A novel strategy for synthesis of amphiphilic π-shaped copolymers by RAFT polymerization pdf

A novel strategy for synthesis of amphiphilic p -shaped

copolymers by RAFT polymerization

De-Hui Han,Cai-Yuan Pan

*

Department of Polymer Science and Engineering,University of Science and Technology of China,Hefei,Anhui 230026,PR China

Received 12July 2005;received in revised form 10August 2005;accepted 12September 2005

Available online 4November 2005

Abstract

The amphiphilic p -shaped copolymers with narrow molecular weight distribution (M w /M n =1.04–1.09)based on poly-styrene (PSt)and poly(ethylene glycol)have been synthesized successfully.The reversible addition-fragmentation transfer (RAFT)polymerization of St in the presence of dibenzyl trithiocarbonate and N ,N 0-azobis(isobutyronitrile)(AIBN)yielded macro RAFT agent PSt–SC(@S)S–PSt,subsequent reaction with excess maleic anhydride (MAh)at 80°C in tetrahydrofuran a?orded the PSt–MAh–SC(@S)S–MAh–PSt.It was used as RAFT agent in the RAFT polymerization of St,and ?nally the amphiphilic p -shaped copolymers were obtained by the reaction of MAh with hydroxyl-terminated poly(ethylene glycol methyl ether)at 90°C for 48h.Their structures were con?rmed by FT-IR and 1H NMR spectra,and their molecular weight and molecular weight distribution were measured by gel permeation chromatography.ó2005Elsevier Ltd.All rights reserved.

Keywords:Amphiphilic p -shaped copolymers;Controlled radical polymerization;Poly(ethylene glycol methyl ether);Reversible addition-fragmentation transfer polymerization

1.Introduction

Graft copolymers have been extensively studied because of their interesting properties in solution and in bulk,and they have wide applications,such as engineering plastics,thermoplastic elastomers,surface modi?ers,compatibilizers and surface-active agents [1–5].Graft copolymers are composed of a main polymer chain to which one or more side chains are connected via covalent bands.Generally,the branches are randomly distributed along the

polymer backbone.Controlling the number of branches and their position on the backbone are very di?cult,even for the anionic polymerization [6–14].The p -shaped polymer is a special graft copolymer,in which two branches are connected to a polymer chain.Because of their special struc-ture,they show unique morphologies [15]and very interesting rheological properties of entangled poly-mer melts and solution [16].Therefore,they have attracted much attention in both polymer chemistry and polymer physics recently.Based on our knowl-edge,p -shaped copolymers were mainly prepared by anionic polymerization [15,17],no report on the synthesis of p -shaped polymers by controlled free radical polymerization is found in the literature.

0014-3057/$-see front matter ó2005Elsevier Ltd.All rights reserved.doi:10.1016/j.eurpolymj.2005.09.019

*Corresponding author.Tel.:+865513603264;fax:+865513601592.

E-mail address:pcy@https://www.360docs.net/doc/fc15951909.html, (C.-Y.

Pan).

EUROPEAN POLYMER JOURNAL

Due to the limited numbers of vinyl monomers applied in the living anionic polymerization,until now,p-shaped polymers were prepared from only a few kinds of polymers,such as PSt,polyisoprene (PI)and polybutadiene(PBD)[15,17].In addition, after each reaction of living polymer chains with linking agent,troublesome puri?cation for remov-ing contaminants from the crude products is neces-sary because the coupling reactions cannot go to completion and side reactions might occur.There-fore,?nding a new,convenient and more versatile synthetic strategy is still challenging.

In our research program on RAFT polymeriza-tion using trithiocarbonates as RAFT agent,the synthesis of well-de?ned polymers and the polymer-ization mechanisms have been studied[18–24].The synthetic strategy based on MAh as linking agent for yielding MPEG branch has been investigated also[25,26].Thus we try to extend this synthetic strategy to prepare p-shaped https://www.360docs.net/doc/fc15951909.html,-pared to anionic polymerization,the controlled rad-ical polymerizations are more versatile in monomers,and they are easier to operate.Poly(eth-ylene glycol)(PEG)is a water-soluble and crystalliz-able polymer,which leads to very interesting solution and solid-state behaviors[27–29].There-fore,the block and graft copolymers containing the PEG block have been extensively studied [25,30–32],and they demonstrated unique morphol-ogies and properties[33–35].In this paper,we report the synthesis of the amphiphilic p-shaped copolymers,(PSt(PEG)PSt(PEG)PSt)by RAFT polymerization.The polymers obtained were char-acterized by FT-IR,1H NMR spectra and gel per-meation chromatography.

