ps-p4vb

Journal of Polymer Research(2005)12:449–456?Springer2005 DOI:10.1007/s10965-004-5665-2

Synthesis and Characterization of Polystyrene-b-Poly(4-vinyl pyridine)

Block Copolymers by Atom Transfer Radical Polymerization

Chih-Feng Huang,Shiao-Wei Kuo,Jem-Kun Chen and Feng-Chih Chang?

Institute of Applied Chemistry,National Chiao Tung University,Hsin Chu,Taiwan

(?Author for correspondence;Tel.:886-35131512;Fax:886-35131512;E-mail:changfc@https://www.wendangku.net/doc/ed16806229.html,.tw)

Received19May2004;accepted in revised form1November2004

Key words:atom transfer radical polymerization(ATRP),block copolymers,polystyrene

Abstract

We aimed at the synthesis of well-de?ne PS-b-P4VP by using atom transfer radical polymerization in two-step process. First,polystyrenes with benzyl bromide end group(PS-Br;by ATRP)were prepared as macroinitiator for the next ATRP of4-vinyl pyridine and characterized these polymers from1H-NMR and https://www.wendangku.net/doc/ed16806229.html,paring with MALDI-TOF-MS,1H-NMR and GPC analyses,this indicates that the formation of the block copolymer can be observed.During the polymerizations,molecular weight distribution and kinetics have been evaluated from GPC traces and1H-NMR analyses. We further characterized the thermal properties of these block polymers by DSC and TGA.DSC measurement on the PS-b-P4VP block copolymers exhibited two glass transitions,indicating that the resulting block copolymers are phase separated. Two maxima differential peaks were observed on the TGA trace for the PS-b-P4VP block copolymers might be assigned to the decomposition of the P4VP blocks at380?C and the PS blocks at higher temperature.

Introduction

The desire to control polymer properties through the synthe-sis of block copolymers and complex macromolecular archi-tectures is a continuing theme throughout polymer chemistry [1,2].Block copolymers are remarkable self-assembling systems that can assume a wide variety of morphologies including lamellar,hexagonal-packed cylindrical,and body-centered cubic micellar structures,depending on the relative volume fractions of the blocks[3,4].This clear picture of the morphology as a function of composition has primar-ily emerged from the investigation of diblock copolymers. The block copolymers with well-de?ned structures,such as molecular weight(MW)and molecular weight distribution (MWD),composition,architecture and end group function-ality,are very important,and this has been carried out by the following three methods[5]:(1)sequential monomer addition,(2)coupling reaction of“living”polymer chains, and(3)mechanism transformation.The development of ionic polymerization methods allowed for the preparation of well-de?ned polymers with controlled chain end func-tionalities and the synthesis of well-de?ned block and graft copolymers[6–9].However,these polymerizations have to be carried out with nearly complete exclusion of moisture and air,and often at very low temperature.Moreover,only a few numbers of monomers can be polymerized,and the presence of functional monomers can cause undesired side reactions.Recently,Matyjaszewski has reported that atom transfer radical polymerization(ATRP)can be used to syn-thesize polymers with narrow molecular weight distribution (MWD)[10],well-de?ned block copolymers[11,12],and star polymers[13,14].The ATRP process uses an alkyl halide as initiator and a metal in its lower oxidation state with complexing ligands[15–21].The process involves the successive transfer of the halide from the dormant poly-mer chain to the ligated metal complex,thus establishing a dynamic equilibrium between active and dormant species (Scheme1).This controlled radical polymerization allows for the polymerization of a wide range of monomers such as styrenes[22,23],acrylates[24,25],methacrylates[26,27], and various functional monomers.

It has been demonstrated that a block copolymer of P4VP has the ability to form self-assembly supramolecular structure[28],high complexibility with metal ion[29,30], and electrical conducting property[31].Polymerization of 4VP posses a very challenging problem for ATRP because both4VP and P4VP are strong coordinating ligands that can compete for the binding the metal catalysts in these systems.Since the monomer is normally present in large excess over the employed ligand,there is a possibility of the formation of pyridine-coordinated metal ion complexes in the polymerization solution.Pyridine-coordinated copper complexes are not effective catalysts for ATRP.For ex-ample,addition of5vol%pyridine to the polymerization solution of styrene catalyzed by CuBr complexed by4,4 -di(5-nonyl);2,2 -bipyridine(dNbpy)signi?cantly slowed down the polymerization rate[21].

