Ring-opening polymerization behavior of l -lactide catalyzed by aluminum alkyl catalysts

Journal of Industrial and Engineering Chemistry 19 (2013) 1137–1143

Contents lists available at SciVerse ScienceDirect

Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec

Ring-opening polymerization behavior of L-lactide catalyzed by aluminum alkyl catalysts Ji Yun Yoo a, Youngjo Kim b, Young Soo Ko a,* a b

Department of Chemical Engineering, Kongju National University, Budaedong 275, Cheonan, Chungnam 314-701, Republic of Korea Department of Chemistry, Chungbuk National University, 52 Naesudong-ro, Heungdeok-gu, Cheongju, Chungbuk 361-763, Republic of Korea

A R T I C L E I N F O

Article history: Received 27 November 2012 Accepted 6 December 2012 Available online 14 December 2012 Keywords: Biodegradable polymer Poly(lactide) Aluminum alkyls Ring-opening polymerization

A B S T R A C T

The L-lactide polymerization using trimethylaluminum (TMA), triethylaluminum (TEAL), trioctylaluminum (TOA), triisobutylaluminum (TIBA), and aluminum isopropoxide (Al(O-i-Pr)3) were studied to produce a PLA. The conversion, turnover frequency (TOF), and molecular weight of TIBA were higher than those of Al(O-i-Pr)3 with the polymerization time of 5 min. In the case of 15 min, the conversion of aluminum alkyls was lower than that of Al(O-i-Pr)3, but the molecular weight of PLA produced by aluminum alkyl was relatively high in comparison with the catalytic activity of Al(O-i-Pr)3. TMA, TEAL, and TIBA seemed to have lower propagation rates and much lower termination rates than Al(O-i-Pr)3. ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

1. Introduction Polymeric materials are being applied to a variety of application areas including molding and packaging materials, but they are well known to be unable to decompose in convenient ways. Due to environmental concerns, there has been a lot of effort to replace the conventional plastic materials with biodegradable materials for the last several decades. However, they are quite expensive compared to conventional materials, resulting in limited application. However, their price competiveness has started to improve in relation to petroleum-based plastic materials due to the sharp increase in oil prices [1,2]. There are conventional biodegradable polymers such as polylactide (PLA), polycaprolactone (PCL), aliphatic polyester (AP) and polyhydroxy-butyrate (PHB), and PLA is known to be the most promising one which is synthesized from biomass materials such as lactic acid [3–5]. Biodegradable polymeric materials are said to decompose to CO2, water, and other light byproducts. Lactic acid, a monomer of PLA is produced from the starch of corns by fermentation processes, and it is reported to have two different isomers, L- and D-lactic acid. The isomer form is also known to influence the melting point and physical strength of PLA. PLA synthesized from L-lactic acid was reported to have higher crystallinity and a higher melting point with better mechanical strength [6–8].

* Corresponding author. Tel.: +82 41 521 9364. E-mail address: [email protected] (Y.S. Ko).

There are two routes to synthesize PLA; one is condensation polymerization of lactic acid and the other is ring opening polymerization with lactide, a dimer form of lactic acid. The high molecular weight of PLA could be synthesized with the ring-opening polymerization of PLA which should have a versatile catalyst [9–12]. Usually, a variety of organometallic compounds, such as Sn, Al, Ti, Zn, and Mg show the coordination-insertion polymerization mechanism [13–20]. Sn-octanoate is a conventional catalyst for the ringopening polymerization of L-lactide for high activity and molecular weight, but the poisonous property of Sn is a disadvantage, and it could also cause the environmental issues in the future. This has led to the intensive development of other metal-type organometallic catalysts for the past decade, and organometallic Al compounds have received a great deal of attention from many research groups, and a representative Al compound for L-lactide polymerization is aluminum isopropoxide.[21] However, there have not been intensive studies on aluminum alkyls for the L-lactide polymerization even though those could be applied to the commercial production without special design and synthesis of the Al complexes. Therefore, it is very interesting to observe the polymerization performance of aluminum alkyls in the L-lactide polymerization for the commercial purpose. In this paper, aluminum alkyls such as trimethylaluminium (TMA), trimethylaluminum (TEAL), trioctylaluminum (TOA), and triisobutylaluminum (TIBA) were employed together with aluminum isopropoxide (Al(O-i-Pr)3) as the catalysts for the Llactide ring-opening polymerization to investigate the effect of alkyl groups on polymerization performances in detail. The resulting PLA was analyzed by gel permeation chromatography

