLA side chains suffers a reduced damage of properties via hydrolysis.

From this viewpoint, a graft copolymer containing PLA as side chains is a desirable candidate in this line. To date, some papers reported the synthesis of PLA graft copolymers using a (meth)acryloyl-polymerizable PLA macromonomer as a copolymerization monomer. These macromonomers were prepared via the dehydration between methacrylic acid and oligo-LA having OH group at one end and COOH group at the other end.26,27 Other syntheses of the macromonomers were the Sn-catalyzed ring-opening polymerization of lactide initiated by 2-hydroxyethyl methacrylate,13,14,28−32 and the Sn-catalyzed ring-opening polymerization of lactide, followed by methacrylation of the terminal OH group of PLA via urethane bond formation using 2-isocyanate ethyl methacrylate.33 In addition, such a graft copolymer was prepared via the Sn-catalyzed ring-opening polymerization of lactide initiated from poly(MMA-co-HEMA).34 It is to be mentioned that the characteristic nature of PLA is stereocomplex formation between enantiomeric D-PLA and L-PLA chains.35−39 By utilizing this nature, graft copolymers derived from dextran (Dex-g-PDLA and Dex-g-PLLA) were employed to prepare a mechanically tenacious and tough material, which is regarded as applicable for an implantable biocompatible material.40,41 Our recent paper reported the first synthesis of a poly(alkyl methacrylate-graf t-lactic acid) (PRMA-g-PLA) miniemulsion using a PLA macromonomer method.13 The present study is its extension and is concerned with polymers having PLA side chains as indicated in Figure 1B, all of which are derived from PLA macromonomers. Thus, the following are the main 3758

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Scheme 1. Outline of Whole Reaction Processes

AIBN (3.0 mol %) in toluene (1.0 mL) were reacted at 70 °C for 24 h under dry nitrogen. For isolating the product copolymer, chloroform was used as good solvent for three runs, and as poor solvent n-hexane for code 1 and methanol for codes 2 and 3 were employed. For the reaction of code 4, a mixture of BMA (0.24 g), MM30 (0.76 g), and AIBN (6.6 mg, 2.0 mol % for the total monomer) in 2.0 mL of 1,4-dioxane was reacted at 60 °C for 24 h under dry nitrogen. The reaction mixture was poured into a large amount of methanol, and the precipitates were separated and dried at 40 °C in vacuo for 12 h. For the reaction of code 5, a mixture of MMA (4.8 g), MM6 (6.7 g), and AIBN (0.098 g, 1.0 mol % for the total monomer) in 100 mL of 1,4-dioxane was reacted at 60 °C for 24 h under dry nitrogen. After the reaction, the reaction mixture was poured into a large amount of diethyl ether to precipitate the product, which was separated and dried in vacuo at 40 °C for 12 h. Synthesis of Comb Polymer by Solution Homopolymerization. A typical comb polymer of PMM12 (code 5 in Table 6) was prepared by heating a mixture of MM12 (1.0 g), 1,4-dioxane (2.0 mL, solvent), and AIBN (16 mg, 10 mol % for the monomer) in a 15 mL test tube at 70 °C for 24 h under nitrogen. Product comb polymers were isolated by pouring the reaction mixture into a large amount of diethyl ether to precipitate powdery polymeric materials, which were separated and dried under vacuum at 40 °C for 12 h. For the reaction of code 9, methanol was used for the polymer precipitation, instead of diethyl ether. 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 24 h to give a cast film of the copolymer. The film samples from the solution polymerization were prepared from casting the 0.3 g of the polymer dissolved in 5.0 mL of methylene chloride. Analytical Methods. 1H NMR measurements were recorded on a spectrometer ARX-500 or 600 (500 or 600 MHz, Bruker Co., Germany). ESI-TOF-MS analysis was performed by using a microTOF instrument (ESI-TOF-MS) (Bruker Co.).

