Polymer International Polym Int 55:558–564 (2006)

Synthesis and characterizationof four-arm star-shaped polymers andcopolymersTzong-Liu Wang1∗ and Ching-Guey Tseng2

1Department of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung 811, Taiwan, Republic of China2Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan, Republicof China

Abstract: Four-arm star-shaped polymers and copolymers were obtained by transition metal-catalyzed atom-transfer radical polymerization (ATRP). The polymers were characterized by FTIR and 1H-NMR spectroscopy.Gel permeation chromatography results indicated the formation of polystyrene and polystyrene-block-poly(methylmethacrylate) (PS-b-PMMA) arms with controlled molecular weights. In dilute solution, the linear polymers hadhigher inherent viscosities than star-shaped ones. Thermogravimetric analysis showed a similar degradationmechanism for linear and star-shaped polymers. Differential scanning calorimetry indicated the successfulformation of diblock star-shaped copolymers. 2006 Society of Chemical Industry

Keywords: atom-transfer radical polymerization; four-arm; star-shaped polymers; viscosity

INTRODUCTIONSince star-shaped polymers, which are structuresof linked polymers with a core of small molecularweight, were first proposed by Schaefgen and Floryin 1948,1 these polymers have drawn increasinginterest because their viscosity is lower than thatof the corresponding linear polymers.2 Comparedwith the corresponding linear polymers of similarmolecular weight, star-shaped polymers also havelower melting points and solution viscosities andsuperior mechanical properties.3–10 Furthermore, theyare more processable. Therefore, some star-shapedpolymers have been used as rheology modifiers orpressure-sensitive adhesives.

Star-shaped polymers can be synthesized in twomain ways: ‘arm-first’ and ‘core-first’. The arm-firstapproach consists of reacting preformed, terminallyfunctionalized polymer chains with a multifunctionalquencher molecule that forms the core of the star.11–15

However, the reaction of several polymer chains witha single core molecule is often difficult becauseof steric hindrance, which can lead to stars withmissing or unattached arms. The alternative core-first approach involves growing polymers from amultifunctional initiator or precursor polymer.16,17

Here, the core reacts only with monomers. Onceinitiation is overcome, the steric problems diminishas the chains grow, and the distances between thereactive centers increase.

To synthesize well-defined stars, the use ofinitiators with precise functionality is essential.

Furthermore, regular star-shaped polymers cannot besuccessfully synthesized by means of conventionalfree-radical polymerization, as it would inevitablyresult in crosslinked materials owing to uncontrolledtermination reactions. Among the variety of methodsreported, living polymerization seems to be the mostsuitable for the synthesis of branched polymers withcontrolled structure.18–21

The concept of living polymerization was firstreported by Szwarc in 1956,22 and it appearsto be the simplest technique to reach the goalof achieving control over not only the molecularweight and polydispersity of polymer chains butalso their composition, architecture and end-groupfunctionalities. Living polymerization has also beenextended to free-radical polymerization, which isgenerally much more tolerant of moisture and otherimpurities and is therefore of great commercialimportance.23 As transition metal-catalyzed atom-transfer radical polymerization (ATRP) has beenproven to be effective for a wide range of monomersboth in bulk and in a variety of solvents, and performedat moderate temperatures (80–120 ◦C), it is emergingas a powerful technique for the polymer industry,allowing for the efficient synthesis of novel, tailor-made materials.24–26 Furthermore, the method allowsthe production of new classes of inorganic copolymerswhich generally possess properties that are differentfrom those of organic polymers.27,28

In this article, a star-shaped polymer and itsdiblock copolymer with narrow molecular weight

∗ Correspondence to: Tzong-Liu Wang, Department of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung 811, Taiwan, Republicof ChinaE-mail: tlwang@nuk.edu.twContract/grant sponsor: National Science Council of the Republic of China; contract/grant number: NSC 90-2216-E-151-003(Received 3 September 2005; revised version received 18 October 2005; accepted 13 December 2005)DOI: 10.1002/pi.2008

2006 Society of Chemical Industry. Polym Int 0959–8103/2006/$30.00

Synthesis and characterization of star-shaped polymers

distribution were synthesized successfully by transitionmetal-catalyzed ATRP through core-first technology.At first, a multifunctional initiator was prepared viathe reaction of 2,4,6,8-tetramethylcyclotetrasiloxane(TMCTS) with 4-vinyl benzyl chloride (VBC) byusing H2PtCl6/tetrahydrofuran (THF) solution ascatalyst. After becoming a ‘living’ free radical, themultifunctional initiator was then used to initiatestyrene monomer to form a star-shaped polymerby a catalyst system of CuCl and N, N, N ′, N ′, N ′-pentamethyldiethylenetriamine (PMDETA). Finally,the star-shaped polymer reacted with anothermonomer, methyl methacrylate (MMA), to form adiblock star-shaped copolymer by ATRP.