1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2012.12.010

J.Y. Yoo et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 1137–1143

1138

(GPC), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FT-IR) to compare the catalyst performance. 2. Experimental 2.1. Materials (3S)-cis-3,6-dimethyl-1,4-dioxane-2,5-dione (L-lactide, Aldrich) was used without recrystallization. All catalysts were purchased from Aldrich. Toluene was distilled from sodium and benzophenone before use. Dichloromethane (MC, JT Baker) and methanol (Samchun) was used without purification. Nitrogen was purified using two columns of Fisher RIDOX and a molecular sieve 5A/13X. All catalysts were employed as shown in Table 1. 2.2. Bulk polymerization of L-lactide All reactions were carried out under a nitrogen atmosphere using the glove box and schlenk technique. 2 g of L-lactide was added to a 20 ml vial in a glove box and the catalyst was added under a nitrogen atmosphere using a schlenk line. The polymerization temperature was maintained at 130 8C using an oil bath. Lump-polymer was dissolved using MC and precipitated in an excess amount of methanol. After washing with methanol, the polymer was obtained by filtering. The obtained polymer was dried in a vacuum oven at 40 8C for 1 h. The polymerization times were 5 and 15 min. 2.3. Solution polymerization of L-lactide All reactions were carried out under a nitrogen atmosphere using the glove box and schlenk technique. L-lactide (1 g) was added to a 20 ml vial in a glove box. The catalyst (0.15 mmol) and toluene (5 ml) were added under a nitrogen atmosphere using a schlenk line. The polymerization temperature was maintained at 110 8C using an oil bath. The reaction product was precipitated in an excess amount of methanol. After washing with methanol, the polymer was obtained by filtering. The obtained polymer was dried in a vacuum oven at 40 8C for 1 h. The polymerization times were 0.5, 1, 2, 3, and 6 h. 2.4. Characterization Thermal characterization was carried out using differential scanning calorimetry (DSC, TA 2010) in a nitrogen atmosphere at a heating rate of 5 8C/min.

Molecular weights were determined by gel permeation chromatography (GPC). The instrument was equipped with a Waters 1515 pump, a Waters 2414 RI detector, and two styragel columns (HR 4E and HR 5E). The columns were eluted with tetrahydrofuran (1 ml/min rate at 40 8C). Polystyrene standards were used for calibration. FT-IR spectra were recorded using a Nicolet 6700 spectrometer. 3. Results and discussion 3.1. Comparison of catalyst performance of aluminum alkyl with aluminum alkoxy A variety of aluminum alkyls, namely, trimethylaluminum (TMA), triethylaluminum (TEAL), triisobutylaluminum (TIBA), and trioctylaluminum (TOA) were employed to investigate the ringopening polymerization behaviors together with Al(O-i-Pr)3, a reference Al catalyst for comparison purposes. Fig. 1 shows the plausible polymerization mechanism of aluminum alkyl catalysts. In the initiation step of polymerization mechanism the aluminum atom of catalysts is coordinated with the oxygen of L-lactide, resulting in Al–O bond, electronegative aluminum and electropositive carbon. The resulting carbocation of L-lactide would be interacted with the aluminum atom of catalyst, and then the ring of L-lactide is broken to be connected to the aluminum in a linear form. The lactide-connected aluminum catalyst follows the similar propagation mechanism to the aluminum alkoxide catalysts as shown in Fig. 1 [22,23]. Therefore, the initiation of ring opening polymerization with aluminum alkyls is different from that of aluminum alkoxides, influencing the total polymerization rate depending on the rate of initiation. Table 2 shows the bulk polymerization results of L-lactide with five different Al catalysts in terms of the amount of catalyst in the feed. TIBA, showing the highest activity among five catalysts, was polymerized for 5 min, and other catalysts were polymerized for 15 min for comparison. Also, Al(O-i-Pr)3 was polymerized for 5 min like TIBA for comparison purposes. With the polymerization time of 5 min, TIBA always showed higher conversion and higher molecular weight than Al(O-i-Pr)3 over the whole range of the amount of catalyst in the feed. As shown in Table 2, TIBA showed increased conversion but decreased molecular weight as the amount of catalyst in the feed increased. Fig. 2 shows that the turnover frequency (moles of L-lactide consumed/moles of catalyst hour) of TIBA is much higher than that of Al(O-i-Pr)3 in the range of 0.02–0.15 mmol of the amount of catalyst in the feed. The molecular weight of TIBA with 0.15 mmol was twice that of Al(O-i-Pr)3.