Molecular weight of polymers was measured by a gel permeation chromatography (GPC) instrument (GL-7400 series, GL Science Inc., Japan) with a refractive index (RI) detector using chloroform eluent at a column temperature of 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 He−Ne atmosphere. Differential scanning calorimetric (DSC) analysis was carried out using a DSC-50 (Shimadzu Co. Kyoto, Japan) under N2 flow (20 mL/ min) with increasing the temperature at a rate of 20 °C/min. Liquid chromatography−mass spectroscopy (LC−MS) was measured by using an apparatus LCMS-QP8000α (Shimadzu) by APCI (atmospheric pressure chemical ionization) method with a positive mode: LC part (Shimadzu), degasser (DGU-12AM), UV detector (SPD-10Avp), and column heater (CTO-10Avp). The column used was an ODS column: mobile phase, acetonitrile/ultrapure water (9/ 1); flow rate, 0.2 mL/min; column temperature, 40 °C; concentration of samples, 1.0 mg/mL. Physical properties of the film were measured on an Autograph AGIS (Shimadzu) with a strain application change rate of 25 mm/min for the film length of 50 mm. Pencil hardness of the film was measured on the pencil hardness measurement apparatus (Imoto Co., Kyoto, Japan) by using Mitsubishi pencils Hi-uni (6B − B, HB, F, H, and 2H) with fixing at an angle 45° ± 1° by loading 750 G.

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RESULTS AND DISCUSSION The synthesis route in the present study is outlined as three types of reactions shown in Scheme 1. First, ring-opening polymerization of L-lactide was carried out initiated by 2hydroxyethyl methacrylate (HEMA) with Sn(Oct)2 catalyst to produce a methacryloyl-polymerizable PLA macromonomer (MMm) (reaction 1). Then, radical copolymerization of MMm 3759

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Table 1. Synthesis and Characterization of Macromonomers product macromonomer code

reaction feed:a lactide/HEMA (molar ratio)

MMmb

isolated yield (%)

m valuec

1 2 3 4 5 6

2.0/1.0 3.0/1.0 4.0/1.0 6.0/1.0 9.0/1.0 15.0/1.0

MM4 MM6 MM8 MM12 MM18 MM30

78 92 86 89 90 71

4.1 5.7 8.1 11.7 17.9 29.5

Mnd (m value) 386 551 732 981 1450 2870

(3.6) (5.8) (8.4) (11.8) (18.3) (37.9)

Mwd

functe (%)

Tgf (°C)

Tmf (°C)

528 720 1010 1350 2000 3690

99 98 97 98 99 97

−27 −17 −12 −8 30 38

58 105 131

Reactions at 110 °C for 3 h in bulk under N2, with catalyst Sn(Oct)2 0.6 mol % for HEMA. bThe m value of MMm denotes the feed molar ratio (Llactide/HEMA) × 2. cDetermined by 1H NMR. dDetermined by GPC. eContent of methacryloyl group in the macromonomer determined by 1H NMR. fDetermined by DSC analysis. a

with n-butyl methacrylate (BMA), n-butyl acrylate (BA), or methyl methacrylate (MMA) as an alkyl (meth)acrylate to afford a graft copolymer (PBMA-, PBA-, or PMMA-g-PLAm) (reaction 2) and radical homopolymerization of MMm to produce comb polymers (PMMm) (reaction 3) were performed. I. Synthesis and Characterization of Macromonomer (MMm). The MMm synthesis via Sn-catalyzed ring-opening polymerization of L-lactide initiated by HEMA was performed according to reaction 1.13,14,28−32 The synthesis reaction was carried out at 110 °C in bulk for 3 h. The chain length (m value of MMm) was controlled by the feed molar ratio of L-lactide/ HEMA. Macromonomers (MMm) with six different chain lengths were obtained in almost quantitative to high yields (Table 1). The m value of six MMm products was determined by 1H NMR spectroscopy. As a typical example, Figure 2A shows the 600 MHz 1H NMR spectrum of product MM18, which was