2.Experimental

2.1.Materials

Poly(ethylene glycol methyl ether)(Fluka, PEGM,M n,PEGM=2000g/mol,M w/M n=1.02) was freed of water by azeotropic distillation of ben-zene.All the other reagents(analytical grade)were purchased from The First Shanghai Chemical Reagents Co.THF was distilled from a purple sodium ketyl solution.St was washed with an aque-ous solution of sodium hydroxide(5wt%)three times and then with water until neutralization.After dried over anhydrous magnesium sulfate,it was dis-tilled at reduced pressure.Maleic anhydride(MAh) was recrystallized from anhydrous chloroform.The initiator,N,N0-azobis(isobutyronitrile)(AIBN)was recrystallized from ethanol.

2.2.Characterization

Molecular weight and molecular weight distribu-tion were determined on Waters150C gel perme-ation chromatography(GPC)with ultra-styragel columns(500,103and104A?),using monodispersed polystyrenes as calibration standards.THF was used as eluent at a?ow rate of1mL/min,and the calibration was carried out based on polystyrene standard.Infrared spectra were recorded on a Bru-ker VECTOR-22infrared spectrometer.1H nuclear magnetic resonance(NMR)spectra were obtained from Bruker-300MHz nuclear magnetic resonance (NMR)instrument using CDCl3as solvent and tetra-methylsilane as internal reference.

2.3.Synthesis of dibenzyl trithiocarbonate (DBTTC)

Dibenzyl trithiocarbonate(DBTTC)was synthe-sized according to the procedure described in the lit-erature[19].1H NMR(300MHz,CDCl3,d,ppm): 4.67(s,4H,2–CH2–),7.15–7.25(m,10H,2aro-matic H).

2.4.RAFT polymerization of St using DBTTC

as RAFT agent

A typical procedure(sample PSt1in Table1)is as follows.Into a5mL sealed polymerization tube with magnetic stirring bar,DBTTC(0.11g;

0.38mmol),AIBN(0.005g,0.03mmol),St(3.12g, 0.03mol),and THF(1.6mL)were added,and the tube was closed with three-way stopcocks.After degassed by three freeze–vacuum–thaw cycles,the tube was sealed under vacuum,and then immersed in an oil bath thermostated at110°C while stirring. After polymerization for a prescribed time,the tube was cooled down to room temperature rapidly,and the polymer was precipitated by pouring the solu-tion in THF into methanol.The polymer,PSt–SC(S)S–PSt was collected by?ltration,and then dried in vacuum oven for24h.

2.5.Reaction of PSt–SC(S)S–PSt with MAh

Sample PSt4in Table2(4g,0.91mmol),MAh (2g,20mmol)and THF(10mL)were added into a10-mL glass tube,followed by three freeze–vac-

508 D.-H.Han,C.-Y.Pan/European Polymer Journal42(2006)507–515

uum–thaw cycles.The tube was sealed under vac-uum and immersed in an oil bath thermostated at 80°C.After reaction for 24h,the tube was cooled to room temperature,the MAh-inserted PSt,PSt–MAh–SC(S)S–MAh–PSt was puri?ed by precipi-tated from petroleum ether (bp 30–60°C)three times for removal of excess MAh.The product (3.3g)was obtained in 80%yield.

2.6.RAFT polymerization of St using PSt–MAh–SC(S)S–MAh–PSt as RAFT agent

Into a 10mL glass polymerization tube,the sam-ple 2a in Table 2(M n (GPC)=4600g/mol,400.0mg),St (2.0g)and AIBN (1.4mg)were added.After three freeze–vacuum–thaw cycles,the polymerization tube was sealed under vacuum.The polymerization was carried out at 110°C for a prescribed time.After the glass tube was cooled down to room temperature rapidly,the product was puri?ed by precipitated from petroleum ether (bp 30–60°C).