Table 1 Aluminum organometallic catalysts employed in this study. Abb.

Al(O-i-Pr)3

TMA

TEAL

Aluminum isopropoxide

Trimethylaluminum

Triethylaluminum

Structure

Name Abb.

TIBA

TOA

Triisobutylaluminum

Trioctylaluminum

Structure

Name

J.Y. Yoo et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 1137–1143

1139

Fig. 1. Polymerization mechanism of L-lactide using Al alkyl compounds. 400 TIBA Al(O-i -Pr)3

1

TOF(h- )

300

200

100

0 0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

Catalyst in feed(mmol) Fig. 2. Change in the TOF as a function of the amount of catalyst in bulk polymerization for 5 min.

Other Al catalysts excluding TIBA were tested with the polymerization time of 15 min, and their results are shown in Fig. 3. Above 0.05 mmol of Al(O-i-Pr)3 in the feed, its turnover frequency (TOF) was higher than those of TMA, TEAL, and TOA, but it started to decrease above 0.1 mmol of Al(O-i-Pr)3. Interestingly, TMA showed a higher TOF than Al(O-i-Pr)3 at 0.02 mmol, and it decreased above 0.05 mmol. The conversion of TMA increased slowly, but the TOF decreased as the amount of TMA in the feed increased. All Al catalysts showed decrease TOF above 0.1 mmol. As shown in Fig. 4, the TMA amount of 0.02 mmol gave the largest molecular weight, 32,600 g/mol among all catalysts, but the molecular weight decreased as the amount of TMA increased. This might have resulted from the increase in active sites in the polymerization medium as the amount of TMA in the feed increased. The molecular weight of Al(O-i-Pr)3 increased below the amount of 0.1 mmol, but no significant changes in molecular weight were observed above 0.1 mmol. TOA and TEAL showed increased molecular weight as

J.Y. Yoo et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 1137–1143

1140

Table 2 The results of PLA bulk polymerization using various aluminum organometallic compounds catalysts.a Polymer (g)

Con. (%)

Mn (g/mol)

Mw (g/mol)

PDI

Tm (8C)

DHf (J/g)

5 5 5 5

0 0.21 0.32 0.36

0 10.4 15.9 18.0

– 14,700 10,800 8500

– 16,900 12,500 9600

– 1.2 1.2 1.1

– 160.0 154.9 149.2

– 63.2 59.5 57.3

0.02 0.05 0.10 0.15 0.02 0.05 0.10 0.15

5 5 5 5 15 15 15 15

0 0 0 0.09 0 0.39 1.09 1.33

0 0 0 4.4 0 19.4 54.5 66.4

– – – 4600 – 9700 15,500 14,600

– – – 5000 – 10,500 17,500 17,300

– – – 1.1 – 1.1 1.1 1.2

– – – 134.9 – 158.2 165.1 164.2

– – – 41.7 – 54.3 65.2 63.0

TMA

0.02 0.05 0.10 0.15

15 15 15 15

0.14 0.17 0.20 0.24

6.9 8.2 9.8 12.2

32,600 30,200 15,100 11,700

36,200 34,200 17,400 13,300

1.1 1.1 1.2 1.1

171.7 170.6 164.4 159.7

60.7 49.6 58.8 43.8

TEAL

0.02 0.05 0.10 0.15

15 15 15 15

0 0.01 0.12 0.04

0 0.2 6.0 1.8

– 11,200 24,300 17,300

– 12,200 31,600 22,800

– 1.1 1.3 1.3

–

–

163.0 157.8

50.9 45.2

0.02 0.05 0.10 0.15

15 15 15 15

0 0.04 0.22 0.25

0 2.1 11.0 12.5

– 20,300 26,900 21,200

– 25,800 34,500 26,200

– 1.3 1.3 1.2

– 165.0 167.6 165.3

– 40.2 55.4 47.2

Catalyst name

Catalyst (mmol)

TIBA

0.02 0.05 0.10 0.15

Al(O-i-Pr)3

TOA

a b

Time (min)

b

b

Bulk polymerization condition: L-lactide = 2 g, temperature = 130 8C. Not measured.