obtained by the reaction of L-lactide/HEMA = 9.0/1.0 (code 5), so that the lactide units might become 18, being therefore expressed as MM18 for the product. The monomer conversion was quantitative (>96% from the 1H NMR analysis). The peak assignments shown in Figure 2A clearly support the structure of MM18. From the peak integration ratio due to methine protons (peaks c and d), the average m value of MM18 was obtained as m = 17.9, the value being almost equal to that calculated from the feed ratio. Further, from the integration ratio of the olefin proton (peak a or b) and the terminal methine proton (peaks d), the methacryloyl-group content, which corresponds to the functionality (%), was calculated and proved to be almost quantitative (99%). Figure 2B demonstrates the 150 MHz 13C NMR spectrum of MM18, with adding the chemical shift values of two peaks g + h due to −OCH2CH2O− of δ 62.0 and δ 63.1, in which all the peak assignments are plausibly performed. Thus, the structure of MM18 well accords with that shown in reaction 1. The molecular weight and the m value of the products MMm were also determined by GPC. The m values of MMm obtained were in good agreement with those of 1H NMR analysis, except for MM30, which showed a little higher m value than that obtained by 1H NMR. As indicated in Figure 3, the Tg value of MMm products increased with elongation of the LA chain length from −27 °C (m = 4) to +38 °C (m = 30). When the chain length of MMm becomes longer, the products start to exhibit Tm like MM12 = 58 °C, MM18 = 105 °C, and MM30 = 131 °C. These phenomena are in accord with those generally observed for optically pure PLA chains.17−25

Figure 2. (A) 600 MHz 1H NMR spectrum of MM18 (CDCl3) and (B) 150 MHz 13C NMR spectrum of MM18 (CDCl3).

Figure 3. DSC charts for the determination of Tg and Tm values of the macromonomers. 3760

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Figure 4 shows the ESI-TOF-MS chart of MM8, showing the highest peak at m = 8. The peak top value of m/e 729

Figure 4. ESI-TOF-MS chart of MM8.

corresponds to the sum of mass due to the structure of MM8 of 729 (130 of HEMA + 8 × 72, the repeat unit mass, plus 23 of Na+). In addition to the major peak, the minor peak observed at 745 is due to the mass of 706 plus 39 (K+). The major and minor peaks situation is similar to other 10 peaks, which appear at every 72 intervals. Transesterification. The Sn-catalyzed ring-opening polymerization of lactide afforded not only the even-numbered units but also the odd-numbered products, as argued from the mechanistic views.42−45 The following observations clearly show the experimental results. Figure 5A indicates the LC−MS chart of the reaction mixture after 1 h at 110 °C, exhibiting three peaks due to even numbers m = 4, 6, and 8, of the MMm structure as normally expected.42−45 Interestingly, after 2 h at the same temperature (Figure 5B); however, in addition to these three peaks, two small peaks due to odd numbers m = 5 and 7 started to appear. Then, after 3 h (Figure 5C), a peak due to m = 9 newly appeared, a total of six peaks of m = 4, 5, 6, 7, 8, and 9 being observed. These chronological experimental results show the extensive occurrence of transesterification to yield a mixture of even and odd numbered MMm products according to the progress of the reaction. At the beginning of the reaction where the lactide is abundant, its ring-opening propagation reaction is prevailing, while at the later stage of the reaction the lactide concentration becomes less and the linear LA chain becomes increased, and then the transesterification starts to occur in addition to the ring-opening polymerization of the lactide. The transesterification involves the attack of the active site of Sn-OR species onto the carbonyl carbon of the linear PLA ester chain. Therefore, eventually the ring-opening polymerization and the transesterification will be dynamically in equilibrium to give PLA having a statistical molecular distribution. It is obvious that similar transesterification occurred in the reaction shown in Figure 4, where even numbered PLA chains are formed more than those of odd numbered ones, probably due to a shorter reaction time for reaching to the equilibrium. II. Miniemulsion Radical Copolymerization to Graft Copolymer. In our previous paper, MM6 was copolymerized

Figure 5. LC−MS charts of the Sn-catalyzed ring-opening polymerization; reaction time of (A) 1 h, (B) 2 h, and (C) 3 h, with the feed ratio of L-lactide/HEMA = 3/1 at 110 °C.