2.7.Preparation of p -shaped copolymer

The sample 3b in Table 3(M n (GPC)=22,100g/mol,5.0g),PEGM (M n =2000g/mol,2.5g)and THF (15mL)were added into a 25mL one-necked round ?ask equipped with a magnetic bar.After ?ushed with nitrogen for a while,the reaction was carried out at 90°C for 48h.When the reaction mixture was cooled down to room temperature,the reaction solution in THF was dialyzed against water for seven days.After evaporation of water under vacuum,the polymer was puri?ed by precip-itated from petroleum ether (bp 30–60°C)three times for removal of unreacted PEGM.3.Results and discussion

3.1.RAFT polymerization of St using DBTTC as RAFT agent

In our research program on investigation of RAFT polymerizations in the presence of

Table 1

RAFT polymerization of St with DBTTC at 110°C in THF a No.DBTTC (mmol)AIBN (mmol)THF (g)t (h)Conv b (%)M n (g/mol)M w /M n d (th)c (GPC)d PSt10.380.03 1.582927002900 1.20PSt20.380.03 1.5123734003700 1.18PSt30.380.03 1.5164338004000 1.18PSt40.380.03 1.5204641004400 1.14PSt50.280.02 1.2164346004900 1.11PSt6

0.23

0.014

1.2

16

41

5800

5900

1.10

a St:0.03mole.

b

Conv (conversion)was measured with gravimetric method.c

Theoretical number-average molecular weight,M n (th)was calculated according to M n (th)=Conv ·N M ·M M /N DBTTC +M DBTTC ,where N M and N DBTTC are initial moles of monomer and DBTTC,M M and M DBTTC are the molecular weights of monomer and DBTTC,respectively.d

Measured by GPC.

Table 2

Conditions and results of insertion reaction of MAh into PSt a Sample PSt T (°C)t (h)PSt–MAh–SC(S)S–MAh–PSt M w /M n d No.b M n (GPC)M n (NMR)c M n (GPC)d 2a PSt44400802447004600 1.102b PSt54900802454005200 1.102c

PSt6

5900

80

24

6400

6100

1.08

a The insertion reactions of MAh into the PSt were carried out at 80°C in THF for 24h.Feed ratio:MAh/PSt =20:1(molar ratio).

b PSt used in the reactions are the same as those listed in Table 1.

c M n (NMR)was calculate

d according to Eq.(1).d

Measured by GPC.

D.-H.Han,C.-Y.Pan /European Polymer Journal 42(2006)507–515

509

trithiocarbonates,a series of polymers and triblock copolymers with well-de?ned structures,controlled molecular weight,narrow molecular weight distri-bution(M w/M n)have been obtained[19–26].The RAFT polymerizations of St with DBTTC and AIBN as RAFT agent and initiator,respectively, were carried out,the conditions and results are listed in Table1.The molecular weight of the PSt was increased with the evolution of polymerization, and the narrow PSt(M w/M n=1.14–1.20)was obtained.For con?rmation of the polymer struc-ture,their1H NMR spectra were measured and a typical spectrum is shown in Fig.1(A).Except char-acteristic signals of PSt,we can?nd a signal at d=4.25ppm ascribed to the methine proton next to trithiocarbonate group.The GPC curve of this polymer(almost the same with Fig.2(B))shows a single and symmetrical.All these facts indicate the formation of PSt–SC(S)S–PSt.

3.2.Insertion of MAh into the PSt chain

In order to prepare graft copolymers with branches at controlled position,a convenient method is insertion of MAh into the polymer backbone. This reaction has been studied already,and the re-sult demonstrated that only one MAh unit could be inserted into dithiobenzoate-terminated PSt chain to form PSt–MAh–SC(S)Ph[25,36].In order to prepare p-shaped copolymer,inserting two MAh units into the backbone is necessary.So we choose the PSt with trithiocarbonate group in the middle of polymer backbone.For getting two MAh groups in both sides of trithiocarbonate group in every macromolecule,excess MAh was used in the insertion reaction of polymer1as shown in Scheme1.The unreacted MAh can be easily re-moved by repeating precipitation from petroleum ether(bp30–60°C).In comparison with its precur-sor,PSt–SC(S)S–PSt(M n(GPC)=4400),slight

molecular weight increase(M n(GPC)=4600)of

the MAh-inserted polymer,PSt–MAh–SC(S)S–

MAh–PSt obtained in Fig.2(B)is related not only

to the molecular weight of MAh-inserted,but also

to the repeating precipitation.Thus the reaction

between MAh and PSt–SC(S)S–PSt cannot be pre-

sumed based on this slight increase of molecular

weight.For con?rming the insertion reaction to

form the polymers,PSt–MAh–SC(S)S–MAh–PSt,

their1H NMR spectra were measured,and a typical 1H NMR spectrum of the insertion reaction prod-uct,the sample2a in Table2is shown in

Fig.1(B).The signals at6.0–7.2and1.2–2.5ppm

are the characteristic signals of PSt.A new signal

a at d=3.7ppm corresponds to the methine proton

of MAh unit next to the trithiocarbonate group

appears,and the signal at4.25ppm in Fig.1(A)dis-

appeared completely in Fig.1(B).This indicates the

cleavage of C–S bond between St unit and trithio-

carbonate group and the formation of a new C–S

bond between the trithiocarbonate and MAh

groups.