EXPERIMENTALMaterialsMMA (EP grade) and styrene (EP grade) werepurchased from Nihon Shiyaku Industries, Ltd. Beforebeing used, MMA, styrene and anisole (Fluka, reagentgrade) were distilled under reduced pressure. Cuprouschloride (CuCl, EP grade), purchased from UnionChemical Works, Ltd, was purified by washingwith glacial acetic acid several times and oncewith anhydrous ether, then filtered under N2 anddried in an oven to give a white powder. VBC(reagent grade, Acros), glacial acetic acid (EP grade,Nihon Shiyaku) and hydrogen hexachloroplatinate(IV) hexahydrate (reagent grade, Showa Chemicals)were used as received. Reagent grade 2,2′-bipyridine(bipy), PMDETA and TMCTS from Aldrich wereused as received.

Synthesis of the multifunctional star coresiloxane initiatorTo a preflamed round-bottomed flask were added5 g (20.8 mmol) of TMCTS, 12.7 g (83.2 mmol) ofVBC and 2 mL of 0.12 mol L−1 catalyst solution(H2PtCl6/THF), sequentially. The mixture was stirredfor 24 h at 45 ◦C, and then cooled to roomtemperature. The solvent, toluene (10 mL), wascharged into the reaction flask, which was then stirredfor a further 24 h at 50 ◦C. The reaction is outlinedin Scheme 1. The four-arm multifunctional star core(SC) siloxane initiator was obtained by a rotaryevaporator to remove unreacted VBC and catalyst.

Synthesis of SC-star-PS star-shaped polymersby ATRPThe polymer was synthesized in a three-necked round-bottomed flask equipped with a magnetic stirrer incor-porating an N2 inlet valve. Styrene monomer (14.4 g,138 mmol), CuCl/PMDETA catalyst composed of0.137 g (1.38 mmol) of CuCl and 0.239 g (1.38 mmol)of PMDETA, 0.044 g (0.69 mmol) of copper powderand 41 mL (35.5 g) of solvent (toluene) (with a weightratio of reactants to solvent of 3:7) were introducedinto the flask and thoroughly mixed. After the flaskwas purged with high-purity N2 at room tempera-ture to remove moisture, 0.5 g (0.587 mmol) of theSC multifunctional initiator was added to the mix-ture, and the flask was purged with N2 once again.The resultant mixture was then heated to 110 ◦Cwith stirring for 20 h to give the star-shaped poly-mer SC-star-PS (SsPS). The yield was estimated to

Scheme 1. Reaction pathway for the synthesis of the four-arm star core (SC) siloxane initiator.

Scheme 2. Reaction pathway for the synthesis of SC-star-PS (SsPS).

Polym Int 55:558–564 (2006) 559DOI: 10.1002/pi

T-L Wang, C-G Tseng

Scheme 3. Reaction pathway for the synthesis of SsPS-b-PMMA.

be 93.0%. Scheme 2 outlines the synthesis of SsPS byatom-transfer radical polymerization.

Synthesis of SsPS-b-PMMA diblock star-shapedcopolymers by ATRPScheme 3 outlines the atom-transfer radical polymer-ization of SsPS-b-PMMA. After 6.28 g (62.7 mmol) ofMMA monomer was added to the reaction flask, it waspurged with high-purity N2 to remove moisture. Then,the CuCl/PMDETA catalyst composed of 0.062 g(0.627 mmol) of CuCl and 0.109 g (0.627 mmol) ofPMDETA, and 0.020 g (0.314 mmol) of copper pow-der were weighed into the flask, and the flask waspurged with N2 once again. The solvent (toluene)(38 mL, 32.9 g) was added into the reactor. After themixture was thoroughly mixed, 9.98 g (0.414 mmol)of the star-shaped polymer SsPS was added and usedas an initiator, and the flask was purged with N2 oncemore. The polymerization was carried out at 110 ◦Cwith stirring for 20 h to obtain SsPS-b-PMMA. Thepolymer, recovered by precipitation and filtration, anddried in vacuo overnight, was obtained in 63.4% yield.