In this paper,we aimed at the synthesis of well-de?ne PS-b-P4VP by using atom transfer radical polymerization in two-step process by using commercial available ligands. First,polystyrenes with benzyl bromide end group(PS-Br; by ATRP)is prepared as macroinitiator for the next ATRP

450Chih-Feng Huang et

al.

Scheme1.Dynamic equilibrium that exists between the propagating and dormant species when a metal complex is used as the reversible halogen atom transfer reagent.

of the4-vinyl pyridine and characterized these polymers by1H-NMR and MALDI-TOF.During the polymerizations, molecular weight distribution and kinetics have been eval-uated by GPC traces and1H-NMR analyses.We further characterize the thermal properties of these block polymers by DSC and TGA.

Experimental

Materials

Styrene(S)and4-vinylpyridine(4-VP)were distilled from calcium hydride before use.Copper(I)bromide(CuBr) was stirred in glacial acetic acid overnight,?ltered,and then rinsed with absolute ethanol under a blanket of ar-gon and dried under vacuum at80?C for three days. All solvents were distilled prior to use.N,N,N ,N ,N -pentamethyldiethylenetriamine(PMDETA),2,2 -bipyridine (Bipy),4,4 -dinonyl-2,2 -dipyridyl(dNBipy)were used as received.N,N -dimethylformamide(DMF)and toluene were distilled from magnesium sulfate and sodium/benzophenone immediately before use.All chemicals were purchased from Aldrich Chemical Co.(Milwaukee,WI).

Preparation of PS-Br Macroinitiator by the ATRP of Styrene

A typical polymerization is as follows:CuBr(0.1mmol) was placed into a dry25-mL round-bottom?ask equipped with a stirring bar.Toluene(10mL),styrene(30mmol) and PMDETA(0.1mmol)were added sequentially and the solution was stirred for20min to form the Cu com-plex.The initiator,1-phenylethyl bromide,(0.1mmol) was then added.This whole process was carried out in a nitrogen-?lled dry box.The mixture was degassed with three freeze-thaw cycles.Polymerization was carried out at an ap-propriate temperature in an oil bath.The reaction mixture turned dark green immediately and became progressively more viscous.Upon completion of the reaction,the mixture was diluted?ve-fold with tetrahydrofuran(THF)and stirred with of Amberlite IR-120(H form)cation-exchange resin (3–5g)for30–60min to remove the catalyst.The mixture was then passed through a neutral alumina column and pre-cipitated into ten-fold of methanol.The resulting polymers were?ltered and dried overnight at60?C under vacuum.Preparation of PS-b-P4VP by the ATRP of4-VP with PS-Br Macroinitiator

A typical procedure for the synthesis of PS-b-P4VP was as follows.Prior to the sequential polymerization,the PS-Br macroinitiator was dried overnight in a vacuum oven at 50?C.In a?ame-dried,two-necked?ask,CuX(0.1mmol) was placed into a dry25-mL round-bottom?ask equipped with a stirring bar.4-VP(20mmol),DMF(4M)and a desired amount of ligand were added sequentially and the solution was stirred for20min to form the Cu complex. The macroinitiator(0.1mmol)was then added.This whole process was performed in a nitrogen-?lled dry box.The mix-ture was degassed with three freeze-thaw cycles.An aliquot of the solution(ca.0.1mL)was removed and then polymer-ization was carried out at an appropriate temperature in an oil bath.The reaction mixture turned dark green immedi-ately and became progressively more viscous.Periodically, aliquots(0.1mL)were removed for analysis.Exotherms of 2–4?C were typically observed,indicating that polymeriza-tion occurred.Upon completion of the reaction,the mixture was diluted?ve-fold with DMF and stirred with of Am-berlite IR-120(H form)cation-exchange resin(3–5g)for 30–60min to remove the catalyst.The mixture was then passed through an alumina column and precipitated into ten-fold of10%H2O/methanol.This puri?cation protocol resulted in the loss of up to5～10%of the polymer as a result of adsorption.The resulting polymers were redissolve in DMF and precipitated into ten-fold of ether.The resulting polymers were?ltered and dried overnight at60?C under vacuum.These procedures were repeated twice to obtain the pure block copolymer.