Assuming each side of trithiocarbonate group is

inserted with one MAh unit and almost no trithio-

carbonate group is lost during the MAh inserting

reaction,the M n(NMR)of the polymer2(Scheme

1)can be calculated according to Eq.(1).

M neNMRT?e2I7:2–6:30=5I3:7T?104t290t98?2

e1THere I7.32–6.30,and I3.7are the integral values of sig-nals at d=7.32–6.30and3.7ppm,290and98are the molecular weights of DBTTC and MAh,respec-tively.The results calculated are listed in Table2, and for comparison,the M n(GPC)s are also listed in Table2.Well agreement between M n(NMR) and M n(GPC)may indicate that the PSt chains are not a?ected by the insertion reaction.Fig.3(B)

Table3

Conditions and results for RAFT polymerization of St with PSt–MAh–S–C(@S)–S–MAh–PSt as RAFT agent

Sample Polymer a T(°C)t(h)Conv b(%)M n(th)c M n(GPC)d M w/M n d

3a2a11061682009940 1.10

3b2a110163418,00022,100 1.05

3c2b11061411,00011,600 1.06

3d2b110163322,00023,800 1.05

a The polymer samples are the same as those listed in Table2.

b Conversion was measured with gravimetri

c method.

c Theoretical number-average molecular weight,M

n

(th)was calculated according to M n(th)=Conv·N M·M M/N polymer+M n,polymer(th), where N M and N polymer are initial moles of St and the polymer;M M and M n,polymer(th)are the molecular weights of monomer and polymer.

d Measured by GPC.

510 D.-H.Han,C.-Y.Pan/European Polymer Journal42(2006)507–515

is a typical FT-IR spectrum of the sample2a in Table2.For comparison,the FT-IR spectrum of its precursor is also shown in Fig.3(A).We will?nd a new strong absorption band at$1790cmà1in Fig.3(B),which is the characteristic carbonyl stretching band of the anhydride,con?rming the existence of anhydride group in PSt backbone.3.3.Further RAFT polymerization of St using polymer2as RAFT agent

Several well-de?ned triblock copolymers have been successfully prepared by RAFT polymeriza-tion of the second monomer with those polymers containing trithiocarbonate group as RAFT agent

D.-H.Han,C.-Y.Pan/European Polymer Journal42(2006)507–515511

[18,20,37,38].The results showed that the RAFT polymerization of St with macro RAFT agent is of living nature[18].Therefore,we conducted the RAFT polymerization of St with PSt–MAh–S–C(@S)S–MAh–PSt as RAFT agent.The conditions and results are listed in Table3.The plot of ln([M]0/ [M])against polymerization time a?orded a straight line as shown in Fig.4,indicating the constant con-centration of propagating chain radicals during the polymerization.Linear growth of the molecular weight with the increase of monomer conversion as shown in Fig.5demonstrates that the molecular weights of the polymers obtained can be controlled. The GPC curves of the products obtained were mea-sured,and the data are listed in Table3.The results show that all the polymers obtained have narrow molecular weight distribution(M w/M n=1.05–1.10 in Table3).All these facts con?rm that the RAFT polymerization of St using PSt–MAh–SC(@S)S–MAh–PSt as RAFT agent is a controlled radical polymerization.Therefore the grafting positions of the branching chains on the polymer backbone can be adjusted by varying the chain length of the second PSt formed.For verifying the structure formed,the1H NMR and FT-IR spectra of PSt–MAh–PSt–SC(@S)S–PSt–MAh–PSt were mea-sured,Figs.1(C)and3(C)show1H NMR and FT-IR spectra of the block polymer,the sample 3a in Table3.The signal at d=3.7ppm corre-sponding to the methine proton of MAh group next to trithiocarbonate group in Fig.1(B)disappears completely,and the signal at d=4.25ppm is

512 D.-H.Han,C.-Y.Pan/European Polymer Journal42(2006)507–515

observed in Fig.1(C).A strong absorption band at m=1790cmà1is still exist in Fig.3(C),indicating that the MAh group in the PSt backbone remained after the RAFT polymerization.

