Preparation of amphiphilic graft copolymer with

polyisoprene backbone by combination of anionic

polymerization and ‘‘click’’ reaction

Fei Shao, Xu Feng Ni *, Zhi Quan Shen *

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering,

Zhejiang University, Hangzhou 310027, China

Received 8 October 2011

Available online 26 January 2012

Abstract

A novel graft copolymer consisting of polyisoprene backbone and hydrophilic side chain with carbamic acid ester functional

group was prepared via thiol-ene ‘‘click’’ reaction and alcohol-isocyanate reactions. Polyisoprene was synthesized by anionic

polymerization using n-butyl lithium as initiator, and the pendant hydroxyl groups were introduced by the thiol-ene reaction of

mercaptoethanol with the double bond of 1, 2-addition units of PI backbone in the presence of radical initiator azobisisobutyr-

onitrile. Isocyanate end group capped poly(ethylene glycol) (mPEG-NCO) was grafted onto the PI backbone through alcohol-

isocyanate reaction between the pendant hydroxyl groups and isocyanate group of mPEG-NCO. The structure of the graft

copolymer were characterized and confirmed by means of size-exclusion chromatography, 1H NMR and FTIR spectroscopy.

# 2011 Xu Feng Ni. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

Keywords: Graft-copolymer; Thiol-ene addition; Isocyanate; Click reaction

Amphiphilic block or graft copolymers consisting of both hydrophilic and hydrophobic segments have attracted

considerable attention for their ability to form nano-structures such as micelles and other aggregates [1–3].

Polyisoprene(PI) has been used as a classical soft segment in the preparation of multiblock and multiconstitution

copolymers, and bring the copolymers with special properties [2,4–9]. Besides, the reactivity of retained double bonds

which either internal along the backbone or as pendant group from the polyisoprene chain offers many opportunities

for functionalization of copolymers having polyisoprene segments. The reactions between isocyanates and active

hydrogen containing functional groups [10], as well as the thiol-ene addition reaction are efficient and quantitative

under certain reaction conditions, which exhibit many of the attributes described for ‘‘click’’ chemistry [11–14]. In our

previous work, a graft copolymer consisting of poly(n-octylallene-co-styrene) as backbone and poly(e-caprolactone)

as side chains was synthesized with the combination of thiol-ene addition and the ring-opening polymerization of e-caprolactone [15]. Herein, hydrophobic PI was functionalized with hydroxyl group by the thiol-ene reaction between

mercaptoethanol and PI to provide the modified polymer PI-OH. Coupled with isocyanate end group functionalized

mPEG (mPEG-NCO), the amphiphilic graft copolymer with PI backbone was prepared as shown in Scheme 1.

www.elsevier.com/locate/cclet

Available online at www.sciencedirect.com

Chinese Chemical Letters 23 (2012) 347–350

* Corresponding authors.

E-mail addresses: xufengni@zju.edu.cn (X.F. Ni), zhiquan_shen@163.com (Z.Q. Shen).

1001-8417/$ – see front matter # 2011 Xu Feng Ni. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

doi:10.1016/j.cclet.2011.12.003

Polyisoprene with 20.5% 1, 2-, 66.7% 3, 4-, and 12.8% 1, 4-addition units was synthesized by anionic

polymerization of isoprene using n-BuLi as initiator (98% yield). As shown in Scheme 1, polyisoprene with pendant

hydroxyl groups (PI-OH) can be prepared by reaction of polyisoprene with mercaptoethanol (RSH) in the presence of

azobisisobutyronitrile (AIBN) as radical initiator with the molar ratio of [RSH]0/[C C]0/[AIBN]0 = 2.5:1:0.33 in a

toluene solution of PI (�3 wt.%). After stirred at 70 8C for 24 h under a dry argon atmosphere, the reaction mixture

was concentrated and purified thrice by dissolving/precipitation with toluene/methanol to remove the unreacted

mercaptoethanol, and the obtained product PI-OH with 86% yield was dried in vacuo to a constant weight. Compared

with that of PI, it could be found from the 1H NMR spectrum of PI-OH, the resonance signals at d 5.7 (–CH CH2) and

d 4.85 (–CH CH2) which assigned to the protons of 1, 2-addition units of PI were completely disappeared. In addition,

the intensity of signals at d 5.03 (–CH C(CH3)–) of 1,4-addition units and d 4.6–4.8 (–C(CH3) CH2) of 3,4-addition

units did not show any change, and new thioether linkages (–CH2SCH2–) at d 2.5–2.8 [16] were observed. These

results indicated that the thiol-ene addition reaction was carried out only on the double bonds of 1,2-addition units of

PI backbone.

To a flask with 50 mL dried toluene, 2 g (0.0036 mol) mPEG-550, 0.57 mL (0.004 mol) toluylene-2,4-

diisocyanate (TDI) and approximately 300 ppm of DBTDL was added. The mixture was allowed at 40 8Covernight under a dry argon atmosphere. After concentration, the reaction mixture was precipitated several times

in excessive ice anhydrous ether, and mPEG-NCO was obtained when anhydrous ether was removed. 30 mL

toluene was added to the flask to redissolve mPEG-NCO, the toluene solution of PI-OH (0.5 g) was dropwised into

the flask, and the flask was heated at 60 8C overnight in the presence of DBTDL. After removing the solvent from

the reaction mixture, the graft copolymer was precipitated and washed by hexane several times, and was purified

by dialyzing to remove unreacted mPEG-NCO. From the 1H NMR spectrum of copolymer, the signals appear at

8.75 ppm assigned to the –NHCOO– group of isocyanate as the linkage of PEG and PI confirms the successful

synthesis of copolymer. Additionally, it was observed that the methylene protons (HOCH2CH2S–) at d 2.48 and

(HOCH2CH2S–) at d 3.67 were shifted to d 2.72 and d 4.25, respectively when the urethane bond (–OCONH–) was

formed.