Synthesis of linear PS polymers and diblockPS-b-PMMA copolymers by ATRPThe corresponding linear PS polymers and diblock PS-b-PMMA copolymers with similar molecular weightsas those of the star-shaped polymers were preparedin a similar procedure by ATRP except that the SCinitiator was changed to 1-bromoethyl benzene.

Purification of the polymeric productsIn order to obtain accurate analytical results, thepolymeric products were further purified to removeresidual unreacted monomers and catalysts. The solidpolymeric products were dissolved in small amountsof dimethylformamide (DMF) and the solutions werepoured into appropriate amounts of solvent mixtures(DMF:CH3OH of 1:10) to precipitate out the solidpolymeric products. The impurities were removedthrough these procedures, and the precipitated pureproducts were dried in an oven.

CharacterizationFTIR spectra of the samples were obtained using aBio-Rad FTS 165 Fourier-transform infrared spec-trometer over the frequency range 4000–400 cm−1 ata resolution of 4 cm−1. The synthesized compoundsand polymers were dissolved in deuterated chloroform(CDCl3), dimethylsulfoxide-d6 (DMSO-d6) or N,N-dimethylformamide-d7 (DMF-d7), and then charac-terized with 1H-NMR using a Varian UNITY INOVA-500 FT-NMR spectrometer. Number-average (Mn)and weight-average (Mw) molecular weights andmolecular weight distributions (MWDs) (Mw/Mn)were determined by gel permeation chromatography(GPC) using a Waters liquid chromatograph equippedwith a 410 RI detector and three µ-Styragel columns.Inherent viscosity (ηinh) measurements were carriedout at a solution concentration of 0.5 g polymer per100 mL of solvent at 30 ◦C using a Cannon-Fenskeviscosimeter. Thermogravimetric analysis (TGA) ofthe star-shaped polymers was carried out on powdersplaced in a platinum sample pan using a TA Instru-ments SDT-2960 analyzer. Sample powders rangingfrom 4 to 5 mg were loaded into the platinum panand sealed in the sample chamber. The samples wereheated from 50 to 600 ◦C under an N2 atmosphereat the rate of 10 ◦C min−1. Differential scanningcalorimetry (DSC) scans from 30 to 200 ◦C wereobtained using a TA Instruments modulated DSC2920 analyzer at a heating rate of 5 ◦C min−1 under adry N2 purge.

RESULTS AND DISCUSSIONSynthesis of the four-arm SC siloxane initiatorAs mentioned above, conventional free-radical poly-merization would result in crosslinked materials owingto uncontrolled termination reactions, and thus thestar-shaped polymers were synthesized by ATRP tech-niques. In this research, TMCTS was used as thestarting material for the synthesis of the four-armsiloxane initiator. The reaction was carried out by ahydrosilation reaction with VBC under mild condi-tions. During the synthetic stage, the H2PtCl6/THFcatalyst should be added after TMCTS and VBC,and the solvent (toluene) should be added in the next

560 Polym Int 55:558–564 (2006)DOI: 10.1002/pi

Synthesis and characterization of star-shaped polymers

Figure 1. FTIR spectra of star-shaped polymers: (a) SC; (b) SsPS;(c) SsPS-b-PMMA.

stage, otherwise the reaction would not go to com-pletion. Excess TMCTS, VBC and catalyst should beavoided. In addition, the reaction temperature shouldbe kept below 60 ◦C to minimize side reactions.

Figure 1(a) shows the FTIR spectrum of SC. Theabsorption bands at 1080 and 800 cm−1 are ascribedto the presence of Si–O bonds and C–Cl bonds,respectively. This may confirm that the formationof SC was via the reaction of TMCTS and VBC.Furthermore, the absence of an absorption band at2162 cm−1 for Si–H bonds also indicates that thereaction was complete.

The 1H-NMR spectrum of SC is shown in Fig. 2.The protons of Si–CH3, CH of the benzene ring,CH2Cl, Si–CH2 and CH2 adjacent to the benzenering are shown at 0.01–0.16, 7.16–7.34, 4.60,1.38 and 2.65 ppm, respectively, thereby confirmingthe formation of SC. SC has a structural formulaof product A as shown in Scheme 1. Actually,the structure is that of the product that wasaimed for.