the amounts of TOA and TEAL in the feed increased, but it decreased slightly above 0.1 mmol. These results indicate that the molecular weight of PLA produced with Al catalysts is strongly dependent on their TOF. However, it was also observed that the absolute value of TOF is not directly related to the molecular weight value, and aluminum alkyls could produce higher molecular weight PLA than aluminum alkoxy catalysts. The PLA produced with aluminum alkyls were analyzed by GPC and DSC to investigate the polymerization behavior in detail. TEAL, TIBA, and TOA were able to produce PLA with GPC and DSC curves that were shaped differently from those of Al(O-i-Pr)3. As shown in Fig. 5, the molecular weight distribution (MWD) curve of TEAL changed to bimodal-type as the molecular weight of PLA increased, and the higher molecular weight part in the bimodal curve clearly appeared. Figs. 6 and 7 show the GPC and DSC curves of PLA produced by TOA and TIBA. Both TOA and TIBA produced shouldertype curves in the higher molecular weight part. Figs. 8 and 9 also

show that the melting curves of TEAL and TOA become broader and bimodal, respectively, as the molecular weight of PLA increases. The dependence of the melting point on the molecular weight of PLA produced by TMA is shown in Fig. 10. The increase in the amount of catalyst in the feed brought about the decreased molecular weight and melting point in a linear manner [24]. The PLA produced with Al catalysts was characterized by FT-IR as shown in Fig. 11. All PLA samples had C–O–C ester bonds as seen in the 1180 and 1211 cm 1 peaks and the CH3 branch in 1384 and 1453 cm 1. In addition, the peak of the carbonyl group in ester linkage was shown in 1750 cm 1, indicating that PLA was successfully synthesized with all Al catalysts [25]. 3.2. Time dependence of L-lactide solution polymerization with aluminum alkyls Based on the L-lactide bulk polymerization described above, Llactide solution polymerization was carried out to investigate the

350 Al(O-i -Pr)3 300

TMA TEAL TOA

TMA TEAL TOA

30000

Mn (g/mol)

TOF(h-1)

250

Al(O-i -Pr)3

40000

200 150

20000

100 10000

50 0 0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

Catalyst in feed(mmol) Fig. 3. Change in the TOF as a function of the amount of catalyst in bulk polymerization for 15 min.

0 0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

Catalyst in feed(mmol) Fig. 4. Change in the Mn as a function of the amount of catalyst in bulk polymerization for 15 min.

J.Y. Yoo et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 1137–1143

1141

0.5 TEAL 0.15mmol Mn=17300

TEAL 0.1mmol

Heat Flow (W/g)

RI response

0.0

Mn=24300

TEAL 0.1mmol

-1.0 -1.5

TEAL 0.15mmol Mn=17300

-2.0

TEAL 0.05mmol Mn=11200

-2.5 100

10

12

14

16

Mn=24300

-0.5

120

140

18

160

180

Temperature(¡É)

Elution volume (mL)

Fig. 8. DSC curve of PLA prepared with TEAL.

Fig. 5. GPC curve of PLA prepared with TEAL.

0

TOA 0.1mmol

Heat Flow (W/g)

RI response

-1

Mn=26900

TOA 0.15mmol Mn=22100

TOA 0.05mmol Mn=20300 TOA 0.15mmol Mn=22100

-2 TOA 0.1mmol

Mn=26900

-3

-4

TOA 0.05mmol Mn=20300 120

10

12

14

16

130

140

150

160

170

180

Temperature(¡É)

18

Fig. 9. DSC curve of PLA prepared with TOA.

Elution volume (mL) Fig. 6. GPC curve of PLA prepared with TOA.

time dependence of aluminum alkyl catalysts. Table 3 shows the results from the solution polymerization of L-lactide of Al(O-i-Pr)3, TMA, TEAL, and TIBA in relation to polymerization time. As shown in Fig. 12, the time to reach the conversion of 80% was in the order of Al(O-i-Pr)3 < TIBA < TEAL < TMA. The conversion of Al(O-i-Pr)3

changed insignificantly after the polymerization time of 1 h. Al(Oi-Pr)3 had the higher activity in the early stage of polymerization, but aluminum alkyls did not. As the length of the alkyls shortened, the initial polymerization activity was low, but most aluminum alkyls reached the conversion of 80% in 3 h. TIBA had lower initial acitivity than Al(O-i-Pr)3, but its activity was similar to that of Al(O-i-Pr)3 after the polymerization time of 1 h. TMA had the slowest activation rate, and it increased until 6 h. Fig. 13 shows the molecular weight dependence on the polymerization time. 180

Tm

40000

10

TIBA

0.05mmol Mn=14700

TIBA

0.1mmol

TIBA

0.15mmol Mn=8500

12

Mn=10900

14

30000

Tm(¡É)

160

20000

Mn(g/mol)

RI response

Mn

140 10000

16

18

120 0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0 0.18

TMA in feed(mmol)

Elution volume (mL) Fig. 7. GPC curve of PLA prepared with TIBA.