The size-exclusion chromatography (SEC) trace of the copolymer was unimodal and symmetrical with narrow

molecular weight distribution as shown in Fig. 1, indicate that neither PI nor PEG homopolymer was retained in

copolymer. The molecular weight of the graft polymer after the ‘‘graft onto reactions’’ was higher than that of the

hydroxyl functionalized polymer PI-OH, and the PDI was broadened slightly.

The polymers were also traced by FTIR spectra as shown in Fig. 2. The characteristic carbon–carbon double bond

stretch absorption at 1645 cm�1 was observed in all three samples. The broad band at 3170–3607 cm�1 was attributed

to the absorption of –OH groups introduced on PI, and the absorption for urethane group (–OCONH–) on graft

copolymer at 3300 cm�1(–NH–), 1724 cm�1(–CO–), 1537 cm�1 (–CONH–) was also discriminated clearly. The FT-

IR results further confirmed that the synthesis of the graft copolymer, and the functional groups transformation on PI

main chain by thiol-ene addition reaction were successful.

F. Shao et al. / Chinese Chemical Letters 23 (2012) 347–350348

Scheme 1. Synthetic procedures of amphiphilic graft copolymer with polyisoprene backbone. Reagents and conditions: (a) AIBN, mercaptoethanol,

70 8C; (b) mPEG, dibutyltin dilaurate (DBTDL), 40 8C; (c) DBTDL, 60 8C.

In a conclusion, an amphiphilic graft copolymer with PEG as hydrophilic branch and polyisoprene as hydrophobic

backbone was synthesized by the combination of thiol-ene radical ‘‘click’’ reaction and alcohol-isocyanate reaction

between polyisoprene and PEG via a controlled approach.

Acknowledgments

The authors gratefully acknowledge the financial supports of the Special Funds for Major Basic Research Projects

(No. G2011CB606001), Zhejiang Provincial Top Key Discipline of New Materials and Process Engineering (No.

20110926).

References

[1] K. Letchford, H. Burt, Eur. J. Pharm. Biopharm. 65 (2007) 259.

[2] J.L. Huang, G.W. Wang, X.S. Fan, J. Polym. Sci. Part A, Polym. Chem. 48 (2010) 3797.

F. Shao et al. / Chinese Chemical Letters 23 (2012) 347–350 349

18171615141312

Elution time (min)

213

1 PI2 PI-OH3 graft copolymer

Fig. 1. SEC traces of PI(1, Mn = 0.89 � 104, PDI = 1.07), PI-OH(2, Mn = 0.98 � 104, PDI = 1.08), and graft copolymer (3, Mn = 1.84 � 104,

PDI = 1.13). Conditions: THF as eluent at a flow rate of 1.0 mL/min, 40 8C, monodispered polystyrene as calibration standard.

5001000150020002500300035004000

1645

1537

3300

c

b

Wavenumber (cm-1)

a

1724

Fig. 2. The FTIR spectra of (a) PI; (b) PI-OH; (c) graft copolymer.

[3] Q.J. Zhang, W.P. Zhu, Z.Q. Shen, Chin. Chem. Lett. 21 (2010) 1255.

[4] I. LaRue, M. Adam, M. da Silva, et al. Macromolecules 37 (2004) 5002.

[5] D.S. Germack, S. Harrisson, G.O. Brown, et al. J. Polym. Sci. Part A, Polym. Chem. 44 (2006) 5218.

[6] J.K. Wegrzyn, T. Stephan, R. Lau, et al. J. Polym. Sci. Part A, Polym. Chem. 43 (2005) 2977.

[7] Z.H. Lu, G.J. Liu, F.T. Liu, J. Appl. Polym. Sci. 90 (2003) 2785.

[8] K.S. Murthy, Q.G. Ma, E.E. Remsen, et al. J. Mater. Chem. 13 (2003) 2785.

[9] M.K. Mahanthappa, L.S. Lim, M.A. Hillmyer, et al. Macromolecules 40 (2007) 1585.

[10] J. Shin, H. Matsushima, J.W. Chan, et al. Macromolecules 42 (2009) 3294.

[11] J.D. Flores, C.E. Hoyle, C.L. McCormick, Polym. Chem. 1 (2010) 213.

[12] K.B. Sharpless, H.C. Kolb, M.G. Finn, Angew. Chem. Int. Ed. 40 (2001) 2004.

[13] A. Dondoni, Angew. Chem. Int. Ed. 47 (2008) 8995.

[14] C.E. Hoyle, C.N. Bowman, Angew. Chem. Int. Ed. 49 (2010) 1540.

[15] X.F. Ni, W.W. Zhu, Z.Q. Shen, Polymer 51 (2010) 2548.

[16] H. Schlaad, J. Justynska, Z. Hordyjewicz, Polymer 46 (2005) 12057.

F. Shao et al. / Chinese Chemical Letters 23 (2012) 347–350350