However, some extra peaks were also present in theNMR spectrum, which correspond to Si–CH–CH3and Si–CH–CH3 observed at ca 0.85 and 2.18 ppm,respectively. This may indicate that side reactionstook place. A Si–C coupling reaction might alsohave occurred at the α-carbon of the vinyl group (i.e.the carbon atom of the double bond adjacent to thebenzene ring). From the peak area ratios of Si–CH2 toSi–CH–CH3 and of Si–CH2– CH2 to Si–CH–CH3,the weight ratio of product A to B was found to beca 7:3. This ratio indicates that SC of structure Awas predominantly obtained although a side reactionoccurred. In addition, both types of SC were effectivein the initiation step of ATRP.

Synthesis of SC-star-PSThe four-arm siloxane initiator bearing benzyl chloridemoieties was capable of initiating atom-transfer livingradical polymerization. The monomer employed forthe first block was styrene. In this reaction, two

Figure 2. 1H-NMR spectrum of the four-arm SC siloxane initiator.

Polym Int 55:558–564 (2006) 561DOI: 10.1002/pi

T-L Wang, C-G Tseng

kinds of catalysts, CuCl/bipy and CuCl/PMDETA,were tested. It was found that products with broaderMWDs and lower molecular weights were obtainedwhen CuCl/bipy was used. So, CuCl/PMDETA wasselected as the catalyst for the synthesis of SsPS. Toavoid getting products with broader MWD and lowermolecular weight, the initiator, SC, should not beadded first at the beginning of the reaction.

The reaction was conducted at 110 ± 3 ◦C whentoluene was used as solvent. Temperatures below105 ◦C were unfavourable because fewer or even noproducts were obtained, and molecular weights werebroader.

In general, ATRP can be carried out either inbulk, in solution, or in a heterogeneous system. Thesolvent effect on the polymer syntheses by ATRP hasbeen studied in our group.29 In this experiment, theamount of solvent (toluene) had significant effectson the molecular weight and MWD of the products.Without solvent, products with broader MWDs andhigher molecular weights would be obtained. Thiscould be attributed to higher polymerization rates forbulk polymerization. However, excess solvent reducedthe yield to 50–70%, and resulted in slightly lowermolecular weights. This might be due to chain transferto solvent, catalyst poisoning by the solvent30 orsolvent-assisted side reactions, such as elimination ofHX from polystyryl halides.31 In the present article,it was found that products with lower polydispersities

could be obtained when the weight ratio of reactantsto solvent was less than unity. A weight ratio of ca 3:7for reactants to solvent is suggested.

Figure 1(b) shows the FTIR spectrum of SC-star-PS (SsPS), with the same absorption bands as thosefor PS. In addition, the absorption band at 1070 cm−1

confirms the presence of Si–O bonds. This confirmsthat the synthesis of SsPS was successful.

The 1H-NMR of SsPS is shown in Fig. 3(a).In order to observe the initiator fragments, thesample used here had a low molecular weight (Mn =4800 g mol−1). Except for the resonance peaks arisingfrom SC, the 1H-NMR spectrum is very similar tothat of PS. This fact also confirms that the synthesisof SsPS was successful.

Synthesis of SsPS-b-PMMAThe reaction phenomena were similar to those withthe synthesis of SsPS. The amount of solvent (toluene)also had a significant effect on the molecular weightand MWD of the products. It was found that productswith lower polydispersities could be obtained whenthe weight ratio of solvent to reactant was about two.For example, when the polymerization was carriedout using a higher weight ratio of 3:1 for solvent toreactants, a lower yield (ca 50%) with a lower Mn and ahigher polydispersity was obtained. On the other hand,when the polymerization was performed with a weightratio of 1:2 for solvent to reactants, a higher yield (ca

Figure 3. 1H-NMR spectra of star-shaped polymers: (a) SsPS; (b) SsPS-b-PMMA.

562 Polym Int 55:558–564 (2006)DOI: 10.1002/pi

Synthesis and characterization of star-shaped polymers

78%) with a higher Mn and a higher polydispersitywas obtained. The reason has been mentioned above.

The FTIR spectrum of SsPS-b-PMMA is shown inFig. 1(c). In addition to the same absorption bandsas those of PS, the spectrum shows absorption bandsat 1730 and 1140 cm−1. The former is assigned tocarbonyl C=O stretching, while the latter correspondsto O=C–O absorption. This spectrum shows thepresence of aromatic structures (PS) and ester linkagesof PMMA.