radically with BMA monomer to produce a “biobased” polymer in a stable miniemulsion system.13 It was aimed in this study to examine the applicability of the macromonomers with different chain length for a miniemulsion system. Therefore, to obtain stable emulsion systems more extensively by radical copolymerization, the above six MMm macromonomers have been prepared as a comonomer. In the copolymerization, BMA, BA, or MMA was employed as the vinyl monomer (reaction 2, Scheme 1). It was important to select an appropriate surfactant; here sodium dodecyl sulfate (SDS) and sodium dioctyl sulfosuccinate (PEREX), both anionic, were found effective. To form a miniemulsion system, ultrasound sonication was applied to the mixture of comonomers and surfactant in water solvent, before the copolymerization, as reported.13 Table 2 summarizes the miniemulsion copolymerization results. In all of the copolymerization runs of reaction 2, the feed molar ratio of the vinyl monomer/MMm was adjusted such that the biomass content (PLA component for the total polymer weight) would be 34 wt %. As the macromonomer, four kinds, MM4, MM6, MM8, and MM12, were used, and as vinyl monomer, BMA and BA were employed. As to the expression of the product graft copolymers, radical copolymerization of BMA and MM4, for example, produces PBMA-co-PMM4, which on the basis of the structure corresponds to PBMA-g-PLA4. In this study the latter expression is used hereafter as shown generally in reaction 2. With using 1.0 or 3.0 wt % of the surfactant, all copolymerizations employing MM4, MM6, and MM8 as comonomer afforded a stable miniemulsion system before and after the reaction. However, the copolymerization system using MM12 as comonomer gave a miniemulsion before the 3761

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Table 2. Miniemulsion Radical Copolymerization of Alkyl (Meth)acrylate (BMA or BA) with MMm to Graft Copolymersa copolymerization reaction feeda

product graft copolymerb

before polymerization

after polymerization

structure expression

SDS (3.0)

261

175

BMA/MM6 (83/17)

SDS (1.0)

231

223

3

BMA/MM6 (83/17)

SDS (3.0)

265

176

4e

BMA/MM6 (83/17)

PEREX (1.0)

220

333

5

BMA/MM6 (83/17)

PEREX (3.0)

267

168

6

BMA/MM8 (87/13)

SDS (3.0)

258

105

7

BMA/MM12 (92/8)

SDS (3.0)

244

78f

8 9

BA/MM6 (85/15) BA/MM6 (85/15)

PEREX (1.0) PEREX (3.0)

113 113

99 95

PBMA-gPLA4 PBMA-gPLA6 PBMA-gPLA6 PBMA-gPLA6 PBMA-gPLA6 PBMA-gPLA8 PBMA-gPLA12 PBA-g-PLA6 PBA-g-PLA6

code

alkyl (meth)acrylate/[MMm] in feed (mol/mol)b

1

BMA/MM4 (75/25)

2

e

surfactant (wt % of the total monomers)

av particle diamc

Mnd (×10−4)

Mwd (×10−4)

polymer aggregates (wt %) 0.0

15.9

21.1

0.0

13.5

23.3

0.0

16.4

19.8

0.0

12.9

22.0

0.0 0.0 3.9f

4.95 9.54

10.1 19.3

0.0 0.0

a Reaction at 85 °C for 0.5 h in H2O with adding the surfactant and KPS radical initiator. bBiomass content was adjusted in the initial reaction feed as 34 wt % in all runs. cDetermined by DLS. dDetermined by GPC. eData taken from ref 13. fAggregated precipitates were formed in 3.9 wt % amount.

Figure 6. Size distributions measured by DLS: Copolymerization between BMA and MM8 (code 6), (A) the comonomer mixture solution with 3.0 wt % SDS (d = 258 nm) and (B) the radically copolymerized product solution with 3.0 wt % SDS (d = 105 nm). Copolymerization between BA and MM6 (code 8), (C) the comonomer mixture solution with 1.0 wt % PEREX (d = 113 nm) and (D) the radically copolymerized product solution with 1.0 wt % PEREX (d = 99 nm). Copolymerization between BA and MM6 (code 9), (E) the comonomer mixture solution with 3.0 wt % PEREX (d = 113 nm) and (F) the radically copolymerized product solution with 3.0 wt % PEREX (d = 95 nm).