Characterization

Matrix-assisted laser desorption/ionization time-of-?ight mass spectrometry(MALDI-TOF-MS)spectra was per-formed on a PerSeptive Biosystems V oyager DE-STR equipped with2-m linear and3-m re?ector?ight tubes and a337-nm nitrogen laser(pulse width,3ns),along with a delayed extraction capability.All experiments were carried out at an accelerating potential of20kV in both linear and re-?ector modes.In general,mass spectra from256laser shots were accumulated summed to produce a?nal spectrum.An-giotensin I(human;M w=1296.5)(BACHEM)and insulin

Synthesis and Characterization of Copolymers451

(bovine pancreas28.3;M w=5733.50)(Nacalai)were used as internal standards to calibrate the mass scale.Sam-ples for MALDI-TOF-MS analysis were prepared by mixing the polymer,a matrix,and a cationizing agent in a sol-vent.For example,a PS sample(approximately10mg/mL) was dissolved in THF.The matrix[1,8-dihydroxy-9(10H)-anthracenone;dithranol]and the cationizing agent(sodium tri?uoroacetate)were also dissolved separately in THF (30and10mg/mL,respectively).A30μL portion of the PS solution,90μL of the matrix solution,and30μL of the cationizing agent were mixed in a glass vial.The weight ratio of polymer/matrix/cationizing agent was thus1/9/1. Then0.5μL portions of the mixture were deposited onto 10–20wells of the gold-coated sample plate and dried in air at room temperature.NMR spectra were recorded on a Brucker AM500Spectrometer and were measured in DMSO-d6.Molecular weights and molecular weight distri-butions were determined by gel permeation chromatography (GPC)using a Waters510HPLC–equipped with a410 Differential Refractometer Index(RI)and a UV detector in series,and three Ultrastyragel columns(100,500,and 103?)connected in series in order of increasing pore size –using DMF as an eluent at a?ow rate of1.0mL/min. The molecular weight calibration curve was obtained using polystyrene standards.Thermal analysis was carried out on a DSC instrument from DuPont(model910DSC-9000con-troller)with a scan rate of20?C/min and temperature range of20–200?C in nitrogen atmosphere.Approximately5–10mg sample was weighted and sealed in an aluminum pan. The samples were quickly cooled to room temperature from the?rst scan and then scanned between30and280?C at a scan rate of20?C/min.The glass transition temperature is taken as the midpoint of the heat capacity transition between the upper and lower points of deviation from the extrapolated glass and liquid lines.FTIR spectroscopy measurements were made from a NaCl disk using a Nicolet Avatar320 FT-IR Spectrometer,with32scans collected at a resolu-tion of1cm?1.A DMF solution containing the sample was cast onto a NaCl disk and dried under conditions similar to those used in the bulk preparation.The sample chamber was purged with nitrogen to maintain the?lm’s dryness.Thermo-gravimetric analysis(TGA)experiments were performed by using a DuPont TGA-51thermogravimetric instrument.The temperature was increased from30to800?C at a heating rate of20?C/min under nitrogen atmosphere.The degra-dation temperature was de?ned as the temperature at the maximum of the differential thermogravimetric curve.

Results and Discussion

The synthetic approach for the PS-b-P4VP block copoly-mers is depicted in Scheme2.Initially,it was decided to grow a well-de?ned polystyrene chain from the phenylethyl bromide initiator and resulted chain end functionalized PS acted as a polymeric initiator for the next controlled poly-merization of4-vinylpyridine.Figure1shows MALDI-TOF-MS and GPC curves of PS macroinitiators obtained from the different polymerization time interval.It can be observed that variation of the PS molecular weight based on the MALDI-TOF-MS spectrum matches well with that from the gel permeation chromatography(GPC).The signals of every series in MALDI-TOF-MS spectrum are separated by104Da,corresponding to the molecular weight of a styrene unit.As observed by both MALDI-TOF-MS and GPC,peak maxima of these spectra clearly shift to high molecular weight with increasing monomer conversion.

Sequentially,we use the PS-Br macroinitiator(M n= 8000)to further polymerize with4-vinylpyridine.The struc-ture of the block copolymer was characterized by1H-NMR spectroscopy.Figure2illustrates the NMR spectra of PS-Br,PS-b-P4VP,and P4VP polymers.The spectrum of the block copolymer shows signal superposition of PS segments with attached segments of4-vinyl https://www.wendangku.net/doc/ed16806229.html,paring with MALDI-TOF-MS and GPC analyses,the formation of the PS-b-P4VP block copolymer can be identi?ed.