Figure 3(b) shows the 1H-NMR spectrum of SsPS-b-PMMA. As already done with SsPS, a low molecularweight sample (Mn = 8600 g mol−1) was used tocharacterize the initiator fragments. Except that peaksfrom SC and PS are observed, the incorporation ofPMMA is evident by the presence of the resonancepeak at δ = 3.63 ppm representing COOCH3. Theseresults clearly show the presence of styrene and methylmethacrylate.

GPC analysis and viscosity measurementsAs seen from Table 1, the GPC results show that SsPSmacroinitiator and SsPS-b-PMMA were successfullysynthesized. By using ATRP, products with narrowMWD were obtained. The theoretical molecularweights for SsPS and SsPS-b-PMMA can be calculatedfrom the following equation:

Mn,theo = MWmonomer × [M]0

[I]0

× conversion + MWinitiator

where [M]0 is the initial concentration of the monomerand [I]0 is the initial concentration of the initiator.From the above equation, the theoretical number-average molecular weights, Mn,theo, for SsPS and SsPS-b-PMMA are 23 600 and 33 700 g mol−1, respectively.The initiator efficiencies (Mn,theo/Mn,GPC) were closeto 1.0 in both cases. These results suggest thatthe polymerization was well controlled. In addition,only one sharp peak was observed for each product,which indicates that the number of arms of thesynthesized star-shaped polymers were close to thatexpected. For comparison, linear polymers withsimilar molecular weights were also prepared byATRP. As seen from this table, it is apparent that star-shaped polymers have much lower viscosities than thecorresponding linear polymers. This can be attributed

Table 1. Properties of the synthesized star-shaped and linear

polymers

SampleMn

(g mol−1)

Mw/Mn

ηinh

(dL g−1)aYield(%)

SC-star-PS 24 100 1.20 0.07 93.0SsPS-b-PMMA 34 800 1.29 0.42 63.4PS 24 000 1.15 0.12 84.8PS-b-PMMA 36 100 1.34 1.48 69.1

a Inherent viscosity at 30 ◦C in toluene.

0 100 200 300 400 500 600

0

20

40

60

80

100

SsPS-b-PMMAPS-b-PMMASsPSPSSC

Wei

ght

loss

(%

)

Temperature (°C)

Figure 4. TGA curves of linear and star-shaped polymers under N2

atmosphere.

to smaller hydrodynamic radii in toluene for star-shaped polymers, indicating more compact forms ofthis type of material.

Thermal analysisThe thermal stability of the star-shaped materialsand the corresponding linear polymers in N2 isillustrated in Fig. 4. All of the TGA curves exceptthat of SC display a one-step mechanism fordegradation. It is apparent that SsPS has a higherdegradation temperature than the correspondingSsPS-b-PMMA diblock copolymer. Since PMMAhas a lower thermal stability than PS, this resultis acceptable. This may imply that the PMMAsegment has been successfully incorporated in thestar-shaped copolymer. In comparison with the star-shaped polymer and copolymer, a similar trend forthe phenomena of thermal stability is observed forthe corresponding linear polymer and copolymer. Onthe other hand, the multifunctional star core has amulti-step degradation mechanism and a very highchar residue at 600 ◦C. As seen from the structure ofSC, the initial degradation may arise from the VBCsegments while the higher char residue may be due tothe inorganic part of siloxane.

Differential scanning calorimetryThe DSC curves are shown in Fig. 5 for the star-shaped materials and linear polymers. For SsPS, theglass transition temperature (Tg) is observed at ca102 ◦C. However, when PMMA was incorporatedinto the copolymer, the Tg was higher at ca 110 ◦C.Since PMMA has a higher Tg than PS with a similarmolecular weight, this indicates that the diblock star-shaped copolymer was successfully synthesized. Thecorresponding linear polymers PS and PS-b-PMMAshow a similar trend. The former has a Tg at ca103 ◦C, while the latter has a higher Tg at ca 105 ◦C.Moreover, both Tg values of the linear polymers areaccompanied by a melting peak, which is not observedfor the star-shaped materials. This can be attributedto the phenomenon of physical ageing.

Polym Int 55:558–564 (2006) 563DOI: 10.1002/pi

T-L Wang, C-G Tseng

0 20 40 60 80 100 120 140 160 180 200 220

SsPSSsPS-b-PMMAPSPS-b-PMMA

End

oE

xoH

eat f

low

(W

g−1

)

Temperature (°C)

Figure 5. DSC curves of linear and star-shaped polymers.