wt % of SDS).13 After radical copolymerization, the product solution showed d = 105 nm (Figure 6B). This smaller diameter value after the copolymerization is normally observed and 3.0 wt % of SDS (a larger amount) affected more on this tendency. By using BA comonomer, on the other side, particle size was much smaller both before and after the copolymerization; before the reaction, d = 113 nm for both 1.0 and 3.0 wt % of PEREX (Figure 6C and Figure 6E), and after the reaction, d = 99 nm for 1.0 wt % (Figure 6D) and d = 95 nm for 3.0 wt % (Figure 6F) (reaction codes 8 and 9). BA is of acryl type monomer and hence steric effect is noticeable compared with that of BMA of methacryl type. Probably, BA lacking methyl group is able to form compact particles and copolymers with a more flexible main chain than BMA having methyl group. The diameter size of these particles belongs to a miniemulsion criterion.46−51

reaction, while a stable miniemulsion system was not obtained after the reaction, with forming a small portion (3.9 wt %) of polymer aggregates (code 7). As a monomer for emulsion polymerization, therefore, the average chain length longer than 12 was not appropriate for the copolymer to be emulsified. A reason for the aggregates may be due to the too hydrophobic nature of a longer PLA chain or surfactant of SDS probably not being effective enough even with 3.0 wt %. Moreover, MM12 contains PLAm chains from m = 4 to 20 revealed by MS analysis, and PLAm graft chains of higher m value exhibit crystalline nature to probably prevent the complete emulsification of the copolymer. Figure 6A shows a size distribution of the monomer droplets from BMA and MM8 with 3.0 wt % SDS (code 6 of Table 2) which was measured by the dynamic light scattering (DLS) method, showing an average particle diameter d = 258 nm. This particle size is little larger than that of MM6 (231 nm with 1.0 3762

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Product graft copolymers possessed high molecular weight Mn values from 1.29 × 105 to 1.64 × 105 from BMA monomer, while Mn values of graft copolymers from BA monomer obtained were 4.95 × 104 and 9.54 × 104 (codes 8 and 9, Table 2). Thus, MMm macromonomers with m value less than 8 can be used as a suitable comonomer for miniemulsion systems with the surfactant 1.0 wt % or 3.0 wt % to give a wide variety of graft copolymers having various biomass contents for wide applications. Figure 8 indicates DSC curves for determination of Tg values of three graft copolymers (PBMA-g-PLAm); they are 35 °C for

Furthermore, molecular weights of copolymers are high enough for various applications: Mn values around 1.29−1.64 × 105 for the BMA copolymers and around 4.95−9.54 × 104 for the BA copolymers, respectively (Table 2). Graft copolymer samples were isolated from the miniemulsion. Structures of PBMA graft copolymers (codes 1−7 in Table 2) were confirmed similarly conducted by 1H NMR

Figure 8. DSC measurement curves of PBMA-g-PLAm for determination of Tg value.

m = 4, 37 °C for m = 6, and 40 °C for m = 8. The results are also included in Table 3. The Tg value of PBMA is 20 °C,52 and Table 3. Physical Properties of Graft Copolymers a

code

Figure 7. (A) 500 MHz 1H NMR spectrum of BA (CDCl3), (B) 500 MHz 1H NMR spectrum of PBA-g-PLA6 (CDCl3), and (C) 125 MHz 13 C NMR spectrum of PBA-g-PLA6 (CDCl3).

1 3

spectroscopy as previously reported.13,14 Figure 7 demonstrates 1 H NMR spectra of BA monomer (Figure 7A) and PBA-gPLA6 (Figure 7B, code 8 in Table 2) as well as 13C NMR spectrum of PBA-g-PLA6 (Figure 7C). In Figure 7C, two very small peaks f′ + f″ due to −OCH2CH2O− are separately seen at δ 62.4 and δ 62.5. In these figures, peak assignments are given, all of which reasonably support the structures of BA monomer and PBA-g-PLA6, respectively. For reference, the spectrum of BMA homopolymer may be useful as reported before.13,14 It is considered that the radical copolymerization of BMA or BA with MMm occurred almost randomly, producing quantitatively copolymers of PBMA-g-PLAm or PBA-g-PLAm. This situation can be seen from the 1H NMR analysis; for example, in spectrum (B) in Figure 7, the integral ratio of peak d (OCH2) due to BA unit over peak a (OCCHCH3O) due to MM6 was 6.0, which did not much change before (the feed molar ratio 85/15 = 5.7) and after the copolymerization. The GPC chart showed a single peak, strongly suggesting that the copolymerization actually took place to give a graft copolymer in a random fashion. An example of the GPC chart is found in the Supporting Information, Figure I.