In a typical ATRP,the concentration of the active species remains unchanged throughout the reaction that can be veri-?ed by a linear semilogarithmic plot of monomer conversion vs.time as shown in Figure3where conversion is calculated from1H-NMR of the feeding monomer initiated from PS-Br macroinitiator.If a stronger binding ligand was used,such as the tridentate PMDETA,a faster polymerization rate was ob-served.Under similar conditions,a PMDETA to CuBr ratio of6:1was needed to maintain a relatively fast polymer-ization rate to obtain a monomer conversion of50%after 4h.In contrast,at a PMDETA to CuBr ratio of1:1, the polymerization was slower as shown in Figures3(a) and3(b).This result is quite similar with the earlier results on the polymerization of4VP[31].It is the evidence for the competitive coordination of4VP monomer to copper.When polymerization of4VP was carried out using macroinitiator and CuBr complexed by dNBipy as the catalyst in a ratio of 1:1[Figure3(d)],a very slow polymerization rate was ob-served.This is due to the same pyridine unit that functions as ligand and monomer,and hence,the dynamics equilibrium of the halogen atom transfer process will be disturbed.This undesired interaction will in?uence the polymerization rate and the molecular weight distribution during ATRP process. Even at a dNBipy to CuBr ratio of6:1as shown in Fig-ure3(c),it still showed a slower polymerization rate than that of PMDETA to CuBr ratio of1:1.Various ligands, contents,reaction time,molecular masses(M w),yield and PDI are summarized in Table1.Overall,PMDETA sys-tem is the most ef?cient ligand for the polymerization of 4-vinylpyridine.

Figure4shows the dependence of the molecular weight of the block copolymers versus monomer conversion ini-tiated by macroinitiator.The drawn line represents the theoretical molecular weight,M n(th),calculated from:

M n(th)=

[M]

[I]0

×M w(mon)+M w(init).(1) We observe a near-linear increase in the measured num-ber average molecular weight(M n)vs.monomer conversion up to～75%and the evolution of molecular weight agrees with the theoretical value,indicating that a living/controlled polymerization process proceeds in solution.

452Chih-Feng Huang et

al.

Scheme2.Reaction scheme for the syntheses of block copolymers.

The polymerization of4-vinylpyridine initiated by PS macroinitiator was examined with and without halogen ex-change.Figure5shows the kinetics plots of semilogarithmic of monomer conversion vs.time.The rate of polymerization without halogen exchange was faster than that with halogen exchange.The rate of polymerization in ATRP depends on the concentration of propagating radicals and is function of both the initiation ef?ciency and the concentration of de-activator in the system.For PS-Br/CuCl system,therefore, lower concentration of propagating radicals is obtained.The molecular weight also displayed a near-linear dependence on conversion(Figure6).The molecular weight and poly-dispersity for the PS-b-P4VP block copolymers prepared with halogen exchange were lower than those without halo-gen exchange.The molecular weight distribution yielded by the PS-Br/CuX initiation system is worth to mention.The Cu(II)Br bond is relatively weaker than that of Cu(II)Cl,re-sulting in faster deactivation of the propagating radical and lower polydispersity.This result is in accordance with the earlier results on the inhibition of radical polymerization of MMA by Cu(II)Br2and Cu(II)Cl2and trapping of alkyl rad-icals[33,34].Thus,for chain extension from PS to4VP, introduction of halogen exchange technique should yield a more precisely controlled block copolymer than that without the halogen exchange.

Thermal behavior of block copolymers was examined by differential scanning calorimetry(DSC)in the range of0 to200?C.The temperature at the midpoint of the baseline shift was de?ned as the bulk glass transition temperature, T g.As shown in Figure7,DSC measurement on the PS-b-P4VP block copolymers exhibited two glass transitions, indicating that the resulting block copolymers are phase sep-arated.Typical T g’s of PS,and P4VP are reported to be100, and150?C.Obviously,the two glass transition temperatures are quite similar to those of respective homopolymers.This result indicates that the block copolymer is in a microphase-

Synthesis and Characterization of Copolymers453

https://www.wendangku.net/doc/ed16806229.html,paring MALDI-TOF-MS and GPC of PS macroinitiators with the progress of polymerization:(a)4h;(b)8h;(c)12h;(d)16h.

Figure2.1H-NMR spectra of(a)PS,(b)PS-b-P4VP and(c)P4VP homo and block copolymer in DMSO-d6.

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Chih-Feng Huang et al.