Fig. 10. Changes in the Tm and the Mn of PLA as a function of the amount of catalyst in the bulk polymerization with TMA.

J.Y. Yoo et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 1137–1143

1142

Table 3 The results of PLA solution polymerization using various aluminum organometallic compounds catalysts.a Catalyst name

Time (h)

Al(O-i-Pr)3

TMA

TEAL

TIBA

a

Conversion (%)

Mn (g/mol)

Mn (g/mol)

PDI

Tm (8C) b

0.63 0.84 0.82 0.82 0.80

63.3 84.1 82.1 81.6 80.0

7200 6400 6500 6300 6900

9200 9100 8500 8300 9300

1.3 1.4 1.3 1.3 1.3

0.5 1 2 3 6

0 0.24 0.59 0.73 0.90

0 23.7 59.3 72.7 90.3

– 5000 8800 10,700 11,800

– 5800 10,800 24,200 15,500

– 1.2 1.2 1.3 1.3

– 147.6

0.5 1 2 3 6

0 0.20 0.69 0.89 0.91

0 20.4 69.4 89.2 90.7

– 5900 11,300 12,400 12,600

6900 13,800 15,600 15,300

– 1.2 1.2 1.3 1.2

– 135.9

0.23 0.73 0.90 0.88

23.1 72.7 90.3 87.7

7200 9800 13,500 11,800

9000 12,700 18,100 15,300

DH (J/g) b

0.5 1 2 3 6

0.5 1 2 3

161.7

56.9

b

b

158.8

45.2

b

b

– 41.4

b

b

163.5

51.6

b

b

– 36.2

b

b

154.2

40.0

b

b

b

1.3 1.3 1.3 1.3

b

153.8

47.1

b

b

154.9

49.7

Solution polymerization condition: L-lactide = 1 g, catalyst = 0.15 mmol, toluene = 5 ml, and temperature = 110 8C. Not measured.

% Transmittance

b

Polymer (g)

Interestingly, Al(O-i-Pr)3 reached the maximum molecular weight at 0.5 h, similar to its conversion, and it showed no significant change from 0.5 h, while the molecular weights of PLA produced by aluminum alkyls increased as the conversion increased. However, no more increase in molecular weight was observed after the conversion of 80% for aluminum alkyls. Very interestingly, all aluminum alkyls were able to produce double the molecular weight of Al(O-i-Pr)3 above the conversion of 80%. TIBA could produce molecular weight PLA 40% larger than that of Al(O-i-Pr)3 under similar conversion. This could indicate that the aluminum alkyls could produce larger molecular weight PLA than Al(O-i-Pr)3 by solution polymerization similar to the bulk polymerization. This can be explained as follow. The aluminum alkyls have no oxygen atom in their ligand, causing retarding of the propagation rate, resulting in lower conversion than that of Al(O-i-Pr)3. However, the molecular weight is proportional to the ratio of the propagation rate to the termination rate, meaning that a higher molecular weight can occur with a much lower termination rate although a catalyst has a lower propagation rate. TMA, TEAL, and TIBA seem to have a lower propagation rate than Al(O-i-Pr)3, but they could have

Al(O-i -Pr)3 TMA TEAL TOA TIBA

4000

3000

2000

1000

1 Wavenumbers(cm- )

Fig. 11. FT-IR spectra of PLA by bulk polymerization.

100

16000 14000

80

60

Mn(g/mol)

conversion(%)

12000

40

10000 8000 6000

Al(O-i -Pr)3 TMA TEAL TIBA

20

0

4000

Al(O-i -Pr)3

2000

TMA TEAL TIBA

0

0

1

2

3

4

5

6

7

Time(h) Fig. 12. Change in the conversion as a function of the reaction time in solution polymerization.

0

1

2

3

4

5

6

7

Time(h) Fig. 13. Change in the Mn as a function of the reaction time in solution polymerization.