CONCLUSIONSStar-shaped polymers and diblock copolymers withnarrow molecular weight distributions and lowerviscosities were synthesized successfully by transi-tion metal-catalyzed ATRP through core-first tech-nology. The multifunctional initiator SC, preparedby the reaction of TMCTS with VBC and usingH2PtCl6/THF solution as catalyst, was used in ATRPwith styrene to form the star-shaped polymer SsPSwith CuCl/PMDETA as a catalyst. In addition, thediblock star-shaped copolymer SsPS-b-PMMA couldbe synthesized using SsPS as a macroinitiator andthe star core. TGA results showed that SsPS had ahigher degradation temperature than the correspond-ing diblock copolymer SsPS-b-PMMA and thereforeconfirmed that a diblock star-shaped copolymer wassuccessfully synthesized. In addition, DSC studiesindicated that the star-shaped polymers were amor-phous materials and had similar glass transition tem-peratures to those of the linear polymers.

ACKNOWLEDGEMENTSWe gratefully acknowledge the support of the NationalScience Council of the Republic of China with grantNSC 90-2216-E-151-003.

REFERENCES1 Schaefgen JR and Flory PJ, J Am Chem Soc 70:2709 (1948).2 Kim YH, in Encyclopedia of Polymeric Materials, ed. by

Salamone JC. CRC Press Inc., Boca Raton, p. 3049 (1996).

3 Takano A, Okada M and Nose T, Macromolecules 25:3596(1992).

4 Chujo Y, Sada K, Kaisaki T and Saegusa T, Polym J 24:1301(1992).

5 Kanaoka S, Sueoka M, Sawamoto M and Higashimura T, JPolym Sci, Polym Chem 31:2513 (1993).

6 Raphael E, Pincus P and Fredrickson GH, Macromoleclues26:1996 (1993).

7 Naka K, Konishi G, Kotera K and Chujo Y, Polym Bull 41:263(1998).

8 Choi YK, Bae YH and Kim SW, Macromolecules 31:8766(1998).

9 Ruckenstein E and Zhang H, Macromolecules 32:3979 (1999).10 Deng F, Bisht KS, Gross RA and Kaplan DL, Macromolecules

32:5159 (1999).11 Zhang X, Xia KJH and Matyjaszewski K, Macromolecules

33:2340 (2000).12 Xia JH, Zhang X and Matyjaszewski K, Macromolecules 32:4482

(1999).13 Baek K, Kamigaito M and Sawamoto M, Macromolecules 34:215

(2001).14 Baek K, Kamigaito M and Sawamoto M, Macromolecules

35:1493 (2002).15 Bosman AW, Vestberg R, Heumann A, Frechet JMJ and

Hawker CJ, J Am Chem Soc 125:715 (2003).16 Ueda J, Kamigaito M and Sawamoto M, Macromolecules

31:6762 (1998).17 Ueda J, Kamigaito M and Sawamoto M, Macromolecules 31:557

(1998).18 Wang JS, Greszta D and Matyjaszewski K, Polym Mater Sci Eng

73:416 (1995).19 Cloutet E, Fillaut JL, Astruc D and Gnanou Y, Macromolecules

31:6748 (1998).20 Angot S, Murthy KS, Taton D and Gnanou Y, Macromolecules

31:7218 (1998).21 Ueda J, Matsuyama M, Kamigaito M and Sawamoto M, Macro-

molecules 31:557 (1998).22 Szwarc M, Nature 178:1168 (1956).23 Mishra MK and Yagci Y, in Handbook of Radical Vinyl

Polymerization, ed. by Mishra M. Marcel Dekker, New York,ch. 10 (1998).

24 Wang JS and Matyjaszewski K, Macromolecules 28:7901 (1995).25 Wang JS and Matyjaszewski K, J Am Chem Soc 117:5614

(1995).26 Patten TE, Xia J, Abernathy T and Matyjaszewski K, Science

272:866 (1996).27 Yu WH, Kang ET, Neoh KG and Zhu SP, J Phys Chem B

107:10 198 (2003).28 Miller PJ and Matyjaszewski K, Macromolecules 32:8760 (1999).29 Wang TL, Liu YZ, Jeng BC and Cai YC, J Polym Res 12:67

(2005).30 Matyjaszewski K, Patten TE and Xia J, J Am Chem Soc 119:674

(1997).31 Matyjaszewski K, Davis KA, Patten TE and Wei M, Tetrahedron

53:15 321 (1997).

564 Polym Int 55:558–564 (2006)DOI: 10.1002/pi