6

graft copolymer PBMA-gPLA4 PBMA-gPLA6 PBMA-gPLA8

Tg (°C)

Young’s modulus (kgf/cm2)

tensile strength (kgf/cm2)

elongation at break (%)

35

1020

25.5

453

37

1582

36.7

415

40

2582

58.2

360

a

The code number of the sample copolymer used denotes that of Table 2.

hence, these Tg values were much enhanced by the graft chain. The value is affected by the graft chain length: the longer the graft chain, the higher the value gradually. This tendency is the same as observed for the Tg value of the macromonomer (Table 1), reflecting the highly optical PLA chain length. Table 3 includes some physical properties of three graft copolymer samples of PBMA-g-PLAm. The results demonstrate that, with the same content of PLA graft chains in the graft polymers (all three containing 34 wt % PLA component), the physical strength is higher with the longer graft chain (m = 8) than with the shorter graft chain (m = 4), whereas the elongation property is higher in nature with the shorter chain than with the longer chain. That is, when the total amount of the PLA component is equal, the longer graft chains yet with smaller number of the chain govern the bulk nature of the copolymer, rather than that the shorter graft chains yet with 3763

dx.doi.org/10.1021/bm301212a | Biomacromolecules 2012, 13, 3757−3768

Biomacromolecules

Article

Table 4. Radical Copolymerization of Alkyl Methacrylate (BMA or MMA) with MMm to Graft Copolymers in Toluene or 1,4Dioxane Solution product graft copolymer biomass content (wt %) code

copolymerizationa in feed B(M)MA/ MMm (mol/mol)

1

BMA/MM6 (83/17)

2

BMA/MM12 (92/8)

3

BMA/MM18 (94/6)

4d

BMA/MM30 (83/17)

5

MMA/MM6 (80/20)

structure expression PBMA-gPLA6 PBMA-gPLA12 PBMA-gPLA18 PBMA-gPLA30 PMMA-gPLA6

isolated yield (%)

Mnb (×10−4)

Mwb (×10−4)

B(M)MA/MMm ratio in copolymerc (mol/mol)

from the feed

from the copolymerc

80

2.25

6.89

82/18

34

34

70

5.40

9.20

92/8

34

34

63

3.69

6.62

95/5

34

31

82

6.95

84/16

72

71

54

4.90

82/18

45

42

12.7 8.90

Toluene as solvent for codes 1−3 with AIBN initiator at 70 °C for 24 h, and 1,4-dioxane as solvent for codes 4 and 5 with AIBN initiator at 60 °C for 24 h. bDetermined by GPC. cCalculated values by the 1H NMR analysis. dThe isolated graft copolymer contained a small amount of unreacted MM30 (3 wt %). a

larger number of the chain do. This is a good example to demonstrate the property relationship of graft copolymers between the graft chain length and the number of the graft chain. III. Solution Radical Copolymerization to Graft Copolymer. Since MMm monomers have been found not suitable for forming the miniemulsion system when m ≥ 12, solution copolymerization was performed in toluene or 1,4dioxane. All the copolymerizations (reaction 2, Scheme 1) proceeded homogeneously to produce soluble graft copolymers in good to high yields with molecular weight range Mn 2.25− 6.95 × 104 as shown in Table 4. Both toluene and 1,4-dioxane solvents were not so much different in terms of the product yield and molecular weight. In codes 1−3, the initial feed ratio was adjusted so that the biomass content of the product copolymer may be 34 wt % as in the case of the above miniemulsion system.13 The biomass contents of the product copolymers determined by the 1H NMR analysis were in all cases identical with or very close to those values of the initial feed ratio. As a typical example of structural confirmation, Figure 9 demonstrates the (A) 1H NMR spectrum and (B) 13C NMR spectrum of PBMA-g-PLA18 of sample code 3 in Table 4. From the integrated ratio of peak d (OCH2) due to the BMA unit and peak a (OCCHCH3O) due to the MM18 unit, the molar unit content ratio was calculated as BMA/MM18 = 16.0, being almost identical with that of the feed ratio of 15.7 (94/6). All peak assignments are given in the figures, which clearly support the graft copolymer structure. It is to be noted that, as shown in Figure 9A, the peak position and shape of 1H signals of OCH2CH2O shifted from monomer (a triplet type peak at δ 4.35 shown in Figure 2 A) to polymer (two triplet-like peaks at δ 4.35 and at δ 4.14); these signals’ region of the polymer is given in a box as expanded in Figure 9A. It is to be added that in Figure 9B, the chemical shift values of two separate very small peaks f′ + f″ due to −OCH2CH2O− are δ 62.4 and δ 62.6. This situation is very similar to that of Figure 7C. Also, Figure 10 shows the 1H NMR spectrum of PMMA-gPLA6, confirming the copolymer structure. From the intensity ratio of peak d (OCH3) due to the MMA unit and peak a (O CCHCH3O) due to the MM6 unit, the content of the MMA unit and the MM6 unit in the graft copolymer was calculated as 4.6, being roughly close to the feed ratio of 4.0 (80/20).