Table 1.Block copolymerization conditions a from PS b to 4VP at 80?C by ATRP Sample Ligand c I :C :L d Time (h)Yield (%)M w (GPC)e PDI 1PMDETA 1:1:153015300 1.332PMDETA 1:1:194818500 1.303PMDETA 1:1:645320300 1.324PMDETA 1:1:697423700 1.305dNBipy 1:1:1569400 1.436dNBipy 1:1:191110800 1.427dNBipy 1:1:641712000 1.438dNBipy 1:1:694817000 1.419

Bipy

1:1:6

24

8

10200

1.37

a Monomer:4vinylpyridine (4VP);solvent:N,N -dimethylformamide (DMF);mole ratio of monomer/macro-initiator:200:1.

b PS macroinitiator:M n =8000,M w

=9600,PDI =1.20.

c Ligand:PMDETA =N,N,N ,N ,N -pentamethyldiethylenetriamine;dNBipy =

4,4 -dinonyl-2,2 -dipyridy;Bipy =2,2 -bipyridine.d Mole ratio of initiator to copper bromide to ligand.e Polydispersity index from GPC

traces.

Figure 3.Semilogarithmic kinetic plot for the ATRP of 4VP with various amount of ligand at 80?C [conditions:monomer (4M),solvent:DMF,CuBr catalyst,PS macroinitiator M n =

8000].

Figure 4.The dependence of the molecular weights and polydispersity of the block copolymers on monomer conversion under different reaction conditions.The linear line represents the theoretical M n vs conversion.

separated state.A large repulsion between the two different segments exists for the PS-b-P4VP system.

Thermal stability of block copolymers was examined by thermogravimetric analyzer (TGA)in the range of 30

to

Figure 5.Semilogarithmic kinetic plot for the ATRP of 4VP with various amount of ligand at 80?C by (?)with or ( )without halogen exchange [conditions:monomer (4M);ligand:PMDETA;solvent:DMF,CuBr or CuCl catalyst,PS macroinitiator M n =8000]

.

Figure 6.The dependence of the molecular weights and polydispersity of PS-b-P4VP copolymers on the monomer conversion with (?)and with-out ( )halogen exchange under different reaction conditions.[Conditions:same as in Figure 5.]

Synthesis and Characterization of Copolymers

455

Figure 7.DSC thermograms of PS and PS-b-P4VP block

copolymer.

Figure 8.TGA thermograms of PS,P4VP and PS-b-P4VP block copoly-mer under N 2atmosphere.

800?C.

As shown in Figure 8(c),TGA analysis of the block

copolymer (sample 3,Table 1)shows a two-step thermal decomposition.In Figure 8(d),which was obtained by dif-ferentiating curve (c),we can observe two maximum points at 380and 431?C,respectively.The parent homopolymers have typically decomposed maximum points at 430?C for PS (M n =8000)and 400?C for P4VP (M n =13000)under nitrogen atmosphere,respectively.Therefore,the two maxima observed on the TGA trace for the PS-b-P4VP block copolymer can be assigned to the decomposition of the P4VP blocks at 380?C and the PS blocks at higher temperature.Conclusion

The formation of block copolymers of styrene and 4-vinyl-pyridine was investigated by using ATRP.We used of ATRP with commercial available ligands to syntheses well con-trolled block copolymers from styrene and 4-vinylpyridine monomers.MALDI-TOF-MS,1H-NMR and GPC analyses verify that the successful synthesis of the PS-b-P4VP block copolymers.The kinetic study shows slower polymerization rate when using the CuBr/dNBipy catalyst system because the same pyridine unit functions as ligand and monomer.On

the contrary,faster polymerization rate is obtained by us-ing the CuBr/PMDETA catalyst system.In the preparation of block copolymers starting from a PS macroinitiator,the use of halogen exchange technique results in better struc-tural control for the polymerization of 4-vinylpyridine than that without the halogen exchange.DSC measurement of the PS-b-P4VP block copolymer exhibits two glass transitions,indicating that the resulting block copolymers are phase sep-arated in condensed state.Two maximum differential peaks are found on the TGA trace for the PS-b-P4VP block copoly-mer as the result of the decomposition of the P4VP blocks at 380?C and the PS blocks at higher temperature.

Acknowledgements

This research was ?nancially supported by the National Sci-ence Council,Taiwan,Republic of China,under Contract Nos.NSC-90-2216-E-009-026.

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