3.4.Synthesis of p-shaped copolymers by

esteri?cation with PEGM

The esteri?cation of MAh-inserted PSt,PSt–MAh–PSt–SC(@S)S–PSt–MAh–PSt with PEGM was carried out according to the procedure similar to the Kallitsis method[36].After the reaction solu-tion in THF was dialyzed against water for seven days for removal of the excess MPEG,and the p-shaped copolymers were obtained by precipitation from petroleum ether(bp30–60°C).The synthetic conditions and results are listed in Table4.The GPC curves of the polymers4obtained were mea-sured;all curves are single and symmetrical as shown in Fig.2(D).This indicates the complete sep-aration of MPEG from the polymers.Fig.1(D)is 1H NMR spectrum of the sample4a in Table4.

We can see that signals at d=3.64and4.26ppm corresponding to the ether methylene protons and ester methylene protons of PEGM,respectively, except the characteristic signals of PSt at6.0–7.2 and1.2–2.5ppm,which evidences the reaction of MAh with PEGM.Assuming that there are two MAh groups in each PSt macromolecule,and the PEG used in this reaction is unknown,we can calcu-late the molecular weight of the PEG according to Eq.(2).

M n;PEG?e5I3:64=4I7:32–6:30T?DP St?44t15e2THere I7.32–6.30,and I3.64are the integral values of signals at d=7.32–6.30and3.64ppm,respectively; DP St means the number-average degree of PSt main chain;44and15are the molecular weights of ethyl-ene oxide and the methyl group.The results calcu-lated are listed in https://www.360docs.net/doc/fc15951909.html,pare the calculated values(M n=2300–2100g/mole)with the M n= 2000of MPEG,we can conclude that almost all anhydride groups in the polymer3were converted to ester groups.For further con?rmation,FT-IR spectrum was measured.As shown in Fig.3(D), the characteristic absorption band of MAh group at m=1790cmà1is disappeared completely,and a

D.-H.Han,C.-Y.Pan/European Polymer Journal42(2006)507–515513

new characteristic carbonyl-stretching band of ester at 1730cm à1appears,which evidences the esteri?-cation reaction of PEGM with MAh groups in the PSt chains.All the facts verify that the PEGM has been grafted onto the PSt backbone at anhydride sites,and the p -shaped copolymers have been pre-pared successively.4.Conclusion

A more convenient synthetic method for prepa-ration of p -shaped copolymers has been developed.MAh group can be inserted into the C–S bond between PSt block and trithiocarbonate group by the reaction of PSt–SC(@S)S–PSt with MAh at 80°C.For obtaining PSt–MAh–SC(S)S–MAh–PSt completely,addition of excess MAh is needed.This e?cient RAFT agent can be used in the successive RAFT polymerization of St,and the PSt–MAh–PSt–SC(S)S–PSt–MAh–PSt was produced.The polymerization is of controlled nature,and varying the chain length of the centered PSt can adjust the MAh position on the polymer backbone.The p -shaped copolymers can be achieved by the reaction of the anhydride group of MAh groups with the hydroxyl group of PEGM.The advantages of this method are the molecular weight and distribution of two branches in the p -shaped copolymers can be controlled,and their positions on the polymer backbone can be adjusted by variation of PSt block chain length.Acknowledgement

This work is supported by Specialized Research Fund for the Doctoral Program of Higher Educa-tion (20020358055).

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Table 4

Conditions and results for synthesis of p -shaped copolymers by esteri?cation of PSt–MAh–PSt–S–C(@S)–S–PSt–MAh–PSt with MPEG a Sample Polymer b MPEG M n (GPC)c M w /M n c M n (th)d M n,PEG e 4a 3a 2000230012,800 1.074b 3b 2000220028,200 1.054c 3a 2000210016,700 1.094d

3b

2000

2300

26,300

1.04

a The esteri?cation was carried out in THF solution at 90°C for 48h (polymer/PEGM =1:10,molar ratio).

b The polymer samples are the same as those listed in Table 3.

c Measure

d by GPC.

d Th

e M n (th)o

f the MPEG purchased was labeled.e

M n,PEG was calculated according to Eq.(2).

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