Figure 9. (A) 600 MHz 1H NMR spectrum and (B) 150 MHz NMR spectrum of PBMA-g-PLA18 (both in CDCl3).

13

C

All these copolymers are considered to be of random copolymer structure as discussed in the miniemulsion copolymerization part, since the polymerizable group of all three monomers (BMA, MMA, and MMm) is of the methacryloyl type. However, a small amount of unreacted MM30 was observed after the copolymerization (code 4); therefore, it is possible that MMm with a larger m value is lower in polymerizability than low molecular weight monomers like BMA and MMA. In order to shed light on this point of radical copolymerizability in more detail, the monomer conversion−reaction time 3764

dx.doi.org/10.1021/bm301212a | Biomacromolecules 2012, 13, 3757−3768

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Article

Figure 10. 500 MHz 1H NMR spectrum of PMMA-g-PLA6 (CDCl3).

and polymerizability of MMm, however, needs further detailed investigations. The Tg values of the graft copolymers in Table 5 were determined by DSC measurement as indicated in Figure 11. All

profile was examined. Two model reactions were carried out with using a comonomer combination of BMA and MM6. First, a mixture of BMA (0.34 g, 2.4 mmol) and MM6 (0.66 g, 1.2 mmol) in 1.0 mL of toluene was subjected to copolymerization by employing 3.0 wt % of AIBN initiator at 70 °C with stirring under dry nitrogen, where the feed monomer molar ratio was 2.0:1.0. The value of monomer consumption was followed by 1 H NMR spectroscopy by determining the integrated value of vinyl proton peaks at δ 5.60 due to MM6 and at δ 5.54 due to BMA by using the peak centered at δ 5.18 due to the proton (CH3)CH as an internal standard for the integration (see Supporting Information, Figure II). Both monomers were consumed in a very close rate with reaction time: 4% (5 min), 16% (10 min), 36% (20 min), 49% (30 min), and 92% (120 min), where the consumption % values are given as the average value of the two monomers at the respective reaction time. The second model reaction used the monomer feed ratio, BMA:MM6 = 4.0:1.0 with using similar other reaction parameters. Again, plots of the consumption of monomer versus reaction time showed a similar profile of these two monomers: Results were in consumption 15% (10 min), 38% (20 min), 51% (30 min), 61% (40 min), 69% (50 min), 75% (60 min), and 93% (120 min) (see Supporting Information, Figure III). These two reactions clearly demonstrate that BMA and MM6 have very close radical polymerizability. Furthermore, GPC analysis of product copolymer samples from the two reactions at the reaction time 30 min showed a single peak, having Mn 4.19 × 104 (Mw 7.74 × 104) for the first reaction and Mn 3.59 × 104 (Mw 7.30 × 104) for the second reaction, respectively (see Supporting Information, Figure IV). All these observations provide strong support that monomers of BMA, MMA, and MMm having a methacryloyl-polymerizable group show a close radical copolymerizability to produce copolymers PB(M)MA-g-PLAm of a random structure, when the m value of MMm is smaller than ∼18, judging the results of Table 4. It may be said at present that the polymerizability of MMm is affected by the m value, i.e., as the m value becomes larger, the substituent PLA chain group becomes bulkier, and radical polymerizability of MMm becomes lower due to the larger steric hindrance of the substituent. To clarify quantitatively the relationship between the m value of MMm

Table 5. Properties of Graft Copolymers codea

graft copolymer

Tg (°C)

pencil hardness

1 2 3 4 5

PBMA-g-PLA6 PBMA-g-PLA12 PBMA-g-PLA18 PBMA-g-PLA30 PMMA-g-PLA6

25 31 36 50 67