J.Y. Yoo et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 1137–1143

a much lower termination rate, giving rise to the higher molecular weight with aluminum alkyls. 4. Conclusions The aluminum alkyls, TMA, TEAL, TIBA, and TOA were employed to investigate the characteristics in ring-opening polymerization together with Al(O-i-Pr)3, a reference Al catalyst. TIBA always showed higher conversion and higher molecular weight than Al(O-i-Pr)3. TMA showed a higher TOF than Al(O-iPr)3 at 0.02 mmol, and the amount of TMA in the feed of 0.02 mmol gave the largest molecular weight, 32,600 g/mol. The aluminum alkyls could produce higher molecular weight PLA than aluminum alkoxy catalysts. The shapes of the GPC and DSC curves of PLA produced by aluminum alkyls are different from those of aluminum alkoxy. The time to reach the conversion of 80% in L-lactide solution polymerization was in the order of Al(O-iPr)3 < TIBA < TEAL < TMA. The molecular weights of PLA produced by aluminum alkyls increased as the conversion increased. All aluminum alkyls were able to produce double the molecular weight of Al(O-i-Pr)3 above the conversion of 80% in solution polymerization. TMA, TEAL, and TIBA seem to have lower propagation rates than Al(O-i-Pr)3, but they could have much lower termination rates, giving rise to the higher molecular weight with aluminum alkyls. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Education, Science and Technology (2010-001376).

1143

References [1] A.A. Shah, F. Hasan, A. Hameed, Biotechnology Advances 26 (2008) 246. [2] Y. Tokiwa, B.P. Calabia, C.U. Ugwu, S. Aiba, International Journal of Molecular Sciences 10 (2009) 3722. [3] O. Coulembier, P. Degee, J.L. Hedrick, P. Dubois, Progress in Polymer Science 31 (2006) 723. [4] Y. Tokiwa, B.P. Calabia, Journal of Polymers and the Environment 15 (2007) 259. [5] V. Siracusa, P. Rocculi, S. Romani, M.D. Rosa, Food Science & Technology 19 (2008) 634. [6] T.M. Quynha, H. Mitomoa, N. Nagasawab, Y. Wadab, F. Yoshiib, M. Tamadab, European Polymer Journal 43 (2007) 1779. [7] H. Tsuji, Macromolecular Bioscience 5 (2005) 569. [8] J. Slager, A.J. Domb, Advanced Drug Delivery Reviews 55 (2003) 549. [9] D. Garlotta, Journal of Polymers and the Environment 9 (2001) 63. [10] A.J.R. Lasprilla, G.A.R. Martinez, B.H. Lunelli, A.L. Jardini, R.M. Filho, Biotechnology Advances 30 (2012) 321. [11] R.E. Drumright, P.R. Gruber, D.E. Henton, Advanced Materials 12 (2000) 1841. [12] S.H. Hyon, K. Jamshidi, Y. Ikada, Biomaterials 18 (1997) 1503. [13] K.B. Aubrecht, M.A. Hillmyer, W.B. Tolman, Macromolecules 35 (2002) 644. [14] P. Purnama, Y.M. Jung, C.H. Hong, D.S. Han, S.H. Kim, Macromolecular Research 20 (2012) 515. [15] S. Gendler, S. Segal, I. Goldberg, Z. Goldschmidt, M. Kol, Inorganic Chemistry 45 (2006) 4783. [16] H.Y. Chen, H.Y. Tang, C.C. Lin, Polymer 48 (2007) 2257. [17] A. John, V. Katiyar, K. Pang, M.M. Shaikh, H. Nanavati, P. Ghosh, Polyhedron 26 (2007) 4033. [18] L. Piao, M. Deng, X. Chen, L. Jiang, X. Jing, Polymer 44 (2003) 2331. [19] M.L. Shueh, Y.S. Wang, B.H. Huang, C.Y. Kuo, C.C. Lin, Macromolecules 37 (2004) 5155. [20] N.Y. Kim, Y.S. Yun, J.Y. Lee, C. Choochottiros, H.R. Pyo, I.J. Chin, H.J. Jin, Macromolecular Research 19 (2011) 943. [21] Y.H. Noh, Y.S. Ko, Polymer (Korea) 36 (2012) 53. [22] F.E. Kohn, J.G.V. Ommen, J. Feijen, European Polymer Journal 19 (1983) 1081. [23] A. Khanna, Y.S. Sudha, S. Pillai, S.S. Rath, Journal of Molecular Modeling 14 (2008) 367. [24] K. Jamshidi, S.H. Hyon, Y. Ikada, Polymer 29 (1988) 2229. [25] T. Furnkawa, H. Sato, R. Murakami, J. Zhang, Y.X. Duan, I. Noda, S. Ochiai, Y. Ozaki, Macromolecules 38 (2005) 6445.